Adaptive Transmitter Precoding for Time Division Duplex CDMA in Fading Multipath Channels: Strategy and Analysis
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Adaptive Transmitter Precoding for Time Division Duplex CDMA in Fading Multipath Channels: Strategy and Analysis Daryl Reynolds Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506-6109, USA Email: [email protected]
Anders Høst-Madsen Department of Electrical Engineering, University of Hawaii, Honolulu, HI 96822, USA Email: [email protected]
Xiaodong Wang Department of Electrical Engineering, Columbia University, New York, NY 10027, USA Email: [email protected] Received 31 January 2002 and in revised form 7 May 2002 The recently developed blind adaptive techniques for multiuser detection in code division multiple access (CDMA) systems offer an attractive compromise of performance and complexity. However, the desire to further reduce complexity at the mobile unit has led to the investigation of techniques that move signal processing from the mobile unit to the base station. In this paper, we investigate transmitter precoding for downlink time division duplex (TDD) code division multiple access (CDMA) communications. In particular, we develop a linear minimum mean square error precoding strategy using blind channel estimation for fading multipath channels that allows for simple matched filtering at the mobile unit and is easy to make adaptive. We also present a performance analysis using tools developed for the analysis of conventional (receiver-based) linear blind multiuser detection in unknown channels. We compare the analytical and simulation results to traditional receiver-based blind multiuser detection. It is seen that transmitter precoding offers a reasonable alternative for TDD-mode CDMA when minimizing computational complexity at the mobile unit is a priority. Keywords and phrases: multiuser detection, transmitter precoding, wireless communications, blind channel estimation.
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INTRODUCTION
The demand for capacity and performance in multiple access wireless systems has spurred the development of sophisticated signal processing techniques for signal reception [1, 2]. However, the goal of maintaining low cost and complexity, especially at the mobile unit, is as important as ever. As a result, researchers have recently begun investigating signal processing techniques that move computational complexity from the mobile unit to the base station, where it can be managed more efficiently. Generally speaking, these techniques involve some kind of transmitter-based multiuser interference cancellation at the base station (precoding) and simple linear processing, for example, matched filtering, at the mobile unit. They are particularly appealing for time division duplex code division multiple access (TDDCDMA) [3] since the same carrier is used for both uplink
and downlink in different time slots. Hence, the downlink channel can be estimated at the base station using the uplink signals [4]. In [5], the authors considered transmitter precoding for synchronous CDMA over additive white Gaussian noise (AWGN) channels. They also present an extension to mu
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