GB2415323A

GB2415323A - Reducing unnecessary handovers in a telecommunications system

Info

Publication number: GB2415323A
Application number: GB0413330A
Authority: GB
Inventors: Tubagus Rizal; Yat-Lung Kwong
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2004-06-15
Filing date: 2004-06-15
Publication date: 2005-12-21
Anticipated expiration: 2024-06-15
Also published as: GB0413330D0; CN101385363B; GB2415323B; EP1759540A2; EP1759540A4; CN101385363A; WO2005125232A3; WO2005125232A2

Abstract

Within a telecommunication cell there are two frequency bands available for use, 900 MHz and 1800 MHz, of which a mobile user is assigned to. If it appears that the quality of the other frequency is better, then the system can hand over the mobile user to the other frequency, known as intra cell hand over. However, if the quality is not actually better, this can result in a user being passed back and fore between the frequencies repeatedly when it would be more beneficial to remain on a single frequency (known as ping pong handover). This reduces power efficiency and increases system resources. This is overcome by counting the number of handovers within a certain time. If this number exceeds a predetermined threshold, then no handovers are allowed to take place for a second predetermined period of time.

Description

24 1 5323 - 1

CELLULAR COMMUNICATIONS SYSTEM

Field of the Invention

The present invention relates to cellular radio communications systems.

The present invention relates in particular to handover between frequency bands in cellular radio communications systems. The pr esent invention relates in particular, but not exclusively, to handover between the 900 MHz frequency band and the 1800 MHz frequency band in Global System for Mobile Telecommunication (GSM) systems.

Background of the Invention

In a cellular radio comm unications system, the area over whicl1 service is provided is divided into a number of smaller areas called cells. Typically each cell is served from a base transceiver station (BTS) which has an antenna or antennas for transmission to and reception from a plurality of user stations, often mobile stations, such as mobile telephones. An established harmonised cellular radio communications system is the Global System for Mobile Telecommunication (GSM).

In GSM, there are two frequency bands available for use. One of the frequency bands ranges approximately from 880 MHz to 960 MHz, and is known as the 90() MHz band. The other frequency band ranges approximately from 1710 MHz to 1880 MHz, and is known as the 1800 MHz band. The 1800 MHz band was previously used b y a system known as Digital Communication Cellular System (DCS). The GSM standard allows these two frequency bands to work 2 simultaneously within one cell, which can be arranged in such way that 900 MHz band will tend to cover the far end of the cell cover age footprint and 1800 MHz band will tend to cover the near end of the cell coverage footprint. Far end area of cell coverage may sometimes be referred to as "outer" band and near end area of cell coverage may sometimes be referred to as "inner" band.

In GSM systems, the 9()0 MHz frequency band tends to be more robust.

Therefore, a call is preferably allocated to this frequency band. However, in order to accommodate all calls, it is desirable to handover calls to the 1800 MHz frequency band where possible. I n conventional GSM systems such a frequency l O band handover is made when the radio frequency (RF) receive level is sufficiently high that the system, as conventionally operated, may in effect assume that the receive level will also be sufficiently high on th e less robust 1800 MHz band. When handover to the 1800 MHz frequency band has been implemented, if the actual receive level on the 1800 MHz band is in reality, or l 5 becomes, inadequate, then the call is handed back over to the 900 MHz band. In conventional systems, the procedure can then be repeated, i.e. handed over then back and so on, and this is known as ping -pony handover. This is undesirable, as the call quality may be reduced and also the process is inefficient in terms of use of system resources.

Summary of the Invention

The present invention tends to avoid, or reduce the occurrence of, such ping-pony handover. - 3

In a first aspect, the present invention provides a method of operating a cellular communications system, as claimed in claim 1.

In a further aspect, the present invention provides a storage medium storing processor-implemeutable instructions, as claimed in claim 5.

In a further aspect, the present invention provides apparatus for operating a cellular communications system, as claimed in claim 6.

In a further aspect, the present invention provides a base station for a cellular communications system, as claimed in claim 10.

Further aspects are as claimed in the dependent claims.

l O The present invention tends to alleviate or resolve the abo ve mentioned problems arising from the conventional handover arrangement.

Brief Description of the Drawings

l 5 Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is schematic illustration of part of a GSM cellular communications system; FIG. 2 is a schematic illustration of frequency bands which are employed for a radio link of the GSM cellular communications system of FIG. 1; and FIG. 3 is a flowchart illustratin g process steps carried out in an embodiment of the present invention. - 4

Description of Preferred Embodiments

FIG. 1 is a schematic illustration of part of a GSM cellular communications system 1. The cellular communications system 1 comprises a large numb er of user stations. For clarity, only one user station, which in this example is a mobile station (MS) 2, in the form of a mobile telephone, is shown in FIG. 1.

The cellular communications system 1 further comprises a plurality of base transceiver station s (BTSs) which provide radio communication with the user stations. The area served by a respective BTS is called a cell. For clarity, only l O one BTS, namely BTS 4, which is the BTS communicating with MS 2 in this example, is shown in FIG. l. In this embodime nt, the BTS 4 comprises a frequency band handover control module 6, whose operation will be described in detail later below.

The BTS 4 and the MS 2 communicate via a radio link 8 established l 5 between them. The radio link 8 is conveniently considered as com prising an uplink, i.e. a channel at a specific radio frequency in the direction from the MS 2 to the BTS 4, and a downlink, i.e. a channel at a specific radio frequency in the direction from the BTS 4 to the MS 2. Further details of the radio link 8 will be described below with reference to FIG. 2.

The cellular communications system further comprises a plurality of base station controllers (BSCs). Each BSC is coupled to, and controls, one or more BTSs. For clarity, only one BSC, namely BSC 10, which is the BSC coupled to and controlling the BTS 4, is shown in FIG. 1. The BTS 4 and the BSC 10 together form a base station system (BSS). In this example the BTS 4 and the BSC 10 are located away from each other, but they may alternatively be co -located.

The cellular communications system further comprises a plurality of mobile services switching centres (MSCs). Each MSC is coupled to one or more BSCs. For clarity, only one MSC, namely MSC 12, which is the MSC coupled to the BSC 10, is shown in FIG. 1. The MSC 1 2 is further coupled to a public switched telephone network (PSTN) 14. The PSTN 14 may be connected to other communication networks.

The cellular communications system 1 further comprises an operations and maintenance centre (OMC) 16 coupled to each MSC, including therefore being coupled to the MSC 12 (other systems may comprise more than one OMC) .

l 0 In operation, the MSC 12 provides interconnection and routing of calls, both within the cellular communication system 1 (in co -operation with other MSCs, not shown) and to external elements via the PSTN 14, such as a landline telephone connected to the PSTN 14. The BTS 4, under the control of the BSC 10, transmits downlink radio signals to, and receives uplink radio signals from, the l 5 MS2. The OMC] 6 is used by t he system operator to configure and maintain the cellular radio communications system l. The different system elements described above communicate with each other using interfaces specified in the GSM

specifications.

FIG. 2 is a schematic illustration of t he frequency bands which are 2() employed for the radio link 8. A first frequency band is the 900 MHz band, indicated in FIG. 2 by reference numeral 9, and ranging approximately from 880 MHz to 960 MHz. A second frequency band is the 1800 MT Jz band, indicated in FIG. 2 by reference numeral 18, and ranging approximately from 1710 MHz to 1880 Ml Iz. - 6

In the case of the 900 MHz band, discrete frequencies within the range of approximately 88() MHz to 915 MHz (indicated in FIG. 2 by reference numeral 9u) are used for the uplink part of the radio link 8 i.e. transmission from the MS 2 to the BT5 4, and discrete frequencies within the range of approximately 925 MHz to 960 MHz (indicated in FIG. 2 by reference numeral 9d) are used for the downlink part of the radio link 8 i.e. transmission from the BTS 4 to the MS 2. In the case of a given call, the respective downlink and uplink discrete frequencies used are separated by 45 MHz, and as a pair correspond to an Absolute Radio Frequency Channel Number (ARFCN).

l 0 In the case of the 1800 MHz band, discrete frequencies within the range of approximately 1710 MHz to 1785 MHz (indicated in FIG. 2 by reference numeral 18u) are used for the uplink part of the radio link 8 i.e. transmission from the MS 2 to the BTS 4, and discrete frequ encies within the range of approximately 1805 MHz to 1880 MHz (indicated in FIG. 2 by reference numeral fed) are used for the downlink part of the radio link 8 i.e. transmission from the BTS 4 to the MS 2. In the case of a given call, the respective downli nk and uplink discrete frequencies used are separated by 45 MHz, and as a pair correspond to an Absolute Radio Frequency Channel Number (ARFCN).

In operation, communication content between the BTS 4 and the MS 2 includes the content of a call, e.g. voice o r data, and control signalling. This content is arranged on various different logical channels by use of different timeslots available from the time division multiplexed access (TDMA) operation of the GSM cellular communications system 1. One set of these logical channels is known as the Broadcast Control Channel (BCCH). In the GSM cellular communications system 1, although the two frequency bands described above - 7 are used, the BCCH is only provided on the 900 MHz band. The terminology "idle mode" is used to name the situation when the MS 2 is switched on and in signalling communication with the BTS 4, but not engaged in a call. When the MS is in idle mode, the MS 2 and the BTS 4 perform signalling communication using the 900 MHz band. The signalling commun ication includes the BTS 4 broadcasting, on the BCCH, signalling information required fro the MS 2 to set up a call.

When a call is set up, at the beginning of the call the 900 MHz band is used. During the call, the MS 2 measures the strength of the signa l received from I O the BTS 4, and, every 48() ms, sends a received signal strength report to the BTS 4.

Based on the received signal strength (achieved in the 900 MHz band), the BTS 4 calculates a predicted level of what the received signal strength would be if the 1800 MI--lz band were used instead of the 900 MHz band. If the calculation results in a predicted level higher than a predetermined threshold, i.e. if the l 5 predicted signal strength in the 1800 MHz is deemed to appear sufficiently high, then the call is handed over to the 1800 MHz frequency band.

The above description of the operation of the GSM cellular communications system I corresponds to conventional operation, and further details are as specified in the GSM specification, except for any difference s described below.

However, in this embodiment, the GSM cellular communication system l has been modified to alleviate a problem that can arise with the next stages of conventional operation. For the sake of comparison, the situation in a conventional sys tern will now be described briefly. In conventional systems and operation, after the call has been handed over to the 1800 MHz band, the MS 2 - 8 continues to measure the strength of the signal received from the BTS 4, and continues, every 480 ms, to send a rec eived signal strength report to the BTS 4. If the received signal strength is below a predetermined threshold, i.e. if the received signal strength is deemed insufficient, then the call is handed back to the 900 MHz band. Then, as the call continues back o n the 900 MHz band, the MS 2 continues to send signal strength reports to the BTS 4, and the BTS 4 again calculates predicted signal strength levels on the 1800 MHz band, and if sufficiently high, passes the call to the 1800 MHz band again. This process ma y be repeated continuously, resulting in a ping -pony effect of repeated hand over l 0 between the two frequency bands.

The ping-pony effect described in the preceding paragraph is alleviated in this embodiment by virtue of modifying the BTS 4, as will now be described in more detail.

In this embodiment, the BTS 4 has been adapted, by provision of a l 5 frequency band handover control module 6, to offer, and provide for, adaptation of handover control between the 900 MHz and 1800 MHz frequency bands, as will be described in more detail below.

However, this adaptation may be implemented in any suitable manner to provide suitable apparatus or operation. The module may consist of a single discrete entity added to a conventional BTS, or may alternatively be formed by adapting existing parts of a conventional BTS, for example by reprogramming of a one or more processors therein. As sucil the required adaptation may be implemented in the form of processor - implementable instructions stored on a storage medium, such as a flo ppy disk, hard disk, PROM, RAM or any combination of these or other storage media. Furthermore, whether a separate entity or an adaptation of existing parts or a combination of these, the module may be implemented in the form of hardware, firmware, soflwa re, or any combination of these.

It is also within the contemplation of the invention that such adaptation of handover control may alternatively be controlled, implemented in full or implemented in part by a module added to or formed by adaptation of any other suitable part of the communication system 1. For example, this may be implemented instead at the BSC 10 or in a combined base station sub -system comprising functionality of both a BSC and a BTS. Further, in the case of other l O network infrastructures, implementation may be at any appropriate node such as any otl1er appropriate type of base station, base station controller etc. Alternatively the various steps involved in determining and carrying out such adaptation (as will be described in more detail bel ow) can be carried out by various components distributed at different locations or entities within any l 5 suitable network or system.

FIG. 3 is a flowchart illustrating the process steps carried out in this embodimcn t.

At step s2, the BTS 4 and the MS 2 set u p a call on the 900 MHz band, L'Sing conventional GSM proccdurcs. The call then continues in this stage according to conventional procedures, i.e. handover between the 900 MHz band and the 1800 MHz band, and back, may occur depending on measured signal strength levels and predicted signal strength levels according to the processes described above. This is represented in the flowchart of FIG. 3 by a step sit, in which the frequency band handover control module 6 allows handovers. - 10

At step s6, the frequency ban d handover control module 6 counts the number of handovers, in either direction, between the 900 MHz band and the 1800 MHz band, in a given period of time, which may conveniently be called a first period of time. In this embodiment, this first period of ti me is 4 seconds.

At step s8, the frequency band handover control module 6 determines whether the number of handovers counted at step s4 is greater than a predetermined count threshold, which in this embodiment is 3 handovers. If the number of handovers co unfed at step s4 is equal to or less than the count threshold, i.e. number of ha ndovers < 3, then the process returns to step s4 and is l O repeated.

However, if the number of handovers counted at step s4 is greater than the count threshold, i.e. number of handovers > 3, then the process instead moves to step slO. At step s10, the frequ ency band handover control module 6 blocks any further handovers, in either direction, between the 900 MHz band and the l 5 1800 MHz band, for a given period of time, which may conveniently be called a second period of time. In this embodiment, this second per ted of time is 1 minu te.

After the second period of time has elapsed, the process returns to step s4 and is repeated, i.e. the frequency band handover control module 6 again allows handovers between the 900 MHz band and the 1800 MHz band to take place.

In this embodiment the process is repeated until the call is ended.

I bus, in summary, over the course of a call, the frequency band handover control module 6 repeatedly monitors the number of handovers that take place in a first time period, and prohibits ha ndovers for a second time period if the number of handovers in the first time period exceeds a predetermined threshold. - 11

It is noted that steps s4 and s6 effectively take place in parallel, but have been represented as separate steps in the above account a nd FIG. 3 for the sake of representing the overall process in a simple flowchart form.

In the above described embodiment, the following values are used: first period of time (i.e. time number of handovers is counted) = 4 seconds; threshold level for alla wed number of handovers = 3; and second period of time (i.e. time further handovers are blocked) = 1 minute. However, in other embodiments different values may be used, and will be selected according to the skilled person according to the requirements of t he particular system and circumstances under l O consideration. A further possibility is that any one or any combination of these values may be varied in an adaptive manner during operation of the system.

In the above described embodiments, when the number o f handovers in the first time period is greater than the predetermined threshold, handovers are blocked for a second period of time which is predefined (e.g. 1 minute in the first l 5 embodiment). In other embodiments, however, handovers may be blocked for the remainder of the call.

In the above described embodiments, when handovers are blocked, the call is left on whichever frequency band the call is at the time of determining to block further handovers. However, in other embodiments, when it is determined to block further handovers, the call is first returned to the more robust band, e.g. the 900 MHz band, before further handovers are then blocked.

In the above embodiments, when the number of handovers is counted over the course of the first period of time, the total number of handovers in both directions, i.e. both from the 900 MHz band to the 1800 Ml-lz band and from the 180() MHz band to the 900 MHz band. However, in other embodiments, just the - 12 handovers in one direction may be counted (and the threshold love l for allowed number of handovers set accordingly).

In the above embodiments, the process shown in FIG. 3 starts immediately the call is set up (or as soon as is practicable after the call is set up) and is continued throughout the duration of the call. H owever, in other embodiments, this need not be the case, and the handover counting/preventing process may instead be implemented for only one or more portions of a call. For example, the process may only start after a call has been conducted for a given a mount of time, and/or may be stopped, or paused for a given length of time, after a given l O duration of call. In the latter case, the process may be stopped or paused after a given duration of call dependent upon whether any, or a given number, of handovcr blocks have been implemcuted in the process to that point.

In the above embodiments the invention is applied to a GSM cellular communication system in which the two frequency bands are the 900 MHz band l 5 and the 1800 MHz band. However, the invention may also be used in other cellular communications systems with plural frequency bands between which calls may be handed over and back in a repeated fashion. - 1 3

Claims

1. A method of operating a cellular communications system (1) in which a call is handed over be twcen a first frequency band (9) and a second frequency band (18) according to a predicted signal strength on the second frequency band (18) calculated from a signal strength measured on the first frequency band (9), and in which the call is handed back fr om the second frequency band (18) to the first frequency band (9) according to a signal strength measured on the second frequency band (18); the method comprising: counting the handovers between the frequency bands or to one of the frequency bands that ta ke place in a first period of time; comparing the counted number of handovers with a predetermined threshold level of allowed handovers; and if the counted number of handovers exceeds the predetermined threshold level of allowed number of handovers, blo eking further handovers for a second period of time.

2. A method according to claim 1, wherein the first period of time is a predefined length of time.

3. A method according to claim I, wherein the second period of time is a predefined length of time.

4. A method according to any of claims 1 to 3, wherein the cellular communications system ( I) is a GSM system, the first frequency band (9) is the - 1 4 900 MHz GSM band, and the second frequency band (18) is the 1800 MHz GSM band.

5. A storage medium storing pr ocessor-implementable instructions for controlling one or more processors to carry out the method of any of claims 1 to 4.

6. Apparatus for operating a cellular communications system (1) in which a call is handed over between a first frequency band (9) an d a second frequency band (18) according to a predicted signal strength on the second frequency band (18) calculated from a signal strength measured on the first frequency band (9), and in which the call is handed back from the second frequency band (18) t o the first frequency band (9) according to a signal strength measured on the second frequency band ( 18); the apparatus comprising: means for counting handovers between the frequency bands or to one frequency band that take place in a first period of time; means for comparing the counted number of handovers with a predetermined threshold level of allowed number of handovers; and means for blocking, if the counted number of handovers exceeds the predetermined threshold level of allowed number of handover s, further handovers for a second period of time.

7. Apparatus according to claim 6, wherein the first period of time is a predefined length of time. - ]5

8. Apparatus according to claim 6, wherein the second period of time is a predefined length of time.

9. Apparatus according to any of claims 6 to 8, wherein the cellular communications system (I) is a GSM system, the first frequency band (9) is the 900 MHz GSM band, and the second frequency band (18) is the 1800 MHz GSM band.

10. A base station (4) for a cellular communications system comprising apparatus (6) according to any of claims 6 to 9.

11. A method of operating a cellular communications system substantially as hereinbefore described with reference to the accompanying drawings.

l 5

12. Apparatus for o Berating a cellular communications system substantially as hereinbefore described with reference to the accompanying drawings.