IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS

Abstract: Described herein is a method of determining when propagation delays measured in both directions between two base stations <i>i</i> and <i>j</i> relate to the earliest path and not to a later path. Once synchronisation has been achieved, the measurement of the propagation delay is compared with predetermined limits and if falling within those limits is stored. Subsequent measurements of propagation delay are also compared with the predetermined limits and are only stored if they fall within those limits and are better than the currently stored measurement.

Description: IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS The present invention relates to improvements in or relating to mobile telecommunications systems. The UMTS terrestrial radio access time division duplex (UTRA TDD) mode is based on a combination of code division multiple access (CDMA) and hybrid time division multiple access (TDM A). UMTS is an acronym for universal mobile telecommunication system as will be understood by persons skilled in the art. Reliable operation in the UTRA TDD mode, incorporating the combined TD-CDMA multiple access scheme, requires synchronisation between base stations within a compliant telecommunications system. Moreover, the mode also requires the provision of position information for the mobile stations affiliated to each base station. Synchronisation between base stations is also desirable in order to maximise system capacity. To these ends, the synchronisation of base stations must be achieved at the levels of time slots, frames and multi-frames, where a multi-frame is a repeating cycle of a number of frames. Co-pending British patent application nos. 0007143.1 and 0007144.9 describe a method of providing synchronisation between a plurality of base stations in a telecommunications system which comprises providing a random access channel in each cell. A mobile station receives a resource unit via the random access channel and a base station uses the random access channel in one cell to transmit a synchronisation signal to other base stations within the system. The base stations are synchronised together by means of transmissions between base stations. This involves measuring timing differences or offsets between the base stations in both directions to remove the effect of propagation delays. The timing differences or offsets are reported back to a central radio network controller (RNC) which computes the necessary timing updates and signals these back to the base stations. This method could theoretically achieve perfect synchronisation provided the measurements are accurate and provided the clocks are all running at the same rate. Co-pending patent application nos. GB 9927919.2 and GB 0010363.0 (Our Ref: 1999P04886) filed concurrently herewith describe a method for performing timing measurements. However, this method is subject to potential errors due to noise appearing as the strongest measurement or due to the earliest path failing to be consistently identified. It is therefore an object of the present invention to provide a method in which erroneous measurements can significantly be reduced. In accordance with one aspect of the present invention, there is provided a method of synchronising measurements of propagation delay between two base stations, the method comprising the steps of:- a) achieving synchronisation between the two base stations; b) calculating an initial propagation delay; c) storing the initial propagation delay; d) calculating a subsequent propagation delay; e) determining if the subsequent propagation delay falls within predetermined limits; f) using the subsequent propagation delay if within the predetermined limits; g) storing the subsequent propagation delay if within the predetermined limits and below the initial propagation delay; and h) repeating steps d) to g). Advantageously, step c) comprises determining if the initial propagation delay meets the requirements of:- where τ™ and T™ are respectively the minimum and maximum predetermined limits of acceptable propagation delays; and di j and dμ are respectively the time differences between base stations i and 7 in opposite directions. In accordance with the present invention the RNC can filter the measurements for consistency and thus significantly reduce the utilisation of erroneous measurements. In the present invention, two way measurements are taken between two base stations and the result comprises a component relating to propagation delay and a component relating to time difference. Ideally, such a two way measurement should cancel out the propagation delay component, but this is only possible if the same path is measured in both cases as the only way to be consistent is to make the measurements on the earliest path. However, it is initially possible to measure the earliest path in one direction and to measure a later path in the other direction, but as synchronisation is achieved, the earliest path will be measured in both directions. Moreover, it is possible to lose the earliest path due to fading, and there is switching between the earliest and a later path. It is therefore desirable to know when measurements are based on a later path and to ignore them. When two way measurements are performed, they each consist of two components. Consider two base stations with indices i and 7. When base station^ transmits to base station i, it measures a time difference, drf.- d. . = δ. . +τ. . (1) where Si j is the time by which the clock at base station i is ahead of the clock at base station 7 and Ti j = Tμ is the propagation delay between the two base stations. Also, for the measurement in the opposite direction:- d 7 ■.1■ = δ J ■.ι■ +T 1-*- d 1.*. = -δ L-I. +τ '..I. ( ^2 ') Adding together equation (1) and (2) gives d. +d .. τ ',.I, =- ' • ' " For a given deployment, τ itj is bounded. Simplistically, τ itj > 0 and ti j < d ma Jc where d max is the maximum range between base stations and c is the speed of light. It may be necessary to increase d max artificially to cover the case where the line of sight path is unavailable. Alternatively, if the location of base stations i and 7 are known, as might be necessary for providing location services, the bounds on -zj can be more tightly specified. In accordance with the method of the present invention, a test is now performed using these bounds, that is:- d . +d .. c <-^ — - < τ ?r Any pairs of measurements of timing differences d tj and dμ between two base stations which fail this test are deemed unreliable and are not used. Note that it may be necessary to widen the limits slightly to allow for the clock drift which occurs between measuring d tj and measuring dμ since these measurements could be made at significantly different times. Suppose that the initial window for measuring the time was 10 6 chips, and suppose further that τ™f = 0 and τ™ = 50. In this case, the probability of a measurement in error falling within the test window is greatly reduced. Moreover, even when it does fall within the test window, its impact is small because the error itself must be relatively small. As the synchronisation process continues, it becomes possible to refine the test by comparing new measurements with old measurements. For example, let d t n j be the nth measurement of di . Then, d n . +d n . *u - 2 It is then possible to evaluate d n+l +d n+l τ » - A < - 1 ^_ < r" ,. + Δ where Δ is a small margin. Of course, in this case, there is no guarantee that d" and d . were better measurements that dff and d * f . However, this process can be used to detect when the timing measurement detects a path later than the earliest path. This is achieved by storing previous values of τN , and when a new measurement is performed and the test becomes τ ι m ,ι in < τ ι m ,ι. <τ ι",ι. +Δ where Δ typically corresponds to 0.25 chips. If the test is true, then the stored value of r" y is replaced with τ" y . and the measurement is used. If the test is not true, then the stored value of τ". is not replaced with τ" y . and the measurement is not used. In this way, the stored value tends towards being based upon the earliest path so that only measurements based on the earliest paths are used. Because of the drift between clocks, the method may be too stringent to perform during the early stages of synchronisation. In this case, wider limits (or no limits, if appropriate) can initially be used. When a reasonable degree of synchronisation is detected, then the more stringent test can be applied. This reasonable degree of synchronisation can be detected, for example, by evaluating the magnitude of the computed timing updates. It will readily be appreciated that the present invention is only useful when synchronisation has been achieved otherwise there will be an apparent increase in the propagation delay which is not related to measuring a later path. It will be understood that the method of the present invention allows the measured propagation delay which is used to determine the earliest path to decrease, but not to increase.

Claims: CLAIMS: 1. A method of synchronising measurements of propagation delay between two base stations, the method comprising the steps of:- a) achieving synchronisation between the two base stations; b) calculating an initial propagation delay; c) storing the initial propagation delay; d) calculating a subsequent propagation delay; e) determining if the subsequent propagation delay falls within predetermined limits; f) using the subsequent propagation delay if within the predetermined limits; g) storing the subsequent propagation delay if within the predetermined limits and below the initial propagation delay; and h) repeating steps d) to g). 2. A method according to claim 1, wherein step c) comprises determining if the initial propagation delay meets the requirements of:- where r/J" and r°** are respectively the minimum and maximum predetermined limits of acceptable propagation delays; and d itj and dμ are respectively the time differences between base stations i and 7 in opposite directions. 3. A method of synchronising measurements of propagation delay between two base stations substantially as hereinbefore described.

Inventors: HULBERT ANTHONY PETER, HULBERT, ANTHONY, PETER
Applicants: HULBERT ANTHONY PETER, ROKE MANOR RESEARCH, HULBERT, ANTHONY, PETER, ROKE MANOR RESEARCH LIMITED
Application year: 2000
Main classification: H04B7
IPC codes: H04B7