MMSE multiuser detection for multi-rate wideband CDMA communications

By generalizing and extending the effective user data model for multi-rate access, we precisely study the general performance of various proposed multi-rate schemes for wideband CDMA systems. A linear multiuser receiver structure with antenna arrays that can fit all the considered multi-rate schemes is therefore presented. With the extended data model and the proposed receiver structure, each multi-rate scheme is only distinguished by the bandwidth requirement and the spreading codes used. The numerical results suggest that the decision on employing which multi-rate scheme can be viewed as a trade-off between the power consumptions and the hardware complexity of the receivers, as well as operating scenarios. I . INTRODUCTION The future cellular communication systems are expected to provide multimedia services with different data rates ranging from low rate services, e.g. speech or facsimile, to high rate services, e.g. image or video. There are several proposed schemes to support multiple data rate transmissions in a direct sequence wideband code division multiple access (DS-CDMA) communication system [l, 21, and these multi-rate schemes can be classified into two categories of system architecture. Multi-code (MC) and variable spreading length (VSL) access support multi-rate data by changing the number or the processing gain of the spreading codes assigned to each user with the same chip rate. On the other hand, variable chip rate (VCR) and variable chip rate spaced spreading (VCRSS) transmissions accommodate the multi-rate characteristics of the information by changing the bandwidth allocated to each user with the same processing gain. To detect the different or even time-varying data rate signals of each user, the receiver structure of conventional single-rate systems has to be modified to extract the information from the received signals. There has been a lot of research discussing the issue of multi-rate detection [l-71. However, only single-user detection or multiuser detection with temporal processing over frequencyselective channel with small delay spread and negligible intersymbol interference has been considered. In this paper, we briefly review these multi-rate schemes and proceed a general linear multiuser detection research in general frequency-selective fading channels. The linear MMSE estimator has the minimum Bayesian mean square error and the minimum noise variance [8] for linear processing estimators. It is also shown in [9] that by using multiple antennas at the receiver and exploiting the spatial diversity, the output signal-to-interference plus This research was partially supported by a research grant from the Institute of Information Industry and the Ministry of Education under the contract 89-E-FA06-2-4. 0-7803-6465-5/00 $10.00