Semianalytic BER evaluation by simulation for noisy nonlinearbandpass channels

The bit error rate (BER) of channels including a memoryless bandpass nonlinearity is evaluated by simulation. This would typically be an onboard travelling wave tube (TWT) amplifier in a satellite repeater, when the noise at the input of such a nonlinear element is non-negligible. The usual evaluation technique, error counting, requires a large amount of computer time if small error probabilities are to be estimated. It is shown that faster semianalytic procedures can be used, provided that a proper model for the nonlinear element is adopted. The output process is decomposed into a signal component plus an additional term representing an equivalent noise component, and an equivalent nonlinear transformation, relating the input useful signal to the output signal component, is derived. In addition, several modes for the probability density function (PDF) of the uplink noise component at the output of the transmission chain are discussed and compared. The procedure has been tested on a transparent satellite link using 4-CPSK modulation format. The results compare well with those of the error-counting technique if a composite rectangular PDF with exponential tails as adopted