Joint sequential channel estimation and multiuser detection for uplink CDMA over multipath fading
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We consider joint channel estimation and data detection in uplink asynchronous CDMA systems employing aperiodic (long) spreading sequences in the presence of unknown multipath fading. Since maximum-likelihood sequence estimation is too complex to perform, multiuser receivers are proposed based on the sequential expectation-maximization (EM) algorithm. With prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. The snapshot estimates of each path are further tracked by linear minimum-mean-squared-error (MMSE) filters and the user data are detected by a maximum-likelihood sequence detector, given the channel estimates. The proposed receivers that use the exact expressions have a computational complexity O(2 K ) per bit, where K is the number of users. Using the traditional EM algorithm, we derive low-complexity approximations which have a complexity of O(K 2 ) per bit. Simulation results demonstrate that the proposed receivers offer substantial performance gains over conventional pilot-symbol assisted techniques and achieve a performance close to the known-channel bounds. Furthermore, the proposed receivers even outperform the single-user RAKE receiver with Nyquist pilot-insertion rate in a single-user environment.
