Do I need a backhaul unit?

I have a 9000AP (7.3.6) transmitting from a T1 using Pacific-Wireless 120-degree sectorized antennas VPOL, set to transmit at 906MHz. There is an area centered about 5.5 miles away where I would like to set up another 9000AP using an omni-directional antenna - this to serve an area that cannot be reached by the first transmitter because of the topography of the area (getting behind a hill).

My plan is to set up a 9000SM connected to the new 9000AP. There could be 20-30 subscribers connected to this new 9000AP and I have questions about the viability of this configuration. Since all of this is tied to a T1 with 1.5Mbps capacity, I ask myself what I’m worried about since the SM and AP have more capacity then 1.5Mbps. However, I also assume that the way in which the SM handles bursts, for example, could adversely affect the way in which multiple subscribers on a single SM will perform.

Currently, there are 14 subscribers on this AP but that could grow. At some point when I’m reaching the saturation point of the T1, I’ll migrate to bonded 2xT1s.

Are there ways in which the SM can be configured to minimize this impact, or, put another way, to maximize the performance of this configuration? Obviously, using a backhaul unit would be more effective but a great deal more expensive.

Any advice you can provide would be great. Thanks.

If I read this correctly, you want to tie a new SM to your old original SM and link to a new 900 AP? If so we did this when we first setup our system. It worked but the problem was we were causing self interference and had sync problems (we had no CMMs at the time). One of the APs would drop almost all users almost every weekend so to get away from this we installed the 5.7 BHs with CMM Micros at each site. The 5.7s are rock solid with signals in the high 50s low 60s and jitter of 0 or 1 at all sites (2 @ 12 miles out, 1 18 miles and one 24 miles out). Using the BH and CMM cleared up many frustrations but if you cant afford to do it now now, it can be done other ways. Use 906 at your old AP and run the new one at the upper end (920 or so). Depending on where you are the upper end of the band may perform less than down near 920 if paging or cellular has a heavy presence.

This is a supported technique called “Remote AP”.

There is an application paper in the Moto Library on how to set this up including carrying sync to the remote AP.

There are three non-overlapping channels at 906, 915, and 924. The idea of using as much separation as possible is a good one.

Since you are using V-Pol Sector antennas, using a H-Pol Omni will further decrease the possibility of inter-tower self interference.

Before you buy anything, take an SM with an 8dB panel antenna to the proposed tower and coverage area and do some spectrum analysis readings. Take some readings at the tower, and then drive around and take some readings with the antenna pointed at the tower. Take each measurement with the antenna H-Pol, and V-Pol.

This will give you an idea of the noise in the area, potential sources of interference, and a baseline to look at later in case something develops.

We are currently feeding 2 AP sites with SM’s instead of using BH’s. We are using 2.4 SM’s to feed the 900 AP’s because we can get them with LOS and the 2.4 SM’s have better throughput.

Another ISP we are working with in another area of the province IS using 900 SM’s to feed remote 900 AP’s with up to 40 customers on a single AP using a HPOL Omni. The average subscriber speed is .75 Mbps with a 3 Mbps pipe to the uplink the SM’s are feeding off. The entire network is also using Software Scheduling so I think you could expect similar results even with the 1.5 Mbps T1 feeding your AP.

Graham MacDonald
Manager - Wireless Services
TNC Wireless

Thank you all for the leads and suggestions. I plan on getting a bucket lift and a long mast to do the site survey and will travel around and see what kind of connections I get in the area.

With the weather forecast full of rain for the next week, it will be a while before I get to this, but I’ll report back on my progress.

Thanks again.