In single-user massive multiple input multiple outputs (SU-MMIMO), both the mobile handset and base station are equipped with a large number of antennas (in excess of 100 or even 1000) to enhance the spectral efficiency or bits/transmission per user. It is feasible to have a large number of antennas in the mobile handset in millimetre wave (mm-wave) frequencies (~300 GHz) due to their small size. While multi-user massive MIMO (MU-MMIMO), where the base station has a large number of antennas and each of a large number of handsets have only a single antenna, has been extensively studied, research on SU-MMIMO is yet to take off. While beamforming is the cornerstone to the success of MU-MMIMO, the availability of a rich scattering channel is fundamental to the operation of SU-MMIMO. Thus, the requirements of MU-MMIMO and SU-MMIMO are contradictory. Moreover, SU-MMIMO promises a much larger spectral efficiency per user compared to MU-MMIMO, which is essential as we move towards data rates of the order of 100 gigabits/sec (Gbps) per user in 5G and beyond. High data rates are required for applications such as self-driving cars, remote surgery, remote teaching, and streaming high quality (8K) video, among others.

The main challenge in SU-MIMO lies in improving the propagation characteristics of mm-waves. This could be achieved by the use of smart reflectors to obtain a rich scattering channel. The other challenges are data detection, channel estimation, and carrier and timing synchronisation for a large number of antennas. Investigations on the use of quantum computers to perform a high volume of signal processing tasks in parallel are of great importance. Due to the presence of a large number of antennas at the transmitter and receiver, it is sufficient to use small constellations, like quadrature phase shift keying (QPSK), which have the minimum peak-to-average power ratio (PAPR) and can easily incorporate powerful error correcting codes like turbo and low density parity checks (LDPC).

The aim of this Special Issue is to collate original research and review articles discussing the state of the art in the field. The availability of open source testbeds would provide a much needed impetus to the research and commercialisation of SU-MIMO. Articles on security using quantum communications are also welcome. We hope that this Special Issue will provide a forum for researchers and practitioners in all aspects of SU-MMIMO.

Potential topics include but are not limited to the following:

• Antennas and propagation in mm-wave frequencies
• The use of smart reflectors to provide a rich scattering channel for the operation of SU-MMIMO
• Development of discrete-time transceiver algorithms for efficient and error-free transmission and detection
• Signal processing using quantum computers
• Security using quantum communications
• Open source testbeds for 5G and beyond for the research and development of SU-MIMO
• Multiple access techniques