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Typical microwave links tend to be 25 miles or less in length, but sometimes they have to be longer – much longer. For example, inter-island jumps in the Caribbean and elsewhere require longer links, as do situations like pipelines where builders try to build a network with as few hops as possible. Oil rigs are another example, where a rig in the Gulf of Mexico needs to communicate with the shore. So how does a microwave link become an ultra-long link? In this article, we’ll take a look at the considerations for building ultra-long links.
Link distance is based on transmit power, modulation, receive sensitivity, and path parameters. IP traffic is more bandwidth-intensive, leading designers to use higher modulations (eg, 16, 64, 256 QAM), which affects path length. The higher the modulation, the shorter the path length and the lower the reliability. IP traffic is a bit more tolerant than TDM because it retransmits lost packets, so sometimes IP link designers can live with lower path availability, or do adaptive coding and modulation, which allows radios to switch from one modulation level to another.
Let us now examine some considerations and techniques for the design of ultra-long microwave links.
The first consideration is site selection. A long link has to overcome the curvature of the earth, so it dictates quite high sites such as mountain peaks or tall buildings. Some of the longer links have antennas located 1,000 feet above sea level.
Another factor is the selection of the frequency. The advantage of a higher frequency is that you can use a smaller antenna and still get the same antenna gain, but the trade-off is that the higher frequencies attenuate faster – the signal does not propagate as well. far. Higher frequencies are good for dense urban areas where you need a lot of bandwidth but don’t need a lot of distance; lower frequencies are better when you need a lot of distance. Most ultra-long links use frequencies of 6 GHz in the United States and frequencies of 7 GHz or 8 GHz in the rest of the world.
Another consideration is the size of the antenna. The intuitive idea is that the bigger it is, the better it is when it comes to antennas, but that isn’t necessarily true. Up to about 10 feet, the larger the antenna, the greater the gain and the further the path will go or the more path reliability you will have, but the larger the antenna, the smaller the beamwidth. With large antennas, it will be difficult to align the beam and keep it aligned. When you get to a 12 foot antenna, it’s like trying to line up a laser beam. Larger antennas also require more robust tower designs, so instead of a 12-foot antenna that acts like a sail in the wind, designers can choose an 8-foot antenna instead. The tower should be designed to keep the antenna in the way, so you need a sturdy tower that won’t sway.
Microwave links are also sensitive to variations in atmospheric conditions which can cause bending of the microwave path. The temperature and density of the air are the two factors – you get an effect where the lower layers of the atmosphere are denser and the upper layers are thinner, which tends to sag the mic path. -waves like a clothesline. This is called the “K-factor”, and it can change from day to night because there is a more pronounced temperature gradient during the day and a more even environment at night as the air cools. And as the K factor changes from day to night, the microwave beam can become defocused and you can lose the signal. There must be a beamwidth large enough to cope with the changing conditions due to the K factor. To overcome the K factor, designers place the antennas higher to pass over the lower layers of the atmosphere.
In addition to the high placement of the antennas, spatial diversity is another technique used by designers to create links longer than 25 miles. Spatial diversity is the idea of using two antennas instead of one to receive a signal. You would have two antennas on the tower, usually about 30 feet apart vertically for 6 GHz systems, and you would typically transmit on the top antenna and have a receiver on both antennas. When you get a signal fade on one antenna, the signals are decoupled enough with the vertical spacing that the other antenna still has a good signal. For example, instead of using a 12-foot antenna on a longer path, you can use two eight-foot antennas to create spatial diversity.
Another challenge to overcome is the multipath propagation delay. Multipath propagation delay refers to the microwave signal taking different paths. The main signal goes directly between the antennas, but you can have minor signals taking longer paths and they have more delay between the antennas. There may be reflective layers in the atmosphere that can actually cause a path to obscure. There may also be reflections from the ground – for example, when farmers flood their fields, those fields turn into mirrors, causing interference and very long delays. The solution to multipath delay is to use spatial diversity and position the antenna higher on the tower to get away from the reflection.
The planning software gives you the azimuth and tilt angle of the antenna, the signal strength to expect and the path availability. It helps designers understand whether or not to use ACM to reduce modulation levels to maintain link reliability. It also identifies the roughness of the terrain and any blockages. Smooth terrain has more chance of reflection, so rough terrain increases the reliability of the path. Path loss is a commonly used brand of path planning software.
When deploying the link, the alignment of the antenna is crucial. Techs start by aiming at the compass and level to get a rough alignment based on the original path design, then they adjust the antenna back and forth and up and down until they can maximize the reception signal. Initial path calculations indicate what the receiving signal should be, so technicians will adjust the antenna alignment until they are near optimal signal strength.
The design of ultra-long microwave links is a very specialized skill that requires a thorough understanding of the considerations discussed above. It’s about having the right conditions and the right antenna sites, choosing the right antennas and sizing them correctly, and using spatial diversity to overcome multipath propagation. Using these techniques, it is possible to create links that stretch for 120 miles or more.