JPH0897749A - Quadrature spread spectrum communication system and code division multiple access system - Google Patents

Abstract

PURPOSE: To improve the transmission characteristic by using a correlation processing means with gate between a spread demodulation code and a signal in a spread code length block after the synchronization between a reception signal and a spread demodulation code is established in spread demodulation processing. and to reduce interference between spread spectrum signals. CONSTITUTION: A guard block code addition section 10 at a transmitter side adds a code of a guard time block to a modified M sequence to generate a spread spectral code. Then input information bit 1 is modulated by an information modulation section 2 and subject to spread spectrum processing at a spread spectrum modulation section 3 based on the spread code to be a spread spectrum signal 4. Furthermore, a spread spectrum code generating section 17 at a receiver side generates a spread spectrum signal for spread spectrum demodulation and a spread spectrum demodulation section 15 applies gated correlation processing to the signal and a received spread spectrum signal 14 by a spread spectrum demodulation section 15 to attain spread spectrum modulation without interference. Then an information demodulation section 16 demodulates the information to provide an output of output bit information 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal spread spectrum communication system and a code division multiple access to which the orthogonal spread spectrum system is applied to reduce mutual interference between spread spectrum signals in a spread spectrum communication system and improve transmission characteristics. About the method.

[0002]

2. Description of the Related Art In a code division multiple access (CDMA) system using a spread spectrum (SS) communication system, transmission characteristics are deteriorated due to mutual interference between spread spectrum (SS) signals, and channel capacity is limited. The orthogonal SS communication method is used as one of the measures. A conventional orthogonal SS communication system generally uses an orthogonal code composed of a Walsh function as a spread spectrum code, but in addition to this, an M sequence with DC bias (modified M
(Sumiyoshi, Tanimoto, Komai: "Theoretical Study of Synchronous Spread Spectrum Multiplexing", IEICE Technical Report, CS81-11, April 1981). In addition, a method using Manchester coded orthogonal sequences (Habuchi, Hasegawa, Hakura, Haneishi: “Code division multiplexing method using Manchester coded orthogonal sequences”, IEICE Transactions, B-II, J74-B-II, No. 5, May 1991). However, the method using the orthogonal code composed of the Walsh function and the Manchester encoded orthogonal sequence is generally orthogonal only in a specific synchronization state.

An outline of an orthogonal SS communication system using a modified M-sequence will be described. The autocorrelation function Q (τ) of the M sequence of code length N is as shown in FIG. 1, and the size of the side lobe is −1 / N. However, if an appropriate DC component α is added to the transmitted and received M-sequence, the side lobe of the autocorrelation function R (τ) can be made completely zero as shown in FIG. Here, α is a document (Sumiyoshi, Tanimoto, Komai: "Theoretical study of synchronous spread spectrum multiplexing communication", IEICE Technical Report,
CS81-11, April 1981), it is a function of code length N. FIG. 2 shows an example of the modified M series when N = 15 and α = 0.2. In the figure, an example of the modified M-sequence is shown by a numerical sequence and its time waveform, and is shown in a simplified manner by delimiting a temporal minimum constituent unit (chip) of the sequence.

In such a method of assigning a phase-shifted version of the modified M-sequence as a spread code of each SS signal, no mutual interference occurs between the SS signals (the spread spectrum codes are orthogonal), so that the SS signals are not synchronized. It is expected that the signals will be mutually orthogonal. However, considering even the modulation by the information bits, the SS signals are mutually orthogonal only in the synchronous state.

[0005]

The conventional orthogonal SS communication system has a problem that the form of use is limited when synchronization can be easily established because the characteristic deterioration due to the loss of orthogonality is significant in the asynchronous state. Therefore, it is an object to realize an SS communication scheme that is orthogonal even in an asynchronous state, and to remove restrictions on a usage form. Another object is to realize an orthogonal code division multiple access system by using an orthogonal SS communication system.

[0006] In order to achieve the above goal, the cause of the loss of orthogonality in an asynchronous state in an SS communication system using a modified M-sequence will first be clarified. FIG. 3 shows the state of the correlation output with respect to the timing of the spread demodulation code when the SS signal is synchronous (spread code and information bits are synchronous) and asynchronous. On the left side of the figure, the timing relationship between the information bits of the desired SS signal S (t) and the undesired SS signal U (t) and the spreading code is shown. In the figure, each signal is not shown by its waveform, but is shown by dividing it into bits and chips for simplification, and the spreading codes C _D (t) and C _U (t) of the desired SS signal and the undesired SS signal are shown. One cycle is shown separated by a solid line.
Here, each spreading code is a modified M-sequence code relatively phase-shifted as described above. It should be noted that one cycle of the spreading code and the bit interval match, and the spreading code length T _C and the bit interval T _B are equal. Also, the chip serving as the phase reference of the modified M series is shown in black, and the phase difference (time difference) is shown as T _P. On the right side of the figure, the correlation output between each SS signal and the spreading demodulation code C _D (t) is shown with respect to the relative timing with the spreading code. It should be noted that the correlation values other than the point where the autocorrelation peak value and the correlation value are 0 in the figure are conceptual and not necessarily accurate. Further, the hatching of the correlation output conceptually indicates that the correlation value differs depending on the preceding and succeeding bits (bit polarity) and the like (there is a range of values).

In FIG. 3, desired SS and spreading code C _D (t)
The correlation with and takes a peak value at the spread code synchronization timing (t _S = 0). In addition, interference S in the case of bit synchronization
The correlation between S and C _D (t) becomes 0 at t _S = 0. On the other hand, the correlation in the case of bit asynchronous (synchronization timing error t _D ) does not become 0 at t _S = 0. However, when there is no modulation by information (inversion of the polarity of the spreading code), even if a timing error occurs, the correlation becomes 0 at t _s = 0. Therefore, the loss of orthogonality in the direct sequence SS system using the modified M sequence is an effect of partial correlation between the preceding and subsequent bits and the spread code, and can be interpreted as a kind of inter-code interference. The loss of orthogonality due to such causes
This is a problem of the SS communication method using the modified M series.

The present invention devises a method of selecting a spread spectrum code and a technique of despreading a spectrum to remove such a cause of loss of orthogonality, thereby enabling an SS communication system using a modified M sequence to be asynchronously performed. The purpose is to make it orthogonal. It is another object of the present invention to orthogonalize the code division multiple access system in a quasi-synchronous state.

[0009]

In order to achieve the above object, in the orthogonal spread spectrum communication system of the present invention, (a) the spread code length is not set equal to the bit interval, but the bit interval is set to the spread code length. And (b) extending the information signal and extending the spreading code by periodic repetition of the spreading code instead of making the guard interval a non-signal interval. (C) third means for selecting and allocating a spread code group having a larger phase shift amount than the synchronization timing error of the spread spectrum signal from the modified M sequence, and (d) receiving in the spread demodulation. After establishing synchronization between the signal and the spread demodulation code, the spread demodulation code and fourth means for performing correlation processing (correlation processing with a gate) in the spread code length section can be used in an asynchronous state. It was also realized orthogonalized in.

Further, in the code division multiple access system of the present invention, (a) the maximum value of the round-trip propagation time between the base station and the mobile station is set as the synchronization timing error, and accordingly, the guard interval is set and the spread code is set. First means for selecting a group and setting a range of gated correlation processing; and (b) S for mobile station transmission.
Second means for making the spread code timing of the S signal dependent on the spread code timing of the code division multiplexed signal of the mobile station reception at the base station transmission, and (c) establishing synchronization for the spread demodulation at the base station reception. , Third means for realizing by finely adjusting the timing with reference to the transmission timing by utilizing the fact that the timing of the code division multiple access signal is in a quasi-synchronous relationship with the timing of the base station transmission. ,
Orthogonalization is realized in quasi-synchronous state.

[0011]

When the setting of the guard section and the correlation processing with the gate are introduced, the state of the cross-correlation with the spread code when the SS signal is synchronous and asynchronous is as shown in FIG. The notation in the figure is almost the same as that in FIG.
The timing relationship of the S signal is shown. On the left side of the figure, one cycle of the spreading code is shown by a thick solid line and the bit section is shown by a thin solid line. Where the bit interval T
_B is set to be longer than the spreading code length T _C , and the difference is the guard interval (time T _G ). The guard section is not a non-signal section but a section where the information code and the spreading code are extended. In addition, it is assumed that the spread code is subjected to periodic repetition. On the other hand, the right side of the figure shows the state of the correlation output with the spread demodulation code in the spread demodulation. In this spread demodulation, a spread demodulation code and a correlation process (gated correlation process) are performed in the _TC section. Note that a similar technique is to introduce a guard interval (B.Floch, R.Lassallele) for delay countermeasures in an orthogonal multicarrier system (OFDM) that performs demodulation by Fourier transform (FFT).
and D.Castelain, "Digital sound broadcasting to mob
ile receivers, "IEEE Trans. Consum. Electro., Vol.
35, No. 3, Aug. 1989.), but there is no example used for spread demodulation of spread spectrum signals.

Desired SS and spreading code shown on the right side of FIG.
The correlation with C _D (t) is the synchronization timing (t _S =
The peak value is taken at 0). Further, the correlation between the interference SS and _CD (t) in the case of bit synchronization is 0 at t _S = 0 and its periphery (± T _G section). On the other hand, the correlation is 0 at t _S = 0 when the synchronous timing error t _D is smaller than the guard time T _G even in the case of bit asynchronous. Although the orthogonalization is possible in most cases by the above means, the phase difference between the spreading codes of the undesired SS signal and the desired SS signal is canceled by the synchronization timing error, and when the spreading codes of both are synchronized, The correlation becomes very large. This phenomenon can be avoided by selecting a spreading code group in which the mutual phase shift amount is equal to or more than the synchronization error from the modified M sequence.

Another problem is that although orthogonalization is possible, a large correlation output with unsynchronized undesired SS signals makes it difficult to synchronize with desired signals when the number of multiplexed SS signals increases. The issue of becoming (Sumiyoshi, Tanimoto, Komai:
"Theoretical Study of Synchronous Spread Spectrum Multiplexing System",
IEICE Technical Report, CS81-11, April 1981
Mon) is pointed out. However, this problem is not a problem of spreading code synchronization purely, but a problem of channel identification or wireless station identification that is integrated with synchronization in the normal SS system. Therefore, it can be dealt with as a problem of channel identification.

Further, if the synchronization timing error is large, it is necessary to set a large guard time, and there is a problem that the number of usable orthogonal spreading codes decreases. For this reason,
From a practical point of view, the range in which the proposed method is effective is that it is used in a bit quasi-synchronous state (a state where the synchronization error is sufficiently smaller than the bit interval) and is applied to a system in which the bit quasi-synchronous state can be easily realized. Limited. However, in a satellite communication system and a mobile communication system, such a problem can be solved by a method in which a mobile station performs SS transmission in a subordinate synchronization with an SS signal transmitted by a base station.

[0015]

FIG. 5 is a block diagram showing an embodiment of an orthogonal spread spectrum communication system according to claims 1 and 2 of the present invention. The figure is
The configuration on the transmission side (spread spectrum signal generation method) and the configuration on the reception side (spread spectrum signal demodulation method) are shown.

On the transmitting side, the code timing generator 6 generates the spread code timing signal 11 from the data clock 5, and the spread code clock generator 7
Generates a clock signal 12 for the spreading code. With the timing signal 11 and the clock signal 12 as the time reference, the spread code phase setting unit 8 sets the phase shift amount of the spread code, and the modified M sequence generation unit 9 generates the modified M sequence. Next, the guard interval code adding section 10 generates a spread spectrum code in which the code of the guard time interval is added to the modified M sequence. On the other hand, the input information bit 1 is information-modulated by the information modulator 2 and then spread-spectrum spread by the spread-spectrum modulator 3 to be a spread-spectrum signal 4. Although the configuration on the transmitting side shows generation of a desired SS signal, non-desired SS signals generated by the same configuration are combined to become a reception signal.

On the other hand, on the receiving side, the synchronization timing detector 20 detects the desired SS from the received spread spectrum signal 14.
After the timing synchronized with the spread code of the signal is detected and the timing control section adjusts the timing based on this timing, the code clock timing generation section 18
Generates a clock signal. Next, a spread spectrum signal for spread demodulation is generated by a spread spectrum code generation section 17, and the spread demodulation section 15 performs a gated correlation process between the spread spectrum signal and the received spread spectrum signal 14 so that no mutual interference occurs. Performs spread demodulation. Here, a complex correlation is taken as the correlation, and processing for removing the influence of the carrier phase is performed. The non-desired SS signal maintains orthogonality regardless of the carrier phase difference from the desired SS signal. afterwards,
The information demodulation unit 16 demodulates information and outputs the output bit information 22
Is output.

In the method shown in FIG. 5, when establishing synchronization with the desired SS signal, there is a possibility that, in some cases, erroneous synchronization with a non-desired SS signal having a spreading code with a small mutual phase difference other than the desired SS signal may occur. Therefore, the channel identification unit 21 performs channel identification based on the output bit information, specifically, detects false synchronization, and based on the result, the timing control unit 19 adjusts the timing of the spread demodulation code until the synchronization becomes normal. The synchronization with the desired SS signal is established.

Next, an embodiment of the orthogonal spectrum communication system according to claim 3 of the present invention, in which the spread spectrum communication system is applied as a countermeasure against multipath delay distortion, will be described. Originally, the direct spread SS method was excellent in multi-wave resistance and capable of path diversity, but had a problem of low spectrum utilization efficiency. A spreading code is selected in consideration of a delay time difference in multiplex wave propagation, and code division multiplexing is performed. As a result, the orthogonality of the code-division-multiplexed spread spectrum signal can be maintained in the multipath propagation environment, and the spectrum utilization efficiency can be improved.

Next, an embodiment of a code division multiple access system according to claim 4 of the present invention in which the orthogonal spectrum communication system is applied to a mobile communication system will be described.

First, FIG. 6 shows an overall configuration of a system assumed when the code division multiple access system is applied to mobile communication. The assumed system is composed of a base station and a number of mobile stations. The downlink (base station to mobile station) is code division multiplexed (CDM) in a synchronized state, and the uplink (mobile station to base station) is in a semi-synchronized state. Operates in code division multiple access (CDMA). As a CDMA spreading code, a spreading code group created by phase shift of a modified M sequence is used. In another cell, a spreading code group created from another modified M sequence is used. On the other hand, the transmission timing of the spreading code of the mobile station is synchronized with the CDM reception signal timing.
The timing error between CDMA signal of the base station reception becomes less than or equal to the maximum value T _MAX of the propagation time of the round trip between the base station and the mobile station. It is assumed that bit quasi-synchronization, that is, the round-trip propagation time T _MAX is sufficiently larger than the bit interval T _B. Hereinafter, the configuration of the transmission / reception system for the uplink operated in quasi-synchronization will be described.

FIG. 7 shows a block diagram of a transmission system of a mobile station for CDMA which is orthogonal in quasi-synchronization. In the transmission system, information modulation is performed from the input information bit 1 by the information modulator 2, a spread code is generated by the spread spectrum code generator 17, and a spread spectrum signal is generated by the spread spectrum modulator 3. The timing of the spread code generated by the code clock generator 18 is controlled by the reception code clock timing 25. The generated SS signal is optionally combined with the SS signal generated by another system of spread spectrum signal generation unit and the multiplexing unit 24.
There is also a form that is multiplexed and transmitted in.

In the spreading modulation shown in FIG. 7, a guard interval is set and a spreading code group is selected so that orthogonality can be maintained with respect to a timing error at the time of receiving a base station. Timing error between CDMA signals received by the base station is 2
FIG. 8 shows the setting of the guard section and the selection of the spreading code group in the case of within the chip. In the figure, the modified M sequence length is 3
It is set to 1. The spreading code group is selected from M sequences having a mutual phase shift of 3 chips or more, and the number of codes is 10. The setting of the timing error within two chips is sufficiently realistic. For example, in the case of using a direct spreading SS system with a chip speed of 1 M chip / s in a micro cell having a cell radius of 300 m (maximum propagation delay of 1 μs), the timing error is within 2 chips.

FIG. 9 shows a configuration diagram of a receiving system of a base station for CDMA which is asynchronous and orthogonal. In the base station, the received spread spectrum signal is distributed by the signal distribution unit 26 and supplied from each mobile station to the spread spectrum reception unit 27 prepared for each of the multiple access SS signals. The synchronization in spread demodulation takes advantage of the fact that the timing of each received signal is in a semi-synchronous relationship with the transmission clock timing 28 of base station transmission,
It is realized by the timing control unit 19 by finely adjusting the timing. In some cases, a method may be considered in which the synchronization timing detection unit 20 separately detects the synchronization of the spread code.

The received SS signal is subjected to a gated correlation process with a spread demodulation code by a spread demodulation unit 15 and spread demodulation is performed without mutual interference. Then, the information demodulation unit 1
In step 6, information demodulation is performed, and output bit information 22 is output.
As a measure against erroneous synchronization with an undesired SS signal, the channel identification unit performs channel identification based on the output bit information, and the timing control unit 19 adjusts the timing of the spreading demodulation code according to the result, and synchronizes with the desired SS signal. .

The above-described embodiment of the quasi-synchronous and orthogonal code division multiple access system has been applied to a cellular system in which orthogonality is achieved in the same cell. Next, an embodiment of the code division multiple access system according to claim 4 of the present invention in which orthogonality between adjacent cells is maintained will be described. By establishing synchronization between SS communication signals transmitted from adjacent base stations, a quasi-synchronous state can be realized in the downlink, that is, in the mobile station reception. Also in the uplink, a quasi-synchronous state can be realized by making the spread code timing of the SS signal transmitted by the mobile station dependent on the spread code timing of the mobile station reception by the base station transmission. Therefore, the synchronization timing error is set to be larger than the maximum value of the round-trip propagation time difference between the base station and the mobile station, and accordingly, the setting of the guard section, the selection of the spreading code group, and the setting of the range of the gated correlation processing are performed. By doing so, orthogonalization can be realized even between adjacent cells.

Next, another embodiment of the code division multiple access system according to claim 4 of the present invention in which orthogonality between adjacent cells is maintained. When orthogonality is maintained between adjacent cells, the same desired SS signal is transmitted from the adjacent cells, and each signal is independently demodulated and diversity-combined, so that path diversity or site diversity can be reduced by mutual interference. It can be realized without getting up.

[0028]

By means of the orthogonal spread spectrum communication system of the present invention, mutual interference between spread spectrum signals can be reduced without presupposing that the signals are synchronized with each other. It is possible to improve. In addition, a countermeasure against a multipath delay distortion by the spread spectrum communication method can be realized without significantly impairing the spectrum use efficiency.

On the other hand, the code division multiple access method of the present invention can be expected to increase the channel capacity of a CDMA cellular system in the mobile communication field. Further, unlike the non-orthogonal conventional method, the "far problem" is negligible, so that the burden of transmission power control can be greatly reduced. Furthermore, it becomes easy to realize a quasi-synchronous and orthogonal system. In addition, it is possible to realize a system orthogonal to adjacent cells and to effectively realize site diversity.

[Brief description of drawings]

FIG. 1 is a diagram of an autocorrelation function of an M-sequence code and a modified M-sequence code related to the description of the related art.

FIG. 2 is a diagram showing an example of a modified M sequence related to the description of the conventional technique.

FIG. 3 is a diagram showing a state of loss of orthogonality due to bit asynchronization in a modified M sequence related to the problem of the conventional technique.

FIG. 4 is a diagram showing a countermeasure against loss of orthogonality in bit asynchronous operation according to the means for solving the problem of the present invention.

FIG. 5 is a configuration diagram of a transmitting side and a receiving side according to an embodiment of claims 1 and 2 of the present invention.

FIG. 6 is a diagram of a micro cell CDMA cellular system which is supposed to be applied in an embodiment of claim 3 of the present invention.

FIG. 7 is a configuration diagram of a CDMA transmission system of a mobile station according to an embodiment of the present invention.

FIG. 8 is a diagram showing an example of setting a guard interval and selecting a spreading code according to an embodiment of claim 3 of the present invention.

FIG. 9 is a configuration diagram of a CDMA receiving system of a base station according to an embodiment of the present invention.

[Explanation of symbols]

 1 Input Bit Information 2 Information Modulation Section 3 Spread Spectrum Modulation Section 4 Spread Spectrum Signal 5 Clock Signal 6 Code Timing Generation Section 7 Spread Code Clock Generation Section 8 Spread Code Phase Setting Section 9 Modified M Sequence Generation Section 10 Guard Time Section Code Addition Section 11 timing signal 12 clock signal 13 spread spectrum code generation unit 14 received spread spectrum signal 15 spread demodulation unit 16 information demodulation unit 17 spread spectrum code generation unit 18 code clock generation unit 19 timing control unit 20 synchronization timing detection unit 21 channel identification unit 22 Output bit information 23 Spread spectrum signal generation unit 24 Multiplexing unit 25 Reception code clock timing 26 Signal distribution unit 27 Spread spectrum reception unit 28 Transmission code clock timing

Claims (5)

[Claims] 1. A modified M sequence, that is, a sequence in which a side lobe of an autocorrelation function is completely zero by adding a DC bias to the M sequence in order to reduce mutual interference between spread spectrum signals. In the orthogonal spread spectrum communication system using as a spreading code, (a) first means for introducing a guard interval by making the bit interval longer than the spreading code length rather than setting the spreading code length equal to the bit spacing. And (b) second means for extending the information signal and extending the spread code by periodically repeating the spread code instead of setting the guard section as a no-signal section, and (c) synchronization timing of the spread spectrum signal. Third means for selecting and allocating a spreading code group having a mutual phase shift amount larger than an error from a modified M sequence, and (d) spreading demodulation,
By using the spread demodulation code and the fourth means for performing correlation processing (correlation processing with gate) in the spread code length section after establishing synchronization between the received signal and the spread demodulation code, orthogonality is achieved even in an asynchronous state. An orthogonal spread spectrum communication method characterized by maintaining the property. 2. A modified M sequence, that is, a sequence in which a side lobe of an autocorrelation function is completely zero by adding a DC bias to the M sequence in order to reduce mutual interference between spread spectrum signals. In the orthogonal spread spectrum communication system using as a spreading code, (a) first means for performing channel identification from the information demodulated signal to detect false synchronization, and (b) timing of the spreading demodulation code in the case of false synchronization. 2. The orthogonal spread spectrum communication system according to claim 1, wherein synchronization between the spread spectrum demodulation code and the desired spread spectrum signal in spread spectrum demodulation is correctly established by using the second means for fine adjustment. 3. In an orthogonal spread spectrum communication system used as a countermeasure against multipath delay distortion, (a) first means for selecting a spread code group in consideration of a delay time difference due to multipath propagation, and (b) selection. And a second means for performing spread spectrum modulation of the information signal sequence by each spread code of the spread code group and multiplexing the generated orthogonal spread spectrum signal, thereby overcoming the disadvantage of low spectrum utilization efficiency. The orthogonal spread spectrum communication system according to claim 1, wherein the information transmission rate per used band is improved. 4. The orthogonal spread spectrum communication system according to claim 1 or 2 is applied to a mobile communication system composed of a base station and a large number of mobile stations, and a downlink from the base station to the mobile stations is synchronized. In the code division multiple access system in which the uplink from the mobile station to the base station is operated in a semi-synchronous state, (a)
Set the maximum value of the round-trip propagation time between the base station and the mobile station to the synchronization timing error, and accordingly set the guard interval,
A first means for selecting a spread code group and setting a range of gated correlation processing, and (b) a spread code timing of an SS signal transmitted by a mobile station, a base station transmission of a code division multiplexed signal received by a mobile station. The second means that is dependent on the spread code timing and (c) the establishment of synchronization for spread demodulation at the base station reception have a quasi-synchronous relationship with the timing of the code division multiple access signal and the transmission timing of the base station. And a third means for realizing fine adjustment of the timing based on the transmission timing by utilizing the above, and the orthogonality is maintained in the quasi-synchronized state. . 5. The orthogonal spread spectrum communication system according to claim 1 or 2 is applied to a cellular system composed of a plurality of base stations and a large number of mobile stations, and a downlink from the base stations to the mobile stations is synchronized. In the code division multiple access system that operates the uplink from the mobile station to the base station in a semi-synchronous state,
(A) First means for synchronizing the spread spectrum signals transmitted from the adjacent base stations with each other, and (b) Synchronization timing based on the maximum value of the round-trip propagation time difference between the base station and the mobile station including the adjacent cells. Second means for setting a large error and accordingly setting a guard interval, a spreading code group and a range of gated correlation processing, and (c) an adjacent cell from the selected spreading code group 5. The code division multiple access method according to claim 4, wherein orthogonality is maintained in a quasi-synchronized state including adjacent cells by using a third means for allocating spreading codes.