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The bit metric is used for soft-decision decoding in bit-interleaved coded modulation (BICM) systems. The complexity of bit metric generation increases rapidly for high-order modulation. Proposed is a nonlinear function and a recursive formula to generate the bit metric for the pulse amplitude modulation (PAM) signal with any modulation order. This method can reduce the computational complexity greatly and has similar performance compared with the optimal bit metric. This method can also be used for a quadrature amplitude modulation signal which can be decomposed into two independent PAM signals.