  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 very large number of antennas increases the BS's hardware
complexity and 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 superdirectivity and nonuniform inter-element spacing.
Specifically, we exploit the mutual coupling between closely spaced elements to
form superdirective pairs. A unique property of them is that all require the
same excitation amplitude, and thus can be driven by a single radio frequency
chain akin to conventional phased arrays. Moreover, they facilitate multi-port
impedance matching, which ensures maximum power transfer for any beamforming
angle. After addressing the implementation issues of superdirectivity, we show
that the number of BS antennas can be effectively reduced without sacrificing
the achievable rate. Simulation results demonstrate that our design offers huge
EE gains compared to uncoupled arrays with uniform spacing, and hence could be
a radical solution for future THz systems.
