The performance of CDMA system using π/4-shift QPSK and π/2-shift BPSK with the nonlinearity of HPA

We evaluate the bit-error rate performance of direct-sequence spread spectrum (DS/SS) system using π/4-shift QPSK and π/2-shift BPSK and observe the effect of the nonlinearity of the high power amplifier (HPA) of transmitters which occurs when the HPA is operated near its saturation region. The hard limiter is chosen as the example of the nonlinear amplifier and the numerical results are reported by using coherent as well as differential detection at the receiving end. With the employment of either π/4-shift QPSK or π/2-shift BPSK, the DS/SS signal is not much influenced by the nonlinearity of the HPA as the two modulation schemes have the advantage of nearly constant envelope