Methods of code division of channels. Frequency time and code division of signals

Transmission systems with time division of channels.

Construction of transmission systems with time division of channels (TSD). Theorem of Kotelnikov. Types of pulse modulation. Comparative analysis of the types of pulse modulation and their scope.

The idea of ​​time division of channels is that the elements of the primary signal belonging to the i-th channel are transmitted in non-overlapping time intervals free from the signals of other channels over a common line.

Most of the primary signals are analog (continuous) and the idea of ​​the TRC determines the need for a sampling operation.

This operation is performed in accordance with the Kotelnikov theorem. It is formulated as follows: any time-continuous signal with a frequency-limited spectrum can be represented by a sequence of its samples (instantaneous values) taken over a time interval:

T D = 1/F D , F D ≥ 2F B .

Each signal is given its own timeslot.

The sampling operation is carried out using channel electronic keys

Rice. 8.1. Structural diagram of a time division transmission system

Time interval between the nearest pulses of the group signal T K is called a timeslot or time slot (Time Slot). It follows from the principle of temporal combining of signals that transmission in such systems is carried out in cycles, that is, periodically in the form of groups of N gr = N + n impulses, where N– number of information signals, n– the number of service signals (synchronization pulses - IS, intercom, control and calls). Then the value of the channel interval:

Δt K = T D /N gr .

Fig.8.2. To an explanation of the method of time division of channels.

With time division of channels, the following types of modulation are possible:

1.AIM - amplitude-pulse modulation;

2.PWM - pulse width modulation;

3.PIM - pulse-phase modulation;

4.PFM - frequency-pulse modulation.

With AIM, the periodic sequence of pulses changes in accordance with the change in the modulating signal. Distinguish (AIM -1) amplitude-pulse modulation of the first kind (with it, the tops of the pulses change in accordance with the modulating signal) With (AIM -2) amplitude modulation of the second kind, the top of the pulses is flat and is equal to the amplitude of the pulse at the moment of sampling. When the pulse ratio is more than ten, the differences between AIM-1 and AIM-2 disappear. AIM modulation is easy to implement, but has low noise immunity, since any interference changes the pulse amplitude and distorts the shape of the restored signal. AIM is usually used as an intermediate type of modulation when converting an analog signal to digital.

With PWM, the signal spectrum changes depending on the signal duration. The minimum signal level corresponds to the minimum pulse duration and, accordingly, the maximum signal spectrum.

In this case, the amplitude of the pulses remains unchanged. With one-way PWM (OSWM), the change in duration occurs only by moving

one of the rear or front fronts. With two-way PWM, the change in duration occurs relative to the clock point. More noise-resistant transmission method in comparison with AIM. To get rid of amplitude distortion, an amplitude limiter is used. PWM is used in SMEs of impulse radio communications, as well as in some radio telemetry systems, telecontrol and telemechanics systems.

PPM is a type of temporal pulse modulation.

There are several types of FIM

PIM of the 1st kind With it, the time shift of the pulses is proportional to the value of the modulating signal at the moment the pulse appears. FIM-2 pulse modulation in which the time shift is proportional to the value of the modulating signal at clock points. PIM-2 is usually used. With negative values ​​​​of the modulating signal, the pulses are shifted to the left, and with positive values ​​\u200b\u200bto the right.

In equipment with TRC and analog modulation methods, FIM has received the greatest use, since when using it, it is possible to reduce the interfering effect of additive noise and interference by limiting the pulses in amplitude on both sides, as well as to optimally match the constant pulse duration with the channel bandwidth. It is in the transmission systems with the VRC that the FIM is mainly used.

With PFM, the pulse repetition rate changes depending on the amplitude of the modulating signal.

Questions for self-control.

1. How does the Kotelnikov theorem sound?

2. Why is the Kotelnikov theorem applicable only to continuous signals with a limited spectrum?

3. What is AIM-1 and AIM-2, what is their difference?

4. PWM - modulation, ways to implement the advantages and disadvantages?

5. FIM - modulation, ways to implement the advantages and disadvantages?

6. Appointment of low-pass filters switched on at the input of channel amplitude-pulse modulators.

7. Appointment of low-pass filters switched on at the output of channel selectors.

8. The need for synchronous operation of channel amplitude-pulse modulators and channel selectors.

Lecture 6 Code division methods

(multiplexing and multiple access); P principle and main feature CDMA ; direct spread spectrum; m multi-channel spread spectrum; spectrum hopping; spectrum hopping; Pthe order of passage of voice data in the mobile station until it is sent on the air; uh evolution of cellular communication systems using CDMA technology.

6.1 Classification of transmission systems using a single resource

Any signal occupies a certain frequency band, exists for some time, has limited energy and propagates in a certain region of space. In accordance with this, four types of channel resource are distinguished: frequency, temporal, energy and spatial.

The problem of effective use of the common channel resource has become more acute due to the need to provide communication in conditions of uneven and unpredictable consumer requests over time. When deciding this problems apply multiplexing and multiple access methods. The concepts of "multiplexing" and "multiple access" are similar in that they involve the distribution of a resource between users. At the same time, there are significant differences between them. At multiplexingthe resource of the communication channel is distributed throughgeneral terminal equipment, forming e group signal S Σ (t ) . At multiple access, S Σ (t ) formed as a resultsignal summationusers directly in the channel (Figure 6.1 ). In this pictureIS is the source of the message, TX is the transmitter, RRP is the receiver, PS is the recipient of the message). Multiple access is typical for satellite channels, radio channels, mobile communication channels.

Figure 6.1 – Multiple access transmission system

M multiplexing is based on common hardware, a multiple access (MA) uses certain procedures (protocols) implemented using software stored in the memory of each terminal. On pic unke 6. 2 presents multiplexing methods.

In most cases formultiplexingchannel, a message source is allocated a special signal called a channel signal. Message-modulated channel signals are combined to form a group signal. S gr (t) . If the union operation is linear, then S gr (t) \u003d S Σ (t) . will be a linear group signal. It is usually formed by linear summation of the modulated channel signals.

Rice unok 6. 2 - Multiplexing methods

In the systems of the so-called combinational compaction, a group signal is formed by means of a certain logical (non-linear) processing, as a result of which each element of the generated signal displays information (combination of symbols) from all ICs. A classic example of such a system is the double frequency telegraphy system. Four frequencies are used to transmit four combinations of symbols on two channels: f 1 - 00, f 2 - 01, f 3 - 10, f 4 - 11.

Line group splitter S Σ (t) is a set of linear selective circuits, each of which selects only its own channel signal and, ideally, does not react at all to other channel signals. To implement such an ideal separation, it is necessary and sufficient that the modulated channel signals form an ensemble of linearly independent signals. Ensembles of orthogonal signals are usually used as such signals.

In the class of linear multiplexing, according to the type of the distinguishing feature of the channel signal, time division of channels (TDM), frequency division (FDM) and division of channels in the form of signals, called code division of channels (CDC), are distinguished. Instead of the term "separation", the term "seal" is also used. With FDM, the frequency band of the common channelΔf divided into several narrower bandsΔfi , each of which forms an IS channel. With VRK, the entire bandΔf is provided alternately at certain intervals to various sources for the transmission of messages. With QKD, there is no division of the common channel between the ISs, neither in frequency nor in time. The channel signals of different ICs, overlapping in time and frequency, remain orthogonal due to the difference in shape, which ensures their separation.

Combinations of these methods are possible. So, in mobile communication as a methodmultiple accesswidely used combinations of FDC and CRC, CRC and CRC. In the first combination, each frequency channel is provided to several users for certain periods of time. With the second combination in the frequency bandΔf form channels with time division, which are provided to several users on the principles of QKD.

When organizing multi-channel information transmission, channel signals can be distributed in a predetermined way between message sources. Such a seal is called a fixed channel seal. The corresponding multi-channel transmission system will also be called a system withassigned channels. Such an organization of multichannel information transmission is also possible, when channel signals are not distributed in advance between sources, but are allocated to each source as needed. Such a seal is called a seal withloose channels. Obviously, for the correct separation of channels in systems with non-dedicated channels, it is necessary to somehow transmit address information to the receiving side.

Basic concepts and definitions introduced for multichannel systems are also applicable to systemsmultiple access(MD) . To date, a large number of various MD methods have been studied and proposed. They differ in the way in which the shared channel resource is allocated (fixed or dynamic), in the nature of the decision-making processes (centralized or distributed), and in the degree to which the access mode is adapted to changing conditions.

Multiple access is typical for satellite channels (in this case, the term "multiple access" is used), radio channels (packet radio communication), mobile communication channels, as well as for multipoint telephone lines, local networks.

All existing DM methods can be grouped and the method of managing the distribution of the common channel resource can be chosen as the classification basis (Fig. Unok 6. 3).

Rice unok 6. 3 - Multiple Access Methods

Random access protocols.With random DM, the entire resource of the communication channel is represented as one channel, access to which occurs randomly, as a result of which a collision of packets of transmitted information is possible. Correspondents are invited to perform a certain sequence of actions in order to resolve the conflict. Each user can optionally send data to the channel without explicitly negotiating with other users. The presence of feedback allows the interacting correspondents to control the passage of the transmitted information.

There are two options for implementing a random access strategy: without carrier sensing and with carrier sensing.

random accessno carrier sensingconsists in the fact that if it is necessary to transmit data, the user terminal immediately starts transmitting packets. Since the packets are not synchronized with each other, they can overlap, which causes mutual interference. When such a collision occurs, confirmed by a feedback signal, the terminals retransmit the corrupted packets. In order to avoid repetition of collisions, the time intervals before the start of retransmission at each terminal are chosen randomly.

random accesswith carrier senseimplies the ability to control the transmission of information by other correspondents. In the absence of data transmission, unoccupied time slots are available for the transmission of their information. In the event of a collision, users delay the transmission of packets by a time intervalΔt . There are currently two types of protocol:persistent and unsteady. The difference lies in the fact that in the first case, users of moving objects, detecting collisions, start transmission immediately, and in the second case, after a certain time interval.

Fixed resource pinning protocolschannels provide a static distribution of the channel resource between users. The most typical representatives of this type of protocols are frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA).

Fixed link resource pinning cannot meet the dynamically changing requirements of network users, i.e. has tight control.

Methods resource assignment on demandallow you to get rid of the shortcomings inherent in the above methods, but require detailed and clear information about the requirements of network users. Based on the nature of the decision-making processes, on-demand resource allocation methods are divided intocentralized and distributed.

Centralizedon-demand resource assignment methods are characterized by the presence of transmission requests from the message source terminals. The decision to grant the resource is made by the central station. The corresponding protocols are distinguished by the presence of redundancy channels rigidly assigned to each mobile object and the presence of a central control station. The protocols are characterized by a high value of the base station bandwidth utilization factor, however, they are critical to disruptions in the functioning of the control system.

Distributed on-demand resource assignment methods differ in that all users perform the same operations without resorting to the central station, and use additional service information that is exchanged with each other. All algorithms with distributed control require the exchange of control information between users. The protocols are characterized by the rigid assignment of redundancy channels to a moving object. At the same time, each object has a table of assignment of request channels, therefore, any mobile object at any time has information about the state of the entire network.

Combined methods are combinations of the previous resource allocation methods, and implement strategies in which the choice of method is adaptive for different users in order to obtain characteristics of the used channel resource close to optimal. As a criterion of optimality, as a rule, the coefficient of utilization of the channel capacity is taken. Based on the protocols of this type, the parameters are adjusted to the specific situation in the network.

Thus, each of the considered methods of resource allocation has advantages and disadvantages. In practice, it is advisable to have the entire set of methods and to carry out an adaptive transition from one method to another under certain changes in operating conditions.

6.2 Principle and main feature CDMA

Popular The principle of operation of cellular communication systems (CCS) with code division of channels can be explained as follows. im an example om . Suppose you are sitting instation waiting room. On every cameo there are two people. One couple speaks English to each other, the other speaks Russian, the third speaks German, and so on. So in the hall everyone is talking at the same time in same frequency range (speech from 3 kHz to 20 kHz), while you, talking with your opponent, understand only him, but hear everyone.

Principles of code division of CDMA communication channelsare based on the use of wideband signals (WBS), the bandwidth of which significantly exceeds the bandwidth required for conventional messaging, such as in narrowband frequency division multiplexing (FDMA) systems. The main characteristic of the SPS is base signal, defined as the product of the width of its spectrum F for its duration T :

B= F*T

As a result of multiplying the pseudo-random noise source signal with the information signal, the energy of the latter is distributed over a wide frequency band, i.e., its spectrum is expanded. In radio devices built X by Spread Spectrum technology(spread spectrum),the spread of the spectrum of the transmitted signal is carried out using a pseudo-random sequence (Pseudorandom Number, PN), which specifies the distribution algorithm.Each receiver must know the coding sequence to decode the message. Devices with different PNs don't actually "hear" each other. Since the signal power is distributed over a wide band, the signal itself is "hidden" in noise and, in terms of its spectral characteristics, also resembles noise in a radio channel.

The broadband transmission method is described in detail by K. Shannon, who introduced the concept of channel capacity and established a relationship between the possibility of error-free transmission of information over a channel with a given signal-to-noise ratio and the frequency band allocated for information transmission. For any given signal-to-noise ratio, a low transmission error rate is achieved by increasing the bandwidth available for information transmission.

In digital communication systems that transmit information in the form of binary symbols, the duration of the NPN T and message rate With related by the ratio T = 1/C . Therefore, the signal base B=F/C characterizes the broadening of the NLS spectrum (S shps ) relative to the message spectrum.The spectrum width is determined by the minimum pulse duration ( t 0 ), i.e. F \u003d 1 / t 0 and B \u003d T / t 0 \u003d F / Δ f (Δ f is the width of the spectrum of the information signal).

Broadening the frequency spectrum of transmitted digital messages can be carried out by different methods and/or their combination. We list the main ones:

1. direct spread of the frequency spectrum ( DSSS-CDMA);
2. with multi-channel spread spectrum(MC-CDMA)
3. carrier frequency hopping ( FHSS-CDMA).

6. 3 Direct spread spectrum - DSSS (Direct Sequence Spread Spectrum)

Traffic channels with this method of media separation are created using eat wideband code-modulated radio signal - noise-like a signal transmitted to a common channel for other similar transmitters, in a single wide frequency range. As a result of the operation of several transmitters, the air in a given frequency range becomes even more noise-like. Each transmitter modulates the signal using a separate numerical value currently assigned to each user. code , a receiver set to a similar code, you divides from the total radio signal the part that is intended for this receiver. Explicitly missing temporary or frequent channel separation, each subscriber constantly uses the entire width of the channel, transmitting a signal in a common frequency range, and receiving a signal from a common frequency range. At the same time, broadband reception and transmission channels are on different frequency ranges and do not interfere with each other. The frequency band of one channel is very wide, conversations subscribers are superimposed on each other, but since their signal modulation codes are different, they can be differentiated by the hardware and software of the receiver.

Technics spread spectrumallows you to increase throughput at a constant signal strength. The transmitted data is combined with a faster noise-like pseudo-random signal using a bitwise mutually exclusive OR operation.(xor – addition modulo 2) (figure 6.4). Data signal with pulse width Tb combined with the OR operation(added modulo 2)with a signal code, the pulse duration of which is equal to T c (width bandwidthproportional 1/T, where T - time of transmission of one bit), therefore the bandwidth of the data signal is equal to 1/ T b , and the bandwidth of the received signal is 1/ T c . Since T c is much less than T b , the bandwidth of the received signal is much larger than that of the original transmitted data signal. Value T b / T c is the signal base and, to some extent, determines the upper limit of the number of users supported by the base station in one temporarily .

Figure 6.4 - Code coding of a discrete signal (time domain)

At using the method DSSS-CDMA narrowband signal (Fig. Unok 6.5 ) is multiplied by a pseudo-random sequence (PRS) with a repetition period T , including N bit sequence duration t o everyone. In this case, the NPS base is numerically equal to the number of PSS elements B \u003d N * t 0 / t 0 \u003d N.

Picture 6.5 - Block diagram of code coding and signal spectrum

Thus, for a carrier phase shiftduring phase manipulationfast bit stream is used. The bandwidth is artificially expanded by increasing the data rate (increasing the number of transmitted bits).This is done by replacing each information bit with a burst of ten or more bits.called "chips". At the same time, the frequency band also expands proportionally. Such bit sequences are called noise-like or PN . These binary sequences are specially generated in such a way that the number of zeros and ones in them is approximately equal. Each of the zero bits of the information stream is replaced by a PN code, and the ones by an inverted PN code. This modulation called modulation with bit inversion. This mixing results in a PN signal.. In the correlator, a non-inverted PN code closely matching the local PN code generates a bit of information " 0 ". At the same time, the sequence corresponding to " 1 ", leads to complete decorrelations , since the PN code is inverted for this information bit. Thus, the correlator will produce a stream of ones for the inverted PN sequence and a stream of zeros for the non-inverted one, which will mean the restoration of the transmitted information. Sometimes a 180-degree phase shift is used to transmit the resulting bit stream, which is called binary phase-shift keying (BPSK). Or (most often) the transmission is implemented by quadrature phase-shift keying (QPSK), that is, two bits (a number from 0 to 4) encoded by four different phase shifts of the carrier frequency are transmitted simultaneously. A transmitter with one PN code cannot generate exactly the same sidebands (spectral components) as another transmitter using a different PN code.

Reception of the NPS is carried out by an optimal receiver, which for a signal with a floor ness known parameters calculates the correlation integral

z =∫ x (t ) u (t ) dt ,

where x(t) - input signal, which is the sum of the useful signal u (t) and interference n (t) (in the case of white noise). Then the value z compared to the threshold Z . The value of the correlation integral is found using a correlator or a matched filter. The correlator "compresses" the spectrum of the wideband input signal by multiplying it by the reference copy u(t) followed by filtering, which leads to an improvement in the signal-to-noise ratio at the output of the correlator in AT times relative to the entrance.

The resulting signal-to-noise gain at the receiver output is a function of the ratio of the broadband to baseband signal bandwidths: the greater the spread, the greater the gain. In the time domain, this is a function of the ratio of the bit rate in the radio channel to the bit rate of the underlying information signal. For IS-95 standard(first standard CDMA) ratio is 128times, or 21 dB. This allows the system to operate with interference levels up to 18 dB higher than the desired signal, since signal processing at the receiver output requires only 3 dB of signal level above the interference level. In real conditions, the level of interference is much less. In addition, spreading the signal spectrum (up to 1.23 MHz) can be considered as an application of receive frequency diversity techniques. The signal during propagation in the 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 varying notch bandwidth (usually no more than 300 kHz). In the AMPS standard(analogue mobile 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 signal recovery in the receiver(Figure 6.6) . In the AMPS standard bandwidth of one channel 30 kHz, in GSM - 200 kHz).

Figure 6.6 - The impact of narrowband interference (a) and fading (b) on a broadband signal.

An extremely useful property of DSSS devices is that due to the very low power level his signal they are practicallydo not interfere with conventional radio devices(narrowband high power), since these latter take a broadband signal for noise within the acceptable range. On the other side - conventional devices do not interfere with broadband ones, since their high-power signals "noise" each only in their own narrow channel and cannot drown out the entire broadband signal. It is as if with a thin pencil, but a large written letter would be shaded with a bold felt-tip pen - if the strokes are not in a row, we can read the letter.

As a result, we can say that the use of broadband technologies makes it possible to use the same part of the radio spectrum twice - conventional narrowband devices and "on top of them" - broadband.

Summing up, we can highlight the following important properties of NSS technology, at least for the direct sequence method:

P noise immunity;

small interference with other devices;

to broadcast confidentiality;

uh economy in mass production;

in the ability to reuse the same part of the spectrum.

6.4 Multichannel spread spectrum MC-CDMA (Multi Carrier)

This method is a variation of DSSS. In 1993, the Institute for Communications Technology introduced a new synchronous sharing scheme. The proposed scheme combines the advantages of the DS-CDMA technique with efficient Orthogonal Frequency Division Multiplexing ( OFDM ). The new sharing scheme is referred to as multi-frequency CDMA ( MC-CDMA) or as OFDM-CDMA , and features high flexibility and bandwidth efficiency comparable to DS-CDMA.

In the MC-CDMA system, the bits after channel coding are converted to chips by multiplying with the user separation code sequence, which is necessary to minimize interference between subscribers. To form these codes, Walsh orthogonal functions are used. The key property of the MC-CDMA system is that all chips associated with one bit of code are transmittedin parallel in narrowband subchannels, using OFDM.

This can be visualized by considering this technology based on the 802.11 standard.(Radio Ethernet) . Imagine that the entire "wide" frequency band is divided into a certain number of subchannels - (according to the 802.11 standard - 11 channels). Each transmitted bit of information is converted, according to a certain algorithm, into a sequence of 11 bits, these 11 bits are transmitted simultaneously and in parallel, using all 11 subchannels. On reception, the received bit sequence is decoded using the same algorithm as the encoding. Another pair of receiver-transmitter may use a different encoding-decoding algorithm, and there may be a lot of such different algorithms.

The obvious result of applying this method is the protection of transmitted information from eavesdropping (a "foreign" receiver uses a different algorithm and will not be able to decode information not from its own transmitter). But another property of the described method turned out to be more important. It lies in the fact that thanks to the 11-fold redundancy transmission can be dispensed withvery low power signal(compared to the signal strength level when using conventional narrowband technology),without increasing the size of the antennas. In this case, the ratio of the transmitted signal level to the level noise , (i.e., random or intentional interference), so that the transmitted signal is already, as it were, indistinguishable in the general noise. But thanks to its 11x redundancy, the receiving device will still be able to recognize it. This isabout the same as written on 11 sheets the same word and some sheets turned out to be written in illegible handwriting, others half-erased or on a burnt piece of paper - but still, in most cases, we will be able to determine what kind of word it is by comparing all 11 copies.

At this stage, for MS-CDMA systems, a frequency band of 1, 25 MHz divided into 512 subcarriers. They have been found in testing to be less sensitive to the "near-far" problem than DS-CDMA systems.

6.5 Frequency hopping spectrum spreading

Carrier frequency hopping in the third way (figure 6.7 ), is carried out by rapidly tuning the output frequency of the synthesizer in accordance with the law of formation of a pseudo-random sequence (Frequency N opping Spread Spectrum CDMA - FHSS-CDMA). Each carrier frequency and its associated sidebands must remain within the bandwidth specified by the FCC.(Federal Communications Commission). Only when the intended receiver knows the transmitter's frequency hopping sequence can its receiver follow those frequency hops.

Rice UNO 6.7 - Carrier frequency hopping spread spectrum

When coding according to the frequency hopping method (FHSS), the entire frequency band allocated for transmissions is divided into a certain number of subchannels (according to the 802.11 standard, these channels are 79). Each transmitter uses only one of these subchannels at a time, regularly hopping from one subchannel to another. The 802.11 standard does not fix the frequency of such jumps - it can be set differently in each country. These jumps occur synchronously at transmitter and receiver in a predetermined pseudo-random sequence known to both; it is clear that without knowing the switching sequence, it is also impossible to receive a transmission.

The other transmitter-receiver pair will use a different frequency switching sequence, set independently of the first. There can be many such sequences in one frequency band and in one line-of-sight territory (in one "cell"). It is clear that with an increase in the number of simultaneous transmissions, the probability of collisions also increases, when, for example, two transmitters simultaneously jumped to frequency No. 45, each in accordance with their sequence, and drowned out each other. For cases where two transmitters attempt to use the same frequency at the same time, a collision resolution protocol is provided in which the transmitter attempts to resend data on the next frequency in sequence.

6 . 6 CDMA based networks

History and general provisions

1991 - Qualcomm developed the draft IS-95 standard.

1993 - The Telecommunication Industry Association (TIA) approved the basic version of IS-95, and in July 1993 the US Federal Communications Commission (FCC) recognized Qualcomm's proposed digital cellular technology as the IS-95 standard based on CDMA.

1995 - Operation of the first commercial cellular mobile communication system on CDMA IS-95 technology in Hong Kong.

Networks and devices using code division multiple access are built on the basis of standards developed by TIA. Basically these are the standards:

IS-95 CDMA - radio interface; IS-96 CDMA - Voice Services;

IS-97 CDMA - mobile station;IS-98 CDMA - base station;

IS-99 CDMA - data services.

Based on a series of standards, the 2nd generation cdma One station was implemented. These ideas were further developed in the 3rd generation CDMA - 2000 broadband system standard.

Basic services : p data and voice transmission at speeds of 9.6 Kbps, 4.8 Kbps, 2.4 Kbps; m long distance call; R oaming (national and international); w blowing call ; P call forwarding (no answer, busy); to conference call ; and Call waiting message indicator; voice mail ; t Text transmission and reception of messages.

Network architecture

In Figure 6. 8 a generalized block diagram of the CDMA IS-95 cellular mobile radio network is given.

The main elements of this network (BTS, BSC, MSC, OMC) are identical in composition to the elements used in cellular networks with time division of channels (for example, GSM). The main difference is that the CDMA IS-95 network includes quality evaluation and block selection devices (SU - Selector Unit). In addition, to implement the procedure for soft switching between base stations controlled by different controllers (BSC), transmission lines between SU ​​and BSC (Inter BSC Soft handover) are introduced. In the switching center of mobile objects (MSC), a transcoder converter (TCE - Transcoder Equipment) has been added, which converts speech signal samples, data format from one digital format to another.

Qualcomm's CDMA system is designed to operate in the 800 MHz frequency band. She is built according to the method of direct spreading of the frequency spectrum based on the use of 64 types of sequences formed according to the law of Walsh functions. For the transmission of voice messages, a speech converting device with the CELP algorithm with a conversion rate of 8000 bps (9600 bps in the channel) was selected. Operating modes at speeds of 4800, 2400, 1200 bps are possible.

The standard uses separate processing of reflected signals arriving with different delays and their subsequent weight addition, which significantly reduces the negative impact of the multipath effect. With separate processing of rays in each receiving channel on the base stations 4 parallel correlators are used, and 3 correlators at the mobile station. The presence of correlators operating in parallel makes it possible to implement a soft "handover" mode when moving from cell to cell.

Rice unok 6. 8 - CDMA network architecture

The soft "handover" mode occurs by controlling the mobile station with two or more base stations. The transcoder, which is part of the main equipment, evaluates the quality of signal reception from two base stations sequentially frame by frame. The process of selecting the best frame results in that the resulting signal can be generated by continuous switching and subsequent "gluing" of frames received by different base stations participating in the "handover".

Traffic and control channels

In CDMA, channels for transmission from a base station to a mobile station are called forward. The channels for receiving information from the mobile base station are called reverse (Reverse). For the return link, IS-95 defines a frequency band from 824 to 849 MHz. For the direct channel - 869-894 MHz. The forward and reverse channels are separated by an interval of 45 MHz. User data is packed and transmitted in a channel with a bandwidth of 1.2288 Mbps. The load capacity of the direct channel is 128 telephone connections with a traffic speed of 9.6 Kbps. The composition of channels in CDMA in the IS-95 standard is shown in unke rice 6. 9 .

In the standard IS-95 uses different types of modulation for the forward and reverse channels. In the forward channel, the base station transmits data simultaneously for all users in the cell, using different codes for each user to separate the channels. The pilot signal is also transmitted, it has a higher power level, providing users with the ability to synchronize ation.

Rice unok 6. 9 - CDMA traffic and control channels

In the reverse direction, the mobile stations respond asynchronously (without using a pilot), with the same power level arriving at the base station from each mobile station. This mode is possible due to power control and power control of mobile subscribers over the service channel.

Direct channels

The data on the forward traffic channel is grouped into a 20 ms frame. The user data after precoding and formatting is interleaved to adjust the current data rate, which may vary. Then the spectrum of the signal is expanded by multiplying with one of the 64 pseudo-random sequences (based on Walsh functions) to a value of 1.2288 Mbps. Each mobile subscriber is assigned a PSP, with the help of which th his data will be separated from the data of other subscribers. The orthogonality of the SRP is ensured by the simultaneous synchronous coding of all channels in the cell (i.e., the fragments used at each moment of time are orthogonal). As already mentioned, a pilot signal (code) is transmitted in the system so that the mobile terminal can control the characteristics of the channel, receive timestamps, providing phase synchronization for coherent detection. For global network synchronization, the system also uses radio tags from GPS(Global Position System)-satellites.

Composition of direct channels

The Pilot Channel is designed to establish initial synchronization, control the signal level of the base station in time, frequency and phase, and identify the base station.

The Synchronizing Channel (SCH) maintains the pilot signal emission level, as well as the phase of the pseudo-random sequence of the base station. The synchronization channel transmits clock signals to mobile terminals at 1200 baud.

Short Message Broadcast Channel, Paging Channel is used to call the mobile station. The number of channels is up to 7 per cell. After receiving the call signal, the mobile station transmits an acknowledgment signal to the base station. After that, information about the connection establishment and assignment of the communication channel is transmitted to the mobile station via the broadcast call channel. Operates at 9600, 4800, 2400 baud.

Direct traffic channel (FTCH - Forward Traffic Channel) is designed to transmit voice messages and data, as well as control information from the base station to the mobile; passes any user data.

CDMA uses two types of channels to provide different communication services. The first of them is called the main, and the second - additional. The services provided through this pair of channels depend on the communication scheme. Channels can be adapted for a specific service and operate at different frame sizes using any of the two speed ranges: RS-1 (1200, 2400, 4800 and 9600 bps) or RS-2 (1800, 3600, 7200 and 14400 bps). Determination and selection of the reception rate is carried out automatically.

Each logical channel is assigned a different Walsh code, as indicated on unke rice 6.10 . In total, there can be 64 logical channels in one physical channel, since there are 64 Walsh sequences to which logical channels are assigned, and each of them has a length of 64 bits. Of all 64 channels:

1. the first Walsh code (W0) to which the pilot channel corresponds is assigned to the 1st channel;
2. the next channel is assigned a thirty-second Walsh code (W32), the next seven channels are also assigned their Walsh sequences (W1, W2, W3, W4, W5, W6, W7) to which the call channels correspond;
3. 55 channels are intended for data transmission over the direct traffic channel.

Rice unok 6. 10 - Structure of direct channels

The composition of the return channels

Access Channel (ACH - Access Channel) provides communication between the mobile station and the base station when the mobile station is not yet using the traffic channel. The Access Channel is used to establish calls and respond to Paging Channel messages, commands, and network registration requests. Access channels are combined (combined) with call channels.

Reverse Traffic Channel (RTCH) provides transmission of voice messages and control information from the mobile station to the base station.

Main characteristics systems

MS transmission frequency range	824.040 - 848.860 MHz
BTS transmission frequency range	869.040 – 893.970 MHz
Relative carrier jitter BTS	+/- 5*10 -8
Relative carrier jitter MS	+/- 2,5*10 -6
Type of carrier frequency modulation	QPSK(BTS), O-QPSK(MS)
Spectrum width of emitted signal:- 3 dB 40 dB	1.25 MHz 1.50 MHz
SNR clock frequency M-function	1.2288 MHz
Number of BTS channels on 1 carrier frequency	1 pilot channel 1 sync channel 7 person channels. call 55 communication channels
Number of MS channels	1 access channel 1 communication channel
Transfer rate in channels : - sync In a personal call and access channel In communication channels	1200 bps 9600, 4800 bps 9600, 4800, 2400, 1200 bps
Encoding in BTS transmission channels	Convolutional code R=1/2, K=9
Encoding in MS transmission channels	Convolutional code R=1/3, K=9
Information bit energy ratio required for reception	6-7 dB
Maximum effective radiated power BTS	50 W
Maximum effective radiated power MS	6.3 - 1.0 W

6.7 The order of passage of voice data in the mobile station until sending to the air

R Let's look at the block diagram of the reverse traffic channel(Figure 6.11) . In the forward and reverse channels, this pattern is repeated; depending on which channel is currently in use, some blocks of this scheme are excluded.

1. The speech signal enters the speech codec - at this stage, the speech signal is digitized and compressed according to the CELP algorithm.

The principle is this. The data stream is written to the matrix row by row. Once the matrix is ​​filled, start tsya her transmission cha by columns. Consequently, when several bits of information are distorted in a row on the air, when receiving a packet of errors, passing through the inverse matrix, it is converted into single errors.

Figure 6.11 - Structural diagram reverse traffic channel

4. Next, the signal enters the coding block (from eavesdropping) - a mask (sequence) of 42 bits is superimposed on the information. This mask is secret. With unauthorized interception of data on the air, it is impossible to decode the signal without knowing the mask. The method of sorting through all possible values ​​is not effective. when generating this mask, going through all possible values, you will have to generate 8, 7 trillion masks 42 bits long.

5. Walsh code multiplication block - the digital data stream is multiplied by a sequence of bits generated by the Walsh function.

At this stage of signal encoding, the frequency spectrum is expanded, i.e. each bit of information is encoded by a Walsh sequence, 64 bits long. That. the data rate in the channel is increased by 64 times. Consequently, in the signal modulation block, the speed of signal manipulation increases, hence the expansion of the frequency spectrum.

The Walsh function is also responsible for filtering out unnecessary information from other subscribers. At the start of a communication session, the subscriber is assigned the frequency at which he will work and one (out of 64 possible) logical channels, which determines the Walsh function. At the moment of acceptance, the signal passes according to the scheme in the opposite direction. The received signal is multiplied by the Walsh code sequence. The result of multiplication is used to calculate the correlation integral.

If the Z threshold satisfies the limit value, then the signal is ours. The sequence of Walsh functions are orthogonal and have good correlation and autocorrelation properties, so the probability of confusing your signal with someone else's is 0, 01 %.

6. The block for multiplying the signal by two M-functions (M1 - 15 bits long, M2 - 42 bits long) or they are also called PSP-pseudo-random sequences - the block is designed to mix the signal for the modulation block. Each assigned frequency is assigned a different M-function.

7. Signal modulation block - the CDMA standard uses phase modulation FM4, OFM4.

Benefits of CDMA

1. High spectral efficiency. CDMA allows you to serve more subscribers in the same frequency band than other types of separation ( TDMA, FDMA).
2. Flexible resource allocation. With code division, there is no strict limit on the number of channels. With an increase in the number of subscribers, the probability of decoding errors gradually increases, which leads to a decrease in the quality of the channel, but not to a denial of service.
3. AT high channel security. It is difficult to select the desired channel without knowing its code, because in The entire frequency band is uniformly filled with a noise-like signal.
4. CDMA phones have lower peak power and are therefore possibly less harmful.

6.8 The evolution of cellular communication systems using CDMA technology

Currently, CDMA equipment is the newest and most expensive, but at the same time the most reliable and most secure. The European Community, within the framework of the RACE research program, is developing the CODIT project to create one of the variants of the Universal Mobile Telecommunications System (UMTS) on the principle of code division using broadband direct spread signals.

The main difference of the CODIT concept will be the efficient and flexible use of the frequency resource. As we explained earlier, the wideband CDMA signal is practically unaffected by narrowband interference. Due to this property, the CODIT standard will additionally use guard intervals between carrier frequencies for data transmission.

The CDMA code division technology, due to its high spectral efficiency, is a radical solution for the further evolution of cellular communication systems.

CDMA2000 is the standard 3G in the evolution of networks cdmaOne (based on IS-95 ). While maintaining the basic principles laid down by the version IS-95A , CDMA technology is constantly evolving.

The subsequent development of CDMA technology occurs within the framework of CDMA2000 technology. When building a mobile communication system based on CDMA2000 1X technology, the first phase provides data transmission at a speed of up to 153 kbps, which allows you to provide voice services, short message transmission, e-mail, Internet, databases, data and still images.

Transition to the next phase CDMA2000 1X EV-DO using the same frequency band of 1.23 MHz, the transmission rate is up to 2.4 Mbps in the forward channel and up to 153 kbps in the reverse channel, which makes this communication system compliant with 3G requirements and makes it possible to provide the widest range of services up to real-time video transmission.

The next phase of the development of the standard in the direction of increasing network capacity and data transmission is 1XEV-DO Rev A : Data transmission at speeds up to 3.1 Mbps towards the subscriber and up to 1.8 Mbps from the subscriber. Operators will be able to provide the same services as based on the Rev. 0, and, in addition, to transmit voice, data and broadcast over IP networks. There are already several such operating networks in the world.

Developers of CDMA communication equipment have launched a new phase - 1XEV-DO Rev B , - in order to achieve the following speeds on one frequency channel: 4.9 Mbps to the subscriber and 2.4 Mbps from the subscriber. In addition, it will be possible to combine several frequency channels to increase the speed. For example, combining 15 frequency channels (the maximum possible number) will allow reaching speeds of 73.5 Mbps to the subscriber and 27 Mbps from the subscriber. The use of such networks is an improved performance of time-sensitive applications such as VoIP , Push to Talk, video telephony, online gaming, etc.

The main components of the commercial success of the CDMA2000 system are a wider service area, high speech quality (almost equivalent to wired systems), flexibility and low cost of introducing new services, high noise immunity, communication channel stability from interception and listening.

Also, an important role is played by the low radiated power of radio transmitters of subscriber devices. So, for CDMA2000 systems, the maximum radiated power is 250 mW. For comparison: in GSM-900 systems, this figure is 2 W (in a pulse, when using GPRS + EDGE withmaximum filling; maximum when averaging over time during a normal conversation is about 200 mW). In GSM-1800 systems - 1 W (in a pulse, the average is slightly less than 100 mW).

In time division multiplexing (TDM), the signals of each channel are sampled and their instantaneous values ​​are transmitted sequentially in time. Thus, each message is transmitted in short pulses - discretes. On one communication line for a certain period of time - the repetition period, which is allotted for transmission, it is possible to transmit the corresponding number of such messages.

Structural diagram of the system for transmitting information from the VRC. On fig. 4.3 shows a simplified block diagram of a system with a VRC. A message, for example, in telephone communication in the form of sound signals, enters the P in, where the sound vibrations are converted into electrical ones. The distributors of the transmitting P1 and receiving P2 sides must work synchronously and in phase. Switching of distributors is carried out from the pulses coming from GTI. At the end of each cycle, a phasing pulse enters the communication line to ensure the in-phase operation of both distributors. The synchronism of their operation is ensured by the stability of the GTI frequency of the transmitting and receiving sides.

The distributor connects circuits in series to transmit messages over the appropriate channel. Since little time is allocated for the transmission of messages, short pulses will follow along the communication line, the duration of which is determined by the time the distributor connected this circuit. On the receiving side, due to the synchronous and in-phase operation of the distributors, short pulses are fed to P vy x, where the reverse conversion of electrical signals into sound ones takes place.

With TRC, between the signals of each channel transmitted sequentially in time over the communication line, a guard time interval is introduced (Fig. 4.4), which is necessary to eliminate the mutual influence (overlap) of the channels. The latter occurs due to the presence of phase-frequency distortions in the communication line, which causes uneven propagation time of signals of different frequencies.

The number of channels in TRC depends on the duration of the channel pulses and the frequency of their repetition, which, when transmitting continuous messages, is determined by the Kotelnikov theorem on the transformation of continuous signals into discrete ones.

Thus, the total number of channels in the TRC

(4.1)

where T p - repetition period;
- the duration of the synphasing pulse; - duration of the protective interval; - the duration of the channel pulse.

Bandwidth required by an organization P channels during TRC, is determined by the minimum duration of the channel pulse
, which depends on the number of organized communication channels and the nature of the message, is determined from the expression

(4.2)

where K p is a coefficient depending on the shape of the pulse (for a rectangular pulse K p ~0.7).

Let's determine the frequency band required, for example, to organize 12 telephone channels for TDC. The duration of the pulse when organizing 12 telephone channels over the communication line is determined from the following considerations. The repetition period T p \u003d 1 / f p, where f p is the repetition frequency, which is determined by the expression f p \u003d 2f max \u003d 2 3400 \u003d 6800 Hz. Here f max = 3400 Hz is the maximum frequency when transmitting telephone messages. For transmission, f p \u003d 8000 Hz is taken. Then f p \u003d 1/8000 \u003d 125 μs.

From expression (4.1)

Substituting the values ​​of T p = 125 μs and n=12 into the last expression, we obtain
1 µs. Knowing the duration of the channel pulse
and taking K p = 0.7 from expression (4.2), we find

Thus, the frequency band for organizing 12 telephone channels in FDM significantly exceeds the frequency band required for organizing the same number of channels in FDM, which is 48 kHz (12(3400 + 600) = 48000 Hz, where 600 Hz is the frequency band allocated for filtering adjacent channels).

Therefore, the use of VRC for the transmission of analog messages (for example, telephone, facsimile, television) has a number of limitations. At the same time, the transmission of discrete messages (telegraph, telemechanics, data transmission) during the TRC provides significant advantages. This is explained by the fact that discrete signals for these types of messages have a significant duration, and the frequency spectrum of such signals is located in the lower part of the frequency range, therefore, the duration and repetition period of channel pulses can be relatively large, which significantly reduces the required frequency band.

In TDM, various types of channel modulation can be used to match a message with a communication channel.

The disadvantages of the TRC should include a relatively wide frequency band required for the transmission of messages; the complexity of switching equipment (distributors) when organizing a significant number of communication channels and the need to correct the phase-frequency characteristics of the communication line to eliminate the mutual influence of communication channels.

Multichannel systems with TDM are widely used for the transmission of analog and discrete information.

It is convenient to explain the principle of temporal combining of channels with the help of synchronously rotating distributors on the transmitting and receiving sides (Fig. 8.9).

The main stages of the formation of a group signal are shown in Fig. 8.10.

Information from sources of analog signals is fed to the inputs of the corresponding individual pulse modulators AIM (PWM, FIM). The generated signal samples at the output of the first pulse modulator () (Fig. 8.10, c), at the output of the second pulse modulator () (Fig. 8.10, d) are taken at the same interval, but with such a time shift that they do not overlap.

Then the transmitting distributor reads the pulses from all sources, forming a signal (Fig. 8.10, e), the spectrum of which, using a group modulator (GM), is transferred to the frequency range allocated for this communication line. The group signal, transmitted over the communication line, carries information from both the first and the second source simultaneously. On the receiving side, from the output of the group demodulator (GD), the group signal pulses arrive at the rotating contacts of the receiving distributor to form channel sequences, etc. from which, at the output of pulse detectors, continuous signals are formed that arrive at the recipients of messages.

It should be emphasized that Fig. 8.9 serves only to illustrate the idea of ​​time multiplexing and does not reflect current switching techniques. In reality, the temporary sealing apparatus dispenses with mechanical distributors, which are replaced by electronic distributors that perform the same functions (Fig. 8.11).

Fig.8.11. Scheme of multichannel communication with VRC.

The outputs of all pulse modulators are connected to "their own" electronic switches, the operation of which is controlled by the distributor of switching impulses. In turn, the distributor is started from the clock pulse generator.

Time separation of signals is carried out by a device, a simplified block diagram of which is shown in fig. 8.11. The received group radio signal in the group demodulator is converted into a group pulse video sequence and is fed simultaneously to the inputs of the sync signal extractor and channel electronic switches.

The process of time division is carried out in two stages. At the first stage of the system entering into synchronism, the search, detection and selection of synchronization signals take place, after which the distributor of channel switching pulses is launched. The distributor generates at its outputs pulses of the required duration and in such a sequence that only one electronic switch of the corresponding channel opens in each channel interval.

At the second stage, each channel pulse is demodulated, after which the signals of the received channels are fed to the recipients of analog information.

In the case of time division of channels, the synchronization system plays the most important role, the algorithm of which is chosen individually each time for the accepted method of pulse modulation, the method of time combining channels, the structure of synchronization signals, etc.

The communication line is the most expensive element of the communication system. Therefore, it is advisable to carry out multi-channel information transmission over it, since with an increase in the number of channels N, its throughput increases S. Poich. condition must be met:

H K - performance of the k-th channel.

The main problem of multichannel transmission is the separation of channel signals on the receiving side. Let us formulate the conditions for this division.

Let it be necessary to organize the simultaneous transmission of several messages over a common (group) channel, each of which is described by the expression

(7.1.1)

Taking into account the formula (7.1.1.) we obtain:

In other words, the receiver has selective properties with respect to the signal Sk(t).

Considering the issue of separation of signals, frequency, phase, time separation of channels, as well as separation of signals by shape and other features are distinguished.

Second study question

Frequency division channels

A block diagram of a multichannel communication system (MCS) with frequency division channels (FCD) is shown in Fig. 7.1.1, where it is indicated: IS - signal source, Mi - modulator, Фi - filter of the i-th channel, Σ - signal adder, GN - carrier generator, TX transmitter, LS - communication line, IP - source of interference, PRM - receiver, D - detector, PS - message receiver.

Fig.7.1.1. Structural diagram of a multichannel communication system

With FDM, the carrier signals have different frequencies fi (subcarriers) and are separated by an interval greater than or equal to the width of the spectrum of the modulated channel signal. Therefore, the modulated channel signals occupy non-overlapping frequency bands and are orthogonal to each other. The latter are summed (compacted in frequency) in block Σ forming a group signal, which modulates the oscillation of the main carrier frequency fn in block M.

All known techniques can be used to modulate the channel carriers. But more economically, the bandwidth of the communication line is used with single-sideband modulation (SSB AM), since in this case the width of the spectrum of the modulated signal is minimal and equal to the width of the spectrum of the transmitted message. In the second stage of modulation (group signal), OBP AM is also more often used in wired communication channels.

Such a signal with double modulation, after amplification in the TX block, is transmitted over the communication line to the RX receiver, where it undergoes an inverse conversion process, i.e. demodulation of the signal along the carrier in block D to obtain a group signal, extracting channel signals from it by bandpass filters Fi and demodulation of the latter in blocks Di. The central frequencies of the bandpass filters Fi are equal to the frequencies of the channel carriers, and their transparency bands are equal to the width of the spectrum of the modulated signals. The deviation of the real characteristics of the bandpass filters from the ideal ones should not affect the quality of signal separation, therefore, guard frequency intervals between channels are used. Each of the filters Ф reception must pass without attenuation only those frequencies that belong to the signal of a given channel. The signal frequencies of all other channels must be suppressed by the filter.

The frequency separation of signals by ideal bandpass filters can be mathematically represented as follows:

where g k is the impulse response of an ideal bandpass filter that passes without distortion the frequency band of the k-th channel.

The main advantages of the CHRK: simplicity of technical implementation, high noise immunity, the possibility of organizing any number of channels. Disadvantages: the inevitable expansion of the used frequency band with an increase in the number of channels, the relatively low efficiency of using the bandwidth of the communication line due to filtering losses; cumbersome and high cost of equipment, mainly due to a large number of filters (the cost of filters reaches 40% of the cost of a system with FDM). In railway transport, an MCS with a K-24T type PMC has been developed, in which small-sized electromechanical filters are used.

Third study question