Since the introduction of fifth-generation new radio (5G-NR) systems in Third Generation Partnership Project (3GPP) Release 15, swift progress has been made in to evolve 5G. Recently, 3GPP began work on Release 18, a.k.a. 5G-Advanced, to standardize systems beyond 5G. From its predecessors, 5G-NR can be distinguished in several different aspects. One critical aspect is the design of the radio access network (RAN) via massive multiple-input multiple-output (MIMO) technology, resulting in superior coverage, spectral efficiency, and reliability. This paper makes several important contributions. We first provide a comprehensive overview of the evolution of standardized massive MIMO features from 3GPP Release 15 to 17 for both time/frequency-division duplex (TDD/FDD) operation across both 3GPP frequency ranges, i.e., FR-1 (microwave) and FR-2 (millimeter-wave). In particular, we present the common massive MIMO architectures in commercial radio products, analyze the progress on channel state information (CSI) acquisition frameworks for single and multi-user (UE) operation, study beam management/indication frameworks for FR-2 bands, and present enhancements for uplink CSI. Secondly, by comparing concepts proposed for the 3GPP Release 18 Work Item relative to those for contemporaneous releases, we shed light on the emerging problems requiring imminent attention at the physical layer. These include advanced codebook design (for FDD) and sounding reference signal design (for TDD systems) optimized for coherent joint transmission (CJT) from multiple transmission/reception points (multi-TRPs); advancements in uplink demodulation reference signal design to support higher-order massive MIMO, enhancements for mobility to provide accurate CSI estimates, and unified transmission configuration indicator (TCI) framework tailored for FR-2-based multi-TRPs to reduce beam switch latency. For each of these concepts, we provide comprehensive system level simulation results to highlight their performance gains relative to systems standardized in Release 15 to 17. Thirdly, via real-world field trials in an outdoor urban environment at Shanghai Jiaotong University, China, we demonstrate the gains in practice of multi-TRP CJT with real-time constraints relative to single TRP transmissions. To the best of our knowledge, a contribution of this type, which amalgamates massive MIMO evolution with novel concepts and results for 3GPP Release 18 has been missing from the literature.

https://arxiv.org/ftp/arxiv/papers/2210/2210.08218.pdf