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 N₀, 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 N₀ matched filter input were multiplied by the factor (6N/π)(1.5 N+1-K) for K⩽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