WO2009151355A1

WO2009151355A1 - Method and arrangement for performing handover in a wireless communication system

Info

Publication number: WO2009151355A1
Application number: PCT/SE2008/000604
Authority: WO
Inventors: Benny Lennartson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-06-10
Filing date: 2008-10-22
Publication date: 2009-12-17

Abstract

Method and arrangement in a base band unit (150) for performing a handover of a wireless communication device (110) from a first remote radio unit (120) to a second remote radio unit (130). The first remote radio unit and the second remote radio unit are connected to the base band unit. They may also exchange wireless signals with the wireless communication device over a first and a second radio link respectively. The method is initialised and performed without need for firstly receiving an initiation signal from the wireless communication device. The method comprises obtaining a first and a second radio link quality measure. The method further comprises determining to initiate a handover from the first radio link to the second radio link. Further yet, the method comprises sending the same base band signals to the second remote radio unit, as to the first remote radio unit and then terminating the sending of base band signals to the first remote radio unit.

Description

METHOD AND ARRANGEMENT FOR PERFORMING HANDOVER IN A WIRELESS COMMUNICATION SYSTEM

TECHNICAL FIELD

The present invention relates to a method and an arrangement in a base band unit. More particularly, the present invention relates to a mechanism for performing a handover of a wireless communication device in a wireless communication system.

BACKGROUND

Functionality to handle user mobility is a fundamental component in cellular networks. From a service quality perspective, such functionality must ensure that service continuity is maintained as users of wireless communication devices such as e.g. mobile telephones move from one cell to another during an active session, and that each new session is established in a sufficiently good radio environment. From a spectral efficiency perspective, such functionality should ensure that an active user is always served by the most appropriate Remote Radio Unit (RRU) or remote radio units, which often means the closest remote radio unit, in a radio sense. Thus a handover may have to be performed from time to time as a wireless communication device move between different cells in order to avoid call termination when the wireless communication device gets outside the range of the first cell.

However, handover may also be performed e.g. when the capacity for connecting new calls of a cell is used up and an existing or new call from a wireless communication device, which is located in an area overlapped by another cell. In such scenario, that call may be transferred to the other cell in order to free-up some capacity in the first cell for other wireless communication devices, who can only be connected to the first cell.

A handover may further be made in order to avoid interference. The channel used by a particular wireless communication device may be interfered by another wireless communication device using the same channel in a different, adjacent cell. In such scenario, the call may be transferred to a different channel in the same cell or to a different channel in another cell in order to avoid the interference in serving cell and/or adjacent cells. In the Global System for Mobile Telecommunications (GSM), the handover can either be an un-synchronized handover or a synchronized handover.

In the un-synchronized handover case the wireless communication device receive a HANDOVER COMMAND from the serving cell that contains the target cell frequencies, Hopping sequence and other properties for the target cell. After receiving this message, the wireless communication device changes the hopping parameters and sends an HANDOVER ACCESS in the target cell. Further signalling is performed on the target cell and the call is proceeding.

The synchronized handover does not contain any HANDOVER ACCESS message and in this case, the handover procedure will be simplified.

However, both un-synchronized and synchronized handovers are not as robust as one could wish. Handover drops are common when performing handovers in GSM according to the existing methods.

SUMMARY

It is an object of the present invention to provide an improved performance in a wireless communication system.

The object is achieved by a method in a base band unit for performing a handover of a wireless communication device from a first remote radio unit to a second remote radio unit. The first remote radio unit and second remote radio unit are connected to the base band unit. The first remote radio unit and second remote radio unit are also adapted to receive base band signals from the base band unit. Further, the first remote radio unit and second remote radio unit are adapted to exchange wireless signals with the wireless communication device over a first and a second radio link respectively. The method is initialised and performed without need for firstly receiving an initiation signal from the wireless communication device. The method comprises the step of obtaining a first radio link quality measure based on a wireless signal sent over a first radio link between the wireless communication device and the first remote radio unit. The method also comprises the step of obtaining a second radio link quality measure based on a wireless signal sent over a second radio link between the wireless communication device and the second remote radio unit. Further, the method comprises the step of determining to initiate a handover from the first radio link to the second radio link. The decision is based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit, or the traffic in the second remote radio unit. Further yet, the method comprises the step of sending the same base band signals to the second remote radio unit, as to the first remote radio unit. In addition, the method comprises the step of terminating the sending of base band signals to the first remote radio unit.

The object is also achieved by an arrangement in a base band unit. The arrangement is adapted to perform a handover of a wireless communication device from a first remote radio unit to a second remote radio unit. The first remote radio unit and the second remote radio unit are connected to the base band unit. Also, the first remote radio unit and the second remote radio unit are adapted to receive base band signals from the base band unit. Further, the first remote radio unit and the second remote radio unit are adapted to exchange wireless signals with the wireless communication device over a first and a second radio link respectively. The arrangement comprises an obtaining unit. The obtaining unit is adapted to obtain a first radio link quality measure based on a wireless signal sent between the wireless communication device and the first remote radio unit, and a second radio link quality measure based on a wireless signal sent between the wireless communication device and the second remote radio unit. The arrangement also comprises a determination unit. The determination unit is adapted to determine the moment to initiate the handover from the first radio link to the second radio link. The handover decision may be based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit or the traffic in the second remote radio unit. Further, the arrangement comprises a sender. The sender is adapted to send the same base band signals to the second remote radio unit, as to the first remote radio unit. The sender is further adapted to terminate the sending of base band signals to the first remote radio unit.

Thanks to the present method and arrangement in a base band unit, the handover of a wireless communication device from one remote radio unit to another is possible to make without the need of firstly receiving a signal for initiating the handover from the wireless communication device. In fact no information at all has to be communicated between the base band unit and the wireless communication device over the air interface in order to perform the handover. As the communication over the air interface is in particular sensitive for interference in a typical handover situation, where the wireless communication device is situated at the edge of the cell, the risk of a handover drop is reduced. Thereby the overall system performance in the wireless communication system is improved.

An advantage of the present method and arrangement for handover is that the number of handover drops may be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more in detail in relation to the enclosed drawings, in which:

Figure 1 is a schematic block diagram illustrating a wireless communication system according to an embodiment of the present solution.

Figure 2 is a schematic block diagram illustrating an embodiment of the present method.

Figure 3 is a combined signalling and flowchart illustrating radio signal transmission according to some embodiments.

Figure 4 is a combined signalling and flowchart illustrating radio signal transmission according to some embodiments.

Figure 5 is a flow chart depicting a method in a base band unit, according to some embodiments.

Figure 6 is a block diagram depicting an arrangement in a base band unit, according to some embodiments.

DETAILED DESCRIPTION The invention is defined as a method and an arrangement in a base band unit which may be put into practice in the embodiments described below. This invention may, however, be embodied in many different forms and should not be constructed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It should be understood that there is no intent to limit the present method and arrangement in a base band unit to any of the particular forms disclosed, but on the contrary, the present method and arrangement in a base band unit is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the claims.

Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

Figure 1 is a schematic illustration over a wireless communication system 100 and a wireless communication device 110 comprised within the wireless communication system 100. Further, the wireless communication system 100 may comprise a first remote radio unit 120, a second remote radio unit 130 and a third remote radio unit 140.

Although three remote radio units 120, 130, 140 is shown in Figure 1 , it is to be understood that another configuration of remote radio units may be connected through, for example, a mobile switching centre and other network nodes, to define the wireless communication system 100. Further, any, some or all of the remote radio units 120, 130, 140 may be referred to as e.g. a base station, an access point, a NodeB, an evolved Node B (eNodeB) and/or a base transceiver station, Access Point Base Station, base station router, etc depending e.g. of the radio access technology and terminology used.

In some embodiments, the wireless communication device 110 may be represented by a wireless communication terminal, a mobile cellular telephone, a Personal Communications Systems terminal, a Personal Digital Assistant (PDA), a laptop, a computer, user equipment (UE) or any other kind of device capable of managing radio resources.

The wireless communication system 100 may be based on a technology such as e.g. Global System for Mobile Telecommunications (GSM), just to mention a none limiting example. It is to be noted however, that the invention is not in any way limited to be performed exclusively in a wireless communication system 100, but may be performed in a wireless communication system 100 wherein some nodes are wirelessly connected and some 5 nodes have a wired connection.

The wireless communication system 100 may further according to some embodiments comprise an optional control node. The control node may be e.g. a Base Station Controller (BSC). The base station controller is a governing element in the wireless

10 communication system 100, responsible for control of remote radio units, which are connected to the base station controller. The base station controller may further for example carry out radio resource management; some of the mobility management functions and may be the point where encryption may be done before user data is sent to and from the wireless communication device 110.

15

The wireless communication device 110 may communicate with other wireless communication devices, or network nodes not shown in Figure 1 , via any, some or all of the remote radio units 120, 130, 140 comprised within the wireless communication system 100.

20

The wireless communication system 100 further comprises a Base Band Unit (BBU) 150. The base band unit 150 is situated in one place and may provide base band to several remote radio units 120, 130, 140. The remote radio units 120, 130, 140 may be placed several kilometres away from each other and the base band unit 150. The base band unit

25 150 may provide base band signals, I/Q signal constellation, to e.g. three different cells.

With this concept it is possible to create a synchronized radio network in e.g. GSM. It is thus possible to fully avoid co-channel interference in a synchronized radio network.

30 Figure 2 illustrates a base band unit 150 that is connected to a first remote radio unit 120 and a second remote radio unit 130, according to some embodiments. The two remote radio units 120, 130 are covering two different areas with a cell border 200 in between. The first remote radio unit 120 covers a first cell 121 while the second remote radio unit 130 covers a second cell 132.

35 In the illustrated scenario, the wireless communication unit 110 has been situated within the first cell 121, covered by the first remote radio unit 120 and is now approaching the second cell 132, covered by the second remote radio unit 130.

5 The base band unit 150 and the connected remote radio units 120, 130 allows the possibility to send the same downlink signal constellation (I/Q) to two different cells 121 , 132 at the same time. Thus it is possible to perform a smooth handover from the first cell 121 , covered by the first remote radio unit 120 to the second cell 132, covered by the second remote radio unit 130.

10

Figure 3 is a combined signalling and flowchart that depicts the transmission of signals between a wireless communication device 110, a first remote radio unit 120 and a second remote radio unit 130, according to some embodiments, when performing a smooth handover. The wireless communication device 110 is initially connected to the first cell

15 121 of the first remote radio unit 120.

310

The downlink signal strength from the traffic channel on the first cell 121 and the Broadcast Control Channel (BCCH) on the second cell 132 is reported through the 20 MEASUREMENT REPORT message from the wireless communication device 110. This information may be used to initiate a smooth handover. Also the uplink signal strength from the wireless communication device 110 to the first remote radio unit 120 is measured here.

25 320

At this point 320, the second cell 132 is tuning in to the current hopping sequence of the wireless communication device 110 and measure the uplink signal quality. This tuning could be done periodically at all time for all wireless communication devices served by neighbouring cells. This information together with the uplink signal strength from the first 30 cell 121 may be also be used to initiate a smooth handover.

330

At this point in time 330, the smooth handover is initiated. The base band unit 150 starts to send the same I/Q stream for this call to both the first cell 121 and the second cell 132. The

35 wireless communication device 110 will receive the two downlink signals, one from the first remote radio unit 120 and one from the second remote radio unit 130. As long as both these signals are within the equalizer window of the wireless communication device 110, then the wireless communication device 110 will be able to handle this in the receiver.

340 At this point in time 340, the uplink signal can be decoded in both cells 121 , 132 and thus uplink diversity could be enabled. As the wireless communication device 110 moves further in to the target cell 132, the MEΞASUREMENT REPORTS will show how the downlink signal strength for both cells 121 , 132 and how far from the cell border 200 the wireless communication device 110 is situated. Also, the uplink signal strength measured in the first cell 121 and the second cell 132 could be used to determine how far over the cell border 200 the wireless communication device 110 is.

350

The wireless communication device 110 is at this point in time 350 so far into the second cell 132 that it is not necessary or beneficial to have a connection to the first cell 121 any more. At this time the base band unit 150 stops transmitting (I/Q) to the first cell 121.

If the timing advance difference is too large between two cells 121 , 132 participating in a smooth handover then there could be interference in the reception at the wireless communication device 110. The timing advance difference should not be larger than the equalizer window of the wireless communication device 110. However, it may be possible to tune the base band signals in such a way that they are fully synchronized. One way to tune this is to create an artificial delay for remote radio units 120, 130 that are close to the base band unit 150. This delay may be the same as the maximum delay in a MAIO Cluster, according to some embodiments.

If the cell border 200 is not geographically between the cells 121 , 132 performing the handover, the timing advance difference may be a problem. For example if the wireless communication device 110 is geographically much closer to the serving cell 121 than the target cell 132. However, the equalizer window in GSM allows the two signals to be separated in time for 3 bit periods. This corresponds to a distance difference between the signals of 5 kilometres. Yet a solution to possible problems occurring due to a too big difference in timing advance may be to let the target cell 132 also measure and estimate which timing advance a certain call would get if that call was served by cell 132 instead of the serving cell 121. A comparison may also be made with respect to the difference in timing advance between the serving cell 121 and the estimated timing advance at the target cell 132. If the difference in timing advance is too big, it may be decided to initiate a hard handover instead of a smooth handover.

The above described handover may with certain advantage be performed within the Mobile Allocation Index Offset (MAIO) cluster, according to some embodiments.

A MAIO cluster is a group of GSM cells that are frame synchronized and the TDMA frames perfectly overlap, timeslot O is transmitted at the same time from all cells in the MAIO cluster.

Also, the same frame number is used at the same time in all cells in the MAIO cluster. The frequency hopping sequence and hopping frequencies are equal for all wireless communication devices in cells covered by the MAIO cluster.

Finally, in these cells a MAIO value may only be used once.

However, when performing handover between different MAIO clusters, information concerning hopping pattern may be comprised with certain advantage, according to some embodiments. Thus signalling may be used.

MAIO refers to a frequency offset separating traffic channels. When a GSM wireless communication device 110 is served by a cell 121 , 132 that is hopping over a set of frequencies, the separate traffic channels hop over the allocated frequencies according to a Hopping Sequence Number (HSN). The traffic channels with the same HSN hop over the same frequencies in the same order but are separated in frequency by a mobile allocation index offset (MAIO). Thus the same frequencies may be used by adjacent cells 121 , 132, but in different time slots.

The smallest amount of interference on a site may be achieved if it is possible to synchronize all channels on the site and separate the channels using the MAIO.

After a smooth handover, into the target cell 132, there may be adjacent channel interference. This is dependent e.g. on the MAIO planning in the MAIO cluster. However, an un-synchronized intra cell handover could be initiated, and thus allocating the wireless communication device 110 to a more appropriate channel. With base band unit 150 and remote radio units 120, 130 it is possible to transmit to all traffic channels within the MAIO -cluster, at all time. This may thus result in the same effect from an end user point of view, as seen for the user of the wireless communication device 110, as described above. However, this may be a less convenient solution as a lot of unnecessary signalling and energy is transmitted from the remote radio units 120, 130.

An advantage with the method illustrated in Figure 3 is that the same downlink signals are sent to the wireless communication device 110 both over the first remote radio unit 120 and the second remote radio unit 130. Thus simultaneous channels in the first cell 121 and the second cell 132 are maintained and the call could only fail if all of the channels are interfered or fade at the same time. Fading and interference over the two different channels are uncorrelated, why the reliability of the connection becomes higher.

In a handover situation, the wireless communication device 110 may typically be situated at the cell border 200 of the cell 121, 132, enduring transmission conditions which may not be the best. The signals received both from the first remote radio unit 120 and the second remote radio unit 130 may then be used to improve the signal reception and interpretation and/or improve to the reliability of the call.

Figure 4 is a combined signalling and flowchart that depicts the transmission of signals between a wireless communication device 110, a first remote radio unit 120 and a second remote radio unit 130, according to some alternative embodiments, when performing a smooth handover. The wireless communication device 110 is initially connected to the first cell 121 of the first remote radio unit 120.

410

The downlink signal strength from the traffic channel on the first cell 121 and the Broadcast Control Channel (BCCH) on the second cell 132 is reported through the MEASUREMENT REPORT message from the wireless communication device 110. This information may be used to initiate a smooth handover. Also the uplink signal strength from the wireless communication device 110 to the first remote radio unit 120 is measured here.

420 At this point 420, the second cell 132 is tuning in to the current hopping sequence of the wireless communication device 110 and measure the uplink signal quality. This tuning could be done periodically at all time for all wireless communication devices served by neighbouring cells. This information together with the uplink signal strength from the first cell 121 may be also be used to initiate a smooth handover.

430

At this point in time 430, the smooth handover is initiated. The base band unit 150 starts to send the I/Q stream for this call to the second cell 132 and stop sending it to the first cell 121. The wireless communication device 110 will receive the downlink signals, from the second remote radio unit 130. According to the embodiments illustrated in Figure 4, no sending in parallel over both the first cell 121 and the second cell 132 is performed.

An advantage with the solution proposed in Figure 4 is that transmitting resources are saved, in comparison with the previously described embodiment, described in association with Figure 3.

The above described handover may with certain advantage be performed within the MAIO cluster, according to some embodiments. However, when performing handover between different MAIO clusters, information concerning hopping pattern may be comprised with certain advantage, according to some embodiments.

After a smooth handover, into the new cell 132, there may be adjacent channel interference. This is dependent e.g. on the MAIO planning in the MAIO cluster. However, an un- synchronized intra cell handover could be initiated, and thus allocating the wireless communication device 110 to a more appropriate channel.

Figure 5 is a flow chart illustrating a method in a base band unit 150, for performing a handover of a wireless communication device 110 from a first remote radio unit 120 to a second remote radio unit 130. The first remote radio unit 120 and the second remote radio unit 130 are connected to the base band unit 150. Both the first remote radio unit 120 and the second remote radio unit 130 are adapted to receive base band signals from the base band unit 150. Also, both the first remote radio unit 120 and the second remote radio unit 130 are adapted to exchange wireless signals with the wireless communication device 110 over a first and a second radio link respectively. The method is initialised and performed without need for firstly receiving an initiation signal from the wireless communication device 110, nor sending any initiation signal to the wireless communication device 110. The base band signals to be sent may according to some embodiments comprise information concerning which time slot for the remote radio unit 120, 130 to allocate for the wireless communication device 110, and which Mobile Allocation Index Offset (MAIO) value to use in that allocated time slot.

To appropriately perform the handover, the method may comprise a number of method steps 501-505. It is however to be noted that the method steps 501-505 may be performed in another chronological order than the enumeration indicates and that some of them, e.g. step 504 and step 505, or even all steps may be performed simultaneously or in an altered, arbitrarily rearranged, decomposed or even completely reversed chronological order. The method may comprise the following steps:

Step 501 A first radio link quality measure is obtained, based on a wireless signal sent over a first radio link between the wireless communication device 110 and the first remote radio unit 120.

According to some embodiments, the first radio link quality measure may be received from the wireless communication device 110.

However, according to some embodiments, the first radio link quality measure may be obtained from the first remote radio unit 120.

The step of obtaining the first radio link quality measure may be performed continuously, according to some embodiments.

Step 502

A second radio link quality measure is obtained, based on a wireless signal sent over a second radio link between the wireless communication device 110 and the second remote radio unit 130.

According to some embodiments, the second radio link quality measure may be received from the wireless communication device 110. However, according to some embodiments, the second radio link quality measure may be obtained from the second remote radio unit 130.

The step of obtaining the second radio link quality measure may be performed continuously, according to some embodiments.

Step 503

It is determined to initiate a handover from the first radio link to the second radio link based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit 120 and/or the traffic in the second remote radio unit 130.

The handover may be performed due to several reasons. One is that the radio link quality is better on one radio link than another radio link. However, a handover may also be made e.g. in order to balance the load between the remote radio units 120, 130, according to some embodiments.

Step 504

The same base band signals are sent to the second remote radio unit 130, as to the first remote radio unit 120.

Step 505

The sending of base band signals to the first remote radio unit 120 is terminated. Thus the base band signals are continuously only sent to the second remote radio unit 130.

According to some optional embodiments, the step of sending the same base band signals to the second remote radio unit 130, as to the first remote radio unit 120, and terminating the sending of base band signals to the first remote radio unit 120, may be performed simultaneously.

According to some optional embodiments, the sending of base band signals to the first remote radio unit 120 is terminated only if the obtained second wireless radio link quality is essentially better than the obtained first wireless radio link quality.

Thus, according to some embodiments, the sending of base band signals to the first remote radio unit 120 is terminated only if:

Q2 > k ^■ Q1 Where Q2 is the obtained second wireless radio link quality, Q1 is the obtained first wireless radio link quality and k is a factor which may be set to a predetermined value. As a non limiting example only, the factor k may be set to e.g. 2.

Figure 6 schematically depicts an embodiment of an arrangement 600 in a base band unit 150, adapted to perform the above described method steps 501-505. The arrangement 600 in the base band unit 150 is thus adapted to perform a handover of a wireless communication device 110 from a first remote radio unit 120 to a second remote radio unit 130. The first remote radio unit 120 and the second remote radio unit 130 are connected to the base band unit 150. Further, the first remote radio unit 120 and the second remote radio unit 130 are adapted to receive base band signals from the base band unit 150. The first remote radio unit 120 and the second remote radio unit 130 are also adapted to exchange wireless signals with the wireless communication device 110 over a first and a second radio link, respectively.

The arrangement 600 in the base band unit 150 comprises e.g. an obtaining unit 601. The obtaining unit 601 is adapted to obtain a first radio link quality measure based on a wireless signal sent between the wireless communication device 110 and the first remote radio unit 120, and a second radio link quality measure based on a wireless signal sent between the wireless communication device 110 and the second remote radio unit 130.

According to some embodiments, the first and the second radio link quality measure may be received from the wireless communication device 110.

However, according to some embodiments, the first radio link quality measure may be obtained from the first remote radio unit 120 and the second radio link quality measure may be obtained from the second remote radio unit 130.

The first and second radio link quality measures may be obtained continuously, according to some embodiments.

The arrangement 600 in the base band unit 150 also comprises a determination unit 603.

The determination unit 603 is adapted to initiate a handover from the first radio link to the second radio link based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit 120 and/or the traffic in the second remote radio unit 130.

The handover may be performed due to several reasons. One is that the radio link quality 5 is better on one radio link than another radio link. However, a handover may also be made e.g. in order to balance the load between the remote radio units 120, 130, according to some embodiments.

According to some embodiments, the determination unit 603 is adapted to determine that 10 the obtained second radio link quality is better than the obtained first radio link quality.

The arrangement 600 in the base band unit 150 also comprises a sender 604. The sender 604 is adapted to send the same base band signals to the second remote radio unit 130, as to the first remote radio unit 120, and is further adapted to terminate the sending of 15 base band signals to the first remote radio unit 120.

For the sake of clarity and in order not to unnecessarily obscure the functionality and advantages of the present method and arrangement 600, any internal electronics of the base band unit 150, not necessary for performing the present method according to steps 20 501-505 has been omitted from Figure 6.

However, according to some embodiments, the arrangement 600 in the base band unit 150 may also comprise e.g. a control unit 610. According to some embodiments, the control unit 610 may be a Central Processing Unit (CPU), a microprocessor, a Peripheral 25 Interface Controller (PIC) microcontroller or any other appropriate device which may be adapted to interpret computer program instructions and processes data.

Further, according to some embodiments, the arrangement 600 may comprise a memory unit. The optional memory unit may be a primary storage memory unit such as a

30 processor register, a cache memory, a Random Access Memory (RAM) or similar. The memory unit may however in some embodiments be a secondary memory unit such as a Read Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), programmable read-only memory (PROM) or erasable programmable readonly memory (EPROM) or a hard disk drive. The memory unit may however in some

35 embodiments be an off-line storage memory unit, a flash memory, a USB memory or a memory card. The memory unit may in some embodiments be a Network-attached storage (NAS) or in fact any other appropriate medium such as a disk or a tape that can hold machine readable data.

It is to be noted that the described units 601-610 comprised within the arrangement 600 in the base band unit 150 are to be regarded as separate logical entities but not with necessity separate physical entities. Any, some or all of the units 601-610 may be comprised or co-arranged within the same physical unit. However, in order to facilitate the understanding of the functionality of the arrangement 600 in the base band unit 150, the comprised units 601-610 are illustrated as separate physical units in Figure 6.

Some particular embodiments

The present method in the base band unit 150 for performing the handover of the wireless communication device 110 may be implemented through one or more processors 610 in the base band unit 150, together with computer program code for performing the functions of the method. The computer program code may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the methods according to the present solution when being loaded into the processor unit 610. The data carrier may be a CD ROM disc, a memory stick, or any other appropriate medium such as a disk or tape that can hold machine readable data. The computer program code may furthermore be provided as pure program code on a server and downloaded to the base band unit 150 remotely.

Thus a computer readable medium encoded with a computer program for performing the handover of the wireless communication device 110 from the first remote radio unit 120 to a second remote radio unit 130 may perform the method steps according to steps 501- 505.

Further, a computer readable medium is provided, encoded with a computer program for performing the handover of the wireless communication device 110 from the first remote radio unit 120 to a second remote radio unit 130. The first remote radio unit 120 and the second remote radio unit 130 are connected to the base band unit 150. The first remote radio unit 120 and the second remote radio unit 130 are also adapted to receive base band signals from the base band unit 150 and to exchange wireless signals with the wireless communication device 110 over a first and a second radio link respectively. The computer program comprises computer program code configured to make the processor 610, comprised within the base band unit 150 perform the step of a first radio link quality measure based on a wireless signal sent over a first radio link between the wireless communication device 110 and the first remote radio unit 120.

Also, the computer program code is configured to make the processor 610 perform the step of obtaining a second radio link quality measure based on a wireless signal sent over a second radio link between the wireless communication device 110 and the second remote radio unit 130.

Further the computer program code is configured to make the processor 610 perform the step of determining to initiate a handover from the first radio link to the second radio link based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit 120 and/or the traffic in the second remote radio unit 130.

The computer program code is furthermore configured to make the processor 610 perform the step of sending the same base band signals to the second remote radio unit 130, as to the first remote radio unit 120.

In addition, the computer program code is also configured to make the processor 610 perform the step of terminating the sending of base band signals to the first remote radio unit 120.

As will be appreciated by one of skill in the art, the present invention may be embodied as an arrangement 600 in a base band unit 150, a method in a base band unit 150 or computer program products. Accordingly, the present invention may take the form of an entirely hardware embodiment, a software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a "circuit" or "module." Furthermore, the present invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium. Any suitable computer readable medium may be utilized including hard disks, CD-ROMs, optical storage devices, a transmission media such as those supporting the Internet or an intranet, or magnetic storage devices.

Computer program code for carrying out operations of the present methods may be written in any arbitrary programming language such as Java®, Smalltalk or C++. However, the computer program code for carrying out the steps of the present method may also be written in any conventional procedural programming languages, such as the "C" programming language and/or a lower level assembler language. The program code may execute entirely on the base band unit 150, partly on the base band unit 150, as a stand-alone software package, partly on the base band unit 150 and partly on a remote computing device or entirely on the remote computing device. In the latter scenario, the remote computing device may be connected to the base band unit 150 through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer, for example, through the Internet using an Internet Service Provider.

Furthermore, the present methods were described in part above with reference to flowchart illustrations of methods and/or block diagrams of the arrangement 600 in a base band unit 150 and/or computer program products according to embodiments of the invention. It will be understood that each block of the various flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

Some clarification concerning interpretation of used terminology and expressions The terminology used in the detailed description of the particular exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements.

As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Further, as used herein, the common abbreviation "e.g.", which derives from the Latin phrase "exempli gratia," may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. If used herein, the common abbreviation "i.e.", which derives from the Latin phrase "id est," may be used to specify a particular item from a more general recitation. The common abbreviation "etc.", which derives from the Latin expression "et cetera" meaning "and other things" or "and so on" may be used herein to indicate that further features, similar to the ones that have just been enumerated, exist, which features however are omitted herein for brevity and/or clarity and/or in order to not unnecessarily obscure the comprehension of the presented subject matter.

As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes," "comprises," "including" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. Furthermore, "connected" or "coupled" as used herein may include wirelessly connected or coupled. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Claims

1. Method in a base band unit (150) for performing a handover of a wireless communication device (110) from a first remote radio unit (120) to a second remote radio unit (130), which first remote radio unit (120) and second remote radio unit (130) are connected to the base band unit (150) and are adapted to receive base band signals from the base band unit (150) and to exchange wireless signals with the wireless communication device (110) over a first and a second radio link respectively, the method is initialised and performed without need for firstly receiving an initiation signal from the wireless communication device (110), the method comprises the steps of: obtaining (501) a first radio link quality measure based on a wireless signal sent over a first radio link between the wireless communication device (110) and the first remote radio unit (120), obtaining (502) a second radio link quality measure based on a wireless signal sent over a second radio link between the wireless communication device (110) and the second remote radio unit (130), determining (503) to initiate a handover from the first radio link to the second radio link based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit (120) and/or the traffic in the second remote radio unit (130), sending (504) the same base band signals to the second remote radio unit (130), as to the first remote radio unit (120), and terminating (505) the sending of base band signals to the first remote radio unit (120).

2. Method according to claim 1 , wherein the steps of obtaining (501 , 502) the first and the second radio link quality measures comprises receiving said measures from the wireless communication device (110).

3. Method according to claim 1 , wherein the step of obtaining (501 ) the first radio link quality measure comprises receiving said value from the first remote radio unit (120), and wherein the step of obtaining (502) the second radio link quality measure comprises receiving said measure from the second remote radio unit (130).

4. Method according to any of the previous claims 1-3, wherein the steps of obtaining (501 , 502) the first and second radio link quality measures are performed continuously.

5. Method according to any of the previous claims 1-4, wherein the steps of: sending (504) the same base band signals to the second remote radio unit (130), as to the first remote radio unit (120), and terminating (505) the sending of base band signals to the first remote radio unit (120), are performed simultaneously.

6. Method according to any of the previous claims 1-4, wherein the step of terminating (505) the sending of base band signals to the first remote radio unit (120) is performed only if the obtained second wireless radio link quality is essentially better than the obtained first wireless radio link quality.

7. Method according to any of the previous claims 1-6, wherein the base band signals to be sent comprises information concerning which time slot for the remote radio unit (120, 130) to allocate for the wireless communication device (110), and which Mobile Allocation Index Offset "MAIO" value to use in that allocated time slot.

8. Arrangement (600) in a base band unit (150) adapted to perform a handover of a wireless communication device (110) from a first remote radio unit (120) to a second remote radio unit (130), which first remote radio unit (120) and second remote radio unit (130) are connected to the base band unit (150) and are adapted to receive base band signals from the base band unit (150) and to exchange wireless signals with the wireless communication device (110) over a first and a second radio link respectively, the arrangement (600) comprises: an obtaining unit (601), adapted to obtain a first radio link quality measure based on a wireless signal sent between the wireless communication device (110) and the first remote radio unit (120), and a second radio link quality measure based on a wireless signal sent between the wireless communication device (110) and the second remote radio unit (130), a determination unit (603), adapted to determine the moment to initiate a handover from the first radio link to the second radio link based on either the first radio link quality, the second radio link quality, the traffic in the first remote radio unit (120) and/or the traffic in the second remote radio unit (130), a sender (604), adapted to send the same base band signals to the second remote radio unit (130), as to the first remote radio unit (120), and further adapted to terminate the sending of base band signals to the first remote radio unit (120).