Sum-SE for Multigroup Multicast Cell-Free Massive MIMO With Multi-Antenna Users and Low-Resolution DACs

In this letter, we investigate the impact of multi-antenna on both users and access points (APs), and low-resolution digital-to-analog converters (DACs) on multigroup multicast cell-free massive multi-input multi-output (MIMO) systems. With the distributed conjugate beamforming receiver and the generalized additive quantization noise model, we derive a closed-form expression for the achievable downlink sum spectral efficiency (SE), which provides us a vigorous tool for easily evaluating the impacts of key parameters. Due to the joint influence of channel hardening property, multi-antenna gain, and intra-group interference, the research becomes sophisticated particularly. Typically, when the sum-SE is small, the multicast is better than the unicast, and vice versa. Besides, it reveals that the full-resolution can be replaced by 5-bit low-resolution DACs to achieve the same performance, and the additional antenna at users can effectively enhance the system performance.