GB2377130A

GB2377130A - Handover criteria for mobile transceivers

Info

Publication number: GB2377130A
Application number: GB0116004A
Authority: GB
Inventors: Mark Shahaf; Salomon Serfaty; Nimrod Glazer
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-06-29
Filing date: 2001-06-29
Publication date: 2002-12-31
Also published as: GB0116004D0

Abstract

A mobile radio communications transceiver M1, M2, M3, which is operable in a communications system to communicate with one of a plurality of base transceivers B1, B2, B3 etc. in a cellular network, the mobile transceiver operable to receive signals from candidate base transceivers within its range of possible communication and to extract and compare information from the received signals to select one of the base transceivers with which the mobile transceivers is to communicate with, if necessary by handover from the currently selected base transceiver, wherein the mobile transceiver is operable so that the information extracted and compared includes information describing estimates of the distances from the mobile transceiver to the candidate base transceivers. Furthermore the mobile transceiver may use information about both the distance to and quality of service of (for example signal strength) the base transceivers to compile and maintain a preference table of candidate base transceivers. Furthermore the mobile transceiver may calculate a rating value based on signal strength and distance measurements of each of the base transceivers to determine their ranking in the preference table.

Description

COMMUNICATIONS TRANSCEIVERS AND SYSTEMS Field of the Invention The present invention relates to communication transceivers and systems. In particular, it relates to mobile radio transceivers and systems providing radio communication to and from such transceivers.

Background of the Invention Mobile radio transceivers are also referred to herein as mobile stations (MSs). The terms'mobile transceiver'and'mobile station (MS) 'are intended to include within their meaning mobile and portable telephones and mobile and portable radios. Systems which provide communications to or from MSs by fixed or base transceivers known in the art as'base transceiver stations (BTSs) 'arranged to give coverage in a network of regions known as cells are referred to herein as cellular radio communications systems. The present invention relates specifically to such systems as well as to MSs for use in such systems.

Subscribers to cellular communication systems having MSs generally require a high grade of service and reliable connection while using the system for various services including telephony calls, dispatch calls and data transfers and possibly more advanced communications such as to transmit video images. In this specification the expression'subscriber'includes members of a user group who are collectively permitted to use the system, e. g. because they belong to a common company or organisation for whose benefit the system is operated, as well as individuals who subscribe directly to an operator of the system. Cellular systems comprise a

network of BTSs each covering a certain geographical area known in the art as a cell each to communicate with MSs in the cell covered. There is likely to be an overlap of some particular cells (especially at the edge of coverage of a particular cell) of a given network and the areas where the cells overlap are thereby covered by more than a single BTS. However, in order for correct operation of the system, it is necessary for the system to select a BTS which is the one which will serve each MS. In order to maintain the connection to a MS that is on the move, the designation of the BTS that is to provide service to the MS will periodically change as the MS moves throughout the coverage regions or cells of the different BTSs. The process of exchanging the serving BTS for another is referred to in the art as 'handover'.

The handover process varies according to the specific type of communication procedures being used for implementation of a given system. However, the process of deciding which BTS will be chosen always requires some system resources which are similar in nature. In modern systems, the procedure by which evaluation of the signals and services from BTSs available to serve a given MS are made, and the selection and identification of the target candidate BTS to which the MS should connect, is assigned to the MS. This monitoring and selection activity is referred to in the art as Mobile Assisted Hand-Over (MAHO). Therefore, in order to operate such an activity, some resources must be provided in the MSs to monitor and evaluate from time to time the signals which can be received from BTSs other than that currently selected as the BTS to serve the MS.

In the prior art, the decision on whether or not handover by a MS should be made to a particular BTS is

based on received signal quality from that and other BTSs. The inventors of the present invention have recognised that various potential problems may occur which degrade the performance of the cellular network using such a procedure. These potential problems are associated with the MS selecting a BTS which is located a long distance from the MS as the one which is to serve the MS. This anomalous situation can occur as follows.

Firstly, the topography of the region in which a MS is located may be such that in some spots the MS will receive a good signal from a BTS which is at a relatively long distance from the MS. This can occur, for example, in a hilly terrain, when at the top of a hill, the MS can be in the direct line-of-sight path of a signal sent by a distant BTS. Secondly, a MS may be airborne causing the MS to be in a direct line-of-sight path of a signal from a distant BTS. The received signal from a distant BTS might have superior quality to that from a nearer BTS and the MS will then decide to switch to the distant BTS even though it can operate at adequate quality using the service of a nearer BTS. Such a decision to switch may lead to several problems in the overall performance of the cellular system as follows.

A first example of a problem that can be encountered is in a system deploying Time Division Multiple Access (TDMA) for its air interface radio communications. TDMA is a procedure in which a signal is divided into time periods (frames and slots) and certain time periods are assigned for access to specific MSs. The MS transmission timing is driven by the receive timing of the downlink signal from the serving BTS. Where the BTS is located a long distance from the MS there is a propagation delay which may cause the MS transmit timeslot to be

significantly delayed causing the transmission from the MS to interfere with the transmission carried out by the BTS in the next BTS transmission timeslot.

Another example of a potential problem where the MS switches to be served by a distant BTS relates to systems in which the minimisation of co-channel interference is being carefully planned by deployment of various transmission power control schemes. This is especially critical in (but not limited to) systems deploying Code Division Multiple Access (CDMA) procedures which are widely used in many networks. In systems of this kind, switching the MS to be served by a distant BTS may require the MS and the BTS both to increase their transmission power and this will introduce a greater likelihood of co-channel interference that will significantly degrade both system performance and system capacity.

Another example of a potential problem is where a MS switches to be served by a distant BTS and there are one or more BTSs in other cells between the distant BTS and the MS. This situation is likely to cause a multiplicity of handovers to take place. From a system management point of view, the number of handovers that have to take place is preferably kept to a minimum. Keeping the number to a minimum will reduce both the location update-signalling load on the system and the number of interruptions of service to subscribers.

There are several known techniques that can provide very limited solutions to the problems described above. These are as follows: (i) Timing advance control. This is designed to deal with transmission delays in TDMA systems. The

system measures the timing of each MS and may issue signals instructing certain MSs to advance or delay their transmission timing. The difficulty is that not all TDMA standards support this mechanism and, even when it is supported, it introduces significant complexity and reliability issues since some of the instruction signals might be lost owing to transmission errors introduced by the radio communications channel. Moreover, this technique cannot be applied where the transmission time from the MS to a distant BTS exceeds the synchronisation capabilities of the BTS.

(ii) Guard time. The TDMA structure of transport channels (bursts) introduces some reserved times at the timeslot boundaries to allow certain delay without resulting overlap between transmission made in adjacent timeslots. With careful planning this might resolve some of the typical operational problems but it will fail to handle the problems associated with anomalies caused by regional topography or when the MS is airborne, as described earlier; (iii) Enforced handover. The system is designed to recognise that either the MS transmission delay exceeds a prescribed limit or the MS transmission power exceeds a prescribed value. Where one of these situations is detected, the system will send a signal to the MS instructing it to make a handover with the expectation that it will find a more suitable alternative BTS. This mechanism might work in some situations. However it will result in an increased number of handovers which as described earlier will create an unnecessary signalling load on the air

interface and will increase the number of service interruptions.

It is therefore an object of the present invention to provide a system and mobile transceiver for use in cellular radio communications in which handover is carried out in an improved manner in which the problems described above are reduced or eliminated.

Summary of the invention According to the present invention in a first aspect there is provided a mobile radio communications transceiver which is operable to communicate with one of a plurality of base transceivers arranged in a cellular network, the mobile transceiver being operable to receive signals from base transceivers within its range of possible communication and to extract and compare information from the received signals to select one of the base transceivers as that with which the mobile transceiver is to communicate with, if necessary by handover from another of the base transceivers, wherein the mobile transceiver is operable so that the information which it extracts and compares includes information relating to estimates of the distances from the mobile transceiver to certain of the base transceivers.

The information extracted and compared by the mobile transceiver,'MS', may include also information about the quality of signal or quality of service which can be received from the base transceivers,'BTSs'as in the prior art. Such information may relate to one or more of received signal strength, bit error rate, message error rate, frame error rate, carrier to interference ratio, signal to noise ratio and BTS service capability

(e. g. BTS load, BTS priority, call type support, security support and others). Information about BTS service capability may be incorporated in a known manner by the BTS in the signals that it transmits.

Thus, the MS may be operable to compile a table of BTSs in order of preference rating with the most preferred at the top of the table using information both about the distance from the MS to the BTS and about the quality of signal received at the MS or the quality of service which the BTS can provide to the MS. There are various ways in which such a table may be compiled using the distance and signal or service quality information.

For example, the table may list in order of distance from the MS those BTSs which can provide a quality of received signal and/or service at the MS of better than a pre-determined minimum quality.

Another example is to define the rating y of BTSs in the compiled table as y = aR + (1/D), where R is a measurement of received signal quality and/or service quality, e. g. received signal strength, from a given BTS, D is the estimated distance from the MS to that BTS and a and are parameters such as constants which are pre-determined and fixed for the complete system of BTSs and MSs.

The information extracted and employed to compile the said table is desirably refreshed from time-to-time and the said table is thereby updated.

Thus, a BTS which is not included in the table, e. g. because the quality of its signal as received by the MS is not of sufficient quality, may later be included in the table when its received signal quality has improved sufficiently.

The MS according to the first aspect of the invention may be operable in a known manner: (i) to select the BTS which is at the top of the rating table according to whichever selection criteria are employed by the MS to rank or list the BTSs in the table in order of preference; and (ii) to arrange that communications to and from the MS are with that selected BTS, ie. that BTS becomes the serving BTS, until at some later time one of the other BTSs reaches the top of the preference table. In practice the MS will tune its transmitter and receiver to enable communications to and from the selected BTS to take place.

The compilation of the said table by the MS may be carried out by a signal processor contained within the transceiver. The processor may be programmed to receive numbers representing the measurements of the various parameters, i. e. relating to estimated distance and signal or service quality from the various BTSs as referred to earlier, and to operate an algorithm which carries out the required rating and ranking procedure to produce the said preference table. Conveniently, the MS may be such that its signal processor operates in a digital form.

According to the present invention in a second aspect there is provided a radio communications system including a plurality of mobile radio communications transceivers and a plurality of base transceivers arranged in a cellular network to communicate with the mobile transceivers, each of the mobile transceivers being operable to receive signals from base transceivers within its range of possible communication and to extract and compare information from the received signals to select one of the base

transceivers as the one with which the mobile transceiver is to communicate with, if necessary by handover from another of the base transceivers, wherein each of the mobile transceivers is operable so that the information which it extracts and compares includes information describing an estimate of the distance from the mobile transceiver to certain of the base transceivers.

The information about estimates of distances from a given mobile transceiver,'MS'to base transceivers 'BTSs'within its range of communication may be obtained using a time of arrival method. In such a method, each of the BTSs may transmit a reference signal at a point in time which is known by the receiving MS and the receiving MS may thereby compare the time of arrival of the reference signals from certain BTSs against that from the presently serving BTS as a reference. The results may be compiled as time differences (either positive or negative) between the time of arrival of the reference signal from the currently serving BTS to the MS and the time of arrival of the reference signals from certain other BTSs to the MS.

It is pointed out that the distance from the MS to a given BTS does not have to be measured precisely.

The values assigned by the MS to the relative distances to the BTSs from the MS can be estimates sufficiently accurate to enable the BTS rating and ranking procedure described earlier to be carried out.

The MS according to the first aspect of the invention and each of the MSs in the system according to the second aspect of the invention may be operable so that for the purpose of receiving information

about the quality of signals or services which can be received from various BTSs, the MS is operable periodically to set its receiver so as to receive signals from each of a set of candidate BTSs in order to extract the required information.

The MS according to the first aspect of the invention and the system according to the second aspect of the invention, including the MSs and BTSs of such a system, may operate in accordance with the TETRA (Terrestrial Trunked Radio) standard industry procedures defined by ETSI (European Telecommunications Standards Institute).

The present invention unexpectedly provides several benefits in a cellular communications system and in the MSs employed therein. The problems arising in prior art systems associated with handovers to distant BTSs as described earlier are reduced or eliminated. Thus, the number of handovers required is statistically reduced. This reduces the number of interruptions to the service provided to the subscribers and reduces the signalling load on the system. The overall interference in the cellular system is reduced which in turn allows the average transmission power to be reduced. For MSs, this allows the battery life of the transceiver to be extended. Capacity problems in CDMA systems caused by power control imbalance when a distant BTS is selected are reduced and collisions and interference between incoming and outgoing signals at a BTS owing to long propagation delays are reduced or avoided.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Brief Description of the Drawings

Figure 1 is a schematic representation of an arrangement of cells in a cellular communications system.

Figure 2 is a schematic representation of the positions of transceivers in the system shown in Figure 1.

Description of an embodiment of the Invention As seen in Figure 1, an arrangement of cells in a cellular system comprise cells 1 to 10. The cells 1 to 10 are mutually overlapping coverage regions which have different sizes to cater for the expected different number of MSs to be serviced in each cell.

Each of the cells 1 to 10 has at its centre a BTS and Figure 2 shows the corresponding mutual arrangement of these BTSs to give the cells 1 to 10.

The BTSs corresponding to the cells 1 to 10 respectively are indicated in Figure 2 as BTSs 1 to 10. Figure 2 also shows three MSs Ml, M2 and M3. In practice there are likely to be a large number of MSs within each of the cells 1 to 10.

Each of the MSs operates so as to receive signals from neighbouring BTSs in the set of BTSs 1 to 10 and extracts the difference in time of arrival of reference signals from neighbouring BTSs as compared with that from the currently serving BTS. In Figure 2, the BTS B9 is the currently serving BTS for the MS M1. The MS M1 can receive signals from the BTSs Bl, B10 and B5 as well as from B9. The MS M1 will calculate the relative time differences of reference signals from these BTSs to the MS M1 to estimate the relative distances from these BTSs to Ml. The MS M1 will also estimate the received signal strength at M1 from these BTSs.

The BTS B5 is located a long distance from the MS Ml but can give the strongest received signal at M1 because of local geographical topography. In the prior art the BTS B5 would thereby be chosen as the BTS to serve the MS M1 and this would then lead to one or more of the various problems associated with handover to a distant BTS as described earlier.

However, because relative estimated distance to BTSs is also taken into account by MS1 in accordance with an embodiment of the invention, the BTS B5 is not chosen because of the long distance from M1 to B5 and instead the selection of BTS to serve M1 is between the nearer BTSs Bl, B10 and B9. This selection is carried out using a preference ranking based on a combination of signal strength and relative distance in one of the ways described earlier.

The compilation of the said table by the MS M1 may be carried out by a digital signal processor contained within the MS Ml programmed to receive numbers representing the distance and signal quality measurements from the various candidate BTSs as referred to earlier and to operate an algorithm which carries out the required ranking procedure to produce the said preference table.

Claims

CLAIMS 1. A mobile radio communications transceiver which is operable to communicate with one of a plurality of base transceivers arranged in a cellular network, the mobile transceiver being operable to receive signals from base transceivers within its range of possible communication and to extract and compare information from the received signals to select one of the base transceivers as the one with which the mobile transceiver is to communicate with, if necessary by handover from another of the base transceivers, wherein the mobile transceiver is operable so that the information which it extracts and compares includes information describing estimates of the distances from the mobile transceiver to certain of the base transceivers.
2. A mobile transceiver according to claim 1 and which is operable so that the information extracted and compared by the mobile transceiver includes also information about the quality of signal received from the base transceivers or about the quality of service which can be received from the base transceivers.
3. A mobile transceiver according to claim 2 and wherein the mobile transceiver is operable so that the said information includes information about one or more of received signal strength, bit error rate, message error rate, frame error rate, carrier to interference ratio, signal to noise ratio and BTS service capability.
4. A mobile transceiver according to claim 1, claim 2 or claim 3 and which is operable to compile a table ranking base transceivers with which the mobile transceiver may communicate in order of preference

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using information both about the distance to the base transceiver and about the quality of signal received at the mobile station or about the quality of service that the base transceiver can provide.
5. A transceiver according to claim 4 and wherein the mobile transceiver is operable so that the said table lists in order of distance from the mobile station those base transceivers providing a received signal quality at the mobile station of better than a predetermined minimum quality.
6. A mobile transceiver according to claim 4 and wherein the mobile transceiver is operable to produce a rating value y of base transceivers in the compiled table as given by y = aR + ss (l/D), where R is received signal strength from a given base transceiver, D is the distance from mobile transceiver to that base transceiver and a and ss are parameters which are pre-determined and fixed for the complete system of base transceivers and mobile transceivers.
7. A mobile transceiver according to any one of claims 4 to 6 and wherein the mobile transceiver is operable such that the information it employs to compile the said table is refreshed from time-to-time and the said table is thereby updated.
8. A mobile transceiver according to any one of the preceding claims and which is operable to arrange that communications are made to and from that mobile transceiver to and from the base transceiver which is selected to be the preferred base transceiver until at a later time one of the other base transceivers becomes the preferred base transceiver.
9. A mobile transceiver according to any one of the preceding claims, which is operable so that

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information about estimates of the distance from the mobile transceiver to base transceivers within its range of communication is obtained using a time of arrival method in which the time of arrival from the base transceivers to the mobile transceiver is compared with that from the currently selected and serving base transceiver to the mobile transceiver.
10. A mobile transceiver according to any one of the preceding claims and which is operable periodically to set its receiver so as to receive signals from each of a set of candidate base transceivers in order to extract the information from such signals required to carry out the base transceiver selection.
11. A mobile transceiver according to any one of the preceding claims and which includes a digital signal processor which is operable to carry out the base transceiver selection using digital signals representing the said information.
12. A mobile transceiver according to any one of the preceding claims and which operates in accordance with the TETRA (Terrestrial Trunked Radio) standard industry procedures defined by ETSI (European Telecommunications Standards Institute).
13. A radio communications system including a plurality of mobile radio communications transceivers and a plurality of base transceivers arranged in a cellular network to communicate with the mobile transceivers, each of the mobile transceivers being operable to receive signals from base transceivers within its range of possible communication and to extract and compare information from the received signals to select one of the base transceivers as the one with which the mobile transceiver is to communicate with, if necessary by handover from

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another of the base transceivers, wherein each of the mobile transceivers is operable so that the information which it extracts and compares includes information describing the location of the base transceivers.