Guest Editorial: Evolution of 5G NR MIMO Standards

The multi-input multi-output (MIMO) technology is proven salient in cellular wireless communications standards for improving end-user experience by increasing the overall spectral efficiency. With the advent of antenna technologies such as active antennas and two-dimensional arrays, deploying base stations with tens or hundreds of antenna elements has become feasible and it is an essential component for successful deployments of 5G New Radio (NR) technologies standardized by the 3rd Generation Partnership Project (3GPP). Also included in 5G NR is the use of mmWave bands where an analog-beam-based transmission and reception paradigm, termed the beam management, poses a new set of challenges and requirements. The initial release of 5G NR, i.e., Rel-15 includes the support of high-resolution channel state information (CSI) to facilitate efficient multi-user MIMO operation, multiple transmit/receive points (multi-TRP) operation, and beam management. Such features are enabled by basic components such as reference signals for facilitating demodulation and CSI measurements, CSI reporting mechanisms for link adaptation and scheduling, transmission schemes for downlink and uplink, and control signaling. 5G NR MIMO has been enhanced in 3GPP Rel-16 and Rel-17 standards. Following the recent completion of Rel-17 standards, 5G NR MIMO would be further enhanced in Rel-18 taking into account various requests to enable new products and more flexible network deployments.