Cooperative sensing with dependent observations on BPSK signal: To quantize amplitude or sign

This paper discusses a typical cooperative sensing model where the local observations are conditionally dependent. The primary user sends a binary phase-shifted keying modulated signal and hard combining strategy is implemented among second users. Two low-complexity local quantization schemes, corresponding to the choice of quantizing either the amplitude or the sign as the one-bit reported message, are compared. We show that the optimal fusion rule for quantizing the sign is non-monotone, which is different from the design for quantizing amplitude scheme. Based on large deviation theory, we finish the parameter optimization for both schemes and use the error exponent to evaluate the asymptotical performance. Several closed-form expressions are derived. Numerical results show that to quantize the sign is asymptotically optimal in low-SNR regime, while to quantize the amplitude performs well in high-SNR regime. This work shows that the dependence among local observations may greatly help the final decision in some case, and the non-monotone fusion rule may achieve better performance than the monotone one for dependent observations.