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Location Services in Wireless Communications Systems

Thu, 09/26/2002 - 11:33am

A brief overview of location services used to meet FCC E911 requirements in mobile phone systems

by Charles A. Murphy, Senior Staff Systems Engineer, Motorola — WBSG Systems and Architecture Group

Introduction
On June 12, 1996 the Federal Communications Commission (FCC) announced that it would impose regulations on the wireless mobile phone industry to ensure that all 911 calls be accompanied by an Automatic Location Identification (ALI). In the ensuing months the regulations have been defined and the methods of meeting the requirements have multiplied. This article will present a brief overview of some of the location services (LCS) being considered and implemented in the wireless industry.

Phase I of the FCC plan required that by April 1, 1998 all carriers provide the cell tower location (Cell-Identity or Cell of Origin (COO)) that received the 911 call. This would allow emergency units to know the location of the caller to within a few miles. Phase II of the plan required all carriers to provide the location of the caller within 50 meters for 67% of all 911 calls and 150 meters for 95% of all 911 calls for handset solutions such as GPS. For network solutions such as TDOA the carrier must provide the caller's location to within 100 meters for 67% of all 911 calls and 300 meters for 95% of all 911 calls. The Phase II deadline was originally October 1, 2001, but the FCC granted waivers to all carriers.

The industry has narrowed the search for a location technique to a few technologies to begin to meet the FCC phase II mandate. For time diversity systems such as GSM, Enhanced Observed Time Difference (E-OTD) has been chosen. E-OTD takes advantage of the unique timing capabilities in GSM to calculate location data.

For CDMA systems, Assisted-GPS (AGPS) and Advanced Forward Link Triangulation (AFLT) have been chosen as the methods for providing location data. Some CDMA carriers have announced the release of mobile phones with GPS capabilities, but at the time that this article was written none of the carriers fully supported Phase II ALI requests.

W-CMDA systems are moving to a location services standard called Observed Time Difference Of Arrival (OTDOA). OTDOA has been found to not provide adequate location accuracy, but an enhancement called IPDL has been added to improve performance.

Many of the location services can be combined to take advantage of the strengths of each. TDOA services such as E-OTS and OTDOA are being combined with GPS to give indoor availability and GPS accuracy. These systems are referred to as hybrids.

GPS and AGPS

How it works
In 1978 the US department of defense activated the GPS system, which today consists of 24 operational satellites. Each satellite transmits a unique code at 1575.42MHz using the Code Division Multiple Access (CDMA) signaling method and Bi-Phase Shift Keying (BPSK) modulation. The civilian signal is spread using a Course Acquisition (CA) Pseudo-random Noise (PN) code at a chipping rate of 1.023MHz.

When the GPS receiver has acquired 4 satellite signals, location, time and altitude can be calculated. To calculate location the receiver uses a technique called triangulation. To do this, the receiver must know how long it took the signal to travel from the satellite to earth and where each satellite is at that instant in time. The location of each satellite is stored in the GPS receiver almanac. The satellite signals contain updates as to where the satellites are located. With each known satellite location a circle can be drawn on the face of the earth. The location of the receiver is the point at which 3 circles intersect. The 4th satellite is used to calculate exact GPS time and altitude.

AGPS/Differential-GPS
Assisted-GPS relies on accurate time, ephemeris and almanac information given by a network. Many wireless mobile phone systems have a GPS unit at the base station from which the voice/data signal is being transmitted. This GPS unit collects all of the information that a GPS unit needs to begin calculating position and sends it to the mobile phone. The GPS in the mobile phone then makes a slight correction in location and it is running. Network assistance decreases the Time to First Fix (TTFF) from 30-60 seconds to as little as 1 second under strong signal conditions.

Since in most cases the base station receiver knows its exact location, it can send error corrections caused by the atmosphere and weather. This is known as differential GPS (DGPS). Differential GPS can improve accuracy to 3-5 meters.

Advantages/Disadvantages
GPS has been proven to be the most accurate location solution available today for wireless handsets. In autonomous or assisted mode the accuracy is less than 10 meters. If Wide Area Augmentation System (WAAS) is used the accuracy can be as low as 2.5 meters.

GPS is the only location technology available today that can provide altitude information. This can be very important if the E911 caller is located in a high-rise building.

GPS works very well when it has a clear view of the sky. When a GPS receiver is moved indoors it may have difficulty acquiring the satellite signals. AGPS improves the receiver sensitivity by allowing more time for integration rather than searching for satellites since the network supplies this data.

Adding GPS to a mobile phone means incorporating a separate 1575MHz receiver, usually in the form of a RF front-end chip, a baseband chip, a patch antenna and software. In some cases, the GPS baseband is integrated. This solution can be very costly.

Time Difference of Arrival (TDOA)

How it works
TDOA is a method by which the location of the handset is determined by the difference in time it takes a signal to travel from the handset to multiple cell towers. The signal will arrive at a near tower before it arrives at a far away tower. The difference in time that it takes the signal to arrive is used to determine the location of the handset. For this method to work the tower locations must be known and the base stations must be synchronized in time.

Location is calculated by use of triangulation if three towers are used. If only 2 towers are available the accuracy suffers greatly, but the location can be calculated by ratios. For example, if a handset is located half way between two towers the signal will arrive at both towers at the same time and the position can then be said to be equal distant between the towers.

Advantages/Disadvantages
TDOA uses signals that are normally transmitted by the mobile phone such as a voice phone call. No special "burst" signals have to be transmitted for TDOA to operate. This allows TDOA to work with current handsets with no modification. The base stations must be updated to calculate the time difference, but no changes are required in the handset.

TDOA can also provide better reception in low signal environments, because the receive signal is much more powerful than in the case of GPS. In-building location determination is difficult with any system, because there is no direct line of sight to the transmitter, but a TDOA unit is more likely to receive a signal than a GPS unit.

Since TDOA operates by measuring the time it takes signals to travel from the handset to the cell tower, multipath can be a big problem. The more the signal reflects off structures before it arrives at the tower, the longer it will take for that signal to get there. This is particularly a problem in urban environments. When a signal is transmitted from within a large grouping of buildings, it is almost certain that it will be reflected multiple times before it arrives at the tower.

TDOA is not able to provide the accuracy of GPS. Multipath causes errors as discussed above, but the altitude of the handset relative to the tower is also a problem. If one tower is located high on a mountain and another in a valley the distance can highly skewed by the relative altitudes of the towers.

TDOA-based technologies such as E-OTD are preferred by many carriers, because of concerns of relying on the US government to provide GPS service. GPS could potentially be deactivated at any time by the US, whereas TDOA systems are fully controlled by the carrier. 

Enhanced Observed Time Difference (E-OTD)
E-OTD, based on TDOA technology, has become the location service of choice for GSM carriers. E-OTD differs from TDOA, as explained above, in that the base stations send out bursts and the handset measures the time differences. An E-OTD unit can calculate position in the handset (MS-based), or it can collect timing data and send it to the base station to be calculated (MS-assisted). In order for E-OTD to work properly, signals from 3 cell towers with known locations are required. To help calibrate the measurements, E-OTD uses Location Measurement Units (LMU), which are GSM receivers set up at known surveyed locations to duplicate the measurement the handset will make and to compensate for non-synchronized base stations.

Observed Time Difference Of Arrival - Idle Period DownLink (OTDOA-IPDL)
OTDOA, also based on TDOA technology, is the chosen location service for W-CDMA systems. OTDOA works on the same principles as E-OTD where the time difference is measured by the handset. A study published in 2001 showed that OTDOA would not provide the accuracy needed to meet E911 requirements because of near tower "jams" to the signals from the far away towers. The "near-far" problem causes blank areas, or "doughnut holes" around the near towers. IPDL has been adopted by the 3GPP to rectify this problem. IPDL requires that each tower disable its downlink once every 150 time slots. This allows the handset to "hear" the quieter, more distant towers and make a better judgment as to its location. As in E-OTD, LMU stations may be used to increase accuracy.

Advanced Forward Link Triangulation (AFLT)
AFLT is the CDMA equivalent to E-OTD. AFLT measures the phase of the pseudo random noise (PN) code and the receive signal strength (RSS) to make a decision about location. CDMA base stations are all synchronized with each other and with the handsets using GPS time making code phase measurements a valuable factor. Handset to base station synchronization alleviates the need for LMU's. Angle of Arrival (AOA) can be added to AFLT to increase accuracy.

Local Positioning Technologies

All of the technologies discussed so far measured timing or signal strength of incoming signals from far away sources such as cell towers or satellites. Another future technology, called local positioning, will use near by sources to determine position.

Bluetooth local positioning devices will communicate with peer devices or networks to determine the users location. A handset could retrieve data from another handset that has a good GPS fix or with a network with known coordinates. A network could calibrate itself with handsets that have position data. The Bluetooth SIG has setup a workgroup to investigate this technology.

Ultra-Wide Band (UWB) technology has gained a lot of momentum over the past few years, and one area of interest is location services. UWB will have an infrastructure similar to a wireless mobile network. UWB communicates using pulses that are 1-2nS long, which is nearly 1000 times shorter than a GPS chip. UWB may be able to provide indoor location accuracy as good as GPS outdoor accuracy. One point of contention with UWB is that it may interfere with GPS systems if it is allowed in the GPS frequency band. So far the FCC has restricted UWB from operating at 1575MHz.

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