Balancing costs: Size Does Matter! Microwave Radio Power vs Antenna Size

For this discussion, let’s talk about some key metrics the FCC requires on licensed radio paths and then look at the total cost of ownership (TCO) for every element on that path. This may sound a bit cliché; but, really, in this case, higher up-front investment will usually save you tens of thousands of dollars in the future.

The most obvious cost of deployment is the costs of the radio, but without thoroughly evaluating the features and capabilities of those radios, the results can be extremely detrimental to the experience you deliver to your clients as a service provider. The customers that rely on the Internet everyday will be the first ones to let you know when they’re not happy with speed and reliability so the last thing you want is a network link that is unstable—which is usually caused by a weather event. You need a link that can maintain a solid, uninterrupted connection, under any circumstance, for years to come.

In order to have a baseline of features that combat weather and environmental changes over a licensed link, the FCC has put certain requirements in place for microwave paths to help maintain a reliable link. One major feature is known as Adaptive Coded Modulation (ACM). ACM allows a microwave radio to adapt to changing weather and atmospheric conditions that may hinder “receive” signals across the entire length of the path. When ACM is present on a microwave link, its sole duty is to keep the microwave link up and stable through an entire weather event. The technology is so widely used that more than 90% of deployments today have ACM built into the licenses assigned by the FCC.

Since ACM technology is now a critical ‘must-have’ feature present on a licensed link, equipment data rates are only acceptable for filing when used for Adaptive Modulation systems where accepted requirements are met 99.95% of the time. With that requirement in mind, a microwave link must be designed to minimally deliver that availability when connecting two sites over a specific distance if ACM is a needed feature.

An ideal microwave path design will always factor in additional fade margin or ‘head room’ for a path before the radios want to modulate downwards to control connectivity during some type of weather event. That said, there are only two ways to increase your licensed paths reliability… bigger antennas, or the radios output power. When you want to deliver optimum signals with good fade margins, high reliability, and high throughput--especially over longer distances – the radio transmit power is going to play a major role.

Every true full-duplex microwave link has two sets of internal radios in each terminal; one radio is dedicated for RF transmission, the other is dedicated for RF retrieval. Both internal radios are always talking and operating on separate frequencies of one another, this is also why licensed microwave is rated in a full-duplex throughput fashion. The throughput speeds are the same for upload and download.

The transmit radios RF power output is measured in ‘dBm’. The receiving radios will ‘listen’ to the signal coming from transmit radio on the other side, and report that signal with an RSL (Receive Signal Level) value, also measured in dBm. The RSL directly reflects how well the receive radios can ‘hear’ the transmit radios through the antennas between each side. The RSL number is normally represented as a negative value (Ie: -41dBm), so it is important to remember that the higher the negative value (further from zero), your transmit signal is weaker; the lower the negative value (closer to zero), your transmit signal is stronger.

Because antennas have static amounts of output (dBi) gain at a certain frequencies, the link will want to drop if there is not enough radio transmit power behind the antenna to push through changes in weather (ruining the signal a radio hears). Meanwhile, ACM radios that can adapt and adhere to changing weather conditions will allow the radio to modulate into different schemes (Ie: From 512QAM to 256QAM) without hiccup, thus maintaining the link connection.

Modulation schemes are steps in throughput capacity that are delivered based on receive signal levels seen by the receiver radios in each terminal. Each modulation step has a receive level “threshold” that needs to be ‘heard’ in order to deliver ‘X’ throughput at said modulation. When a path is designed, a Pathloss report is generally used for reference and planning. The Pathloss report will give you target RSL numbers to hit when aligning your antennas. The report also lists what a radio’s receive level threshold is for each modulation scheme when ACM is a factor. The receiver (RX) threshold, minus the RSL (both measured on a negative scale) is how one figures out the fade margins for each modulation step.

For Example: If a radios receiver threshold (RX) for a 256QAM modulation scheme is a low -62.50dBm, and the path analyzed is reporting a strong RSL of -40.59dBm when the antennas are aligned properly. The difference between the two values is the expected fade margin before the link steps down in modulation. For this example, the answer would be 21.91dB of fade margin. Thus meaning the radios RSL would have to fade (during weather) from a strong signal level of -40.59dBm to a weaker signal of -62.49dBm before stepping down to 128QAM modulation--dropping throughput delivered, but maintaining the link connection without missing a packet.

The RF signal coming off an antenna is known as ‘gain’ and measured in ‘dBi’. An antenna’s size, in diameter, is in direct correlation to the power it can emit into RF signal, i.e. the larger the antennas diameter is, the more gain (dBi) is delivered, the smaller the diameter, the less gain is delivered.

But…size does matter… An important point to remember is the size of an antenna is always going to directly impact the cost of the antenna itself. Additionally it will cost more to transport it to your site and to hang it on a tower, building, or structure. If you need to rent or lease space on a tower or building, the space occupied by the antenna is generally charged according to size which is billed on a monthly/quarterly reoccurring cycle. In time, that reoccurring cost will equal thousands upon thousands of dollars for rented ‘air’. The wind loading stress larger antennas put on a tower are always a critical fact to think about, it is very important to look at the limits to tower capacity, and consider not only for now, but in the future when additional gear is added to them. One antenna too much, and your tower could potentially be laying perpendicular to the terrain it’s on.

When you combine the two measurements from a microwave radios transmit power plus the antenna gain (dBi+dBm), you’ll figure what is most commonly known as ‘total system gain’. The higher your total system gain, generally the higher your link availability will be over longer distances. When a microwave antenna is aligned correctly, the RSL numbers are generally going to be within a dB or two of the link analysis populated by a wireless point-to-point network engineer. If the path is designed properly, a weather event will press on the radios to modulate down according to the RSL they’re seeing from the opposing ends transmitter, at that time, the ACM takes over and functions as it’s supposed to. This will result in a link that stays online 99.999% of the time, never completely losing the link and going ‘out of service’.

Total system gain should always be a driving consideration when determining if you should invest up front in radios with higher amounts of output power or run larger antennas. Generally, the smarter investment for most operators needing to save money on real estate is radios that deliver higher power vs. running antennas that cause more stress on the tower or structure they are mounted too.