DS-CDMA communications in fast fading channels

Time-selective fading due to Doppler spread is a major difficulty in wireless mobile communication systems. Because code division multiple access (CDMA) systems tend to have larger symbol durations than time division multiple access (TDMA) systems, the effect of time-selective fading can be more severe in CDMA systems. For example, consider an IS-95 CDMA system operating in the personal communication systems (PCS) band at a carrier frequency around 2GHz. For a mobile user moving at 60miles/hr., the Doppler spread is about 200Hz. According to Jakes model, this corresponds to a coherence time of 2ms. For a symbol rate at 9600sym./s, fading coefficients separated by 20 symbols are almost uncorrelated. For this example, it is no longer reasonable to approximate the channel as time-invariant. Hence, the detection and interference cancellation schemes based on the time-invariant channel assumption may not be applicable.

In this work, we propose a decision-feedback MAP receiver for CDMA channels with time-selective fading. The receiver consists of a sequence-matched filter and a MAP demodulator. Outputs samples from the matched filter are fed into the MAP demodulator. The MAP demodulator exploits the channel memory by delaying the decision and using a sequence of observations. This receiver also rejects MAI and estimates channel fading coefficients implicitly to give good demodulation decisions. Simulation results show that the BER performance of the receiver is good under various channel conditions. Extensions to include the use of adaptive antenna arrays and to track time-varying channels have also been explored.

Publications

  1. S. Tsai, T. F. Wong, and J. S. Lehnert, "DS-CDMA System with Joint Channel Estimation and MAP Detection in Time-Selective Fading," IEEE Journal on Selected Areas in Communications, vol. 19, no. 1, pp. 121-131, Jan. 2001.
  2. T. F. Wong, Q. Zhang, and J. S. Lehnert, "Decision-Feedback MAP Receiver for Time-Selective Fading CDMA Channels," IEEE Transactions on Communications, vol. 48, no. 5, pp. 829-840, May 2000.
  3. S. Tsai, T. F. Wong, and J. S. Lehnert, "Joint Channel Estimation and MAP Detection in Time-Selective Fading CDMA Channels," in Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC '99), vol. 3, pp. 1484-1488, New Orleans, LA, Sep. 1999.
  4. T. F. Wong, Q. Zhang, and J. S. Lehnert, "MAP Demodulator for Time-Selective Fading CDMA Channels," in Proceedings of the IEEE Military Communications Conference (MILCOM '98), pp. 355-359, Boston, MA, Oct. 1998.

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Last modified: 2/22/01