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 may use line-of-sight paths or engineered relay arrangements. Feasible distance depends on frequency, path clearance, terrain, antennas, permitted power, fade margin, capacity and availability targets; it should be established through path engineering rather than a generic maximum.
Licensed vs. License-Free/Unlicensed operation. UK spectrum use is governed by current Ofcom licensing and licence-exemption rules. Each band can carry conditions covering purpose, EIRP, location, coordination and technical controls; licence-exempt does not simply mean operation below one power threshold. The appropriate route depends on interference risk, capacity and availability requirements and must be checked for the proposed equipment and sites.
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. FSO throughput is product- and configuration-specific. Compare current vendor data against the required usable capacity, interfaces, distance and atmospheric availability rather than relying on a fixed list of line rates.
Distance. FSO distance and availability depend on the selected optics, alignment, path length, local fog and precipitation data, building movement and the required fade margin. A link should be engineered from product data and site-specific atmospheric conditions.
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.
