A wireless communication unit usually forms part of a wireless communication system. The wireless communication unit communicates through a wireless communication network, which also forms part of the wireless communication system.
An example of a wireless communication unit is a mobile telephone in a mobile telephone system. The terms ‘mobile communication unit’ and ‘wireless communication unit’ are often used interchangeably. However, the term ‘wireless communication unit’ may comprise a wide variety of devices, such as laptops and personal digital assistants that can communicate wirelessly.
The wireless communication network normally comprises a network of base stations. Each base station enables communication within an area referred to as a cell-footprint. Each cell-footprint may comprise multiple sectors. There are usually three sectors served by one base station. Each sector may be served by a dedicated antenna, co-located with the base station.
The term ‘communicating’ includes a variety of forms of communication. These forms include, but are not limited to, speech or data communication sessions on traffic channels, and communication on the control channel. So, for example, a communication in a mobile telephone network may not require the user of a mobile telephone to actually place or receive a call. Instead, the communication may involve only the intermittent receipt by a mobile telephone of data, for example over the wireless communication system's control channel.
Typically, many wireless communication units move around. Information about the location of a wireless communication unit is commonly referred to as ‘geo-location’ information. Geo-location information can be derived in many ways. In particular, various forms of measurement information can be processed to provide an estimate of the location of the wireless mobile communication unit.
At any particular time, different forms of measurement information may be available from:
(i) The wireless communication unit;
(ii) The wireless communication network, and particularly from one or more base stations with which the wireless communication unit is communicating; or
(iii) Both of (i) and (ii).
Considering the measurement information in more detail, this information may be available either:
(i) Directly. This means that the measurement information is included in the measurement made. The measurement may be made either by the wireless communication unit, or by another part of the wireless communication system, such as the wireless communication network.
(ii) Indirectly. This means that the measurement information is derived from the measurements made. An example would be an estimate of the distance between a wireless communication unit and the base station of a wireless communication system. Such an estimate might be calculated by multiplying the speed of propagation of the signal by a measured time difference between transmission and receipt of a signal.
Wireless communication networks fall into two broad categories:
(i) Synchronous networks, such as Code Division Multiple Access systems, e.g. CDMA 2000. In synchronous networks, the timing offset between different base stations is constant. The amount of the offset is known to wireless mobile communication units that are using the network. In the example of CDMA2000, the timing offset is both known and constant, because each base station's timing is locked to a Global Positioning System satellite.
(ii) Asynchronous networks, such as the Universal Mobile Telephone System (UMTS). In asynchronous networks, the timing offset between different base stations is not constant. Wireless mobile communication units in asynchronous networks are not provided with information about the timing offset between base station timing references. In addition, these references drift over time, relative both to absolute timing references, as well as to each other.
In a synchronous wireless communication network, timing information may be used for measuring the geo-location of a wireless communication unit. In order to understand this, it is first necessary to consider the relationship between distance and time for a signal passing from a base station to a wireless communication unit.
This relationship can be expressed in an equation, which is of the form of equation [1] below:Distance=(Speed of light)×[(Measured time)−(Timing offset)]  [1]
Where:
‘Distance’ is the distance from the base station to the wireless communication unit.
‘Measured time’ is the amount of time that the communication appears to have taken to travel from the base station to the wireless communication unit. The measured time may be the difference between a time stamp embedded in the communication by the base station and an absolute reference time at which the wireless communication unit receives the communication. The absolute reference might be the correct time for the time zone in which the base station is located.
‘Timing offset’ is the amount of time by which the timing reference of the base station differs from the absolute reference.
Equation [1] can be re-arranged and then re-written as equation [2] below:Tm=Tpath+Tb  [2]
Where:
Tm is the ‘Measured time’
Tpath is the ratio of ‘Distance’/(Speed of light). This is the time it takes a radio signal to travel the path from the base station to the wireless communication unit.
Tb is the base station ‘Timing offset’, the amount of time by which the timing reference of the base station differs from the absolute reference.
FIG. 1 illustrates one technique that can be used to perform geo-location in a prior art mobile communications network 100.
FIG. 1 shows a wireless communication network 100. Wireless communication network 100 may be either a synchronous or an asynchronous network. FIG. 1 also illustrates various values of Tpath. FIG. 1 shows a first base station B1, see reference 120, a second base station B2, with reference 130, and a third base station B3, with reference 140. Wireless communication unit 110 is able to receive signals from all three base stations.
The time taken for signals to reach wireless communications unit 110 from base station B1 is Tpath(1). The time taken for signals to reach wireless communication unit 110 from base station B2 is Tpath(2). The time taken for signals to reach wireless communication unit 110 from base station B3 is Tpath(3).
Base station B1 has a timing offset Tb(1) relative to the absolute reference, which is the true local time. Base station B2 has a timing offset Tb(2) relative to the absolute reference. Base station B3 has a timing offset Tb(3) relative to the absolute reference.
Using these parameters, it is possible to apply equation [2] to signals received by wireless communications unit 110 from each of the three base stations. The resulting equations are the following set of equations [3]-[5]:Tm(1)=Tpath(1)+Tb(1)  [3]Tm(2)=Tpath(2)+Tb(2)  [4]Tm(3)=Tpath(3)+Tb(3)  [5]
Tm is measured by the wireless unit 110 in each case. Tm may be calculated, for example, by taking the difference between a time stamp embedded in the communication from a base station, and an absolute reference time at which the wireless communication unit 110 receives the communication. Tm is therefore known.
If wireless communication network 100 is a synchronous network, the offset timing values for each base station are known and constant. So each of Tb(1), Tb(2) and Tb(3) is known.
Using the measured values for Tm(1), Tm(2) and Tm(3), it is therefore possible to solve each of equations [3]-[5] above for the values of Tpath(1), Tpath(2) and Tpath(3).
Using the relation Tpath=‘Distance’/(Speed of light), each of the three Tpath values can be turned into a measurement of the distance from the wireless communication unit 110 to each base station.
Possession of this distance information allows for the geo-location of subscribers using a variety of well-known techniques. In a real wireless communication network, a mixture of relative and absolute distance information is likely to be available. However, if wireless communication network 100 is an asynchronous network, then the values of Tb(1), Tb(2) and Tb(3) are likely to be unknown, and to vary over time. This prevents solution of the equations [3]-[5].
So in an asynchronous network, it is much more difficult to derive a measurement of the location of a wireless communication unit from signal timing information. Geo-location techniques that are used in synchronous networks cannot be applied directly to asynchronous wireless communication systems. In effect, the lack of a known and constant timing offset Tb for each base station deprives these techniques of a key piece of information. This can greatly limit geo-location accuracy in asynchronous networks. In real networks of the prior art, this problem is usually considered insoluble.
Prior art Canadian patent application CA2600700 (A1) does describe a ‘Method and system for facilitating timing of base stations in an asynchronous CDMA mobile communications system’. In the arrangement of CA2600700 (A1), the determination of the perceived timing offset between base stations is necessary to support communications objectives. This allows two base stations to send the same information to a mobile, so that it arrives at the same time. The information concerned might be, for example, a portion of speech. This is always done on a mobile-by-mobile basis, and the information or results from one mobile do not influence any operations at a different mobile. CA2600700 describes how the calculation of this timing can be done at one or more mobiles.
Prior art United States patent application US2006239391 (A1) describes ‘Evaluating base station timing in an asynchronous network’. In this arrangement, base station timing is determined. However, this determination requires prior knowledge of the locations of the mobiles.
Prior art International patent application WO2005002124 (A2) describes a ‘Method for sparse network deployment accuracy enhancements’. The approach provides location information. It relies on received power levels from different cell sites, which asynchronous systems will make available. In general terms, the mobile is expected to be closer to strongly-received cell sites, and further away from weakly-received cell sites. These assumptions can be fair approximations, under some special circumstances. However, a precise geo-location strategy will be limited, due to highly-variable signal fading effects. An example of such an effect would be that commonly experienced inside buildings. When a user is inside a building, one cell site may appear weaker than another, due to different numbers of brick walls lying in the signal paths to each cell site. As a consequence, equidistant cell sites may provide very different signal strengths to the user in the building. A measurement of distance based on received and transmitted signal powers is therefore of limited applicability.