EP1759540A2

EP1759540A2 - Cellular communications system

Info

Publication number: EP1759540A2
Application number: EP05791278A
Authority: EP
Inventors: Tubagus Motorola RIZAL; Yat-Lung Motorola KWONG
Original assignee: Motorola Inc
Current assignee: Motorola Mobility LLC
Priority date: 2004-06-15
Filing date: 2005-06-06
Publication date: 2007-03-07
Also published as: EP1759540A4; GB0413330D0; CN101385363A; GB2415323B; WO2005125232A3; WO2005125232A2; GB2415323A; CN101385363B

Abstract

A method, and apparatus, for operating a cellular communications system (1) in which a call is handed over between 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 from the second frequency band (18) to the first frequency band (9) according to a signal strength measured on the second frequency band (18); comprising: counting the number of handovers between the frequency bands that take place in a first period of time; comparing the counted number of handovers with a predetermined threshold level of allowed number of handovers ; and blocking, if the counted number of handovers exceeds the predetermined threshold level of allowed number of handovers, further handovers for a second period of time.

Description

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 present 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 communications system, the area over which 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 900 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 by a system known as Digital Communication Cellular System (DCS). The GSM standard allows these two frequency bands to work 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 coverage 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 900 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. In conventional GSM systems such a frequency 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 the 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 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-pong 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-pong handover. 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-implementable 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. The present invention tends to alleviate or resolve the above mentioned problems arising from the conventional handover arrangement.

Brief Description of the Drawings

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 illustrating process steps carried out in an embodiment of the present invention.

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 number 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 stations (BTSs) which provide radio communication with the user stations. The area served by a respective BTS is called a cell. For clarity, only one BTS, namely BTS 4, which is the BTS communicating with MS 2 in this example, is shown in FIG. 1. In this embodiment, 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 between them. The radio link 8 is conveniently considered as comprising 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 12 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). 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 MS2. The OMC 16 is used by the system operator to configure and maintain the cellular radio communications system 1. The different system elements described above communicate with each other using interfaces specified in the GSM specifications. FIG. 2 is a schematic illustration of the frequency bands which are 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 MHz band, indicated in FIG. 2 by reference numeral 18, and ranging approximately from 1710 MHz to 1880 MHz. In the case of the 900 MHz band, discrete frequencies within the range of approximately 880 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 BTS 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). 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 frequencies within the range of approximately 1805 MHz to 1880 MHz (indicated in FIG. 2 by reference numeral 18d) 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). In operation, communication content between the BTS 4 and the MS 2 includes the content of a call, e.g. voice or 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 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 2 is in idle mode, the MS 2 and the BTS 4 perform signalling communication using the 900 MHz band. The signalling communication 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 signal received from the BTS 4, and, every 480 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 MHz 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 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 1 corresponds to conventional operation, and further details are as specified in the GSM specification, except for any differences described below. However, in this embodiment, the GSM cellular communication system 1 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 system 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 continues to measure the strength of the signal received from the BTS 4, and continues, every 480 ms, to send a received 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 on 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 may be repeated continuously, resulting in a ping-pong effect of repeated hand over between the two frequency bands. The ping-pong 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 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 such the required adaptation may be implemented in the form of processor-implementable instructions stored on a storage medium, such as a floppy 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, software, 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 subsystem comprising functionality of both a BSC and a BTS. Further, in the case of other network infrastructures, implementation may be at any appropriate node such as any other 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 below) can be carried out by various components distributed at different locations or entities within any suitable network or system. FIG. 3 is a flowchart illustrating the process steps carried out in this embodiment. At step s2, the BTS 4 and the MS 2 set up a call on the 900 MHz band, using conventional GSM procedures. 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 s4, in which the frequency band handover control module 6 allows handovers. At step s6, the frequency band 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 time 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 counted at step s4 is equal to or less than the count threshold, i.e. number of handovers < 3, then the process returns to step s4 and is 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 slO, the frequency band handover control module 6 blocks any further handovers, in either direction, between the 900 MHz band and the 1800 MHz band, for a given period of time, which may conveniently be called a second period of time. In this embodiment, this second period of time is 1 minute. 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. Thus, 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 handovers for a second time period if the number of handovers in the first time period exceeds a predetermined threshold. It is noted that steps s4 and s6 effectively take place in parallel, but have been represented as separate steps in the above account and 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 allowed 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 the particular system and circumstances under 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 of 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 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 MHz band and from the 1800 MHz band to the 900 MHz band. However, in other embodiments, just the handovers in one direction may be counted (and the threshold level 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. However, 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 amount of time, and/or may be stopped, or paused for a given length of time, after a given 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 handover blocks have been implemented 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 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.

Claims

1. A method of operating a cellular communications system (1) in which a call is handed over between 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 from 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 take 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, blocking 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 1, wherein the second period of time is a predefined length of time.

4. A method according to claim 1, wherein the cellular communications system (1) 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.

5. An apparatus for operating a cellular communications system (1) in which a call is handed over between 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 from the second frequency band (18) to 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 handovers, further handovers for a second period of time.

6. An apparatus according to claim 5, wherein the first period of time is a predefined length of time.

7. An apparatus according to claim 5, wherein the second period of time is a predefined length of time.

8. An apparatus according to any of claim 5, wherein the cellular communications system (1) 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.