Wireless and microwave point to point links are widely used as a quick-to-deploy and cost effective alternative to fiber optic cabling for interconnecting the network of two sites with distances of few hundred meters and up to 50 km or more.
However like any other solution and probably more than many others, establishing a reliable and high-quality microwave point to point link can be quite challenging, and if it is not properly designed and implemented, it can cause major quality issues such as lower throughputs, link instability, and longer than expected latency.
In this article, I have tried to very briefly explain some key factors that should be considered in a proper point to point microwave link design – so that IT managers and engineers who are not experts in the field would have enough idea to enable them to properly evaluate and control such work.
What is a good point to point microwave link?
The quality of a point to point microwave link can be determined by below measurements:
1) Signal to Noise Ratio (SNR):
This ratio is measured by dB and shows the strength of signal vs the noise level for that frequency channel. The higher the value, the better but it should be at least 20 dB.
2) Bit Error Rate (BER)
This figure shows the % of bits of data with errors vs the total number of bits that have been transmitted during a period of time. The value is usually expressed as 10 to a negative power. The lower this figure, the better is the link quality. Good BER rates are usually in range of 10 -8 or better.
3) Bandwidth Throughput
This is the actual amount of data that can be transferred per second and is expressed by bits per second – for example a bandwidth throughput of 100 Mbps means about 100 megabits of data can be transferred by the link in every second. Obviously the larger this figure, the better the link.
Link latency determines how much time it would take to transfer the data – for a good microwave link, the latency should be fixed and not going over 2-3 ms. The easiest way to check the latency is to ping the destination device.
5) Link Availability
This parameter is expressed in % and determines for what % of time the link has been established over a certain period of time, usually in a 12 months period. A reliable microwave link should have link availability as good as 99.999%.
As microwave links can be well affected by time of day as well as many other geographical factors, for critical links it would be important to have a constant test of at least 48 hours.
5 Key Factors for a Stable Microwave Link
Below are 5 key factors you would need to ensure about for having a reliable and stable microwave link:
1) Frequency Selection
Microwave links range from 2.4GHz to 42GHz spectrum. The higher the frequency, the higher the available capacity but at the same time, the effective range is lowered and the link would be more susceptible to rain or high humidity. To use a frequency, a license should usually be obtained from the legal authorities of the country. There are also a few frequency bands that are “license-free” – mainly 2.4GHz, 5GHz and 24GHz.
While these license-free bands are in much greater use, professional solutions more depend on utilizing licensed frequencies which would guarantee a free-to-use spectrum greatly improving link reliability.
2) Calculating Capacity
The required capacity (bandwidth throughput) of a point to point microwave link is a key design parameter.
As the capacity increases, you would need to design the link for a higher SNR, resulting the need for stronger equipment and antennas.
3) Calculation of Line of Sight and Path Loss
For point to point microwave links, the antenna on the two sides should be in line of sight of each other. The line of sight can be limited by natural or man-made obstacles and also by the earth’s curvature which limits the practical distance of microwave links to 50-60kms (which would call for 100m tower heights and large dish antennas to achieve).
There are now many computer applications that can accurately predict the line of sight and path loss however a visual survey by an experienced engineer is also necessary.
4) Interference and Fading
Interference and fading is another issue that if not handled correctly, can considerably affect the link reliability.
Apart from issues such as Fresnel zone, rain fade, and multipath fading which require proper consideration during the path loss calculation, there are also other factors causing interference such as installing the radios adjacent to other radios which would greatly affect the receiving sensitivity of the radio (like when you try to hear a weak voice when standing beside a big speaker that is playing music).
The reliability of the link can be greatly increased by applying redundancy. In frequencies of 7GHz and above, dual redundant radios can be connected to the same antenna.
But this is not possible in lower frequencies and two independent radio links should be installed with sufficient frequency and space diversity.
Who can design and implement a successful point to point microwave link?
Design and implementation of a successful and reliable point to point microwave link requires good theoretical knowledge about RF design and antennas, as well as good deal of practical experience.
The concepts mentioned above are the primary information you can ask an implementer to make sure they have the required knowledge and expertise. You should also ensure you receive clear test reports for the established link.