General requirements for building an ieee 802.11 network. All existing Wi-Fi network standards

Currently, mainly three standards of the IEEE 802.11 group are widely used (presented in Table 1.1)

Table 1.1 - Main characteristics of IEEE 802.11 group standards

1.3.1 IEEE 802.11g standard

The IEEE 802.11g standard, adopted in 2003, is a logical development of the 802.11b standard and involves data transmission in the same frequency range, but at higher speeds. Additionally, 802.11g is fully compatible with 802.11b, meaning any 802.11g device must be able to work with 802.11b devices. The maximum data transfer rate in the 802.11g standard is 54 Mbit/s. During the development of the 802.11g standard, two competing technologies were considered: the OFDM orthogonal frequency division method, borrowed from the 802.11a standard and proposed for consideration by Intersil, and the PBCC binary packet convolutional coding method, proposed by Texas Instruments. As a result, the 802.11g standard contains a compromise solution: OFDM and CCK technologies are used as base technologies, and the optional use of PBCC technology is provided.

The idea of ​​convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The incoming sequence of information bits is converted in a convolutional encoder so that each input bit corresponds to more than one output bit. That is, the convolutional encoder adds certain redundant information to the original sequence. If, for example, each input bit corresponds to two output bits, then we talk about convolutional coding with a rate of 1/2. If every two input bits correspond to three output bits, then the convolutional coding speed will be 2/3.

Any convolutional encoder is built on the basis of several sequentially connected memory cells and XOR gates. The number of storage cells determines the number of possible encoder states. If, for example, a convolutional encoder uses six memory cells, then the encoder stores information about six previous signal states, and taking into account the value of the input bit, we find that such an encoder uses seven bits of the input sequence. Such a convolutional encoder is called a seven-state encoder.

The output bits generated by the convolutional encoder are determined by XOR operations between the values ​​of the input bit and the bits stored in the storage cells, that is, the value of each output bit generated depends not only on the incoming information bit, but also on several previous bits.

The main advantage of convolutional encoders is the noise immunity of the sequence they generate. The fact is that with redundant coding, even in the event of reception errors, the original bit sequence can be accurately restored. A Viterbi decoder is used at the receiver side to restore the original bit sequence.

The dibit generated in the convolutional encoder is subsequently used as a transmitted symbol, but it is first subjected to phase modulation. Moreover, depending on the transmission speed, binary, quadrature or even eight-position phase modulation is possible.

Unlike DSSS technologies (Barker codes, SSK sequences), convolutional coding technology does not use spectrum broadening technology through the use of noise-like sequences, however, spectrum broadening to standard 22 MHz is also provided in this case. To do this, variations of possible QPSK and BPSK signal constellations are used.

The considered PBCC coding method is optionally used in the 802.11b protocol at speeds of 5.5 and 11 Mbit/s. Similarly, in the 802.11g protocol for transmission speeds of 5.5 and 11 Mbit/s, this method is also used optionally. In general, due to the compatibility of the 802.11b and 802.11g protocols, the encoding technologies and speeds provided by the 802.11b protocol are also supported in the 802.11g protocol. In this regard, up to a speed of 11 Mbps, the 802.11b and 802.11g protocols are the same, except that the 802.11g protocol provides speeds that the 802.11b protocol does not.

Optionally, in the 802.11g protocol, PBCC technology can be used at transmission rates of 22 and 33 Mbit/s.

For a speed of 22 Mbit/s, compared to the PBCC scheme we have already considered, data transmission has two features. First of all, 8-position phase modulation (8-PSK) is used, that is, the signal phase can take on eight different values, which allows three bits to be encoded in one symbol. In addition, a puncture encoder (Puncture) has been added to the circuit, with the exception of the convolutional encoder. The meaning of this solution is quite simple: the redundancy of the convolutional encoder, equal to 2 (for each input bit there are two output bits), is quite high and under certain noise conditions it is unnecessary, so the redundancy can be reduced so that, for example, every two input bits correspond to three output bits . For this, you can, of course, develop an appropriate convolutional encoder, but it is better to add a special puncture encoder to the circuit, which will simply destroy extra bits. Let's say a puncture encoder removes one bit from every four input bits. Then every four incoming bits will correspond to three outgoing ones. The speed of such an encoder is 4/3. If such an encoder is used in conjunction with a convolutional encoder with a speed of 1/2, then the total encoding speed will be 2/3, that is, for every two input bits there will be three output bits.

PBCC technology is optional in the IEEE 802.11g standard, and OFDM technology is mandatory. In order to understand the essence of OFDM technology, let's take a closer look at the multipath interference that occurs when signals propagate in an open environment.

The effect of multipath signal interference is that, as a result of multiple reflections from natural obstacles, the same signal can reach the receiver in different ways. But different propagation paths differ from each other in length, and therefore the signal attenuation will not be the same for them. Consequently, at the receiving point, the resulting signal represents the interference of many signals having different amplitudes and shifted relative to each other in time, which is equivalent to the addition of signals with different phases.

The consequence of multipath interference is distortion of the received signal. Multipath interference is inherent in any type of signal, but it has a particularly negative effect on wideband signals, since when using a broadband signal, as a result of interference, certain frequencies add up in phase, which leads to an increase in the signal, and some, on the contrary, out of phase, causing a weakening of the signal at a given frequency.

Speaking about multipath interference that occurs during signal transmission, two extreme cases are noted. In the first of them, the maximum delay between signals does not exceed the duration of one symbol and interference occurs within one transmitted symbol. In the second, the maximum delay between signals is greater than the duration of one symbol, so as a result of interference, signals representing different symbols are added, and so-called inter-symbol interference (ISI) occurs.

It is intersymbol interference that has the most negative effect on signal distortion. Since a symbol is a discrete state of a signal, characterized by the values ​​of carrier frequency, amplitude and phase, the amplitude and phase of the signal change for different symbols, and therefore it is extremely difficult to restore the original signal.

For this reason, at high data rates, a data encoding method called Orthogonal Frequency Division Multiplexing (OFDM) is used. Its essence lies in the fact that the stream of transmitted data is distributed over many frequency subchannels and transmission is carried out in parallel on all such subchannels. In this case, a high transmission speed is achieved precisely due to the simultaneous transmission of data over all channels, while the transmission speed in a separate subchannel may be low.

Due to the fact that the data transmission rate in each of the frequency subchannels can be made not too high, the prerequisites are created for effective suppression of intersymbol interference.

Frequency division of channels requires that an individual channel be narrow enough to minimize signal distortion, but at the same time wide enough to provide the required transmission speed. In addition, to economically use the entire bandwidth of a channel divided into subchannels, it is desirable to arrange the frequency subchannels as close to each other as possible, but at the same time avoid interchannel interference to ensure their complete independence. Frequency channels that satisfy the above requirements are called orthogonal. The carrier signals of all frequency subchannels are orthogonal to each other. It is important that the orthogonality of the carrier signals guarantees the frequency independence of the channels from each other, and therefore the absence of inter-channel interference.

This method of dividing a wideband channel into orthogonal frequency subchannels is called orthogonal frequency division multiplexing (OFDM). To implement it in transmitting devices, an inverse fast Fourier transform (IFFT) is used, which transforms the previously multiplexed n -channels signal from time O th representation into frequency.

One of the key advantages of the OFDM method is the combination of high transmission speed with effective resistance to multipath propagation. Of course, OFDM technology itself does not eliminate multipath propagation, but it creates the prerequisites for eliminating the effect of intersymbol interference. The fact is that an integral part of OFDM technology is the Guard Interval (GI) - a cyclic repetition of the end of the symbol, attached at the beginning of the symbol.

The guard interval creates pauses between individual symbols, and if its duration exceeds the maximum signal delay time due to multipath propagation, then intersymbol interference does not occur.

When using OFDM technology, the duration of the guard interval is one-fourth of the duration of the symbol itself. In this case, the symbol has a duration of 3.2 μs, and the guard interval is 0.8 μs. Thus, the duration of the symbol together with the guard interval is 4 μs.

The 802.11g protocol uses binary and quadrature phase modulation BPSK and QPSK at low bit rates. When using BPSK modulation, only one information bit is encoded in one symbol, and when using QPSK modulation, two information bits are encoded. BPSK modulation is used to transmit data at speeds of 6 and 9 Mbit/s, and QPSK modulation is used at speeds of 12 and 18 Mbit/s.

For transmission at higher speeds, quadrature amplitude modulation QAM (Quadrature Amplitude Modulation) is used, in which information is encoded by changing the phase and amplitude of the signal. The 802.11g protocol uses 16-QAM and 64-QAM modulation. The first modulation involves 16 different signal states, which allows 4 bits to be encoded in one symbol; the second - 64 possible signal states, which makes it possible to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.

1.3.2 IEEE 802.11a standard

The IEEE 802.11a standard provides data transfer rates of up to 54 Mbit/s. Unlike the basic standard, the 802.11a specifications provide for operation in the new 5 GHz frequency range. Orthogonal frequency multiplexing (OFDM) was chosen as a signal modulation method, which ensures high communication stability in conditions of multipath signal propagation.

In accordance with FCC rules, the UNII frequency range is divided into three 100-MHz sub-bands, differing in maximum emission power limits. The low band (5.15 to 5.25 GHz) provides only 50 mW of power, the middle (5.25 to 5.35 GHz) 250 mW, and the high (5.725 to 5.825 GHz) 1 W. The use of three frequency subbands with a total width of 300 MHz makes the IEEE 802.11a standard the most broadband of the 802.11 family of standards and allows the entire frequency range to be divided into 12 channels, each of which has a width of 20 MHz, with eight of them lying in the 200 MHz range from 5 .15 to 5.35 GHz, and the remaining four channels are in the 100 MHz range from 5.725 to 5.825 GHz (Figure 1.3). At the same time, the four upper frequency channels, which provide the highest transmission power, are used primarily for transmitting signals outdoors.

Figure 1.3 - Division of the UNII range into 12 frequency subbands

The IEEE 802.11a standard is based on the Orthogonal Frequency Division Multiplexing (OFDM) technique. To separate the channels, an inverse Fourier transform is used with a window of 64 frequency subchannels. Since each of the 12 channels defined in the 802.11a standard is 20 MHz wide, each orthogonal frequency subchannel (subcarrier) is 312.5 kHz wide. However, out of 64 orthogonal subchannels, only 52 are used, with 48 of them used for data transmission (Data Tones), and the rest for transmission of service information (Pilot Tones).

In terms of modulation technology, the 802.11a protocol is not much different from 802.11g. At low bit rates, binary and quadrature phase modulation BPSK and QPSK are used to modulate subcarrier frequencies. When using BPSK modulation, only one information bit is encoded in one symbol. Accordingly, when using QPSK modulation, that is, when the phase of the signal can take four different values, two information bits are encoded in one symbol. BPSK modulation is used to transmit data at 6 and 9 Mbps, and QPSK modulation is used at 12 and 18 Mbps.

To transmit at higher speeds, the IEEE 802.11a standard uses 16-QAM and 64-QAM quadrature amplitude modulation. In the first case there are 16 different signal states, which allows you to encode 4 bits in one symbol, and in the second there are already 64 possible signal states, which allows you to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at speeds of 24 and 36 Mbit/s, and 64-QAM modulation is used at speeds of 48 and 54 Mbit/s.

The information capacity of an OFDM symbol is determined by the type of modulation and the number of subcarriers. Since 48 subcarriers are used for data transmission, the capacity of an OFDM symbol is 48 x Nb, where Nb is the binary logarithm of the number of modulation positions, or, more simply, the number of bits that are encoded in one symbol in one subchannel. Accordingly, the OFDM symbol capacity ranges from 48 to 288 bits.

The sequence of processing input data (bits) in the IEEE 802.11a standard is as follows. Initially, the input data stream is subjected to a standard scrambling operation. After this, the data stream is fed to the convolutional encoder. The convolutional coding rate (in combination with puncture coding) can be 1/2, 2/3 or 3/4. Since the convolutional coding rate can be different, when using the same type of modulation, the data transmission rate is different. Consider, for example, BPSK modulation, where the data rate is 6 or 9 Mbit/s. The duration of one symbol together with the guard interval is 4 μs, which means that the pulse repetition rate will be 250 kHz. Considering that one bit is encoded in each subchannel, and there are 48 such subchannels in total, we obtain that the total data transfer rate will be 250 kHz x 48 channels = 12 MHz. If the convolutional coding speed is 1/2 (one service bit is added for each information bit), the information speed will be half the full speed, that is, 6 Mbit/s. At a convolutional coding rate of 3/4, for every three information bits one service bit is added, so in this case the useful (information) speed is 3/4 of the full speed, that is, 9 Mbit/s. Similarly, each modulation type corresponds to two different transmission rates (Table 1.2).

Table 1.2 - Relationship between transmission rates and modulation type in the 802.11a standard

After convolutional encoding, the bit stream is subjected to interleaving, or interleaving. Its essence is to change the order of bits within one OFDM symbol. To do this, the sequence of input bits is divided into blocks whose length is equal to the number of bits in the OFDM symbol (NCBPS). Next, according to a certain algorithm, a two-stage rearrangement of bits in each block is performed. In the first stage, the bits are rearranged so that adjacent bits are transmitted on non-adjacent subcarriers when transmitting an OFDM symbol. The bit swapping algorithm at this stage is equivalent to the following procedure. Initially, a block of bits of length NCBPS is written row by row into a matrix containing 16 rows and NCBPS/16 rows. Next, the bits are read from this matrix, but in rows (or in the same way as they were written, but from a transposed matrix). As a result of this operation, initially adjacent bits will be transmitted on non-adjacent subcarriers.

This is followed by a second bit permutation step, the purpose of which is to ensure that adjacent bits do not simultaneously appear in the least significant bits of the groups defining the modulation symbol in the signal constellation. That is, after the second stage of permutation, adjacent bits appear alternately in the high and low digits of the groups. This is done in order to improve the noise immunity of the transmitted signal.

After interleaving, the bit sequence is divided into groups according to the number of positions of the selected modulation type and OFDM symbols are formed.

The generated OFDM symbols are subjected to fast Fourier transform, resulting in the formation of output in-phase and quadrature signals, which are then subjected to standard processing - modulation.

1.3.3 IEEE 802.11n standard

This standard was approved on September 11, 2009. 802.11n is comparable in transmission speed to wired standards. The maximum transfer speed of the 802.11n standard is approximately 5 times higher than the performance of classic Wi-Fi.

The following main advantages of the 802.11n standard can be noted:

– high data transfer speed (about 300 Mbit/s);

– uniform, stable, reliable and high-quality coverage of the station’s coverage area, the absence of uncovered areas;

– compatibility with previous versions of the Wi-Fi standard.

Flaws:

– high power consumption;

– two operating ranges (possible replacement of equipment);

– more complicated and larger equipment.

The increase in transmission speed in the IEEE 802.11n standard is achieved, firstly, by doubling the channel width from 20 to 40 MHz, and secondly, by implementing MIMO technology.

MIMO (Multiple Input Multiple Output) technology involves the use of multiple transmitting and receiving antennas. By analogy, traditional systems, that is, systems with one transmitting and one receiving antenna, are called SISO (Single Input Single Output).

The IEEE 802.11n standard is based on OFDM-MIMO technology. Many of the technical details implemented in it are borrowed from the 802.11a standard, but the IEEE 802.11n standard provides for the use of both the frequency range adopted for the IEEE 802.11a standard and the frequency range adopted for the IEEE 802.11b/g standards. That is, devices that support the IEEE 802.11n standard can operate in the frequency range of either 5 or 2.4 GHz.

Figure 1.4 - Implementation principle of MIMO technology

The transmitted sequence is divided into parallel streams, from which the original signal is restored at the receiving end. This is where some complexity arises - each antenna receives a superposition of signals that must be separated from each other. For this purpose, a specially developed spatial signal detection algorithm is used at the receiving end. This algorithm is based on subcarrier allocation and turns out to be more complex the greater the number of subcarriers. The only disadvantage of using MIMO is the complexity and bulk of the system and, as a result, higher energy consumption. To ensure compatibility between MIMO stations and traditional stations, three operating modes are provided:

Legacy mode.

Mixed mode.

Green field mode.

Each operating mode has its own preamble structure - a service field of the packet that indicates the start of transmission and serves to synchronize the receiver and transmitter. The preamble contains information about the packet length and its type, including the type of modulation, the selected encoding method, and all encoding parameters. To avoid conflicts in the operation of MIMO and conventional stations (with one antenna), during exchange between MIMO stations, the packet is accompanied by a special preamble and header. Upon receiving such information, stations operating in legacy mode defer transmission until the end of the session between MIMO stations. In addition, the preamble structure defines some of the primary tasks of the receiver, such as estimating received signal strength for automatic gain control, detecting the start of a packet, and time and frequency offset.

Operating modes of MIMO stations.

Legacy mode. This mode is designed to ensure exchange between two stations with one antenna. Information is transmitted via 802.11a protocols. If the transmitter is a MIMO station and the receiver is a regular station, then the transmitting system uses only one antenna and the transmission process is the same as in previous versions of the Wi-Fi standard. If the transmission goes in the opposite direction - from a conventional station to a multi-antenna station, then the MIMO station uses many receiving antennas, but in this case the transmission speed is not maximum. The structure of the preamble in this mode is the same as in the 802.11a version.

Mixed mode. In this mode, exchange is carried out both between MIMO systems and between conventional stations. Because of this, MIMO systems generate two types of packets, depending on the type of receiver. With regular stations the operation is slow because they do not support high speed operation, and between MIMO it is much faster, but the transmission speed is lower than in green field mode. The preamble in the packet from a regular station is the same as in the 802.11a standard, but in the MIMO packet it is slightly modified. If the transmitter is a MIMO system, then each antenna does not transmit the entire preamble, but a cyclically shifted one. Due to this, the power consumption of the station is reduced, and the channel is used more efficiently. However, not all legacy stations can operate in this mode. The fact is that if the device synchronization algorithm is based on cross-correlation, then loss of synchronization will occur.

Green field mode. This mode takes full advantage of MIMO systems. Transmission is only possible between multi-antenna stations with legacy receivers. When a MIMO system is transmitting, conventional stations wait for the channel to become free to avoid collisions. In green field mode, signal reception from systems operating according to the first two schemes is possible, but transmission to them is not. This was done in order to exclude single-antenna stations from the exchange and thereby increase the speed of operation. The packets are accompanied by preambles, which are only supported by MIMO stations. All these measures allow you to get the most out of MIMO-OFDM systems. All modes of operation must be protected from interference from adjacent station operation to prevent signal distortion. At the physical layer of the OSI model, special fields are used for this in the preamble structure, which notify the station that a transmission is in progress and a certain waiting time is required. Some protection methods are also adopted at the data link layer. Depending on the bandwidth used, operating modes are classified as follows:

1. Inherited mode. This mode is needed to harmonize with previous versions of Wi-Fi. It is very similar to 802.11a/g both in hardware and bandwidth, which is 20 MHz.

2. Double inherited mode. The devices use a 40 MHz bandwidth, with the same data sent on the upper and lower channels (each 20 MHz wide), but with a 90° phase shift. The structure of the packet is based on the fact that the receiver is a regular station. Signal duplication reduces distortion, thereby increasing transmission speed.

3. High throughput mode. The devices support both frequency bands - 20 and 40 MHz. In this mode, stations exchange only MIMO packets. Network speed is maximum.

4. Top channel mode. This mode uses only the upper half of the 40 MHz band. Stations can exchange any packets.

5. Bottom channel mode. This mode uses only the lower half of the 40 MHz band. Stations can also exchange any packets.

Methods for increasing performance.

The data transfer speed depends on many factors (Table 1.3) and, above all, on the bandwidth. The wider it is, the higher the exchange speed. The second factor is the number of parallel threads. In the 802.11n standard, the maximum number of channels is 4. The type of modulation and coding method are also of great importance. Anti-jamming codes that are typically used in networks require some redundancy. If there are too many security bits, the transmission speed of useful information will decrease. In the 802.11n standard, the maximum relative encoding rate is up to 5/6, that is, there is one redundant bit for every 5 data bits. Table 3 shows the exchange rates for QAM and BPSK quadrature modulation. It can be seen that with other identical parameters, QAM modulation provides much higher operating speed.

Table 1.3 - Data transfer rate for different types of modulation

802.11n transmitters and receivers

The IEEE 802.11n standard allows the use of up to four antennas at the access point and wireless adapter. Mandatory mode implies support for two antennas at the access point and one antenna and wireless adapter. The IEEE 802.11n standard provides both standard 20 MHz and double-width channels. The general block diagram of the transmitter is shown in Figure 1.5. The transmitted data passes through a scrambler, which inserts extra zeros or ones into the code (called pseudo-random noise masking) to avoid long sequences of identical characters. The data is then divided into N streams and sent to a forward error correction (FEC) encoder. For systems with one or two antennas, N = 1, and if three or four transmit channels are used, then N = 2.

Figure 1.5 - General structure of a MIMO-OFDM transmitter

The encoded sequence is divided into separate spatial streams. The bits in each stream are interleaved (to eliminate block errors) and then modulated. Next, space-time streams are formed, which pass through the inverse fast Fourier transform block and arrive at the antennas. The number of space-time streams is equal to the number of antennas. The structure of the receiver is similar to the structure of the transmitter shown in Figure 1.6, but all actions are performed in the reverse order.

Figure 1.6 - General structure of a MIMO-OFD receiver

IEEE 802.11 standard (Wi-Fi)

Wireless networks of the IEEE 802.11 standard operate in two bands: 2.4......2.483 GHz and in several bands near 5 GHz, which are unlicensed. In this case, several topologies are possible:

• independent basic service areas (independent basic sets, IBSSs),
• basic service sets (BSSs),
• extended service sets (ESSs).

An independent base service area is a group of stations operating in accordance with the 802.11 standard that communicate directly with one another. IBSS is also called an episodic or ad-hoc network. In Fig. Figure 6.8 shows how three stations equipped with 802.11 wireless network interface cards (NICs) can form an IBSS and communicate directly with each other.

Rice. 6.8. Episodic (ad-hoc) network

The technology of basic service areas requires the presence of a special station: access points AP (access point). The access point is the central point of communication for all BSS stations. Client stations do not communicate directly with each other. Instead, they transmit messages to the access point, and it then forwards information packets to the destination station. The access point may have an uplink port through which the BSS is connected to a wired network (for example, an Ethernet uplink for Internet access). Therefore, BSS is called an infrastructure network. In Fig. Figure 6.9 shows a typical BSS infrastructure.

Rice. 6.9. Wireless LAN with infrastructure

Multiple BSS infrastructures can be connected via their uplink interfaces. Where the 802.11 standard is in effect, the uplink interface connects the BSS to the distribution system (DS). Multiple BSSs interconnected through a distribution system form an extended service area (ESS). The uplink to the distribution system does not necessarily have to use a wired connection. The specifications of the 802.11 standard allow this channel to be built as a wireless channel. But more often, the uplinks to the distribution system are wired Ethernet links. In Fig. Figure 6.10 provides an example of a practical implementation of ESS.

An area covered by a BSS or ESS with Internet access is called a hot spot. “Hot spots” are created in hotels, airports, restaurants, student dormitories and just on the streets. At the end of 2004, there were about 50,000 “hot spots” operating in the world, and the number of users reached
50 million people. The rapid expansion of WLAN services and the large number of equipment manufacturers requires compatibility between hardware and software offered by different companies. For this purpose, the WECA (Wireless Ethernet Compatibility Alliance) organization was created in 1999, which was soon transformed into the Wi-Fi Alliance. It includes developers and manufacturers of 802.11 equipment, network operators, and experts. The main goal of the alliance is to certify manufactured equipment in order to ensure the interaction of Wi-Fi devices produced by different companies.

Rice. 6.10. Extended ESS Wireless LAN Service Area

The 802.11 standard has 3 variants: 802.11a, b and g. In all options, information is transmitted in batch mode, in separate frames (packets).

802.11b equipment operates in the range of 2.4....2.483 GHz. As mentioned, this range is unlicensed and there are many other systems and devices operating in it. To reduce the impact of interference in 802.11b networks, 2 methods have been proposed. The first is the use, as in the Bluetooth standard, of frequency jumping when transmitting each next frame. However, in practice, another method is usually used: direct spectrum expansion by filling information symbols with a scrambling code.

In the classic 802.11b version, information is transmitted in the form of symbols at a rate of 1 Msym/s. With 2-PM modulation, the information transmission rate in the frame is 1 Mbit/s, and with 4-PM, 2 Mbit/s. When using direct spread spectrum, each symbol is filled with an m-chip sequence of 11 chips (Barker code): +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 . The chip speed in the radio channel is 11 Mchip/s, and the radio channel width is 22 MHz. In the 2.4 GHz range, the central frequencies of 13 radio channels are fixed: 2412, 2417, 2422, 2427, 2432, 2437, 2442, 2447, 2452, 2457, 2462, 2467 and 2472 MHz. Upon reception, the signal is subjected to correlation processing, which significantly reduces the influence of interference, as in cellular communication standards with code division of channels.

Using a broadband channel allows you to increase the data transfer rate with a high signal-to-interference ratio (15 – 17 dB). In this case, scrambling is abandoned, and data is transmitted at a symbol rate of 11 Msym/s with 4-PSK modulation. To improve the quality of communication during transmission, redundant coding is used using complementary code Keying. The frame data rate can be 11 or 5.5 Mbit/s.

The maximum transmitter power of 802.11b devices is 100 mW in Europe, and 1 W in the USA.

802.11a devices operate in three sub-bands at 5 GHz. In the 5.15….5.25 GHz sub-band the transmission power is limited to 50 mW, in the 5.25…. 5.35 GHz – 250 mW, and in the sub-range 5.725....5.825 GHz –
1 W. In these ranges, 12 channels with a width of 20 MHz are allocated.

The advantage of the 802.11a standard compared to 802.11b is the increased data transfer rate per frame: from 6 to 54 Mbit/s. To do this, the 802.11a standard uses OFDM modulation: Orthogonal Frequency Division Multiplexing - multiplexing with orthogonal frequency division. This method is used to eliminate inter-symbol interference at high data rates. Let's give a typical example.

Let there be a transmission over the radio channel with a symbol rate of B = 40 Msym/s. When transmitting on a single carrier frequency, the symbol duration is s. Let's imagine the situation of transmitting such a signal in a large room (station, airport, shopping center - Fig. 6.11).

Fig.6.11. Multipath propagation of signals

In order for the forward and backward rays to arrive with a delay of 1 symbol, the difference in their paths must be only m. Such a delay can be observed even in a fairly large room. To eliminate the problem of inter-symbol interference, the symbol length should be increased by 10, or even better, by 100 times. Then intersymbol interference will be noticeable with a path difference of 750 m. Hence the idea underlying OFDM: split a high-speed data stream into many separate streams (dozens!), transmit each of the substreams at its own frequency (subcarrier), increasing the symbol length to units milliseconds

The generalized symbol is the sum of the symbols transmitted to N S subcarriers. All subcarriers can use different types of modulation: 2-PM, 4-PM, 8-PM, 16-QAM or 64-QAM. The timing diagram of the OFDM signal is shown in Fig. 6.12, where the number i individual subcarriers are labeled.

Rice. 6.12. OFDM signal structure

The characters are specially separated from each other by pauses lasting T r so that in the case of a multipath signal, neighboring symbols do not “crawl” onto each other.

The total OFDM signal at can be represented as:

, (6.1)

where is the complex amplitude of one transmitted signal,

ts– the start time of each individual character,

T s– symbol duration.

The spectral picture of the OFDM signal is shown in Fig. 6.13.

Rice. 6.13. OFDM signal spectrum

In order to be able to distinguish between signals transmitted on adjacent subcarriers during reception, all signals must be mutually orthogonal. This condition is satisfied if the distance between adjacent subcarriers is .

When transmitting (forming) an OFDM signal, an inverse discrete Fourier transform (inverse FFT) is used; upon reception – direct discrete Fourier transform (FFT). The OFDM signal is generated at a lower frequency with subsequent spectrum transfer to the frequency of the radio channel.

The 802.11a standard uses 48 subcarriers (52 in total) to transmit information. Symbol Duration T s=3.2 µs, pause duration Tp=0.8 µs. Distance between adjacent frequencies MHz. With 2-PM modulation on each subcarrier, data rate (without security coding)

When moving to multi-position modulation methods

Mbit/s,

Mbps.

Depending on the interference situation, the 802.11a standard provides for the use of adaptive modulation and coding schemes. The main characteristics of the standard are given in table. 6.4.

Table 6.4

The 802.11g standard combines the capabilities of the 802.11a and b standards in the 2.4...2.483 GHz range. The main characteristics of the standard are given in table. 6.5. In addition to SSC and OFDM, the standard uses redundant binary packet convolutional coding PBCC (packet binary convolutional coding) at a number of speeds.

Table 6.5

Access to the network of subscriber stations and the ability to transmit frames in 802.11 networks is carried out using coordinate functions. When using distributed coordinate function DCF (distributed coordination function) all stations have the same priority and occupy the channel based on contention with rollback timers. The operating principle of DCF is illustrated in Fig. 6.14.

Rice. 6.14. Operation of stations in DCF mode

Operating stations listen to the radio channel and wait until it becomes free (carrier transmission stops). In Fig. 6.14, station 3 transmits first, and stations 1, 2 and 5 are ready to transmit. Upon completion of station 3’s frame, a mandatory inter-frame gap DIFS (34...50 μs) follows, after which the stations that are ready to transmit their packets begin the race. Each station starts a competition timer, where random numbers are set inside the competition window: 0.....7, 0...63, and then up to 127, 255, 511, 1023. From the moment the competition begins, the timers are read at a clock speed of 9...20 µs . The station that resets the timer first occupies the channel (station 2 in Fig. 6.14). The rest remember the contents of their timers (rollback) until the next competition. During the transmission process, collisions are possible when two stations simultaneously reset their timers (stations 4 and 5 in Fig. 6.14). This causes the contention window to expand, followed by frame retransmission.

The actual DCF-based access algorithm uses a more reliable procedure (Figure 6.15). The winning station sends a short request packet to the RTS receiver - Request to Send, which receives confirmation of the recipient’s readiness to receive CTS – Clear to Send. This is followed by the transmission of an information frame. The cycle ends with a packet confirming (or not acknowledging) the receipt of an ACK frame. This is how files are exchanged using the TCP/IP protocol.

Rice. 6.15. DCF based network access procedure

In the transmission cycle, RTS - CTS - Data - ACK frames are separated by short interframe intervals SIFS (10...16 μs). Stations that do not participate in the exchange, based on the information contained in RTS and CTS frames about the duration of the transmission cycle, establish their NAV vectors (network allocation vector). NAV is the timer read time during which the station is in “sleep” mode and does not participate in contention until NAV becomes 0.

The considered access method is used when reading files from the Internet. However, it does not allow the transmission of streaming video and, especially, IP telephony, where permissible signal delays are strictly limited. The new IEEE 802.11e standard provides support for four classes of traffic in Wi-Fi networks, arranged in order of priority:

Voice – telephony with transmission quality at the level of long-distance communication,

Video – television transmission,

Best Effort – reading Internet files,

Background – transfer files with low priority.

This classification corresponds to the service classes of 3rd generation mobile networks, which makes it possible to organize the interaction of mobile and Wi-Fi networks. Implementation of the 802.11e standard is possible only in networks with access points, where they use point coordinate function PCF (point coordination function). The operating principle of a PCF-based network is illustrated in Fig. 6.16.

The transfer process is determined by the AP. The transmission time is divided into superframes, the duration of which is set adaptively by the AR and can be changed during the transmission process. At the beginning of each superframe, the AP transmits a beacon frame. It sets the duration of the superframe, the maximum size of data frames, and the contention-free period. At this time, information is exchanged between the access point and stations only by polling the AP (the station itself cannot occupy the channel). Simultaneously with sending a polling frame, the AP can send an information frame to the station. The AP marks the end of the contention-free period by sending a CF-End frame. Stations, including APs, then occupy the channel on a contention basis. This access method allows you to organize the transmission of data packets at a constant speed, which is necessary for telephone and streaming traffic.

Rice. 6.16. PCF based data transfer

It must be said that the point coordinate function PCF does not fully provide QoS parameters. To support the required quality of services, a special standard 802.11e has been developed. It introduces the concept of AC access categories, which are derived from the 802.1D group of standards and define priority levels. In total, there are 4 access categories (Table 6.6): voice (Voice), video (Video), best attempt (Best Effort) and background (Background). Each category is associated with a corresponding data type.

Table 6.6

The 802.11e standard defines a new type of media access to ensure quality of service - hybrid coordinate function (hybrid coordination function, HCF). HCF defines two mechanisms for accessing the environment:

· Contention-based channel access;

· Controlled channel access.

Contention-based channel access corresponds to extended distributed channel access ( enhanced distributed channel access, EDCA), and a controlled channel access corresponds to an HCF-controlled channel access ( HCF controlled channel access, HCCA). 802.11e still has two phases of operation within a superframe—contention periods (CP) and contention-free periods (CFP). EDCA is used only in CP and HCCA is used in both periods. HCF combines PCF and DCF methods, which is why it is called hybrid. The result of the MAC architecture transformation is shown in Fig. 6.17.

Rice. 6.17 MAC architecture

A station that acts as a central coordinator for all stations within a basic QoS-enabled service set ( QoS supporting BSS, QBSS), is called a hybrid coordinator ( hybrid coordinator). It, like the point coordinator, is located inside the access point. Client stations that support QoS are called QSTA.

An 802.11e station that is granted access to the medium shall not use radio resources of longer duration than that defined in the standard. This new introduction is called transferability ( transmission opportunity, TXOP). TXOP is the interval during which a station has the right to transmit packets. It is defined by its start time and duration. TXOP existing in contention-based media access is called EDCA-TXOP. Similarly, TXOP existing in controlled media access is called HCCA-TXOP. The duration of EDCA-TXOP is limited by the TXOPlimit parameter, the value of which is constantly transmitted through a certain beacon frame field information element.

Another improvement to the standard is that no station can transmit when it is time to transmit a beacon frame. This reduces the expected beacon latency, which gives the hybrid coordinator better control over the environment, especially when the optional CFP is used after the beacon frame.

In the new standard, a station can transmit packets directly to another station in QBSS without communicating with an access point. In the old standard, within a network with infrastructure, all data exchange packets between stations went only through the access point.

EDCA's support for QoS provides concepts such as access categories and multiple independent rollback objects ( backoff entities). Multiple concurrent fallback objects can exist within each 802.11e station, with these objects assigned different priorities according to a set of specific access category parameters ( EDCA parameter set). As mentioned above, there are four access categories, respectively, in each station there are four rollback objects (Fig. 6.18). The EDCA set of parameters prioritizes access to the medium by defining individual inter-frame gaps, contention windows, and other parameters.

Rice. 6.18. Four access categories in one station

Each access category has its own interframe intervals ( arbitration interframe space, AIFS), similar to DIFS, but of different durations. In addition, the size of the contention window changes depending on the priority of the traffic.

6. 5. IEEE 802.16 - WiMAX standard

WiMAX-Worldwide Interoperability for Microwave Access

Table 6.7

Main characteristics of the WiMAX standard

Table 6.8

The non-profit organization WiMAX (World Interoperability for Microwave Access - interaction of network access equipment at ultra-high frequencies around the world) was formed to promote the development of wireless equipment for access to broadband networks based on the IEEE 802.16 specification for wireless area networks, certification of such equipment for compatibility and interoperability, as well as speeding up its entry into the market.

The 802.16 standard provides for operation in the ranges of 2...11 GHz and 10-66 GHz (Fig. 6.1). In the range of 10-66 GHz, radio communication is possible only in the case of direct visibility between points. In this range, direct carrier modulation (single carrier mode) is used.

In the range of 2...11 GHz, the radio interface specifications allow for the possibility of solving radio communication problems in conditions of multipath propagation and in the absence of line of sight (NLOS - Non-Line-Of-Sight). The WMAN-SC2 radio interface uses single carrier modulation, the WMAN radio interface – OFDM – orthogonal frequency modulation (OFDM - Orthogonal Frequency Division Multiplexing) with fast Fourier transform of 256 points and up to 2048 points. Certified frequency ranges for fixed and mobile WiMAX profiles are shown in Fig. 1.

Fixed WiMAX profiles– 3.5 GHz (FDD): 3.5; 7; (256)

3.5 GHz (TDD): 3.5; 7; (256)

5.8 GHz (TDD): 10 (256)

Mobile WiMAX profiles- 2.3 – 2.4 GHz: 5 (512); 10 (1024); 8.75 (1024);

all TDD 2.305 – 2.320 GHz: 3.5 (512); 5 (512)

2.345 – 2.360 GHz: 10 (1024)

2.496 – 2.69 GHz: 5 (512); 10 (1024)

3.3 – 3.4 GHz: 5 (512); 7 (1024); 10 (1024)

3.4 – 3.8 GHz: 5 (512)

3.4 – 3.6 GHz: 7 (1024)

3.6 – 3.8 GHz: 10 (1024)

In addition to those indicated, it is possible to allocate channels in the 5.7 GHz bands,
1.710 – 1.755: 2.110 – 2.155 GHz.

The 802.16 standard uses the following interfaces:

1. WirelessMAN-SC (10 – 66 GHz)

2. WirelessMAN-SCa (2 – 11 GHz; licensed bands)

3. WirelessMAN-OFDM (2 – 11 GHz; licensed bands)

6. WirelessMAN-OFDMA - Orthogonal Frequency Division Multiple Access

(2 – 11 GHz; licensed bands)

5. WirelessHUMAN (2 – 11 GHz; unlicensed bands)

Interfaces 3 and 5 provide Mesh capabilities - organizing networks with a full topology to speed up traffic transmission.

The inverse Fourier transform determines the shape of the OFDM signal. The useful duration of a symbol is considered to be Tb. The last part Tg of the symbol period, called the guard interval, is used to eliminate the effects of multipath propagation of orthogonal signal components (Fig. 6.19).

Rice. 6.19. Single Frequency Symbol Format

In the frequency domain, the signal is characterized by spectral characteristics (Fig. 6.20). It contains subcarriers for data transmission, pilot signals, and guard intervals are located at the edges of the band.

Rice. 6.20. Description of the signal in the frequency domain

The OFDM symbol is characterized by the following parameters:

BW – nominal channel bandwidth.

Nused - number of used subcarriers.

N is the sampling coefficient. This parameter, in conjunction with BW and Nused, determines the subcarrier spacing and symbol duration. The required values ​​of this parameter are defined in Table 6.6.

G is the ratio of the duration of the guard interval (prefix) to the useful time. This value can be 1/4, 1/8, 1/16, 1/32 Tb.

NFFT: number of Fourier transform points,

Sending frequency: Fs=floor (n*BW/0.008)*0.008 (BW-bandwidth in MHz),

-∆f: subcarrier spacing, defined as: Fs/NFFT,

Tb= 1/∆f – symbol transformation duration,

Tg=G*Tb – duration of the guard interval (CP),

Ts=Tb+Tg – OFDM symbol duration,

Ts/NFFT - sampling interval.

The main parameters of OFDM channels of the 802.16a standard are given in table. 6.9.

Table 6.9.

The duration of symbols depending on the channel bandwidth is given in table. 6.10.

Table 6.10

Modulation and coding schemes for the 802.16-2004 standard are summarized in table. 6.11.

Table 6.11

The values ​​of transmission rates depending on the type of modulation and code rate are given in Table. 6.12, and the requirements for the signal-to-noise ratio at the receiver input for various modulation and coding schemes are in Table. 6.13.

Table 6.12

Table 6.13

Data at the physical layer is transmitted as a continuous sequence of frames. Each frame has a fixed duration (2 (2.5) ... 20 ms), so its information capacity depends on the symbol rate and modulation method. A frame consists of a preamble, a control section and a sequence of data packets. IEEE 802.16 networks are full-duplex. Both frequency FDD and time TDD separation of uplink and downlink channels are possible.

With temporary duplexing of channels, the frame is divided into downstream and upstream subframes (their ratio can be flexibly changed during operation depending on the bandwidth needs for upstream and downstream channels), separated by a special guard interval. With frequency duplexing, the upstream and downstream channels are transmitted on two carriers (Fig. 6.21)

Rice. 6.21. Frame structure for TDD and FDD

In the downlink, information from the base station is transmitted as a sequence of packets. For each packet, you can set the modulation method and data encoding scheme - i.e. choose between transmission speed and reliability. TDM packets are transmitted simultaneously to all subscriber stations, each of them receives the entire information flow and selects “its own” packets. So that subscriber stations can distinguish one packet from another, downlink (DL-MAP) and uplink (UL-MAP) channel maps are transmitted in the control section (Fig. 6.22).

Fig.6.22. Downlink channel structure.

The downlink map specifies the frame duration, frame number, number of packets in the downlink subframe, and the starting point and profile type of each packet. The start point is measured in so-called physical slots, each physical slot being equal to four modulation symbols.

A packet profile is a list of its parameters, including modulation method, FEC coding type (with coding scheme parameters), as well as the range of signal-to-noise ratio values ​​in the receiving channel of a particular station for which this profile can be applied. The base station periodically broadcasts a list of profiles in the form of special control messages (downlink and uplink DCD/UCD channel descriptors), and each profile is assigned a number, which is used in the downlink channel map.

Subscriber stations gain access to the transmission medium through the TDMA (Time Division Multiple Access) mechanism. To do this, in the upstream subframe for the AS, the base station reserves special time intervals - slots (Fig. 6.23). Information about the allocation of slots between speakers is recorded in the UL-MAP uplink map, broadcast in each frame. UL-MAP is functionally similar to DL-MAP - it reports how many slots there are in a subframe, the starting point and connection ID for each, and the profile types of all packets. The UL-MAP message of the current frame may refer to either the current frame or a subsequent frame. The modulation rate (symbol rate) in the uplink must be the same as in the downlink. Note that, unlike downstream TDM packets, each packet in the uplink begins with a preamble - a sync sequence of 16 or 32 QPSK symbols.

Rice. 6.23. Uplink channel structure

Examples of frame structure with TDD are shown in Fig. 6.24.

Rice. 6.24. Example OFDM frame structure with TDD

In the upstream channel, in addition to the slots assigned by the BS for certain ASs, there are intervals during which the AS can transmit a message for initial registration in the network or to request a change in channel bandwidth (providing channels on demand DAMA - Demand Assigned Multiple Access).

The physical layer of the IEEE 802.16 standard provides direct delivery of data streams between the BS and the AS. All tasks associated with the formation of the structures of this data, as well as managing the operation of the system, are solved at the MAC (Medium Access Control) level. IEEE 802.16 standard equipment forms a transport environment for various applications (services).

WiMAX networks support 4 types of traffic, differing in reliability and latency requirements:

UGS – Unsolicited Grant Service – real-time transmission of telephony (E1) and VoIP signals and streams. The permissible delay is less than 5 - 10 ms in one direction at BER = 10 -6 ... 10 -6.

rtPS – Real Time Polling Service – real-time streams with variable length packets (MPEG video).

nrtPS – Non-Real-Time Polling Service – support for variable length streams when transferring files in broadband mode.

BE – Best Effort – other traffic.

IEEE 802.11 -- a set of communication standards for communication in the wireless local area network of frequency bands 0.9; 2.4; 3.6 and 5 GHz.

It is better known to users by the name Wi-Fi, which is actually a brand proposed and promoted by the Wi-Fi Alliance. It has become widespread due to its development in mobile electronic computing devices: PDAs and laptops.

Institute of Electrical and Electronics Engineers - IEEE (English Institute of Electrical and Electronics Engineers) (I triple E - "I triple E") - an international non-profit association of specialists in the field of technology, a world leader in the development of standards for radio electronics and electrical engineering.

Initially, the IEEE 802.11 standard assumed the ability to transmit data over a radio channel at a speed of no more than 1 Mbit/s and, optionally, at a speed of 2 Mbit/s. One of the first high-speed wireless network standards - IEEE 802.11a - defines transmission speeds of up to 54 Mbit/s gross. The operating range of the standard is 5 GHz.

Contrary to its name, the IEEE 802.11b standard, adopted in 1999, is not a continuation of the 802.11a standard, since they use different technologies: DSSS (more precisely, its improved version HR-DSSS), DSSS technology (direct sequence spread spectrum), in 802.11b versus OFDM, OFDM (English Orthogonal frequency-division multiplexing - multiplexing with orthogonal frequency division of channels), in 802.11a. The standard provides for the use of the unlicensed 2.4 GHz frequency range. Transfer speed up to 11 Mbit/s.

IEEE 802.11b products from various manufacturers are tested for compatibility and certified by the Wireless Ethernet Compatibility Alliance (WECA), now better known as the Wi-Fi Alliance. Compatible wireless products that have been tested by the Wi-Fi Alliance may be labeled with the Wi-Fi symbol.

For a long time, IEEE 802.11b was a common standard on the basis of which most wireless local area networks were built. Now its place has been taken by the IEEE 802.11g standard, which is gradually being replaced by the high-speed IEEE 802.11n.

The draft IEEE 802.11g standard was approved in October 2002. This standard uses the 2.4 GHz frequency band, providing connection speeds up to 54 Mbps (gross) and thus surpassing the IEEE 802.11b standard, which provides connection speeds up to 11 Mbit/s. In addition, it guarantees backward compatibility with the 802.11b standard. Backwards compatibility of the IEEE 802.11g standard can be implemented in DSSS modulation mode, in which the connection speed will be limited to eleven megabits per second, or in OFDM modulation mode, in which the speed can reach 54 Mbit/s. Thus, this standard is the most suitable for building wireless networks.

The widespread use of wireless communication technologies in our time is simply amazing. IEEE 802.11 technology deserves a separate topic. It is almost impossible to find a place in the city where a laptop or tablet does not “find” at least one Wi-Fi network. In any cafe, multi-storey building or office you can find several broadcasts. It is very difficult to underestimate the touch of convenience that this technology provides us.

The Wi-Fi that we use today has come a long and thorny path for the user convenience to which we are all accustomed. Many standards with their own transmission features and frequency ranges have formed something without which it is difficult to imagine the life of an IT specialist or just a modern person. We will not plunge into history, but only note that at the moment the 802.11g and 802.11n standards, which operate in the 2.4 GHz band, are actively used. There are many sources of interference to wireless networks in everyday life, but they are not the main problem. The culprit of most of the inconveniences is the Wi-Fi point itself, or, to be more precise, a large number of them close to each other. Due to the popularity of this technology and the high density of broadcasting sites, users may encounter some difficulties in their work. Large concentrations of wireless networks can cause frequencies to overlap, causing transmission speeds to slow down or the connection to be lost altogether. This significant shortcoming caused by the popularization of wireless technology was one of the loud bells in WECA to implement the IEEE 802.11ac standard.

The new IEEE 802.11n wireless standard has been talked about for several years now. This is understandable, because one of the main disadvantages of the existing IEEE 802.11a/b/g wireless communication standards is the data transfer speed is too low. Indeed, the theoretical throughput of IEEE 802.11a/g protocols is only 54 Mbit/s, and the actual data transfer rate does not exceed 25 Mbit/s. The new wireless communication standard IEEE 802.11n should provide transmission speeds of up to 300 Mbit/s, which looks very tempting compared to 54 Mbit/s. Of course, the actual data transfer rate in the IEEE 802.11n standard, as test results show, does not exceed 100 Mbit/s, but even in this case, the actual data transfer speed is four times higher than in the IEEE 802.11g standard. The IEEE 802.11n standard has not yet been fully adopted (this should happen before the end of 2007), but almost all wireless equipment manufacturers have already begun producing devices compatible with the draft version of the IEEE 802.11n standard.
In this article we will look at the basic provisions of the new IEEE 802.11n standard and its main differences from the 802.11a/b/g standards.

We have already talked about the 802.11a/b/g wireless communication standards in some detail on the pages of our magazine. Therefore, in this article we will not describe them in detail; however, in order for the main differences between the new standard and its predecessors to be obvious, we will have to make a digest of previously published articles on this topic.

Considering the history of wireless communication standards used to create wireless local area networks (WLAN), it is probably worth recalling the IEEE 802.11 standard, which, although no longer found in its pure form, is the progenitor of all other wireless communication standards for networks WLAN.

IEEE 802.11 standard

The 802.11 standard provides for the use of a frequency range from 2400 to 2483.5 MHz, that is, a range of 83.5 MHz wide, divided into several frequency subchannels.

The 802.11 standard is based on the technology of spreading the spectrum (Spread Spectrum, SS), which implies that the initially narrow-band (in terms of spectrum width) useful information signal is converted during transmission in such a way that its spectrum is much wider than the spectrum of the original signal. Simultaneously with the broadening of the signal spectrum, a redistribution of the spectral energy density of the signal occurs - the signal energy is also “spread out” across the spectrum.

The 802.11 protocol uses Direct Sequence Spread Spectrum (DSSS) technology. Its essence lies in the fact that to broaden the spectrum of an initially narrow-band signal, a chip sequence, which is a sequence of rectangular pulses, is built into each transmitted information bit. If the duration of one chip pulse is n times less than the duration of the information bit, then the width of the spectrum of the converted signal will be n times the width of the spectrum of the original signal. In this case, the amplitude of the transmitted signal will decrease by n once.

The chip sequences embedded in the information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and is difficult to distinguish from natural noise.

It’s clear how to broaden the signal spectrum and make it indistinguishable from natural noise. To do this, in principle, you can use an arbitrary (random) chip sequence. However, the question arises of how to receive such a signal. After all, if it becomes noise-like, then isolating a useful information signal from it is not so easy, if not impossible. Nevertheless, this can be done, but for this you need to select the chip sequence accordingly. Chip sequences used to broaden the signal spectrum must satisfy certain autocorrelation requirements. In mathematics, autocorrelation refers to the degree to which a function is similar to itself at different points in time. If you select a chip sequence for which the autocorrelation function will have a pronounced peak for only one point in time, then such an information signal can be distinguished at the noise level. To do this, the received signal is multiplied by the chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal again becomes narrow-band, so it is filtered in a narrow frequency band equal to twice the transmission rate. Any interference that falls within the band of the original broadband signal, after multiplication by the chip sequence, on the contrary, becomes broadband and is cut off by filters, and only part of the interference falls into the narrow information band; its power is significantly less than the interference acting at the receiver input.

There are quite a lot of chip sequences that meet the specified autocorrelation requirements, but the so-called Barker codes are of particular interest to us, since they are used in the 802.11 protocol. Barker codes have the best noise-like properties among known pseudo-random sequences, which has led to their widespread use. The 802.11 family of protocols uses Barker code that is 11 chips long.

In order to transmit a signal, the information sequence of bits in the receiver is added modulo 2 (mod 2) with the 11-chip Barker code using an XOR (exclusive OR) gate. Thus, a logical one is transmitted by a direct Barker sequence, and a logical zero by an inverse sequence.

The 802.11 standard provides two speed modes - 1 and 2 Mbit/s.

With an information speed of 1 Mbit/s, the speed of individual Barker sequence chips is 11x106 chips per second, and the spectrum width of such a signal is 22 MHz.

Considering that the width of the frequency range is 83.5 MHz, we find that a total of three non-overlapping frequency channels can fit in this frequency range. The entire frequency range, however, is usually divided into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz from each other. For example, the first channel occupies the frequency range from 2400 to 2423 MHz and is centered relative to the frequency of 2412 MHz. The second channel is centered relative to the frequency of 2417 MHz, and the last, 11th channel is centered relative to the frequency of 2462 MHz. When viewed this way, channels 1, 6 and 11 do not overlap with each other and have a 3 MHz gap relative to each other. It is these three channels that can be used independently of each other.

To modulate a sinusoidal carrier signal at a data rate of 1 Mbit/s, relative binary phase modulation (DBPSK) is used.

In this case, information encoding occurs due to a phase shift of the sinusoidal signal relative to the previous signal state. Binary phase modulation provides two possible phase shift values ​​- 0 and p. Then a logical zero can be transmitted by an in-phase signal (the phase shift is 0), and a logical one can be transmitted by a signal that is phase shifted by p.

An information speed of 1 Mbit/s is mandatory in the IEEE 802.11 standard (Basic Access Rate), but a speed of 2 Mbit/s (Enhanced Access Rate) is optionally possible. To transmit data at this speed, the same DSSS technology with 11-chip Barker codes is used, but Differential Quadrature Phase Shift Key is used to modulate the carrier wave.

In conclusion, considering the physical layer of the 802.11 protocol, we note that at an information speed of 2 Mbit/s, the speed of individual chips of the Barker sequence remains the same, that is, 11x106 chips per second, and therefore the width of the spectrum of the transmitted signal does not change.

IEEE 802.11b standard

The IEEE 802.11 standard was replaced by the IEEE 802.11b standard, which was adopted in July 1999. This standard is a kind of extension of the basic 802.11 protocol and, in addition to speeds of 1 and 2 Mbit/s, provides speeds of 5.5 and 11 Mbit/s, for which so-called complementary codes (Complementary Code Keying, CCK) are used.

Complementary codes, or CCK sequences, have the property that the sum of their autocorrelation functions for any cyclic shift other than zero is always zero, so they, like Barker codes, can be used to recognize a signal from a background of noise.

The main difference between CCK sequences and the previously discussed Barker codes is that there is not a strictly defined sequence through which either a logical zero or a one can be encoded, but a whole set of sequences. This circumstance makes it possible to encode several information bits in one transmitted symbol and thereby increases the information transmission speed.

The IEEE 802.11b standard deals with complex complementary 8-chip sequences defined on a set of complex elements taking values ​​(1, –1, +j, –j}.

Complex signal representation is a convenient mathematical tool for representing a phase-modulated signal. Thus, a sequence value equal to 1 corresponds to a signal in phase with the generator signal, and a sequence value equal to –1 corresponds to an antiphase signal; sequence value equal j- a signal phase-shifted by p/2, and the value is equal to – j, - signal phase shifted by –p/2.

Each element of the CCK sequence is a complex number, the value of which is determined using a rather complex algorithm. There are a total of 64 sets of possible CCK sequences, with the choice of each determined by the sequence of input bits. To uniquely select one CCK sequence, six input bits are required. Thus, the IEEE 802.11b protocol uses one of 64 possible eight-bit CKK sequences when encoding each character.

At a speed of 5.5 Mbit/s, 4 bits of data are simultaneously encoded in one symbol, and at a speed of 11 Mbit/s - 8 bits of data. In both cases, the symbolic transmission rate is 1.385x106 symbols per second (11/8 = 5.5/4 = 1.385), and taking into account that each character is specified by an 8-chip sequence, we find that in both cases the transmission speed of individual chips is 11x106 chips per second. Accordingly, the signal spectrum width at speeds of both 11 and 5.5 Mbit/s is 22 MHz.

IEEE 802.11g standard

The IEEE 802.11g standard, adopted in 2003, is a logical development of the 802.11b standard and involves data transmission in the same frequency range, but at higher speeds. Additionally, 802.11g is fully compatible with 802.11b, meaning any 802.11g device must be able to work with 802.11b devices. The maximum data transfer rate in the 802.11g standard is 54 Mbit/s.

Two competing technologies were considered during the development of the 802.11g standard: the orthogonal frequency division OFDM method, borrowed from the 802.11a standard and proposed by Intersil, and the binary packet convolutional coding method PBCC, proposed by Texas Instruments. As a result, the 802.11g standard contains a compromise solution: OFDM and CCK technologies are used as base technologies, and the optional use of PBCC technology is provided.

The idea of ​​convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The incoming sequence of information bits is converted in a convolutional encoder so that each input bit corresponds to more than one output bit. That is, the convolutional encoder adds certain redundant information to the original sequence. If, for example, each input bit corresponds to two output bits, then we talk about convolutional coding with a speed r= 1/2. If every two input bits correspond to three output bits, then it will be 2/3.

Any convolutional encoder is built on the basis of several sequentially connected memory cells and XOR gates. The number of storage cells determines the number of possible encoder states. If, for example, a convolutional encoder uses six memory cells, then the encoder stores information about six previous signal states, and taking into account the value of the input bit, we find that such an encoder uses seven bits of the input sequence. Such a convolutional encoder is called a seven-state encoder ( K = 7).

The output bits generated by the convolutional encoder are determined by XOR operations between the values ​​of the input bit and the bits stored in the storage cells, that is, the value of each output bit generated depends not only on the incoming information bit, but also on several previous bits.

PBCC technology uses seven-state convolutional encoders ( K= 7) with speed r = 1/2.

The main advantage of convolutional encoders is the noise immunity of the sequence they generate. The fact is that with redundant coding, even in the event of reception errors, the original bit sequence can be accurately restored. A Viterbi decoder is used at the receiver side to restore the original bit sequence.

The dibit generated in the convolutional encoder is subsequently used as a transmitted symbol, but it is first subjected to phase modulation. Moreover, depending on the transmission speed, binary, quadrature or even eight-position phase modulation is possible.

Unlike DSSS technologies (Barker codes, SSK sequences), convolutional coding technology does not use spectrum broadening technology through the use of noise-like sequences, however, spectrum broadening to standard 22 MHz is also provided in this case. To do this, variations of possible QPSK and BPSK signal constellations are used.

The considered PBCC coding method is optionally used in the 802.11b protocol at speeds of 5.5 and 11 Mbit/s. Similarly, in the 802.11g protocol for transmission speeds of 5.5 and 11 Mbit/s, this method is also used optionally. In general, due to the compatibility of the 802.11b and 802.11g protocols, the encoding technologies and speeds provided by the 802.11b protocol are also supported in the 802.11g protocol. In this regard, up to a speed of 11 Mbps, the 802.11b and 802.11g protocols are the same, except that the 802.11g protocol provides speeds that the 802.11b protocol does not.

Optionally, in the 802.11g protocol, PBCC technology can be used at transmission rates of 22 and 33 Mbit/s.

For a speed of 22 Mbit/s, compared to the PBCC scheme we have already considered, data transmission has two features. First of all, 8-position phase modulation (8-PSK) is used, that is, the signal phase can take on eight different values, which allows three bits to be encoded in one symbol. In addition, a puncture encoder (Puncture) has been added to the circuit, with the exception of the convolutional encoder. The meaning of this solution is quite simple: the redundancy of the convolutional encoder, equal to 2 (for each input bit there are two output bits), is quite high and under certain noise conditions it is unnecessary, so the redundancy can be reduced so that, for example, every two input bits correspond to three output bits . For this, you can, of course, develop an appropriate convolutional encoder, but it is better to add a special puncture encoder to the circuit, which will simply destroy extra bits.

Let's say a puncture encoder removes one bit from every four input bits. Then every four incoming bits will correspond to three outgoing ones. The speed of such an encoder is 4/3. If such an encoder is used in conjunction with a convolutional encoder with a speed of 1/2, then the total encoding speed will be 2/3, that is, for every two input bits there will be three output bits.

As already noted, PBCC technology is optional in the IEEE 802.11g standard, and OFDM technology is mandatory. In order to understand the essence of OFDM technology, let's take a closer look at the multipath interference that occurs when signals propagate in an open environment.

The effect of multipath signal interference is that, as a result of multiple reflections from natural obstacles, the same signal can reach the receiver in different ways. But different propagation paths differ from each other in length, and therefore the signal attenuation will not be the same for them. Consequently, at the receiving point, the resulting signal represents the interference of many signals having different amplitudes and shifted relative to each other in time, which is equivalent to the addition of signals with different phases.

The consequence of multipath interference is distortion of the received signal. Multipath interference is inherent in any type of signal, but it has a particularly negative effect on wideband signals, since when using a broadband signal, as a result of interference, certain frequencies add up in phase, which leads to an increase in the signal, and some, on the contrary, out of phase, causing a weakening of the signal at a given frequency.

Speaking about multipath interference that occurs during signal transmission, two extreme cases are noted. In the first of them, the maximum delay between signals does not exceed the duration of one symbol and interference occurs within one transmitted symbol. In the second, the maximum delay between signals is greater than the duration of one symbol, so as a result of interference, signals representing different symbols are added, and so-called inter-symbol interference (ISI) occurs.

It is intersymbol interference that has the most negative effect on signal distortion. Since a symbol is a discrete state of a signal, characterized by the values ​​of carrier frequency, amplitude and phase, the amplitude and phase of the signal change for different symbols, and therefore it is extremely difficult to restore the original signal.

For this reason, at high data rates, a data encoding method called Orthogonal Frequency Division Multiplexing (OFDM) is used. Its essence lies in the fact that the stream of transmitted data is distributed over many frequency subchannels and transmission is carried out in parallel on all such subchannels. In this case, a high transmission speed is achieved precisely due to the simultaneous transmission of data over all channels, while the transmission speed in a separate subchannel may be low.

Due to the fact that the data transmission rate in each of the frequency subchannels can be made not too high, the prerequisites are created for effective suppression of intersymbol interference.

Frequency division of channels requires that an individual channel be narrow enough to minimize signal distortion, but at the same time wide enough to provide the required transmission speed. In addition, to economically use the entire bandwidth of a channel divided into subchannels, it is desirable to arrange the frequency subchannels as close to each other as possible, but at the same time avoid interchannel interference to ensure their complete independence. Frequency channels that satisfy the above requirements are called orthogonal. The carrier signals of all frequency subchannels are orthogonal to each other. It is important that the orthogonality of the carrier signals guarantees the frequency independence of the channels from each other, and therefore the absence of inter-channel interference.

This method of dividing a wideband channel into orthogonal frequency subchannels is called orthogonal frequency division multiplexing (OFDM). To implement it in transmitting devices, an inverse fast Fourier transform (IFFT) is used, which transforms the previously multiplexed n-channels signal from time O th representation into frequency.

One of the key advantages of the OFDM method is the combination of high transmission speed with effective resistance to multipath propagation. Of course, OFDM technology itself does not eliminate multipath propagation, but it creates the prerequisites for eliminating the effect of intersymbol interference. The fact is that an integral part of OFDM technology is the Guard Interval (GI) - a cyclic repetition of the end of the symbol, attached at the beginning of the symbol.

The guard interval creates pauses between individual symbols, and if its duration exceeds the maximum signal delay time due to multipath propagation, then intersymbol interference does not occur.

When using OFDM technology, the duration of the guard interval is one-fourth of the duration of the symbol itself. In this case, the symbol has a duration of 3.2 μs, and the guard interval is 0.8 μs. Thus, the duration of the symbol together with the guard interval is 4 μs.

Speaking about the OFDM frequency division technology used at various speeds in the 802.11g protocol, we have not yet touched upon the issue of the carrier signal modulation method.

The 802.11g protocol uses binary and quadrature phase modulation BPSK and QPSK at low bit rates. When using BPSK modulation, only one information bit is encoded in one symbol, and when using QPSK modulation, two information bits are encoded. BPSK modulation is used to transmit data at speeds of 6 and 9 Mbit/s, and QPSK modulation is used at speeds of 12 and 18 Mbit/s.

For transmission at higher speeds, quadrature amplitude modulation QAM (Quadrature Amplitude Modulation) is used, in which information is encoded by changing the phase and amplitude of the signal. The 802.11g protocol uses 16-QAM and 64-QAM modulation. The first modulation involves 16 different signal states, which allows 4 bits to be encoded in one symbol; the second - 64 possible signal states, which makes it possible to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.

In addition to the use of CCK, OFDM and PBCC coding, the IEEE 802.11g standard also optionally provides various hybrid coding options.

In order to understand the essence of this term, remember that any transmitted data packet contains a header (preamble) with service information and a data field. When referring to a packet in CCK format, it means that the header and data of the frame are transmitted in CCK format. Similarly, with OFDM technology, the frame header and data are transmitted using OFDM encoding. Hybrid encoding means that different encoding technologies can be used for the frame header and data fields. For example, when using CCK-OFDM technology, the frame header is encoded using CCK codes, but the frame data itself is transmitted using multi-frequency OFDM encoding. Thus, CCK-OFDM technology is a kind of hybrid of CCK and OFDM. However, this is not the only hybrid technology - when using PBCC packet coding, the frame header is transmitted using CCK codes, and the frame data is encoded using PBCC.

IEEE 802.11a standard

The IEEE 802.11b and IEEE 802.11g standards discussed above refer to the 2.4 GHz frequency range (from 2.4 to 2.4835 GHz), and the IEEE 802.11a standard, adopted in 1999, involves the use of a higher frequency range (from 5 .15 to 5.350 GHz and 5.725 to 5.825 GHz). In the USA, this range is called the Unlicensed National Information Infrastructure (UNII) range.

In accordance with FCC rules, the UNII frequency range is divided into three 100-MHz sub-bands, differing in maximum emission power limits. The low band (5.15 to 5.25 GHz) provides only 50 mW of power, the middle (5.25 to 5.35 GHz) 250 mW, and the high (5.725 to 5.825 GHz) 1 W. The use of three frequency subbands with a total width of 300 MHz makes the IEEE 802.11a standard the most broadband of the 802.11 family of standards and allows the entire frequency range to be divided into 12 channels, each of which has a width of 20 MHz, with eight of them lying in the 200 MHz range from 5 .15 to 5.35 GHz, and the remaining four channels are in the 100 MHz range from 5.725 to 5.825 GHz (Fig. 1). At the same time, the four upper frequency channels, which provide the highest transmission power, are used primarily for transmitting signals outdoors.

Rice. 1. Division of the UNII range into 12 frequency subbands

The IEEE 802.11a standard is based on the Orthogonal Frequency Division Multiplexing (OFDM) technique. To separate the channels, an inverse Fourier transform is used with a window of 64 frequency subchannels. Since each of the 12 channels defined in the 802.11a standard is 20 MHz wide, each orthogonal frequency subchannel (subcarrier) is 312.5 kHz wide. However, out of 64 orthogonal subchannels, only 52 are used, with 48 of them used for data transmission (Data Tones), and the rest for transmission of service information (Pilot Tones).

In terms of modulation technology, the 802.11a protocol is not much different from 802.11g. At low bit rates, binary and quadrature phase modulation BPSK and QPSK are used to modulate subcarrier frequencies. When using BPSK modulation, only one information bit is encoded in one symbol. Accordingly, when using QPSK modulation, that is, when the phase of the signal can take four different values, two information bits are encoded in one symbol. BPSK modulation is used to transmit data at 6 and 9 Mbps, and QPSK modulation is used at 12 and 18 Mbps.

To transmit at higher speeds, the IEEE 802.11a standard uses 16-QAM and 64-QAM quadrature amplitude modulation. In the first case there are 16 different signal states, which allows you to encode 4 bits in one symbol, and in the second there are already 64 possible signal states, which allows you to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at speeds of 24 and 36 Mbit/s, and 64-QAM modulation is used at speeds of 48 and 54 Mbit/s.

The information capacity of an OFDM symbol is determined by the type of modulation and the number of subcarriers. Since 48 subcarriers are used for data transmission, the capacity of an OFDM symbol is 48 x Nb, where Nb is the binary logarithm of the number of modulation positions, or, more simply, the number of bits that are encoded in one symbol in one subchannel. Accordingly, the OFDM symbol capacity ranges from 48 to 288 bits.

The sequence of processing input data (bits) in the IEEE 802.11a standard is as follows. Initially, the input data stream is subjected to a standard scrambling operation. After this, the data stream is fed to the convolutional encoder. The convolutional coding rate (in combination with puncture coding) can be 1/2, 2/3 or 3/4.

Since the convolutional coding rate can be different, when using the same type of modulation, the data transmission rate is different.

Consider, for example, BPSK modulation, where the data rate is 6 or 9 Mbit/s. The duration of one symbol together with the guard interval is 4 μs, which means that the pulse repetition rate will be 250 kHz. Considering that one bit is encoded in each subchannel, and there are 48 such subchannels in total, we obtain that the total data transfer rate will be 250 kHz x 48 channels = 12 MHz. If the convolutional coding speed is 1/2 (one service bit is added for each information bit), the information speed will be half the full speed, that is, 6 Mbit/s. At a convolutional coding rate of 3/4, for every three information bits one service bit is added, so in this case the useful (information) speed is 3/4 of the full speed, that is, 9 Mbit/s.

Similarly, each modulation type corresponds to two different transmission rates (Table 1).

Table 1. Relationship between transmission rates
and modulation type in the 802.11a standard

After convolutional encoding, the bit stream is subjected to interleaving, or interleaving. Its essence is to change the order of bits within one OFDM symbol. To do this, the sequence of input bits is divided into blocks whose length is equal to the number of bits in the OFDM symbol (NCBPS). Next, according to a certain algorithm, a two-stage rearrangement of bits in each block is performed. In the first stage, the bits are rearranged so that adjacent bits are transmitted on non-adjacent subcarriers when transmitting an OFDM symbol. The bit swapping algorithm at this stage is equivalent to the following procedure. Initially, a block of bits of length NCBPS is written row by row into a matrix containing 16 rows and NCBPS/16 rows. Next, the bits are read from this matrix, but in rows (or in the same way as they were written, but from a transposed matrix). As a result of this operation, initially adjacent bits will be transmitted on non-adjacent subcarriers.

This is followed by a second bit permutation step, the purpose of which is to ensure that adjacent bits do not simultaneously appear in the least significant bits of the groups defining the modulation symbol in the signal constellation. That is, after the second stage of permutation, adjacent bits appear alternately in the high and low digits of the groups. This is done in order to improve the noise immunity of the transmitted signal.

After interleaving, the bit sequence is divided into groups according to the number of positions of the selected modulation type and OFDM symbols are formed.

The generated OFDM symbols are subjected to fast Fourier transform, resulting in the formation of output in-phase and quadrature signals, which are then subjected to standard processing - modulation.

IEEE 802.11n standard

Development of the IEEE 802.11n standard officially began on September 11, 2002, that is, one year before the final adoption of the IEEE 802.11g standard. In the second half of 2003, the IEEE 802.11n Task Group (802.11 TGn) was created, whose task was to develop a new wireless communication standard at speeds above 100 Mbit/s. Another task group, 802.15.3a, also dealt with the same task. By 2005, the processes of developing a single solution in each of the groups had reached a dead end. In the 802.15.3a group, there was a confrontation between Motorola and all other members of the group, and members of the IEEE 802.11n group split into two approximately identical camps: WWiSE (World Wide Spectrum Efficiency) and TGn Sync. The WWiSE group was led by Aigro Networks, and the TGn Sync group was led by Intel. In each of the groups, for a long time, none of the alternative options could get the 75% of votes necessary for its approval.

After almost three years of unsuccessful opposition and attempts to work out a compromise solution that would suit everyone, the 802.15.3a group members voted almost unanimously to eliminate the 802.15.3a project. Members of the IEEE 802.11n project turned out to be more flexible - they managed to agree and create a unified proposal that would suit everyone. As a result, on January 19, 2006, at a regular conference held in Kona, Hawaii, a draft specification of the IEEE 802.11n standard was approved. Of the 188 members of the working group, 184 were in favor of adopting the standard, and four abstained. The main provisions of the approved document will form the basis for the final specification of the new standard.

The IEEE 802.11n standard is based on OFDM-MIMO technology. Many of the technical details implemented in it are borrowed from the 802.11a standard, but the IEEE 802.11n standard provides for the use of both the frequency range adopted for the IEEE 802.11a standard and the frequency range adopted for the IEEE 802.11b/g standards. That is, devices that support the IEEE 802.11n standard can operate in either the 5 or 2.4 GHz frequency range, with the specific implementation depending on the country. For Russia, IEEE 802.11n devices will support the 2.4 GHz frequency range.

The increase in transmission speed in the IEEE 802.11n standard is achieved, firstly, by doubling the channel width from 20 to 40 MHz, and secondly, by implementing MIMO technology.

MIMO (Multiple Input Multiple Output) technology involves the use of multiple transmitting and receiving antennas. By analogy, traditional systems, that is, systems with one transmitting and one receiving antenna, are called SISO (Single Input Single Output).

Theoretically, a MIMO system with n transmitting and n receiving antennas can provide peak throughput in n times larger than SISO systems. This is achieved by the transmitter breaking the data stream into independent bit sequences and transmitting them simultaneously using an array of antennas. This transmission technique is called spatial multiplexing. Note that all antennas transmit data independently of each other in the same frequency range.

Consider, for example, a MIMO system consisting of n transmitting and m receiving antennas (Fig. 2).

Rice. 2. Implementation principle of MIMO technology

The transmitter in such a system sends n independent signals using n antennas On the receiving side, each m antenna receives signals that are a superposition n signals from all transmitting antennas. So the signal R1, received by the first antenna, can be represented as:

Writing similar equations for each receiving antenna, we obtain the following system:

Or, rewriting this expression in matrix form:

Where [ H] - transfer matrix describing the MIMO communication channel.

In order for the decoder on the receiving side to be able to correctly reconstruct all signals, it must first determine the coefficients hij, characterizing each of m x n transmission channels. To determine the coefficients hij MIMO technology uses a packet preamble.

Having determined the coefficients of the transfer matrix, you can easily restore the transmitted signal:

Where [ H]–1 - matrix inverse to the transfer matrix [ H].

It is important to note that in MIMO technology, the use of multiple transmitting and receiving antennas makes it possible to increase the throughput of a communication channel by implementing several spatially separated subchannels, while data is transmitted in the same frequency range.

MIMO technology does not affect the data encoding method in any way and, in principle, can be used in combination with any methods of physical and logical data encoding.

MIMO technology was first described in the IEEE 802.16 standard. This standard allows the use of MISO technology, that is, several transmitting antennas and one receiving antenna. The IEEE 802.11n standard allows the use of up to four antennas at the access point and wireless adapter. Mandatory mode implies support for two antennas at the access point and one antenna and wireless adapter.

The IEEE 802.11n standard provides both standard 20 MHz and double-width channels. However, the use of 40 MHz channels is an optional feature of the standard, since the use of such channels may contravene the laws of some countries.

The 802.11n standard provides two transmission modes: standard transmission mode (L) and high throughput (HT) mode. In traditional transmission modes, 52 frequency OFDM subchannels (frequency subcarriers) are used, of which 48 are used for data transmission, and the rest for transmission of service information.

In modes with increased capacity with a channel width of 20 MHz, 56 frequency subchannels are used, of which 52 are used for data transmission, and four channels are pilot. Thus, even when using a 20 MHz channel, increasing the frequency subchannels from 48 to 52 allows an 8% increase in transmission speed.

When using a double-width channel, that is, a 40 MHz channel, in standard transmission mode the broadcast is actually carried out on a double channel. Accordingly, the number of frequency subcarriers doubles (104 subchannels, of which 96 are information). Thanks to this, the transfer speed increases by 100%.

When using a 40-MHz channel and high-bandwidth mode, 114 frequency subchannels are used, of which 108 are information subchannels and six are pilot ones. Accordingly, this allows you to increase the transmission speed by 125%.

Table 2. Relationship between transmission rates and modulation type
and convolutional coding speed in the 802.11n standard
(20 MHz channel width, HT mode (52 frequency subchannels))

Two more circumstances due to which the IEEE 802.11n standard increases the transmission speed are a reduction in the duration of the GI guard interval in OGDM symbols from 0.8 to 0.4 μs and an increase in the speed of convolutional coding. Recall that in the IEEE 802.11a protocol, the maximum convolutional coding rate is 3/4, that is, for every three input bits one more is added. In the IEEE 802.11n protocol, the maximum convolutional coding rate is 5/6, that is, every five input bits in the convolutional encoder are converted into six output bits. The relationship between transmission rates, modulation type and convolutional coding rate for a standard 20 MHz wide channel is given in Table. 2.

The IEEE (Institute of Electrical and Electronic Engineers) is developing WiFi 802.11 standards.

IEEE 802.11 is the base standard for Wi-Fi networks, which defines a set of protocols for the lowest transfer rates.

IEEE 802.11b- describes b O higher transmission speeds and introduces more technological restrictions. This standard was widely promoted by WECA ( Wireless Ethernet Compatibility Alliance ) and was originally called WiFi .
Frequency channels in the 2.4GHz spectrum are used ().
Ratified in 1999.
RF technology used: DSSS.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel: 1, 2, 5.5, 11 Mbps,

IEEE 802.11a- describes significantly higher transfer rates than 802.11b.
Frequency channels in the 5GHz frequency spectrum are used. ProtocolNot compatible with 802.11 b.
Ratified in 1999.
RF technology used: OFDM.
Coding: Conversion Coding.
Modulations: BPSK, QPSK, 16-QAM, 64-QAM.
Maximum data transfer rates in the channel: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.

IEEE 802.11g - describes data transfer rates equivalent to 802.11a.
Frequency channels in the 2.4GHz spectrum are used. The protocol is compatible with 802.11b.
Ratified in 2003.
RF technologies used: DSSS and OFDM.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel:
- 1, 2, 5.5, 11 Mbps on DSSS and
- 6, 9, 12, 18, 24, 36, 48, 54 Mbps on OFDM.

IEEE 802.11n- the most advanced commercial WiFi standard, currently officially approved for import and use in the Russian Federation (802.11ac is still being developed by the regulator). 802.11n uses frequency channels in the 2.4GHz and 5GHz WiFi frequency spectrums. Compatible with 11b/11 a/11g . Although it is recommended to build networks targeting only 802.11n, because... requires configuration of special protective modes if backward compatibility with legacy standards is required. This leads to a large increase in signal information anda significant reduction in the available useful performance of the air interface. Actually, even one WiFi 802.11g or 802.11b client will require special configuration of the entire network and its immediate significant degradation in terms of aggregated performance.
The WiFi 802.11n standard itself was released on September 11, 2009.
WiFi frequency channels with a width of 20MHz and 40MHz (2x20MHz) are supported.
RF technology used: OFDM.
OFDM MIMO (Multiple Input Multiple Output) technology is used up to the 4x4 level (4xTransmitter and 4xReceiver). In this case, a minimum of 2xTransmitter per Access Point and 1xTransmitter per user device.
Examples of possible MCS (Modulation & Coding Scheme) for 802.11n, as well as the maximum theoretical transfer rates in the radio channel are presented in the following table:

Here SGI is the guard intervals between frames.
Spatial Streams is the number of spatial streams.
Type is the modulation type.
Data Rate is the maximum theoretical data transfer rate in the radio channel in Mbit/sec.

It is important to emphasize that the indicated speeds correspond to the concept of channel rate and are the maximum value using a given set of technologies within the framework of the described standard (in fact, these values, as you probably noticed, are written by manufacturers on the boxes of home WiFi devices in stores). But in real life, these values ​​are not achievable due to the specifics of the WiFi 802.11 standard technology itself. For example, “political correctness” in terms of ensuring CSMA/CA is strongly influenced here (WiFi devices constantly listen to the air and cannot transmit if the transmission medium is busy), the need to confirm each unicast frame, the half-duplex nature of all WiFi standards and only 802.11ac/Wave-2 will be able to start bypassing this, etc. Therefore, the practical efficiency of legacy 802.11 b/g/a standards never exceeds 50% under ideal conditions (for example, for 802.11g the maximum speed per subscriber is usually no higher than 22Mb/s), and for 802.11n efficiency can be up to 60%. If the network operates in protected mode, which often happens due to the mixed presence of different WiFi chips on different devices on the network, then even the indicated relative efficiency can drop by 2-3 times. This applies, for example, to a mix of Wi-Fi devices with 802.11b, 802.11g chips on a network with WiFi 802.11g access points, or a WiFi 802.11g/802.11b device on a network with WiFi 802.11n access points, etc. Read more about .

In addition to the basic WiFi standards 802.11a, b, g, n, additional standards exist and are used to implement various service functions:

. 802.11d. To adapt various WiFi standard devices to specific country conditions. Within the regulatory framework of each state, ranges often vary and may even differ depending on geographic location. The WiFi IEEE 802.11d standard allows you to adjust frequency bands in devices from different manufacturers using special options introduced into the media access control protocols.

. 802.11e. Describes QoS quality classes for the transmission of various media files and, in general, various media content. Adaptation of the MAC layer for 802.11e determines the quality of, for example, simultaneous transmission of audio and video.

. 802.11f. Aimed at unifying the parameters of Wi-Fi access points from different manufacturers. The standard allows the user to work with different networks when moving between coverage areas of individual networks.

. 802.11h. Used to prevent problems with weather and military radars by dynamically reducing the emitted power of Wi-Fi equipment or dynamically switching to another frequency channel when a trigger signal is detected (in most European countries, ground stations tracking weather and communications satellites, as well as military radars operate in ranges close to 5 MHz). This standard is a necessary ETSI requirement for equipment approved for use in the European Union.

. 802.11i. The first iterations of the 802.11 WiFi standards used the WEP algorithm to secure Wi-Fi networks. It was believed that this method could provide confidentiality and protection of the transmitted data of authorized wireless users from eavesdropping. Now this protection can be hacked in just a few minutes. Therefore, the 802.11i standard developed new methods for protecting Wi-Fi networks, implemented at both the physical and software levels. Currently, to organize a security system in Wi-Fi 802.11 networks, it is recommended to use Wi-Fi Protected Access (WPA) algorithms. They also provide compatibility between wireless devices of different standards and modifications. WPA protocols use an advanced RC4 encryption scheme and a mandatory authentication method using EAP. The stability and security of modern Wi-Fi networks is determined by privacy verification and data encryption protocols (RSNA, TKIP, CCMP, AES). The most recommended approach is to use WPA2 with AES encryption (and don't forget 802.1x using tunneling mechanisms such as EAP-TLS, TTLS, etc.). .

. 802.11k. This standard is actually aimed at implementing load balancing in the radio subsystem of a Wi-Fi network. Typically, in a wireless LAN, the subscriber device usually connects to the access point that provides the strongest signal. This often leads to network congestion at one point, when many users connect to one Access Point at once. To control such situations, the 802.11k standard proposes a mechanism that limits the number of subscribers connected to one Access Point and makes it possible to create conditions under which new users will join another AP even despite a weaker signal from it. In this case, the aggregated network throughput increases due to more efficient use of resources.

. 802.11m. Amendments and corrections for the entire group of 802.11 standards are combined and summarized in a separate document under the general name 802.11m. The first release of 802.11m was in 2007, then in 2011, etc.

. 802.11p. Determines the interaction of Wi-Fi equipment moving at speeds of up to 200 km/h past stationary WiFi Access Points located at a distance of up to 1 km. Part of the Wireless Access in Vehicular Environment (WAVE) standard. WAVE standards define an architecture and a complementary set of utility functions and interfaces that provide a secure radio communications mechanism between moving vehicles. These standards are developed for applications such as traffic management, traffic safety monitoring, automated payment collection, vehicle navigation and routing, etc.

. 802.11s. A standard for implementing mesh networks (), where any device can serve as both a router and an access point. If the nearest access point is overloaded, data is redirected to the nearest unloaded node. In this case, a data packet is transferred (packet transfer) from one node to another until it reaches its final destination. This standard introduces new protocols at the MAC and PHY levels that support broadcast and multicast (transfer), as well as unicast delivery over a self-configuring system of Wi-Fi access points. For this purpose, the standard introduced a four-address frame format. Examples of implementation of WiFi Mesh networks: , .

. 802.11t. The standard was created to institutionalize the process of testing solutions of the IEEE 802.11 standard. Testing methods, methods of measurement and processing of results (treatment), requirements for testing equipment are described.

. 802.11u. Defines procedures for interaction of Wi-Fi standard networks with external networks. The standard must define access protocols, priority protocols and prohibition protocols for working with external networks. At the moment, a large movement has formed around this standard, both in terms of developing solutions - Hotspot 2.0, and in terms of organizing inter-network roaming - a group of interested operators has been created and is growing, who jointly resolve roaming issues for their Wi-Fi networks in dialogue (WBA Alliance ). Read more about Hotspot 2.0 in our articles: , .

. 802.11v. The standard should include amendments aimed at improving the network management systems of the IEEE 802.11 standard. Modernization at the MAC and PHY levels should allow the configuration of client devices connected to the network to be centralized and streamlined.

. 802.11y. Additional communication standard for the frequency range 3.65-3.70 GHz. Designed for latest generation devices operating with external antennas at speeds up to 54 Mbit/s at a distance of up to 5 km in open space. The standard is not fully completed.

802.11w. Defines methods and procedures for improving the protection and security of the media access control (MAC) layer. The standard protocols structure a system for monitoring data integrity, the authenticity of their source, the prohibition of unauthorized reproduction and copying, data confidentiality and other protection measures. The standard introduces management frame protection (MFP: Management Frame Protection), and additional security measures help neutralize external attacks, such as DoS. A little more on MFP here: . In addition, these measures will ensure security for the most sensitive network information that will be transmitted over networks that support IEEE 802.11r, k, y.

802.11ac. A new WiFi standard that operates only in the 5GHz frequency band and provides significantly faster O higher speeds both for an individual WiFi client and for a WiFi Access Point. See our article for more details.

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