GB2316579A

GB2316579A - Handover management for cellular network

Info

Publication number: GB2316579A
Application number: GB9617759A
Authority: GB
Inventors: Howard Thomas; Rupinder Singh Oberoi
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1996-08-24
Filing date: 1996-08-24
Publication date: 1998-02-25
Anticipated expiration: 2016-08-24
Also published as: SE519294C2; GB9617759D0; FI116355B; FI973443A; GB2316579B; SE9703028L; FR2752659B1; FI973443A0; FR2752659A1; DE19736085A1; SE9703028D0

Abstract

A handover management system for a hierarchical cellular network (eg GSM) in which handover between levels is controlled on the basis of the rate of change of candidate cell identification data reported back to a serving base station by a mobile unit. Rapid changes identify a mobile station which is moving relatively rapidly and is accordingly better allocated to a higher level base station than to a lower level station from which handover may soon be necessary again. The levels may be macrocells and microcells. Handover may be to the cell which has been a candidate for the longest time.

Description

2316579 E[ANDOVER MANAGEMENT SYSTEM FOR CELLULAR NETWORK

Field of the Invention 5

This invention relates generally to cellular radio communication networks and more particularly to the management of handover of communication with a mobile station from one base station to another in such a network.

Background of the Invention

In a typical cellular radio network the area over which service is to be offered is divided into a number of smaller areas called cells, each of which is served from its own base station. Each cell has its own antenna or antennas for transmission to and reception from a user station, normally a mobile station. When a user station moves out of the area covered by one cell carrying a particular call the call is handed over to one of a number of neighbouring cells.

The process of handover can be separated into three phases. Firstly, identifying that a handover may be required, secondly, identifying a suitable base station handover candidate and, finally, switching the call from one base station to the other.

Conventionally the determination as to whether handover is required is made by monitoring the quality of the signal, comparing the quality with predetermined thresholds, and initiating handover when the monitored quality falls below a predetermined threshold. The process may also take into account measurements made of the signal quality of neighbouring cells and handover may be initiated on the basis of the relative level of the signal 2 received from the serving cell and from the neighbour cells.

The general principles governing the management of handover in a typical network operating under the GSM (Global System for Mobile Communications) protocol are described in the publication entitled "European digital cellular communications system (Phase 2): Radio subsystem link control," GSM 05.08 version 4.9.0, dated 15 April 1994, by the European Telecommunications Standards Institute (ETSI).

The capacity of a cell is determined by the number of channels, i.e. frequencies, available to the owner of the service. Since this number is limited, cell areas may be reduced to increase capacity since this permits available frequencies to be reused without intolerable interference being introduced into the network. Thus in rural areas where traffic density is low, cells tend to be large while in urban areas traffic density tends to be higher requiring the use of smaller cells.

For such high density traffic areas it has been proposed that a multi layer cell structure be provided consisting of an overlay of conventional cells defined as macro cells, with an underlay of smaller cells, defined as micro cells, serving small areas such as blocks within a district. A further layer of still smaller cells, defined as pico cells, may be further provided to serve individual buildings or floors within a building.

In such a network, referred to as a hierarchical network, a number of factors such as movement of a mobile station within a cell make the processing of handover in accordance with the conventional techniques referred to above unsatisfactory. In particular if a mobile station is moving at such a speed that it stays within a lower 3 layer cell for a very short time the number of handovers required will become unmanageably high. Further, problems can arise with the identification of suitable handover candidate stations for rapidly moving mobile stations because the updating of candidate station identification codes cannot be performed rapidly enough. There accordingly arises a need in hierarchical systems for an improved handover management system.

The invention takes advantage of the fact that cellular networks typically provide data, usually gathered by the mobile station, which identifies neighbouring stations with the highest potential ability to provide a reliable link with a given mobile station. This data is is updated on a regular basis. For example in a GSM network a base station supplies a mobile station currently being served with a list of other neighbouring base stations (the BA list) which the mobile station is instructed to monitor and provide periodic reports to the base station. of signal quality.

Six-cary of the Invention According to the invention there is provided a handover management system for a hierarchical cellular radio network having a plurality of base stations each able to collect data from mobile stations within its service area identifying potential candidate base stations for handover on the basis of current potential signal quality, including rate measuring means for measuring the rate of change of potential candidate stations so identified, comparison means for comparing such rate with a - predetermined threshold value and handover control means responsive to said comparison means to maintain 4 communication between a mobile station and a higher level base station in the hierarchy or to permit handover to a lower layer on the basis of the comparison.

In a network operating in accordance with the GSM protocol each mobile station is required to report the identities of the six best potential handover candidates and to transmit a base station identification code (BSIC) to the base station for each of these candidates. In such a system the reported identities can conveniently be monitored continuously at the base station and if the number of changes in cell identity, for cells classed as micro cells, falls below a predetermined threshold, the handover control means may hand the mobile station to a is lower level in the hierarchy.

A handover management system embodying the invention will typically be incorporated into a base station forming part of the network.

There is further provided a method of managing handover in a hierarchical cellular radio network having a plurality of base stations each able to collect data from mobile stations within its service area identifying potential candidate base stations for handover on the basis of current potential signal quality, the method comprising measuring the rate of change of potential candidate stations so identified, comparing such rate with a predetermined threshold value and maintaining communication between a mobile station and a higher level base station in the hierarchy or permitting handover to a lower layer on the basis of the comparison.

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 illustrates in simplified form a hierarchical cellular network FIG.2 is a flow chart showing the implementation of the present invention in a GSM cellular network.

Detailed Description of the Preferred Embodiment

Referring first to FIG.1 two conventional cells of a cellular network, macro cell 1 and macro cell 2, are shown. Within macro cell 1 an underlay of micro cells is provided typically representing low power base stations with antennas situated for example below average roof top height. The numbers within the micro cells represent the frequencies used by the respective base stations; in the example shown sixteen micro cells share four frequencies.

Since these micro cells will typically be two hundred to five hundred metres in extent, the power used can be maintained at a very low level subject to achieving specific levels of in-building penetration. If the traffic density is particularly high, a lower level of still smaller cells of even lower power, referred to as pico cells, serving individual buildings or even floors of a building, may be provided. As radio propagation is effectively delimited by buildings lining a street or by the walls and partitions of individual buildings it will be appreciated that the distance between potentially interfering stations can often be reduced in this situation. However as the cell size becomes smaller the effects of movement of a mobile station within a cell dramatically increase.

Further since cell coverage overlaps to a much greater extent in a hierarchical system than in a non hierarchical 6 system the situation may frequently arise where a macro cell will have micro cell or pico cell neighbours using identical frequencies. In this situation the neighbours can be distinguished only by coded identification such as 5 the BSIC code used in the GSM system referred to above.

FIG.2. shows an embodiment of the invention in a base station of a hierarchical system which implements the GSM protocol. Such a base station includes a processor operating under the control of programme modules to perform signal processing and handover control operations required by the GSM specification. While dedicated processing means may be provided to perform the operations described below in relation to the invention, sufficient capacity will normally be available in the existing processor. In the preferred embodiment additional programme modules are provided which control the existing processor to perform the steps described with reference to FIG.2.

Within a GSM system each mobile unit is required to monitor a number of carrier frequencies determined by information broadcast by the base station currently serving that unit. This information includes a list of frequencies (the BA list) but, as will be understood from the description of a hierarchical system above with reference to FIG.l., a list of frequencies alone does not uniquely determine which cell the mobile unit is currently monitoring. The mobile unit is accordingly instructed to decode the cell identification code (BSIC) for each signal monitored and to check that it conforms to the neighbour list information broadcast by the base station currently serving the unit. After the initial decoding, at update intervals which cannot exceed ten seconds under the GSM protocol, the mobile unit is required to decode the cell identity codes of the cell that it is monitoring to verify that these cells are still in the broadcast neighbour list. Additionally the mobile unit is required under the GSM 7 protocol to report the six strongest of the frequencies monitored together with their identification codes as the periodic updating takes place.

Referring to FIG.2. the monitoring process described above takes place at step (1). In accordance with the invention, at step (2) the number of changes in BSIC identifications over a predetermined measuring interval T is determined. In the present embodiment, weightings are associated with the BSIC samples according to the timing of the sample and a progressive averaging process is applied to the samples taking account of the weightings on the socalled 'leaky bucket, principle. This process develops an average value over the interval T which is updated as new samples are taken and old samples are weighted out of the process.

At step (3) a comparison is made between the average obtained in step (2) and a predetermined threshold number- If the average is above the predetermined threshold the monitoring process is continued for a further interval T. However if the average is below the predetermined threshold handover is initiated at step (4). At this point an assessment is made at step (5) of the potential performance of available microcell handover candidates on the basis of signal strength as indicated for example by the value of the GSM parameter RxIev. In order to provide a reliable decision this value is taken over a predetermined number of measurements n and a selection of the handover candidate is made. Assuming that this candidate is available for handover the process is completed at step (6).

The threshold number and measuring intervals can be set to address various traffic conditions. However they will normally be set to maintain an acceptable level of handover activity for user stations moving within the 8 network in accordance with rules established by the network operator.

In order to determine the handover candidates with a greater degree of precision, registers may be provided to record the length of time for which each potential candidate cell has provided appropriate performance, measured for example by the number n of updating intervals T over which such performance has been measured. 10 While there has been described in connection with step (5) in FIG.2. a process of monitoring based on signal strength it will be appreciated that other parameters such as signal quality as indicated by the number of bit errors is in the transmission (in a GSM system the parameter rxqual) may be taken into account.

9

Claims

A handover management system for a hierarchical cellular radio network having a plurality of base stations each able to collect data from mobile stations within its service area identifying potential candidate base stations for handover on the basis of current potential signal quality, including rate measuring means (2) for measuring the rate of change of potential candidate stations so identified, comparison means (3) for comparing such rate with a predetermined threshold value and handover control means (4,5) responsive to said comparison means to maintain communication between a mobile station and a higher level base station in the hierarchy or to permit handover to a lower layer on the basis of the comparison.
2. A system as claimed in Claim 1, for a cellular radio network operating under the GSM protocol, n which said rate measuring means is adapted to measure the rate of change of base station identity (BSIC) codes in reports made by a mobile station to a serving base station.
3. A system as claimed in Claim 2 in which said rate measuring means is a progressive averaging processor adapted to operate on the leaky bucket' principle.
4. A system as claimed in Claim 3 in which said rate measuring means is adapted to identify the base station which has remained a candidate for a longer period of time than the other candidates, and said handover control means (5) is responsive to said rate measuring means to select such candidate for handover.
5. A handover management system for a hierarchical cellular radio network, substantially as described with reference to the accompanying drawings.
6. A base station for a hierarchical cellular radio network including a handover management system as claimed in any preceding claim. 5
7. A method of managing handover in a hierarchical cellular radio network having a plurality of base stations each able to collect data from mobile stations within its service area identifying potential candidate base stations for handover on the basis of current potential signal quality, the method comprising measuring the rate of change of potential candidate stations so identified, comparing such rate with a predetermined threshold value and maintaining communication between a mobile station and a higher level base station in the hierarchy or permitting handover to a lower layer on the basis of the comparison.
8. A method as claimed in Claim 7, for a cellular radio network operating under GSM protocol, in which the rate of change of base station identity (BSIC) codes reported by a mobile station to a base station in accordance with said protocol is averaged over a predetermined time period to produce a rate value for comparison with said predetermined threshold value.
9. A method as claimed in Claim 8 in which said rate value is developed using the leaky bucket, principle.
10. A method of managing handover in a hierarchical cellular radio network, substantially as described with reference to the accompanying drawings.