Abstract: The demand for wireless data has been driving network capacity to double about every two years for the past 50 years, if not 100 years, and this has come to be known as Cooper’s Law. In recent years, this trend has accelerated as a greater proportion of the population adopts wireless devices with ever greater capabilities, including tablets that support HD video and other advanced capabilities. Many cellular operators have tried to adapt this trend by throttling data rates, backing away from all-you-can-eat data plans, and offloading to WiFi. Over the next decade, further increases in demand are expected, and this issue of ZTE Communications examines millimeter wave communications as one technology that may answer the call.

Historically, the ever-growing demand for data capacity has been met by adding more spectrum and improving spectral efficiency, but spectrum reuse employing smaller cells has been by far the most popular means of adding network capacity. Deploying cells with ever greater density is a simple way of adding capacity to a network. Increasing the number of cells in the network increases the network capacity without increasing the capacity per cell. However, this approach becomes cost-prohibitive in part because it is expensive to roll out all these cells and provide them with a quality backhaul connection, for example, fiber. A less-expensive means of adding network capacity is needed in the long term.

As cells become smaller and link distances have become shorter, an alternative to adding capacity and reducing deployment costs is to use much higher carrier frequencies. Shorter link distances, which come with smaller cells, combined with recent advances in millimeter wave transceivers and antennas opens the door for the use of millimeter wave spectrum in cellular systems. An obvious benefit to this is the availability of a huge amount of spectrum. The 60 GHz unlicensed band alone offers 5-9 GHz of bandwidth (the exact amount depends on country), and there are many other millimeter wave and terahertz bands that have potential. Another great benefit of millimeter wave carriers is that high-gain, highly directional, electrically steerable antennas can be very small and greatly reduce interference. The wide bandwidths and narrow steerable beams enable low-cost deployment based on a wireless backhaul.

WirelessHD devices with 60 GHz phased array antennas are already on the market, and WiGig/802.11ad devices are on their way. ABI research predicts that by 2016 one third of all WiFi products will be tri-band (2.4/5/60 GHz). Although WiGig is intended to be an indoor, short-link technology (~10m), it may be an important standard used as a starting point for larger networks to use millimeter wave communications. Mass production of devices such as these will continue to drive costs down for millimeter wave radios and antennas that should extend to longer links.

The 60 GHz unlicensed band is of particular interest because of the growing ecosystem being built around consumer electronics that support WirelessHD and WiGig. However, the fact that the band is unlicensed means that it is riskier for cellular service providers to adopt. Molecular oxygen absorption at 60 GHz creates further confusion as some argue that these losses limit link distance. Others argue that reduced interference is worth it. Below 60 GHz, the LMDS bands are of interest and are underutilized; however, they offer less total spectrum than the 60 GHz unlicensed band. Recent technological advances may soon enable communications well above 100 GHz and into the terahertz region above 300 GHz, where allocations have not yet been made by regulators, and even greater bandwidths could become available. Some agreement on a band will be needed in order to make good progress.

Of course, there are also great challenges with millimeter wave systems. Although link distances in a line-of-sight environment might be easily closed with millimeter wave technology, the environment poses particular problems. Millimeter waves do not generally penetrate through buildings or diffract around them. Furthermore, humans are great blockers of millimeter waves and tend move around more than buildings. The problem of cost-effective routing around buildings and people will be one of the larger problems.

In this special issue, we examine the role that millimeter wave communication could play in cellular and cellular hybrid networks in access and backhaul. The first paper provides an introduction to the potential use of millimeter waves in a large network context and provides a preliminary simulation study. The second paper provides an overview of the 802.11ad/WiGig MAC and PHY. The third paper provides an experimental study of human blocking of millimeter wave propagation. The fourth paper describes the design and measurements of a 60 GHz LTCC phased array antenna with integrated waveguide distribution network that could be suitable for backhaul applications. The fifth paper considers the use of MIMO techniques for millimeter wave in line-of-sight conditions.

We are grateful to the authors who made contributions to this special issue and to the reviewers who spent their valuable time to provide valuable and constructive feedback. We hope that you find this special issue interesting and useful.

We are grateful to the authors who made contributions to this special issue and to the reviewers who spent their valuable time to provide valuable and constructive feedback. We hope that you find this special issue interesting and useful.

Key words: Millimeter Wave Communication, 802.11, Hybrid Networks, Cellular