WO2001008436A1

WO2001008436A1 - Method and system for measuring during a call set-up procedure

Info

Publication number: WO2001008436A1
Application number: PCT/EP1999/005354
Authority: WO
Inventors: Brendan Patrick Ludden; Luis Lopes
Original assignee: Motorola Limited
Priority date: 1998-06-10
Filing date: 1999-07-21
Publication date: 2001-02-01
Also published as: GB9812406D0; GB2338375A

Abstract

The invention provides a system for obtaining measurements in a GSM system. There is provided a system which withholds the assignment command during a call set-up thereby causing a mobile station (101) to transmit fill frames. These are detected by receiver stations (115, 117, 119) where measurements are performed and a characteristic is determined. The invention is applicable to determine the position of the mobile station from the time of arrival of the received fill frames.

Description

METHOD AND SYSTEM FOR MEASURING DURING A CALL SET-UP PROCEDURE

Field of the Invention

The present invention relates to a GSM (Global System for Mobile communication) mobile communication system and a method of operation thereof. The invention is applicable but not limited to determination of the position of mobile stations.

Background of the Invention

In a cellular mobile communication system such as the Global System for

Mobile Communication (GSM),each of the mobile stations communicate with a typically fixed base station. Communication from the mobile station to the base station is known as uplink and communication from the base station to the mobile station is known as downlink. The total coverage area of the system is divided into a number of separate cells each covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. As a mobile station moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the mobile station and the base station of the first cell to being between the mobile station the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.

All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the fixed network. A call from a mobile station is routed through the fixed network to the destination specific for this call. If the call is between two mobile stations of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other mobile station currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a mobile station and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.

GSM has been developed in order to provide an efficient use of the allocated frequency spectrum and to provide high quality calls. A number of measurements made by the mobile stations and the base stations have been defined in order to provide sufficient information for efficient allocation of the mobile stations and to ensure reliable handover when moving between cells. The standards do not include specific provisions for enabling non-serving cells to identify or indeed measure the transmissions from the mobile stations.

However, for some applications it can be advantageous to identify and measure transmissions from mobile stations. An example is the determination of the position of a mobile station. Techniques are known which by measuring the transmission delays between a mobile station and three or more fixed stations can determine the position of the mobile. An example is the triangulation technique used for example in the Global

Positioning System (GPS). The transmission can either originate at the base stations or at the mobile stations. However, a position determination which can be implemented in the fixed infrastructure (i.e. base stations and fixed network) without requiring any modifications of a standard mobile station has the advantage if not requiring all current mobile stations to be replaced. This can be achieved by determining the position from measurements of the standard transmissions of the mobile station.

Techniques for facilitating and improving measurements of mobile station transmissions are thus advantageous.

Summary of the Invention

The invention seeks to provide a GSM system with improved possibility of performing measurements on mobile station transmissions.

According to a first aspect of the invention there is provided a GSM mobile communication system comprising means for withholding a call assignment command during a call set-up causing a mobile station to transmit at one fill frame; at least one receiver station arranged to detect at least the one fill frame transmitted from the mobile station; and a processor for determining a characteristic of at least the one fill frame received by the receiver station.

Preferably, the characteristic is a time of arrival, a direction of arrival, a signal level, a signal to noise ratio, a bit error rate or a rate of fading.

According to another aspect of the invention the detection of the at least one fill frame is by comparison to a local replica of a fill frame, preferably by correlation.

According to a different aspect of the invention a position of the mobile station is determined from the characteristic of at least the one fill frame. According to one aspect of the invention, the call set-up is subsequently abandoned and the position of the mobile station can be reported to the mobile station by a data service such as the Short Message Service.

According to another aspect of the invention there is provided a method of performing measurements in GSM mobile communication system comprising the steps of withholding a call assignment command during a call set-up causing a mobile station to transmit at least one fill frame; detecting at least one fill frame transmitted from the mobile station at least one receiver station; and determining a characteristic of at least the one fill frame received by the receiver station.

Without requiring modifications to existing GSM standards, the invention thus by withholding call assignment commands enables control of mobile transmissions with known data contents and approximate transmission times.

Brief Description of the Drawings

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawing, in which:

FIG. 1 is an illustration of a GSM communication system according to the invention;

FIG. 2 is an illustration of a call set-up procedure for GSM according to prior art.

Description of a Preferred Embodiment FIG. 1 illustrates an example of a GSM communication system according to the invention. A mobile station 101 is located within a cell 103 served by a base station 105. Neighbouring cells 107,109 are served by other base stations 111,113.

Receiving stations 115,117,119 are included to receive and measure characteristics of the transmissions from the mobile station. For example if the propagation delay of the paths 121,123 and 125 are measured it is possible to determine the position (or location) of the mobile station from the position of the receiving stations and these propagation delays as is well known in the art. Preferably the receiving stations will communicate the result of their measurements to a processor 126 where the position determination is performed. This processor 126 can be located at the receiver stations, the base stations, in the fixed network or distributed in the system. Furthermore, the position determination can be performed from receiver stations in the same cell or in different cells. Preferably, the number and location of the receiver stations are so that the transmissions from mobile stations throughout the coverage area can be received simultaneously from as many receiver stations as is required for the specific algorithm. Specifically, the receiver stations may be co-located with the base stations or the indeed the base stations themselves may perform the function of the receiver stations.

According to the invention a technique is provided for performing measurements of mobile station transmissions during a call set-up procedure.

In GSM call set-up can be either mobile or initiated in the fixed network. For a mobile station initiating the call, the simplified protocol is shown in FIG. 2 which is according to prior art. At Tl the mobile station (MS) requests a channel and the base station (BTS) responds at T2 by assigning a temporary channel (SDCCH) on which to complete the call set-up. The mobile station confirms by using this channel at T3. At T4 the mobile station transmits a set-up message containing call information such as type of call and number being called. This message is confirmed by a call 'proceeding message' from the fixed network. The fixed network then determines a suitable traffic channel and transmits this to the mobile station in the ' Assignment Command' message. The mobile station responds on the allocated traffic channel by sending an 'Assignment Complete' message and the call set-up completes as defined in the standards. A description of the call set-up routine in GSM can be found in M Mouly and M-B Pautet's The GSM System for Mobile Communications pub Cell & Sys, Palaiseau, France, ISBN2-9507190- 0-7.

When determining the position of the mobile station it is beneficial to control the number of repeats of the transmission and to have information of the approximate time of transmission and of the content of transmission.

According to a feature of the invention, the call set-up procedure is followed as for standard GSM call set-up except that the assignment command is withheld and not transmitted to the mobile station. According to the GSM specifications, the mobile station will transmit fill frames when waiting for the assignment command. The content of these fill frames are known and the approximate transmission times are also known. The fixed network can furthermore control the number of retransmissions by transmitting an assignment command when a sufficient number of retransmissions have occurred.

Accordingly, the receiving stations are directed to detect the fill frames being received, preferably by receiving around the expected time of arrival and comparing the receiving signal with the knowledge of what has been transmitted and a characteristic of the reception of the fill frame is generated. As a specific example where the position of the mobile station is determined during a mobile initiated call set-up, the fixed network withholds the assignment command and communicates the approximate time of transmission and the contents of the fill frame to three receiving stations. The receiving stations compare the received signal with the expected signal in a time window around the expected time of arrival and determine the time of arrival from the time offset yielding the closest similarity. The receiving stations then communicate the determined time of arrival to a processor which determines the position of the mobile station from the times of arrival and the position of the receiving station by for example the triangulation technique well-known in the art. If the absolute time of transmission is not known, the relative time of arrival from three receiving stations allows a position determination in two dimensions and four receiving stations will allow position determination in three dimensions.

The detection of fill frames is preferably by comparison where the received signal is compared to a local replica of the expected signal to be received when a fill frame is transmitted. The comparison can be performed as a correlation between the two signals. For a sampled signal a correlation over N samples is given by

n J n

where x_n are the samples of the received signal and y_n are the samples of the local replica.

The detection of the fill frames and the determination of a characteristic of the fill frame received will preferably be performed simultaneously. For example, the correlation is repeated with small timing offsets within the time window where the fill frame is expected. A strong correlation for a given timing offset will indicate that the fill frame has been detected at this time. The correlation will thus simultaneously indicate that the fill frame has been detected and provide a determination of the time of arrival with a timing resolution equivalent to the steps of the timing offset. Also the magnitude of the correlation will be an indication of the signal strength of the received fill frame.

The comparison and correlation allows the receiver station to separate the signal of the mobile station from unwanted noise and interference. For a fill frame the receiver station can perform the correlation over an entire time slot as all data of the fill frame are known. However, for a GSM transmission containing data only the data in the midamble can be known and a correlation can thus only be extended over the midamble. This substantially increases the variance of the correlation due to noise and interference, and the transmission of fill frames thus allow significantly increased reliability and accuracy.

Other characteristics which can be determined include :

1. the direction of arrival which can be determined using for example by using a antenna array and suitable algorithm such as the 'Beamforming Algorithm' which is well known in the art. 2. a signal noise ratio which for example can be estimated between the ratio of the correlation and the total signal energy received 3. a bit error rate which for example can be determined by demodulating the received signal and comparing the data to the known data of the fill frame. 4. a rate of fading which for example can be determined from the rate of variation of the correlation.

Specifically, different parameters can be measured either at the same receiving station or at different receiving stations. As an example, the time of arrival and direction of arrival may be determined by one receiving station and the time of arrival by another receiving station and the position of the mobile station determined from these parameters.

It will be obvious to the skilled person how the invention can be applied to the situation of a fixed network originating call.

According to a preferred feature of the invention, the call may be abandoned subsequent to the transmission of fill frames. Thereby the measurements can be performed without the requirement that a call setup is completed that the user or fixed network subsequently will terminate if the desire is not to make a call but only to generate a position estimate. The fixed network may thus instigate measurements of the mobile station to determine the position of or the radio conditions for the mobile station with alerting the user. Alternatively, the user may instigate measurements, for example to determine his current position, without needing to make a call. The fixed network can report the determined position back to the mobile station preferably by use of one of the data services defined in GSM such as the Short Message Service (SMS) or the General Packet Radio Service (GPRS) or the Unstructured Supplementary Service Data (USSD). This will provide a fast, cheap and user friendly technique for a mobile station user to determine his position.

The invention thus provides the possibility of performing measurements of a number of characteristics of the transmissions from a mobile station as received by receiver stations. The measurements can be used for position determination or can provide extra information about the radio environment to the communication system.

Claims

1. A GSM mobile communication system comprising: means for withholding a call assignment command during a call set-up causing a mobile station (101) to transmit at least one fill frame; at least one receiver station (115,117,119) arranged to detect at least the one fill frame transmitted from the mobile station (101); and a processor for determining a characteristic of at least the one fill frame received by the receiver station (115,117,119).

2. A GSM mobile communication system as claimed in claim 1 wherein the characteristic is chosen from the group of: a) a time of arrival b) a direction of arrival c) a signal level d) a signal to noise ratio e) a bit error rate f) a rate of fading.

3. A GSM mobile communication system as claimed in claim 1 wherein detection of at least the one fill frame is by comparison to a local replica of a fill frame.

4. A GSM mobile communication system as claimed in claim 3 wherein comparison is by correlation between the local replica and at least the one fill frame.

5. A GSM mobile communication system as claimed in claim 1 wherein a position of the mobile station (101) is determined from the characteristic of at least the one fill frame.

6. A GSM mobile communication system as claimed in claim 5 further comprising: a plurality of receiver stations (115,117,119) each arranged to detect at least the one fill frame transmitted from the mobile station (101); means for determining a characteristic of at least the one fill frame received by each receiver station (115,117,119); and a processor for determining the position of the mobile station (101) in response to the characteristics determined by the receiver stations (115,117,119).

7. A GSM mobile communication system as claimed in claim 5 wherein the call set-up is subsequently abandoned.

8. A GSM mobile communication system as claimed in claim 5 wherein the position of the mobile station is reported to the mobile station by a data service.

9. A method of performing measurements in GSM mobile communication system comprising the steps of: withholding a call assignment command during a call set-up causing a mobile station (101) to transmit at least one fill frame; detecting at least the one fill frame transmitted from the mobile station at least one receiver station (115,117,119); and determining a characteristic of at least the one fill frame received by the receiver station (115,117,119).

10. A method of performing measurements in GSM mobile communication system according to claim 9 wherein a position of the mobile station is determined from the characteristic of at least the one fill frame.