Expansion of the spectrum by frequency hopping. Basic Concepts of Spread Spectrum Systems
SPREAD SPECTRUM SYSTEMS
The term spread spectrum has been used in numerous military and commercial communications systems. In spread spectrum systems, each message carrier signal requires significantly more radio frequency bandwidth than a conventional modulated signal. A wider frequency band allows you to obtain some useful properties and characteristics that are difficult to achieve by other means.
Spread spectrum is a method of generating a spread spectrum signal by using an additional modulation stage to not only broaden the spectrum of the signal, but also reduce its influence on other signals. Additional modulation has nothing to do with the transmitted message.
Broadband systems are used due to the following potential benefits:
Increased noise immunity;
Possibility of providing code division of channels for multiple access based on it in systems using CDMA technology;
Energy secrecy due to low level spectral density;
High resolution when measuring distances;
Communication security;
Ability to withstand the effects of intentional interference;
Increased capacity and spectral efficiency in some cellular personal communications systems;
A gradual decrease in communication quality with an increase in the number of users simultaneously occupying the same HF channel;
Low cost of sale;
Availability of modern element base (integrated circuits).
Figure 6.1 – Structure of a direct spread spectrum system
According to the architecture and types of modulation used, spread spectrum systems can be divided into the following main groups.
Pseudo-random sequence (PRS)-based direct spread spectrum, including CDMA systems,
Frequency agility (frequency hopping), including CDMA systems with slow and fast frequency agility,
Carrier Sense Multiple Access (CSMA),
With restructuring of the time position of signals (“jumping” time),
With linear frequency modulation of signals (chip modulation),
With mixed spectrum spreading methods.
Direct spectrum expansion using pseudorandom sequences
Figure 6.1 shows a conceptual diagram of a direct spread spectrum system based on pseudo-random sequences (a - PSK signal transmitter with subsequent spectrum, b - baseband spread spectrum transmitter, c - receiver). In the first modulator, phase shift keying (PSK) of the intermediate frequency signal is carried out using binary digital signal transmitted message d(t) in non-return to zero (NRZ) format with symbol frequency f b = 1/T b .
Within one cell of a mobile radio communication system, as a rule, there are several subscribers using the communication simultaneously, each of them using the same carrier frequency RF and occupying the same frequency band RF.
The process of generating spread spectrum signals in multiple access systems occurs in two stages: modulation and spread spectrum (or secondary modulation via PSP). Secondary modulation is carried out using the ideal multiplication operation g(t)s(t). With this multiplication, an amplitude-modulated two-way signal with a suppressed carrier is formed. The first and second modulators can be swapped without changing the potential characteristics of the system.
The spread spectrum signal g(t)s(t) is upconverted to the desired radio frequency. Although up- and down-frequency conversion is an almost necessary process for most systems, it is not a critical step. Therefore, in the future we will assume that the signal g(t)s(t) is transmitted and received at an intermediate frequency, excluding the up and down frequency conversion subsystem from consideration.
Thus, the receiver input receives a sum of M independent spread spectrum signals occupying the same RF band.
The concept of spread spectrum systems through software tuning of the operating frequency is in many ways similar to the concept of direct spread spectrum systems. Here, the binary PSP generator controls the frequency synthesizer, with the help of which a transition (“jump”) is made from one frequency to another of the many available frequencies. Thus, here the effect of spectrum expansion is achieved through pseudo-random tuning of the carrier frequency, the value of which is selected from the available frequencies f1,...,fN, where N can reach values of several thousand or more. If the rate of message tuning (the rate of frequency change) exceeds the message transmission rate, then we have a system with fast frequency tuning. If the speed of adjustment less speed message transmission, so that several bits are transmitted in the tuning interval, then we have a system with slow frequency tuning.
If an ensemble of uncorrelated PSP signals is selected, then after the spectrum compression operation only the modulated useful signal is retained. All other signals, being uncorrelated, retain broadband and have a spectral width exceeding the cutoff bandwidth of the demodulator filter. Figure 6.2 shows simplified timing and spectral diagrams that qualitatively illustrate the processes of expansion and compression of the signal spectrum. In particular, they lack a carrier signal.
Figure 6.2 - Diagrams for spectrum expansion
In spread spectrum systems, by tuning the operating frequency, the latter remains constant during each tuning interval, but changes abruptly from interval to interval. Transmission frequencies are generated by a digital frequency synthesizer controlled by a code (“words”) arriving in serial or parallel view and containing m binary symbols (bits) Each m-bit word or part thereof corresponds to one of M = 2m frequencies. Although there are M = 2m, m = 2, 3 frequencies available for frequency tuning, not all of them are necessarily used in a particular system. Systems with spectrum expansion by software tuning of the operating frequency are divided into systems with slow, fast and medium tuning speeds.
In systems with slow tuning, the tuning rate fh is less than the message transmission rate fb. Thus, in the tuning interval, two message bits or more (in some systems over 1000 in some systems) can be transmitted before changing to another frequency. In systems with medium tuning speed, the tuning speed is equal to the transmission speed. The most widely used systems are systems with fast and slow adjustment of the operating frequency.
To synchronize receivers when receiving spread spectrum signals, three synchronization devices may be required:
Carrier phase synchronization (carrier recovery);
Symbolic synchronization (clock frequency recovery);
Time synchronization of generators that generate code or pseudo-random sequences.
Time synchronization is provided in two stages, during which the following is performed:
Search (initial, coarse synchronization);
Tracking (precise synchronization).
Figure 6.3 shows block diagrams transmitting and receiving parts of the system with frequency tuning.
Figure 6.3 - System with software frequency tuning
The GSM standard uses spectral-efficient Gaussian minimum shift keying (GMSK). The manipulation is called Gaussian because the sequence of information bits before the modulator passes through a low-pass filter (LPF) with a Gaussian characteristic, which results in a significant reduction in the frequency band of the emitted radio signal. The formation of a GMSK radio signal is carried out in such a way that in the interval of one information bit the carrier phase changes by 90°. This is the smallest possible phase change detectable with a given type of modulation. Continuously changing the phase of a sinusoidal signal results in frequency modulation with a discrete change in frequency. The use of a Gaussian filter makes it possible to obtain “smooth transitions” with a discrete change in frequency. The GSM standard uses GMSK modulation with a normalized bandwidth VT = 0.3, where IN- filter bandwidth at level -3 dB, T- duration of 1 bit of digital message. The functional diagram of the modulator is shown in Figure 6.4.
Figure 6.4 - Functional diagram of the modulator
The basis of the GMSK signal shaper is a quadrature (1/Q) modulator. The circuit consists of two multipliers and one adder. The purpose of this circuit is to provide continuous accurate phase modulation. One multiplier changes the amplitude of a sinusoidal oscillation, and the second – a cosine oscillation. The input signal before the multiplier is divided into two quadrature components. The decomposition occurs in two blocks designated "sin" and "cos".
Diagrams illustrating the formation of a GMSK signal are shown in Figure 4.9.
GMSK modulation has the following properties that are preferred by: mobile communications:
The envelope is constant in level, which allows the use of efficient transmitting devices with power amplifiers in class C mode;
Compact spectrum at the output of the power amplifier of the transmitting device, which provides low level out-of-band radiation;
Good characteristics noise immunity of the communication channel.
Figure 6.5 - GMSK signal generation
Speech processing. Speech processing in the GSM standard is carried out in order to ensure high quality of transmitted messages and implement additional service capabilities. Speech processing is carried out within the framework of the adopted system of intermittent transmission of speech (Discontinuous Transmission - DTX), which ensures that the transmitter is turned on when the user begins a conversation, and turns it off during pauses and at the end of the conversation. DTX is controlled by a Voice Activity Detector (VAD), which detects and distinguishes speech with noise and noise without speech, even when the noise level is comparable to the speech level. The intermittent speech transmission system also includes a device for generating comfortable noise, which is turned on and listened to during pauses in speech when the transmitter is turned off. It has been experimentally proven that turning off background noise at the receiver output in pauses when the transmitter is turned off, irritates the subscriber and reduces speech intelligibility, so the use of comfortable noise in pauses is considered necessary. The DTX process in the receiver involves interpolating speech fragments lost due to errors in the channel.
In order to send a high-power radio signal in the microwave range, you need an expensive transmitter with an amplifier and an expensive large-diameter antenna. In order to receive a low-power signal without interference, you also need an expensive large antenna and an expensive receiver with an amplifier.
This is the case when using a conventional “narrowband” radio signal, when transmission occurs at one specific frequency, or more precisely, in a narrow band of the radio spectrum surrounding this frequency (frequency channel). The picture is further complicated by various mutual interferences between high-power narrowband signals transmitted close to each other or at similar frequencies. In particular, a narrowband signal can simply be jammed (accidentally or intentionally) by a transmitter of sufficient power tuned to the same frequency.
It was this vulnerability to interference from conventional radio signals that led to the development, first for military applications, of a completely different principle of radio transmission called technology broadband signal, or noise-like signal (both versions of the term correspond to the abbreviation Spread Spectrum). After many years of successful defense use, this technology has found civilian applications, and it is in that capacity that it will be discussed here.
It was found that in addition to its characteristic properties (its own noise immunity and low level of generated interference), this technology turned out to be relatively cheap for mass production. Cost-effectiveness occurs due to the fact that all the complexity of broadband technology is programmed into several microelectronic components (“chips”), and the cost of microelectronics in mass production is very low. As for the remaining components of broadband devices - microwave electronics, antennas - they are cheaper and simpler than in the usual “narrowband” case, due to the extremely low power of the radio signals used.
The idea of Spread Spectrum is that a much wider frequency band is used to transmit information than is required for conventional (in a narrow frequency channel) transmission. Two fundamentally different methods for using such a wide frequency band have been developed - the Direct Sequence Spread Spectrum (DSSS) method and the Frequency Hopping Spread Spectrum (FHSS) method. Both of these methods are provided by the 802.11 (Radio-Ethernet) standard.
Current state wireless communication determined by the situation with the IEEE 802.11 standard. The standard is being developed and improved by the Wireless Working Group. local networks(Working Group for Wireless Local Area Networks) standards committee of the Institute of Electrical and Electronic Engineers (IEEE), chaired by Vic Hayes of Lucent Technologies. The group has about a hundred members with a decisive vote and about fifty with an advisory vote; they represent virtually all equipment manufacturers, as well as research centers and universities. Four times a year, the group meets in plenary sessions and makes decisions to improve the standard.
The standard defines one type of MAC layer media access protocol and three different protocols for physical (PHY) links.
The MAC layer defines the basic components of the network architecture and the list of services provided by this layer. There are two standard architecture options wireless networks:
Independent “ad-hoc” configuration, where stations can communicate directly with each other. The area of such a network and functionality limited.
An infrastructure configuration in which stations communicate through an access point, either operating standalone or connected to a cable network. The standard defines the radio channel interface between stations and the access point. Access points can be connected to each other using radio bridges or cable network segments.
The standard establishes a protocol for using a single transmission medium, called Carrier Sense Multiple Access Collision Avoidance (CSMA/CA). The likelihood of collisions for wireless nodes is minimized by sending to all nodes in advance short message(ready to send, RTS) about the recipient and duration of the upcoming transmission. Nodes delay transmission for a time equal to the advertised message duration. The receiving station responds to the RTS with a message (CTS), which tells the sending node whether the medium is clear and whether the node is ready to receive. After receiving a data packet, the node sends an acknowledgment (ACK) of error-free reception. If the ACK is not received, the data packet will be retransmitted.
The specification provided by the standard requires the data to be divided into packets equipped with control and addressing information. This information, which takes up about 30 bytes, is followed by an information block up to 2048 bytes long. This is followed by a 4-byte CRC code of the information block. The standard recommends using packets of 400 bytes in length for an FHSS physical channel and 1500 or 2048 for a DSSS channel.
The standard provides for data security, including authentication (to verify that a node entering the network is authorized in it) and data encryption using the RC4 algorithm with a 40-bit key. For laptop computers, the standard provides for a power saving mode: putting the device into a “sleep” mode and bringing it out of this state for a short time necessary to receive a service signal from network nodes starting transmission. There is also a roaming mode that allows a mobile subscriber to move between access points without losing connection.
Spectrum extension
On physical level The standard allows the use of one of two types of radio channels and one type of infrared channel. Both types of radio channels use spread spectrum technology, which reduces the average power spectral density of the signal by distributing energy over a frequency band wider than necessary to provide a given transmission rate. This technology reduces the level of interference generated and provides increased reception immunity to interference.
The first type of radio channel is Frequency Hopping Spread Spectrum (FHSS) Radio PHY. A transfer rate of 1 Mbit/s is provided (optional 2 Mbit/s). The 1 Mbit/s version uses two-level Gaussian frequency modulation (2GFSK), while the 2 Mbit/s version uses four-level Gaussian frequency modulation (4GFSK). At a speed of 1 Mbit/s, the signal frequency changes over a message symbol duration of 1 μs, according to the Gaussian law, from the nominal value to a value of +170 kHz and returns to the nominal value. To transmit zero, the signal frequency is changed to –170 kHz. For 2 Mbps there are four levels of frequency offset (+225, +75, –75, –225 kHz), so each chip (symbol) carries two message bits. The signal spectrum width with such modulation is 1 MHz, regardless of the transmission speed. This makes it possible to use 79 frequency positions for transmission in the range from 2402 to 2480 MHz in 1 MHz steps. To expand the spectrum, the signal frequency changes according to a pseudo-random law at least once every 400 ms.
The second type of radio channel is Direct Sequence Spread Spectrum (DSSS) Radio PHY. This option provides for transmission at speeds of 1 and 2 Mbit/s. At a transfer rate of 1 Mbit/s, binary phase shift keying (BPSK) is used. The one bit is represented by an 11-element Barker code of the form 11100010010, and the zero bit is represented by an inverse Barker code. Elementary symbols of the Barker code do not carry information; bits are transmitted at once by the entire Barker code - direct or inverse. This allows you to give the signal noise properties that provide noise immunity. The spectrum width of such a signal is 22 MHz. For speeds of 2 Mbit/s, the standard provides quadrature phase shift keying - QPSK. In this case, two bits are transmitted during the duration of the message symbol. For this you need not two, but four different signal. Therefore, together with the main carrier vibration, an additional one is used, shifted in phase by 90° relative to it. The phase of each of these oscillations is controlled by a direct or inverse Barker sequence, and both oscillations are added. Thus, over the duration of a symbol, the signal has four degrees of freedom, allowing two bits to be transmitted. In this case, the transmission speed is doubled while maintaining the same frequency band as with binary transmission. The DSSS signal uses one of 14 overlapping frequency bands, defined by the standard in the general frequency band 83.5 MHz.
For infrared channel(Infrared PHY) standard provides a speed of 1 Mbit/s (optional 2 Mbit/s) with pulse-position modulation. This type of channel is not of great interest, since the transmission range provided for by the standard does not exceed 20 m.
There are several different spread spectrum technologies, but to further understand the 802.11 protocol, we only need to take a closer look at Direct Sequence Spread Spectrum (DSSS).
DSSS technology
With potential coding, information bits - logical zeros and ones - are transmitted as rectangular voltage pulses. A rectangular pulse of duration T has a spectrum whose width is inversely proportional to the pulse duration. Therefore, the shorter the duration of the information bit, the larger the spectrum occupied by such a signal.
To deliberately broaden the spectrum of an initially narrow-band signal, DSSS technology literally embeds a sequence of so-called chips into each transmitted information bit (logical 0 or 1). If information bits - logical zeros or ones - during potential encoding of information can be represented as a sequence rectangular pulses, then each individual chip is also a rectangular pulse, but its duration is several times less than the duration of the information bit. The sequence of chips is a sequence of rectangular pulses, that is, zeros and ones, but these zeros and ones are not informational. Since the duration of one chip is n times less than the duration of the information bit, the width of the spectrum of the converted signal will be n times more width spectrum of the original signal. In this case, the amplitude of the transmitted signal will decrease by n times.
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 is 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: 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. It turns out that it is possible, but for this you need to select the chip sequence accordingly. Chip sequences used to broaden the signal spectrum must satisfy certain autocorrelation requirements. The term autocorrelation in mathematics refers to the degree of similarity of a function 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 will be possible to isolate at the noise level. To do this, the received signal is multiplied in the receiver by the same chip sequence, that is, the autocorrelation function of the signal is calculated. As a result, the signal again becomes narrowband, so it is filtered in a narrow frequency band and any interference falling into the band of the original broadband signal, after multiplying 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, according to power is significantly less than the interference acting at the receiver input (Fig. 7.1).
Barker codes
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 a Barker code that is 11 chips long (11100010010).
In order to transmit a signal, a logical one is transmitted by a direct Barker sequence, and a logical zero by an inverse sequence.
Speed 1 Mbps
The 802.11 standard provides two speed modes: 1 and 2 Mbit/s. To encode data at the physical layer, the DSSS method with 11-chip Barker codes is used. With an information speed of 1 Mbit/s, the speed of individual Barker sequence chips is 11×106 chip/s, 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 in total 3 non-overlapping frequency channel. All frequency range However, it is customary to divide it into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz apart. 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 at 2417 MHz, and the last, channel 11, is centered at 2462 MHz. When viewed this way, the first, sixth and 11th channels do not overlap with each other and have a 3 megahertz gap relative to each other. It is these three channels that can be used independently of each other.
Differential Binary Phase Shift Key (DBPSK) is used to modulate a sinusoidal carrier signal (a process necessary to inform the carrier signal). 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 π. 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 π.
Speed 2 Mbps
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 shiftey is used to modulate the carrier wave. With relative quadrature phase modulation, the phase shift can take four different values: 0, π/2, π and 3π/2. Using four different signal states, it is possible to encode a sequence of two information bits (dibits) in one discrete state and thereby double the information transmission rate. For example, dibit 00 may correspond to a phase shift of 0; dibit 01 - phase shift equal to π/2; dibit 11 - phase shift equal to π; dibit 10 - phase shift equal to 3π/2.
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, 11 × 10 6 chip/s, and therefore the width of the spectrum of the transmitted signal does not change.
7.2 7.2 Physical layer of the 802.11b/b+ protocol
The IEEE 802.11b protocol, adopted in July 1999, 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. To operate at speeds of 1 and 2 Mbit/s, spectrum spreading technology using Barker codes is used, and for speeds of 5.5 and 11 Mbit/s so-called complementary codes (Complementary Code Keying, CCK) are used.
CCK sequences
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.
The IEEE 802.11b standard deals with complex complementary 8-chip sequences defined on a set of complex elements.
Here it is worth making a small lyrical digression so as not to alienate the reader by the complexity of the mathematical apparatus used. The mathematics of complex numbers can evoke a lot of negative memories, being associated with something completely abstract. But in in this case everything is quite simple. A complex representation of a signal is just a convenient mathematical apparatus for representing a phase-modulated signal.
Using a set of complex elements (1, –1, j, –j), it is possible to form eight complex numbers that are identical in magnitude but differ in phase. That is, the elements of the 8-chip CCK sequence can take one of the following eight values: 1, –1, j, –j, 1+j, 1–j, –1+j, –1–j. 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 could be encoded, but a whole set of sequences. Considering that each element of an 8-sip sequence can take one of eight values depending on the phase value, it is clear that 8 8 =16777216 sequence options can be combined, however, not all of them will be complementary. But even taking into account the requirement of complementarity, a fairly large number of different CCK sequences can be formed. This circumstance makes it possible to encode several information bits in one transmitted symbol and thereby increase the information transmission speed.
Generally speaking, using CCK codes allows you to encode 8 bits per character at 11 Mbit/s and 4 bits per character at 5.5 Mbit/s. Moreover, in both cases symbol rate transmission is 1.385×10 6 symbols per second (11/8 = 5.5/4 = 1.385), and taking into account that each symbol is specified by an 8-chip sequence, we obtain that in both cases the speed of individual chips is 11×10 6 chips per second. Accordingly, the signal spectrum width at both speeds of 11 Mbit/s and 5.5 Mbit/s is 22 MHz.
Considering the possible transmission speeds of 5.5 and 11 Mbit/s in the 802.11b protocol, we have so far left without addressing the question of why a speed of 5.5 Mbit/s is needed if the use of CCK sequences allows data to be transmitted at a speed of 11 Mbit/s . Theoretically, this is true, but only if you do not take into account the interference environment. IN real conditions The noise level of transmission channels and, accordingly, the ratio of noise and signal levels may be such that transmission at a high information speed, that is, when many information bits are encoded in one symbol, may be impossible due to their erroneous recognition. Without going into mathematical details, we only note that the higher the noise level of communication channels, the less information speed transfers. It is important that the receiver and transmitter correctly analyze the interference environment and select an acceptable transmission rate.
Related information.
Method frequency hopping spectrum extension (FHSS - Frequency Hopping Spread Spectrum) is based on a constant change of carrier within a wide frequency range.
The carrier frequency F1, ..., FN changes randomly after a certain period of time, called cutoff period (chip) , in accordance with the selected algorithm for generating a pseudo-random sequence. Modulation is applied at each frequency (FSK or PSK). Transmission on one frequency is carried out for a fixed time interval, during which a certain portion of data (Data) is transmitted. At the beginning of each transmission period, sync bits are used to synchronize the receiver with the transmitter, which reduces the useful transmission rate.
Depending on the rate of change of the carrier, there are 2 spectrum extension modes:
· slow spectrum spreading – several bits are transmitted in one cut-off period;
· fast spectrum spreading - one bit is transmitted over several cutoff periods, that is, repeated several times.
In the first case data transmission period less chip transfer period, in the second – more.
The fast spread spectrum method provides more reliable data transmission in the presence of interference due to the repeated repetition of the same bit value at different frequencies, but is more complex to implement than the slow spread spectrum method.
Direct serial spread spectrum
The Direct Sequence Spread Spectrum (DSSS) method is as follows.
Each "one" bit in the transmitted data is replaced by a binary sequence of N bit, which is called spreading sequence , and the “zero” bit is encoded by the inverse value of the spreading sequence. In this case, the transmission clock speed increases by N times, therefore, the signal spectrum also expands by N once.
Knowing allocated for wireless transmission(communication line) frequency range, you can select the baud rate and value accordingly N so that the signal spectrum fills the entire range.
The main purpose of DSSS coding, like FHSS, is to increase noise immunity.
Chip speed– transmission speed of the resulting code.
Expansion coefficient– number of bits N in an expanding sequence. Usually N is in the range from 10 to 100. The more N, the larger the spectrum of the transmitted signal.
DSSS is less immune to interference than fast spread spectrum.
Code Division Multiple Access
Spread spectrum techniques are widely used in cellular networks, in particular, when implementing the CDMA (Code Division Multiple Access) access method – code division multiple access . CDMA can be used in conjunction with FHSS, but in wireless networks it is more often used with DSSS.
Each network node uses its own spreading sequence, which is selected so that the receiving node can extract data from the total signal.
The advantage of CDMA is the increased security and secrecy of data transmission: without knowing the spreading sequence, it is impossible to receive a signal, and sometimes even detect its presence.
WiFi technology. WiMax technology. Wireless personal networks. Bluetooth technology. ZigBee technology. Wireless sensor networks. Comparison of wireless technologies.
WiFi technology
Wireless LAN (WLAN) technology is defined by the IEEE 802.11 protocol stack, which describes a physical layer and a data link layer with two sublayers: MAC and LLC.
At the physical layer, several specification options are defined that differ:
· frequency range used;
· coding method;
· data transfer speed.
Options for building wireless LANs of the 802.11 standard, called WiFi.
IEEE 802.11 (option 1):
· transmission medium – IR radiation;
· line-of-sight transmission;
· 3 radiation propagation options are used:
Omnidirectional antenna;
Reflection from the ceiling;
Focal directional radiation (“point-to-point”).
IEEE 802.11 (option 2):
· coding method – FHSS: up to 79 frequency ranges wide
1 MHz, the duration of each of which is 400 ms (Fig. 3.49);
· in 2 signal states it is provided throughput transmission medium at 1 Mbit/s, with 4 – 2 Mbit/s.
IEEE 802.11 (option 3):
· transmission medium – microwave range 2.4 GHz;
· coding method – DSSS with 11-bit code as spreading sequence: 10110111000.
IEEE 802.11a:
1) frequency range – 5 GHz;
2) transmission speeds: 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s;
3) coding method – OFDM.
Flaws:
· too much expensive equipment;
· in some countries, frequencies in this range are subject to licensing.
IEEE 802.11b:
1) frequency range – 2.4 GHz;
2) transmission speed: up to 11 Mbit/s;
3) coding method - modernized DSSS.
IEEE 802.11g:
1) frequency range – 2.4 GHz;
2) maximum speed transmission: up to 54 Mbit/s;
3) coding method – OFDM.
In September 2009, the IEEE 802.11n standard was approved. Its use will increase the data transfer speed by almost four times compared to devices of 802.11g standards. Theoretically, 802.11n is capable of providing data transfer rates of up to 600 Mbps. The range of IEEE 802.11 wireless networks is up to 100 meters.
WiMax technology
WiMax high-bandwidth wireless broadband access technology is represented by the IEEE 802.16 group of standards and was originally intended for building long-distance (up to 50 km) wireless networks belonging to the class of regional or metropolitan networks.
IEEE 802.16 or IEEE 802.16-2001 (December 2001), the first point-to-multipoint standard, was focused on operating in the spectrum from 10 to 66 GHz and, as a result, required the transmitter and receiver to be in line of sight, which is a significant drawback, especially in urban conditions. According to the described specifications, the 802.16 network could serve up to 60 clients at T-1 channel speed (1.554 Mbit/s).
Later, IEEE 802.16a, IEEE 802.16-2004 and IEEE 802.16e (mobile WiMax) standards appeared, in which the requirement for line of sight between the transmitter and receiver was removed.
Main parameters of the listed standards WiMax technologies.
Let's look at the main technology differences WiMax from WiFi.
1. Low mobility. The standard was originally developed for fixed wireless communications on long distances and provided for user mobility within the building. It was only in 2005 that the IEEE 802.16e standard was developed, aimed at mobile users. Currently, new specifications 802.16f and 802.16h are being developed for access networks that support the operation of mobile clients at speeds of up to 300 km/h.
2. Use of better radio receivers and transmitters causes higher costs for network construction. 3. Long distances data transmission requires solving a number of specific problems: signal generation different power, use of multiple modulation schemes, information security problems.
4. Large number of users in one cell.
5. Higher throughput provided to the user.
6. High quality servicing multimedia traffic.
It was originally believed that IEEE 802.11 – mobile analogue Ethernet, 802.16 – wireless landline analogue cable television. However, the emergence and development of WiMax (IEEE 802.16e) technology to support mobile users makes this statement controversial.
Initially, the spread spectrum method was created for intelligence and military purposes. The main idea of the method is to distribute the information signal over a wide radio band, which ultimately makes it much more difficult to suppress or intercept the signal. The first spread spectrum scheme developed is known as frequency hopping technique. A more modern spread spectrum scheme is the direct serial spread method. Both methods are used in different standards and wireless communications products.
Frequency Hopping Spread Spectrum (FHSS)
To ensure that radio traffic could not be intercepted or suppressed by narrow-band noise, it was proposed to transmit with a constant change of carrier within a wide frequency range. As a result, the signal power was distributed over the entire range, and listening to a specific frequency produced only a small amount of noise. The sequence of carrier frequencies was pseudo-random, known only to the transmitter and receiver. An attempt to suppress a signal in some narrow range also did not degrade the signal too much, since only a small part of the information was suppressed.
The idea of this method is illustrated in Fig. 1.10.
For a fixed period of time, transmission is carried out on a constant carrier frequency. At each carrier frequency, standard modulation methods, such as FSK or PSK. In order for the receiver to synchronize with the transmitter, sync bits are transmitted for a period of time to indicate the start of each transmission period. So the useful speed of this encoding method is lower due to the constant synchronization overhead.
Rice. 1.10.
The carrier frequency changes in accordance with the numbers of frequency subchannels generated by the pseudo-random number algorithm. Pseudorandom sequence depends on some parameter called initial number. If the receiver and transmitter know the algorithm and the value of the seed, then they change frequencies in the same sequence, called a pseudo-random frequency hopping sequence.
If the frequency of subchannel changes is lower than the data transmission rate in the channel, then this mode is called slow spectrum expansion(Fig. 1.11a); otherwise we are dealing with rapid spectrum expansion(Fig. 1.11b).
The fast spread spectrum method is more resistant to interference because the narrowband interference that suppresses the signal in a particular subchannel does not result in bit loss because its value is repeated several times in different frequency subchannels. In this mode, the effect of intersymbol interference does not appear, because by the time the signal delayed along one of the paths arrives, the system has time to switch to another frequency.
The slow spectrum spreading method does not have this property, but it is simpler to implement and involves less overhead.
FHSS methods are used in IEEE 802.11 and Bluetooth wireless technologies.
In FHSS, the approach to using the frequency range is different from other coding methods - instead of economically using a narrow bandwidth, an attempt is made to occupy the entire available range. At first glance, this does not seem very effective - after all, only one channel is working in the range at any given time. However, the latter statement is not always true - spread spectrum codes can also be used to multiplex multiple channels over a wide range. In particular, FHSS methods make it possible to organize the simultaneous operation of several channels by selecting for each channel such pseudorandom sequences so that at each moment of time each channel operates at its own frequency (of course, this can only be done if the number of channels does not exceed the number of frequency subchannels).
Direct Sequence Spread Spectrum (DSSS)
Direct Sequential Spread Spectrum also uses the entire frequency range allocated to a single wireless link. Unlike the FHSS method, the entire frequency range is occupied not by constant switching from frequency to frequency, but by replacing each bit of information with N-bits, so that the clock speed of signal transmission increases by N times. And this, in turn, means that the signal spectrum also expands N times. It is enough to select the data rate and N value appropriately so that the signal spectrum fills the entire range.
The purpose of coding with the DSSS method is the same as with the FHSS method - to increase immunity to interference. Narrowband interference will distort only certain frequencies of the signal spectrum, so that the receiver is likely to be able to correctly recognize the transmitted information.
The code that replaces the binary unit of the original information is called spreading sequence, and each bit of such a sequence is a chip.
Accordingly, the transmission rate of the resulting code is called chip speed. A binary zero is encoded by the inverse of the spreading sequence. Receivers must know the spreading sequence that the transmitter uses in order to understand the information being transmitted.
The number of bits in the spreading sequence determines the spreading factor source code. As with FHSS, any kind of modulation, such as BFSK, can be used to encode the result code bits.
The larger the spreading factor, the wider the spectrum of the resulting signal and the higher the degree of interference suppression. But at the same time, the spectrum occupied by the channel increases. Typically the expansion factor ranges from 10 to 100.
Initially, the spread spectrum method was created for intelligence and military purposes. The main idea of the method is to distribute the information signal over a wide radio band, which ultimately makes it much more difficult to suppress or intercept the signal. The first spread spectrum scheme developed is known as frequency hopping technique. A more modern spread spectrum scheme is the direct serial spread method. Both methods are used in various wireless standards and products.
Frequency Hopping Spread Spectrum (FHSS)
To ensure that radio traffic could not be intercepted or suppressed by narrow-band noise, it was proposed to transmit with a constant change of carrier within a wide frequency range. As a result, the signal power was distributed over the entire range, and listening to a specific frequency produced only a small amount of noise. The sequence of carrier frequencies was pseudo-random, known only to the transmitter and receiver. An attempt to suppress a signal in some narrow range also did not degrade the signal too much, since only a small part of the information was suppressed.
The idea of this method is illustrated in Fig. 1.10.
For a fixed period of time, transmission is carried out on a constant carrier frequency. At each carrier frequency, they are used to transmit discrete information. standard methods modulations such as FSK or PSK. In order for the receiver to synchronize with the transmitter, sync bits are transmitted for a period of time to indicate the start of each transmission period. So the useful speed of this encoding method is lower due to the constant synchronization overhead.
Rice. 1.10. Spectrum expansion by frequency hopping
The carrier frequency changes in accordance with the numbers of frequency subchannels generated by the pseudo-random number algorithm. The pseudorandom sequence depends on some parameter called initial number. If the receiver and transmitter know the algorithm and the value of the seed, then they change frequencies in the same sequence, called a pseudo-random frequency hopping sequence.
If the frequency of subchannel changes is lower than the data transmission rate in the channel, then this mode is called slow spectrum expansion(Fig. 1.11a); otherwise we are dealing with rapid spectrum expansion(Fig. 1.11b).
The fast spread spectrum method is more resistant to interference because the narrowband interference that suppresses the signal in a particular subchannel does not result in bit loss because its value is repeated several times in different frequency subchannels. In this mode, the effect of intersymbol interference does not appear, because by the time the signal delayed along one of the paths arrives, the system has time to switch to another frequency.
The slow spectrum spreading method does not have this property, but it is simpler to implement and involves less overhead.
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Rice. 1.11. Relationship between data rate and subchannel change frequencyFHSS methods are used in IEEE 802.11 and Bluetooth wireless technologies.
In FHSS, the approach to using the frequency range is different from other encoding methods - instead of economically using a narrow bandwidth, an attempt is made to occupy the entire available range. At first glance, this does not seem very effective - after all, only one channel is operating in the range at any given time. However, the latter statement is not always true - spread spectrum codes can also be used to multiplex multiple channels over a wide range. In particular, FHSS methods allow you to organize the simultaneous operation of several channels by selecting such pseudo-random sequences for each channel so that at each moment of time each channel operates at its own frequency (of course, this can only be done if the number of channels does not exceed the number of frequency subchannels).
Direct Sequence Spread Spectrum (DSSS)
Direct Sequential Spread Spectrum also uses the entire frequency range allocated to a single wireless link. Unlike the FHSS method, the entire frequency range is occupied not by constant switching from frequency to frequency, but by replacing each bit of information with N-bits, so that the clock speed of signal transmission increases by N times. And this, in turn, means that the signal spectrum also expands N times. It is enough to select the data rate and N value appropriately so that the signal spectrum fills the entire range.
The purpose of DSSS coding is the same as FHSS - to increase immunity to interference. Narrowband interference will distort only certain frequencies of the signal spectrum, so that the receiver is likely to be able to correctly recognize the transmitted information.
The code that replaces the binary unit of the original information is called spreading sequence, and each bit of such a sequence is a chip.
Accordingly, the transmission rate of the resulting code is called chip speed. A binary zero is encoded by the inverse of the spreading sequence. Receivers must know the spreading sequence that the transmitter uses in order to understand the information being transmitted.
The number of bits in the spreading sequence determines the spreading factor of the source code. As with FHSS, any type of modulation, such as BFSK, can be used to encode the bits of the result code.
The larger the spreading factor, the wider the spectrum of the resulting signal and the higher the degree of interference suppression. But at the same time, the spectrum occupied by the channel increases. Typically the expansion factor ranges from 10 to 100.