Road trip for implementing E-OTD

Enhanced Observed Time Difference (E-OTD) has become the de facto technology for E-911 Phase II implementation among U.S. GSM carriers. The trend began with VoiceStream, (www.voicestream.com) the only GSM carrier that has received FCC (www.fcc.gov) approval to implement E-OTD.

Other current and future GSM carriers, including AT&T Wireless, (www.attws.com) Carolina PCS, (www.carolinaphone.com), Cingular (www.cingular.com), Conestoga Wireless ( www.callconestoga.com/cwc/) and PCS One (www.pcsone.com) intend to use E-OTD to meet Phase II accuracy requirements and have made that known to the commission.

E-OTD is a hybrid technology that uses both the handset and the network to determine a caller’s location. The technology compares arrival times of wireless phone signals to find the caller. E-OTD requires minor software upgrades to the network and E-OTD chips are being included in many forthcoming GSM phones.

The first step in implementing E-OTD is installing location mobile units (LMUs) at sites’ base stations.

“LMUs serve as reference points for the base stations,” said Kevin McCracken, Nortel Networks (www.nortelnetworks.com) senior manager of wireless service networking marketing. “The base stations sync off these reference points to set the baseline so they (carriers) are then able to measure the time differences there.”

That information gets relayed to the serving mobile location center (SMLC), where the caller’s location is determined.

“The SMLC has all of the mathematical algorithms that take the sensitivity measurements and crank them out into longitude and latitude,” McCracken said.

Cambridge Positioning Systems (www.cursor-system.com) is the company behind the LMU and SMLC portion of E-OTD. Dubbed Cursor, the technology is expected to provide 50-meter accuracy on GSM networks.

The final network component of E-OTD is the gateway mobile location center (GMLC). The GMLC interfaces to public safety answering points’ (PSAPs) emergency networks. Nortel’s GMLC also will allow third-party location application providers to interface there and obtain location-based services subscribers’ coordinates.

The LMUs will need to be installed in a large portion of carriers’ cell sites, McCracken said. Basic engineering issues, such as coverage and capacity, will determine which sites need LMUs. Different types of LMUs also can be deployed in areas with high or low density.

Of course, the more cell sites a carrier employs, the more LMUs the carrier will have to install. This presents an ironic fortune to AT&T Wireless and Cingular, carriers that are migrating to GSM networks.

“They are going to be visiting all of their base stations or building new cell sites to upgrade to GSM, so they will have people out there doing all of the manual labor,” McCracken said.

Those carriers will be able to launch E-OTD-ready GSM networks. Compared to switching to GSM, installing E-OTD will be a minor endeavor, McCracken added. Current GSM carriers such as VoiceStream will have to hit the road to visit their existing sites to install the LMUs. That could prove to be an arduous task for the largest American GSM carrier. Furthermore, the more sites a GSM carrier has, the more LMUs it will require (read: higher cost). Compared to 2.5G evolution, however, E-OTD’s cost will be a minor addition, McCracken said.