The performance of QPSK in low-SNR interference channels

We investigate the low-SNR sum rate performance of QPSK for symmetric interference channels. The QPSK performance is described by the minimum energy per bit and the wideband slope pertaining to the sum capacity. Comparing this with the minimum energy per bit and wideband slope of corresponding interference channels using optimal inputs, we find that QPSK achieves optimal performance in all of the cases where exact sum capacities are known. We also show that a simplified Han-Kobayashi scheme is suboptimal in the low-SNR regime when the input alphabet is the whole set of complex numbers.