Secrecy Energy Efficiency of Massive MIMO AF Relaying System with Low-Resolution ADCs

Abstract

This paper studies the secrecy energy efficiency problem for a massive multiple-input multiple-output (MIMO) relaying system. Specifically, the imperfect analog-to-digital converters (ADCs), i.e., ADCs with limited resolution, are employed at the relay, which will cause quantization noise. For the relay strategy, we adopt the amplify-and-forward (AF) relaying protocol. Meanwhile, a null-space artificial noise (AN) is also injected by the relay. To measure the quantization noise, we use the additive quantization noise model (AQNM), which models the quantization error as independent Gaussian noise. Based on this model, the ergodic secrecy rate of the considered relaying system is studied. To maximize the ergodic secrecy rate, we study the power allocation problem at the relay, which allocates power between the forwarded signal and the null-space AN. With the derived optimal power allocation parameter, we then focus on the secrecy energy efficiency, that is, the secrecy rate normalized by the total power consumed at the relay. By using a reasonable energy consumption model, the energy consumption of the ADCs and the transmit power are both included. Numerical results are provided to show that low-resolution ADCs with quantization bits being one or two can achieve the highest secrecy energy efficiency. In addition, higher-resolution ADCs are preferred when the number of antennas becomes smaller.