FI94819B - Communication method and CDMA / FDMA radio system - Google Patents

Description

94819

Communication method and CDMA / FDMA radio system

The invention relates to a data transmission method using combined CDMA / FDMA multipurpose.

One of the key problems in the design and implementation of mobile phone systems is the efficient use of frequency. With the ever-increasing volume of radio traffic, this problem has become increasingly important. The more efficient a 10-camera system is able to take advantage of a given frequency band, the greater the potential number of users in the system. In the development of new mobile telephone systems, radio channel multi-use methods are a key area of research.

15 The first generation of mobile networks has been implemented using the FDMA method. The second generation, which takes advantage of digital data transmission, is designed on the basis of combined FDMA / TDMA technology.

A third method of multi-use of a radio channel, the CDMA-20 method, has recently been applied to cellular networks. CDMA is a multi-use method based on spread spectrum technology, which has several known advantages over previous methods, such as the absence of the need for frequency planning and the efficiency of frequency band use.

In FDMA, users are distinguished from each other by frequency; each user data signal is allocated a separate frequency band. In TDMA, the frequency band is divided into successive time slots and each user's data signal is transmitted in its own repetitive time slot. There may be several such frequency bands in use, in the case of combined FDMA / TDMA.

In CDMA, a user's narrowband data signal is modulated with a more wideband spreading code than the data signal over a relatively wide band. For example, 1.25 MHz, 10 MHz 294819 and 50 MHz bandwidths have been used in the known 35 test systems. The spreading code consists of several bits. The bit rate of the spreading code is much higher than that of the data signal, and as a difference from the data bits and symbols, the bits of the spreading code are called chips. The already 5 user data symbol is multiplied by all chips of the spreading code. In this case, the narrowband data signal propagates to the frequency band used. Each user has their own spreading code. Multiple users transmit simultaneously in the same frequency band and the data signals 10 are separated at the receivers based on the spreading code.

The correlators in the receivers synchronize to the desired signal, which they recognize based on the spreading code. The correlators return the data signal 15 to the original narrow frequency band. The signals coming to the receiver, which are modulated by the second spreading code, do not ideally correlate in the receiver, but retain their wide band. The aim is to select the spreading codes used by the system in such a way that they are uncorrelated with respect to each other, i.e. orthogonal.

It is typical for a cellular network environment that the signal traveling between the user and the base station does not follow a single direct route, but depending on the characteristics of the environment. 25 travels several routes of different lengths from the transmitter to the receiver. Such multipath propagation occurs even if there is a direct line of sight between the base station and the mobile station. This multipath propagation is mainly due to signal reflections from surrounding surfaces. Signals traveling on different paths 30 have different travel time delays, and. they thus arrive at the receiver in different stages.

In general, the spreading codes are not orthogonal to all possible delay values. Thus, differently delayed signals interfere with the detection of other signals. Thus, users interfere with each other, and this disturbance is called multi-use disturbance. CDMA is an interference limited system. As the number of users increases, and thus also the effect of multi-use interference, the signal-to-noise ratio of the connections deteriorates. With a certain number of users, the signal-to-noise ratio increases to such an extent that the connection becomes more difficult to maintain, and users can no longer be added other than at the expense of the quality of existing connections. Typically, one radio channel with a bandwidth of, for example, 1.25 MHz may have a maximum of 10 simultaneously 30-40 CDMA connections. For example, if more channel capacity is desired at a base station, this means the introduction of a new radio channel.

Although CDMA technology per se is more efficient in terms of spectrum utilization than previous technologies, the broadband radio channels required by the CDMA / FDMA system still consume a considerable amount of frequency bandwidth, especially in areas with high capacity requirements. Thus, there is a need for even more efficient use of the spectrum.

20 In a cellular network environment, users are randomly located at the base station and relative to each other. The effect of multi-use interference is particularly evident in connections from mobile phones to the base station. Telephones near a base station may completely obscure the transmission of remote stations 25 if the power control of the telephones is not accurate, as even a small correlation of a strong signal can cause a large interference in the detection of a weak signal. This phenomenon is called the near-far problem. The aim of the power control of the telephones is that the power received at the base of all telephones should be the same depending on the ·. distance from the base station. However, precise power control is difficult to implement due to, for example, the rapidly changing nature of the radio channel.

The problems caused by multi-use interference could be eliminated if the users' signals were completely orthogonal, i.e., did not correlate. For this reason, efforts have been made to develop code families with as little cross-correlation between codes as possible.

The object of the present invention is to implement a data-5 transmission method in which the multi-use interference and the problems caused by it can be substantially reduced by using certain types of codes and their properties and thus achieve higher capacity and spectrum efficiency in the data transmission system.

This is achieved by the method according to the invention, characterized in that complementary codes are used as CDMA spreading codes and in that interleaved frequencies are used in at least some of the frequency channels, the frequency difference between which is selected as a function of the frequency deviation of the CDMA spreading codes that the transmissions on the frequency channels used are synchronized with each other.

The invention also relates to a CDMA / FDMA radio system with several frequency channels. The system is characterized in that the CDMA spreading codes used in the system are complementary codes, and that at least some of the frequency channels are interleaved frequencies, the frequency difference between the frequency channels are synchronized with each other.

The invention utilizes a spreading code family whose cross-correlation properties are ideal under certain boundary conditions. Such a family of codes is called complementary codes. Complementary codes are characterized in that they are formed of several members of the same length, so that the sum of the mutual cross-correlations of these codes obtained by combining them is zero with lime delay values. The complementary codes are thus completely orthogonal to each other. However, the terms from which the complementary codes used as spreading codes are compiled are not in themselves orthogonal to each other at all delay values.

Therefore, each member of the complementary code must be transmitted on an uncorrelated channel to avoid interference. In practice, this can be done in a number of different ways. One possibility is to transmit code members on 10 different frequencies. In this case, the distance between the frequency bands should be at least equal to the coherence bandwidth. In the case of CDMA, a more sensible way is to distinguish code members in the time domain. Sufficient protection time should be left between members to cover timing inaccuracies, delay variance, and the effect of propagation delays. Other ways to implement uncorrelated channels is to use different polarization levels or carrier quadrature components.

A significant advantage of the invention is that when complementary codes are used, multi-user signals can be transmitted in the same frequency band without multi-use interference. This is possible if all users transmit at the same rate, i.e. synchronously, so that the different members of the complementary codes do not overlap and correlate in 25 transmissions. correlate with the core and thus do not interfere with each other. The radio channels of the CDMA / FDMA system can be selected so that the frequency difference between the carriers of adjacent radio channels is the same as the frequency difference between two such zeros, i.e. less than the bandwidth required by the spreading code 35 used, whereby the bands of adjacent radio channels 6 94819 overlap. With this "interleaving technique", the frequency spectrum can be utilized much more efficiently than in previous CDMA / FDMA systems.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 shows an example case of two users, Figures 2a to 2d show an example of a two user system, Figure 3 shows an example of single user data signal spreading, Figures 4a and 4b show complementary code inaccuracy functions, Fig. 5a shows the frequency band allocation of a conventional CDMA system, Fig. 5b shows an example of interleaved frequency band allocation, Fig. 6 shows an example of an implementation of a receiver according to the invention and Figs. 7a and 7b an example of an implementation of a transmitter according to the invention.

In the present invention, the spreading codes are complementary codes, which are discussed in more detail in B.P. Schweitzer, Generalized Complementary Code 25 Sets, 1971, dissertation, University of California, Los ** Angeles, USA, 87 pages.

In the data transmission system, each user must have his own spreading code. Thus, if the capacity of the system is K users, there must be K ortho-30 gonal codes that can be implemented as complementary codes, which in turn each consist of K members. K uncorrelated channels are required to transmit each code member.

Figure 1 shows by way of example a system for two users. Each user has their own 7,94819 codes, each with two members, 14a, 14b and 15a, 15b, respectively. The members of the codes are transmitted on uncorrelated transmission channels 10 and 11, which can be implemented, for example, at different frequencies or time slots. At the receiving end 5, there are filters 16a, 16b and 17a, 17b, respectively, arranged for the codes used, the output signals of which are combined to obtain user data signals S18 and S19.

In the following, the signals of the two-user system according to Fig. 1 will be discussed by way of example. Assume-10 that the system uses BPSK modulation. The modulation modes used are indicated by "+" and M-M characters. Assume that the system uses the following two mutually orthogonal complementary code pairs: 3 ^ = +++ - and s12 = ++ - + and s21 = + - ++ and s22 = + ---. Of these, 15 sn and s12 are used by user 1, and s21 and s22 are used by user 2, respectively. The impulse responses of the filters matched in reception are: user 1 filter 16a: - +++ and filter 16b: + - ++ and user 2 filter 17a: ++ - + and filter 17b: --- +. If the user data signals are assumed to be binary, d1 = 1 and d2 = 0, then the waveforms of user 1 are at the output of 14a +++ - and 14b at the output ++ - +. Respectively, the waveforms of user 2 are + - ++ at output 15a and + --- at output 15b.

Figures 2a to 2d show the signals at the receiver 25 assuming that the radio channel is ideal, i.e. ** no interference and noise. Confirmation is assumed to be one sex. Figure 2a shows the signals at the outputs of the matched filters 16a and 16b and the summed signal 18 when transmitted by the user 1. Accordingly, Fig. 2b shows 30 signals at the outputs of the matched filters 17a and 17b and a summed signal 19 when transmitted by the user 2. For each user, an waveform similar to the ideal impulse response is obtained, from which the transmitted data symbol can be expressed. Fig. 2c shows the signals 35 at the outputs of the matched filters 16a and 16b and the 8,94819 summed signal S18 when transmitted by the user 2. Correspondingly, Fig. 2d shows the signals at the outputs of the matched filters 17a and 17b as well as the summed signal S19 when transmitted by the user 1. Thus, no waveform is detected in either of the summed-5 signals, i.e., multi-use interference does not occur. This is due to the orthogonality of the spreading codes.

In a preferred embodiment of the invention, the transmission of the single user data signal takes place in the same way as in the example of Fig. 3. Assume that the system uses a complementary code family consisting of K members Si, S2, ..., SK. Each member is N chips in length. The duration of each chip is Tc seconds. The code of user i is thus in the form S, -l, S {2, ..., S {k, where S, -k 15 (k = 1, 2, ..., K) is one of the members S1, S2 ,. .., SK, so that each code is orthogonal to the other codes.

Figure 3 shows the transmission of a single signal burst in which n data bits or symbols bn are transmitted. At the beginning of the burst 20, each data symbol bn is modulated by the first term ε, Ι of the spreading code. The duration of this transmission is nNTc seconds. A transmission pause is then taken. The purpose of the transmission pause is to distinguish consecutive code members in the time plane. Although the complementary codes are orthogonal to each other, the members of the codes per se are not * 'uncorrelated at all delay values. Therefore, the duration of the transmission pause should be long enough so that due to multipath propagation in the transmission and timing inaccuracies, no cross-correlation between code members is visible at the receiver. The required transmission pause duration can be defined as TM + TR + ΤΔ seconds, where TM is the multipath, TR is the difference in signal propagation delays, and ΤΔ is the timing inaccuracy. After the transmission pause, each data symbol bn is in turn modulated by the next member 8, -2 of the spreading code, after which a corresponding transmission pause is again held. In this way, the data symbols are modulated alternately by all members of the spreading code.

Thus, the invention uses a combined CDMA / FDMA-5 method. There are several frequency bands in use, and in each frequency band the users are distinguished from each other based on the spreading code. The combination of CDMA and FDMA is known per se, but in the solution according to the invention the method of selecting the frequency bands to be used is new and considerably more efficient in terms of frequency efficiency compared to previous methods. The selection is based on the zeros of the cross-correlation functions of the complementary codes calculated by the transmission frequency deviations.

Figure 4a shows, by way of example, the auto and cross-precision functions calculated for the complementary codes. The inaccuracy function describes the auto- and cross-correlation when the codes have a frequency deviation from each other. In that example, each code has four members.

20 Each code member consists of four chips. The scattering ratio is thus sixteen. The first solid line curve in Figure 4a illustrates the auto inaccuracy function of the code. The horizontal axis is scaled as follows: 1.0 corresponds to a phase inversion of 2 * π: η during the chip, i.e. a maximum of 0.25 on a 25 horizontal axis corresponds to a »r / 2 or 90” degree phase change due to a frequency error. In the same figure, the dashed line shows the corresponding cross-inaccuracy functions. As will be seen, the curves show three common zeros in this example case. The frequency deviations corresponding to these zero-30 points can be taken as the channel intervals of the system, i.e. frequency channels (carriers) can be introduced in the system for the frequencies indicated by these zero points. These frequencies are called interlaced frequencies.

10 94819

Figure 4b shows similar curves to Figure 4a, but the maximum value of the horizontal axis has now been increased to 1.0. It is noted that non-zero components occur at intervals of tr / 2, which in this Example-5 case prevents the use of more than four adjacent interlaced frequencies. The following interlaced frequencies can only be used when the transmission filters of the transmitters isolate the frequency bands from each other, ie after 2 * π: η.

This corresponds to the normal frequency band difference of FDMA.

Thus, in the example case, one bit is transmitted with four-chip spreading, and according to Fig. 4a, four interleaved frequencies are available. The channel spacing, i.e. the difference between frequencies, is the bit rate. However, the frequency band of each channel is much larger than the bit-15 band, so the spectra of the channels are almost overlapping. Thus, if the spreading ratio, which was thus four in the example described above, is increased, the number of interleaved frequency channels used increases accordingly, but the channel spacing remains the same.

Figure 5a shows a conventional method of allocating different frequency channels for use by a CDMA system. In the example shown in the figure, three frequency ranges are used, 50, 51 and 52, the center frequencies of which are fa, fb and fc, respectively.

In order to ensure that transmissions at different frequencies do not interfere with each other, the spectra at different center frequencies must not overlap in accordance with the principle of “FDMA”.

Figure 5b illustrates a method according to the invention for dividing frequency channels and frequencies used in relation to the width of the spectrum. The figure is drawn only to illustrate the method, and is not an authentic pattern produced by a spectrum analyzer. The spectrum of the transmission at frequency f0 is in the region 53 width. The method according to the invention has also introduced interleaved frequencies f1, f2 and f4. The corresponding spectra are plotted as regions 54, 55, 35, and 56, respectively. It is observed that the spectra of the signals at the different 11,94819 interleaved frequencies nearly overlap, but since the carrier frequencies are selected based on the zero points of the spreading code inaccuracy functions, no interference occurs. The next group of interleaved carrier-5 wave frequencies is at frequencies f4, f5, f6 and f7 and the corresponding spectra 57, 58, 59 and 60. Compared to Fig. 5a, it can be seen that the frequencies can be used more efficiently by the method according to the invention.

The interleaved frequencies may be used within the same cell 10 or may be divided into different cells. Since the network is synchronized, there is no multi-use interference between the interleaved carrier frequencies between the cells either, assuming that in the scaled transmission pause between the members of the spreading code, the factor T „covers the difference in propagation delays between the cells as well. In practice, this fact limits the size of the cells in the system to some extent, because it does not make sense to dimension the transmission pause indefinitely.

The same frequencies can also be reused in different cells, as long as those cells are far enough apart so that interference at the same frequency does not occur.

This is implemented in FDMA by conventional frequency planning means, whereby the number of frequencies corresponds to the reuse cluster. In principle, some interference is obtained from outside the cluster 25, but due to the long propagation delay, the orthogonality condition is not valid and the interference is reflected as a conventional CDMA multipurpose interference, i.e. it is close to white noise.

. Conventional CDMA uses detection gain against 30 multi-use interference. In the method according to the invention, the multi-use disturbance does not occur except for disturbances coming from outside the re-use cluster.

Thus, in theory, detection gain is only needed to overcome the thermal noise and the above-mentioned interference. On the other hand, the propagation delay spread control, i.e. the cell size limitation, can be mitigated if the performance of the system is good due to, for example, high detection gain.

Figure 6 shows a simplified structure of a base station receiver operating by the method according to the invention. For simplicity, the figure includes two receiver units 61a and 61b. In an actual base station, the number of receivers is much larger. The receiver typically includes a filter 62a, 62b, a first multiplier 63a, 63b, a second multiplier 64a, 64b, a radio frequency oscillator OSC1, 0SC2, and a demodulator 66a, 66b, and means for generating a spreading code 65a, 65b. For clarity, only one spreading code demodulator unit 64-66 is marked on the pattern receivers. The actual CDMA receiver generally has a plurality of such spreading code demodulator units for each connection (CDMA channel), so that a plurality of multipath components can be received and summed in a manner typical of CDMA. In addition, multiple CDMA channels are typically transmitted on the same carrier, each with its own set of spreading code demodulator units 20, to all of which the output signal of the common multiplier 63 is distributed.

Figures 7a and 7b show a simplified structure of two transmitters operating by the method according to the invention. The transmitter typically includes a filter "72a, 72b, a first multiplier 73a, 73b, a second multiplier 74a, 74b, a radio frequency oscillator 0SC1", an OSC2 ', and means for generating a spreading code 75a, 75b. and a spreading code unit 75, the output signal of which is input to a common multiplier 73.

It is assumed that the receiver 61a receives the signal from the mobile station transmitter 71a and the receiver 61b 35 receives the signal from the mobile station transmitter 71b.

134819 The transmitter 71a, 71b receives a user data signal 76a, 76b which is multiplied by a user spreading code in the multiplier 74a, 74b. The output signal of the multiplier 74a, 74b is modulated by the radio frequency signal from the oscillator 0SC1 ', 0SC2' in the multiplier 73a, 73b. This modulated signal is applied by a bandpass filter 72a, 7a2b filtered to the antenna 70a, 70b. The radio frequencies of oscillators 0SC1 and OSC2 are selected to zero of the cross-path inaccuracy functions of the spreading codes used in a manner specific to the invention. For the sake of example, assume that each of the connections described above is on a different frequency. In a real situation, there are, of course, multiple users on the same frequency. Denote the transmission frequency f0 of OSC1 ', and the transmission frequency f0 of OSC2'. Then, according to the invention, the frequencies of OSC1 'and OSC2' differ from each other by a bit frequency, and their spectra are almost overlapping according to Fig. 5b.

The signal received at the receivers 6a and 61b, respectively, after filtering by the bandpass filters is first multiplied by the radio frequency signal obtained from the oscillator OSC'1, OSC2, and then by the user spreading code in the multiplier 63a, 63b, after which the data can be demodulated. The frequencies of the oscillators OSC1 and OSC2 differ from each other by 25 bit as described above.

The accompanying drawings and the accompanying description are intended to illustrate the present invention only. The details of the method according to the invention may vary within the scope of the appended claims.

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Claims (7)

94819 A data transmission method using combined CDMA / FDMA multipurpose, characterized in that A method according to claim 1, characterized in that a transmission burst comprising a certain number of data bits or symbols is transmitted in the transmission, which is spread modulated by one code member (S {1) of the complementary CDMA code, followed by a transmission pause, the next burst is transmitted , which is modulated by another member of the 20 complementary CDMA codes, followed by a transmission pause, etc. Method according to claim 2, characterized in that the length of the transmission pause is TM + TR + TA, where TH is the multipath difference, T „is the intra-network signal propagation delay difference and TA is the inaccuracy of the users' time synchronization. Method according to Claim 1, characterized in that several groups of interleaved frequencies are used. A method according to claim 1, characterized in that the frequency difference between the interleaved frequency groups used is at least equal to the bandwidth of the transmitter filter. Complementary codes are used as CDMA spreading codes, and that interlaced frequencies are used in at least some of the frequency channels, the frequency difference between which is selected from the zero points of the auto- and cross-correlation functions calculated as a function of the frequency-10 deviation of the CDMA spreading codes used on the frequency channels, and 6. A CDMA / FDMA radio system with a plurality of 35 frequency channels, characterized in that the system-:. The CDMA spreading codes used in OilU 1 ii ^> 94819 are complementary codes, and that at least some of the frequency channels are interleaved frequencies with a frequency difference between the frequency deviation functions of the CDMA spreading codes used as frequency channels, is synchronized with each other. Radio system according to Claim 6, characterized in that the system has a plurality of groups of 10 interleaved frequencies. 3 · • I ie 94819