Massive Multiple-Input, Multiple-Output: Instantaneous versus statistical channel state information-based power allocation [Lecture Notes]

Massive multiple-input, multiple-output (mMIMO) has emerged as a key enabler for 5G and beyond wireless communications, offering the potential to meet the ever-growing demands of an increasingly connected world. Unlike traditional MIMO systems that employ a limited number of antennas at the base stations (BSs), mMIMO utilizes a significantly larger number of antennas, often numbering in the hundreds or even thousands. This massive antenna deployment enables simultaneously serving many tens of users without any need for additional frequency resources. Overall, by leveraging spatial multiplexing and precoding/combining techniques in conjunction with efficient power allocation, mMIMO can mitigate interference, enhance signal quality, and hence, achieve remarkable gains in both spectral and energy efficiency. More specifically, the optimization of power allocation plays a crucial role in minimizing interference within mMIMO systems, offering two principal implementation avenues: instantaneous channel state information (CSI) and statistical CSI-based schemes. Instantaneous CSI-based power control schemes exhibit notable performance gains, particularly in scenarios characterized by significant variations in channel quality across individual subcarriers. These variations arise due to differing channel conditions, such as fading, that affect each subcarrier independently. By adapting power allocation based on instantaneous CSI, the system can more effectively optimize transmission parameters, improving overall system performance. The deployment of instantaneous CSI-based power control schemes necessitates computationally intensive signal processing operations, requiring substantial resources to handle real-time CSI updates and the associated overhead. Conversely, statistical CSI-based schemes enable efficient implementation of advanced power allocation algorithms within large-scale mMIMO systems, where the algorithms are updated much less frequently. Nevertheless, these schemes may deviate from optimal results in certain practical mMIMO configurations, necessitating the adoption of instantaneous CSI-based schemes. In addition, they may be limited in practical implementation where instantaneous CSI-based resource allocation and management schemes are widely adopted. This “Lecture Notes” article provides a comprehensive comparison between statistical CSI-based power allocation and instantaneous CSI-based power allocation designs for mMIMO systems from performance, complexity, and practical implementation aspects.