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How many channels do I need for optimum frequency re-use?
To provide adequate coverage for a Field Area Network, there is a requirement to have enough channels allocated in your system design to allow for repeated frequency re-use over the FAN operational area.
“Our experience from deploying a number of Field Area Networks and large SCADA networks has shown that you require 14 channels as a minimum to allow for frequency re-use in the presence of terrain, fading and ducting,” says Doug McConnell, CTO at Mimomax.
In addition to these 14 channels, however, it is also recommended to have some additional channels spare so that at the boundary of the operational area, you can avoid using similar channels to adjacent users sharing the same spectrum. This flexibility is particularly important in scenarios where radios are deployed right up to the boundary of the licensed area.
Some key considerations:
1) Frequency band limitations
For some frequency bands, the narrow slices of spectrum will determine the channel size you can use. Looking at the 700MHz Upper A Block, there are only paired 1MHz blocks so if frequency re-use is a priority, you will need more than 14 channels and the widest channel size you can use is 50kHz.
“Even in other bands where the spectrum would allow for wider channel sizes (i.e. 100kHz), I would recommend you still aim to maximize the number of channels in your system. From my experience, the most powerful tool you have is to have spare channels to utilize when you really need it,” says Mr McConnell.
2) Terrain limitations
There are cases where the re-use of some channels are extremely limited due to the terrain. For example, a high site on a mountain may be required to be used to fill in some tricky coverage areas. Given their wide coverage due to their height, however, these sites substantially reduce the ability to re-use that channel elsewhere in the network.
“This scenario puts pressure on the overall number of channels required in your network if one of them can only be used once or twice across an entire operational area. If you only start with 14 channels and one of them is effectively used up for this high site, you then end up needing to re-use the other 13 channels more than might be ideal.”
Even with the best network planning in the world, in a Field Area Network you are likely to encounter self-interference – most typically due to terrain or atmospheric effects.
“In this situation you could look to re-plan the whole system, but we have found you may simply shift the problem elsewhere. The better solution is to have some channels spare and use those for any offending equipment.”
Utilities have a requirement to have their teleprotection circuits or SCADA networks working – all the time, not some of the time. While cellular systems may look at aggregate throughput, this approach will not work for mission critical communications.
“Even impact from low probability scenarios such as weather induced ducting can still take you below your required availability stats. Therefore, designing a system for high availability means you need to have allowed for spare channels to utilize if you find your availability stats are suffering.”