WO2009002241A1

WO2009002241A1 - Adaptive handover in a cellular wireless access network

Info

Publication number: WO2009002241A1
Application number: PCT/SE2007/050465
Authority: WO
Inventors: Magnus Gannholm; Magnus Johansson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-06-25
Filing date: 2007-06-25
Publication date: 2008-12-31

Abstract

The invention discloses a method (300) for use in a cellular network (100) with a plurality of cells (110, 120, 130) in which there can be Mobile Stations, MS (132), with one base station, BS (111, 121, 131 ), per cell. In the network, an MS can be handed over from a first BS (131 ) of a first cell (130) to a second BS (121 ) of a second cell (120) if certain criteria are fulfilled. The method (300) comprises measuring (310) the traffic load in at least said first and second cells, and calculating (320) the difference between the load in the first cell and the load in the second cell. The method also comprises lowering (340) the criteria for handover from the first BS to the second BS if the load difference between the first cell and the second cell exceeds (330) a first threshold value (T1 ).

Description

TITLE

Adaptive handover in a cellular wireless access network.

TECHNICAL FIELD The present invention discloses a method and a device for adaptive handover in a cellular wireless access network.

BACKGROUND

Cellular wireless access networks such as, for example, GSM, WCDMA and Mobile WiMAX networks, comprise a number of Base Stations, BSs, with each BS covering a geographical area, a so called cell. In each cell, there can be a number of mobile terminals or stations, MSs, and the traffic to and from an MS in a cell is routed via the BS of that cell.

The MSs in the network can move between the cells, and there will be so called "handover" made of an MS from the BS of one cell to the BS of another cell as the MS moves between those cells.

An MS which is in the so called active state can have a number of services ongoing, such as, for example, voice calls, video streaming or best effort file download or upload. When there is a handover of the MS between BSs, the end user, i.e. the user of the MS, expects all ongoing services to be maintained, which is known as session continuity.

In order to maintain session continuity during handover, the cells of the network must have a certain degree of overlap, and that handover will be performed when an MS is in an area covered by both the serving cell/BS and the target cell/BS.

Since most cells in the network will have a geographical overlap with one or more neighbouring cells, there will be a number of MSs, either fixed or mobile, which are located in areas where two or more cells overlap. If or when the utilization rate of the network in question is high, it would be advantageous to enable traffic load sharing between cells, in the sense that MSs located in "overlap areas" could be handled by the BS with the best capacity for doing so, i.e. usually the BS which has the lowest traffic load. In this way, a BS with a high traffic load will be aided by neighbouring BSs, and will be better able to provide a high quality of ongoing services for its remaining MSs.

In some systems, handover of an MS from one BS to another is initiated by the network, while, for example, in the WiMAX system, handovers are normally initiated by the MSs. However, even in WiMAX systems, the network controls handover decisions to a certain degree, by letting the BSs broadcast so called "triggers", which are handover criteria, to their respective MSs.

European Patent EP 981 919 describes a handover method for so called "cell breathing" based on dynamically adjusted handover thresholds. However, the method proposed by that document seems to have a drawback in that a so called "base station controller" is needed to perform the method, the drawback being that such a component may not be present in all systems.

In addition, the method proposed by EP 981 919 seems to have an additional drawback in that the handover method proposed may result in dropped calls or a reduced quality of service subsequent to a handover, since handovers may be made to base stations which in fact have a traffic load which is at least as high as the base station from which the handover is made.

SUMMARY As has been shown above, there is thus a need for a way of making handovers between base stations in a cellular wireless access network in such a way that the traffic load of the base stations involved is taken into account, and preferably also so that the traffic load is distributed between the base stations in a better way than that offered by present methods.

This need is addressed by the present invention in that it discloses a method for use in a cellular wireless access network in which there is a plurality of cells, in each of which there can be a number of Mobile Stations, MSs, with one base station, BS, per cell.

In the system in which the invention may be applied, the traffic to and from the MSs in a cell is routed via the BS of that cell, and an MS can be handed over from a first BS of a first cell to a second BS of a second cell.

A handover is carried out if certain criteria are fulfilled, and according to the method of the invention, the traffic load in at least the first and second cells is measured, and the difference between the load in the first cell and the load in the second cell is calculated.

According to the method of the invention, the criteria for handover from the first BS to the second BS are lowered if the load difference between the first cell and the second cell exceeds a first threshold value, so that handovers from the first BS to the second BS will occur more easily than before the lowering of the criteria.

Thus, according to the invention, if the first cell has a high traffic load and the second cell has a low traffic load, handovers may be directed from the BS of the first cell to the BS of the second cell, while, if both cells are, for example, equally busy, handovers will not be directed from the first cell to the second cell.

Suitably, the criteria for handover from the second BS to the first BS may be made more restrictive if the load difference between the first and the second cell exceeds a second threshold value, so that if the first BS "hands over" MSs to the second BS, the first BS cell will not have those or other MSs handed back to it by the second cell.

The first and second threshold values may be different from each other, or they may be essentially equal to each other.

Thus, handovers may be directed from the BS of one cell to the BS of another cell based on the difference in traffic load between the cells, which means that the load can be shared between cells in a better way than previously. This and other advantages of the present invention will become even clearer from the following detailed description.

The invention also discloses a Base Station which essentially functions according to the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows an example of a system in which the invention may be applied, and

Fig 2 shows an example of an application of the invention, and Fig 3 shows a flowchart of a method of the invention, and Fig 4 shows a schematic block diagram of a base station of the invention, and

Fig 5 shows an alternative feature of the invention.

DETAILED DESCRIPTION

Fig 1 shows an overview of a part of a system 100 in which the invention may be applied. It should be pointed out that although the invention will be described in the following using system terminology which is mainly taken from the WiMAX standard, this is by way of example only, in order to facilitate the reader's understanding of the invention. The invention can be applied to a variety of different wireless cellular access systems, although in some systems a slightly different terminology may be used for the components in the system.

Thus, for example, the Base Stations in the system which will be described below correspond to what is referred to as NodeB or eNodeB in some systems, and as Base Transceiver Stations in other systems. However, the person skilled in the field, reading this text, will recognize which system components of the system of the examples below that correspond to similar system components of other systems or standards.

Returning now to the system 100 of fig 1 , the system is a cellular wireless access network with a plurality of cells, three of which, 110, 120, 130, are shown in fig 1. In each cell there can be a number of Mobile Stations, MSs, and as an example of this, fig 1 shows one MS 132 in cell 130.

In addition, the system 100 comprises a number of Base Stations, BSs, one for each of the three cells 110, 120, 130, the corresponding base stations of these cells being shown as 111 , 121 and 131 in fig 1. The function of the BSs varies between different standards, but in most systems the traffic to and from an MS in a cell is routed via the BS of that cell.

As shown in fig 1 , the areas covered by the cells of the system overlap to some extent. As is well known to those skilled in the art, an MS can be

"handed over" from one cell to another cell, meaning that the BS through which the traffic of an MS is routed will be changed. A hand over is performed if certain hand over criteria are fulfilled, such as, for example, if the signal strength received by an MS from a "new" BS exceeds the strength of the signal received by its present BS by a certain amount, and does so for a certain amount of time. A hand over may be initiated by different nodes in the system, depending on the system standard. In the WiMAX standard, a handover is normally initiated by the MS, with the network having a degree of control over hand over decisions by broadcasting so called "triggers" to the MSs as part of the system information.

WiMAX triggers will typically have the following format:

• If the RSSI (Received Signal Strength Indicator) in the serving cell/BS is less than X dBm: start measuring the CINR (Carrier to Interference Noise Ratio) of neighbour cells/BSs.

• If the CINR of a neighbour cell/BS is more than Y dB higher than the CINR in the serving cell/BS: initiate handover.

The triggers are typically provided as configuration parameters, based on network planning.

As shown in fig 1 , and as mentioned previously, most cells in the system 100 will have some degree of overlap with other cells in the system. Thus, if one imagines a situation in which there is a high utilization rate of the network/system 100, there may still be a large difference in the traffic load between overlapping cells. In such a situation, it would be advantageous to find a solution by means of which the traffic load could be distributed between overlapping cells, since this would enable even the cell with the highest traffic load to offer a higher QoS (Quality of Service) to the MS in that cell.

A solution to this is offered by the present invention in the following manner: the traffic load is measured in a number of cells in the system, two of which will be used as an example, these two cells being exemplified, with renewed reference to fig 1, by cell 130 as the first cell and cell 120 as the second cell. Thus, the traffic load is measured in a first 130 and a second 120 cell, and the difference between the traffic load in the first cell 130 and the load in the second cell 120 is then calculated. If the load difference between the first cell 130 and the second cell 120 exceeds a certain first threshold value, the criteria for handover from the first cell 130 to the second cell 120, or more exactly, from the BS 131 of the first cell to the BS 121 of the second cell are lowered, which means that handovers from the first BS 131 to the second BS 121 will occur more easily than before the lowering of the criteria.

Thus, if the first cell 130 has a traffic load which is more than a certain threshold above that of the second cell 120, handovers can be directed from the BS 131 of the first cell to the BS 121 of the second cell. This will ensure not only that a busy BS 131 can direct MSs away from itself, but also that a busy BS 131 can direct MSs towards a BS 121 which is less busy.

The varying of said criteria is suitably carried out by letting a BS transmit new or updated criteria to the MSs in its cell.

The traffic load in a cell can be measured in a variety of ways which are well known to those skilled in the art, so the exact nature of the measurements will not be describe in depth here, but some examples of parameters which may serve as indicators of traffic load are:

• The air interface utilization rate, usually expressed as a percentage between 0% and 100%.

• The traffic transport quality, usually expressed as an integer from 0 to 10.

• The number of active MSs in a cell.

• The transport network utilization rate and/or congestion level. In a particularly preferred embodiment, in order to ensure that a busy BS 131 does not receive a stream of MSs from a less busy BS 121 , the criteria for handover from the second BS 121 to the first BS 131 can be made more restrictive if the load difference between the first 130 and the second 120 cell exceeds a second threshold value. This second threshold value, which is used to decide if handovers should be held back from the second cell 120 to the first cell 130 may be the same as the first threshold mentioned above, or it may differ, depending on the effect which it is desired to obtain, but in a preferred embodiment, the two thresholds are essentially the same.

The measurement of the traffic load in the first and second cells, and the calculation of the difference in traffic load between the cells can be carried out in a number of ways, as will be realized by those skilled in the art. In a preferred embodiment, however, each of the BSs 131 , 121 , measures the traffic load in its cell, and then exchanges this data with the other BS, following which each BS calculates the difference, and sets the appropriate handover criteria according to the thresholds.

The exchange of measurement data between the BSs can be done via a third node in the system, such as, in a WiMAX system, the so called ASN, Access Service Network, Gateway. As an alternative, the exchange can be done directly between the BSs, if such a connection is allowed by the system standard.

Alternatives to this include, for example, letting the ASN Gateway or another third node in the system receive the measurement reports from the BSs, and then calculate the difference, following which the difference is communicated from the third node to the BSs, which then set the appropriate handover criteria according to the thresholds. The thresholds may be programmed in the BSs upon system installation, as an alternative to which they can be communicated to the BSs from the network via, for example, the ASN Gateway adaptively during the operation of the network.

Above, the terms "lowering the criteria" and "making the criteria more restrictive" have been used. In a preferred embodiment, there is a set number of criteria, so "lowering" or "making more restrictive" should be interpreted as moving between these criteria. As an alternative, the criteria could be changed in any increment between a maximum and a minimum value, although this is not envisioned at present.

In order to attempt to always achieve optimal hand over criteria, the traffic load measurements in the cells in the network may be made not only once, but at specified intervals, typically tens of seconds, so that the hand over criteria may be adapted to the present traffic situation in the two cells.

In order to achieve a "hysteresis effect", in a certain embodiment of the invention, if the criteria for handover from the first BS 131 to the second BS 121 have been lowered as the result of a traffic load measurement, and the traffic load difference between the first 130 and the second 120 cells in a subsequent measurement is equal to or below a third threshold value, the criteria for handover from the first BS 131 to the second BS 121 are made more restrictive, so that handovers from the first BS 131 to the second BS 121 will occur less easily than before the increase.

Thus, there is a threshold for "reversing" the decision to lower the criteria, which suitably differs from the threshold for the lowering as such. This will be explained in more detail in the following.

In order to further make it possible to keep the handover criteria updated, if the criteria for handover from the second BS 121 to the first BS 131 have been made more restrictive as the result of a traffic load measurement, and the load difference between the first 130 and the second 120 cell in a subsequent measurement is equal to or below a fourth threshold value, the criteria for handover from the second BS 121 to the first BS 131 are made less restrictive, so that handovers from the second BS 121 to the first BS 131 can occur more easily. Again, it can be seen that the threshold for "reversing" the decision to make the criteria more restrictive differs from the threshold for making the criteria more restrictive as such. This will also be explained in more detail in the following. It can be mentioned that the third and fourth thresholds can be made essentially equal.

Turning now to fig 2, there is shown a graph, as a function of time, of the difference, "Δ Load", in traffic load between the first 130 and the second 120 cells of fig 1 , these cells merely being mentioned as an example. The use of the different criteria and the thresholds for changing the handover criteria will be explained with reference to fig 2.

It can be mentioned that in the example used in fig 2, the first threshold for making handovers from the first BS to the second BS occur more easily is set to be the same as the second threshold for making handover from the second BS to the first BS more restrictive. Thus, T1 and T2 are shown next to each other in fig 2.

Initially, since the load difference is below T1 and T2, a "normal" criteria level C1 is used for making handovers from the first BS 131 to the second BS 121 , as well as for making handovers in the opposite direction, i.e. from the second BS 121 to the first BS 131.

However, as the load difference increases, at a point in time ti it exceeds the thresholds T1 , T2, and then a second criteria level C2 is used for making handover to/from the first and the second BS. This criteria level C2 makes it easier for MSs to initiate handover from the first BS to the second BS, while being more restrictive in the other direction, i.e. handovers from the second BS to the first BS.

As has been mentioned, a third and a fourth threshold are used in order to "reverse" the decision to go to criteria level C2, the third and fourth thresholds preferably being different from the first and second thresholds, in order to obtain a hysteresis effect. However, as has been mentioned, it is perfectly possible to let the third and fourth thresholds be essentially equal to each other, which is shown in fig 2, T3 and T4 being shown next to each other.

Thus, at a subsequent point in time t₂, the load difference falls below the third and fourth thresholds T3, T4, and the "normal" criteria level C1 is applied again. At a subsequent point in time t₃, the load difference exceeds T1 and T2 again, and thus the criteria level C2 is applied, in order to direct traffic from the first BS 131 to the second BS 121 , while decreasing the amount of handovers in the opposite direction.

Also shown in fig 2 is a feature of one embodiment of the invention which has not been mentioned previously: At a certain point in time U, the load difference exceeds not only the thresholds T1 , T2, but in fact rises beyond a level defined by a fifth threshold T5. The threshold T5 is used to illustrate the fact that additional threshold can be introduced according to the invention.

Thus, when the load difference passes the threshold T5, a third criteria level C3 is used, as is also indicated in fig 2. The third criteria level C3 implies a further lowering of the criteria for handovers from the first BS to the second BS, as compared to C2 and C1 , while making it even more difficult to perform handovers in the opposite direction as compared to C1 and C2.

In order to maintain the hysteresis effect, a sixth threshold T6 is also used in conjunction with T5, for leaving the criteria level C3 and instead going to C2. This occurs when the load difference falls below T6 at a point in time ts. At a subsequent point in time tβ, the load difference has also fallen below T3, T4, so the criteria level is changed to C1.

Fig 3 shows a schematic flow chart 300 of some steps of a method according to the invention. Steps which are options or alternatives are shown with dashed lines.

As shown in fig 3, the method comprises the step 310 of measuring the traffic load in at least the first 130 and second 120 cells shown in fig 1 , and the step

320 of calculating the difference, "Δ", between those loads. If the load difference "Δ" between the first cell and the second cell exceeds a first threshold value T1 , the comparison between Δ and T1 being shown in step

330, the criteria for handover from the BS 131 of the first cell 130 to the BS 121 of the second cell 120 are lowered, as shown in step 340, so that handovers from the first BS to the second BS will occur more easily than before the lowering of the criteria.

As shown in step 350, the criteria for handover in the "reverse" direction, i.e. from the second BS 121 to the first BS 131 may, in a particular embodiment of the invention, be made more restrictive if the load difference between the first and the second cell exceeds a second threshold value.

The second threshold value T2 may be essentially equal to the first threshold value T1 , as shown in step 360.

As shown in step 370, the traffic load measurements may be "renewed", i.e. they may be made at specified intervals, and the handover criteria may be "updated" according to the new measurement results, so that, for example, if the criteria for handover from the first BS to the second BS have been lowered as the result of a traffic load measurement, and the traffic load difference between the first and the second cells then, in a subsequent measurement, is equal to or below a third threshold value, the criteria for handover from the first BS to the second BS will be made more restrictive, so that handovers from the first BS to the second BS will occur less easily than before the increase.

In addition, if the criteria for handover from the second BS to the first BS have been made more restrictive as the result of a traffic load measurement, and the load difference between the first and the second cell in a subsequent measurement is equal to or below a fourth threshold value, the criteria for handover from the second BS to the first BS may be made less restrictive, so that handovers from the second BS to the first BS can occur more easily.

As shown in step 380, the third and the fourth thresholds, T3 and T4, may be essentially equal to each other.

Fig 4 is a block diagram of a Base Station 400 of the invention. As indicated in fig 4, the Base Station 400 comprises an antenna 410, which can be seen as a generic symbol for an antenna which can serve either as a receive or a transmit antenna, or as one integrated transmit/receive antenna.

The Base Station 400 also comprises a receive part, Rx, 420, and a transmit part, 430, Tx, both of which are connected to the antenna 410, as well as comprising a computer 440 such as a microprocessor and memory means 450. If the Base Station communicates with other units, such as "higher" nodes in the system via other means than radio, the Base Station will comprise an interface for such communication, which is indicated in box 460.

As indicated in fig 4, this interface is suitably controlled by the computer 440, and the traffic which is exchanged via the interface 460 is also suitably routed via the computer 440. Thus, in the Base Station 400, all of the components mentioned in the preceding paragraph, with the possible exception of the interface 460, may be used for routing the traffic to and from the MSs in a first cell in the network in which the Base Station is used. The computer 440 may be used for measuring the traffic load in the first cell and the computer 440 may also, together with the transmitter 430 and the antenna 410, be used for lowering the criteria for handover to the second Base Station, i.e. these components may be used to make the necessary calculations and to then transmit the updated criteria to the MSs in the cell.

In a particular embodiment, the Base Station 400 may also use the interface 460 or the antenna 410 together with the receive part 420 to receive information about the traffic load in the second cell from the second Base Station, and the computer 440 may then be used to calculate the difference between the load in the first cell and the load in the second cell.

In another embodiment, the antenna/interface 410 and the transmitter 430 may be used for transmitting the results of the Base Stations traffic load measurements to another node in the system, such as an ASN Gateway, and the antenna/interface 410 and the receive part 420 can then be used to receive criteria for handover to the second Base Station from said node.

If the traffic load measurements in the first cell are to be carried out at specified intervals, this may be done with the aid of the antenna 410 and/or the interface 460, the receiver 420 and the computer 440, following which the computer 440, and suitably also the transmitter 430 and the memory means 450, are used for making the criteria for handover to the second Base Station more restrictive if the criteria for handover to the second Base Station have been lowered as the result of a traffic load measurement and the traffic load difference between the first and the second cells in a subsequent measurement is equal to or below a third threshold value, so that handovers to the second BS will occur less easily than before the increase. The memory means are used since the different criteria are preferably stored in the memory and accessed by the computer 440, and the transmitter 430 and the antenna 410 are used for transmitting updated criteria to the MSs n the cell of the Base Station 400.

Finally, fig 5 shows another principle which may be employed in an embodiment of the present invention: as explained above, and as shown in fig 2, if the load difference between two cells exceeds a certain threshold, handovers from one cell to the other may be "encouraged" by changing the handover criteria. However, in some cases, the load difference, the "Δ", may exceed the threshold, although the actual load of both cells is actually quite low. In such cases, it may not be necessary or desired to change the handover criteria, although the "Δ" exceeds a certain threshold.

In order to prevent a change of handover criteria in a case such as the one described above, the load of the two cells whose "Δ" exceeds the threshold for changing criteria may be taken into consideration, for example by the computer 440 of fig 4. Fig 5 illustrates the concept, with cell load being shown on the vertical axis, as a function of time on the horizontal axis.

The difference in cell load between two cells C2 and C1 , "Δ", exceeds the threshold for changing handover criteria. However, the load of both of the cells C1 and C2 is well below a "comfort level", such as, for example, 50% cell load, as illustrated in fig 5. This, in the embodiment of the invention described at present, the handover criteria will remain unchanged.

However, with two other cells, C4 and C3, the situation is different: the difference in cell load between cells C4 and C3 is the same, "Δ", as that between cells C2 and C1 , but the actual load in at least one of the cells, cell

C4, is above the "comfort level" of 50%. Hence, the handover criteria for handover from C4 to C3 are changed, in a manner which has been described previously in this text.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims. For example, although only a first and a second Base

Station and corresponding cells have been used in the examples given above, this is merely in order to make the description of the invention more understandable, and is not intended to restrict the number of Base Stations, cells or Mobile Stations in a system in which the invention is applied.

Claims

1. A method (300) for use in a cellular wireless access network (100), in which network there is a plurality of cells (110, 120, 130) in each of which there can be a number of Mobile Stations, MS (132), and one base station, BS (111 , 121 , 131), per cell, the traffic to and from the MSs in a cell being routed via the BS of that cell, in which network an MS can be handed over from a first BS (131 ) of a first cell (130) to a second BS (121 ) of a second cell (120), said handover being carried out if certain criteria are fulfilled, the method (300) being characterized in that it comprises measuring (310) the traffic load in at least said first and second cells and calculating (320) the difference between the load in the first cell and the load in the second cell, the method additionally comprising lowering (340) the criteria for handover from the first BS to the second BS if the load difference between the first cell and the second cell exceeds (330) a first threshold value (T1 ), so that handovers from the first BS (131) to the second BS (121) will occur more easily than before the lowering of the criteria.

2. The method (300, 350) of claim 1 , according to which the criteria for handover from the second BS to the first BS are made more restrictive if the load difference between the first and the second cell exceeds a second threshold value.

3. The method (300, 360) of claim 1 or 2, according to which said first and second threshold values are essentially equal to each other.

4. The method (300, 370) of any of claims 1-3, according to which the traffic load measurements in the cells in the network are made at specified intervals, and according to which, if the criteria for handover from the first BS to the second BS have been lowered as the result of a traffic load measurement, and the traffic load difference between the first and the second cells in a subsequent measurement is equal to or below a third threshold value (T3), the criteria for handover from the first BS to the second BS are made more restrictive, so that handovers from the first BS to the second BS will occur less easily than before the increase.

5. The method (300, 370) of claim 4, according to which, if the criteria for handover from the second BS to the first BS have been made more restrictive as the result of a traffic load measurement, and the load difference between the first and the second cell in a subsequent measurement is equal to or below a fourth threshold value (T4), the criteria for handover from the second BS to the first BS are made less restrictive, so that handovers from the second BS to the first BS can occur more easily.

6. The method (300, 380) of claim 4 or 5, according to which said third and fourth threshold values (T3, T4) are essentially equal to each other.

7. The method (300) of any of the previous claims, according to which said calculation (320) of load differences is made in each of said first and second BS, based on traffic load data which is exchanged between the BSs.

8. The method (300) of any of claims 1-6, according to which said calculation (320) of load differences is made in one of said BSs, based on traffic load data which is exchanged between the BSs, and according to which method changes in handover criteria are communicated from the BS in which said calculations are made to the other BSs.

9. The method (300) of any of claims 1-6, according to which said calculation (320) of load differences is made in a node on the network which is external to both said first and second BSs, based on traffic load data which is sent from the BSs, and according to which method changes in handover criteria are communicated from said external node to the BSs.

10. The method (300) of any of the previous claims, according to which the handover criteria for handover between two cells are only changed if the cell load in at least one of said cells is above a certain predefined level.

11. A Base Station (131 , 400) for use in a cellular wireless access network (100), in which network there is a plurality of cells (110, 120, 130) in each of which there can be a number of Mobile Stations, MSs (132), said Base Station (131 , 400) being equipped with means (410, 420, 430, 440, 450, 460) for routing the traffic to and from the MSs in at least a first cell (130) in the network, in which network an MS can be handed over between the Base Station of the invention and a second Base Station (121 ) of a second cell (120), said handover in either direction being carried out if certain criteria are fulfilled, the Base Station (131 , 400) being characterized in that it comprises means (440) for measuring the traffic load in at least said first cell and with means (440, 430, 410) for lowering the criteria for handover to said second Base Station if the load difference between the first cell and the second cell exceeds a first threshold value (T1 ), so that handovers to the second Base Station will occur more easily than before the lowering of the criteria.

12. The Base Station (131 , 400) of claim 11 , further comprising means (410, 420, 460) for receiving information about the traffic load in the second cell from the second Base Station, and with means (440) for calculating the difference between the load in the first cell and the load in the second cell.

13. The Base Station (131 , 400) of claim 11, additionally comprising means (460) for transmitting the results of its traffic load measurements to another node in the system, such as an ASN Gateway, and means ( 460) for receiving criteria for handover to the second Base Station from said node.

14. The Base Station (131 , 400) of any of claims 11-13, further comprising means (410, 420, 440) for making the traffic load measurements in the first cell in the network at specified intervals, and means (410, 430, 440, 450) for making the criteria for handover to the second Base Station more restrictive if the criteria for handover to the second Base Station have been lowered as the result of a traffic load measurement and the traffic load difference between the first and the second cells in a subsequent measurement is equal to or below a third threshold value, so that handovers to the second BS will occur less easily than before the increase.

15. The Base Station (400) of any of claims 1 1-14, further comprising means (440) for measuring if the cell load in at least one of two cells is above a certain predefined level, and for only in that case enabling the handover criteria for handover between said two cells to be changed.