GB2397469A

GB2397469A - Method and apparatus for cell biasing

Info

Publication number: GB2397469A
Application number: GB0301292A
Authority: GB
Inventors: Colleen Cheung; Howard Thomas
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-01-20
Filing date: 2003-01-20
Publication date: 2004-07-21
Anticipated expiration: 2023-01-20
Also published as: GB0301292D0; GB2397469B

Abstract

The invention relates to a system for cell biasing for idle mode subscriber units (207) in a cellular communication system. A base station (201) comprises a load processor (211) for locally determining load information for a cell in response to load related operating conditions of the cell. The load information may be a congestion level of the cell. The load processor (211) is coupled to a bias processor (213) that determines a bias parameter for the cell in response to the load information. The bias parameter may be a signal strength offset value to be applied the signal strength measurements for that cell. The base station (201) further comprises a transmitter (215) for broadcasting the bias parameter. An idle mode subscriber unit (207) measures the signal strength of the surrounding base stations (201, 205) and compensates the measurements by the bias parameter. It then selects the cell having the highest modified signal strength as a serving cell.

Description

1 2397469

METHOD AND APPARATUS FOR CELL BIASING IN A CELLULAR

COMMUNICATION SYSTEM

Field of the invention

The invention relates to a method and apparatus for cell biasing in a cellular communication system and in particular in a Global System of Mobile communication (GSM) system or a Universal Mobile Telecommunication System (UMTS).

Background of the Invention

FIG. 1 illustrates the principle of a conventional cellular communication system 100 in accordance with prior art. A geographical region is divided into a number of cells 101, 103, 105, 107 each of which is served by base station 109, 111, 113, 115. The base stations are interconnected by a fixed network which can communicate data between the base stations 109, 111, 113, 115. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated. In the example of FIG. 1, mobile station 117 is served by base station 109 over radio link 119, mobile station 121 is served by base station 111 over radio link 123 and so on.

As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. For example mobile station 125 is initially served by base station 113 over radio link 127. As it moves towards base station 115 it enters a region of overlapping coverage of the two base stations 113 and 115 and within this overlap region it changes to be supported by base station 115 over radio link 129. As the mobile station 125 moves further into cell 107, it continues to be supported by base station 115.

This is known as a handover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typically an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.

The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently the most ubiquitous cellular communication system is the 2n generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving cell during initial access or handovers. Further description of the GSM TDMA communication system can be found in 'The GSM System for Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de-spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed.

Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

The frequency band allocated for a cellular communication system is typically severely limited, and therefore the resource must be divided effectively between the mobile stations. A fundamental property of a cellular communication system is that the resource is divided geographically by the division into different cells. An important advantage of a cellular communication system is that due to the radio signal attenuation with distance, the interference caused by communication within one cell is negligible in a cell sufficiently far removed, and therefore the resource can be reused in this cell. In order to optimise the available communication capacity in a cellular communication system, it is advantageous to have the mobile stations distributed over different cells in accordance with the available communication capacity.

When mobile stations are idle and not involved in an active call, they select a cell as the serving cell. They then proceed to monitor this cell for control information and paging signals. Hence, mobile stations in idle mode select a serving cell to camp on. Mobiles stations tend to establish calls on the cell that they are currently camped-on. In multi- band networks, one way of ensuring full usage of available network capacity is by handing the mobile stations over to a cell of a particular frequency or layer once the mobile station has established a call or a session. However this entails additional signalling traffic to handover an active connection from one switch in the network to another.

Conventionally, the only way for the network operator to influence the idle mode behaviour of mobile stations is by setting a common offset parameter when configuring the network. These offsets are static, and constitute the operator's view of how the mobile stations preferably behave in an idealized general case. However, if the conditions deviate from the assumptions made, the behaviour of the idle mode mobile stations will typically become sub- optimal resulting in degraded performance of the communication system.

Furthermore, the static offsets are centrally determined and the offset parameters for different cells are determined from a general relationship between different types of cells. Accordingly, the offsets are unable to adapt to changing or local operating conditions.

Hence, an improved system for biasing mobile stations towards cells would be advantageous and in particular an improved system allowing for increased flexibility, variation and granularity in cell biasing would be advantageous.

Summary of the Invention

Accordingly, the Invention seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a method of cell biasing in a cellular communication system comprising the steps of: locally determining load information for a cell in response to load related operating conditions of the cell; determining a cell selection bias parameter for the cell in response to the load information; and broadcasting the cell selection bias parameter.

Accordingly an improved cell biasing is provided in a cellular communication system. Load information is locally determined in response to the load related operating conditions of that cell thereby allowing for the cell biasing to adapt to the currently prevailing local conditions. Accordingly, an improved distribution of idle subscriber units, such as mobile stations, may be achieved thereby improving performance of the communication system and specifically reducing signalling, delay and resource consumption associated with handovers following call establishment. Hence, the invention allows for an improved management of network load and/or utilization of network resources.

The load information is determined locally and is specifically based on the available information. There is thus no requirement for centralised processing and/or information exchange. Hence, it allows for dynamic modification of a cell selection bias parameter in response to locally determined load related operating conditions of a cell.

The load information may specifically be determined in response to the loading of the cell. For example an absolute load value, an average load value and/or a variance of a load value may used.

According to a feature of the invention, the cell selection bias parameter is a measurement offset parameter. This provides for an efficient and low complexity method of cell biasing.

According to another feature of the invention, the cell selection bias parameter is a signal strength offset parameter. This provides for an efficient and low complexity method of cell biasing.

According to another feature of the invention, the method further comprises the step of a subscriber unit performing a cell selection in response to the cell selection bias parameter. Hence, a method is provided wherein the cell selection bias parameter causes subscriber units to be redistributed.

According to another feature of the invention, the step of cell selection comprises compensating a measured cell parameter in response to the cell selection bias parameter. This provides for a simple yet efficient means of performing a cell biasing.

According to another feature of the invention, the cell selection bias parameter is dynamically updated. The cell selection bias parameter may be recalculated and thereby updated at for example regular intervals or when specific events occur. This allows for a distribution of idle subscriber units to be modified and optimised for the currently prevailing operating conditions in the cellular communication system.

According to another feature of the invention, the cell selection bias parameter is dynamically updated in response to variations in the load related operating conditions of the cell. This allows for the cell biasing to be optimised to suit the currently prevailing load conditions of the cell.

According to another feature of the invention, the load related operating conditions comprise a resource usage of the cell. This allows for the cell biasing to be optimised to suit the currently prevailing resource usage of the cell. The resource usage may for example be a relative loading of the cell or a congestion indication.

According to another feature of the invention, the method further comprises the step of locally determining neighbour cell load information related to load conditions of a neighbour cell and the load information is furthermore determined in response to the neighbour cell load information. An improved distribution may be achieved by determining the cell selection bias parameter in response not only to the load related conditions of the current cell but also to load conditions of one or more neighbour cells. Specifically, the cell selection bias parameter may be determined in response to the relative load conditions of the cells, for example such that the cell selection bias parameter causes subscriber units to be biased towards the least loaded cells. Indications of the load conditions of the neighbour cell may be determined from locally available information or may be determined in response to information received from other elements of the communication system.

According to another feature of the invention, the neighbour cell load information is determined in response to a handover rejection indication associated with the neighbour cell. This provides a suitable parameter for determining cell load information of the neighbour cell. The information may furthermore be locally available. For example, the number, ratio and/or frequency of handover rejections related to the neighbour cell provide an indication of the load level (and congestion level) of the neighbour cell. The load information may be combined with time information. For example, the cell selection bias parameter may depend on how many rejections are received in a time period and the time since the last rejection. Additionally, the cell selection bias parameter may be determined by taking into account the cause of the handover rejections.

According to another feature of the invention, the neighbour cell load information is determined in response to a handover request indication associated with the neighbour cell. This provides a suitable parameter for determining cell load information of the neighbour cell. The information may furthermore be locally available. For example, the number, ratio and/or frequency of handover requests from the neighbour cell may provide an indication of the load level (and congestion level) of the neighbour cell. The load information may be combined with time information. For example, the cell selection bias parameter may depend on how many requests are received in a time period and the time since the last request. Additionally, the cell selection bias parameter may be determined by taking into account the cause of the handover requests.

According to another feature of the invention, the load related operating conditions comprise a service characteristic of at least one service supported by the cell. For example, the cell selection bias parameter may be determined by taking into account the priority or quality of service parameters of the load of the cell. This allows for a cell selection bias parameter to be determined that is more accurately adapted to the operating conditions.

According to another feature of the invention, the cell selection bias parameter is common for packet data and circuit switched data and the load related operating conditions relate to the packet data. Accordingly, the cell biasing may be performed such as to improve the distribution for packet services. The cell selection bias parameter may be determined based only on operating conditions associated with packet services. Hence, the method does not require information related to circuit switched services. This permits effective cell biasing for independent packet and circuit switched resource management.

According to another feature of the invention, the cell selection bias parameter is common for packet data and circuit switched data and the load related operating conditions relate to the circuit switched data. Accordingly, the cell biasing may be performed such as to improve the distribution for circuit switched data. The cell selection bias parameter may be determined based only on operating conditions associated with circuit switched data services.

Hence, the method does not require information related to packet services.

This permits effective cell biasing for independent packet and circuit switched resource management.

According to another feature of the invention, the step of locally determining load information is in response to only locally available load information. This allows for a simple implementation. It furthermore allows for the cell biasing to be implemented in some cells independently of other cells. Specifically, it allows for cell biasing to be gradually introduced to existing communication systems and in multi vendor systems.

According to another feature of the invention, the step of locally determining load information is performed in a base station system in response to only information locally available for the base station system. A base station system typically comprises a number of base stations and a controller for the base stations. The controller may be a Base Station Controller (BSC) or a Radio Network Controller (RNC). A base station system is suitable for implementing the method as the required information is readily available within the base station system in most cellular communication systems.

Specifically the local cell load information is known and congestion information for other cells may be available.

According to another feature of the invention, the step of locally determining load information is performed in response to only information locally available for the base station. This allows for simple implementation in existing communication systems and provides efficient and localised optimization.

According to another feature of the invention, the step of locally determining load information is performed in a base station controller in response to only information locally available for the base station controller. This allows for a simple implementation wherein a single implementation may provide cell selection bias parameters for a plurality of cells. The base station controller may be a BSC or an RNC.

According to another feature of the invention, the communication system is a Global System for Mobile (GSM) communication system. Preferably, the cell selection bias parameter is a GSM offset parameter as specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.08.

Hence, the invention allows for a simple and efficient cell biasing in a GSM communication system.

The GSM offset parameter may specifically be one or more of the "CELL_RESELECT_OFFSET "and "GPRS_RESELECT_OFFSET (n)" parameters.

According to another feature of the invention, the communication system is a Universal Mobile Telecommunication System (UMTS). Preferably, the cell selection bias parameter is a UMTS offset parameter as specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.304.

Hence, the invention allows for a simple and efficient cell biasing in a UMTS communication system.

The UMTS offset parameter may be one or more of the "Qoffsetls,n", "Qoffset2s,n", "Qhyst Is, n" and "Qhyst2s,n" parameters.

According to a second aspect of the invention, there is provided an apparatus for cell biasing in a cellular communication system comprising: means for locally determining load information for a cell in response to load related operating conditions of the cell; means for determining a cell selection bias parameter for the cell in response to the load information; and means for broadcasting the cell selection bias parameter.

According to a second aspect of the invention, there is provided a cellular communication system comprising: means for locally determining load information for a cell in response to load related operating conditions of the cell; means for determining a cell selection bias parameter for the cell in response to the load information; means for broadcasting the cell selection bias parameter; and a subscriber unit operable to perform a cell selection in response to the cell selection bias parameter.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 is an illustration of a cellular communication system in accordance with

the prior art;

FIG. 2 is an illustration of part of a cellular communication system in accordance with an embodiment of the invention; FIG. 3 illustrates a flow chart of a method of cell biasing in a cellular communication system in accordance with an embodiment of the invention; and FIG. 4 illustrates a method of cell re-selection in a subscriber unit suitable for a cellular communication system comprising an apparatus for cell biasing in accordance with an embodiment of the invention.

Detailed Description of a Preferred Embodiment of the Invention The following description focuses on an embodiment of the invention applicable to a GSM cellular communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other cellular communication systems including for example UMTS.

FIG. 2 is an illustration of part of a cellular communication system 200 in accordance with an embodiment of the invention.

FIG. 2 illustrates a first base station 201 comprising an apparatus for cell biasing in accordance with an embodiment of the invention. Only the parts of the base station 201 associated with the cell biasing apparatus have been illustrated, and it will be apparent to the person skilled in the art that the base station 201 additionally comprises the required functionality for operating in accordance with the specifications of the cellular communication system, which in this case is a GSM cellular communication system. The base station 201 is connected to a fixed network 203 which is further connected to other base stations of which one neighbour base station 205 is shown. FIG. 2 further illustrates a subscriber unit 207 which is in idle mode and situated in a location where it is able to receive signals from both the first base station 201 and the neighbour base station 205.

The first base station 201 comprises a resource controller 209 which is operable to allocate the available resources to different subscriber units and services. Specifically, individual time slots are in a GSM cellular communication system allocated to active subscriber units when a new call is established or a handover is performed. The resource controller is for brevity and clarity shown to be part of the base station. However, it will be appreciated that resource allocation is typically implemented partly or fully in a Base Station Controller (BSC) in GSM communication systems. Hence, the resource controller 209 may be viewed as the functional module of the base station 201 that implements the resource allocation performed in the BSC.

The resource controller 209 is connected to a load processor 211 which is operable to determine the load information for a cell in response to load related operating conditions of the cell. The load information is determined locally for the cell, and is in the preferred embodiment determined locally in the base station. The load information is determined on the basis of the information that is locally available and is specifically derived from standard GSM parameters and measurements available to the load processor. In the preferred embodiment, the load processor 211 determines the load information from characteristics of the load as derived from the information present in the local resource controller 209.

The load processor 211 is coupled to a bias processor 213, which is operable to determine a cell selection bias parameter for the cell in response to the load information determined by the load processor 211. The bias processor 213 is coupled to a base station transmitter 215 which broadcasts the cell selection bias parameter on the BCCH broadcast channel. Hence, the cell selection bias parameter is broadcast to all subscriber units that are capable of receiving the BCCH signal.

The subscriber unit 207 is capable of receiving signals from both base stations 201, 205 and to select a cell in response to the measured characteristics of the received signals. In addition, the subscriber unit 207 is operable to receive the cell selection bias parameter and use the bias in the selection of the preferred cell. Hence, the determined characteristics of the received base stations 203, 205 are compensated by the cell selection bias parameter before the cell selection is made. The neighbour base station 205 may broadcast a cell selection bias parameter associated with that cell. The neighbour cell selection bias parameter may specifically be derived by the neighbour base station 205 independently operating a cell bias algorithm as described for the first base station. Hence, a suitable cell biasing may be achieved by independently operated local processes associated with the different base stations 201, 205.

The subscriber unit 207 is further operable to camp on to the selected cell and thereby perform a cell reselection. Hence, the distribution of the idle mode subscriber units in the communication system may be achieved through simple, independent and locally operated processes.

FIG. 3 illustrates a flow chart of a method of cell biasing in a cellular communication system in accordance with an embodiment of the invention.

The method will be described with reference to the cell biasing apparatus of the first base station 201 of FIG. 2.

The method starts in step 301 where load information for the first cell of the first base station 201 is determined locally by the load processor 211 in response to load related operating conditions of the cell. In the preferred embodiment, the load related operating conditions specifically comprise a resource usage of the cell. Specifically a congestion indication may be used which provides a measure of how much resource is currently available in the cell. Hence, in the preferred embodiment, the cell load condition of the first cell is determined, and specifically a cell load condition relating to a relative level of congestion is determined. As a specific example suitable for the implementation in a GSM communication system, the base station has a given number of resource units in the form of time slots available for allocation to subscriber units, and the cell load condition is simply determined as the ratio of these time slots that are currently allocated. Hence, in this example, the cell load condition is a relative cell loading between 0 and 100%. The higher the relative cell loading, the closer the first cell is to being congested.

In the preferred embodiment, the load information is determined based only on locally available load information. In the described embodiment, the load related operating conditions are determined based only on information which is guaranteed to be available at the base station in accordance with the specifications of the cellular communication system. Hence, the load related operating conditions arc determined based only on the information that must necessarily be available at the base station in order to comply with the GSM specifications. It will be apparent that the GSM specifications require local knowledge in the base station of the available time slots and allocation of these to subscriber units, and consequently the described method for determining a relative loading has the advantage of requiring only information that is already available at the base station.

In the preferred embodiment, the load related operating conditions may simply consist in a relative loading of the cell. However, in other embodiments, the load related operating conditions may for example comprise not only an average of the relative load but also a statistical measure of the load, such as an average and/or variance of the relative load. This may provide a more suitable load determination in some situations thereby leading to a cell biasing which is more accurately suited to the current conditions.

Step 301 is followed by step 303, wherein a cell selection bias parameter is determined by the bias processor 213 in response to the load information. In a simple embodiment, a specific predefined relationship between the cell selection bias parameter and the load information may simply be used to determine the cell selection bias parameter. The relationship may be expressed as a suitable mathematical function or may be implemented as a look up table in the bias processor 213. In thespecific example described, the cell selection bias parameter is an offset value intended to be applied to the signal strength measurements made by the subscriber unit. As a specific example, the cell selection bias parameter may be an offset which is a linear function (in dB) of the relative cell load starting at +lOdB at a relative cell loading of zero and - 10dB for cell loadings in excess of for example 80%. This will cause a positive bias towards the cell for low loading of the cell and a bias away from the cell at high loading. This will thus tend to bias idle mode subscriber units towards cells having low loading.

Step 303 is followed by step 305 wherein the cell selection bias parameter is broadcast on the BCCH channel of the first base station 201.

In the preferred embodiment the method re-iterates to step 301 following step 303. Accordingly, in the preferred embodiment the method is continuously repeated whereby the cell selection bias parameter is dynamically updated to follow the current load related operating conditions of the cell. Hence, as the load related operating conditions vary, the cell selection bias parameter will automatically be modified accordingly. For example, if the relative load increases in the cell, the cell selection bias parameter decreases thereby causing an increased bias away from the cell and consequently mitigating the impact of the increased loading. Preferably, the method is repeated for many or all cells whereby an automatic bias of idle mode subscriber units towards the cells having relatively low load is achieved.

FIG. 4 illustrates a method of cell re-selection in a subscriber unit suitable for a cellular communication system comprising an apparatus for cell biasing in accordance with an embodiment of the invention. The method will be described with reference to the subscriber unit 207 of FIG. 2.

In step 401, the subscriber unit selects a base station to monitor. For a GSM embodiment, the base station may for example be sequentially selected from the group containing the current serving base station and the base stations of the neighbour list of the serving base station.

Step 401 is followed by step 403 wherein the cell selection bias parameter is received from the base station. Preferably, the cell selection bias parameter is received as part of broadcast and control signals transmitted by the currently selected base station. In the preferred embodiment, all base stations transmit a cell selection bias parameter but in some embodiments only some base stations transmits this. In this case, the bias is only applied to measurements of cells for which a cell selection bias parameter is received.

Step 403 is followed by step 405 wherein the subscriber unit measures a cell parameter, which in the preferred embodiment is the signal strength of the base station. In a GSM embodiment, the received signal level of the BCCH carrier of the selected cell is measured.

Step 405 is followed by step 407 wherein the measured cell parameter is compensated in response to the cell selection bias parameter. In the preferred embodiment, the received signal level is simply modified by adding the cell selection bias parameter for that cell. Thus, if a received signal level of -80dBm has been received and the received cell selection bias parameter is +5dB, a modified received signal level of 75dBm is determined.

Step 407 is followed by step 409 wherein it is determined if all base stations of the group have been processed. If not, the method returns to step 401 wherein the next base station of the group is selected. Otherwise, the method continues in step 411 wherein a base station is selected as the serving base station. The base station is in the preferred embodiment, selected as the base station that has the highest modified received signal strength level of the BCCH signal. In some cases, a hysteresis may be introduced such that the subscriber unit only changes serving cell if the preferred cell has a modified signal strength exceeding that of the current serving cell by a given value. If a new cell is selected by the process, the subscriber unit proceeds to instigate a cell reselection procedure in accordance with the protocols of the given communication system.

In the preferred embodiment, the cell selection bias parameter is a GSM offset parameter as specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.08. Specifically, the CELL_RESELECT_OFFSET may be used as the cell selection bias parameter.

Thus, in this embodiment, an existing bias parameter is determined in accordance with the described method and transmitted by the base station in accordance with the requirements of the GSM specifications. Hence, no modifications or changes to the existing GSM specifications are required.

It will be apparent to the person skilled in the art that the invention is equally applicable to for example the Universal Mobile Telecommunication System (UMTS). In such an embodiment the cell selection bias parameter is preferably an offset parameter specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.304. In one such embodiment, the cell selection bias parameter may comprise a first component associated with the serving cell and a second component associated with the neighbour cells of the serving cell. In this case, the first component is applied by the subscriber unit to measurements of the serving cell and the second component is applied to measurements of the neighbour cells. Preferably, the first component is the "Qoffsetls" parameter and the second component is the "Qoffsetln" parameter.

In other embodiments, the cell selection bias parameter may relate to for example signal to noise measurements rather than just signal strength measurements. In such an embodiment for a UMTS communication system, the cell selection bias parameter may preferably consist in the "Qoffset2s" and "Qoffset2n" parameters defined in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.304. In other embodiments the "Qhystls", "Qhystln", "Qhyst2s" and/or "Qhyst2n" parameters may alternatively or additionally be used.

In the preferred embodiment, the load information is determined only in response to the load conditions of the specific cell. However, in other embodiments, locally available information, from which load indications associated with other cells can be derived, may also be used. In these embodiments, neighbour cell load information is locally determined.

One method for determining such a load information is from a handover rejection indication associated with a neighbour cell. If handovers to a neighbour cell are frequently rejected, this may indicate that the cell has only limited available resource, and thus that it has a high load and possibly is close to congestion. Therefore, if many handovers are rejected by neighbour cells, it may be not be advantageous to bias idle mode subscriber units towards these cells, and accordingly it will be desirable to reduce the bias of the cell selection bias parameter towards other cells. Specifically, the biasing apparatus may determine that the cell selection bias parameter should be increased for an increasing number or ratio of handovers being rejected by neighbour cells.

Similarly, a large number of handover requests from neighbouring cells may indicate that these cells have high loads and may be close to congestion.

Consequently, the load information may advantageously comprise neighbour cell load information determined in response to the characteristics of handover requests received. Specifically, the actual number of handover requests may be included, and the cell selection bias parameter may be increased for increasing numbers of handover requests.

In cellular communication systems, such as GSM and UMTS, the handover requests and rejections furthermore comprise information related to the cause of the requests and rejections. In some embodiments, these parameters may be included in the determination of the cell selection bias parameter. For example, only handover requests and rejections caused by capacity considerations may be included in the determination of the cell selection bias parameter, and handover requests and rejections caused by deteriorating radio conditions may be ignored in the determination.

In some embodiments, the load related operating conditions may comprise a service characteristic of at least one service supported by the cell. For example, the priority of the communications currently being supported by the cell may be used in setting the cell selection bias parameter. Specifically, the higher the priority of the supported communication, the lower the bias towards the cell will be. This will reduce the probability that idle mode subscriber units will establish calls in the cell which may interfere with high priority communications. Similarly, the quality of service levels of services supported by the cell may be taken into account. Thus, the cell selection bias parameter may be reduced for increasingly strict quality of service levels thereby biasing idle mode subscriber units away from the cell and thus reducing the likelihood of new calls preventing the quality of service levels being achieved.

Most current cellular communication systems allows for both circuit switched and packet data services to be set up. For example, GSM systems are typically enhanced by General Packet Radio Services (GPRS) in order to support data packet communication. The packet data and circuit switched data share the same radio resource, although the dynamic resource allocation for circuit switched and packet data is often performed independently by different functional modules. In accordance with an embodiment of the invention, a common cell selection bias parameter may be used that will bias both circuit switched and packet data to or from other cells. However, in the preferred version of such an embodiment, the determination of the cell selection bias parameter is performed based only on the load related packet conditions relating only to packet data or to circuit switched data.

Hence, according to one such embodiment, the cell selection bias parameter is determined based on the packet data operating conditions while the cell selection bias parameter affects both packet data and circuit switched data.

This allows for the packet data resource allocation to independently change the distribution of idle mode subscriber units as required. In another embodiment, the cell selection bias parameter is determined based on the circuit switched operating conditions while the cell selection bias parameter affects both packet data and circuit switched data. This allows for the circuit switch resource allocation to independently change the distribution of idle mode subscriber units as required. As the resource management is frequently performed independently, although the services share the radio resource and consequently impacts on each other, this provides for an advantageous method of influencing the shared operating conditions.

In the preferred embodiment, the method is operated in a base station based on information locally available in the base station. However, in other embodiments, the method may be operated in a base station controller such as a BSC or an RNC. In this case, the load information is preferably determined only in response to information locally available in the base station controller.

In many communication systems, the resource allocation for a base station is performed in the base station controller, and thus the base station controller typically has load information related to each of the base stations controlled.

Accordingly, the base station controller may independently operate the process for each controlled base station. The determined cell selection bias parameter may then be communicated to the respective base stations for broadcasting to the subscriber units. In other embodiments, the functionality may be differently split between the base station controller and base stations.

However, preferably the load information is determined in the base station system based only on information locally available in the base station system The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. However, preferably, the invention is implemented as software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way.

Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

Claims

1. A method of cell biasing in a cellular communication system comprising the steps of locally determining load information for a cell in response to load related operating conditions of the cell; determining a cell selection bias parameter for the cell in response to the load information; and broadcasting the cell selection bias parameter.

2. A method as claimed in claim 1 wherein the cell selection bias parameter is a measurement offset parameter.

3. A method as claimed in claim 1 wherein the cell selection bias parameter is a signal strength offset parameter.

4. A method as claimed in any previous claim further comprising the step of a subscriber unit performing a cell selection in response to the cell selection bias parameter.

5. A method as claimed in claim 4 wherein the step of cell selection comprises compensating a measured cell parameter in response to the cell selection bias parameter.

6. A method as claimed in any previous claim wherein the cell selection bias parameter is dynamically updated.

7. A method as claimed in claim 6 wherein the cell selection bias parameter is dynamically updated in response to variations in the load related operating conditions of the cell.

8. A method as claimed in any previous claim wherein the load related operating conditions comprise a resource usage of the cell.

9. A method as claimed in any previous claim further comprising the step of locally determining neighbour cell load information related to load conditions of a neighbour cell and wherein the load information is furthermore determined in response to the neighbour cell load information.

10. A method as claimed in any previous claim wherein the neighbour cell load information is determined in response to a handover rejection indication associated with the neighbour cell.

11. A method as claimed in any previous claim wherein the neighbour cell load information is determined in response to a handover request indication associated with the neighbour cell.

12. A method as claimed in any previous claim wherein the load related operating conditions comprise a service characteristic of at least one service supported by the cell.

13. A method as claimed in any previous claim wherein the cell selection bias parameter is common for packet data and circuit switched data and wherein the load related operating conditions relate to the packet data.

14. A method as claimed in any previous claim 1 to 13 wherein the cell selection bias parameter is common for packet data and circuit switched data and wherein the load related operating conditions relate to the circuit switched data.

15. A method as claimed in any previous claim wherein the step of locally determining load information is in response to only locally available load information.

16. A method as claimed in claim 15 wherein the step of locally determining load information is performed in a base station system in response to only information locally available for the base station system.

17. A method as claimed in claim 16 wherein the step of locally determining load information is performed in response to only information locally available for the base station.

18. A method as claimed in claim 16 wherein the step of locally determining load information is performed in a base station controller in response to only information locally available for the base station controller.

19. A method as claimed in any previous claim wherein the communication system is a Global System for Mobile (GSM) communication system.

20. A method as claimed in claim 19 wherein the cell selection bias parameter is a GSM offset parameter as specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.08.

21. A method as claimed in any of the claims 1 to 18 wherein the communication system is a Universal Mobile Telecommunication System (UMTS) .

22. A method as claimed in claim 21 wherein the cell selection bias parameter is a UMTS offset parameter as specified in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 05.304.

23. A computer program enabling the carrying out of a method according to any of the claims 1 to 22.

24. A record carrier comprising a computer program as claimed in claim 23.

25. An apparatus for cell biasing in a cellular communication system comprising: means for locally determining load information for a cell in response to load related operating conditions of the cell; means for determining a cell selection bias parameter for the cell in response to the load information; and means for broadcasting the cell selection bias parameter.

26. A cellular communication system comprising: means for locally determining load information for a cell in response to load related operating conditions of the cell; means for determining a cell selection bias parameter for the cell in response to the load information; means for broadcasting the cell selection bias parameter; and a subscriber unit operable to perform a cell selection in response to the cell selection bias parameter.