WO2012102653A1 - Adjusting coverage area of a base station - Google Patents

Abstract

The invention relates to a method and computer program product for adjusting a coverage area provided by a base station as well as to a base station capable of adjusting coverage area. According to the invention the base station (10) comprises an antenna (16) being set to cover a geographical area for forming a cell (C, C'), a cell-control unit configured to obtain an indication of a change in resources that mobile stations (12, 14) are scheduled to use when the antenna covers a first geographical area ("GA1"), and control the antenna to cover a second geographical area ("GA2") based on the indication in order to adjust the coverage area.

Description

ADJUSTING COVERAGE AREA OF A BASE STATION

TECHNICAL FIELD The invention relates to base stations in wireless communication systems. More particularly, the invention relates to a method and computer program product for adjusting a coverage area provided by a base station as well as to a base station capable of adjusting coverage area.

BACKGROUND

Mobile broadband data have grown tremendously over the last years and the evolution in handsets towards more smart phones has stressed the capacity of the networks. To cope with the increased demand on data capacity the network operators have made their networks more dense by adding more and more sites. Since the needed

capacity is unevenly distributed, increased attention has been given to heterogeneous networks (HetNets) , where small (in output power) base station nodes are placed within the coverage area of the traditional macro sites to locally increase the wireless capacity. Here a macro site may form one or more cells, where such a small base station node may form one or more cells within one such macro cell.

Another trend in the mobile world is that operators find it more difficult to find suitable locations for the equipment of the sites. This can be due to public reluctance to live close to transmitting antennas, or due to regulatory restrictions. Added to this is the increased cost of site installations which also points in a direction where more flexible site solutions are very attractive.

However, placing sites in spots that are non-optimal from a capacity or coverage perspective would decrease the performance of the network. When deploying antennas at places that are non-optimal from a coverage/capacity point of view more sites are needed to obtain a reasonable capacity in the network. The antenna installation may be chosen with several practical constraints (cost, accessibility, regulatory, etc.) and does not necessarily match where the traffic is expected.

The network performance can, to some extent, be

increased by careful tuning of different parameters. Much of this tuning requires a site visit, something that is expensive and may in some cases also be

difficult .

There are thus several problems associated with a suboptimal placing of a base station in a communication network .

SUMMARY The invention is therefore directed towards improving the coverage provided by a base station at least temporarily having a suboptimal coverage. One object of the invention is thus to adjust the coverage area provided by a base station when the base station has suboptimal coverage.

This object is according to a first aspect of the invention achieved through a method for adjusting a coverage area provided by a base station comprising an antenna being set to cover a geographical area for forming a cell, where the method comprises obtaining an indication of a change in resources that mobile

stations are scheduled to use when the antenna covers a first geographical area and controlling the antenna to cover a second geographical area based on the

indication in order to adjust the coverage area.

The object is according to a second aspect of the invention achieved through a base station capable of adjusting coverage area. The base station comprises an antenna being set to cover a geographical area for forming a cell and a cell-control unit configured to obtain an indication of a change in resources that mobile stations are scheduled to use when the antenna covers a first geographical area and configured to control the antenna to cover a second geographical area based on the indication in order to adjust the coverage area .

The above-mentioned object is according to a third aspect of the invention achieved through a computer program product for adjusting a coverage area provided by a base station comprising an antenna being set to cover a geographical area for forming a cell. The computer program product comprises computer program code on a data carrier, which computer program code when run on a processor forming a cell-control unit, causes the processor to obtain an indication of a change in resources that mobile stations are scheduled to use when the antenna covers a first geographical area and controls the antenna to cover a second

geographical area based on the indication in order to adjust the coverage area.

The invention has many advantages. It allows the capacity provided by a cell to be moved with users. It is possible to have the coverage area of the cell change, which therefore allows a more efficient use of a base station. This allows a reduction of the number of base stations used in a system. Another advantage is that site selection is less critical. It is possible to later adapt the coverage area of a cell, after being deployed, for improving the coverage and/or capacity of the system.

The base station may here comprise a behaviour- investigating element configured to determine the above-mentioned indication.

The indication may in addition to the above-mentioned resources also be based on the positions of mobile stations in the cell. The indication can also be based on the movement of mobile stations in the cell. According to one variation the controlling of the antenna in the method comprises controlling the antenna to make the cell follow the movement of mobile

stations .

According to the same variation of the invention the cell-control unit is configured to control the antenna to move the cell and follow the movement of mobile stations .

According to another variation of the invention, the method further comprises obtaining a determination of the second geographical area to be covered by the cell after the change to a new coverage area.

The cell-control unit may according to the same

variation of the invention furthermore comprise an area-determining unit configured to determine the geographical area to be covered by the cell after the change to a new coverage area.

According to other variations of the invention the controlling of the antenna comprises changing the location of the coverage of the cell or changing the size of the coverage area of the cell.

The antenna may comprise a set of antenna elements including at least two antenna elements. In yet another variation of the invention, the

controlling of the antenna comprises controlling the antenna elements for forming the cell. According to yet another variation the controlling comprises changing properties of the radio signals passing the set of antenna elements. This changing of properties may comprise applying signal-property modifying measures onto radio signals passing the set of antenna elements, where these signal-property modifying measures are changed when the coverage area is changed.

It should be emphasized that the term

"comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in relation to the enclosed drawings, in which: fig. 1 schematically shows a base station according to the invention providing a cell that is changed through being moved,

fig. 2 shows a block schematic of a base station according to one embodiment of the invention,

fig. 3 shows a flow chart of a general method for adjusting the coverage area of a base station according to a first embodiment of the invention,

fig. 4 shows a block schematic of an antenna-control unit of the base station connected to a cell-control unit and a data-handling unit of the base station, fig. 5 schematically shows a transforming unit

transforming antenna element beams of a base station antenna into narrow beams by means of a Butler matrix, fig. 6 shows a flow chart of a number of method steps being performed in a method for adjusting the coverage area provided by a base station according to a second embodiment of the invention,

fig. 7 schematically shows a base station according to the invention providing a cell that is changed through changing the size of the coverage area of the cell, and fig. 8 schematically shows a computer program product according to an embodiment of the invention in the form of a CD ROM disc. DETAILED DESCRIPTION

In the following description, for purposes of

explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough

understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the

description of the invention with unnecessary detail.

The present invention concerns a base station in a wireless communication system, for instance a mobile communication system like Long-Term Evolution (LTE) , Universal Mobile Telecommunications System (UMTS) and Global System for Mobile Communications (GSM) . These are just a few examples of systems where the invention can be implemented and it should be realized that the invention can just as well be implemented in other types of systems such as Wireless Local Area Network (WLAN) systems. These types of systems are being run by operators, which operators generally have an interest in that their systems are used as much as possible. The deployed systems should thus be efficient in order to minimize the cost for the operator and maximize

revenue. From the perspective of the operator the systems should thus be easy to access for users.

However, also the end users, the users of mobile stations, have a need of better access to services provided via these systems. Thus also the end users benefit from an easier access to such systems.

As mentioned earlier an operator may face a number of problems when deploying a base station in one of the communication systems mentioned above, where the initial coverage area is or can become suboptimal.

It would therefore be very valuable if the coverage area of the radiating equipment could be adjusted after deployment to fit the capacity needs.

More and more sites consist of central base station equipment (main units) placed at a suitable location (low rent, easily accessible, etc.) together with remote radio units (RRUs) which are placed wherever it is convenient. If possible, the RRUs are placed close to the antennas to minimize losses in feeder cables. The next step in this evolution is to combine the radio units and antennas into one so-called active antenna. This building practice is also known as antenna- integrated radios (AIR) . In this concept each antenna element in the antenna column may have its own radio and power amplifier (PA) . This high level of

integration can be obtained since each PA can be of low power. High output power is achieved through coherent combining of all elements, so-called "in-air

combining" .

When deploying antennas at places that are non-optimal from a coverage/capacity point of view more sites are needed to obtain a reasonable capacity in the system. The antenna installation may be chosen with several practical constraints (cost, accessibility, regulatory, etc.) and does not necessarily match where the traffic is expected.

The system performance can, to some extent, be

increased by careful tuning of different parameters. The pointing direction of an antenna may as an example need to be tuned in order to optimize the performance of the system. Much of this tuning requires a site visit, something that is expensive and in some cases may be difficult. If the capacity needs change due to different traffic patterns a retuning is necessary. For example, if people move during the day, ideally the coverage area of the cell should also move in the same way . With today's solutions, e.g. RRUs and passive antennas, this is not possible. The coverage area of the antennas only depends on the actual physical direction of the radiating antenna elements. The antenna tilt can, to some degree, be adjusted remotely by a so-called remote electrical tilt (RET) system.

Fig. 1 here schematically shows a base station 10 of the present invention, which in one variation of the invention is a slave base station that is controlled by a master base station. The base station 10 may be an active antenna (or AIR) that covers a hot spot and may then also be a femto or pico base station, while the master base station may be a macro base station. It is furthermore possible that the master base station provides an umbrella cell, which covers a cell C provided by the slave base station. It should also be realized that in other variations of the invention, the base station 10 may not be controlled by any master base station. .

In fig. 1 there is shown a first geographical area GA1, which the cell C covers at a first current point in time and a second geographical area GA2 which the cell is later moved to and covers at a second later point in time. The cell after the move has been performed is indicated as C . In fig. 1 there is also shown a first and a second mobile station 12 and 14, which are both located at an edge of the cell C before the coverage area has been changed and at the centre of the cell C after the cell has been moved. The mobile stations are thus covered both before and after the cell has been moved. The cell is in this example formed as a beam provided by the base station and the change does in this example only include a change or a shift of the cell so that a new area is covered. The cell size is retained. This means that the size of the coverage area of the cell is retained. In this specific example the change is furthermore an angular change. It should be realized that the change can also be for instance radial or both radial and angular.

Fig. 2 shows a block schematic of one variation of a base station according to the invention. The base station 10 includes a data-handling unit 20 being connected to a base station antenna 16 via an antenna- control unit 22. The data-handling unit is here

typically a unit that handles communication between the base station and mobile station, such as performs coding, decoding, modulation and demodulation,

assignment of mobile stations to communication

channels. In the base station 10 there is furthermore a cell-control unit 18, which cell-control unit 18 comprises a behaviour-investigating element 24 and an area-determining element 26. Here the behaviour- investigating element 24 is connected to the data- handling unit 20 and the area-determining element 26, while the area-determining element 26 is connected to the antenna-control unit 22. The antenna 16 is here a group antenna or an array antenna. It thus includes a set of antenna elements. This set includes at least two elements and in this case it includes four elements, a first antenna element 16A, a second antenna element 16B, a third antenna element 16C and a fourth antenna element 16D. The elements 16A, 16B, 16C and 16D can be placed so that they are aligned with each other in a vertical direction. They can also be placed so that they are aligned with each other in a horizontal direction. They are with advantage provided in the form of a matrix so that both vertical and horizontal alignment is obtained for forming said array. In the example shown here they are shown as being horizontally aligned, which is sufficient when performing an angular shift of the cell.

The general functioning of the base station will now be described with reference to fig. 3, which shows a number of steps in a general method of adjusting the coverage area according to a first embodiment of the invention .

The load of a cell may change during the day, for instance because mobile station users are moving. One scenario is the case where the cell initially covers an urban office area, where there will be a lot of mobile stations in a cell at business hours, almost empty at nights and many mobile stations just outside of the initial coverage of the cell at lunch.

This means that mobile station users when considered as a group, may move around and this fact may be of interest to consider in a cellular system. It would then be very beneficial if the coverage area of the base station, here an AIR node, moves together with the users. A cell that is active and covering a geographical area when there is little activity is less advantageous from an energy-efficiency point of view.

This means that it may be necessary to follow the movement of the mobile stations.

In order to be able to do this, the cell-control unit 18 obtains an indication of a change in mobile station traffic in relation to the cell C when covering a first geographical area GA1, step 28. If the base station 10 is controlled by another entity, like a master base station, this indication could be received by the cell- control unit 18 from the master base station via the data-handling unit 20. The indication could also be determined by the cell-control unit 24 based on traffic data obtained for instance from the data-handling unit 20. Based on this indication the cell-control unit 18 then controls the antenna 16 to change the coverage area, step 30. In the example of fig. 1 the change of coverage area involves the change of the position of the cell C to leave the first geographical area GA1 and instead cover C a second geographical area GA2 while retaining the cell size. The change thus involves a change of the location of the coverage area of the cell. The change of position typically involves

controlling the antenna-control unit 22 to influence the beam formed by the antenna to change, which may be a change in shape such as a change in size, as well as is in position. This means that a change in traffic may lead to the coverage area having to be changed. In the simplest variation of the invention this is all that is done. The coverage area is changed based on an indication of changed traffic such as the change in resources the mobile stations in the first geographical area are scheduled to use, like the number of bits requested in a certain time period from the system by mobile station users.

The new coverage area that is required can here

furthermore be known from previous investigations of user behaviour. This means that if a certain indication is obtained of a change in traffic the cell-control unit may directly select a corresponding new coverage area that has been pre-set for this specific

indication . As an alternative it is possible that the cell-control unit itself determines that a change is to be made, i.e. itself provides the indication and also determines the new coverage area based on the traffic in the cell. The indication of a change in mobile station traffic may thus be obtained because of a change in resources, i.e. a change for instance in the number of connections being set up. It can also be obtained because of a change in scheduled data rate to and from mobile stations.

This indication may here be provided through the cell- control unit 18 obtaining relevant data from the data- handling unit 20. Data for the decisions can as an alternative be obtained from another entity in the system, like a mobile services system centre or the previously described macro base station. As mentioned earlier it is possible that the second geographical area to be covered by the cell is pre^¬ determined. As an alternative it is possible that also the second geographical area is determined, either by the base station 10 or by another node, like the previously mentioned macro base station or base station controller. This means that the base station 10 may also obtain a determination of the geographical area to be covered after the change, which determination may be made by the area-determining element 26 of the cell- control unit 18 or which is received by the cell- control unit from said other node. The change in coverage area furthermore typically comprises influencing the antenna beam. In this respect it comprises influencing the radio signals that pass by the antenna-control unit 22 to the antenna elements 16A, 16B, 16C and 16D for changing the beam. Typically parameters for the antenna beam that can be influenced through changing the properties of the radio signals are amplitude, phase and delay.

This will be described in more detail shortly.

Fig 4 schematically shows an antenna-control unit 22 according to one variation of the invention being connected to the data-handling unit 20 and the cell- control unit 18. More particularly there are a number of radio signal branches, typically as many as there are antenna elements in the antenna. Every antenna element is thus connected to the data-handling unit 20 via a corresponding radio signal branch. There are here furthermore a number of multiplying elements provided in the radio signal branches. In this example there is one multiplying element in each branch. However, it should be realized that it is possible to omit a multiplying element in one or more of the branches. To each multiplying elements there is provided a

corresponding weight, which weight is being set by the cell-control unit 18. The weights are signal-property modifying measures that can be applied to radio signals on the radio signal branches. There is thus a first branch having a first weight Wl that can be set for the first antenna element, a second branch having a second weight W2 that can be set for the second antenna element, a third branch having a third weight W3 that can be set for the third antenna element and a fourth branch having a fourth weight W4 that can be set for the fourth antenna element. In order to determine the movement of the mobile stations in the cell, it is possible to use a Butler matrix. Such a Butler matrix is schematically shown in fig. 5. The Butler matrix can be realized through a transforming element 32 that as an input has the four antenna beams BAE, BBE, BCE and BDE of the antenna elements in the antenna element domain. The

transforming unit 32 then performs a transformation from the antenna element domain to the beam domain, where this transformation may be performed either in RF using an analog Butler matrix or in base band using digital signal processing means such as the Fast

Fourier Transform (FFT) . This allows a mapping of the beams BAE, BBE, BCE and BDE in the antenna element domain provided by the antenna elements 16A, 16B 16C and 16D into separate beams BAB, BBB, BCB and BDB in the beam domain. The reference indicia BAE, BBE, BCE and BDE are here used to indicate that the shown antenna element beams are provided in the antenna element domain. The reference indicia BAB, BBB, BCB and BDB are here used to indicate that the antenna element beams have been transformed into the beam domain. The beams generated by this transformation can then be used to determine the position and movement of mobile stations, which will be described in more detail later.

The functioning of the invention according to a second embodiment will now be made with reference also to fig. 6, which shows a flow chart of a method according to this second embodiment of the invention.

In this second embodiment, the behaviour-investigating element 24 obtains traffic data from the data-handling unit 20, step 34. This data concerns the mobile

stations which are located in the cell C, here

exemplified by the mobile stations 12 and 14. This data, which is indicative of the load, may include data such as data about the resources required by mobile stations in the cell, for instance scheduled bits per second and number of connections. The behaviour- investigating element 24 also obtains positional data, i.e. the positions of the mobile stations in the cell. Positional data can be obtained through conventional radio-network positioning methods, for instance

triangulation or using GPS. Another interesting way is through using the Butler matrix. It is also possible for the base station to estimate, e.g., the directions of arrival of the signals received from each mobile station and through this find their positions relative to the antenna. The latter two ways of determining positions have the advantage of being independent of receiving external positional data. Not all mobile stations do for instance have the possibility to employ GPS .

Also other types of positional data can be obtained. Examples of such other data are the direction of movement and speed of movement of a mobile station. This can be provided by the radio network in the form of, e.g., coordinates of specific mobile stations.

Alternatively, this can be estimated by the base station itself. The direction of arrival can for instance be estimated by the array antenna. An

alternative measurement could be to find in what part of the cell (or coverage area of the antenna) most of the data are served or received. The movement can also be obtained through using the above-mentioned Butler matrix . When using a Butler matrix received signals are

converted to "beam-space" through being transformed from the antenna element domain into the beam domain by the transforming unit 32. This beam-forming method is sometimes termed "grid-of-beams" (GoB) and will give well-defined beams if sufficient correlation exists between the elements in the array. This is usually the case if the antenna elements are placed reasonable close to each other.

The combined coverage of each antenna element defines the cell. The Butler matrix transforms this into orthogonal beams. When a user moves in the coverage area the power measured in each "narrow" beam will indicate where in the "cell" the user is located. By using this measure together with, e.g., served traffic for a certain user it is possible to judge if a user is leaving the coverage of this cell. In this way it is possible to obtain a beam index or direction for each user at various points in time. By comparing a current beam index with previously logged beam indices for a user it is thus possible to determine if a user is moving and also estimate in what direction the user is moving as well as angular speed. Furthermore the more antenna beams there are the better the estimation will become. For example, if a mobile station is detected in beam BBB and later detected more strongly in beam BAB, it is most likely that the user is moving towards the left. Through this technique, it is thus also possible to estimate the direction and speed of movement. Another possible technique to use to determine

positional data is through the timing advance signals provided by the mobile stations.

All the above-mentioned data may furthermore be handled through considering the mobile stations in the cell as a group. This means that the data may be treated through averaging in order to get a measure on all the mobile stations in the cell. It is thus possible to consider the average resources required by a number of mobile stations as well as their average position and average movement when determining if there is to be a change of the coverage of the cell. Here it may also be mentioned that it is also possible to consider time. The decision concerning a change of coverage area can thus also be made based on a point in time being reached .

Based on this traffic data, positional data and

possibly also time, the behaviour-investigating element 24 then determines if there is a change in the pattern of movement of the mobile stations in the cell. This change could be determined in relation to some kind of threshold. If for instance the movement and change in data rise above or fall below one or more corresponding thresholds, then a determination is made that there should be a change of the coverage area, otherwise not. In case there is to be no change in coverage area, step 38, the behaviour-investigating element 24 then stops and no change is being made. However, if there is a change in pattern, step 38, then the behaviour- investigating element provides an indication that there is a change in mobile station traffic in the first geographical area to the area-determining element 26. It also orders the area-determining element 26 to determine the new coverage area. The area-determining element 26 therefore determines the new coverage area, step 42. In doing this it may consider the same data, load or required resources, position and movement of the mobile stations as well as time in order to determine the new coverage area. This means that the behaviour-investigating element may forward the data it has used for determining that a change in coverage area was to be performed.

The area-determining element 26 may for instance find out that the mobile stations 12 and 14 have moved close to the edge of the first geographical area GA1 depicted in fig. 1 and having a direction and movement

indicating that they will move from there into the area shown as the second geographical area GA2. Here it is possible that a business district is located in the first geographical area GA1 while a restaurant area frequented during lunch time is located in the second geographical area GA2. The area-determining element 26 may then determine that the cell is to be moved to cover this second geographical area GA2. Thereafter the area-determining element 26 determines new weights for provision to the multiplying elements of the antenna-control unit 22, step 44, which weights thus cause the antenna elements 16A, 16B, 16C and 16D to change the coverage area. The weights are here selected so that the antenna beams forming the cell will change so that the cell is moved to a new position C' covering the second geographical area GA2. This changing of properties thus comprises applying signal- property modifying measures onto radio signals passing the set of antenna elements 16A, 16B, 16C and 16D, where these signal-property modifying measures are changed when the coverage is changed. This change may be done through providing a weight that influences the physical properties of the radio signal in terms of amplitude, phase and delay. The weights are then received by the antenna-control unit 22 and provided to the multiplying elements for influencing the radio signals transferred between the data-handling unit 20 and the antenna elements 16A, 16B, 16C and 16D. In this way the new weights are thus applied to the radio signals to change the coverage area, step 46.

In this way it is possible to make the base station 10 move the cell and for instance follow the traffic as in the example given above. As the cell is typically surrounded by other cells and the cell boundaries are determined by signal-quality measures such as signal strength, it will be easily understood that a base station that moves a cell will still transmit signals in an area that is left, however weaker than before the move. Furthermore, if the transmissions of another base station forming a neighbouring cell in the area in question are stronger, then this means that the cell size of this neighbouring cell is increased and now includes the area left by the cell. It can thus be seen that the moving of a cell from an area does not mean that that area has been abandoned. The moving of a cell according to the principles of the invention allows a more efficient use of the base station. In the lunching example it is, for instance, possible to use the same base station for covering two hot spots that are "hot" at different points in time. This might otherwise have required two separate base stations. In the example on geographical areas given above, the second geographical area partly overlapped the first geographical area. It should be realized that the invention is not limited to such a partial overlap but the geographical areas may be completely separated.

Furthermore, in the example above the move was a change in cell coverage so that the cell went from covering a first geographical area to covering a second

geographical area while retaining the same cell size. This is typically the case when essentially the same volume of traffic is moving from one geographical area to another, which is typically the case in the lunch example given above. These lunching users would then move back to the first geographical area after

finishing lunch and the cell would move back to the original cell coverage, when the cell covered the first geographical area.

However, after office hours, then it is possible that many mobile users would go home and not stay in either of the cell coverage areas of the previous example. This means that the traffic load may be drastically reduced in both the above-described first and second geographical areas. In this case it can be inefficient to have a small-sized cell. In this case it may

therefore be of interest to instead increase the size of the coverage area of the cell. An example of this is shown in fig. 7, where the size of the cell C covering the first geographical area is increased. The thus enlarged cell is here indicated through the reference indicia C ' . This change may be done based on the load or traffic but in addition also based on time. If, for instance, the load falls below a certain traffic threshold possibly simultaneously with a first point in time being reached, the cell area may get increased. This may also be done through changing the antenna weights. It is for example possible to change the antenna parameters so that a vertical component of the beam is up-tilted to increase the coverage.

Alternatively, it is also possible to widen the created antenna beam to increase the coverage area. Similarly, it is possible that if a certain traffic load threshold is exceeded possibly together with a second point in time being reached, then the cell size may be reduced, since then it is possible that the mobile station users are back at work. If for instance the cell provided by the base station is a small cell within a larger

"umbrella cell", it is also possible to completely switch off the equipment generating the cell. The remaining traffic can then be transferred (or picked up) by the umbrella cell. By this the energy efficiency of the system is increased.

It is furthermore also possible to use position and speed. It is in this case possible to detect the radial movement of user terminals. This can be detected by studying the timing advance (TA) value used by each mobile station. The TA is the time (relative to a reference) when the user starts transmitting uplink data. This is used since signals from different mobile stations need to arrive at the same time at the base station 10 in order to be orthogonal. The TA value can be used to calculate the distance between a mobile station and the base-station antenna, and hence, also be used to find out which tilt angle should be applied in this cell. When most of the load is detected in, e.g., the outer part of the cell, the tilt angle of the active antenna can be changed to achieve better coverage for mobile stations requiring most of the capacity. An alternative way of finding an appropriate tilt angle would be to compare the time delay for the first tap in the estimated channel for one mobile station to that of the other mobile stations. The TA may be too coarse of a measure, and by using the delay of the first tap a finer granularity can be achieved.

Weights corresponding to this tilt angle are then supplied to vertically aligned antenna elements in the antenna .

It is then of course also possible to later decrease the coverage through decreasing the tilt angle if mobile stations again return to the cell centre. The present invention has a number of advantages. One advantage is that the capacity provided by a cell can move with the users. It is possible to have the

coverage area of the cell change, which therefore allows a more efficient use of a base station. Another advantage is that site selection is less critical. It is possible to later, after deployment, adapt a cell for improving the coverage. There are many variations that can be made to the invention apart from those already mentioned. The behaviour-investigating element was in the second embodiment described as determining that a change in coverage area was to be made based on both load and positional data. It is possible that it only does it based on load and possibly also time. Likewise, it is possible that the area-determining element does not consider the load, but only positional data and

possibly also time. Furthermore, since the behaviour- investigating element and area-determining element essentially use the same data for their determinations, they may also be combined into a single element and their determinations being performed jointly.

The antenna elements need furthermore not be placed at the same location as the other elements of the base station. It is also possible that the antenna elements and antenna-control unit are co-located, while the other units are provided at another location. A further possible variation is that all the different units are provided at different locations. Also the elements of the cell-control unit may be split up between different sites .

The cell-control unit may with advantage be provided in the form of a processor with associated program memory including computer program code for performing the functionality of the behaviour-investigating element and the area-determining element. It should be realized that this unit may also be provided in the form of hardware, like for instance in the form of an ASIC circuit. The computer program code may also be provided on a computer-readable means, for instance in the form of a data carrier, like a CD ROM disc or a memory stick, which will implement the function of the above- described cell-control unit when being loaded into the above-mentioned program memory and run by the

processor. One such computer program product in the form of a CD ROM disc 48 with such a computer program code 50 is schematically shown in fig. 8.

The data-handling unit is typically a conventional baseband coding unit in the base station. It may also include scheduling functionality. As such it can be realized in the same way as the cell-control unit. The antenna-control unit may be provided in the form of one or more antenna combiners .

It was previously mentioned that the base station could receive an indication of a change from another entity and select a pre-determined new coverage area based on the received indication. It was also mentioned that the base station could itself determine the indication and itself determine the new coverage area, which is more flexible. However, it is also possible that the

reception of an indication is combined with

determination of a new coverage area and the

determination of an indication is combined with

selecting a pre-determined new coverage area.

The antenna need not be an antenna array, but can be a standard base station antenna, in which case the antenna-control unit may comprise an electrical motor that adjusts the antenna for obtaining the change of coverage area. While the invention has been described in connection with what is presently considered to be most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements. Therefore the invention is only to be limited by the following claims.

Claims

1. A method for adjusting a coverage area provided by a base station (10) comprising an antenna (16) being set to cover a geographical area for forming a cell (C, C ; C, C ' ) , comprising:

obtaining (28; 34, 36) an indication of a change in resources that mobile stations (12, 14) are

scheduled to use when the antenna covers a first geographical area ("GA1") , and controlling (30; 46) the antenna to cover a second geographical area ("GA2") based on said indication in order to adjust the coverage area.

2. The method according to claim 1, wherein the

indication is based on the positions of mobile stations in the cell.

3. The method according to claim 2, wherein the

indication is based on the movement of mobile stations in the cell.

4. The method according to claim 3, wherein controlling the antenna comprises controlling the antenna to make the cell follow the movement of mobile

stations .

5. The method according to any previous claim, wherein the indication is based on a point in time being reached.

6. The method according to any previous claim, further comprising obtaining (42) a determination of the second geographical area to be covered by the cell after the change to a new coverage area.

7. The method according to any previous claim, wherein controlling the antenna (16) comprises changing the location of the coverage area of the cell.

8. The method according to any previous claim, wherein controlling the antenna (16) comprises changing the size of the coverage area of the cell.

9. The method according to any previous claim, wherein said antenna (16) comprises a set of antenna

elements including at least two antenna elements (16A, 16B, 16C, 16D) and controlling the antenna comprises controlling the antenna elements for forming the cell.

10. The method according to claim 9, wherein

controlling antenna elements comprises changing (46) properties of the radio signals passing said set of antenna elements.

11. A base station (10) capable of adjusting coverage area and comprising:

an antenna (16) being set to cover a geographical area for forming a cell (C, C ; C, C ' ) ,

a cell-control unit (18) configured to

obtain an indication of a change in resources that mobile stations (12, 14) are scheduled to use when the antenna covers a first geographical area

("GA1") , and

control the antenna to cover a second geographical area ("GA2") based on said indication in order to adjust the coverage area.

12. The base station according to claim 11, wherein the cell-control unit comprises a behaviour- investigating element (24) configured to determine the indication.

13. The base station according to claim 11 or 12, wherein the indication is determined based on the positions of mobile stations in the cells.

14. The base station according to claim 13, wherein the indication is determined based on the movement of mobile stations in the cell.

15. The base station according to claim 14, wherein the cell-control unit is configured to control the antenna to move the cell and follow the movement of mobile stations.

16. The base station according to any of claims 11 -

15, wherein the cell-control unit further comprises an area-determination element (26) configured to determine the second geographical area to be covered by the cell after the change to a new coverage area.

17. The base station according to any of claims 11 -

16, wherein the cell-control unit is configured to control the antenna (16) to change the position of the cell.

18. The base station according to any of claims 11 - 17, wherein the cell-control unit is configured to control the antenna (16) to change the coverage area of the cell.

19. The base station according to any of claim 11 - 18, wherein said antenna comprises a set of antenna elements (16A, 16B, 16C, 16D) including at least two antenna elements and the cell-control unit is configured to control the antenna elements for forming the cell.

20. The base station according to claim 19, further comprising an antenna-control unit (22) configured to apply signal-property modifying measures onto radio signals passing said set of antenna elements, where the cell-control unit is configured to change these signal-property modifying measures when changing the coverage area.

21. A computer program product for adjusting a

coverage area provided by a base station (10) comprising an antenna (16) being set to cover a geographical area for forming a cell (C, C ; C, C ' ) , the computer program product comprising computer program code (50) on a data carrier (48), which computer program code when run on a processor forming a cell-control unit (18), causes the

processor to: obtain an indication of a change in resources that mobile stations (12, 14) are scheduled to use when the antenna covers a first geographical area

("GA1") , and

control the antenna to cover a second geographical area ("GA2") based on said indication in order to adjust the coverage area.