Can Uplink Weights be Used for Downlink in TDD DS-CDMA Systems with Base Station Antenna Array?

Ying-Chang LIANG  

IEICE TRANSACTIONS on Communications   Vol.E85-B    No.8    pp.1627-1630
Publication Date: 2002/08/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: LETTER
Category: Antenna and Propagation
adaptive beamforming,  time division duplex (TDD),  code division multiple access (CDMA),  power control,  

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For base station antenna array systems with time-division-duplex (TDD) mode, downlink channel responses are equal to uplink channel responses if the duplexing time is small, thus it is often believed that TDD mode simplies downlink beamforming problem as uplink weights can be applied for downlink directly. In this letter, we show that for TDD DS-CDMA systems, even though uplink and downlink channel responses are equal, optimal uplink weights are no longer equal to the optimal downlink ones due to asynchronous property in uplink and synchronous property in downlink, as well as different data rate traffic and QoS requirements. Computer simulations show that for asymmetric traffic, if uplink weights are used for downlink directly, downlink system capacity is less than 50% of that with optimal downlink weights.