Fifth Generation (5G) cellular systems are being designed to communicate over both sub-6 GHz bands (a.k.a. Frequency Range 1) as well as mmWave bands (a.k.a. Frequency Range 2). mmWave spectrum offers abundant bandwidth, which can be used to support multi-Gbps transmission speeds per user. Historically, mmWave bands have been used for fixed/mobile satellite services (FSS/MSS) and local multipoint distribution service (LMDS). Recently, the Federal Communications Commission (FCC) has opened up nearly 11 GHz of the mmWave spectrum for mobile broadband, aiming to support 5G cellular systems, wireless LANs (e.g., Wi-Gig), and others. The newly allocated bands include licensed bands at 24 GHz (24.25-24.45 GHz and 24.75-25.25 GHz), 28 GHz (27.5-28.35 GHz), 37 GHz (37-38.6 GHz), and 39 GHz (38.6-40 GHz), as well as a new unlicensed band at 64-71 GHz – see Figure 1. Combined with the previously introduced 57-64 GHz unlicensed band, this creates almost 18 GHz of new spectrum for next-generation wireless systems. The FCC is also planning to add 15.8 GHz more spectrum at 31.8-33.4 GHz, 42-42.5 GHz, 47.2-50.2 GHz, 71-76 GHz, and 81-86 GHz bands, along with spectrum above 95 GHz.
Figure 1: Licensed and unlicensed bands in the mmWave spectrum.
Despite this abundant capacity, RF communications at mmWave frequencies can be quite challenging, due to high propagation losses, poor penetration, blockage, rain sensitivity, etc. At the same time, the smaller wavelengths make it possible to pack tens or even hundreds of antenna elements into a single device (mobile phone, Small base station, or access point). With proper processing of signals fed into these antennas, electronically steerable, highly directional transmissions can be performed. The severe signal attenuation can, thus, be compensated for by the resulting beamforming and spatial reuse gains.
There are several ways to apply beamforming at mmWave frequencies. Fully digital beamforming relies on the availability of several RF chains. A particular precoding vector is multiplied by the modulated baseband signal of a given RF chain. Analog beamforming, on the other hand, can be performed with a single RF chain. It applies the beamforming weights in the RF domain by controlling phase shifters. Finally, in hybrid (analog/digital) beamforming, the signal processing is divided between the analog and digital domains, allowing comparable performance to digital beamforming but with fewer RF chains. The low power consumption of the analog beamforming makes it a desirable architecture, especially for the user equipment (UE).
