The different location technologies currently being examined by the wireless telecommunications industry are Enhanced Observed Time Difference (E-OTD), Time Difference of Arrival (TDOA), Angle of Arrival (AOA) and Assisted GPS (A-GPS). The TDOA and AOA methods are considered to be network based solutions, while the A-GPS method is handset based. The E-OTD method is considered to be a hybrid network and handset based technology.

Different methods of air interfaces require different location determination techniques. The E-OTD solution is best suited for GSM based networks. The E-OTD technique is already part of the GSM standard, utilizing many of the built-in standard timing measurements that the handsets already perform. A relatively simple software modification is needed in the handset to accommodate the reporting of the timing differences back to the MSC to assist in the location determination process. The TDOA and AOA methods are solutions that will work with the AMPS, TDMA, and CDMA air interfaces, with no modifications needed to existing handsets. This means that the existing subscriber base of legacy handsets will benefit from location determination technology with no direct impact to the customer. The A-GPS method of location determination would involve the subscriber base to acquire a new GPS-equipped handset. This method would not allow legacy handsets to utilize the location determination system.

These different types of location determination technologies represent the complex decision a wireless service provider must make in regards to its compliance with the FCC E-911 Phase II mandate. There are numerous vendors supplying multiple solutions, at different levels of compliance. Technical feasibility, capital and physical space requirements along with network requirements also play a large role in the decision of choosing a particular location technology. The ability to market other location based services in the future (and therefore effect a sort of cost-recovery), also dictates the ultimate solution for location determination. Ultimately the deadlines associated with the Commission’s mandate may dictate the direction a particular wireless provider may follow.

The following report summarizes the several different types of location technologies available for satisfaction of the E-911 Phase II mandate:

Enhanced Observed Time Difference (E-OTD) Location Determination

E-OTD systems operate by placing location receivers or reference beacons, overlaid on the wireless network as a location measurement units (LMU) at multiple sites geographically dispersed in a wide area. Each of these receivers has an accurate timing source and when a signal from at least three base stations is received by an E-OTD software enabled mobile and the LMU, the time differences of arrival of the signal from each cell site at the handset and the LMU are calculated. The differences in time stamps are then combined to produce intersecting hyperbolic lines from which the location is estimated (see Diagram 1). This location method involves new hardware to be located in the network as well as new handsets capable of E-OTD.

Diagram 1:

The TDOA technique works by measuring the exact time of arrival of a handset radio signal at three or more separate cell sites. Because radio waves travel at a fixed known rate (the speed of light), by calculating the difference in arrival time at pairs of cell sites, it is possible to calculate hyperbolas on which the transmitting device is located (see Diagram 2). The TDOA technique typically uses existing receive antennas already present at a cell site. In multi-path environments (urban areas) it may be necessary to make measurements with four antennas to overcome the effects of the multi-path. This location technique works with any handset, including legacy units and requires modifications to the network only.

Diagram 2:

Angle of Arrival (AOA) Location Determination

The AOA technique determines the direction of arrival of a handset’s signal at a cell site. The phase difference of the signal on elements of a calibrated antenna array mounted at a cell site provides the angle of arrival. The intersection of the angles from two or more sites provides the location. Typically the AOA technique is used to augment the TDOA approach of a location system. One example of this is the coverage of a rural highway where the cell site arrangement often in a line along the highway. TDOA-only systems must overcome increased propagation loss for a three-site reception, but by including AOA on the highway sites, an accurate position calculation can be obtained from only two sites. This location technique works with any handset, including legacy units and requires modifications to the network only.

Diagram 3:

Assisted GPS (A-GPS) Location Determination

The A-GPS technique is considered a handset-based location solution. A handset that has been specially equipped with a GPS receiver determines its location based upon signals received from the Global Positioning System. The location is then forwarded to the PSAP during a 911 call (see diagram 4). This basic location determination can be improved upon by implementing the "assisted" portion of the A-GPS technique. This assistance comes from the use of a GPS reference receiver or network of reference receivers, which in turn report all gathered navigation messages and differential correction data for the GPS satellites in view to a location server. The location server then provides aiding data to the handsets on-demand (upon an E-911 call). The aiding data generally consists of a list of satellites in view from the handset and their relative Doppler offsets. (Estimated Doppler can be improved by using the location of the base station communicating with the hand-held device as an approximate handset location.) This small message (approximately 50 bytes) is all the handset needs to know from the location server to extract pseudo-range information from its short snapshot of GPS data. The location server can also have access to a terrain elevation database, which would allow accurate altitude aiding in the location of the handset. The terrain elevation provides essentially an extra range measurement, improving reliability and accuracy (see diagram 5).

Diagram 4 – GPS Location Determination:

Diagram 5 – Assisted GPS: