I saw this post had a lot of views so I just wanted to give everyone an update. We bought an AP w/ 90-degree antenna and 3 SMs w/ integrated antennae (14.5 dBi, I think) and this is what we've seemed to find so far:
A bit different from other Motorola/Cambium Networks stuff, but I think we've partly figured out a good bit of it. What I say here may be simplifying things, but I don't claim to completely understand everything myself. I also may leave out a lot of the stuff that has already been explained in the advertisements/announcements/powerpoint presentations.
Before I go any further, I did see a post where someone was unable to achieve distances greater than 1 mile. We had this problem as well until we realized that our AP transmit power was set to -5. We bumped it to 20 and have been generally liking what we have been seeing.
Okay here we go. Maximum speed, both to a single SM AND total available at the AP, simply put, basically depends on two things:
1. The modulation level of each connected SM--and there are 9
2. The bandwidth of the channel you select.
We'll start with number one. If you are familiar with Canopy PMP 100 900 MHz, this is roughly similar to 1X or 2X, but seems more important to pay attention to here. With 900s you have, say, 3 Mb of total downlink throughput available, but ONLY if everyone is connected at 2X. With 1X, there is about 1.5 Mbps available. So try to follow this scenario with me. If you already know all this then you can probably skip this section. If you see a mistake then feel free to correct me. I haven't had any Motorola/Cambium training. In the beginning I was really only hired as a bottom-tier field tech, so I am meek and lowly of heart.
- 2 SMs (Isaac and Jacob) at 2X at a 1 Mbps tier.
- 1 SM (Esau) at 1X at a 1 Mbps tier.
- The 2 SMs at 2X start downloading at full speed. They are doing great because there is a full 1 Mbps left on the AP, right?
- The 1 SM starts downloading at full speed. Now, all three of the customers drop to below their subscribed speed.
Why? Because Esau is using more LINK BUDGET. See, the AP has to "work" just as hard to send 1.5 Mbps at 1X as it does to send 3 Mbps at 2X. Therefore, as far as Isaac and Jacob are concerned, Esau is actually using 2 Mbps worth of their link budget.
But the PMP 320s, on the other hand, have NINE modulation rates. What this means in practical terms is that a policy should likely be set in place dictating the lowest modulation rate an installer ought to accept. You, as the privileged owner who further has LoS to the tower, might have a 12 Mbps connection; but if you are connected at the highest modulation rate (called "qam64 ctc-5/6 MIMO B"; and btw, that is the ONLY modulation rate that connects at MIMO B) you might STILL be leaving plenty of link budget for the rest of the network even when downloading at full speed. If the practical maximum AP throughput in ideal conditions is, say, 40 Mbps, you are only using a little more than 1/4 of the link budget.
BUT, if you connect an SM at the lowest modulation rate (qpsk-ctc-1/2), then they would use the ENTIRE link budget at the AP
to achieve just 2.6 Mbps.
So, depending on the modulation level, the link budget is affected to varying degrees. There are a few things to note:
- Only at the highest modulation level (where MIMO B becomes available) does the AP have significantly more throughput capability than a single SM.
- Some of the specific numbers I have mentioned here only apply when using the highest channel bandwidth: 10 MHz. Which leads us to item two...
Number two, the channel bandwidth, is a bit more straightforward. 10 MHz, 7 MHz, 5 MHz, and 3.5 MHz are your choices. This is set at the AP, so should probably be decided in the early stages of deployment. If you select 3.5 MHz for better connectivity, but later decide that you want the better throughput of the 10 MHz channel, some of your customers might drop entirely. Others might drop to a lower modulation rate, thereby slowing your network down.
Conversely, if you start at 10 MHz, but then decide that you want the greater range and/or connectivity of the 7 MHz channel (~10 miles vs ~6.9 miles), then it may not be an issue UNLESS you have quite a few customers connected at a high modulation rate. The switch to 7 MHz would then increase link budget usage at absolute speeds, especially for those connected at MIMO B.
Another thing I've found is that, while we generally install the 900s as high as we can, that is not always the best place for the PMP 320 SMs. I think this is because they employ something called "receive diversity." That is, if I understand correctly, they can actually benefit from signal reflections. So, for example, the reason one of our 7-mile non-LoS tests worked better at almost ground level instead of on the roof where the 900 is, was probably because the signal was allowed to reflect off the ground and the metal wall behind it, and possibly the metal roof above it. The signal stats even include two or three numbers for "CINR re-use." CINR = Carrier to Interference-plus-Noise Ratio. (The CINR stat seems more important with PMP 320s than the RSSI/power level is.
Anyway, I can't say this with certainty at the moment, but I THINK CINR re-use might have to do with this concept that I will copy and paste from the manual (the emphasis within the quote is my own):
In field deployments with a rich LOS multipath that can be converted to MRC gain, the dual-receiver architecture of the PMP 320 can achieve the same level of performance at greater distances since the RSSI and CINR levels at the radios can better by 3 dB or more than those listed in the table, which was measured in an environment with no multipath.
That's all for now. Our limited testing, both in town and out, LoS and non-LoS, including shooting through trees and past ridges, has been enough to cause the boss to give us the green light on getting four sectors set up on our main tower.