Superdirective Antenna Pairs for Energy-Efficient Terahertz Massive MIMO

Terahertz (THz) communication is widely deemed the next frontier of wireless networks owing to the abundant spectrum resources in the THz band. Whilst THz signals suffer from severe propagation losses, a massive antenna array can be deployed at the base station (BS) to mitigate those losses through beamforming. Nevertheless, a large number of antennas increases the hardware complexity and circuit power consumption, and hence it can lead to poor energy efficiency (EE). To surmount this fundamental problem, we propose a novel array design based on coupled antenna pairs. Specifically, we exploit the mutual coupling between closely spaced antennas to form superdirective pairs. A unique property of them is that all require the same excitation amplitude, and therefore can be driven by a single radio frequency chain akin to conventional phased arrays. Moreover, they facilitate the implementation of multi-port impedance matching, which ensures maximum power transfer for any beamforming angle. After addressing the hardware-related problems of superdirectivity, we show that the number of BS antennas can be effectively reduced without sacrificing the achievable rate. Simulation results showcase that our design offers huge EE gains compared to uncoupled uniform linear arrays, and hence could be a radical solution for future THz systems.