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To boost the performance of 4-ary pulse amplitude modulated (PAM) at low sig-nal-to-noise ratio (SNR), bistable stochastic resonance (BSR) system is introduced into digital communications system and get a re-liable signal detection scheme. In this paper, we first analyse BSR system for different amplitudes of 4-ary PAM signals. The steady-state of the bistable system will be statistically distinct, and the feasibility of the proposed detection scheme is confirmed. On this basis, we present a detailed study on steady-state transitions of the BSR system, and an explicit expression of the bistable system parameters is derived. By setting the bistable system pa-rameters, bistable system, 4-ary PAM signal, and noise reach the resonance state, and the BSR-based detection scheme is implemented. Moreover, we derive an analytical expression to calculate the symbol error rate (SER) of 4-ary PAM signals with the BSR-based de-tection under additive white Gaussian noise (AWGN). Finally, the simulation results val-idate that BSR-based detection scheme can improve the detection performance while effi-ciently reducing the symbol error rate.