Why is it so hard to establish point-to-point links via microwave?
There are two answers to that question. The first can be summed up in one word: terrain.
Microwave signals are blocked by anything more substantial than blue sky. Hills, buildings, water towers, and trees in or close to the line of site between the two microwave antennas block, reflect, or absorb part or all of the microwave signal. Even rain and snow can degrade data throughput, although this is a relatively minor issue at the frequencies used by MAIPN.
Since our backbone nodes are in high locations, free and clear of nearby obstacles, the biggest challenge to connecting end-users to the backbone is getting over nearby buildings and trees. (Trees in this area tend to grow to a height of 100 feet.)
Here is an example of that. The photo shows a view of Catoctin Ridge in Frederick, MD, from the site of a planned public event at which we were asked to provide broadband service. We happen to have a backbone node on the Ridge at a location just to the left of the flagpole. ((The cell tower is highlighted just as a point of reference.)
In this case, the flagpole and nearby trees are close enough to the line of sight to prevent a solid connection. A 20-foot mast was not high enough to provide the needed clear path. Had we been able to set up a relay station on the roof of the school, we could probably have found a location that would have provided a clear path. Alas, that was considered impractical for this one-day event. At this location, we needed our dish to be at least 50 feet in the air to overcome the nearby obstacles. And this is considered relatively open, flat terrain in our corner of the world.
So, terrain is a big issue. The preceding example should give you an intuitive sense of how this plays out in the real world.
The second biggest issue is interference. To keep costs down, MAIPN uses equipment that operates in what is called the Part 15, or unlicensed microwave band at 5.8 GHz. This is the same band that is used by many Wi-Fi routers, cordless telephones, baby monitors, and garage door openers. At fist glance, one would think that our microwave links would be unusable in our urban and suburban communities due to interference from literally thousands of sources along the path between two of our backbone nodes. However, we have found that we can reduce interference to a tolerable level by siting our backbone nodes well above surrounding buildings and using high-gain dishes with correspondingly narrow beam widths. To the extent that interfering signals do reach our radios, spread-spectrum modulation methods reduce the adverse impact. In addition, the radios utilize adaptive modulation techniques, automatically reducing symbol rates and increasing redundancy whenever bit error rates start to go up. This degrades the overall channel capacity of the connection, but in a way that is transparent to end users other than slower response times in extreme cases.
(to be continued)