Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1997-11-17
2001-03-13
Olms, Douglas W. (Department: 2732)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S347000
Reexamination Certificate
active
06201803
ABSTRACT:
This application is the national stage of PCT/GB96/01039, filed May 1, 1996.
BACKGROUND OF THE INVENTION
This Invention relates to radio location systems. A number of systems are being developed for identifying the location of a mobile unit, using radio propagation characteristics. One such system is the Global Positioning System (GPS), in which a portable unit obtains a position fix using radio transmissions from space satellites. This system is highly accurate, but requires special equipment, and is unreliable in locations having poor visibility of the sky, because several widely separated satellites must be in line-of-sight relationship with the handset for a fix to be obtained.
Several proposals have been made for systems which use the radio propagation characteristics of a cellular radio system to provide a position fix for a cellular radio mobile unit. This would allow the mobile unit itself to act as a position finding device. As is well known, cellular radio systems allow a user having a portable handset (a “mobile unit”) to make and receive telephone calls, either to another mobile unit or to a conventional fixed termination, by means of a radio link. The radio link is established between the mobile unit and one of a network of fixed radio base stations distributed over the area to be covered. The system allows any mobile unit to communicate through any of the base stations; usually the mobile unit will communicate through the base station providing the best quality radio signal.
Because the mobile unit may move during the course of a call, it can become necessary for it to move out of range of the base station with which the call was initially established. Cellular radio systems therefore include handover systems to allow communication to be established with a second base station, and dropped from the first, without interrupting the call itself as perceived by either party to the call. In the system known as GSM, (Global System for Mobile communications), the mobile unit frequently monitors the BCCHs (Broadcast Control CHannels) of the surrounding base stations in order to establish which base station is providing the best signal, and therefore through which base station a new call should be established, or whether a handover should be initiated. This process occurs in both idle and active modes, i.e. there is no need for the user to make a call.
Developments in GPS technology mean that a highly accurate synchronisation source can now be implemented relatively cheaply at each cellular radio base site. A good source of synchronisation has a number of benefits, these include; improved handover, an ability to reduce the effect of interference between neighbouring base stations, and enabling highly accurate radiating frequencies on the radio interface. It should be noted that unlike simple broadcast time signals, the GPS synchronisation signal takes the position of the GPS receiver into account, and can therefore compensate for the time lag caused by the finite speed of radio waves.
European patent Specification EP0320913, (Nokia), describes a system in which timing pulses derived from the GPS system are transmitted from each of three or more base stations, and their different arrival times at the mobile unit are used to identify the position of the unit. This prior art system requires the mobile unit to interrogate each base station in turn, which requires it to hand over communication between the various base stations in order to carry out this interrogation. This requires the use of several traffic channels, or an auxiliary channel and also requires that reliable radio communication can be established with each nearby base station.
In International Patent Application WO95/00821 (Omniplex) each base station transmits synchronised packet data signals. The mobile unit monitors all the base stations' packet data channels simultaneously, which either requires a mobile unit capable of receiving several radio frequencies at once, or that all the base stations transmit their data packets on the same channel. Neither of these features are conventional in a cellular radio system.
Both of these systems also require the transmission of special timing or synchronisation pulses from the cell sites (base stations) to the mobile unit, and the recognition of these pulses by the mobile unit. This requirement not only imposes a signalling overhead on the mobile unit, but it requires additional functionality in the mobile unit to recognise the timing pulses.
SUMMARY OF THE INVENTION
According to the invention, there is provided a method of determining the location of a mobile unit of a cellular radio system having a plurality of base stations, comprising the steps of determining the differences in timing between the base stations' transmissions as measured at the mobile unit, determining from the timing differences the differences in the distance of the mobile unit from each of the base stations, and deriving the location of the mobile unit from the differences in distance so determined, characterised in that the time division frame structures of the control channels of at least some of the base stations within radio range of the mobile unit are synchronised, and the mobile unit determines the differences in timing at the mobile unit of a characteristic feature of the time division frame structure broadcast by the control channel of each base station.
By using the control channel the mobile unit is able to make use of the existing radio link quality monitoring systems used to establish whether a handover should take place, and does not need to establish full communication with any of the base stations.
Preferably the characteristic feature used is a training signal transmitted by each base station, which is correlated with a reference training signal stored by the mobile unit. Such a “synchronisation burst” (SCH), and its correlation process already form part of the GSM standard for characterising the radio path for the purpose of identifying candidate base stations for handover. The method of the invention can therefore make use of these existing signals and correlation analysis programming. However, for position determination it is preferred that the system should use the first identified correlation of the reference signal, rather than the strongest as is use d for handover determination. This ensures that the most direct signal path, rather than a stronger but more indirect path, is used for distance calculation.
The derived location may also be time averaged to minimise the effects of spurious results from reflected signals, which would make the apparent distance between the base station and the mobile unit longer than it really is.
The derived location may be communicated via the cellular radio network to a remote user, instead of to the user of the mobile unit itself. An alarm signal may be transmitted if the derived location corresponds to a predetermined location. According to another aspect of the invention, there is provided a mobile unit for use with a cellular radio system, the mobile unit comprising apparatus for determining the position of the mobile unit; the apparatus comprising means for detecting timing differences between signals received from different radio base stations, and means for determining, from the timing differences, the differences in the distances of the mobile unit from each of the base stations; and means for deriving, from the differences in distance, the location of the mobile unit, characterised in that the mobile unit has means for determining the differences in timing at the mobile unit of a characteristic feature of a time division frame structure broadcast synchronously by the control channel of each base station. The mobile unit may further comprise means for receiving data from the currently serving base station concerning base stations within radio range of the mobile unit, the information including the geographical locations of the base stations.
The cellular radio network may be complementary to the mobile unit of the second aspe
Gannon Stephen M
Goetz Ian
Munday Peter R
British Telecommunications public limited company
Nixon & Vanderhye P.C.
Olms Douglas W.
Pizarro Ricardo M.
LandOfFree
Cellular radio location system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cellular radio location system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cellular radio location system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2535093