A digital receiver for demodulation of CDMA waveforms with a-priori unknown delays and amplitudes
R.A. Iltis · 2002
A digital receiver is presented which provides near-optimum demodulation of code division multiple access (CDMA) waveforms, with simultaneous tracking of amplitudes and delays. It is shown that a CDMA waveform is equivalent to an M-ary symbol sequence, with intersymbol interference of no more than one duration. Thus, the individual users can be jointly demodulated using the Viterbi algorithm (VA), when the amplitudes and delays are known a priori. A modified VA is developed under the assumptions that the amplitudes are unknown, and the delay estimates initialized to within half a chip. The branch metrics of the VA are approximately given by the innovations likelihoods of a bank of extended Kalman filters (EKSs). Each EKF provides an estimate of the CDMA amplitudes and delays, conditioned on a particular survivor sequence. Thus, for binary modulation and N users, 2/sup N/ EKFs and nodes in the VA trellis are required, with 2/sup N/ branches from each node. Simulation results are presented which demonstrate that the algorithm successfully estimates delays and amplitudes, given suitable initialization of the CDMA delay estimates.>