Inband and Out-of-Band Performance Evaluation of Downlink CF-mMIMO-OFDM Under Low-Resolution DACs

Cell-free massive MIMO (CF-mMIMO) technology has proved to offer significant gains in terms of spectral efficiency (SE). However, it can be economically attractive only when adopting low cost and energy-efficient hardware. This hardware includes components such as low-resolution digital-to-analog converters (DACs), and causes signal distortions called hardware impairments (HWI). In this paper, first, we propose an analytical characterization for low-resolution DAC where the exact behavior can be described, allowing to shine a new light on DAC-induced distortions. Based on the well-known Bussgang decomposition, we derive for a known signal distribution through closed-form expressions based on the integer and fractional resolution separately for downlink SE performance of orthogonal frequency multiplexing (OFDM) based CF-mMIMO. Moreover, we propose a new CF-mMIMO-specific framework to observe the out-of-band (OOB) radiations on potential victim users. We point out that OOB radiations is less of a problem in multi-user CF-mMIMO, where the total OOB radiated power is sufficiently low to respect any standard mask. Meanwhile, inband distortions represent a serious problem that have to be tackled.