Downlink observed time difference measurements

Details Downlink observed time difference measurements Downlink observed time difference measurements

1998-11-04

2002-03-12

Maung, Nay (Department: 2744)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S422100, C455S502000, C370S324000, C342S457000

Reexamination Certificate

active

06356763

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to locating the position of a mobile communication unit in a wireless communication network and, more particularly, to making downlink observed time difference measurements.
BACKGROUND OF THE INVENTION
The ability to locate the position of a mobile communication unit operating in a wireless communication system (for example, a cellular communication system) provides many well known advantages. Exemplary uses of such position location capability include security applications, emergency response applications, and travel guidance applications. Several known techniques for providing position location involve the measurement of certain characteristics of communication signals, such as the time of arrival (TOA), the round trip delay, or the angle of arrival of a communication signal. Some of these techniques can be further divided into uplink or downlink approaches. In the uplink category, a base transceiver station (BTS) or other receiver performs the measurements on communication signals originating at a mobile communication unit (or mobile station). In downlink approaches, the mobile station performs the measurements on signals originating at base transceiver stations or other transmitters.
One example of a downlink technique for locating the position of a mobile station is the observed time difference (OTD) technique. This technique will now be described with respect to the Global System for Mobile Communication (GSM), which is exemplary of a cellular communication system in which downlink observed time difference techniques are applicable. The OTD technique is implemented, for example, by having the mobile station measure the time difference between arrival times of selected radio signals transmitted from different base transceiver stations. Assuming the geometry shown in
FIG. 1
, and further assuming that two signals are transmitted simultaneously from the base transceiver stations BTS
1
and BTS
2
, and letting T
1
and T
2
denote the times of arrival of the respective signals at the mobile station, then the observed time difference OTD is given by the following equation:
T
1
-
T
2
=(
d
1
-
d
2
)/
c
,  (Eq. 1)
where d
1
and d
2
are the respective distances from BTS
1
and BTS
2
to the mobile station. The locations of BTS
1
and BTS
2
are known, and the possible locations of the mobile station are described by the hyperbola
15
shown in FIG.
1
. By combining measurements from at least three base transceiver stations, a position estimate for the mobile station can be obtained.
Most conventional cellular communication systems (including GSM systems) are asynchronous, that is, each base transceiver station uses its own internal clock reference to generate its frame and time slot structure. Therefore, the frame structures of the different base transceiver stations will tend to drift in time relative to one another, because clocks are not perfectly stable. As a consequence, an OTD measurement is not really meaningful for locating the position of a mobile station unless the differences in timing between the base transceiver stations being used is known. This difference, often referred to as the real time difference or RTD, represents the actual difference in absolute time between the transmission of respective signals (e.g., respective synchronization bursts in GSM) from respective base transceiver stations, which signals would be transmitted simultaneously if the frame structures of the base transceiver stations were perfectly synchronized.
Among several possible approaches to determine the real time difference RTD between base transceiver stations, two conventional examples are: absolute time stamping in the respective base transceiver stations; and use of stationary reference mobiles located in known positions. In the latter example, the reference mobile measures downlink signals sent from various base transceiver stations. Because the respective distances between the various base transceiver stations and the stationary reference mobile station are known, the expected time difference in arrival times of the respective signals from the base transceiver stations can be easily calculated. The real time difference RTD between base transceiver stations is simply the difference between the expected time difference of arrival and the observed time difference of arrival actually observed at the reference mobile station. The reference mobile station can periodically make the downlink time of arrival measurements and report them to a mobile location node in the network so that the network can maintain an updated record of the RTDs.
The techniques underlying known OTD methods are very similar to procedures used conventionally by mobile stations to synchronize to a serving base transceiver station and make measurements on a number of neighboring base transceiver stations as instructed by the serving cell (as in mobile assisted hand-off operations). The mobile station needs to know which base transceiver stations are to be monitored for OTD measurements. This information can typically be provided in conventional system information messages broadcasted in the cell, for example on a GSM cell's BCCH (broadcast control channels) frequency. This system information typically includes a list of frequencies of neighboring cells which are to be measured. The mobile station scans the designated frequencies to detect a frequency correction burst, which is an easily identifiable burst that appears approximately every 50 ms in GSM.
After successful detection of a frequency correction burst, the mobile station knows that in GSM the next frame will contain a synchronization burst SB. The synchronization burst SB contains the Base Station Identity Code (BSIC) and information indicative of the frame number of the current frame in which the burst SB is occurring. The mobile station measures the time of arrival of the synchronization burst SB at the mobile station relative to the timing of mobile station's own serving cell. Since now the mobile station knows the frame structure of the neighboring base transceiver station relative to its own serving base transceiver station timing, it is possible to repeat the time of arrival measurements to improve accuracy. This procedure is repeated until all frequencies (i.e., all BTSs) on the list have been measured. The observed time difference values recorded by the mobile station are then transferred to a mobile station location node in the cellular system, which node performs the position determination based on the observed time difference values, the real time difference values and the geographic locations of the base transceiver stations.
Because the mobile station does not know when the frequency correction burst (and thus the following synchronization burst SB) will appear, the brute force method described above, namely monitoring for the frequency correction burst, must be used.
The time required to capture a synchronization burst will depend on the measurement mode. OTD measurements can be made, for example, when call setup is being performed on a GSM SDCCH (Stand-alone Dedicated Control Channel), or during idle frames when the mobile station is in call mode, or during speech interrupt. For example, if the mobile station makes the measurements in call mode, then the mobile station can only make measurements during idle frames, which conventionally occur in GSM systems every 120 ms. The probability that a particular synchronization burst will appear within the idle frame is approximately 1 in 10, because the synchronization burst conventionally occurs once every ten frames in GSM. Accordingly, on average, 5 idle frames will be needed, meaning 0.6 seconds per base transceiver station. Thus, if it is desired to measure at least 6 neighboring base transceiver stations, an average measurement time of 3 or 4 seconds will be required, which may be prohibitively long in many applications.
The mobile station is guaranteed to have measured the synchronization burst SB if the mobile st

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