Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1997-07-28
2000-05-30
Pham, Chi H.
Pulse or digital communications
Spread spectrum
Direct sequence
375347, 375140, H04L 2730
Patent
active
060699124
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a diversity receiver and its control method for carrying out diversity reception of a data signal transmitted in accordance with a direct sequence CDMA (code division multiple access) scheme.
Specifically, the present invention is applied to reception in the code division multiple access (CDMA) scheme using a spread spectrum technique, and particularly to a mobile communications field using a cellular configuration.
More specifically, the present invention is applied to the field of a diversity receiving technique which despreads received signals which are input to a plurality of antennas of a diversity receiver placed at a base station, multiplies the despread signals by appropriate weighted coefficients, and combines the resultant products. In particular, the present invention relates to the establishment of synchronization between the diversity receiver in the base station and a mobile station in the base station area, and to the procedure of setting initial values of weighted coefficients.
BACKGROUND ART
The DC-CDMA is a scheme for carrying out communications using a single frequency band shared by multiple users, and spreading codes are employed to identify the individual users. Here, orthogonal codes such as Gold codes are used as the spreading codes for the users.
In the process of despreading in the receiver, the interference signal power from other users are reduced by a factor of average processing gain (PG). In a mobile communications environment (particularly, under reverse link asynchronous environment), the received signals of the users undergo instantaneous fluctuations due to independent fading, short term fluctuations and distance fluctuations. Accordingly, to satisfy the intended reception quality of the users, it is necessary to carry out the transmission power control to keep constant the SIR at the input of the receiver in the base station, the SIR being defined as the ratio of the received signal power of each user to interference signal power from the other simultaneous users.
However, even if the transmission power control is perfect, and hence the SIR at the receiver input is kept constant without fail, the spreading codes are never completely orthogonal under the multipath environment in the mobile communications, and the interference is unavoidable due to cross-correlation having average power reduced by a factor of processing gain per user.
Thus, the interference signal power increases with the number of simultaneous users in the same frequency band. As a result, the capacity in terms of the number of simultaneous users per cell is determined by the receiving quality which in turn is determined by the required quality of the system.
To increase the capacity in terms of the number of simultaneous users, the cross-correlation due to other users must be reduced.
Interference cancellation techniques are proposed as one of the methods to reduce the cross-correlation from other users. As the interference cancellation technique are known: 1. a multi-user detector that demodulates not only the desired signal of the intended channel, but also the signals of other simultaneous users received at the receiver input using the spreading code information of the other users; and 2. a single user detector that minimizes average cross-correlation and noise components from other simultaneous users using the spreading code of only the intended channel. Among these, the single user detector of item 2 corrects a spreading replica code such that the cross-correlation from other users produced in the process of despreading the desired user signal is reduced through quadrature filters in the receiver.
As another technique to reduce the cross-correlation from other users to increase the capacity in terms of the number of simultaneous users, an adaptive diversity technique as shown in FIG. 1 is known. In FIG. 1, reference numerals 101A-101D each designate an antenna, 102A-102D each designate an RF stage, 103A-103D each designate an A/D converter, 104A-1
REFERENCES:
patent: 5524023 (1996-06-01), Tsujimoto
patent: 5598428 (1997-01-01), Sato
patent: 5646964 (1997-07-01), Ushirokawa et al.
patent: 5652765 (1997-07-01), Adachi et al.
patent: 5757845 (1998-05-01), Fukawa et al.
Ando Hidehiro
Higuchi Kenichi
Miki Yoshinori
Sawahashi Mamoru
Tanaka Shinya
Bayard Emmanuel
NTT Mobile Communications Network Inc.
Pham Chi H.
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