# Frequency reuse and colocation

Hello, is it true to say that when using frequency reuse, “back” and “front” both transmit at the same time? Using GPS only for this thread.

If the answer is yes, i’m now confused.

I understand the theory is for APs to transmit at the same time. So if front and back transmit at the same time, is there any difference between the two? Why have front and back, instead of say A or B?

How about “colocation” and what it does? Does it change front/back syncing?

I previously thought this was easy to understand, so why am I confused? I may be dumb of course lol.

This part of my network has 4 APs in a cross configuration - one top, one bottom, one left, one right, like a religious cross. I want all APs on same freq, broadcasting at the same moment. They all same 75/25 windows of course. Does it matter if they’re front or back, and should I use colo even though only one AP per tower?

Thank you

Let me try to help you understand these terms from a deployment point of view.

There are three parts to the formula for using any kind of reuse on any broadband equipment:

1. The sector antenna transmits in every direction. It happens to be about 28-30 dB better in the direction it was designed than in the opposite direction. This is the spec front to back ratio that is on any sectoral antenna datasheet.
2. The GPS synchronization which times the downlink (DL)and uplink (UL) portions of the time division duplex (TDD) protocol. Let’s imagine we have 4 sectors on a tower, azimuths 0,90,180,270 degrees. Without GPS, O degrees may be transmitting, and 90 degrees may be receiving, in which case the energy from 0 degrees will raise the noise floor of 90 degrees a significant amount, even though they may be on different frequencies. So, GPS synchronization times all the sectors so that all 4 are in transmit mode at the same time, and all 4 are receiving uplink from their respective SMs at the same time. In this case, an SM that is in the 180-degree sector receives signal from the 0-degree sector at 28 dB less than the level from the 180-degree sector, since both are transmitting at the same time.
3. The third consideration for frequency reuse to work is the automatic transmit power control in the SM’s. Using our SM in the 180-degree sector as an example, the ATPC in the SM turns the power down so that the 180-degree sector is receiving the UL transmit at the target UL RSSI we configure. Usually that is around -60 dBm. The 0-degree sector receives that SM at -88 dBm (the front to back ratio is 28 dB), which is a very low signal. The desired SMs in the 0-degree sector have adjusted their transmit power so that the 0-degree AP is receiving them at the target RSSI (-60 dBm). In this case, during busy hour when there is the most traffic on the network, the max signal to noise ratio should be 28 dB, which is enough to receive and decode QAM 256 signal.

In this case, the ePMP and PMP 450 systems are designed for reuse=2 channel plan. That is from 0 to 270 degrees, the channels are A, B, A, B. If we are using 40 MHz channels, the idea channel assignment might be A=5740 MHz, and B=5800: This gives us 20 MHz guard band between channel A and B, so that the chance of the 90-degree SM’s interfering with the 0- and 180-degree sectors is reduced. Cambuim says that you can have the channels immediately adjacent, so that means if A=5740, B=5780. Then, when you enable co-location, the system converts some of the immediately adjacent OFDM carriers to null or guard carriers, to increase the interference rejection between those 2 frequencies.

This is the Reader’s Digest condensed version of the explanation. I hope it clears things up a bit. In my experience, most networks need channels A, B, and C to operate properly. AB can be used on one site, AC, on a second overlapping coverage site, and BC on a third overlapping coverage site. Using rotated sectors, antenna downtilt, and DL power setting, a complex network can be built in this way that operates very efficiently.

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Thanks for your time on this Dave. I understand all of that but am really after details on the actual mechanism.

Do front and back radios broadcast at the exact same time? I think yes.

What does enabling colocation do to the sync?

I only have one AP per tower. They all face each other, and no AP can really overlap coverage to any other AP’s subscribers. I believe I should be able to run same freq on all 4 APs.

Thanks

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As far as I know, co-location would be for ePMP co-located with PMP type gear. If you have all ePMP gear, I don’t think that co-location setting comes into play. (As far as I know)

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Enabling co-location on a site that has only one AP only limits your capacity on that site. Single AP on a single site does not requiire co-location parameter to be set.
As for the synchronization, co-location does not do anything to affect synchronization. The AP uses the 1 pps component from the GPS to determine where the start of the frame is in the time domain.
The 1pps also helps stabilize the master clock of the AP normally, so the GPS synchronization also contributes to the frequency stability of the AP.

Thanks Brubble.

I didnt realise there was proprietary sauce involved.

I have a similar network where many towers can see eachother.

What settings do you use for APs on different towers that can see eachother? Im trying to understand if i should use “front” or “back” or if it makes no difference?

Thanks

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Absolutely makes a difference.

Co-location is just off you are colocating with Canopy or Cambium PMP 450 type gear (your’s or a competitor’s).

But yes, Front/Back is there for a reason and it makes a difference. For me - we make ‘North’ as ‘Front’ and we make ‘South’ as ‘Back’. Likewise we make ‘East’ as ‘Front’ and ‘West’ as ‘Back’. And we keep that consistent across our towers, so that a North pointing sector will be Front, and week be facing a South pointing sector 20KM away that is Back (but they will be in different freq anyway.

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