CDMA Sites and GPS

[Jonathan Kramer]

Originally Posted by Andy84094

Will the PN offsets of the repeater site be the same as the offets of the donor site? I'm not completely sure why a carrier would use a repeater site(unless of course in a rural area that doesn't have access to T1 lines, etc.). Sorry I have so many questions.

As for the Psuedo Noise offsets, I'm not sure. I suspect that for pure RF repeating there's no offset at all. Yes, repeaters are used in rural areas and areas without access to T1s, but there are other considerations such as capital budgeting, and efficiently using otherwise excess capacity at the donor site.

j

[chuikov]

Originally Posted by hillbilly44

The little while cone shaped antennas are for GSM companies like Cingular. They use GPS and antenna triangulation to locate a caller for 911 purposes. Some CDMA carriers use GPS in the handset to do the same thing

I worked on a project installing GPS antennas and GSM cabinets at Cingular/AWS sites in New England.

I wondered, why the heck do they need GPS receivers? Did they forget where their sites were?

The triangulation stuff you describe is correct as far as I know. what I regret is not saving the exhaustive pictures of all the 200+ cell sites.

[Jonathan Kramer]

Originally Posted by Andy84094

Will the PN offsets of the repeater site be the same as the offets of the donor site? I'm not completely sure why a carrier would use a repeater site(unless of course in a rural area that doesn't have access to T1 lines, etc.).

Sorry I have so many questions.

I presume the PN will be the same at the repeater site as the donor site for RF repeaters. And, yes, repeaters are dandy solutions where no T1s are available, or the traffic doesn't support a full site.

j

[KiwiSurfer]

AMPS, TDMA, GSM and UMTS sites do not need GPS. However some non-CDMA networks may use GPS to get an accurate time source, so phones can update the time from the network.

Here in New Zealand, Vodafone (GSM/UMTS) only have GPS at switch sites and none of their ordinary cell sites, whereas Telecom (AMPS/TDMA/CDMA) has GPS at all their cell sites.

[Myself248]

In my experience, a site that's backhauled to another site over RF rather than straight to the switch by copper or fiber is still an independent site, in all senses. It doesn't share a PN offset or anything else with the upstream site.

Reason being, the backhaul is done way below the cellular level. The equipment is usually just a t1 radio, using microwave carriers. Glenayre's LYNX series was popular for a while, then they got bought by Western Multiplex, who already had their own line of microwave backhaul equipment. Then Wmux got bought by Proxim and I stopped keeping track.

All those microwave radios just present a plain old T1 interface to the cell equipment, so no changes are needed. Hence, the circuit from the distant site back to the switch might be fiber in places, microwave in places, and copper in places, but end to end, it's just a T1, and neither the switch nor the site is the wiser.

What the GPS is used for depends on the network. The CDMA system is too timing-sensitive for anything else, so an ultra-stable local oscillator (usually double-oven quartz) is disciplined to the Navstar L1 signal and used to keep all the sites' chip clocks in time. In the event of antenna failure or signal fade, the local oscillator can maintain good (~10ppm) timing for a while, typically 24 hours, before drift becomes a problem. The receiver (made by Trimble or HP, in the Nortel CDMA equipment) knows its own location, but doesn't use that information for anything. (Once we begged the switch techs to give us a dump of all the clocks' GPS data, because our site map was so horrid. They said they couldn't access that info!)

In GSM, the cell base transceiver station itself doesn't need a precise clock, but the E911 triangulation equipment does. They're using "differential time of arrival", a method by which three (or more) neighboring cell sites listen to the signal from the handset, and compare its phase and timing against their local clocks. This information is correlated by a central server to infer the location of the handset. Again, all those clocks are synchronized to GPS.

I've seen GPS antennae at Nextel iDEN sites too, and I'm not sure what they use it for. Probably similar. I'll start keeping an eye on whose sites have GPS, and post again if I see anything weird.

[pbw]

Originally Posted by Jonathan Kramer

I presume the PN will be the same at the repeater site as the donor site for RF repeaters. And, yes, repeaters are dandy solutions where no T1s are available, or the traffic doesn't support a full site.

j

Yes and no. In the Norfolk market the tunnel "repeaters" have their own PN Offsets and Base IDs for the CDMA carriers. CIDs and DVCs for the TDMA/GSM folks. In DC the BAM metro system's Base ID and PN Offsets are configured very differently from the rest of the DC/Baltimore network.

However, most above ground repeaters repeat the "donor" cell. Repeaters are used to extend coverage (sorry a "duh" point). Generally it is not a T-1 issue. If you can run power, you can run a T-1. The question becomes quality/reliability and cost of the T-1. In that case the site simply backhauls via microwave (usually unlicensed).

[pbw]

Originally Posted by Myself248

In my experience, a site that's backhauled to another site over RF rather than straight to the switch by copper or fiber is still an independent site, in all senses. It doesn't share a PN offset or anything else with the upstream site.

Reason being, the backhaul is done way below the cellular level. The equipment is usually just a t1 radio, using microwave carriers. Glenayre's LYNX series was popular for a while, then they got bought by Western Multiplex, who already had their own line of microwave backhaul equipment. Then Wmux got bought by Proxim and I stopped keeping track.

All those microwave radios just present a plain old T1 interface to the cell equipment, so no changes are needed. Hence, the circuit from the distant site back to the switch might be fiber in places, microwave in places, and copper in places, but end to end, it's just a T1, and neither the switch nor the site is the wiser.

What the GPS is used for depends on the network. The CDMA system is too timing-sensitive for anything else, so an ultra-stable local oscillator (usually double-oven quartz) is disciplined to the Navstar L1 signal and used to keep all the sites' chip clocks in time. In the event of antenna failure or signal fade, the local oscillator can maintain good (~10ppm) timing for a while, typically 24 hours, before drift becomes a problem. The receiver (made by Trimble or HP, in the Nortel CDMA equipment) knows its own location, but doesn't use that information for anything. (Once we begged the switch techs to give us a dump of all the clocks' GPS data, because our site map was so horrid. They said they couldn't access that info!)

In GSM, the cell base transceiver station itself doesn't need a precise clock, but the E911 triangulation equipment does. They're using "differential time of arrival", a method by which three (or more) neighboring cell sites listen to the signal from the handset, and compare its phase and timing against their local clocks. This information is correlated by a central server to infer the location of the handset. Again, all those clocks are synchronized to GPS.

I've seen GPS antennae at Nextel iDEN sites too, and I'm not sure what they use it for. Probably similar. I'll start keeping an eye on whose sites have GPS, and post again if I see anything weird.

Great post. Re: Nextel. iDEN is based off of TDMA, so it probably is used for the Time Division component. What is interesting is that Nextel employs two at opposite ends of the equipment shelter for redundancy. Good physical diversity to (I've several sites where smaller trees have fallen over top of the shelter).

[pbw]

Originally Posted by Jonathan Kramer

Andy, a repeater site will normally look like a regular cell site, albeit with a smaller equipment footprint. In some cases, diamond-shaped antennas will be used to link to the donor site. In other cases, data lines (copper or fiber) will be used. In most cases, you won't be 100% sure if the site is a repeater site, or a stand-alone site.

j

Heh. But! But! the Sprint PCS ones have site ids with XR instead of XC =] Alltel also prefixes their site ids with R for repeaters, like Cell # R123 or something. All repeaters I have seen used incredibly small foot prints, like the size of a telco demarc. I think the most common ones I have seen are by Repeater Systems and Allten Telecom (now Andrew Corp.).

[pbw]

Originally Posted by Andy84094

INteresting. I didn't know that 1900 carriers needed to place the GPS equipment away from the panels. Then I wonder how come Qwest had their GPS so close to the panels...

If tower space is leased/rented, the owner will actually charge the carrier for the extra GPS antenna on the tower? I didn't know that...Great information, thank you very much.

? I've seen many Sprint PCS sites where the antenna was placed in between the panels. I always thought this was to get a consistently good view of the sky.

[Jonathan Kramer]

Originally Posted by pbw

However, most above ground repeaters repeat the "donor" cell. Repeaters are used to extend coverage (sorry a "duh" point). Generally it is not a T-1 issue. If you can run power, you can run a T-1. The question becomes quality/reliability and cost of the T-1. In that case the site simply backhauls via microwave (usually unlicensed).

The site I recently processed in Thousand Oaks, California (approved last week by its Planning Commission) used 1900 for up/downlink to the donor site several miles away. No 10 GHZ anywhere in site. Purely borrowing O-T-A capacity.

j

[FieldTek]

In my experience with CDMA sites, there is also a lot of use of BDA's (Bidirectional Antenna) this is usually used to fill in small gaps that don't justify a full build out of a site. It can have small antennas for use in a building for example, or it can look like one sector off of an array. It takes up very little space, doesn't require T1 or GPS, and just uses the PN of the donor sector. If it is not installed correctly though it can be a Sys Perf Engineers nightmare...

[chuikov]

Originally Posted by FieldTek

In my experience with CDMA sites, there is also a lot of use of BDA's (Bidirectional Antenna) this is usually used to fill in small gaps that don't justify a full build out of a site. It can have small antennas for use in a building for example, or it can look like one sector off of an array. It takes up very little space, doesn't require T1 or GPS, and just uses the PN of the donor sector. If it is not installed correctly though it can be a Sys Perf Engineers nightmare...

I know Nextel uses a bunch of BDAs around here - like you say, no GPS with the BDAs.

[Yankees368]

Originally Posted by FieldTek

In my experience with CDMA sites, there is also a lot of use of BDA's (Bidirectional Antenna) this is usually used to fill in small gaps that don't justify a full build out of a site. It can have small antennas for use in a building for example, or it can look like one sector off of an array. It takes up very little space, doesn't require T1 or GPS, and just uses the PN of the donor sector. If it is not installed correctly though it can be a Sys Perf Engineers nightmare...

So you are telling me that sprint can add, lets say a "mini-sector" to provide coverage to the gap that they claim my entire town is in? I was on the line with tech support and they claim that there are 5 towers within 3 miles, but I am in a bubble where they do not reach. However I had them check another address about a mile away where I always have full service (about 1/4 miles from the sprint site) and that was listed as a dead zone too.

I have observed that the set-up of the sectors closest to my house looks like it would create a large gap between sectors once you move away from it. http://gallery.wirelessadvisor.com/s...lts&searchid=7 is the site I am refering to, I believe that sprint is the higher site towards the left of the picture. If you look at the set up, there is a very large gap between the sectore facing the camera, and the sectore behind it, almost 180 degres. There is nothing facing perspective right. Any thoughts?

[Yankees368]

Nothing to say, huh?

[FieldTek]

Sorry, was out of town. I wouldn't say it could create a mini-sector to cover a town. It would more or less extend a sector further out. However, all channel elements used could overload the donor sites sector, especially if it was covering an entire town. As far as following out the sectors near your house, while most 3 sector sites have 120 degree coverage per sector that is not always the case. Some carriers use a smaller radius, some use smart antennas to "bend" the signal the direction they want. So trying to map it out isn't always going to give you the best results. If you can't get coverage at your home it may be time to look for a new provider...depending on your contract.

[Yankees368]

Originally Posted by FieldTek

Sorry, was out of town. I wouldn't say it could create a mini-sector to cover a town. It would more or less extend a sector further out. However, all channel elements used could overload the donor sites sector, especially if it was covering an entire town. As far as following out the sectors near your house, while most 3 sector sites have 120 degree coverage per sector that is not always the case. Some carriers use a smaller radius, some use smart antennas to "bend" the signal the direction they want. So trying to map it out isn't always going to give you the best results. If you can't get coverage at your home it may be time to look for a new provider...depending on your contract.

Thanks for the reply. I had no idea that antennas could bend the signals! However, this site is pretty old, so I doubt it has this technology. But anyway, just about every carrier is a no-go at my house. Sprint is ok, but has a BIG problem with the phone not ringing when called. Verizon is OK, so while home from school I set my phone to roam on verizon . Cingular is terrible, not sure about nextel. The only strong carrier is now T-Mobile now that they have put up a very large tower near by. But, I dont like tmobile, so oh well!