5G mobile technology promises to offer fantastic performance, such as 1-10Gbps actual delivered connection speed, and 1000x bandwidth per unit area. The only way to enable this is by using spectrum in the millimetre wave bands (30-300 GHz), and the EHF band (30-100 GHz) in particular.
Two factors have contributed to making mmWave bands available for mobile services. 1) The RF technology for components that can operate in these extremely high frequency bands has matured and is ready for mainstream commercial use. And 2) Increased processor performance will make the extreme demands of 5G signal processing possible in around five years.
Millimetre waves behave differently from radio signals and their propagation resembles visible light more than traditional radio. For example, mmWaves can not penetrate solid objects such as walls. Even the human body, foliage, or glass windows with curtains pose significant problems. Another issue is the signal attenuation by the atmosphere and rainfall, which is higher for mmWaves compared to traditional radio signals. It was previously thought that millimetre waves could only be used with direct line of sight (LoS), and this limitation made the frequency band rather useless for wireless communication. However, field trials have shown that mmWave signals bounce (are reflected) on hard surfaces such as walls in an effective way. This makes it possible to achieve good Non Line of Sight (NLoS) outdoor coverage of at least around 200 meters in an urban environment.
The bouncing creates a lot of multipath propagation but this problem can be overcome with high performance signal processors. The limited reach and air attenuation is actually an advantage if mmWaves are deployed in ultra-dense networks. It will make it easier to reuse the same spectrum as nearby access nodes won’t interfere with each other.
But the real advantage of mmWaves is the abundance of spectrum. In the traditional radio spectrum (0-3 GHz), the mobile industry has until now managed to deliver their services to 4.5 billion people on less than 0.6 GHz of allocated spectrum. If only 5% of the mmWave band (30-300 GHz) was allocated to mobile services, the available spectrum would increase by a factor 25. This abundance could be used for extremely wide carriers that would enable multi Gigabit speed bandwidth.
In the mobile industry’s vision, 5G will provide up to 10 Gbps bandwidth in urban areas with ultra-low latency, almost 100% availability, and seamless fallback to 4G when coverage falters. If and when these targets are achieved, it is claimed that 5G mobile could be a serious competitor to landline fibre access. More outlandish claims are that “everything will be mobile and the fibre network will be abandoned”. There is good reason to be sceptical.
It is not yet clear whether mmWave signals can penetrate buildings from an outdoor access point at all. Can the signal go through a window? Will it be blocked if curtains are drawn or blinds are pulled down? Or will the consumer be expected to mount a 5G repeater unit in his or her window, or outside the window? What if a cat walks in front of the mmWave transceiver on the window sill? These are potential inconveniences and could be a barrier for mainstream market adoption.
If mmWave access nodes are deployed indoors, the wireless backhaul from the indoor node to the core network must have the capability to penetrate walls. This would require a narrow beamforming antenna (array or horn antenna) using lower frequencies than the mmWave bands (the only way to penetrate walls). This antenna can be aimed toward a nearby target node with a connection into the core network. If the access nodes are self-deployed by the users, an array antenna could possibly self-configure and form a beam in the right direction. However, it is unclear if an array antenna can form a beam in any direction (up/down in addition to horizontal). The user might have to manually point the antenna in the right direction. Alternatively, the access node could be equipped with a servo that aligns an internal antenna. Health concerns regarding radiation could be another barrier to adoption as the narrow beam from the access point will generate a strong RF signal that passes through the user’s home. (It is of course possible to connect an indoor 5G access node to the existing in-building fibre directly, but that can hardly be called, “replacing fibre with 5G”.)
Indoor use of 5G based on mmWaves face additional challenges. The signal can propagate between rooms by bouncing off the walls but a closed door will most likely cut the connection. Seamless fallback to Wi-Fi and/or LTE can manage this situation but these legacy technologies will not be able to deliver the same performance as mmWaves.
The demands for fast processing in 5G will be extreme. Higher data rates and lower latency require faster processors. The critical building block for the 5G mmWave technology is narrowbeam MIMO antennas, and they rely heavily on signal processing. In a scenario with extreme data rates and extreme user density, the requirements for processor performance will exceed what today’s processors can deliver. Moore’s Law has already slowed down significantly, and if it comes to a halt, processor capacity could be a barrier for the 5G vision. It is believed that processor technology will not be able to fully meet the requirements for 5G until 2022 or 2023.
The costs of deploying ultra-dense 5G networks will be substantial. If the range of an access node is limited to a few hundred meters, thousands of access nodes will have to be deployed in urban areas. The major cost driver will not be the electronics but installation, cabling, power and maintenance. The traffic from each access node will have to be backhauled into the core network. Probably with Point to Point narrowbeam links which have to be aimed towards a nearby access node that is connected to the core network (via fibre or further mmWave links).
Even though 5G has a fantastic best case performance, the first deployments will be underprovisioned. The number of access nodes will initially be insufficient and when peak traffic exceeds capacity, 5G networks will suffer from the same type of service degradation as 3G and 4G networks. 5G will often fall back to 4G. The operators have a limited investment budget, and they will most likely settle for a slower and less costly rollout of 5G. Users who expect their wireless 5G to replace fibre will be disappointed.
In addition, new civil engineering technologies are reducing the costs of deploying fibre in street ducts. 5G is still at least five years away from a broad market launch and, at that time, the fibre networks will have a much larger user base than they do currently. When fibre service providers begin to face competition from 5G, they will of course lower their prices and offer higher bandwidth to stay competitive. The bandwidth that 5G will deliver in a decade should be compared to what fibre can deliver at that time, not with fibre capacity today. In a competitive market, 5G will not be a viable direct alternative to fibre for at least a decade, if ever.