GB2397468A - Generating a handover decision in response to a desired handover probability in response to a cell load condition - Google Patents
Generating a handover decision in response to a desired handover probability in response to a cell load condition Download PDFInfo
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- GB2397468A GB2397468A GB0301291A GB0301291A GB2397468A GB 2397468 A GB2397468 A GB 2397468A GB 0301291 A GB0301291 A GB 0301291A GB 0301291 A GB0301291 A GB 0301291A GB 2397468 A GB2397468 A GB 2397468A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/16—Performing reselection for specific purposes
- H04W36/22—Performing reselection for specific purposes for handling the traffic
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Abstract
The invention relates to a system for performing handovers in a cellular communication system. A desired handover probability is determined 303 for a cell (201) in response to a cell load condition which specifically is a relative load of the cell (201). A handover decision is generated 307 by a stochastic process resulting in a decision corresponding to the desired handover probability. The desired handover probability for handovers away from the cell (201) increases for increasing congestion, and the desired handover probability for handovers to the cell (201) decreases for decreasing congestion of the cell. If the handover decision is a decision to perform a handover, this is instigated 311. The process is repeated for a plurality of subscriber units thereby providing a flexible and gradual congestion relief that increases for increasing congestion levels.
Description
METHOD AND APPARATUS FOR PERFORMING A HANDOVER IN A
CELLULAR COMMUNICATION SYSTEM
Field of the invention
The invention relates to a method and apparatus for performing a handover 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 116 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 2nd 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 effectively divided between 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.
A frequently encountered problem is where one or more cells are congested and do not have available resource for supporting additional mobile stations.
Congestion causes calls to be dropped and it is therefore desirable to reduce the probability of congestion occurring. Often, additional resource may be available in other cells, and therefore many cellular communication systems comprise algorithms attempting to utilise such resource. Typically, the cellular communication systems comprise congestion relief traffic management algorithms which operate when congestion occurs in a cell. These algorithms seek to distribute traffic across different frequency resources and specifically seek to move traffic to neighbouring cells having available capacity.
However, by the time congestion occurs, radio conditions in the congested cell will already have degraded to an extent that there is an increased likelihood of dropped calls and reduced performance. Typically, congestion algorithms operate by refusing non-imperative handovers to the congested cell and by attempting to handover currently supported traffic to other cells. However, congestion relief is a last resort as there is a high probability of dropped calls when a relatively large number of handovers are triggered at the same time.
Furthermore, once congestion relief is triggered, the cell is already congested or very near to congestion. Additionally, having a fixed congestion threshold at which the congestion relief is used is inflexible and tends to not provide optimal performance in most conditions. For example, if the congestion threshold is set too high there is a low probability that congestion relief is triggered until the performance is significantly reduced, but if the threshold is set too low, the cell's resources are not fully utilised.
Hence, an improved system for performing handovers to improve resource distribution would be advantageous and in particular a system allowing for improved flexibility and performance for cells approaching congestion 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 performing handover in a cellular communication system comprising the steps of: determining a desired handover probability for a cell in response to a cell load condition; generating a handover decision in response to the desired handover probability; and performing a handover if the handover decision is a decision to perform the handover.
The invention thus allows for handovers to be performed in accordance with a desired handover probability which depends on a cell load condition thereby allowing for a flexible and gradual change in resource distribution dependent on the load conditions. For example, the desired handover probability may change with load conditions such that the bias away from a cell increases with increasing load of that cell. Specifically, the bias away from a cell may increase as the cell becomes increasingly congested. The invention thus allows for a gradual resource distribution in response to the load conditions and may thus reduce the probability of needing a dedicated congestion relief algorithm. The desired handover probability for the cell is preferably the same for all subscriber units but may also be different for, for example, different subscriber units, services or handover candidate cells.
According to a feature of the invention, the step of generating the handover decision comprises a stochastic process that determines the handover decision as a decision to perform a handover with a probability substantially equal to the desired handover probability. For example, a random value may be generated for a given handover candidate and compared to a threshold determined from the desired handover probability. Hence, this allows for a simple process that can be performed individually for each handover candidate yet resulting in a desired proportion of handover candidates being handed over.
According to another feature of the invention, the steps of generating the handover decision and performing the handover is repeated for a plurality of subscriber units such that the proportion of subscriber units handed over corresponds to the desired handover probability. Specifically, the ratio of subscriber units handed over is substantially equal to the desired handover probability. For example, if the desired handover probability is 50%, the steps may be repeated for a plurality of subscriber units resulting in substantially half of these handing over or attempting to handover to other cells. Hence, the invention allows for a simple method for controlling the resource distribution.
According to another feature of the invention, the cell load condition is a load condition of the cell. This allows for the congestion level of the cell to be restrained by the invention. For example, increasing the desired handover probability for handovers to other cells for increasing load of the cell will tend to reduce the congestion of the cell and distribute resource more evenly thereby increasing the communication capacity of the entire communication system.
According to another feature of the invention, the cell load condition comprises a load condition of at least one other cell. This may improve the overall resource distribution as the congestion level of the other cell may be taken into account when determining desired handover probabilities for the current cell.
Specifically, a desired handover probability for a handover from the cell to the other cell may be determined by taking into account the relative congestion levels of both cells. The cell load condition may comprise load conditions from several other cells.
According to another feature of the invention the method further comprises the step of determining the load condition of the at least one other cell in response to a handover rejection indication associated with the at least one other cell.
This provides a suitable parameter for determining a load condition of another 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 load condition may be determined by taking into account the cause of the handover rejections. The handover rejection indication may preferably be determined in response to a number of handovers rejected by the least one other cell in a programmable time period and/or a cause value from a handover reject message of at least one other cell According to another feature of the invention, the cell load condition comprises at least one congestion parameter. This allows for determining a suitable desired handover probability for a given congestion level and thus reduces the probability of undesired congestion conditions occurring or prevailing.
According to another feature of the invention, the desired handover probability and the handover decision relates to a handover to the cell. Hence, handovers to the cell may be determined in accordance with a desired handover probability that depends on cell load conditions. Accordingly cell load conditions and specifically congestion levels may be controlled by influencing the handovers to the cell. Specifically, the number of subscriber units handing over to the cell may be reduced as the congestion levels increase thereby reducing the probability of the cell becoming congested.
According to another feature of the invention, the desired handover probability is a desired probability of rejecting a handover to the cell. This allows for the cell to control its own congestion level by rejecting handovers to the cell. It allows for a simple and localised method of controlling the cell loading.
According to another feature of the invention, the method comprises the step of rejecting a handover to the cell if the handover decision is a decision not to perform a handover. This allows for a suitable means of implementing the invention and effecting the resource distribution.
According to another feature of the invention, the desired handover probability is a decreasing function of the cell load. This allows for an advantageous bias towards a desired resource distribution as the higher the cell load the higher the bias is towards not accepting the handover. The performance of the communication system is improved since the higher the congestion level the less likelihood of the load increasing from new subscriber units handing over to the cell. It thus allows for a gradual and flexible congestion mitigation. The decreasing function may be constant in some intervals.
According to another feature of the invention, the desired handover probability and the handover decision relates to a handover from the cell to another cell.
Specifically, the desired handover probability is a desired probability of handing over to the other cell. Hence, handovers from the cell may be determined in accordance with a desired handover probability that depends on cell load conditions. Accordingly cell load conditions and specifically congestion levels may be controlled by influencing the handovers from the cell to other cells. Specifically, the number of subscriber units handing over from the cell to other cells may be increased as the congestion levels increase thereby reducing the probability of the cell becoming congested.
According to another feature of the invention, the desired handover probability is an increasing function of the cell load. This allows for an advantageous bias towards a desired resource distribution as the higher the cell load the higher the bias is towards performing a handover of a subscriber unit from the cell to another cell. The performance of the communication system is improved as the higher the congestion level the higher the likelihood of the loading of the cell being reduced by handing currently served subscriber units over to other cells.
It thus allows for a gradual and flexible congestion mitigation. The increasing function may be constant in some intervals.
According to another feature of the invention, the desired handover probability is zero for a cell loading below a first threshold, one for the cell loading above a second threshold and gradually increasing for a cell loading between the first and second threshold. This provides for a suitable desired handover probability to be determined by a simple and low complexity method.
Specifically, the thresholds may be any suitable values including values corresponding to no loading or to full congestion of the cell.
According to another feature of the invention, the method further comprises the step of determining the first threshold in response to desired operating conditions in the cell. For example, the first threshold may be set by user definable parameters that will achieve a desired trade off between a reduced cell loading and a decreased performance from preventing advantageous handovers. The invention thus allows for a very flexible and efficient way of controlling the operating conditions of a cell.
According to another feature of the invention, the method further comprises the step of determining the second threshold in response to desired operating conditions in the cell. For example, the second threshold may be set by user definable parameters that will achieve a desired trade off between a reduced cell loading and a decreased performance from preventing advantageous handovers. The invention thus allows for a very flexible and efficient way of controlling the operating conditions of a cell.
According to another feature of the invention, the desired operating conditions comprise a cell load. The invention accordingly allows for a flexible and efficient method of biasing the operating conditions towards a desired cell load.
According to another feature of the invention, the desired operating conditions comprise a desired handover ratio. This may allow for a gradual impact of the congestion relief According to another feature of the invention, the method further comprises the step of determining the first threshold in response to characteristics of a neighbour cell associated with the handover. The impact on the service provided to a user may depend on characteristics of the neighbour cell being handed over to or from, and the first threshold may preferably be determined to reflect this. Hence, different trade offs between the cell load and the service impact of handovers may be independently controlled for different types of neighbour cells by setting the first threshold differently for different cells.
According to another feature of the invention, the method further comprises the step of determining the second threshold in response to characteristics of a neighbour cell associated with the handover. The impact on the service provided to a user may depend on characteristics of the neighbour cell being handed over to or from, and the first threshold may preferably be determined to reflect this. Hence, different trade offs between the cell load and the service impact of handovers may be independently controlled for different types of neighbour cells by setting the second threshold differently for different cells.
According to another feature of the invention, the step of determining a desired handover probability is only based on a local cell load condition.
Preferably, no information is received from other cells and the method makes use only of information generated in the cell. Hence, this allows for a simple implementation of the method. Preferably, the method is operated independently by a plurality of cells. Accordingly, an improved resource distribution over a plurality of cells may be achieved without requiring complex or centralised processing or information exchange between cells.
According to another feature of the invention, the desired handover probability is further determined in response to a handovor characteristic. Specifically, the handover characteristic may be a cause of a handover request or a handover rejection. This allows for improved accuracy in determining a suitable desired handover probability to suit the current conditions.
According to another feature of the invention, the desired handover probability is determined in response to a service characteristic of a communication associated with the handover. This allows for a suitable desired handover probability to be determined taking into account the different impact that a handover may have on different services. The service characteristic may for example be a priority characteristic or a quality of service characteristic.
The communication system is preferably a GSM communication system or a 3rd Generation cellular communication system such as a UMTS communication system.
According to a second aspect of the invention, there is provided an apparatus for performing handover in a cellular communication system comprising: means for determining a desired handover probability for a cell in response to a cell load condition; means for generating a handover decision in response to the desired handover probability; and means for performing a handover if the handover decision is a decision to perform the handover.
According to a third aspect of the invention, there is provided a cellular communication system comprising: means for determining a desired handover probability for a cell in response to a cell load condition; means for generating a handover decision in response to the desired handover probability; and means for performing a handover if the handover decision is a decision to perform the handover.
These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described 1 5 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 illustrates part of a cellular communication system in which an embodiment of the invention may be employed; FIG. 3 illustrates a flowchart of a method of performing handovers in accordance with an embodiment of the invention; and FIG. 4 illustrates a graph showing a desired handover probability for handovers from a cell as a function of the relative cell load of the cell 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 but 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 3rd Generation communication systems such as UMTS.
FIG. 2 illustrates part of a cellular communication system in which an embodiment of the invention may be employed. FIG. 2 illustrates a first cell 201 having a base station 203 serving a number of subscriber units of which four subscriber units are shown 205,207,209,211. In addition, FIG. 2 shows two neighbour cells 213,215 of the first cell 201. The first ncighbour cell 213 comprises a base station 217 supporting a plurality of subscriber units of which one is shown 219. The second neighbour cell 215 comprises a base station 221 supporting a plurality of subscriber units (not shown).
In the example shown, the loading of the first cell 201 is substantially higher than the neighbour cells 213,215. In the specific example, the first cell is not yet congested but has a high probability of becoming congested in the near future. Accordingly, it would be advantageous to attempt to distribute the subscriber units more evenly between the different cells thereby reducing the probability of the first cell being congested. However, as the first cell 201 is not congested it is not necessary or desired to enter a congestion relief mode wherein it is attempted to handover as many subscriber units as possible.
In accordance with an embodiment of the invention, a desired handover probability is determined for the first cell 201 in response to a cell load condition. As a simple example, the desired handover probability may be set to nil when the cell loading is below a given level, 50% when it is between a first and a second level and 100% when it is above the second level corresponding to congestion. In the example of FIG. 2, the load of the first cell 201 is such that the desired handover probability is determined as 50%.
In accordance with this embodiment, a subscriber unit is identified as a potential handover candidate. In the specific example of fig. 2, the received measurement reports of subscriber unit 211 indicate that this could potentially be served by the second neighbour cell 215. Accordingly, a handover decision is generated for the subscriber unit 211 in response to the desired handover probability. In the described simple embodiment, the handover decision is generated by a stochastic process that determines the handover decision as a decision to perform a handover with a probability substantially equal to the desired handover probability. In a simple embodiment, a random binary value is generated corresponding to the 50% desired handover probability. If the handover decision is to perform a handover, a handover to the second neighbour is instigated for the subscriber unit 211. If the decision is not to perform a handover, no handover is instigated.
The process is repeated for all subscriber units of the first coil that may potentially be handed over to a neighbour cell. Accordingly a number of subscriber units arc handed over to other cells thereby reducing the loading of the first cell 201 and thus reducing the probability of this reaching congestion.
Furthermore, a gradual congestions mitigation or load reduction or load distribution can be achieved. The extent of the load control can be closely controlled by determining suitable desired handover probabilities.
Furthermore, as the desired handover probability depends on the load conditions, it allows for the level of congestion mitigation to automatically be adjusted according to the congestion level. Additionally, a flexible and efficient load control for one or more cells can be implemented by simple handover decisions which are made individually for each subscriber unit.
In the preferred embodiment, the desired handover probability is determined by the base station 203 of the first cell 201 based only on information derived by this cell. Specifically, only the local cell load condition is taken into account when determining the desired handover probability. Thus, in a simple embodiment, the ratio of available resource units of a cell (e.g. time slots for a GSM communication system) that are currently utilised is determined and a corresponding desired handover probability is generated. Thereby, a gradual congestion relief and improved resource distribution over a plurality of cells is achieved from a local process. Preferably, the process is independently operated in the neighbour cells thereby providing a dynamic and automatic resource distribution across the cells depending on the current relative load conditions in the cells.
In the preferred embodiment, the described approach is not only used to control handovers from the first cell 201 to neighbour cells 213, 215 but also to control handovers from the neighbour cells 213, 215 to the first cell 201.
Hence, the desired handover probability and the handover decision may relate to a handover to the cell. In the example of FIG. 2, the subscriber unit 219 is currently served by the first neighbour cell 213 but the measurement reports indicate that a handover to the first cell 201 may be advantageous.
Accordingly a handover attempt is instigated which results in a handoverrequest being communicated to the first cell. In accordance with the embodiment, the first cell 201 proceeds to determine a desired handover probability in response to a cell load condition and to determine a handover decision for the subscriber unit 219. In the simple example, the desired handover probability may be equal to the desired handover probability for handing over from the first cell. Thus, in the specific example a random binary value may be generated for determining a handover decision. If the handover decision is to perform a handover, the request is accepted but if the handover decision is not to perform a handover, the handover request is rejected.
FIG. 3 illustrates a flowchart of a method of performing handovers in accordance with an embodiment of the invention. The method is applicable to the cellular communication system of FIG. 2 and will be described with reference to this. The method will be described with specific reference to handovers from the first cell to a neighbour cell but it will be apparent that the principles described are equally applicable to handovers from the neighbouring cells to the first cell.
In step 301, a cell load condition is determined. 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. In the described example, 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 which are currently allocated. Hence, in this example, the cell load condition is relative cell loading between 0 and 100%. The higher the relative cell loading, the closer the first coil is to being congested. It will be apparent to the person skilled in the art, that the cell may be considered to be congested when the relative load of the cell is above a given threshold that may be lower than the full load of a 100%.
Step 301 is followed by step 303 wherein a desired handover probability is determined. In the described embodiment, the same desired handover probability is used for all subscriber units but in other embodiments, individual desired handover probabilities may be determined for e.g. each subscriber unit, neighbour cell, service or any combination of some or all of these.
In the preferred embodiment, the desired handover probability for handovers to the first cell is a decreasing function of the relative cell load and the desired handover probability for handovers from the first cell is an increasing function of the relative cell load.
FIG. 4 illustrates a graph showing a desired handover probability for handovers from the first cell as a function of the relative cell load of the first cell.
In accordance with the example of FIG. 4, the desired handover probability is zero for relative loads below a first threshold "low_congest_thr". Accordingly, when the relative load is below the first threshold, no handovers are performed by the process of FIG. 3, and only the conventional handover mechanisms cause handovers to be performed.
In accordance with the example of FIG. 4, the desired handover probability is 100% for relative loads above a second threshold "high_congest_thr".
Accordingly, when the relative load is above the second threshold, it is attempted to handover all possible subscriber units to other cells.
In the example of FIG. 4, the desired handover probability increases linearly between zero and 100% for relative loads between the first and second threshold. Hence, the probability of handing over subscriber units to other cells, and thus the degree of congestion relief introduced, gradually increases for increasing relative loads.
It will be appreciated that similar functions may be used for determining desired handover probabilities for handovers to the first cell. Specifically, a probability of rejecting a handover may be identical to the probability shown in FIG. 4, such that the probability of rejecting a handover for congestion relief is zero below the first threshold, 100% above the second threshold and linearly increasing between the first and second threshold. In this case, the desired handover probability may be taken as the reciprocal of the probability of rejecting a handover, and the desired handover probability for handovers to the first cell may thus be 100% for relative loads below the first threshold, zero above the second threshold and linearly decreasing between the first and second threshold.
In some cases, it may be advantageous to jump to a non-zero probability of handovers at the first threshold, and the function for determining the desired handover probability as a function of the relative load may specifically comprise steps in the desired handover probability.
It will be appreciated that any suitable algorithm or function for determining a desired handover probability as a function of a cell load may be used without detracting from the invention.
Step 303 is followed by step 305 wherein a subscriber unit that may be a candidate for a handover is selected. Specifically, a subscriber unit may be selected if the associated measurement reports indicate that it may successfully be supported by a neighbour cell.
Step 305 is followed by step 307 wherein a handover decision is generated for the candidate subscriber unit in response to the desired handover probability.
In the preferred embodiment, a simple random value is generated and compared to the desired handover probability in order to determine if the handover should be performed. As a specific example, a random number between 1 and 100 may be generated. If the random number is lower than or equal to the desired handover probability (expressed in percentage), the handover decision is to perform a handover of the candidate subscriber unit to another cell.
Step 307 is followed by step 309 wherein it is determined if the handover decision is to perform a handover or not. If the decision is not to perform a handover, the method continues in step 313. If the decision is to perform a handover, the method continues in step 311 wherein the handover of the candidate subscriber unit to another cell is performed. Step 311 is followed by step 313.
In step 313, it is determined if there are any more subscriber units that may be handed over to other cells. If so, the method returns to step 305 wherein a new candidate subscriber unit is selected. In other embodiments, the method may return to step 301 or 303 and determine a new desired handover probability appropriate for the new candidate subscriber unit.
If no more candidate subscriber units are found, the method stops following step 313. In the preferred embodiment, the method is repeated at regular intervals.
In the above described embodiment, the desired handover probability was determined based only on the load conditions of the first cell. This allows for a local operation of the process without requiring information to be obtained from other elements of the communication system. However, it will be clear that many other parameters may additionally or alternatively be included in the determination of the desired handover probability.
In one embodiment, the cell load condition comprises a load condition of at least one other cell. Preferably, the load condition of the neighbouring cells are determined and provided to the method. These load conditions are then used in the determination of a suitable desired handover probability. For example the desired handover probability may be reduced for increasing relative loads of the neighbouring cell or cells.
One method for determining such a load condition of another cell 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 reduce the desired handover probability such that fewer handovers are made to the neighbour cell. Specifically, the desired handover probability for handovers to other cells may be reduced and the desired handover probability for handovers from neighbour cells increased for an increasing number or ratio of handovers being rejected by neighbour cells.
In cellular communication systems, such as GSM and UMTS, the handover rejections furthermore comprise information related to the cause of the rejections. In some embodiments, these parameters may be included in the determination of the desired handover probability. For example, only handover rejections caused by capacity considerations may be included in the determination of the desired handover probability cell, and handover rejections caused by deteriorating radio conditions may be ignored in the determination.
The desired handover probability may further be determined in response to a handover characteristic. The handover characteristic may for example relate to a characteristic of other handovers that have been performed for the cell or to handovers which have been rejected.
Specifically, the GSM cellular communication system comprise information related to the cause for a handover request (as specified in the HANDOVER_REQUEST cause value defined in the GSM specifications) or, as mentioned previously, to the cause of a handover rejection (as specified in the HANDOVER_REQUEST_REJECT cause value defined in the GSM specifications). Hence, a more accurate and suitable desired handover probability may be determined by taking these factors into account. The person skilled in the art will realise that the exact relationship between these parameters and a suitable desired handover probability is a design parameter that may be determined in accordance with the preferences and desired performance for the specific embodiment.
The desired handover probability may also be dependent on a handover characteristic of the handover being considered. For example, the necessity of a handover may be taken into consideration when determining the desired handover probability. Specifically, GSM specifies some handovers as imperative and some handovers as non-imperative with imperative handovers being necessary to support the call whereas nonimperative handovers are desired but not essential for supporting the call. In GSM, the desired handover probability for handovers to the cell may thus be set to 100% for all imperative handovers and in accordance with the previously described approach for non- imperative handover. This will allow all imperative handovers to be performed whereas a desired ratio of non-imperative handovers is rejected. This is equivalent to only operating the described method for non- imperative handovers.
Alternatively or additionally, the desired handover probability may also be determined in response to a service characteristic of a communication associated with the handover.
The impact and effect of a handover may depend on the service, which is provided by the communication being handed over. For example, handing over a high data rate may result in a significant reduction in the load of the first cell, and therefore the desired handover probability may be set higher for high data rate services than for low data rate services. As another example, some cells may not be able to support some services, and therefore a handover may result in the user being provided with a service that does not meet his desires.
In this case the desired handover probability may be reduced for communications that will result in a reduced service level. In other embodiments, the desired handover probability may depend on the quality of service requirements or a priority of the service.
In the preferred embodiment, the desired handover probability for a handover from the first cell is zero for a cell loading below a first threshold, one for the cell loading above a second threshold and gradually increasing for a cell loading between the first and second threshold as described previously and illustrated in FIG. 4. Similarly, in the preferred embodiment the desired handover probability for a handover to the first cell is one for a cell loading below a third threshold, zero for the cell loading above a fourth threshold and gradually decreasing for a cell loading between the third and fourth threshold.
The first and third and/or second and fourth threshold may for example be identical.
Suitable values for the thresholds may be determined in any suitable way but are preferably set so as to provide the desired operating conditions for the cell.
Specifically, a desired cell loading, congestion margin and/or handover ratio may be used in setting the thresholds.
If the thresholds are set too low, performance may be impaired due to advantageous handovers not being performed whereas setting the thresholds too high may result in insufficient congestion relief. The exact value will depend on the exact embodiment and typical operating conditions, and may be set by a person skilled in the art based on e.g. simulation or a trial and error approach.
Preferably, the thresholds are additionally set in response to characteristics of the neighbouring cells to which the subscriber units may be handing over to or from.
For example, the thresholds may be set dependent on whether the neighbour cell is connected to the same Base Station Controller or Packet Control Unit, a different one, or is part of a different cellular communication system (for example a 3rd Generation or wireless LAN system) as these values may affect the degree of service degradation that may arise from the handover.
As another example, the thresholds may be set dependent on the neighbour cell threshold parameter settings, e.g. whether these are the same or are different. The former may be the case when the first cell and the neighbour cell are similar cells whereas the latter will tend to be the case if the cell is of a quantifiable different type, such as a macro cell, micro cell, in-building cell, hot-spot cell or dedicated data service cell etc. It will be appreciated that other handover algorithms and mechanisms typically operate in parallel to the described method. Specifically, the conventional handover algorithms used in cellular communication systems may be operated in parallel to the current method.
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. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.
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 (30)
1. A method of performing handover in a cellular communication system comprising the steps of: determining a desired handover probability for a cell in response to a cell load condition; generating a handover decision in response to the desired handover probability; and performing a handover if the handover decision is a decision to perform the handover.
2. A method as claimed in claim 1 wherein the step of generating the handover decision comprises a stochastic process that determines the handover decision as a decision to perform a handover with a probability substantially equal to the desired handover probability.
3. A method as claimed in claim 1 or 2 wherein the steps of generating the handover decision and performing the handover is repeated for a plurality of subscriber units such that the proportion of subscriber units handed over corresponds to the desired handover probability.
4. A method as claimed in any previous claim wherein the cell load condition is a load condition of the cell.
5. A method as claimed in any previous claim wherein the cell load condition comprises a load condition of at least one other cell.
6. A method as claimed in claim 5 further comprising the step of determining the load condition of the at least one other cell in response to a handover rejection indication associated with the at least one other cell.
7. A method as claimed in any previous claim wherein the cell load condition comprises at least one congestion parameter.
8. A method as claimed in any previous claim wherein the desired handover probability and the handover decision relates to a handover to the cell.
9. A method as claimed in claim 8 wherein the desired handover probability is a desired probability of rejecting a handover to the cell.
10. A method as claimed in claim 9 further comprising the step of rejecting a handover to the cell if the handover decision is a decision not to perform a handover.
11. A method as claimed in any previous claim 8 to 10 wherein the desired handover probability is a decreasing function of the cell load
12. A method as claimed in any previous claim 1 to 7 wherein the desired handover probability and the handover decision relates to a handover from the cell to another cell.
13 A method as claimed in claim 12 wherein the desired handover probability is a desired probability of handing over to the other cell.
14. A method as claimed in claim 13 and 13 wherein as any previous wherein the desired handover probability is an increasing function of the cell load.
15. A method as claimed in any of the claims 12 to 14 wherein the desired handover probability is zero for a cell loading below a first threshold, one for the cell loading above a second threshold and gradually increasing for a cell loading between the first and second threshold.
16. A method as claimed in claim 15 further comprising the step of determining the first threshold in response to desired operating conditions in the cell.
17. A method as claimed in any of the claims 16 or 17 further comprising the step of determining the second threshold in response to desired operating conditions in the cell.
18. A method as claimed in claim 16 or 17 wherein the desired operating conditions comprise a cell load.
19. A method as claimed in any of the claims 15 to 18 wherein the desired operating conditions comprise a desired handover ratio.
20. A method as claimed in any of the claims 15 to 19 further comprising the step of determining the first threshold in response to characteristics of a neighbour cell associated with the handover.
21. A method as claimed in any of the claims 15 to 20 further comprising the step of determining the second threshold in response to characteristics of a neighbour cell associated with the handover.
22. A method as claimed in any previous claim wherein the step of determining a desired handover probability is only based on a local cell load condition.
23. A method as claimed in any previous claim wherein the desired handover probability is further determined in response to a handover characteristic.
24. A method as claimed in any previous claim wherein the desired handover probability is determined in response to a service characteristic of a communication associated with the handover.
25. A method as claimed in any of the previous claims wherein the communication system is a GSM communication system.
26. A method as claimed in any of the previous claims 1 to 24 wherein the communication system is a UMTS communication system.
27. A computer program enabling the carrying out of a method according to any of the previous claims.
28. A record carrier comprising a computer program as claimed in claim 26.
29. An apparatus for performing handover in a cellular communication system comprising: means for determining a desired handover probability for a cell in response to a cell load condition; means for generating a handover decision in response to the desired handover probability; and means for performing a handover if the handover decision is a decision to perform the handover.
30. A cellular communication system comprising: means for determining a desired handover probability for a cell in response to a cell load condition; means for generating a handover decision in response to the desired handover probability; and means for performing a handover if the handover decision is a decision to perform the handover.
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US8971293B2 (en) | 2005-03-11 | 2015-03-03 | Interdigital Technology Corporation | Method and system for station location based neighbor determination and handover probability estimation |
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GB2397468B (en) | 2006-03-15 |
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