The present invention is related to wireless networks, and in particular to determining the location of wireless stations in a wireless network.
Use of wireless networks such as wireless local area networks (WLANs) is becoming widespread. Locating radios in a wireless communication system, such as a WLAN, enables new and enhanced features, such as location-based services and location-aware management. Location-based services include, for example, assigning the correct closest printer to a wireless station of a WLAN.
A WLAN may be ad hoc, in that any station may communicate directly with any other station, or have an infrastructure in which a station (called a “client station” or simply a “client”) can only communicate via an access point (AP)—a station that acts as a base station for a set of clients. The access point is typically coupled to other networks that may be wired or wireless, e.g., to the Internet or to an intranet. That wider network is called the “wired” network herein, and it is to be understood that this wired network may be an internetwork that includes other wireless networks.
WLAN management applications of radiolocation include the location of client stations and the location of rogue access points. See for example, U.S. patent application Ser. No. 10/766,174 filed Jan. 28, 2004 and titled “A METHOD, APPARATUS, AND SOFTWARE PRODUCT FOR DETECTING ROGUE ACCESS POINTS IN A WIRELESS NETWORK” to inventors Olson, et al., for more details of the latter application and how radiolocation may be used to aid rogue access point detection.
A number of techniques have been proposed for radiolocation. Prior art methods are known that rely on the Global Positioning System (GPS). GPS, however, is known to have poor indoor reception and long acquisition time. GPS also requires additional GPS hardware in the wireless station that would increase the cost of stations, e.g., client devices.
Methods based on signal strength also are known. Many such methods, however, require a signal strength map of the area of interest, obtained, for example, by taking time-consuming signal strength measurements at numerous locations in the area of interest. Methods that use RF modeling to determine the signal strength map also are known. The modeling, however, requires detailed input of a building layout, wall location, and construction materials. Some methods also depend on a model of loss of signal strength in the area of interest. In general, while such signal strength methods may work well, particularly if there is a relatively large number of closely spaced access points from which the signal strength measurements can be obtained, there is still a need in the art for alternate or complimentary methods to carry out radiolocation.
Radiolocation methods are known that rely on time difference of arrival (TDOA) estimation. One advantage of TDOA methods is that no prior information is needed on the region of interest. All that are needed are a common clock reference and connections that are line of sight—or near line of sight.
Thus there is an incentive to use TDOA methods for radiolocation.
The speed of electromagnetic radiation (light) is about 1 ft/ns. Therefore, TDOA methods require fine time accuracy. Furthermore, radio hardware that is used in modern WLANs includes sampling a signal at a sampling rate that is typically in the order of 40, 80 MHz or perhaps even 120 MHz, so that the best possible resolution based on individual samples, is ±12.5, ±6.25, ±4.2 ns (in time) or feet (in distance) for the 40, 80, and 120 MHz sampling rates. However, typical IEEE 802.11b, 802.11a and 802.11g signals have a signal bandwidth that is less than or equal to 20 MHz, so that 1/bandwidth is greater than or equal to 50 ns. This in itself is not sufficient to achieve resolution of, say 5 to 10 ft.
Thus there is a need to perform TDOA with finer resolution than is provided based on the timing of an individual sample and based on bandwidth. To do better requires processing a sequence of samples of received data, each of which is associated with a local clock value, e.g., processing a sequence of timestamped received signal samples. Such processing in turn requires a capture of a sequence of received signal samples and the ability to post-process such samples.
There therefore is a need in the art for a method and apparatus that captures, timestamps and distributes as necessary a plurality of samples received in a radio receiver for TDOA location determining.
Local clocks in wireless devices, e.g., clocks that can be used for timestamping the data samples, have some level of drift and other inaccuracies. Such clocks are typically generated using crystals. A crystal that is inherently sufficiently accurate to carry out synchronization useful for TDOA estimation may be prohibitively expensive. Furthermore, any form of independent clock requires at least time offset calibration no matter how accurate. Any free running clock will have an unknown clock offset unless a relative measurement is performed. The quality of the crystal will influence how often such calibration needs to be carried out.
One mechanism that would avoid needing mutual calibration would be some form of physical or wired clock distribution with equal length cables or some mechanism to eliminate clock offsets.
Thus there is a need in the art for a method of TDOA determination using local clocks that are relatively inaccurate, e.g., that drift over time without requiring a mechanism for central clock distribution.