Efficiently structured CDMA receiver with near–far immunity

Abstract

A Gaussian code division multiple access (CDMA) channel is shared by K active users who transmit asynchronously with BPSK modulation by independent binary data streams. A conventional direct sequence CDMA receiver has only limited capability for suppressing cochannel multiple user interference (MUI) from the K – 1 other CDMA signals while attempting to receive and demodulate each CDMA signal. A new and efficient feedback receiver structure is described for coherent demodulation of the K asynchronous CDMA signals. By incorporating adaptive cancellation of the cochannel MU1 in a feedback structure, the new design offers the following advantages: protection of the receiver's synchronization loops as well as its data demodulators against MUI; lower bit error rate (BER) than the conventional receiver for equal-strength as well as near-far MUI; and implementation simplicity close to that of the conventional CDMA receiver. It is shown that all the CDMA signals are canceled to a common level determined by N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , the one-sided power spectral density of the input AWGN; the bandwidth spreading ratio N; and K. The BER for any signal may be computed as if the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at its matched filter input were multiplied by the factor (6N/π)(1.5N + 1 – K) for K ≤ 1.5N. Computer simulations corroborate this result. A straightforward enhancement to the receiver structure is shown to allow it to overcome the deleterious affects of multipath propagation.