Analysis of URLLC-Enabled Hardware-Impaired Massive MIMO Relaying With D2D Users

We consider a massive multiple-input multiple-output (mMIMO) two-way decode-and-forward (DF) relaying system with underlaid device-to-device (D2D) users. The mMIMO relay aids ultra-reliable and low-latency communication (URLLC) between relay users (RUs). The URLLC-enabled D2D users (DUs) communicate without relay intervention, and use the same spectral resources as that of RUs. The relay and users are equipped with low-cost radio frequency (RF) chain and finite-resolution analog-to-digital converters (ADCs), which cause RF distortion and ADC quantization errors, respectively. Considering these impairments, we derive closed-form spectral efficiency (SE) expressions for both RUs and DUs, which observe correlated Rician channels. Our theoretical findings reveal that increasing relay antennas mitigate the DU interference experienced by the RUs. The DUs, in contrast, require additional power scaling at relay to suppress the relay interference. We optimize the non-concave weighted sum SE metric of our system by exploiting minorization-maximization (MM) framework. We show that i) underlaying D2D users not only improve the system SE, but also energy efficiency; and ii) a DF relay can tolerate more hardware impairments than its amplify-forward counterpart.