Guard band recommendations

Hi all,

Interested to know what peoples recommendations are on guard band for ePMP 3000 (and 5Ghz in general), Both GPS sync and non sync ?

I’m thinking 5Mhz as a general rule, but have had other suggestions such as 20Mhz-30Mhz.

Thanks everyone.

I recommended to Jesse to try it with 20mhz and see how it goes. Depends upon the front-to-back ratios of the antennas being used. If using horns it could be quite close but i think he said you guys were using sectors?
With GPS sync enabled you can get quite close.

Hey Ray, That was quick :smiley:
Yeh for this example ePMP 3000 sectors (and some rfelement horns), Front to back ratio in docs suggests “Front-to-Back Ratio 30 dB”. I’m sure one of you rf pros have some way to calculate this. AFAIK (and I don’t claim to know much) a little interference shouldn’t smash the SNR to the point it’s worth sacrificing some of that tasty clear spectrum?

Let me provide a bit more info for anyone with the same question, which is a very good question, BTW. I guess I’m one of those RF pros you mentioned.
Front to back ratio essentially protects your opposite sector SM’s from the “wrong direction” AP. The best situation is a 4 sector site (using sector antennas or horns) with sectors spaced ~90 degrees apart. Use GPS synchronization, and the same TDD split percentage on all sectors within the market area. Channel plan can be reuse=2 (same frequency in opposite directions) or use more channels to relieve co-channel interference (that is self interference on the same frequency) Don’t mix bandwidths…use the same bandwidth for each sector across your market area.
The guard band you are talking about protects the adjacent channel AP receivers from SM’s transmissions. Generally speaking, when a carrier is immediately adjacent to another carrier (same bandwidth), the receivers can provide ~25-28 dB isolation. When you have F1-space-F2 channel plan, when the space is 1 channel width, the receivers will provide about 45 dB isolation. If the space is less than 1 channel bandwidth, then the value ranges from 45 dB to 25 dB isolation.
So how much guard band (if any) do you need? Its actually simple to calculate. Measure the UL RSSI at the AP from a SM within the sector at a short range (500m or so), note the TX power of the SM. Then, move out of the sector with the SM at the same short range, and connect to the same AP. See what the RSSI value and SM TX power now is. The difference in RSSI values plus the difference in TX power is the actual isolation value your antenna system is providing. Apply these values to this formula:
Max UL RSSI minus antenna isolation calculation derived above minus the SNR required for maximum data rate service you provide must be greater than the noise floor you measure at the AP’s. If you need some more isolation, then use adjacent channel + guard band to provide it.
If your system meets this qualification, you will be providing awesome subscriber service!


Quality response Dave. I appreciate your comments and time!
Thanks for clarifying for me :slight_smile:

The quick and dirty rule is… if you’re using Cambium antennas (which generally have 30+dB F/B ratio), for sync’d PMP450 you don’t need a guard band, for sync’d ePMP you need 5MHz guard band. For unsynced radios you should have 10MHz guard band for every 10MHz of channel… e.g. a 40MHz channel will need at least 40MHz guard band.

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