WO2003061132A1

WO2003061132A1 - A correlation method for communication apparatus

Info

Publication number: WO2003061132A1
Application number: PCT/GB2003/000008
Authority: WO
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Limited
Priority date: 2002-01-04
Filing date: 2003-01-03
Publication date: 2003-07-24
Also published as: GB0200093D0; AU2003201634A1; EP1461865A1

Abstract

In a correlation method local interference cancellation is carried out based on the significant paths of the strongest received signal. These are the paths that may cause interference to the weaker signals and the interference cancellation is carried out at relevant parts of the weaker signals. This saves computational time in comparison with cancellation carried out over all points of the signals.

Description

A correlation method for communication apparatus

This invention relates to a correlation method for communication apparatus and it is particularly, but not exclusively, applicable to a correlation method the result of which is used to determine the location of the apparatus.

Cellular communications systems are being increasingly required to provide location services to determine the geographic position of mobiles using the systems. One way in which the position may be determined is for the mobile to time the arrival of signals from three or more base-stations having known positions. This is a similar technique to that employed in the global positioning satellite system (GPS). A satellite-based system has the advantage of course of having line of sight propagation rather than the more problematic multi-path propagation environment experienced by mobiles operating in a cellular system.

In current mobile systems the mobile identifies the signals to be used by performing a correlation of the incoming signals against the code appropriate to each required signal. The effect of this correlation is to suppress unwanted signals since they do not experience the same correlation gain. A problem with this approach is that some of the base stations are relatively distant and others will be relatively close. This is known as the "Near Far Effect". In this effect the difference of the path losses due to the different ranges from the transmitters to the receivers may be large enough to overwhelm the interference suppression that is achievable by correlation. Shadowing and multi-path propagation adds to this problem.

According to the invention there is provided a correlation method for use in communications apparatus which method comprising receiving signals from a plurality of base stations, correlating all the signals against relevant signals in a correlation window, for each correlated signal determining an earliest significant entry, determining the strongest signal, for the determined strongest signal identifying significant paths, about the earliest significant entry for at least some of the other signals performing an interference cancellation using at least one of the paths, processing the at least some of the other signals to determine the earliest significant entry and comparing with the earlier determined significant entries in order to determine if the entries have moved, if the entries have moved repeating the process by using a further identified path.

The interference-cancelled signals may then be used in a position location process or other process.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings in which:

Figure 1 shows a communications system and a mobile operating in accordance with the invention; and

Figure 2 shows method of the invention in schematic block diagram form.

As is shown in figure 1 a communications system 1 includes a number of base-stations 2 to 5 and a mobile station 6. The environment in which the system operates includes a number of topographical features and buildings 7 and 8 which reflect signals. This causes multi-path reception of signals from the base-stations. For example, base-station 4 produces a direct signal 9, and signals 10 and 11 reflected to the mobile by the buildings 7 and 8.

It will be seen from the figure that some base-stations are relatively remote and one in particular base-station 4 is relatively close. Signals will normally be received from that base-station with the greatest strength. However, the multi-path signals will arrive at different times because of the greater path length and these signals will interfere with the signals received from the other base-stations.

The mobile station 6 in a first step 20 correlates across a detection window all the relevant transmitted codes preferably this is done at complex base-band This provides a set of correlation scores over the measurement window which are complex amplitudes. In the next step 21 the earliest significant entry for each signal is determined which in this embodiment is performed by computing the modulus squared of the complex correlation scores and comparing these against a threshold. In step 22 the relative strengths of the received signals are determined. The strongest signal is identified and for this signal the significant paths are identified in step 23. The significant t paths are. those_.that may interfere with the weaker signals.

In step 24 an interference cancellation process is carried out for the significant paths at relevant peaks of the weaker signals. This is therefore a localised interference cancellation by subtracting the cross correlation from the strongest signal or signals to one or more of the weaker signals around the position of the respective earliest peaks for the weaker signals. The interference cancelled weaker signals are then reprocessed in step 25 to determine their earliest peaks and in step 26 the peaks are compared with those earlier determined and if the peaks are in the same position the process stops and the resultant signals used in a location process to determine the position of the mobile station in a known manner. If the peaks have moved the process returns to step 23 to process based on a different identified significant path.

In alternative embodiments of the invention the order of the steps may be changed.

By performing interference cancellation by cancelling the strongest signal or signals in this localised manner significant processing power is saved over that which would be expended if cancellation was carried out over all positions of the weaker signals.

Claims

1. A correlation method for use in communications apparatus which method _. comprising receiving signals from _ap!urality_ of base stations., correlating all the signals against relevant signals in a correlation window, for each correlated signal determining an earliest significant entry, determining the strongest signal, for the determined strongest signal identifying significant paths, about the earliest significant entry for at least some of the other signals performing an interference cancellation using at least one of the paths, processing the at least some of the other signals to determine the earliest significant entry and comparing with the earlier determined significant entries in order to determine if the entries have moved, if the entries have moved repeating the process by using a further identified path.

2. A method as claimed in claim 1 wherein the correlation is carried out at complex base-band.

3. A method as claimed in claim 2 wherein the correlation produces a set of correlation scores which are complex amplitudes.

4. A method as claimed in claim 3 wherein the step of determining the earliest entry comprises computing the modulus squared of the complex correlation scores and comparing these against a threshold.

5. Radio receiving apparatus in use operating in accordance with a method as claimed in any preceding claim.