Joint Transmitter-Receiver Optimization in the Downlink CDMA Systems
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Joint Transmitter-Receiver Optimization in the Downlink CDMA Systems Mohammad Saquib Wireless Communications Research Lab. (WiCoRe), Department of Electrical Engineering, University of Texas at Dallas, Richardson, TX 75083-0688, USA Email: [email protected]
Md Habibul Islam Wireless Communications Research Lab. (WiCoRe), Department of Electrical Engineering, University of Texas at Dallas, Richardson, TX 75083-0688, USA Email: [email protected] Received 23 August 2001 and in revised form 15 March 2002 To maximize the downlink code-division multiple access (CDMA) system capacity, we propose to minimize the total transmitted power of the system subject to users’ signal-to-interference ratio (SIR) requirements via designing optimum transmitter sequences and utilizing linear optimum receivers (minimum mean square error (MMSE) receiver). In our work on joint transmitter-receiver design for the downlink CDMA systems with multiple antennas and multipath channels, we develop several optimization algorithms by considering various system constraints and prove their convergence. We empirically observed that under the optimization algorithm with no constraint on the system, the optimum receiver structure matches the received transmitter sequences. A simulation study is performed to see how the different practical system constraints penalize the system with respect to the optimum algorithm with no constraint on the system. Keywords and phrases: CDMA system, joint transmitter-receiver optimization, MMSE receiver, power control, downlink, multipath and multiple antennas.
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INTRODUCTION
Code-division multiple access (CDMA) systems are being considered to support multimedia traffic in the next generation mobile radio systems, such as CDMA2000 and WCDMA. Voice-based CDMA systems are generally equal in their uplink and downlink traffic, whereas, in future CDMA systems, which will support various types of high data rate image and video traffic with voice messages, the downlink will carry the significant portion of the total system traffic. Therefore, an important area of research is to maximize the downlink capacity via fully utilizing the limited system resources. The capacity of a CDMA system is interference limited. Techniques that control or avoid interference improve the CDMA system capacity. There are three means of controlling interference in a CDMA system: power control, multiuser detection, and beamforming. Power control balances received powers of all users so that no user suffers from excessive interference due to other users in the system. Multiuser detection suppresses interference by exploiting the temporal
structure of the interference, whereas beamforming uses the spatial structure of the interference to cancel it. Recently, several studies [1, 2, 3, 4] have been performed in order to integrate power control with multiuser detection. The motivation of these works was to achieve a performance gain over multiuser detection by providing power control for multiuser detection. In [1, 3], the problem of finding the jointly optim
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