Although standardization is not anticipated before the 2020 timeframe, mobile operators in Japan, South Korea, and the United States are already conducting advanced technology trials and are eager to deploy pre-standard 5G technology before 2020.
In contrast to its predecessors, 5G is a tapestry of technologies aimed at addressing the unique demands of different services, including the Internet of Things, enhanced broadband services, and highly reliability services for critical communications, such as public safety.
5G proposals for enhanced broadband services leverage millimeter-wave technologies, with the aim of capitalizing on vast radio spectrum resources in the vicinity of 30 GHz and above. Historically millimeter wave spectrum has seen limited use, primarily because it has challenging radio propagation characteristics and requires costly radio components.
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Conventional wisdom, and what the “learned” refer to as the “laws of physics”, tells us that when the carrier frequency of a radio system is increased, all other things being equal, the system has reduced coverage. It is for this reason that a cellular network operating for example in the 900-MHz band can achieve the same coverage as another cellular network using 1800-MHz spectrum, with the less radio base stations.
Based on this conventional wisdom, millimeter-wave solutions would have pathetic coverage, unless highly directional antennas are used to focus the signal between the transmitting and receiving devices.
While this conventional wisdom holds true when standard technologies and system designs are used, it is not the full story. Ironically, the laws of physics come to the rescue, with advanced technologies to enable millimeter wave coverage that is sufficient for 5G.
Firstly, with advancements in signal processing, tremendous radio performance gains can be achieved using advanced antenna arrays that consist of many antenna elements to independently capture multi-path (scattered) signals (the “techies” call this massive MIMO) and adaptive beam-forming to direct the antenna gain towards the strongest signal components.
However there is a catch. The performance of these advanced antenna systems generally increases with the number of antenna elements that are used. The length of each antenna element is proportional to wavelength of the radio signals; and for an antenna array to be effective, the antenna elements must be sufficiently spaced relative to the wavelength.
Since the wavelength of the signals used for cellphone networks typically range between 14 and 40 centimeters, large scale antenna arrays are not physically feasible in small form-factor devices. In contrast, the wavelength of millimeter-wave signals is 1cm or less, and therefore large antenna arrays are theoretically feasible in relatively small form-factors.
The radio coverage prediction models that have been developed for cellular systems are derived from extensive measurements and analysis of signals at frequencies of less than 6 GHz.
While efforts have been made to apply these models to millimeter wave technologies, they are inaccurate and fail to account for the salient characteristics of millimeter wave propagation. Most notably, millimeter waves can propagate over much larger distances than previously predicted, by being reflected from objects in the propagation path between the transmitter and receiver, and through enhanced signal scattering.
In the past, designers have not capitalized on these reflected and scattered signals, but rather have relied on line-of-sight (LOS) transmission paths with highly directional antennas. More recently, researchers, have demonstrated that when advanced antenna technologies are used in urban environments, reliable millimeter wave coverage can be achieved over several hundred meters with non-LOS connectivity.
This coverage is comparable to that of conventional cellular small-cells. In LOS scenarios, researchers have observed millimeter wave coverage of up to several kilometers when advanced antenna technologies are used.
The opportunities that 5G creates for millimeter-wave technologies and solutions has not gone unnoticed by mobile industry players and the investment community. Technology vendors, network operators, and even government regulators and incubators are aggressively pursuing technology trials. Investors are targeting players with millimeter wave spectrum holdings and startup technology companies with semiconductor and digital and analog solutions for millimeter wave implementations.
While the laws of physics and field trials give us confidence that the 5G aspirations for millimeter wave technologies are feasible, there are a variety of challenges that the industry must address before mass market millimeter wave deployments are achievable. For example:
- Although millimeter-wave frequencies enable the physical placement of many antenna elements to deliver massive MIMO and beam forming capabilities, analog and digital processing components are needed to active these elements. This is challenging to achieve with cost effective and energy efficient solutions, and an area where significant technology innovation is required.
- Small cells are needed for 5G millimeter-wave deployments. While the mobile industry has had high expectations for small-cells for over a decade, adoption has suffered from site acquisition challenges and excessive deployment costs. It is crucial for the mobile industry to resolve these challenges for the mass market small-cell adoption.
- While vast millimeter-wave spectrum resources are available, there continues to be debate regarding the most suitable bands to use for 5G. If the mobile industry plays its cards right, it could harmonize 5G millimeter-wave spectrum globally. Last year the industry had an ideal opportunity to drive harmonization when the ITU held its World Radio Conference (WRC15) event in Geneva. Unfortunately the results from WRC15 were conspicuously underwhelming, particularly in the context of millimeter-wave spectrum allocations.
- The availability of adequate backhaul is commonly raised as a concern for enhanced broadband services. While this is a genuine concern today, we believe that it can be largely resolved using millimeter-wave technology. However this will depend on adequate operational convergence between the access and backhaul domains of mobile operators.
While 5G enhanced broadband has its challenges, there is clear evidence of market demand. However this requires the mobile industry to take a pragmatic approach to its adoption, with realistic deployment and commercialization objectives.
Phil Marshall is Chief Research Officer at Tolaga Research
This article was first published on Telecom Asia 5G Insights September 2016 edition