Good morning Tim,
I think I can start the discussion here.
Minimum floor RSSI should be determined by your service levels you want to deliver to your customers. The short formula is this - all measurements are in dB, or dBm as indicated by ():
Noise floor of channel bandwith (dBm) + known interference (dB) + SNR required to provide your max service level (dB) + fade margin (dB) = RSSI floor at SM (dBm).
For example, using a 40 MHz channel, with 6 dB of known interference and you want 3 dB of added fade margin, supplying a 100Mb/s best effort service:
Noise floor = -97 dBm + SM noise figure (5dB) = -92 dBm
Known interference = 6dB
Fade margin = 3 dB
Service level = 100 Mb/s DL = 4x MIMO B modulation required = min 17 dB SNR
We add up the values and we see we require minimum -66 dBm to achieve 100 Mb/s DL throughput. Less throughput would be lower modulation level required, so lower minimum RSSI.
The next part of your question’ s answer is statistical. The net performance you can achieve on any particular sector is related to the weighted average of the SNR each SM in that sector is achieving. The easiest way to calculate this is to create a spreadsheet with each sector as a tab. Then in the columns, list the SM connected, the DL SNR reported and the volume of data passed through that SM in a 24 hour period, for example. Using the sumproduct function, you are going to take the average of the SNR reported, weighted by the 24 throughput. The actual formula is “=sumproduct(SNR, Throughtput array)/(throughtput column)”. There are quite a few excel tutorials on how to set this up.
cnMaestro can deliver a report, you edit the report in csv, create a pivot table to filter your data by sector, and run the weighted average. Sinple, yes? LOL. This is 20 years of radio network planning experience summed up in a few paragraphs.
The idea is this…an SM with low SNR, but passing a small amount of traffic has little effect on the sector. An SM with low SNR, and high throughput affects the sector in a more significant way, since the low SNR takes more time to handle the traffic, and therefore is a larger load on the sector.
My experience is that on most multipoint sectors where we have a range of SNR for the connected SM’s from 2x to 8x, the net throughput on the sector is between 50-70% of the spec of the AP. If you are dilligent about connecting SM above your minimum threshold, you can achieve 80-90% of the AP’s rating. A large part of being dilligent about your SM connections is making sure that your SM is connected to the best serving sector. If it connects to any other than the best serving sector, performance will take a hit. I’ve taken networks in the past that the SM was allowed to connect anywhere. By re-aligning SM’s to best serving sector, network net throughput increased by 30%.
I hope this helps. Please ask any questions you might have. I’m pretty sure the first question will be “what about the uplink?” This is a whole new discussion due to ATPC operation.
Dave.
