WO2001015353A1

WO2001015353A1 - A method of measuring a characteristic of a radio signal and apparatus therefor

Info

Publication number: WO2001015353A1
Application number: PCT/EP2000/008055
Authority: WO
Inventors: Brendan Patrick Ludden; Luis Lopes
Original assignee: Motorola Limited
Priority date: 1999-08-26
Filing date: 2000-08-16
Publication date: 2001-03-01
Also published as: GB9920248D0; AU6996900A

Abstract

This invention relates to a method of measuring a characteristic of a radio signal and apparatus therefor, in particular for use in a CDMA cellular communication system. The invention includes means (307) for power down intervals being designated for a plurality of base stations (301, 303, 305). The base stations include means for powering down (323, 325, 327) and means (311, 313, 315) for transmitting a radio signal. In each designated power down time interval some base stations (301, 303, 305) will transmit a signal whereas other base stations (301, 303, 305) will power down. The system also comprises a receiver (309) for measuring a characteristic of the transmitted signal. The time intervals are chosen to be at least partly overlapping thereby allowing a signal from a weak base station to be detected and a characteristic determined when a strong interferer is powering down. The invention is specifically applicable to the Universal Terrestrial Mobile Telephone system (UMTS).

Description

A METHOD OF MEASURING A CHARACTERISTIC OF A RADIO

SIGNAL AND APPARATUS THEREFOR

Field of the Invention

This invention relates to a method of measuring a characteristic of a radio signal and apparatus therefor, in particular for use in a CDMA cellular communication system for mobile communication.

Background of the Invention

In a cellular communication system each of the subscriber units (typically mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.

As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and 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 network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units 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 subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit 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 .

A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the subscriber units and the base stations. This spectrum must be shared between all subscriber units simultaneously using the system.

One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de- spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of subscriber units can be accommodated in the same wideband spectrum by allocating different codes for different subscriber units. Codes are chosen to minimise the interference caused between subscriber units typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in 'Spread Spectrum CDMA Systems for Wireless Communications', Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.

In CDMA systems the despreading of the wanted signal effectively improves the signal to noise ratio of the wanted signal with respect to a non-correlated interferer by a factor equal to the length of the spreading code. This factor is known as the spreading gain. However, if the interferer is sufficiently strong the signal to noise ratio may still be unacceptable for a given length of spreading code. An example of this is where a subscriber unit in a CDMA cellular system is very close to one base station while attempting to receive a signal from a base station further away. Another example is where a subscriber unit very close to a base station transmits a strong signal interfering with subscriber units further away from the base station. The problem is known as the near far problem.

The near far problem is specifically problematic in situations where the signals from several base stations are measured by one receiver. This is the case in some methods of location determination where it is known to determine the location of a subscriber unit by estimating the range from the subscriber unit to a number of fixed base stations with known locations. If the location must be determined in three dimensions, the range between the subscriber unit and at least three base stations must be determined. The method is known as triangulation.

It is known to determine the range between the base stations from the time of arrival of signals transmitted from the base station to the subscriber unit. For example, if a signal is transmitted by a base station to the subscriber unit, the time of arrival will depend on the propagation delay which is proportional to the distance travelled by the radio signal. If the time of transmission is known the relative time of arrival can be calculated and this will correspond to the range between the base station and the subscriber unit. Typically, the subscriber units are not synchronised with the base stations, and the location is determined from the difference in the time of arrival of signals from different base station. In this case signals from at least one additional base stations must be received by the subscriber unit.

In order for this method of location determination to work it is necessary for the subscriber unit to receive signals from several base stations at sufficient signal to noise ratio to determine the time of arrival. However, as mentioned this can often be a problem especially in areas close to base stations and in a CDMA systems. In these systems there will thus typically be substantial areas around each base station where location determination is not possible due to the near far problem.

In order to overcome this problem it has for UMTS been proposed that each base station pseudo randomly switches off transmission during some time intervals. As shown in Fig. 1 UMTS employs a slotted transmission structure with time slots of a 10 msec duration. According to the proposal the base stations will switch off in pseudo random time slots determined in the individual base station. In the example shown base station 1 switches off in time slot 201, base station 2 in time slot 203 and base station 3 in time slot 205. As each interval is chosen pseudo randomly, it is unknown when an interfering base station will switch off and also several base stations may switch off at the same time.

Whilst the proposed scheme goes some way to alleviate these problems it is not optimal. For example as the received signal to noise ratio is not always improved to a useful extent for location measurements. Simulations have shown that a relatively high number of measurements must be received in order to achieve acceptable performance and this has significant impact on the complexity of the handset w.r.t. processing and memory. Furthermore, this requirement results in relatively slow location determination where measurement collection times of around 1 to 20 seconds can be expected. Accordingly a method of measuring radio signals in a communication system would be beneficial.

Summary of the Invention

The current invention seeks to provide an improved method of measuring radio signals in a communication system.

Accordingly there is provided a method for measuring a characteristic of a radio signal in a communication system, the communication system comprising a plurality of base stations and at least one receiver. The method comprising the steps of power down time intervals being designated for a plurality of base stations, at least a first base station powering down in its designated power down time interval and at least a second base station transmitting a radio signal during its designated power down time interval. A receiver measures a characteristic of the radio signal from the base station and wherein the designated power down time intervals are chosen to at least partly overlap.

An apparatus is also provided for measuring a characteristic of a radio signal in a communication system, the communication system having a plurality of base stations and at least one receiver. The apparatus comprises means for power down time intervals being designated for a plurality of base stations and means for at least one base station powering down in its designated power down time interval. At least one base station is operable to transmit a radio signal during its designated power down time interval and a receiver measures a character is itic of the radio signal from the base station. The designated power down time intervals are chosen to at least partly overlap. Herein the expression "power down" can mean either reducing power or switching off power altogether.

Brief Description of the Drawings

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

FIG. 1 is an illustration of a cellular communication system according to prior art;

FIG. 2 is an illustration of the transmission structure of CDMA signals in UMTS according to prior art;

FIG. 3 is an illustration of an embodiment in accordance with the current invention;

FIG. 4 is an illustration of the transmission structure of CDMA signals in UMTS in accordance with an embodiment of the invention; and

FIG. 5 illustrates a method of measuring a characteristic of a radio signal in accordance with an embodiment of the invention

Detailed Description of a Preferred Embodiment

The following description focuses on an embodiment compliant with the current approach for the standardisation of UMTS but it will be apparent that the invention is not limited to this application but is applicable to many other communication systems and in particular to CDMA communication systems.

Fig. 3 illustrates an embodiment in accordance with the invention. In this case the communication system comprises three base stations 301, 303 and 305, means for designating power down intervals for a plurality of base stations 307; and a receiver 309 for measuring a characteristic of the radio signal. Each of the base stations 301,303,305 comprises a transmitter 311,313,315 and an antenna 317,319,321 and means for the base stations to power down in their designated power down time interval 323,325,327.

In the example of a UMTS communication system all base stations simultaneously transmit and receive CDMA signals thereby supporting a number of users. The receiver 309 thus receives signals from a number of base stations simultaneously and due to the near far problem may be unable to detect signals from weaker base stations due to the interference caused by stronger base stations. The receiver 309 may specifically be a receiver for location determination and specifically may be operable to detect signals from a plurality of base stations in order to determine a time of arrival and thus a location estimate based on triangulation. The receiver can be a separate unit but is preferably part of a subscriber unit or a position determination unit.

In accordance with an embodiment of the invention the means for designating power down intervals for a plurality of base stations 307 is operable to designate a number of time slots during which specific base stations will power down. Thus the means 307 for designating power down intervals may designate a certain power down time interval for each of the three base stations during which they will power down and in accordance with the invention these power down time intervals will be chosen so that they overlap at least partly. This illustrated in Fig. 4 where the power down time intervals 401,403,405 for three base stations are chosen as the time interval having a mid time closest to a certain time instant 407. By choosing the time intervals, in this case the time slots, having a mid time closest to a certain time instant the power down time intervals at least partly overlap. Alternatively, power down intervals can be chosen by first designating a specific power down time interval for one base station and subsequently choosing power down time intervals for other base stations as the time interval having the largest overlap with this. In accordance with an embodiment of the invention, a different base stations having also been designated a power down interval closest to the time instant 407 does not power down but instead transmits a signal. This signal is then detected by the receiver which will detect the signal with a high signal to noise ratio since the three other base stations have powered down. The receiver is thus able to estimate time of arrival of this signal, which can be used for location determination.

According to the preferred embodiment of the invention the means 307 for designating power down intervals is also responsible for designating which base stations transmits a signal rather than power down during the designated power down intervals.

Fig. 5 illustrates a method 500 of measuring a characteristic of a radio signal in accordance with an embodiment of the invention. In step 501 power down intervals are designated for a plurality of base stations. Each base station is transmitting according to a certain transmission structure or time structure such as the one shown in Fig. 4 and in the step 501 a certain time interval is designated for each base station affected by the method. Thus the method is not necessarily applied to the whole communication system but can be limited to parts of the communication system. In accordance with the invention the time intervals designated are chosen so that they overlap at least partly. In the case of UMTS a time interval is preferably chosen as a time slot and by choosing the time slot for each base station having a mid time closest to a given time instant it is ensured that the designated power down time intervals of the participating base stations overlap by at least half of a time slot. In step 503 at least one base station powers down in their designated power down time interval thereby reducing the interference caused to signals received from other base stations. In step 505 at least one base station transmits a radio signal during the designated power down interval thereby providing a signal which can be measured by a receiver. In step 507 the receiver measures a characteristic of the radio signal transmitted. It should be noted that the steps are shown in a sequence but it will apparent to a person skilled in the art that the steps may be carried out in different order and specifically that several or all steps can be carried out substantially simultaneously.

In the preferred embodiment the steps are repeated at frequent intervals and specifically the frequency of the various steps can be different. In UMTS a frame structure is employed which repeats every 720 msec. In this case a power down interval is preferably designated for each frame and in each frame base stations will either power down or transmit a measurement signal in the designated time slot. The designation of time slots may also be carried out for each time frame but preferably this step is carried out less frequently and the same relative designation is used for each frame. For example, the designation of power down intervals may be carried out once every ten frames and the designated time slots may be designated as e.g. the third time slot in each time frame for base station 1, the second time slot in each time frame for base station 2 etc.

A number of advantages are obtained by overlapping of the designated power down intervals. These include

• Location accuracy is improved for the same measurement time due to the better C/(I+N) of the signals in the idle slots used for location measurements.

• Idle slot periods can be shorter and less frequent for the same location performance thus minimising impact on the network performance • Less processing is required at the handset for the same location performance. This is due to the significant reduction in the number of bursts needed and length of the correlators required in the less challenging C/(I+N) conditions in known methods.

• Location performance is no longer dependent on system load. • Improved multipath rejection (MPR).

• Measurements are only collected during the power down interval (in known systems the measurements from the serving site must be made separately)

The means 307 for designating power down intervals may be located centrally located as shown in Fig. 3. However it will be apparent that the means 307 can be for example centralised in the fixed network, can be located in one or more base stations or can be distributed in the system.

In a preferred embodiment a central processor is located in the fixed network. The central processor designates power down intervals for a plurality of base stations and communicates the designation to the participating base stations whereas the determination of whether each base station will power down or transmit a signal is decided locally in each base station. In one embodiment each base station randomly or pseudo randomly decides to transmit in some intervals and to power down in other intervals. This will result in some conflicts but the probability of consequent conflicts between the same base stations is unlikely. Alternatively each base station can periodically transmit in a designated interval and preferably the periods of neighbouring base stations are chosen to be offset from each other.

Determining locally whether to power down or transmit has the advantage that it reduces communication from a central processor to the base station. This is especially the case where designation of power down intervals is performed at a much reduced rate in comparison with the frequency of power down intervals. It also provides a simple and robust system with good performance. However conflicts between transmitting base stations may still occur and alternatively it can be centrally decided which base stations are to transmit. In this case each base station is instructed of whether to transmit a signal or power down and this is preferably by means of communication through the fixed network. This embodiment thus increases the communication overhead in the fixed network but has the advantage that performance can be optimised as conflicts between transmitting base stations can be minimised. Furthermore information on which base stations transmit can be communicated to subscriber units thereby allowing them to specifically search for the transmitted signal.

It should be noted that the signal transmitted may possibly be of reduced power compared to normal transmissions and that powering down base stations may continue transmitting at reduced power levels. However, the reduced power level must in comparison to power transmitted by the base station transmitting a radio signal during the designated power down time intervals be sufficiently low to improve visibility of the base station transmitting the radio signal. Specifically, the powering down base stations may continue to transmit pilot signals but switch of traffic signals thereby reducing the total transmitted power.

It will be apparent that any suitable signal can be transmitted by the base station not powering down, including for example a pilot signal or a traffic signal. However, in the preferred embodiment a dedicated measurement signal optimised for detecting the given characteristic is used. For example for location determination a signal suitable for determining the time of arrival can advantageously be used. In such a case a dedicated measurement signal having a very high auto-correlation may thus be used. This will result in optimised detection and estimation of the desired characteristic such as the time of arrival. Alternatively by transmitting traffic signals measurements are made possible without impacted the traffic communication from that base station. In a synchronised communication system where all time slots are aligned the overlap which can be obtained between base station can be virtually as large as the time slots. For an unsynchronised system however this is not possible. Nevertheless, if the degree of non-synchronisation between different base stations is known it is possible for a central controller to select overlapping time slots. For example assuming the length of a time slot is four seconds and it is known that base station 1 is 5 seconds ahead of base station 2, the central processor can designate time slot 3 of base station 1 and time slot 4 of base station 2 as power down intervals . This will result in 3 seconds of overlap between the power down time intervals.

However, in order to use this method knowledge of the timing offset between base stations must be obtained and according to one embodiment of this invention this is achieved by the time offset between the plurality of base stations being determined from transmissions from subscriber units to the base stations. Thus a signal transmitted from a subscriber unit is received by at least two base stations and by comparing the time of arrival of the signals with respect to the local timing of each base station the timing offset between them can be determined.

Alternatively the time offset between the plurality of base stations can be determined from measurement units disposed in the communication system. Thus signals from at least two base stations are transmitted relative to the local timing of each base station and a measurement unit detects when the signals are received. For example the measurement unit can detect when timeslot five was received from respectively base station 1 and 2 and determine the timing offset as the difference between the time of arrivals. The calculated timing offset or the bare measurement data can be transmitted back to the base stations and the central processor.

Claims

1. A method of measuring a characteristic of a radio signal in a communication system comprising a plurality of base stations (101) and at least one receiver (103) comprising the steps of: power down intervals being designated (501) for a of plurality base stations; at least one base stations powering down (503) in their designated power down time interval; at least one base station transmitting (505) a radio signal during the designated interval; a receiver measuring (507) a characteristic of the radio signal from that base station; and wherein the designated power down time intervals are chosen to at least partly overlap.

2. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the signal transmitted is a traffic signal.

3. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the signal transmitted is a measurement signal.

4. A method of measuring a characteristic of a radio signal as claimed in claim 1 further comprising the steps of designating which base stations transmit a signal and which base stations power down; and instructing the base stations to transmit a signal or power down accordingly.

5. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein each base station (101) substantially randomly transmits a signal in the designated interval.

6. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein each base station (101) substantially periodically transmits a signal in the designated interval.

7. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the communication system comprise a non-synchronised network and the time offset between the plurality of base stations is determined from transmissions from subscriber units to the base stations.

8. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the communication system comprise a non-synchronised network and the time offset between the plurality of base stations is determined from measurement units disposed in the communication system.

9. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the characteristic of the radio signal is used for determining the location of the receiver

10. A method of measuring a characteristic of a radio signal as claimed in claim 1 wherein the communication system is a Code Division Multiple Access communication system.

11. An apparatus for measuring a characteristic of a radio signal in a communication system with a plurality of base stations (301) (303) (305) and at least one receiver, the apparatus comprising: means (307) for power down intervals being designated for a of plurality base stations; means (323) for at least one base station powering down in their designated power down time interval; at least one base stations (301) operable to transmit a radio signal during the designated interval; a receiver (309) for measuring a characteristic of the radio signal from that base station; and wherein the designated power down time intervals are chosen to at least partly overlap.