  Millimeter wave (mmWave) communication is a promising technology for the
fifth-generation (5G) wireless system. However, the large number of antennas
used and the wide signal bandwidth in mmWave systems render the conventional
multi-antenna techniques increasingly costly in terms of signal processing
complexity, hardware implementation, and power consumption. In this article, we
investigate cost-effective mmWave communications by first providing an overview
of the main existing techniques that offer different trade-offs between
performance and cost, and then focusing our discussion on a promising new
technique based on the advanced lens antenna array. It is revealed that by
exploiting the angle-dependent energy focusing property of lens arrays,
together with the angular sparsity of the mmWave channels, mmWave lens-antenna
system is able to achieve the capacity-optimal performance with very few
radio-frequency (RF) chains and using the low-complexity single-carrier
transmission, even for wide-band frequency-selective channels. Numerical
results show that the lens-based system significantly outperforms the
state-of-the-art designs for mmWave systems in both spectrum efficiency and
energy efficiency.
