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Operators should consider link availability when exploring the use of higher modulation radios or adaptive coding and modulation (ACM) radios, especially in today's and tomorrow's higher density environments. In the end the ultimate aim is to have increased availability of higher capacity in high density environment with minimum total cost of ownership (TCO). ACM radios are a hot topic in the backhaul industry these days, and with good reason, for they offer some promising benefits based on adaptation to local conditions. Wireless backhaul systems-like many other transmission networks-have been moving to higher modulation schemes to increase capacity.
A general rule about higher modulation, such as 128 or 256 QAM (Quadrature Amplitude Modulation), is that they are more sensitive to distortion from the environment including rain, as compared to lower order modulations. Operators are using these modulation schemes because more bits can be packed into the same amount of bandwidth, thus increasing capacity. If skies are blue and there is no rain, 256 QAM works great. If weather is bad, then a high modulation becomes problematic as the signal begins to get distorted. That is where ACM radios come into play.
ACM radios change the modulation schemes utilized based on local weather conditions (essentially). If it is sunny outside, than higher modulation schemes will be used. When storms roll in and signals start to be distorted, ACM radios move to a lower QAM, which are less susceptible to environmental distortion. Sure, the operator loses the capacity advantages of high QAM at these times, but it is better than losing the signal altogether. That is an aspect of availability. But the class of microwave antenna deployed also impacts link availability even when ACM radios are in use in high density environments.
ACM allows operators to use smaller antennas unless there is a very high amount of interference scenarios. It is reasonable to expect to be able to switch to a one-foot antenna, and using a smaller antenna means lower tower leasing costs. But if you deploy a Class 3 versus Class 4 antenna, there is a significant difference when it comes to signal availability. It's a huge jump in down time.
A case study of microwave links has been done with ACM radios and one-foot microwave antennas of two different ETSI (European Telecommunications Standards Institute) antenna classes: Class 3 and Class 4. Across five modulation rates the Class 4 antennas performed significantly better when it came to link downtime. Both classes of antennas allow for essentially the same throughput rates, but the unique side lobe suppression of Class 4 antennas translates to more link availability. Losing bits means losing revenue and customer base! Less decibels of interference means more data getting through, hence more dollars for an operator.
If you would like more details about the case study, leave me a comment and I will get back to you. If you'd like to learn more about CommScope's Class 4 Sentinel antennas, check out this brochure (PDF). Does anyone have experience utilizing Class 4 microwave antennas? Have you noticed any performance benefits like I describe? Leave a comment if you'd like to share.