Microwave technologies use electromagnetic waves in the microwave band to achieve wireless communication. While the microwave band theoretically includes all frequencies above 300 MHz, in common practice the term is used to describe communication conducted above 3 GHz. These frequencies are referred to as centimetre-wave and millimetre-wave (mmWave) bands.
What is microwave communication?
Microwave communication is most common in the form of point-to-point data links, which provide a cost effective alternative to conventional fibre optic lines. In its most popular usage, data communication is achieved by the use of two highly directional (high gain) antennas which focus a thin beam of electromagnetic waves each other through the open air. This thin beam consists of a channel of closely spaced frequencies which are digitally modulated to carry data. Each microwave antenna is connected to a radio unit which has the task of modulating and demodulating the carrier frequency, that is, sending and receiving the data.
Fundamentally, this is all microwave communication is. In practice however, microwave communication can be extremely complex. Unlike fixed line communication like fibre optics, microwave uses an unprotected transmission medium affected by the environment. Factors that may affect a microwave link include:
- Free Space Path Loss (FSPL)
- Rain / humidity / precipitation
- Ambient temperature
- Air refraction (k-factor)
- High wind (wind load)
- Hostile microwave links
- Solar radiation
- Pollen count
- Vegetation growth
- Terrain reflections
- Equipment ageing
- Vandalism
Properties of microwave frequencies
Below 3 GHz, electromagnetic waves are reasonably simple to handle. Low cost coaxial cables are an effective and economical way to interconnect a modem and antenna. As the wave becomes smaller, its propagation through physical media becomes more affected by ohmic losses due to the Skin Effect, where surface resistance increases as the frequency increases because the thickness of the layer carrying the current gets thinner. While specialised coaxial cables remain in use up through 110 GHz, as their complexity rises so too does their cost. Instead, microwave makes use of waveguides as its preferred transmission line.
For several decades microwave radio units were too large and heavy to install on the tower and instead used an All-Indoor or split IDU-ODU (Indoor Unit-Outdoor Unit) configuration. Waveguides were used to interconnect the radio unit and transmission antenna(s). Waveguides can be thought of in principle as a metal tube, where an electromagnetic wave continuously reflects back and forth off the walls until it reaches the receiver. Waveguides are almost exclusively made of metal and mostly rigid structures. There are certain types of flexible corrugated waveguides that have the ability to bend but are only used where absolutely essential since they degrade wave propagation.
Modern microwave communication
Today, modern microwave radios are almost exclusively in an all-ODU format, connected into wider telecommunications networks using SPF fibre optic cables. Data rates as high as 10 Gb/s are achievable using 8KQAM / 8192QAM modulation schemes. With many conventional microwave bands (such as 26, 28, 39 GHz) being refarmed for 5G, E-band (70 / 80 GHz) mmWave has become a popular choice for high capacity, short range backhaul.
With most countries implementing fibre-optic based national broadband infrastructure, the use of microwave as a primary data connection is falling out of favour. The technology however has become popular as a failover service, and remains critical for last-mile communications. Low cost microwave radios, such as those developed by Ubiquiti, Cambium, and Mikrotik have become a ubiquitous part of the landscape in developing nations, and regions where national broadband infrastructure has yet to reach.
