JPH06152487A - Radio communication system - Google Patents

Abstract

PURPOSE:To provide a radio communication system in a code dividing multiplex (CDMA) system which can increase the number of communication channels by using the limited number of spreading codes. CONSTITUTION:Between a center station and plural terminal stations, communication is performed by using the CDMA. The center station respectively spreads plural information sequences S1-Sn at multipliers 11-1n while using a common spreading code from a spreading code generating circuit 10. These spreading codes are passed through transmission timing adjusting circuits 21-2n and synthesized by a synthesizer 30. Therefore, transmission timing offset multiplexing is performed, and the signals are transmitted to the plural terminal stations at the respectively different kinds of transmission timing. At each terminal station, only one signal transmitted to the present station at the previously decided timing is received among the signals transmitted from the center station and inversely spread by using the same spreading code as the spreading code used for the transmitter of the center station, and the source information sequence is reproduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wireless communication system for performing communication between a central station and a plurality of terminal stations using code division multiplexing (CDMA).

[0002]

2. Description of the Related Art A code division multiplexing (CDMA) system is a communication system in which an information sequence to be transmitted under the same frequency is spread by different spreading codes for each channel and multiplexed, for example, Document 1: Jatec Publishing. The detailed configuration and functions are described in "Spread spectrum communication system" by RCDixon. Here, an outline of a method called direct sequence CDMA will be described.

FIG. 7 shows the configuration of a transmitter in a typical CDMA communication system. n information sequences S1, S
, ..., Sn are input to the multipliers 11, 12, ..., 1n,
Spread codes C1, C2, ..., C from the spread code generation circuit 2
diffused by n. Multipliers 11, 12,
, 1n output signals are combined by the combiner 3 at the same timing to generate a transmission signal.

On the other hand, the signal received at the receiving side is
The same spreading codes C1, C2, ..., Used on the transmitter side
The original information series S1, S2 ..., Despread by Cn,
Sn is generated.

Here, the spreading codes C1, C2, ..., Cn are usually code sequences having good autocorrelation characteristics (where τ is the time offset between two code sequences having a correlation, a strong correlation occurs only at τ = 0. It takes a value and the correlation value is small in other cases)
And a code sequence having a good cross-correlation characteristic, that is, a code sequence having a low correlation with other spread codes (a code sequence having a cross-correlation value smaller than a certain value which is an arbitrary time offset) is selected.

[0006]

However, there are few code sequence classes that satisfy the above-mentioned requirements for spread codes.
Also, the number of spreading codes included in each class is small. Therefore, the conventional CDMA communication system in which a different spreading code is assigned to each communication channel has a drawback that the number of usable communication channels is small and it is not suitable for a wireless communication system such as a mobile communication system having a large subscriber capacity.

An object of the present invention is to provide a CDMA radio communication system which can effectively increase the number of communication channels by using a limited number of spreading codes.

[0008]

According to the present invention, in a wireless communication system for performing communication using code division multiplexing between at least one central station and a plurality of terminal stations, the central station shares a plurality of information sequences. The basic feature is to have a transmission device that spreads using the spreading code of 1 and then transmits to a plurality of terminal stations at different transmission timings.

On the other hand, the terminal station receives one of the signals transmitted from the transmitter of the central station, which has been transmitted to the terminal station at a predetermined transmission timing, and transmits the received signal. The present invention is characterized by having a receiving device that obtains an information sequence by despreading using the same spreading code as that used in the device.

Further, the present invention is characterized in that a plurality of transmitters are installed in one central station and different spread codes are used among the plurality of transmitters.

Further, according to the present invention, the central stations are installed at a plurality of points, and the same spreading code is used by the transmission devices of at least two predetermined central stations located at spatially distant points among the plurality of central stations. It is characterized by being used.

[0012]

As described above, in the CDMA radio communication system of the present invention, a plurality of information sequences are spread at the central station using the same spreading code, and each spread signal is transmitted to a plurality of terminal stations at different transmission timings. By performing transmission timing offset multiplexing in which each spread signal has a unique transmission timing offset, a number of communication channels corresponding to the number of transmission timing offsets are formed using the same spreading code. .

On the other hand, in each terminal station, among the transmitted signals, only the signal transmitted at the predetermined transmission timing to the terminal station, that is, its own station, is received,
The original information sequence is easily reproduced by despreading this using the same spreading code as that used in the transmitter.

Further, if different spread codes are used in a plurality of transmitters installed in one central station, it is possible to use these different spread codes at the same time and perform communication in the same transmission timing group. Multiple communication channels are available.

Further, by using the same spreading code in the transmitters of two central stations located at spatially distant points, so-called spatial reuse of the spreading code is performed, so that transmission timing offset multiplexing is performed. In combination with this, the communication capacity is further increased.

[0016]

【Example】

(Embodiment 1) FIG. 1 is a block diagram showing the configuration of a transmitting device provided in a central station, showing a transmitting device that simultaneously transmits to n terminal stations.

In FIG. 1, the spread code generating circuit 10 is controlled by a control unit 20 and has information sequences S1, S2, ..., S.
A spread code is generated in synchronization with n. Multipliers 11, 12,
, 1n performs a process of spreading the information series S1, S2, ..., Sn by multiplying the information series S1, S2, ..., Sn by the same spreading code supplied from the spreading code generation circuit 10. Transmission timing adjusting circuits 21, 22, ... 2
n is controlled by the control circuit 20, and the multipliers 11, 12,
The transmission timing is adjusted by giving different delay amounts (offsets) corresponding to the n terminal stations to the output signal of 1n. This operation is called a transmission timing offset. The synthesizer 30 synthesizes the output signals of the transmission timing adjusting circuits 21, 22, ... 2n to generate a transmission signal.

Next, the operation of this transmitter will be described. The information sequence S1, S2, ..., Sn to be transmitted is the multiplier 1
, 1n are spread by the same spread signal from the spread signal generating circuit 10 and are delayed by the transmission timing adjusting circuits 21, 22, ..., 2n to adjust the transmission timing, and then combined. A transmission signal that is combined in 30 and transmission timing offset multiplexed is generated.
The generated transmission signal is output from a transmission circuit unit (not shown) and transmitted to n terminal stations via a wireless line.

FIG. 2 shows an example of the above transmission timing offset. , Tn are set within a time defined by one symbol length Ts of the information sequences S1, S2, ..., Sn, and the delay amount corresponding to each reference symbol timing is set. Are provided by the transmission timing adjusting circuits 21, 22 ,. When A is the reference time, the delay amount is d1 at timing T1, d2 at T2, and dn at Tn. The time interval between the transmission timings T1, T2, ..., Tn is set to a value corresponding to the sum of the delay spread and the guard time at the time of reception at the terminal station.

FIG. 3 is a block diagram showing the configuration of a receiving device provided in each of n terminal stations for receiving the signal transmitted from the transmitting device of FIG. 1, and uses a matched filter as a correlator. It is an example. This receiving device includes a matched filter 31, a demodulator 32, a timing extraction circuit 33, and a control unit 34. Matched filter 3
1 detects the correlation between the spread code given from the control unit 34 and the received signal. The timing extraction circuit 33 extracts the reception timing of the signal to the own station from the correlation output of the matched filter 31, and the reception timing is demodulated by the demodulator 32.
To notify.

The demodulator 32 reproduces the original information sequence Si (i = 1, 2, ..., N) from the output of the matched filter 31 and the output of the timing extraction circuit 33. As a demodulator, a RAKE demodulator is typical, for example, Document 2: U.
Grob, ALWelti, E. Zollinger, R. Kung and H. Kaufmann, "
Microcellular Direct-Sequence Spread-Spectrum Radi
o System Using N-Path RAKE Receiver ", IEEE JSAC, Vo
l.SAC-8, No. 5, pp. 772-780, June, 1990.

The control section 34 gives a spread code to the matched filter 31 and gives its own station information at the timing extraction of the timing extraction circuit 33.

Next, the operation of the receiving device provided in this terminal station will be described. The received signal, that is, the signal transmitted from the transmitting device in the central station having the configuration shown in FIG. 1 and received by the receiving device shown in FIG.
Input to 1. The matched filter 31 includes a control unit 34.
Therefore, the spreading code generated by the spreading code generation circuit 10 is given in the transmitting apparatus of FIG. 1, and the despreading process is performed by taking the correlation between this spreading code and the received signal to generate the correlation output.

FIG. 4 shows an example of the correlation output of the matched filter 31, in which the horizontal axis represents time and the vertical axis represents the correlation value. As described above, the reception signal of the receiving apparatus in FIG. 3, that is, the transmission signal output from the transmitting apparatus in FIG. 1 is transmission timing offset multiplexed, so that the correlation occurs at the time positions corresponding to the reference symbol timings T1 to Tn. A place with a large value is detected.

The timing extraction circuit 33 has information relating to the reception timing of a signal transmitted from the control section 34 to the station in advance, for example, the reference time A and the time A in FIG.
From the reference symbol timing Ti (i = 1, 2, ...,
Information of the time length up to n) is known, and the reception timing of the own station is recognized from the output of the matched filter 31 according to this information. Then, the timing extraction circuit 33
When the arrival of this reception timing is detected, it is notified to the demodulator 32. The demodulator 32 reproduces the information sequence Si for the own station from the output of the matched filter 31 at the reception timing of the own station notified from the timing extraction circuit 33.

In the direct spread-code division multiplexing (DS-CDMA) system using spread codes, even if signals to a plurality of terminal stations having the same spread code sequence are transmitted at the same time, if their delay profiles overlap. Otherwise, each terminal station can capture only the profile portion to be received and reproduce the information sequence. This means that even if transmission timing offset multiplexing is performed, the information sequence of each channel can be independently demodulated and decoded.

Therefore, if transmission timing offset multiplexing is performed as in the present embodiment, one spreading code can provide as many independent communication channels as the transmission timing offset number n. As a result, it is possible to obtain a communication channel that is n times as large as the conventional one, by using a limited number of spreading codes having good autocorrelation characteristics and cross-correlation characteristics, and it is possible to realize a CDMA communication system with a large communication capacity. it can.

As the demodulator 32, for example, Reference 3: Akihiro Higashi, Tadashi Matsumoto, "BER Perform.
ance of Adaptive RAKE Diversity (ARD) in DPSK DS / CDM
It is also possible to use the ARD receiver described in "A Mobile Radio", IEICE Technical Report, SST92-16, June 1992, and Japanese Patent Application No. 4-83947 "Spread spectrum receiver". This A
When the RD receiver is used, the transmitted information sequence can be reproduced even if there is some overlap in the delay profiles, so that the transmission timing offset multiplexing can be performed at a narrower interval, and the transmission timing offset number n can be further increased. It is possible to take.

In this embodiment, a matched filter is used as the correlator, but a correlator having a combination of sliding correlators can be used as long as it can detect a plurality of reference symbol timings.

(Second Embodiment) In the first embodiment, the case where one central station uses one spreading code has been described. Therefore, according to the first embodiment, the same number of communication channels as the transmission timing offset number n can be obtained as described above.

On the other hand, in the second embodiment, a plurality of transmitters having the configuration shown in FIG. 1 are installed in one central station, and a plurality of transmitters have the same transmission timing (T1 to Tn). In addition, each transmitter uses different spreading codes. Each spreading code is a code sequence having good auto-correlation characteristics and cross-correlation characteristics as described above. In this case, the number of transmitters installed in one central station is m
Then, the number of usable communication channels will increase to m × n.

FIG. 5 shows an example of such transmission timing and spread code arrangement. In FIG. 5, four spreading codes C1 to C4 are used at n transmission timings T1 to Tn. Therefore, the number of communication channels realized is 4 × n.
Becomes

(Third Embodiment) In the first embodiment, the CD is closed to one central station and transmission timing offset multiplexing is performed.
Although the MA communication system has been described, the third embodiment considers a CDMA communication system including a plurality of central stations.

FIG. 6 shows an example of a plurality of central stations, a plurality of cells formed thereby in a multicellular mobile communication system, and transmission timings and spreading codes used in each central station. T11 to T1n are transmission timings at the central station 1, and similarly Ti1 to Tin are transmission timings at the central station i (i = 1, 2, ..., J). Here, when spreading codes having good auto-correlation characteristics and cross-correlation characteristics as described above are used, mutual interference hardly occurs between channels using different spreading codes, so that transmission is performed between the central stations i. There is no need to change the timing. That is, T11 to T1n,
The transmission timings of T21 to T2n, ..., TJ1 to TJn may partially or wholly coincide.

When the distance is spatially increased to some extent,
Since radio waves are attenuated in inverse proportion to the square of the distance in free space and the square of the distance in urban areas, even if the same spread code is used between such spatially separated points, mutual interference does not occur. The spread code can be spatially reused by utilizing this. That is, for example, as shown in FIG. 6, the central station J spatially sufficiently separated from the central station 1 can use the same spreading code as that of the central station 1.

By using the spatial reuse of spreading codes in this way, it becomes possible to use more spreading codes as the entire communication system. Therefore, by combining the spatial reuse of the spread code and the above-mentioned transmission timing offset multiplexing, the number of usable communication channels can be further increased, and a CDMA communication system with a very large communication capacity can be realized. .

The present invention can be implemented by combining the configurations of the first and second embodiments.

[0038]

As described above, according to the present invention, a plurality of information sequences are spread by using a common spreading code, and then transmission timing offset multiplexing is performed and transmitted to a plurality of terminal stations. As a result, it is possible to form independent communication channels for the number of transmission timing offsets. Therefore, even if a code sequence including only a small number of spreading codes having good autocorrelation and cross-correlation characteristics is used, a communication channel having several times the transmission timing offset can be obtained, so that the communication capacity is large and the channel selection A CDMA communication system with a high degree of freedom can be realized.

Further, a plurality of transmitters are installed in one central station, and different spread codes are used in each transmitter, so that these different spread codes are used at the same time for communication in the same transmission timing group. Therefore, it is possible to obtain (the number of transmitters × the number of transmission timing offsets) communication channels.

Furthermore, by combining transmission timing offset multiplexing and spatial reuse of spreading codes, it is possible to realize a CDMA communication system having a larger communication capacity and a higher frequency utilization efficiency.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a transmission device installed in a central station according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a transmission timing offset in the embodiment.

FIG. 3 is a block diagram showing a configuration of a receiving device installed in a terminal station in the embodiment.

FIG. 4 is a diagram showing an example of a correlation output of the matched filter in FIG.

FIG. 5 is a diagram showing an example of arrangement of transmission timings and spread codes when a plurality of spread codes are used in one central station according to another embodiment of the present invention.

FIG. 6 is a diagram showing an arrangement example of a plurality of central stations, a plurality of cells formed by the central stations, transmission timings of the respective central stations, and spread codes in another embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a transmission device in a conventional CDMA communication system.

[Explanation of symbols]

10 ... Spread code generation circuit 11-1n ... Multiplier 20 ... Control unit 21-2n ... Transmission timing adjustment circuit 30 ... Synthesizer 31 ... Matched filter 32 ... Demodulator 33 ... Timing extraction circuit 34 ... Control unit

Claims (4)

[Claims] 1. A wireless communication system for performing communication using code division multiplexing between at least one central station and a plurality of terminal stations, wherein the central station spreads a plurality of information sequences with the same spreading code. After that, the wireless communication system is provided with a transmitting device for transmitting to the plurality of terminal stations at different transmission timings. 2. A radio communication system for performing communication using code division multiplexing between at least one central station and a plurality of terminal stations, wherein the central station uses a same spreading code for a plurality of information sequences. After spreading, it has a transmitter that transmits to the plurality of terminal stations at different transmission timings, and the terminal station is predetermined for the terminal station among signals transmitted from the transmitter apparatus of the central station. A receiving device that receives one signal transmitted at different transmission timings, despreads the received signal using the same spreading code as that used in the transmitting device, and obtains an information sequence. Wireless communication system. 3. The wireless communication system according to claim 1, wherein the central station has a plurality of transmitters and uses different spreading codes among the plurality of transmitters. 4. The central station is installed at a plurality of points, and the same spreading code is used in transmitting devices of at least two predetermined central stations located at spatially distant points among the plurality of central stations. The wireless communication system according to claim 1, 2 or 3, characterized in that.