Multistage interference cancellation for CDMA applications using M-ary orthogonal moduation

The present invention relates in general to code division multiple access (CDMA) communications systems and, more particularly, to an improved method of interference cancellation for CDMA communications systems using M-ary orthogonal modulation. A multistage or parallel interference canceler (PIC) arrangement is disclosed for use in a communications system using code division multiple access (CDMA) with M-ary orthogonal modulation. All the M symbol waveforms, weighted according to their correlations with a despread signal, are combined to form a reconstructed symbol waveform for each user in each stage. The reconstructed symbol waveforms for all users are summed to form an estimated composite received signal having reduced interference which is subtracted from the composite received signal to generate a residual interference signal (RIS). While the total received signal is despread in the first stage of the PIC, the RIS or cancellation residue signal from the immediately preceding stage is used in succeeding stages of the PIC. A final decision is made by despreading the RIS from the final stage of the PIC, correlating it with the M symbol waveforms using an FHT. A weight vector from the final stage is then added to the output of the FHT. The real part or the squared magnitude of the resulting correlations, depending, respectively, on whether coherent or noncoherent demodulation is used, is then taken. The final symbol decision is made by selecting the symbol corresponding to the maximum component resulting decision statistic vector.