Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-01-20
2003-12-30
Maung, Nay (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C342S400000
Reexamination Certificate
active
06671514
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Invention
The present invention relates, in general, to wireless communications systems, particularly, to a system and method for improved mobile station location, and, more particularly, to a system and method for facilitating the location positioning of a mobile station in a code division multiple access cellular system.
2. Background and Objects of the Present Invention
The evolution of wireless communication over the past century, since Guglielmo Marconi's 1897 demonstration of radio's ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi's discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. The exponential growth of mobile telephony will continue to rise in the coming decades as well, as this wireless network interacts with and eventually overtakes the existing wireline networks.
Pursuant to a recent Federal Communications Commission (FCC) Ruling and Order, cellular phone service providers within the United States must provide by October 2001 the capability to locate the position of a cellular or mobile phone making an emergency (911) call within the provider's system to within 125 meters with about 67% probability, i.e., within one standard statistical deviation. A variety of techniques are currently under study to implement geolocation technology into existing and proposed systems.
Of course, one simple way to get a rough indication of a mobile terminal or station's (MS) position is to determine the identity of the current cell. A more accurate, but still approximate, mechanism is based upon handovers (including soft handovers), by which the propagation time between the MS and an associated base transceiver station (BTS) is measured. The handover method is simple to implement since it involves very little change in the radio part. Further, the various BTSs in such a system do not need an absolute time reference. The handover technique is generally considered unsatisfactory, however, since a handover to two other geographically located BTSs (needed for trilateration) is rare, particularly if the telecommunications system employs 1-cell frequency reuse, as is understood in the art.
An antenna array solution has been proposed by which an MS′ geolocation may be calculated from an estimated direction and the roundtrip delay of the communication signals. Naturally, the inclusion of a Global Positioning System (GPS) device within the MS would solve the geolocation problem, at a cost, however, of excessive computational and receiver complexity in the MS.
Another proposed solution to the above problem is set forth in Applicant's co-pending and commonly owned patent application entitled “System and Method for Positioning a Mobile Station in a CDMA Cellular System”, U.S. Ser. No. 08/951,345, filed Oct. 16, 1997, of which the instant invention is a Continuation-in-a-Part, and which is incorporated herein by reference. In the co-pending application, a modified downlink measurement solution is proposed in which signals by the BTSs and MSs are repeatedly idled in order to improve the reception (or “hearability”) of more remote systems, thereby enabling the exchange of timing information and trilateration therefrom.
For Code Division Multiple Access (CDMA) systems, the downlink measurements are made by the MS on signals transmitted by the BTSs, e.g., within the pilot channel data. These methods, however, require an absolute time reference in (or synchronization of) the BTSs. The downlink method of the aforementioned Continuation-in-Part application obtains the requisite timing references through signal cessations.
Other proposed solutions include uplink measurements made by the BTSs based upon MS signals, e.g., a long, known training sequence. Such methods, however, as with the downlink techniques, require an absolute time reference in the respective BTS(s) or that inter-BTS timing is known. A combined uplink/downlink solution by Applicants' Assignee, which overcomes the timing/synchronization requirement, is set forth in U.S. patent application Ser. No. 08/935,421, entitled “Method and System for Determining Position of Cellular Mobile Terminal”, filed Sep. 23, 1997, which is also incorporated herein by reference. The combined solution set forth in said application involves the utilization of uplink and downlink signal propagation air-times to determine the distance between a mobile station and a base station, thereby avoiding the requirement of the absolute time reference or synchronization.
Finally, power measurement geolocation techniques have been employed which estimate signal path loss and, therefore, distance. Such techniques, however, are not very accurate.
Of the above solutions, only the downlink, uplink and combined downlink/uplink techniques are remotely feasible, but each has fundamental problems. The stand-alone downlink solution, for example, has the inherent problem of hearing a sufficient number of BTSs. This is the so-called “near-far” problem, which is particularly serious in CDMA systems. The combined technique also has some drawbacks in that the positioning may take a considerable time since both downlink and uplink measurements are required. Also, reliability in the combined technique is poorer since hearability is limited to that link (up or down) having the worst performance. Finally, more information bandwidth is required in the combined technique.
With regard to the stand-alone uplink technique, it, too, has some difficulties. For example, the near-far problem in such systems may be combated by increasing the transmitter power of the MS and by transmitting a known signal for a long period of time, all of which has serious effects upon system performance. Transmitting a known sequence requires either overriding speech on the speech channel, potentially causing speech disruption, or sending the pattern in parallel, resulting in increased MS complexity, battery drainage and increased information bandwidth usage. The aforementioned problems worsen if the signal is transmitted over a longer period of time.
As discussed, the above uplink and downlink technique require the known relative timing between the BTSs. Although this problem is immediately solvable by inclusion of a GPS receiver in each BTS, this costly alternative is not feasible for small base station systems (BSS), particularly micro and pico BSSs. Furthermore, reliance upon an independent system means the operator has no control over system performance.
A still further problem, related particularly to a current Wideband CDMA standard (WCDMA), is that although the MS can find the primary and secondary synchronization codes from neighboring BTSs, the particular identities of those transmitting BTSs are unknown, which would mean that any timing information obtained is useless. It is, however, possible to ascertain the correct BTS for each signal by detecting the Broadcast Control Channels (BCCH) of the respective BTSs. This, however, requires that the signal to noise (S/I) ratio be fairly high so as to better capture signals from the more remote BTSs.
By way of background, the basics of a Time Division Multiple Access system and a CDMA system will now be discussed to better illustrate some of the aforementioned problems in geolocation in these two systems, as also set forth in Applicant's co-pending parent application. For example, in Time Division Multiple Access (TDMA) telecommunications systems a mobile terminal or station (MS) is in communication with a given base transceiver station or system (BTS) during only one of typically eight sequential and repeating timeslots. Other MSs communicate
Cedervall Mats
Karlsson Jonas
Maung Nay
Nguyen Tu X
Telefonaktiebolaget LM Ericsson (publ)
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