EFFICIENTLY STRUCTURED CDMA RECEIVER WITH NEAR-FAR IMMUNITY

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 MUI 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-0, 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-0 at its matched filter input were multiplied by the factor (6N/pi)(1.5N + 1 - K)) for K less than or equal to 1.5 N. 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.