Line-of-sight (LoS) vs. non-line-of-sight (NLoS) transmission is the distinction that is most easily made. Some means of transmission work only as long as there is a direct, unobstructed “line of sight” between the antennas of the receiver and the transmitter.
This requirement puts a cap on:
- After a certain distance (which depends on the height of the two antennae), the Earth’s natural curvature causes the two antennae to become invisible to each other. An observer sitting on top of one antenna would not be able to see the other one, because it’s below the horizon.
- Deployment environment. Even over short enough distances, there are obstacles, such as tall hills or broad, tightly-packed groups of tall buildings that render the antennae invisible to each other.
Non-LoS methods do not require a direct, unobstructed line of sight between the two endpoints. However, most non-LoS methods do rely on LoS communication with intermediary nodes called repeaters — so you can think of non-LoS communication as being achieved, more or less, by “stringing together” several LoS sections.
Optical vs. microwave/RF links. Wireless data transmission over non-trivial distances can be carried out by two means, and both of them rely on sending data using electromagnetic waves.
One option is to send data encoded in pulses of light; the other is to send data over a stream of radio waves — typically, though not necessarily, in the microwave region of the spectrum.
Free-Space Optical (FSO) links are the most widely-deployed solution that relies on optic data transmission. FSO links are fast, line-of-sight links with several useful properties.
Microwave links can be either LoS or non-LoS, so they work over higher distances. Traditionally, the maximum distances have been in the 60-100 km region, but there is no hard limit.
Licensed vs. License-Free/Unlicensed operation. The electromagnetic spectrum is divided into several slices or *regions*, based on frequency. Some of these regions, such as the region between 694 and 790 MHz, can only be used by operators who hold a *license*, and only for specific purposes. Other regions can be used without a license, as long as the devices operate below a certain power threshold.
Private operators can be (and often are) granted a license, but the process is not very straightforward, and operating within a licensed band is neither easy nor cheap. When it comes to internal communication/data traffic, operating in a license-free region is far more hassle-free and is sufficient for most businesses.
Three technologies see significant use in the UK today, and we are going to discuss each one of them. These technologies are:
- Free-Space Optical (FSO) links
- Short-distance microwave bridges
- Long-distance microwave link
Free-Space Optical (FSO) Links
FSO is an optic, line-of-sight, license-free data transmission method which uses a tightly-focused beam of light to transmit data between two endpoints. More to the point, it transmits data by modulating a laser beam.
If you think this sounds an awful lot like fiber optics, you’re right — except that signals are transmitted through air, rather than through a dedicated transmission medium. The lack of a dedicated transmission medium (i.e. the cable, made from a material with special optical properties) that you can bend and route in order to guide the light means that there has to be a free, straight path between the two endpoints. But it also means that there is no fibre cable to bury or break.
FSO links use light beams at specific wavelengths in the infrared spectrum, between 750 nm and 1550 nm. These values are not chosen at random — they are chosen so as to be safe for the skin and eyes, and to guarantee good signal propagation properties.
Speed. With today’s technology, FSO links can reach speeds of 622 Mbps, 1.25 Gbps, 2.5 Gbps or even 10 Gbps. This is on-par with other point-to-point wireless technologies.
Distance. The distance that FSO links can cover is limited by atmospheric conditions and power. Typically, FSO equipment can operate reliably, year-round, over distances of up to 1.5 – 2 km, but they are usually deployed over shorter distances, on the order of 400-600 meters.
Reliability. Historically, the reliability of FSO transmission used to be frequently brought into question. However, modern FSO technology can offer carrier-grade reliability over distances typical for urban deployment. Where absolutely necessary, FSO links can be deployed along with a redundant, RF-based LoS solution (i.e. a microwave uplink).
Equipment. FSO links use remarkably compact, self-contained equipment. You will need to install a FSO laser link device at each communication endpoint. This device incorporates both the transmitter and the receiver, and looks somewhat like a bulky CCTV camera.
The only major installation hurdle is that it needs to be tightly secured in place, on a stable surface, so that its beam can be aligned to the receiver at the other end. This is why FSO emitters typically cannot be installed on top of poles and are rarely used for mobile or portable units.
