Čńňî÷íčę: IEEE Communications Magazine • April 1998
http://www.sss-mag.com/pdf/radioloc.pdf
Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning.
Wireless location has received considerable attention over the past few years. A recent Report and Order issued by the U.S. Federal Communications Commission (FCC) in July 1996 requires that all wireless service providers, including cellular, broadband PCS, and wide-area SMR licensees, provide location information to Emergency 911 (E-911) public safety services [1]. These new FCC E-911 requirements have boosted research in wireless location. The basic function of a location system is to gather information about the position of a mobile station (MS) operating in a geographical area and process that information to form a location estimate. A popular approach, known as radiolocation, measures parameters of radio signals that travel between an MS and a set of fixed transceivers, which are subsequently used to derive the location estimate.
Many existing wireless location systems, such as the Global Positioning System (GPS) and Loran C, make use of radiolocation techniques. With these technologies the MS formulates its own position, which can be relayed to a central site. Some approaches employ a cellular network as the transport mechanism for relaying the location estimate [2]. As an alternative to these approaches, cellular networks can be used as the sole means of providing location services, where the MSs are located by measuring the signals traveling to and from a set of fixed cellular base stations (BSs). The signal measurements are used, for example, to determine the length and/or direction of the individual radio paths, and then the MS position is computed from geometric relationships [3].
Radiolocation systems can be implemented in one of two ways. With the first approach, the MS uses signals transmitted by the BSs to calculate its own position, as in GPS. With the second approach, the BSs measure the signals transmitted by the MS and relay them to a central site for processing. The second approach has the advantage of not requiring any modifications or specialized equipment in the MS handset, thus accommodating the large pool of handsets already in use in existing cellular networks.
The remainder of this article presents an overview of wireless location in code-division multiple access (CDMA) cellular networks. The second section discusses the potential applications of wireless location. The third section provides an overview of wireless location methods, followed by the accuracy requirements for specific applications in the fourth section. The remainder of the article discusses wireless location in CDMA cellular networks, including location algorithms in the sixth section, sources of error in the seventh section, and system loading aspects in the final section.
Wireless location using CDMA cellular networks brings with it the possibility of several applications which will benefit businesses as well as consumers. The potential applications include:
— E-911
— Location-sensitive billing
— Fraud detection
— Cellular system design and resource management
— Fleet management and intelligent transportation systems (ITS)
Location information for wireless E-911 calls permits rapid response in situations where callers are disoriented, disabled, unable to speak, or do not know their location. An increasingly large fraction of E-911 calls are placed by cellular phones, which is a direct result of the growing number of cellular subscribers. In 1994, approximately 50,000 wireless E-911 calls per day were made in the United States, a figure that increased to 60,000 in 1996. By the year 2000, it is estimated that this figure will grow to 130,000. A recent study by the state of New Jersey indicated that wireless E-911 calls accounted for 43 percent of all E-911 calls received during wireless location trials [4].
The wireless E-911 services outlined in the 1996 FCC ruling are to be implemented and deployed in two phases. Phase I, to be completed by April 1, 1998, requires that the carriers relay the location of the cell site and/or sector receiving the E- 911 call and the E-911 caller’s telephone number (known as the Automatic Number Identification, or ANI) to the designated Public Safety Answering Point (PSAP), thereby allowing the PSAP to call back if the call is disconnected. Phase II, to be completed by October 1, 2002, requires that wireless carriers be able to report the location of all E-911 callers with an accuracy of 125 m (410 ft) in 67 percent of cases.
Location-sensitive billing provides a wireless carrier the ability to offer different rates depending on whether the wireless terminal is used at home, in the office, or on the road [5]. This will allow wireless carriers to offer new rate choices for their subscribers and offer rates that will bring new subscribers into their customer base. It also enables a carrier to encourage desirable usage behavior by employing location price discrimination. Another lucrative application for location technology is in the ongoing battle against cellular phone fraud. Some carriers estimate that up to 1 percent of their customer base experiences fraud each month. Annual industry fraud ranges in the area of $500 million, all of which is passed on to wireless customers in the form of higher phone usage rates. Without the use of wireless location systems, it is very difficult to find and catch the perpetrators.
Location technology could also be used in wireless system design and for radio resource and mobility management [6,7]. With the ability to locate a wireless call, system planners could dramatically improve their ability to architect cells and wireless systems. Cells could be better positioned and tuned, and spectral efficiency improved. More effective resource management could be obtained through the allocation of channels based on the knowledge of the wireless caller’s location. Moreover, a service provider who may have multiple agreements with PCS, cellular, or satellite carriers could offer its customers the ability to choose a carrier that best suits their needs at any given time and location [8], thereby allowing the service provider to offer its customers a selection of carriers and price advantages.
Wireless location technology is also useful for fleet operations. Many fleet operators already make use of location technology to track their vehicles and operate their fleets more efficiently, thus improving their field service. Police and emergency vehicles, as well as taxi and other service operators, could also improve their field service through the use of location technology. Having knowledge of the location of their vehicles allows a dispatcher to locate the nearest available vehicle, greatly improving response times.
Radiolocation systems can be implemented that are based on either signal strength, angle of arrival (AOA), or time of arrival (TOA) measurements, or their combinations. The signal measurements are used to determine the length or direction of the radio paths to/from an MS from/to multiple BSs. This article only considers the case where the signal measurements are made at the BSs. We note that line of sight (LOS) propagation to the BSs is essential for highly accurate location estimates.
Radiolocation using signal strength is a well known location method that uses a known mathematical model describing the path loss attenuation with distance [9,10]. Since a measurement of signal strength provides a distance estimate between the MS and BS, the MS must lie on a circle centered at the BS. By using multiple BSs, the location of the MS can be determined.
For signal-strength-based location systems, the primary source of error is multipath fading and shadowing. Variations in the signal strength can be as great as 30–40 dB over distances on the order of a half wavelength (1/2-l). Signal strength averaging can help, but low-mobility MSs may not be able to average out the effects of multipath fading, and there will still be the variability due to shadow fading. The errors due to shadow fading can be combatted by using premeasured signal strength contours centered at the BSs [11]. However, this approach assumes a constant physical topography and requires that contours be mapped out for each BS.
Finally, in CDMA cellular systems the MSs are power controlled to combat the near-far effect. Time-division multiple access (TDMA) cellular systems use power control to conserve battery power in the MSs. Therefore, for signal-strength-based systems it is necessary that the transmit power of the MSs be known and controlled with reasonable accuracy.
AOA techniques estimate the MS location by first measuring the AOAs of a signal from an MS at several BSs through the use of antenna arrays. Scattering near and around the MS and BS will alter the measured AOA. In the absence of an LOS signal component, the antenna array will lock on to a reflected signal that may not be coming from the direction of the MS. Even if an LOS component is present, multipath will still interfere with the angle measurement. The accuracy of the AOA method diminishes with increasing distance between the MS and BS due to fundamental limitations of the devices used to measure the arrival angles as well as changing scattering characteristics.
For macrocells, scattering objects are primarily within a small distance of the MS, since the BSs are usually elevated well above the local terrain [12, 13]. Consequently, the signals arrive with a relatively narrow AOA spread at the BSs. Jakes [12] and Gans [14] have modeled this situation by assuming a ring of scatterers about the MS, with the BS situated well outside the ring (Fig. 1). For microcells, the BSs may be placed below rooftop level. Consequently, the BSs will often be surrounded by local scatterers such that the signals arrive at the BSs with a large AOA spread. Thus, while the AOA approach is useful for macrocells, it may be impractical for microcells.
The final class of radiolocation techniques are those based on estimating the TOAs of a signal transmitted by the MS and received at multiple BSs or the time differences of arrival (TDOAs) of a signal received at multiple pairs of BSs. In the TOA approach, the distance between an MS and a BS is measured by finding the one-way propagation time between an MS and a BS. Geometrically, this provides a circle, centered at the BS, on which the MS must lie. By using at least three BSs to resolve ambiguities, the MS’s position is given by the intersection of the circles. In the TDOA approach, differences in the TOAs are used. Since the hyperbola is a curve of constant time difference of arrival for two BSs, the time differences define hyperbolae, with foci at the BSs, on which the MS must lie. Hence, the location of the MS is at the intersection of the hyperbolae. The essential ingredient for the timebased approaches are high-resolution timing measurements. However, it should be noted that LOS propagation conditions are still necessary to achieve high accuracy for the time-based methods. The problem of non-LOS (NLOS) propagation is addressed later.
Several methods have been proposed as means of forming time estimates in wireless systems, including phase estimation, pulse transmission, and spread spectrum techniques. Phase estimating systems employ phase detectors from which TOA information is obtained [15], and requires synchronization at three or more BSs. TOA or TDOA information can be obtained from wideband pulse transmission using correlation techniques [7, 15]. Finally, with spread spectrum signaling, the TOAs or TDOAs can also be determined through the use of a correlation techniques, as will be discussed later. Spread spectrum ranging has been investigated in the literature [16, 17] and is the principle behind GPS [18].
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