With 5G technology on the verge of a massive roll-out, Mobile Operators are currently evaluating their 5G network strategies. According to ITU-R Recommendation M.2083 for IMT2020 and beyond 5G usage scenarios are focused on three critical areas. Enhanced Mobile Broadband (eMBB) services offer ultra-fast data rates and broadband speeds of up to 10 times more than existing 4G networks and are required to support data-driven applications such as 3D/UHD video, video-on-demand, fast downloading, etc. For Massive Machine Type Communications (mMTC), lower data rates are not required, however, a high coverage network has to be deployed to connect many devices and support services such as Internet of Things (IoT) and Smart Cities.
Finally, the 5G area that will support services with the most demanding network requirements is Ultra-reliable, Low-latency Communications (URLLC). These types of services have strict requirements regarding latency and network reliability. 5G systems should be able to provide end-to-end latency as low as 1 ms for applications that require ultra-low response times such as e-Health, remote Surgery, Tactile Internet, and Autonomous Driving.
Microwave radio in 5G
Legacy microwave backhaul bands are in the range of 6-42GHz, while new equipment has been developed in the millimeter-wave region at 60GHz (V-Band) and mainly 80GHz (E-Band). In the new millimeter-wave areas, large amounts of spectrum are available, therefore, offering higher data rates such as the E-Band systems that support up to 10Gbps per link. Various factors define network latency, such as data rate and channel bandwidth. Larger data rates and larger channel bandwidths result in lower end-to-end latency. With the available bandwidths and data rates in the millimeter region, it is evident that operators need to move to Millimeter-wave frequencies to support 5G URLLC applications.
Microwave Vs Fiber
Microwave connectivity has been used for many years in telecom networks. Although fiber penetration is the number one priority for most operators, microwave connectivity is the best alternative in cases where fiber is too expensive or difficult to deploy. Due to the fast time-to-market, and easy installation, microwaves are a low-cost solution. They can also be used complementary in the 5G roll-out until the fiber is deployed extensively. New millimeter-wave bands in the E-Band region offer multi-gigabit speeds. In the meantime, new equipment close to or over 100GHz is already under development in the W (90GHz) and D-Band (140-171 GHz). This equipment will take advantage of the available spectrum in that frequency region, and together with other techniques such as MIMO, XPIC, and Channel Aggregation will provide super-fast connectivity up to 100Gbps per single radio link.
Propagation Latency
A microwave link is a radiocommunications system that uses electromagnetic waves (radio waves) to carry information from one specific point to another. Optical fibers are used to transmit light through a thin fiber of glass to communicate from one location to another. Microwaves (RF), fiber, and visible light are all part of electromagnetic radiation. All Electro-Magnetic (EM) waves travel at the speed of light. Visible and non-visible light, such as radio waves and optical communications, are made of photons and travel at the same speed. In physics, the Speed of Light “c” in a vacuum is precisely 299.792.458 m/sec. This means that for 1km distance, the latency in vacuum is ≈ 3.34 μsec. When light travels in a material, the speed is calculated using the refractive index n of the material, which is the ratio between c and the speed v.
The refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 199,861,639 m/s, which equals to 5 μsec per km. The refractive index of air is about 1.0003, so the speed of light in air is just slightly lower than in vacuum, which is c/1.0003 ≈ 299,702,547m/s that equals 3.34μsec per km. This means that the signals propagate faster when the carrier medium is the air than the actual optical fiber.
Fiber Vs Microwave Latency
Although theoretical carrier latency per kilometer is about 3.4μs for radio and 5μs for fiber, other factors also affect overall network latency. RF units add slightly more latency than a typical fiber switch due to modem processing needed. However, new chipset solutions and data forwarding techniques have been developed that limit the one-way latency down to 50 up to 20 us. This way, the main factor that determines the overall latency remains the link distance. It is obvious that if a link is several or even tens of kilometers long, microwaves have an advantage over fiber. For smaller distances, the opposite is true.
Of course, the network topology is also a significant factor. With new 5G deployments and higher bands for 5G NR, network densification is expected. More base stations (micro or small cells) will be deployed to cover the same geographical areas compared to existing networks. This means that link distances will shorten and this will be of advantage to fiber installations. However, microwaves might still be the preferred solution to suburban or rural connectivity, while at the same time a reliable alternative in city areas until fiber penetration reaches 100%. Furthermore, with the upcoming radios in bands over 100GHz will higher data rates and channel bandwidths, new millimeter-wave radio solutions might become even more attractive over the coming years.
