Recommended GPS Sync Configuration

NOW · November 23, 2015, 3:07am

In reading the configuration guide for GPS sync it recommends that all Downlink/Uplink Ratios on the radios be the same. This has caused me to wonder about a configuration scenario when a tower has a wireless feed and is feeding another location wirelessly.

Would performance be enhanced by having all antennas set to AP even though the Ratios wont match?

Should the "Max Range" on all the AP's reflect the farthest link?

Assuming that there are not overlapping channels and they have a 10mhz buffer between frequencies.

Link Distances:

1. 3 miles

2. 7 miles

3. 5 miles

Antenna Configuration Option #1:

1. AP, 40Mhz, GPS Sync, Downlink/Uplink Ratio 30/70 (Feed to Tower)

2. AP, 40Mhz, GPS Sync, Downlink/Uplink Ratio 75/25 (Feed to Other Tower)

3. AP, 20Mhz, GPS Sync, ABAB Configuration, Downlink/Uplink Ratio 75/25

Antenna Configuration Option #2:

1. SM, 40Mhz, GPS Sync, Downlink/Uplink Ratio 75/25 (Feed to Tower), max range 7

2. AP, 40Mhz, GPS Sync, Downlink/Uplink Ratio 75/25 (Feed to Other Tower), max range 7

3. AP, 20Mhz, GPS Sync, ABAB Configuration, Downlink/Uplink Ratio 75/25, max range 7

matkix · November 26, 2015, 9:26pm

If your plan is to re-use channels you would want all values on all APs to match for the Uplink / Downlink Ratio, and the max distance. As you noted, you would want to find the longest client connection and configure all APs around that for max distance.

If you are running non overlapping channels, you only need 5Mhz between APs, and if your plan is to not run two APs on the same channel you may just consider not using GPS Sync, and placing all APs in Flex mode. You will see about 10% better upload and download in Flex Frame mode.

Your option #2 seems like a okay setup, however if antenna 1, and 2 are going to be in a P2P configuration you may want to just place #1 in Flex mode. I would expect you might want more then 25% of the radio capacity to your tower with respect to your tower feed. If your thought was to Sync it so the 75% is download, then that won't assist in re-use in this tower, but could in the tower feeding it.

I would think something more along these lines might be a good idea:

Antenna Configuration Option #3:

1. SM, 40Mhz Flex Mode (Feed to Tower)

2. AP, 40Mhz, GPS Sync, Downlink/Uplink Ratio 75/25 (Feed to Other Tower), max range 7 (This could be useful if you later on want to feed a second tower)

3. AP, 20Mhz, GPS Sync, ABAB Configuration, Downlink/Uplink Ratio 75/25, max range 7

NOW · November 27, 2015, 6:43am

My main interest in the use of GPS is to reduce noise and then re-use channels by syncing the communication, is that achievable or am I dreaming? From my understanding if the antenna receiving the feed to the tower is a SM then its not going to sync with the rest of the communication on the tower and with a 75/25 it will receive 75% of the radio capacity from its downlink. I'm trying to be as efficient as possible now for future growth later.

matkix · November 27, 2015, 10:32am

That is correct, you will only be able to sync the APs feeding clients on the tower with matching frame time settings with respect to uplink downlink ratios. Your backhaul feeding the site would have different frame transmission times. In our network we simply use a different band for backhaul to the site leaving all of 5ghz free for access.

NOW · November 30, 2015, 3:42am

Mind sharing which frequency / equipment? What does a normal BH link cost with it?

matkix · November 30, 2015, 3:55am

Cambium 820s is what we are moving to.

The FCC fees are about $700, frequency cordination is around $600 and the radios cost around $8-9k with dishes and POEs depending on what POEs, dishes and other items you need. Get full duplex 500 Mbps up and down out of the unit. Normally we use 11Ghz, but shorter shots we also use 18Ghz.

NOW · November 30, 2015, 4:07am

What kind of distance do you get with the 11ghz? How does it operate with rain fade?

matkix · November 30, 2015, 2:57pm

Currently we run our longest link at 10 Miles, we get full modulation and have had no issues dropping below that on 2' dishes. If I had to do the shot over again I would at least have used a 3' on one side. As it's really close to dropping down to the next lowest modulation. Link planner is a good tool to look at what kind of capacity you can expect on the 820S, I've found the real world to actually be slightly better then link planner states it to be.