Microwave and Millimeter-Wave Links

Abstract

AbstractMicrowave links are terrestrial hops providing backhaul to all types of systems whether wired or wireless. They are in operation since 1950s but with the growth of digital wireless, all of them adopted digital modulation techniques with power control. The chapter starts out by asking basic questions on what these links are and how are they designed. Cursory calculations are made on link budget to verify the link budget indicated in Fig. 14.6 where path loss/gain is illustrated.Answer to some questions result in more details about microwave link design and parameters used to provide a reliable link. Other than the RF signal, factors such as wind load on the antenna and the tower it stands on, are considered, and reviewed. Important considerations in microwave links include Fresnel Zone, Earth’s curvature, polarization, terrain, climate, and frequency band. Other considerations that can affect operation of microwave dished are wind load, snow, and ice. Some of them are addressed by active power control available in all digital microwave links.There are reviews of regulations and guidelines that support and extend the use of smaller dishes and higher frequency bands. Historic change is introduction of millimeter band for point-to-point links. These bands directly support the Gigabit throughput rates expected by backhaul networks. All microwave links provide low latency (< 1 ms) unmatched by any wired network. Adaptive Modulation and Coding (ACM) is one that supports active use by regulators and operators since it allows a compromise solution. The compromise in terms of variable throughput rate due to weather conditions, allows smaller antennas (simpler towers, including option of mounting on roof tops), lower power limits over the air allows more links to co-exist. The chapter concludes highlighting growth of microwave links in Asia and shows why over half of world’s cellular systems use them.