CODE DIVISION MULTIPLEX ACCESS MOBILE COMMUNICATION SYSTEM

Abstract: A mobile communication system for communication by a code division multiplex access (CDMA) method between a base station and a plurality of mobile stations, is provided with a transmitter whereby the base station diffuses a plurality of information series (S1 to Sn) by multipliers (11 to 1n) by use of the common diffusion codes from a diffusion code generating circuit (10), and these diffused codes are combined by a synthesizer (30) through transmission timing adjusting circuits (21 to 2n) to effect the transmission timing offset multiplexing and to transmit synthesized signals to a plurality of mobile stations at different transmitting timings. Each mobile station is provided with a receiver which receives signals transmitted from the transmitter of the base station, and inversely diffuses the received signal by use of the same diffusion code as the one used by the transmitter to reproduce the original information series.

Description: TECHNICAL FIELD The present invention relates to a mobile communication system which utilizes a code division multiple access (CDMA) system for communications between a base station and a plurality of mobile stations. PRIOR ART The code division multiple access (CDMA) system is a communication system which multiplexes information sequences to be transmitted at the same frequency, by spreading them with different spreading codes for respective channels; for example, in R.C. Dixon, "Spread Spectrum Communication System," published by Jatec, there are described in detail its system configuration and capabilities. Now, a brief description will be given of a system called a direct sequence CDMA system. Fig. 7 shows the construction of a transmitting device in a typical CDMA communication system. n information sequences S1, S2, ..., Sn are input into multipliers 11, 12, ... 1n, wherein they are spread by spreading codes C1, C2, ..., Cn from a spreading code generator circuit 2, respectively. The output signals from the multipliers 11, 12, ..., 1n are added by an adder 3 at the same timing to generate a transmission signal. On the other hand, the signal received at the receiving side is despread by the same spreading codes C1, C2, ..., Cn as those used in the transmitting device, by which the original information sequences S1, S2, ..., Sn are reconstructed. As the spreading codes C1, C2, ..., Cn, code sequences are usually chosen which are excellent not only in self-correlation characteristic (letting the time offset between two correlating code sequence be represented by τ, the correlation value is large for τ = 0 and small for other values of τ) but also in mutual-correlation characteristic, that is, low in the correlation with other spreading codes (code sequences whose mutual-correlation value is smaller than a certain value which is offset for any given time). On the other hand, there has been proposed a scheme of repeatedly using the same spreading code in a plurality of cells in an application of the CDMA system to a multi-cellular mobile communication system (Pat. Pub. No. 56290/83). It is expected that this scheme will make it possible to increase the number of communication channels and the system capacity. However, very few classes of code sequences satisfy the above-mentioned requirements for spreading codes and the number of spreading codes in each class is also small. Accordingly, the conventional CDMA communication system which assigns a different spreading code to each communication channel is inevitably limited in the number of channels available for communication, and hence is not suitable for use as a radio telecommunication system of large channel capacity such as a mobile radio communication system. With the scheme of reusing the same spreading code in a plurality of cells in the multi-cellular mobile communication system, the reuse of the same spreading code in adjacent cells, not in cells sufficiently far apart, would degrade the channel or speech quality owing to interference or interactions, thus imposing limitations on the increase in the channel capacity. The document EP-A-0 486 834 discloses the use of different delay times. Plural different delay times (corresponding to transmitting timings) are applied to respective spread sequences which have been spread by a single common spreading code in one base station. The document WO-A-92 17989 discloses a wireless communication system including a repeated pattern of cells in which base stations and mobile stations for each cell may be assigned a spread-spectrum code for modulating radio signal communication in that cell. Radio signals used in that cell are spread across a bandwidth sufficiently wide that both the base station and the mobile stations in an adjacent cells may distinguish communication which originates in one cell from another. Adjacent cells may use distinguishable frequencies and distinguishable codes but it is sufficient if adjacent cells use distinguishable frequencies and identical codes. A repeated pattern of cells allows the codes each to be reused in a plurality of cells. The document VEHICULAR TECHNOLOGY SOCIETY, 42nd VTS CONFERENCE FRONTIERS OF TECHNOLOGY, vol.2, May 1992, Denver, US, pp. 736-739: J. Ruprecht "Code rime Division Multiple Access: an Indoor Cellular System" teaches the allotment of spreading codes to a plurality of cells and determination of offset times for a respective cell depending on the size of the cell. In this prior art, however, a plurality of cells are allotted different spreading codes. SUMMARY OF THE INVENTION An object of the present invention is to provide a CDMA mobile communication system which makes it possible to effectively increase the number of communication channels through use of a limited number of spreading codes. In accordance with the present invention, the base stations in a plurality of cells use the same set of spreading codes for spreading information sequences and each base station uses a transmitting timing different from those used by the base stations of other cells. It is possible to transmit a plurality of information sequences by simultaneously using these different spreading codes -- this provides an increased number of communication channels. Preferably, two base stations spatially far apart from each other are allowed to transmit spread signals at the same transmitting timing, which makes it possible to use more communication channels throughout the entire system -- this provides increased system capacity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating the configuration of a transmitting device provided in a base station in an embodiment of the present invention. Fig. 2 is a diagram showing an example of setting a basic symbol timing in the embodiment of Fig. 1. Fig. 3 is a diagram showing an example of spreading codes for use in respective base stations in the embodiment of Fig. 1. Fig. 4 is a block diagram illustrating the configuration of a receiving device provided in a mobile station in the embodiment of Fig. 1. Fig. 5 is a graph showing an example of the correlation output from a matched filter in Fig. 4. Fig. 6 is a diagram for explaining the re-use of basic symbol timing in an embodiment of the present invention. Fig. 7 is a block diagram illustrating the configuration of a transmitting device in a conventional CDMA communication system. Fig. 1 illustrates in block form the construction of a transmitting device which is provided in each base station in this embodiment. The transmitting device is shown to transmit to n mobile stations at the same time. The transmitting device comprises a spreading code generator circuit 110, multipliers 111, 112, ..., 11n, an adder 121, a transmitting timing control circuit 122, a controller 123 and a synchronization timing generator circuit 124. The spreading code generator circuit 110 generates spreading codes C1, C2, ..., Cn in synchronization with the information sequences S1, S2, ..., Sn, respectively. The multipliers 111, 112, ..., 11n multiply the information sequences S1, S2, ..., Sn by the spreading codes C1, C2, ..., Cn from the spreading code generator circuit 110, respectively, thereby spreading the information sequences S1, S2, ..., Sn. The adder 121 adds together the output signals from the multipliers 111, 112, ..., 11n. Based on synchronization timing (basic timing) from the synchronization timing generator 124, the transmitting timing control circuit 122 controls the transmitting timing to transmit the output signal of the adder 121 at a basic symbol timing instructed by the controller 123. Next, the operation of this transmitting device will be described. The information sequences S1, S2, ..., Sn to be transmitted are added together by the adder 121 after being spread by the multipliers 111, 112, ..., 11n with the spreading codes C1, C2, ..., Cn from the spreading code generator circuit 110. The output signal of the adder 121 is controlled in transmitting timing by the transmitting timing control circuit 122 to provide a transmission signal. The thus produced transmission signal is output from a transmitting circuit part, not shown, and is then transmitted via radio channels to n mobile stations. The transmitting timing is adjusted so that the basic symbol timing does not coincide among a plurality of base stations. Fig. 2 shows an example of the setting of the basic symbol timing in each base station. As shown in Fig. 2, a plurality of basic symbol timing points T1, T2, ..., Tn are set in the period of one symbol length Ts of the information sequence, and a delay from the basic timing A of each basic symbol timing is provided by the transmitting timing control circuit 122 for each base station. That is, in the case where the base station 1 transmits at the timing T1, the base station 2 at the timing T2 and the base station Tn at the timing Tn, the timing T1 is delayed by d1 relative to the basic timing A, T2 by d2 and Tn by dn. The interval of these basic symbol timing points T1, T2, ..., Tn is set to a value substantially corresponding to the sum of the delay spread and guard time at the time of receiving a signal by each mobile station. In this embodiment, as shown in Fig. 3, the base stations 1, 2, ..., n are each allowed to use the common spreading codes C1, C2, ..., Cn, making it possible to substantially increase communication channels available for use. Moreover, to allow respective base stations to use common spreading codes provides a significant advantage in terms of system construction and operation that management of the spreading codes is simple and easy or unnecessary. Fig. 4 illustrates in block form the construction of a receiving device which is provided in each of n mobile stations for receiving the signal transmitted from the transmitting device of Fig. 1. The receiving device in this example uses a matched filter as the correlator. This receiving device comprises a matched filter 131, a demodulator 132, a basic symbol timing extracting circuit 133 and a controller 134. The basic symbol timing extracting circuit 133 extracts from the correlated output of the matched filter 131 the basic symbol timing of the signal transmitted to this station and indicates the extracted timing to the demodulator 132. The demodulator 132 uses the outputs from the matched filter 131 and the basic symbol timing extracting circuit 133 to reconstruct the original information sequence Si (i = 1, 2, ..., n). The demodulator may be the RAKE demodulator which is described in detail in, for example,: U. Grob, A.L. Welti, E. Zollinger, R. Kung and H. Kaufmann, "Microcellular Direct-Sequence Spread-Spectrum Radio System Using N-Path RAKE Receiver," IEEE JSAC, Vol. SAC-8, No. 5, pp. 772-780. June, 1990. The controller 134 provides spreading codes to the matched filter 131 and local information to the basic symbol timing extracting circuit 133 when extracting the basic symbol timing. Next, a description will be given of the operation of this receiving device which is provided in each mobile station. The receiving signal, that is, the signal transmitted from the transmitting device of the Fig. 1 construction in the base station and received by the receiving device of Fig. 4, is fed first to the matched filter 131. The matched filter 131, which is being supplied with the same spreading code as that generated by the spreading code generator circuit 110 in the transmitting device of Fig. 1, correlates the spreading code with the received signal to despread the latter, generating a correlated output. Fig. 5 shows an example of the correlated output from the matched filter 131, the abscissa representing time and the ordinate the correlation value. Since the received signal by the receiving device of Fig. 4, that is, the transmission signal from the transmitting device of Fig. 1, was transmitted at offset basic symbol timing points T1 through Tn for each base station as referred to previously, large correlation values are detected at points in time corresponding to the basic symbol timing points T1 through Tn. The basic symbol timing extracting circuit 133 is presupplied, from the controller 134, with information about the basic symbol timing of the signal transmitted to this station, for example, information about the basic symbol timing A and the time length therefrom to the basic symbol timing Ti (i = 1, 2, ..., n); the basic symbol timing extracting circuit uses this information to recognize from the output of the matched filter 131 the basic symbol timing at which the transmitted signal is to be received by this station. Upon detecting that the basic symbol timing is reached, the basic symbol extracting circuit 133 indicates it to the demodulator 132. The demodulator 132 reconstructs the information sequence Si transmitted to this station, from the output of the matched filter 131 at the basic symbol timing indicated from the basic symbol timing extracting circuit 133. In a direct spread-code division multiple access (DS-CDMA) system using spreading codes, even if signals are simultaneously transmitted to a plurality of mobile stations which use the same spreading code sequence, it is possible for each mobile station to reconstruct the original information sequence by capturing that portion of a profile which is to be received by the mobile station, unless the delay profiles of the transmitted signals overlap. This means that even if adjacent cells or adjoining base stations use the same spreading code in the multi-cellular mobile communication system, the information sequences in respective channels can independently be demodulated or decoded unless the basic symbol timing points overlap. Hence, by setting the basic symbol timing different for each of the n base stations as in this embodiment, it is possible to use one spreading code in common to all the base stations and obtain n independent communication channels. By this, it is possible to obtain communication channels of a number n times larger than in the prior art by use of a limited number of spreading codes of excellent self-correlation and mutual-correlation characteristics, permitting the implementation of a CDMA mobile communication system of large channel capacity. Incidentally, the demodulator 132 may also be formed by, for instance, an ARD demodulator such as described in Akihiro Higashi, Tadashi Matsumoto, "BER Performance of Adaptive RAKE Diversity (ARD) in DPSK DS/CDMA Mobile Radio," The Transactions of the Institute of Electronics, Information and Communication Engineers of Japan, SST92-16, June, 1992 or Japanese Pat. Appln. No. 83947/92 "Spread Spectrum Receiver". The use of this ARD demodulator permits reconstruction of the transmitted signal sequence, even if the delay profiles overlap to some extent. Accordingly, the number n of communication channels can be increased by further decreasing the interval between the basic symbol timing points of the respective base stations. While this embodiment uses a matched filter as the correlator, a combined version of sliding correlators can also be used as long as it is able to detect a plurality of basic symbol timing points. Fig. 6 shows an example of a multi-cellular mobile communication system, together with examples of basic symbol timing and spreading codes for use in each base station. T1 is the basic symbol timing in a base station 1 and Ti is similarly the basic symbol timing in a base station i (i = 1, 2, 3, 4, J in Fig. 6). By setting the basic symbol timing different for each base station, no interference occurs between the base stations, and consequently, the same spreading codes C1-Cn can be used in common to different base stations. Furthermore, as described in "Fundamentals of Mobile Communication," edited by the Institute of Electronics, Information and Communication Engineers of Japan , since electric waves attenuate in inverse proportion to the square of distance in the free space and about the fourth power of distance in a city area, no interference occurs between places spatially far apart to some extent, even if the same basic symbol timing is used. This can be utilized to spatially re-use the same basic symbol timing. For example, as shown in Fig. 6, a base station J sufficiently far apart from the base station 1 is allowed to use the same basic symbol timing as that in the base station 1. With such spatial re-use of the basic symbol timing, it is possible to provide an increased number of communication channels available for use throughout the mobile communication system. Accordingly, the combined use of the spatial re-use of the basic symbol timing and the offset of the basic symbol timing will further increase the number of communication channels available for use, permitting implementation of a CDMA mobile communication system of very large channel capacity.

Claims: A code division multiple access mobile communication system which uses code division multiple access for communications between a plurality of mobile stations and a respective base station (1-4) in each of a plurality of cells, wherein the same set of different spreading codes (C1-Cn) and a respective one of a plurality of predetermined delay times corresponding to transmitting timings (T1, T2, T3, T4) are allotted to each cell and wherein said base station (1 - 4) in each of said cells comprises a transmitting device (111-11n, 110, 121-124) adapted to spread each of a plurality of information sequences with a respective one of said different spreading codes to produce a plurality of spread information sequences, to delay the plurality of spread information sequences by the delay time allotted to the respective cell and then to transmit said plurality of the delayed spread information sequences to said plurality of mobile stations. The system according to claim 1, wherein each mobile station has a receiving device (131-134) for receiving a signal from the base station (1-4) in a respective cell, for despreading the received signal with one of said different spreading codes that is allotted to this mobile station to produce a despread signal, and for extracting information from said despread signal at a timing corresponding to the delay time allotted to said respective cell to reproduce an information sequence. The system according to claim 1 or 2, wherein said transmitting devices of said base stations in at least two of said cells, which are spatially far apart from each other, use the same delay time.

Inventors: DOUZONO YOUICHI, MATSUMOTO TADASHI, UMEDA NARUMI, DOUZONO, YOUICHI, MATSUMOTO, TADASHI, UMEDA, NARUMI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1993
Main classification: H04B7
IPC codes: H04J13, H04B7