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The procedure for direct spectrum expansion of drawings. Basics of wireless technology. Spread Spectrum Systems

2.4.2. Method direct expansion spectrum Fundamentals of the theory of mobile and wireless communication

2.4.2. Direct Spread Spectrum Method

An important property of the direct spread spectrum method can be considered that the width of the spectrum of the signal modulating the reference frequency, and therefore the radio signal, is determined mainly not by the transmission speed useful information, but by PSP parameters.

An elementary PSP pulse is called a chip. Each information bit, after multiplying with the memory bandwidth, will be displayed by many chips. (For example, one information bit is displayed by 128 PSP chips.) The speed in the radio channel is determined as the product of the transmission speed at the output of the channel encoder and the number of chips per one bit interval. Typically, the transmission speed of a radio channel is measured in megachips per second (Mchip/s).

Spread spectrum signals are pseudorandom, that is, they have properties similar to those of a random process or noise, although they are formed using completely deterministic algorithms. The PSP is most often binary with elements 0 and 1 and has properties similar to those of a random binary sequence. For example, if on any finite interval the number of zeros is approximately equal to the number of ones, then the autocorrelation function of such a sequence is close to the autocorrelation function of a random binary sequence, in particular, it has small values ​​of the correlation coefficient between copies of the same sequence shifted relative to each other, etc. . This property is used to recognize PSP.

Pseudorandom sequences are usually formed using logical chains that implement deterministic algorithms. In Fig. 2.5 shows an example of such a circuit, which contains a shift register of series-connected elements with two stable states and some logic circuit in the feedback circuit. The binary sequence of characters 0 and 1 stored in the register is shifted to the right in the register when the next clock pulse is applied; the symbol from the last cell of the register is output as the next symbol in the sequence; symbols of all or some register cells are fed into a logical feedback circuit in which the symbol is formed

The repetition period of the clock pulses determines the duration of the elementary symbol (chip) of the sequence. If the feedback loop contains only XOR gates, which are the most commonly used gates, this device is called a linear pseudorandom sequence generator (LPS). In this case, the value of the next symbol at the output of the feedback circuit is determined by the following recurrence relation:

where the symbol “+” denotes summation modulo 2, and the coefficients and the symbols take the values ​​0 or 1. The logical feedback circuit in this case is a modulo 2 adder.

The initial state of the register cells and the structure of the logical feedback circuit completely determine the subsequent state of the register cells. If we take some state of the shift register as the initial one, then through N cycles, this state will repeat again. If at the same time we register a sequence of characters at the output of the cell with number I, then the length of this sequence will be equal to N. On subsequent N in bars this sequence will repeat again, etc.

Number N called the period of the sequence. Meaning N with a fixed register length m depends on the number of non-zero weighting coefficients With and the location of the corresponding taps in the register. For example, from equality (2.6) it follows that if at some point in time the state of all register cells is equal to 0, then all subsequent elements of the sequence at the register output will be zero. There are different non-zero states of the shift register. Consequently, the period of the linear bandwidth generated by the shift register with m cells, cannot exceed characters. PSPs with period , generated by a linear feedback shift register, are called maximum length sequences or, in short, M-sequences. The duration of the PSP repetition period can be tens to hundreds of hours.

The device, the functional diagram of which is shown in Fig. 2.5 can be called a digital machine. If the sequence it generates is described by equation (2.6), then such automata are usually defined by a characteristic polynomial:

where and . The value of the vector completely determines the structure of the PSP generation machine: if the coefficient is , then this means that the output of cell number I not connected to the feedback circuit; at I The th output is connected.

There are a fairly large number of methods for generating pseudorandom sequences, the statistical properties of which are well studied. Their autocorrelation function has a pronounced maximum, and the cross-correlation function has a random noise-like character with a low level of values. New ways of implementing PSP are being developed today.

There are two ways to receive a spread spectrum radio signal. For example, first multiply the original bit sequence from the output of the channel encoder by the PSP signal, thereby expanding the spectrum. Then use the received signal to modulate the oscillations carrier frequency. For the second modulation, phase modulation (BPSK, QPSK) or amplitude-phase (QAM) methods can be used. An example of constructing such a method for generating a radio signal with an extended spectrum is shown in the functional diagram of Fig. 2.6.

Rice. 2.6. Functional diagram spread spectrum radio signal generation

The baseband filter in this circuit is designed to produce a baseband signal with the required power spectral density shape and the required frequency band. However, now at the filter input the signal has a much wider spectrum, so the radio signal also has a IN times wider spectrum than a conventional narrowband radio signal.

A similar result will be obtained if you first modulate the carrier frequency oscillations with a bit sequence using the BPSK, QPSK or QAM methods, and then modulate the received radio signal with PSP pulses.

Direct spectrum expansion is carried out by multiplying information signal to the PSP signal , formed from a pseudo-random sequence throughout the entire communication session. As a result, the modulating signal can be written:

In Fig. Figure 2.7 shows an approximate view of a section of the original bit sequence, the PSP signal and their corresponding spectra.

Rice. 2.7. An approximate view of the relationship between a bit sequence and memory bandwidth

Spread spectrum signals have an interesting feature. When the bit sequence is first multiplied with the PSP signal (in the transmitter), the spectrum is expanded to a band . At the receiver, the input spread spectrum radio signal is fed to the first demodulator, which is also supplied with the same bandwidth that was used in the transmitter. As a result of multiplying the input radio signal with the PSP signal at the output of the first demodulator, a radio signal is obtained, the spectrum of which again narrows and becomes equal in width to the spectrum of the channel bit sequence. It is important to note that during the first multiplication (in the transmitter) of the bit sequence with the PRP signal, the spectrum is expanded, and the second multiplication (in the receiver demodulator) with the same PRP again narrows the spectrum to the original spectrum of channel bits. This property of spread spectrum signals plays a very useful role in reducing the negative impact of interference. Let us assume that there is narrow-band (intentional or random) interference in the radio channel, the spectrum of which is within the extended spectrum of the signal. When interference, together with the signal, hits the receiver input on the first demodulator, the signal will undergo a second multiplication by the PSP, its spectrum will narrow, and the interference will undergo the first multiplication with the PSP, its spectrum will expand and its energy will be “spread out” over a wide frequency range (see Fig. 2.8, A). When the spectrum of the useful signal is isolated by a bandpass filter (for example, at an intermediate frequency), only a small fraction of the interference energy will fall into its band. Therefore, even relatively strong narrowband interference will have a negligible effect.

a – narrowband interference; b – broadband interference

When broadband interference hits the receiver input together with a useful signal (Fig. 2.8, b) after multiplication with the PSP, the spectra of both the signal and the noise will be proportionally narrowed. If they had different stripes and different center frequencies, then the interference and the signal can be separated by a bandpass filter. This immunity to interference makes it attractive to use spread spectrum signals in interference environments.

In conditions of multipath signal propagation, reflected copies will arrive at the receiver input with a delay relative to the main signal. If the delay of the copies is longer than the duration of the chip, then they can be separated from the main signal. In a narrowband signal modulated by bit pulses, the duration of the bit pulse is quite long, and reflected copies of the signal have time to overlap the main signal. The duration of chip pulses is much shorter, so the reflected signals may not overlap with the main signal.

One more property of spread spectrum signals should be noted. Since the width of the extended spectrum of a radio signal from one channel is much greater than the width of the spectrum of the signal obtained by frequency division of channels (narrowband), then with the same emitted power of these radio signals, the power spectral density of the signal with an extended spectrum turns out to be much smaller and may not even exceed the spectral power density of noise. This ensures good secrecy of broadband signals.

It is also important for mobile communication systems that there is no need to solve the problem of frequency distribution between different subscribers, since all subscribers use the same frequency band. For narrowband modulation methods, solving the frequency planning problem is mandatory.

An important characteristic of a broadband signal is its base, the meaning of which is a relative increase in the frequency band transmitted signal in the radio channel compared to the frequency band of the bit (original) signal. Signal base size: . Typically, the signal base is determined in decibels: . In practice, it is more convenient to define the signal base as the product of the width of the spectrum of the original signal and the duration of the elementary symbol of the PSP (chip): . For many reasons, it is convenient to use a PSP chip duration such that the base of the spread spectrum signal is an integer. On the receiving side it is convenient to use the concept processing win, the value of which is numerically equal to the value of the signal base and means a gain due to the reverse narrowing of the spectrum from the expanded to the original: .

Let us briefly list some properties of direct spread spectrum signals that are most important from the point of view of organizing multiple access in communication systems with mobile objects.

· Multiple access. If several subscribers simultaneously use a transmission channel, then several direct spread spectrum signals are simultaneously present in the channel. Each of these signals occupies the entire channel bandwidth. In the signal receiver of a specific subscriber, reverse operation- convolution of this subscriber's signal by using the same pseudo-random signal that was used in the transmitter of this subscriber. This operation concentrates the power of the received wideband signal again in a narrow frequency band equal to the width of the information symbol spectrum. If the cross-correlation function between the pseudo-random signals of a given subscriber and other subscribers is sufficiently small, then during coherent reception only a small fraction of the power of the signals of other subscribers will fall into the information band of the subscriber's receiver. The signal from a specific subscriber will be received correctly.

· Multipath interference. If the pseudo-random signal used to spread the spectrum has an ideal autocorrelation function, the values ​​of which outside the interval are equal to zero, and if the received signal and a copy of this signal in another beam are shifted in time by an amount greater than , then when the signal is folded, its copy can be considered as interfering interference that introduces only a small fraction of the power into the information band.

· Narrowband interference. With coherent reception, the receiver multiplies the received signal by a copy of the pseudo-random signal used to spread the spectrum in the transmitter. Consequently, the operation of spreading the spectrum of narrowband interference will be carried out in the receiver, similar to that which was performed with the information signal in the transmitter. Consequently, the spectrum of narrowband interference at the receiver will be expanded by IN times where IN- spreading factor, so that only a small fraction of the interference power will fall into the information frequency band, in IN times less than the original interference power.

· Probability of interception. Since a direct spread spectrum signal occupies the entire frequency band of the system during the entire transmission time, its radiated power per 1 Hz of bandwidth will be very small. Therefore, detecting such a signal is a very difficult task.

The use of broadband signals has its advantages and disadvantages, which are generally inherent in any method of their formation.

Advantages of broadband signals:

  • generation of the necessary pseudo-random signals can be provided by simple devices (shift registers);
  • spread spectrum operation can be realized simple multiplication or addition digital signals modulo 2;
  • the carrier wave generator is simple, since it is necessary to generate a harmonic carrier wave with only one frequency;
  • coherent signal reception with direct spread spectrum can be realized;
  • there is no need to ensure synchronization between system subscribers.

Disadvantages of broadband signals:

at the input of the base station receiver. Spread spectrum plays a vital role in radio communication technologies. This method

does not fall into any of the categories defined in the previous chapter, since it can be used to transmit both digital and analog data using an analog signal.

Below, after a brief overview, these spread spectrum methods are discussed in detail. In addition, the spread spectrum multiple access method will be explored in this chapter.

As incredible as it may sound, spectrum extension using the frequency tuning method was invented by Hollywood movie star Hedy Lamarr in 1940 at the age of 26. In 1942, Lamarr patented her invention (US Patent 2,292,387 dated August 11, 1942) together with a partner who began to participate in the work a little later. The girl did not receive any profit from the patent, considering the communication method she discovered to be her contribution to the US participation in World War II.

7.1. Spread spectrum concept

In Fig. Figure 7.1 shows the key elements of a spread spectrum system. The input signal is fed to a channel encoder, which generates an analog signal with a relatively narrow bandwidth centered on a specific frequency. The signal is then modulated using a sequence of numbers called a spreading code, or spreading sequence. Typically, although not always, the extension code is generated by a random number generator. As a result of modulation, the bandwidth of the transmitted signal is significantly expanded (in other words, the signal spectrum is expanded). Once received, the signal is demodulated using the same spreading code. The last step is to send the signal to the channel decoder to restore the data.

Rice. 7.1. General scheme digital system spread spectrum communications

Excess spectrum provides the following benefits.

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. More modern scheme spread spectrum is a direct sequential spreading method. Both methods are used in different standards and wireless communications products.

Expansion of the spectrum by frequency hopping ( 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 a certain 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 usable speed


This encoding method is less expensive 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). Method spectrum is more resistant to interference, since narrowband interference that suppresses the signal in a particular subchannel does not result in bit loss, since 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 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 last statement is not always true - spread spectrum codes can also be used to multiplex multiple channels into 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 one wireless line communications. 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 only distort certain frequencies spectrum of the signal, 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 a certain 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 less 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 pseudo-random sequence depends on some parameter called 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

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.

enlarge image
Rice. 1.11. Relationship between data rate and subchannel change frequency

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 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.

Spectrum extension

In this lecture we will look at the basic principles of signal spreading technology.

Spread spectrum is a technology speaking in simple words, in which the modulated signal is represented by a signal with a bandwidth much larger than the bandwidth of the information signal.

Modern mobile communications are based on spread spectrum technology and are widely used under the name “CDMA”.

Consider the CDMA IS-95 (cdmaOne) standard as the most widely used at present. Spread spectrum technology was first proposed for mobile communicators in the 1980s. commercial distribution Qualcomm Inc was the first to tackle this issue by presenting this standard in the DS-CDMA (Direct Sequence Code Division Multiple Access) format. Commercial use of the IS-95 standard began in 1996 in the USA. The abbreviation IS (interim standard) is used for accounting in TIA, and the number means the serial number. From full name The TIA/EIA/IS-95 standard shows that the EIA, which unites seven large US organizations, also took part in its consideration.

Types of multiple access: Multiple access is the problem of numbering users who want to use the same electromagnetic spectrum. It can be solved in several ways:

- Selection with frequency division (signals are distributed only between specific communicators);

- Spatial filtering;

- Frequency Division Multiple Access (FDMA);

- Time Division Multiple Access (TDMA);

- Code Division Multiple Access (CDMA).

TDMA (Time Division Multiple Access - time division multiple access) is a method of using radio frequencies when there are several subscribers in the same frequency interval, different subscribers use different time slots (intervals) for transmission. TDMA provides each user full access to a frequency interval for a short period of time.

FDMA (Frequency Division Multiple Access - frequency division multiple access) - a method of using radio frequencies when in one frequency range there is only one subscriber, different subscribers use different frequencies within a cell.

CDMA (Code Division Multiple Access - code division multiple access) is a mobile communication technology in which transmission channels have a common frequency band, but different code modulation.

Basically CDMA is used as a term for a system of modulating information into a signal having a wider bandwidth, i.e. spectrum expansion. This expansion is carried out through binary "code", which is usually very long, and for most considerations, random in nature. Of course the code is not random, it is quite predictable, and the term pseudo-random (a confusing term in itself) is often used.

One of the fundamental concepts that determines the noise immunity and efficiency of a CDMA system is the “signal base” (in English literature the term “processing gain” is used). Physical meaning This concept is an increase in the frequency band of the transmitted signal relative to the original one (measured in decibels). For spread spectrum systems, the signal base is defined as the ratio of the bandwidths of the emitted and source signals. However, more often the value of the signal base (B) is calculated as the product of the spectrum width (F) and the duration of the elementary symbol (T). For broadband signals, the base is significantly greater than 1 (B>>1). It is clear that the wider the frequency band on the air and the lower the speed of the input signal, the more base signal and, accordingly, higher noise immunity.

However, it is important to understand that the signal base is not a characteristic of the entire CDMA system, but only of its individual channel. Let us explain this with an example. So, with a chip speed of 1.2288 Mchip/s (IS-95) and an information speed of 9.6 kbit/s, the signal base is 21.1 dB (1.2288x103 / 9.6 = 128). The base of a signal is proportional to its transmission speed.

Wideband is a system that transmits a signal that occupies a very wide frequency band, significantly exceeding the minimum frequency bandwidth that is actually required to transmit information. In a wideband system, a source baseband signal (eg a telephone signal) with a bandwidth of only a few kilohertz is distributed over a frequency band that can be several megahertz wide. This is done by double modulating the carrier with a transmitted information signal and a wideband coding signal. The main characteristic of a broadband signal is its base B, defined as the product of the signal spectrum width F and its period T. As a result of multiplying the signal of a pseudorandom noise source with an information signal, the energy of the latter is distributed over a wide frequency band, i.e. its spectrum is expanding.

The technology is optimized for providing high-speed multimedia services such as video, Internet access and video conferencing; provides access speeds of up to 2 Mbit/s over short distances and 384 Kbit/s over long distances with full mobility. Such speed values

Data transmissions require a wide frequency band, so the WCDMA bandwidth is 5 MHz.

Technology can be added to existing networks GSM and PDC what does WCDMA standard most promising in terms of use network resources and global compatibility.

At the transmitter, the narrowband information signal is multiplied by a pseudo-noise reference N-symbol sequence, and the resulting signal is modulated using BPSK or QPSK (direct operation). The base of the resulting signal is equal to the number of symbols of the pseudo-random sequence (B = N). In this case, the use of noise-like signals with a high clock frequency leads to the fact that the original narrow-band

the signal is “spread out” over a wide band and becomes less than the noise level.

At the receiver, the original signal is reconstructed using a pseudo-random sequence of known structure (inverse operation). Other signals arriving at this receiver are perceived as noise.

In a similar way, powerful narrowband interference from other operating transmitters is suppressed. In the receiver, such interference is also “spread out” over a wide frequency band and, after filtering, only slightly degrades the quality of communication. With further digital processing interference can be suppressed completely.

In addition to the most commonly used DS-CDMA method, there are other spectrum extension technologies, for example using multiple carriers - MC-CDMA (Multi-Carrier CDMA) or leapfrogging frequencies - FHCDMA (Frequency Hopping CDMA). The features of these technologies will be discussed in future issues of the magazine.

Real-time digital signal processing prior to RF transmission. The principle of constructing a transmitter/receiver is the same as with DS-CDMA, only the final modulated signal is supplied to the DAC. The transmitter/receiver uses a special filter called a raised cosine filter that minimizes intersymbol distortion by representing a portion of the spectrum simplest form into a cosine wave, raised in such a way that it “sits” on the horizontal axis.

Chipping is any operation by which symbols (bits) are split (chipped) into smaller time intervals. The operations of scrambling, channeling and spreading are the chipping operation.

Scrambling is a reversible transformation of a digital stream without changing the transmission rate using a random sequence. After scrambling, the appearance of “1” and “0” in the output sequence is equally likely. Scrambling is a reversible process, that is Original message can be restored using the reverse algorithm.

Channelization is a reversible transformation of a digital stream by dividing the information signal into chips using a fixed sequence.

Comprehensive presentation.

Note that the complex representation is purely mathematical and is introduced for convenience of notation. Third generation CDMA networks use all three representations in an integrated form. Channeling in the Uplink system is carried out by the first presentation method, and in the Downlink system - by the second.

Each user has a unique spreading/channeling code, most likely an orthogonal Walsh code. For downstream signal transmission, it is taken as a basis real part with a complex representation of the chipped sequence and is transmitted at the same speed. The transmitted encoded signals will be synchronized. Each mobile station knows the scrambling code of the current base station, and its set (and only) spreading code - from here the transmitted data is recovered.

Logical downlink channels include:

Pilot channel;

Synchronization channel;

Personal call channel;

Direct traffic channel.

In the forward channel (from BS to mobile), signal modulation by Walsh functions (binary phase shift keying) is used to distinguish between different physical channels this BS; long PSP modulation (binary phase

manipulation) - for the purpose of encrypting messages; modulation of a short PSP (quadrature phase shift keying of two PSPs of the same period) - to expand the bandwidth and distinguish between signals from different BSs.

Distinction between signals from different stations is ensured by the fact that all BSs use the same pair of short bandwidths, but with a shift of 64 samples between different stations, i.e. There are a total of 511 codes on the network; in this case, all physical channels of one BS have the same sequence phase.

4 types of channels are formed on the BS: pilot signal channel (PI), synchronization channel (SYNC), calling channel (PCN) and traffic channel (TCN).

Signals different channels mutually orthogonal, which guarantees the absence of mutual interference between them on the same BS. Intra-system interference mainly arises from transmitters of other BSs operating at the same frequency, but with a different cyclic shift.

The pilot signal is emitted continuously. To transmit it, the zero-order Walsh function (W0) is used. The pilot signal is a carrier signal that is used by the MS to select work cell(at most powerful signal), and also as a reference for synchronous signal detection information channels. Typically, about 20% of the total power is emitted on the pilot signal, which allows the mobile station (MS) to ensure accurate carrier frequency selection and coherent reception of signals.

In the synchronization channel (SYNC), the input stream at a rate of 1.2 kbit/s is re-encoded into a stream transmitted at a rate of 4.8 kbit/s. The synchronization message contains the technological information necessary to establish initial synchronization on the MS: data on the exact system time, the transmission speed in the PCH channel, and the parameters of the short and long code. The transmission speed in the synchronization channel is lower than in the calling (RSN) or schedule channel (TSN), which increases the reliability of its operation. Upon completion of the synchronization procedure, the MS is tuned to the PCH call channel and constantly monitors it. Function W32 is used to encode the sync channel.

In the reverse channel (uplink) asynchronous option code division implemented in combination with incoherent reception of signals at the BS. This eliminates the need for a pilot channel and a synchronization channel. This leaves only two types of uplink logical channels:

Access channel;

- return traffic channel.

The asynchrony of code division makes it irrational to use Walsh functions as channel-forming sequences (signatures) of physical channels, since with relative time shifts they cannot maintain orthogonality and have very unattractive cross-correlation properties.

The access channel provides connection between the MS and the BS until the MS is tuned to the reverse traffic channel assigned to it. The access channel selection process is random - the MS randomly selects a channel number from a certain range. The access channel is used to register the MS in the network, transmit a request to establish a connection to the BS, respond to commands transmitted over the call channel, etc. The data transfer rate over the access channel is fixed and amounts to 4.8 kbit/s.

The return traffic channel provides transmission speech information and subscriber data, as well as control information from MS to BS, when the MS is already occupying the physical channel allocated to it.

Walsh codes.

IN CDMA standard For code separation of channels, orthogonal Walsh codes are used. Walsh codes are formed from the rows of the Walsh matrix:

The peculiarity of this matrix is ​​that each of its rows is orthogonal to any other or row obtained using the logical negation operation. The IS-95 standard uses a 64th order matrix. A digital filter is used to isolate the signal at the receiver output. With orthogonal signals, the filter can be configured so that its output will always be a logic "0" unless the signal to which it is configured is received. Walsh coding is used in the forward channel (from BS to AT) to separate users. In systems using the IS-95 standard, all speakers operate simultaneously in the same frequency band. Matched filters of BS receivers are quasi-optimal in conditions of mutual interference between subscribers of the same cell and are very sensitive to the “far-close” effect. To maximize the subscriber capacity of the system, it is necessary that the terminals of all subscribers emit a signal of such power that would ensure the same level of signals received by the BS. The more precise the power control, the greater the subscriber capacity of the system.

Pseudo-random sequence.

PSP is a deterministic periodic signal that is known to both correspondents. It has all statistical properties white noise and for a third party it will seem completely random - a pseudo-noise signal. In order for the PSP to be a random process, a number of conditions must be met:

- number binary units should not differ from the number of binary zeros by no more than one element;

- The PSP must have good correlation properties, namely, the levels of the ACF side lobes of such a sequence must have a minimum level.

Many sequences satisfy these properties - Walsh, Barker, Gold sequences, M-sequences and many others.

FCSR (Feedback with carry shift register) - shift register, feedback function and carry register. The length of the shift register is the number of bits. When a bit needs to be retrieved, all the bits in the shift register are shifted to the right by one position. The new leftmost bit and the new value of the carry register are determined by the function of the remaining bits of the shift register and the carry register (their bits are added together). The least significant bit of the result becomes the new leftmost bit, and the remaining bits of the result (except the least significant bit) become the new value of the carry register.

Unlike LFSR, there is a delay for FCSR before it goes into cyclic mode, that is, it begins to generate a cyclically repeated sequence. Depending on the selected initial state, 4 different cases are possible:

1. The initial state may be part maximum period.

2. The initial state may enter the maximum period sequence after some initial delay.

3. The initial state may, after an initial delay, produce a sequence of zeros.

4. The initial state can, after an initial delay, produce a sequence of ones.

Gold's sequence is a pseudo-random sequence formed by adding modulo 2 two pseudo-random sequences.

Kasami is a type of pseudo-random sequence. Used in CDMA. The significance of these sequences comes from their very low cross-correlation. A Kasami code of length N = 2m − 1, where m is an even integer, can be obtained by taking periodic samples from M-

sequences and performing modulo 2 summation on cyclically shifted sequences. Samples are taken every s = 2m / 2 + 1 elements of the M-sequence to form a periodic sequence and then adding this sequence incrementally to the original M-sequence modulo 2 to form s = 2m / 2 Kasami sequences. The cross-correlation function of two Kasami sequences takes values ​​[-1, -s, s-2].

Orthogonal codes

Possibility of adapting the system to different speeds transmission is ensured through the use of so-called channelization codes. The principle of their generation can be illustrated (Fig. 1) with a code tree diagram for orthogonal variable-length codes

(Orthogonal Variable Spreading Factor, OVSF).

Each level of this code tree has its own codewords, the length of each of which is equal to the spreading factor (SF). The complete code tree contains 8 levels (the last, eighth, corresponds to the coefficient SF=256).

The structure of the code tree is such that at each subsequent level the possible number of channel-forming codes is doubled. So, if at level 2 only 2 codes are generated (SF = 2), then at level 3 4 codewords are generated (SF = 4), etc. The ensemble of OVSF codes is not fixed, but depends on the spreading factor SF, i.e., in fact, on the transmission speed of the channel.

The problem of orthogonality.

Suppose there is simple system with two users and two signal paths. The two paths have a relative latency of one chip. Orthogonal Walsh codes are used to propagate the data sequence.

In this case, the receiver will extract two different signal for each user corresponding to two different paths, the relative delay between them will be one chip.

For each user, the receiver will receive two signals from the channel, the desired signal (the PRP is synchronized with this signal) and its delayed version.

The result of narrowing the four received signals in the case of two-channel transmission to two users will be:

B N (bit of interest) from narrowing the desired signal user;

- 0 from narrowing of orthogonal noise-like signals, no interference due to the use of Walsh codes;

- undesirable conditions when the narrowing causes delay of the desired signal and interference.

Multipath.

For a code sequence with ideal correlation properties, the autocorrelation function gives a zero output in the interval , where Tc is the chip time. This means that the wanted signal (main path) and a delayed version of that signal for a time greater than 2Tc are received at the receiver, then, with coherent demodulation/down-spreading conditions, the receiver will identify the delayed signal as interference. In addition, the power level of the delayed signal is less than the useful one due to reflections during multipath, therefore, the delayed signal in the form of interference is “smeared” over the entire bandwidth, and the receiver receives only the useful signal.

The "near - distant" problem.

Despite the high efficiency of CDMA technology, it also has a number of disadvantages. One of them - high sensitivity to the power dispersion of mobile stations. The most difficult situation arises due to the far-near problem, when mobile station, located near the base one, works on high power, creating is unacceptable high level interference when receiving other, “distant” signals, which leads to a decrease bandwidth systems as a whole. This problem exists in all mobile communication systems, but the greatest signal distortion occurs in CDMA systems operating in a common frequency band, which use orthogonal noise-like signals. If these systems did not have power control, they would be significantly inferior in performance cellular networks based on TDMA. Therefore, the key problem in CDMA systems can be considered individual power control of each station.

Detection.

The receiver has access to a code bank that stores all codes allocated to base stations(BS). For a specific user, the BS knows what code to expect and the code is detected by comparing the received sequence with the expected code. The correlation operation is carried out by narrowing, which can be performed in a matched filter. Before correlation can begin, the recipient must know the exact point in time. Synchronization is achieved by using a pilot signal, which is located in front of transmitted information. The pilot signal is the same for all users. When synchronization is completed, the matched filter begins the correlation operation: if the correlation is above a predefined threshold, the matched filter is positive by the user.

Multiplying the received signal and the signal from the same pseudorandom noise source (PRN) that was used in the transmitter compresses the spectrum of the useful signal and simultaneously expands the spectrum of background noise and other sources of interference. The resulting gain in signal-to-noise ratio at the receiver output is a function of the ratio of the broadband and baseband signal bandwidths: the greater the spectrum spread, the greater the gain. In the time domain, this is a function of the ratio of the transmission rate of the digital stream in the radio channel to the transmission rate of the basic information signal. For the 1S-95 standard, the ratio is 128 times, or 21 dB. This allows the system to operate at a level of interference interference that exceeds the level of the useful signal by 18 dB, since signal processing at the receiver output requires the signal level to exceed the interference level by only 3 dB. In real conditions, the level of interference is much less. In addition, expanding the signal spectrum (up to 1.23 MHz) can be considered as an application of frequency diversity reception methods. A signal propagating in a radio path is subject to fading due to the multipath nature of propagation. In the frequency domain, this phenomenon can be represented as the effect of a notch filter with a variable notch bandwidth (usually no more than 300 kHz). In the AMPS standard, this corresponds to the suppression of ten channels, and in the CDMA system, only about 25% of the signal spectrum is suppressed, which does not cause any particular difficulties in restoring the signal in the receiver.

Rake receiver.

Digitized samples input signals are received from the RF input stages and are represented as quadrature I and Q branches (i.e., in the complex number format of the low-pass filter at the receiver output). Code generators and a correlator perform compression and summation of user data transmission symbols. The channel device uses the pilot symbols to estimate the channel state, the effect of which will then be compensated by a phase shifter for the received symbols. The delay is compensated by the difference in the arrival time of the symbols in each path. The Rake adder then adds the compensated channel symbols, thereby providing multipath diversity as a means of combating fading.

Also shown is the matched filter used to determine and update the current multipath delay profile of the channel. This measured and possibly averaged multipath delay profile is then used to sum the highest peak Rake receiver path outputs.

In typical implementations, a Rake receiver performing chip-rate processing (correlator, code generator, matched