Multiplex communications – Wide area network – Packet switching
Patent
1994-06-29
1996-10-22
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 951, H04J 1304
Patent
active
055684726
DESCRIPTION:
BRIEF SUMMARY
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 literature 1: 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. 19 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 generated. 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 .tau., the correlation value is large for .tau.=0 and small for other values of .tau.) 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 (Japanese 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 that are not 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.
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.
The mobile communication system according to the present invention which utilizes the code division multiple access scheme for communications between at least one base station and a plurality of mobile stations, characterized in:
that the base station has a transmitting device which spreads a plurality of information sequences by the same spreading code and then transmits them to the plurality of mobile stations at different timing;
that the mobile stations each have a receiving device which receives that signal from the transmitting devices of the base station which was transmitted thereto at timing predetermined therefor and despreads th
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Ruprecht, J., et al., "Code Time Division Multiple Access: An Indoor Cellular System", Vehicular Technology Society, 42nd VTS Conference Frontiers of Technology, vol. 2, May 1992, pp. 736-739.
Douzono Youichi
Matsumoto Tadashi
Umeda Narumi
Jung Min
NTT Mobile Communications Network Inc.
Olms Douglas W.
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