GB2314731A

GB2314731A - Handover management system for cellular network

Info

Publication number: GB2314731A
Application number: GB9613527A
Authority: GB
Inventors: Behzad Mohebbi; Simon Reza Saunders
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1996-06-27
Filing date: 1996-06-27
Publication date: 1998-01-07
Also published as: GB2314731A8; GB9613527D0

Abstract

To reduce the effect of non-uniform radiation patterns on handover decisions in a cellular radio network using adaptive antennas, a mobile station monitors the beacon signal of its serving base station 302a in addition to those of neighbouring cells 302. The beacon signal defines the cell footprint whereas the traffic channels use an adaptive antenna with a coverage which may at times differ considerably from this. In the GSM network disclosed the traffic channel level and quality are monitored 301 and also a comparison is made between the serving base station beacon signal and those of neighbouring cells to determine the need for handover. In a power budget test 308 this comparison allows an alternative link to be chosen at lower power levels to conserve the mobile station batteries and reduce interference. The comparison is made relative to a fixed pattern so that cell boundaries are maintained and interference is controlled.

Description

HAND OVER MANAGEMENT SYSTEM FOR CELLULAR NETWORK Field of the Invention This invention relates generally to cellular radio communication networks and particularly to communication networks using adaptive antenna systems.

Background of the Invention In frequency division multiplexed (FDM) networks such as those operating under the Global System for Mobile Communication (GSM) protocol, a mobile station (MS) is connected to the network for communication by allocating a duplex traffic channel to establish a link to one of a number of base transceiver stations (BTS) together defining a subsystem of communication cells referred to as a Base Subsystem (BSS). As the mobile station moves within the area served by the Base Subsystem the performance of the link is monitored and compared with the potential performance of alternative links with other Base Transceiver Stations. When the performance of the link deteriorates, for example as a result of movement of the mobile station away from the serving BTS, a handover mechanism within the BSS operates to determine when, and to which alternative link, the MS is to be transferred. The MS plays a significant part in this mechanism by monitoring beacon signals broadcast by neighbouring Base Transceiver Stations on a Broadcast Channel (BCCH) to permit a comparison to be made with the performance of the allocated channel to the serving BTS.

In a typical GSM system the neighbouring base transceiver stations monitored by the mobile stations are determined by a list, known as the Base Station Allocation (BA) list, developed by the Base Subsystem on the basis of cell boundaries of other base transceiver stations which are close to the nominal cell boundary of the serving base transceiver station.

While this arrangement is satisfactory in many systems in current use, a number of problems arise in a system employing adaptive antennas, in which base station antenna beam patterns for traffic channels change rapidly so that the effective coverage of each cell changes with time.

Specifically, measurements made on the traffic channel serving a mobile station, when compared with measurements made on the broadcast channels of potential handover dandidates as listed in the BA list, may give a highly inaccurate indication of the position of the mobile station. The mobile station may stray far out of the nominal cell boundary without triggering a handover, leading to increased interference, and, where predictive techniques are used to create the BA list, an undesirable increase in the size of such list.

In order to make effective use of adaptive antenna techniques in cellular networks it is necessary to provide a handover management system which addresses these problems.

Smeary ry of the Invention According to one aspect of the invention there is provided a method of managing handover in a cellular radio communication network of the type having a number of base stations each adapted to transmit a beacon signal, and in which a mobile station currently being served by a base station over a traffic channel is instructed by said base station to monitor the beacon signals of specified neighbour base stations together with the traffic channel signal to enable a determination to be made of the potential performance of alternative communication links, including the steps of forcing said mobile station to monitor the beacon signal of the serving base station in addition to those of the specified neighbour stations and using such monitored signal to determine the need for handover.

According to a second aspect of the invention there is provided a handover management system for a cellular radio communication network including a number of base stations forming a base subsystem, each base station being adapted to transmit a beacon signal, said base subsystem being adapted to instruct a mobile station currently being served by a base station to monitor the beacon signals of specified base stations neighbouring the serving base station to provide data to control a handover process in said base subsystem, said base subsystem being further adapted to cause said mobile station to monitor the beacon signal of the serving base station and to compare the signal so monitored with the signals of the specified neighbouring stations to determine the need for handover.

In a cellular radio network operating in accordance with the GSM protocol, mobile stations are provided with a list of beacon signals radiated on broadcast channels (BCCH) by specified base stations. In such a network the mobile station may be conveniently instructed to monitor the BCCH of the serving base station by causing that BCCH to be added to the list.

In order that the invention may be well understood a preferred embodiment thereof will now be described with reference to the accompanying drawings.

Brief Description of the Drawings FIG.1 is a schematic diagram of a radio communication network using adaptive antennae.

FIG.2 illustrates the handover management system conventionally used in a cellular network.

FIG.3 shows the handover management system of the preferred embodiment.

Detailed Description of the Preferred Embodiment Referring first to FIG.1 a group of base stations (1-7) together form a network defining a number of cells designed to cover a particular geographical area with overlapping coverage to permit a mobile station travelling within the area to maintain viable communication. Each base station transmits a beacon signal on a broadcast.

channel (BCCH) whose coverage defines the footprint of the cell. In the figure the footprint is shown by dotted lines as generally circular although it will be appreciated that local distortions will occur due to terrain and the presence of obstacles and the actual footprint will in practice depart substantially from the ideal. For the traffic channels each base station uses an adaptive antenna which is controlled to optimise the signal providing an operational link by adjusting the directivity of the antenna dynamically while a mobile station is camped on to a given base station. As shown by the full lines surrounding each base station, this serves to reduce the power radiated in directions away from the mobile station while concentrating the available power in the direction of the mobile station, normally reducing interference and permitting the link to be maintained in situations where a non adaptive antenna would be inadequate. However as is seen particularly from the traffic channel radiation patterns of base stations 1 and 4 the traffic channel signals can develop a pattern of signal strength as a call progresses which differ considerably from the signal strengths of the corresponding broadcast channels. In some situations a usable signal strength from a particular base station can extend beyond the nominal cell boundary of that base station well into the service area of distant cells.

FIG.2 shows the handover system employed conventionally in a GSM network. At 201 a mobile station being served by a given base station continuously monitors the level (RxLev) and the quality (RxQual) of the serving traffic channel. RxLev is conventionally a measure of signal strength whilst RxQual is a measure of error rate which increases as the link deteriorates. At 202 the mobile station measures the corresponding RxLev for the BCCH of neighbouring cells identified in the BA list, which is developed by the base subsystem as described earlier from information on the configuration of the subsystem.

At step 203, in accordance with the GSM protocol, the mobile station reports to the base station subsystem the best six candidates on the BA list for transfer of the call. This is a continuous process and and normally if the quality of the link is maintained at a satisfactory level no handover is instructed by the base subsystem. In order to determine whether a handover is necessary a test is first made at step 205 to determine whether RxLev continues to be satisfactory. If it does not a handover process is initiated at step 206. However the signal strength may remain satisfactory although for reasons such as interference the quality of the link may have deteriorated. Thus at step 207 a test is made to determine whether the error level exceeds an acceptable figure and if so the handover process is again initiated.

Even if the signal strength and quality are acceptable it may still be determined that the call can proceed with an alternative link at power levels less than those currently being used by a predetermined margin. This would enable for example the mobile station to reduce power and thus conserve its batteries while also reducing potential interference. This test, referred to as a power budget (PBGT) test, is made at step 208. Finally a test is made at 209, typically on the basis of the time delay between transmission and reception of information on the channel, to determine whether the distance of the mobile station from the serving base station exceeds a specified amount.

Bearing in mind that the BCCH signals from the various base stations are, as described with reference to Figure 1, broadcast with a fixed pattern since they must be received by mobile stations whose direction is unknown in relation to the base station, any comparison, such as those made in steps 205, 207, 208 and 209, can only be reliable if the behaviour of the traffic channel is similar to that of the BCCH signal. In a GSM system employing non adaptive antennae this can be generally assumed. However if the traffic channel antennae are made adaptive, the traffic channels will generally be highly directive. The interference and signal strength parameters of the traffic channel and the BCCH can then differ widely, leading to unreliable comparisons and undesirable extensions of the size of the BA list.

FIG.3 shows a handover system embodying the present invention. The signal strength and quality of the serving channel are measured as before by the mobile station at step 301. However at step 302 in addition to measuring the signal strength of the BCCH of the specified neighbouring cells the mobile station is caused at 302 (a) to measure the signal strength of the BCCH of the serving cell, by adding this latter BCCH to the BA list. The best six alternative channels are again reported at step 303 and tests made as before on the signal strength and signal quality at 304 and 307 to determine whether the handover process shall be started at 306. However at 308 the power budget test (PBGT) is now made on the basis of the BCCH measurements made at step 302 and 302(a). This enables this comparison to be made relative to a fixed pattern thus allowing the cell boundaries to be properly maintained and permitting interference and candidate neighbours to be properly controlled. A test is made at 309 as before to determine whether the distance of the mobile station from the serving base station exceeds a given margin.

It will be seen that by forcing the mobile station to monitor the broadcast channel of the serving base station in an adaptive antenna system the value of the comparisons made to establish the handover process is considerably enhanced, since like is being compared with like. In a network employing the GSM protocol the enhancement can be simply and effectively achieved by adding the BCCH of the serving base station to the BA list.

What is claimed is:

Claims

1. A method of managing handover in a cellular radio communication network of the type having a number of base stations each adapted to transmit a beacon signal, and in which a mobile station currently being served by a base station over a traffic channel is instructed by said base station to monitor the beacon signals of specified neighbour base stations together with the traffic signal to enable a determination to be made of the potential performance of alternative communication links, including the steps of forcing said mobile station to monitor the beacon signal of the serving base station in addition to those of the specified neighbour stations and using such monitored signal to determine the need for handover.

2. A method as claimed in claim 1 in which said network system is a GSM system and the step of forcing said mobile station to monitor the beacon frequency of said serving base station is performed by adding the broadcast channel (BCCH) of said serving station to the Base Station Allocation (BA) list.

3. A method of managing handover in a cellular radio communication network substantially as described with reference to Figure 3 of the accompanying drawings.

4. A handover management system for a cellular radio communication nework including a number of base stations forming a base subsystem, each base station being adapted to transmit a beacon signal, said base subsystem being adapted to instruct a mobile station currently being served by a base station to monitor the beacon signals of specified base stations neighbouring the serving base station to provide data to control a handover process in said base subsystem, said base subsystem being adapted to cause said mobile station to monitor the beacon signal of the serving base station and to compare the signal so monitored with the signals of the specified neighbouring base stations to determine the need for handover.

5. A system as claimed in claim 4 in which said network is a GSM system in which said base subsystem is adapted to create a Base Station Allocation (BA) list of specified neighbouring cells for transmission to a mobile station, saiid base station subsystem causing said mobile station to monitor the beacon signal of said serving base station by including the broadcast channel (BCCH) of said serving station in said BA list.

6. A handover management system for a cellular radio network substantially as described with reference to Figures 1 and 3 of the accompanying drawings.