A method of defining emission limits including the gradient of an amplitude-probability-distribution curve

Bit-error probability (BEP) of a digital communication system subjected to interference can be approximately derived from the amplitude probability distribution (APD) of the interfering signal. In this paper, it is pointed out that the approximation underestimates maximum BEP of a multilevel modulation scheme when the interference is impulsive, which is undesirable in regard to setting emission limits for the protection of communication systems. In order to avoid the underestimation risk, an expression of exact upper bound of BEP is derived in terms APD by taking account of the probability of multiple bit errors being caused by one symbol error. It was found that the gradient of an APD curve is a key factor in determining the probability of multiple bit errors. On the basis of the derived expression of BEP, a method of defining emission requirements by limiting the gradient of an APD curve is proposed. The validity of the method is clearly demonstrated by numerical simulations of a 64 QAM system.