WO2008131589A1

WO2008131589A1 - Method and system for handover from a source cell to a target cell in a cellular communication system

Info

Publication number: WO2008131589A1
Application number: PCT/CN2007/001428
Authority: WO
Inventors: Martin Bakhuizen
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2007-04-28
Filing date: 2007-04-28
Publication date: 2008-11-06
Also published as: CN101379855A; CN101379855B

Abstract

The present invention relates to a method for communicating a handover command in a handover process in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein, in said handover process, communication with a user entity in said communication is handed over from a source cell to a target cell, and wherein a handover command is transmitted to said user entity from said communication system prior to the handover. The method comprises the step of transmitting said handover command from at least a first radio transmitter belonging to a cell being different from said source cell. The invention also relates to a communication system.

Description

Method and System for Handover from a Source Cell to a Target Cell in a Cellular Communication System

Field of the Technology

The present invention relates to a method for handover from a source cell to a target cell in a cellular communication system according to the preamble of claim 1.

The present invention further relates to a communication system according to the preamble of claim 26, a computer program according to claim 52 and a computer program product according to claim 53.

Background of the Invention

Wireless mobile communication systems are usually cellular, i.e., the total coverage area of such systems is divided into smaller areas wherein each of these smaller areas is associated with a radio base station having one or more antennas (radio transmitters) for providing communication resources via a radio interface for communication with user entities (UEs) in the radio coverage area of the radio base station. The areas are usualiy called cells, and the UEs can, e.g., consist of mobile phones, smartphones or handheld computers having communication capabilities.

A base station with a single radio transmitter is often referred to as an omni cell site having only one cell. However, a base station can also be shared by two or more cells, e.g. the base station can be provided with radio transmitters that transmit in different directions, the coverage area of each transmitter being defined as a cell. This is commonly called a sectorized site, each sector building one cell. A common example is three cells being supported by a sectorized base station. Alternatively, a single cell may utilize two or more radio transmitters for providing radio coverage in a specific area. In the simplest form of a cellular communications network, the cells are located adjacent to each other to jointly provide coverage in the geographical area that the communication system is intended to cover. In more complex networks, however, there are different cell layers, wherein cells in different layers can be at least partially overlapping, and also of different cell size. The cellular network supports mobility by allowing the interface between the UE and the network (i.e., the connection to a radio transmitter) to be transferred from radio transmitter to radio transmitter as the UE moves around in the network, thereby allowing the UE to maintain an ongoing communication. This transfer from one radio transmitter to another (which, in the present description and claims, is referred to as handover) usually take place when the interface quality to the serving cell (hereinafter called source cell) deteriorates and falls below a threshold. In a well planned wireless communication system, interface quality of another cell (or cells) will at the same time increase so that one of these other cells can be selected as the target cell, i.e., as the new source (serving) cell with a maintained communication.

With regard to handovers, there are a rmmber of different types of handovers, which are divided into hard and soft/softer handover.

Hard handover usually means that the UE leaves the source cell before connecting to the target cell. It is also referred to as "break before make" handover. This type of handover is used in e.g. GSM networks, in 3 G UTRAN networks as one option, and is also planned for in the current 3rd Generation Partnership Program (3 GPP) work on defining a future cellular communication system, presently called the Evolved Universal Terrestrial Radio Access (E-UTRA) or Long Term Evolution (LTE). If the UE stays connected with the source cell when connecting to the target cell, also referred to as "make before break", the handover is a soft handover. Accordingly, in this type of handover, the UE is connected to two (or more) cells simultaneously for some period of time. When the cells that the UE is connected to belong to the same base station, the handover is referred to as softer handover. A handover is normally triggered when it is identified that the source cell connection is deteriorating due to the UE moving out of the coverage area of the source cell, and a better cell being identified. However, in addition to identifying the deterioration of the connection between the UE and the source cell, a candidate target cell has to be identified before a decision to make a handover can be made. Further, the source cell base station often needs to consider measurements, measurements performed by the base station as well as the UE, over some period of time, in order to ensure that a handover is not triggered for what merely constitutes short coverage dips. Consequently, if it takes too long time to determine that a handover is required, and to determine a suitable target cell, the current connection may have deteriorated to such extent that a handover command comprising details about, e.g., communication resource(s) to be used in the target cell and/or target cell identity transmitted from the source cell is not properly received by the UE, and with regard to network controlled handover, it is essential that the UE receives the handover command so as to become aware of, e.g., the cell and/or communication resource(s) to which handover will be performed.

For this reason, several methods to increase the probability for receiving the handover command properly have been suggested. In GSM, for example, the source cell can increase the power when transmitting the handover command in order to increase the probability for the message to be received correctly by the UE. In another example, the handover command is repeated without waiting for an acknowledgment.

However, even with the above solutions, there is still a risk that the current communication channel has deteriorated to such extent that the handover command is not received by the UE, and the ongoing communication thereby is lost.

Consequently, there is a need for an improved method for handover from a source cell to a target cell so as to improve the probability of having a successful handover.

Summary of the Invention

It is an object of the present invention to provide a method in a cellular communication system that solves the above mentioned problems. This object is achieved by a method according to the characterizing portion of claim 1.

It is a further object of the present invention to provide a communication system that solves the above mentioned problems. This object is achieved by a communication system according to the characterizing portion of claim 26.

According to the present invention, it is provided a method for communicating a handover command in a handover process in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein in said handover process communication with a user entity in said communication is handed over from a source cell to a target cell, and wherein a handover command is transmitted to said user entity from said communication system prior to the handover. The method comprises the step of transmitting said handover command from at least one radio transmitter in a cell being different from said source cell.

This has the advantage that the possibility of the UE actually receiving the handover command can be substantially improved since the handover command is, as opposed to the prior art, transmitted from a cell normally having a better channel quality from the UE point of view than the source cell. In the simplest form of the invention, the handover command is only transmitted from a cell other than the source cell, e.g., the target cell or any other cell capable of providing higher quality communication than the source cell. Since a handover is normally triggered when it is identified Hhat the source cell connection is deteriorating, which usually is due to the UE moving out of the coverage of the source cell, there is often a cell that is better suited, e.g., having a better signal strength at the UE, for transmitting the handover command. The handover command can, for example, comprise an identity of the target cell to which the communication is to be handed over, and/or information regarding a communication resource to be used for communication with the target cell.

In an alternative embodiment of the present invention, the handover command is transmitted from a plurality of cells, e.g., two, three or more. For example, the information can be transmitted from the source cell and the target cell, or the target cell and another neighbour cell, or cells not constituting either of source or target cells. As will be explained below, the plurality of cells can transmit the handover command either sequentially or in parallel on the same physical communication resource. Further characteristics of the present invention, and advantages thereof, will be evident from the following detailed description of preferred embodiments and appended drawings, which are given by way of example only, and are not to be construed as limiting in any way.

Brief Description of the Drawings

Fig. 1 discloses an example of a E-UTRA system in which the present invention may advantageously be utilised. Fig. 2 discloses an exemplary handover procedure according to the prior art.

Fig. 3 discloses an exemplary handover procedure according to the present invention.

Fig. 4 discloses a farther exemplary handover procedure according to the present invention.

Fig. 5 discloses another exemplary handover procedure according to the present invention.

Detailed Description of the Invention

The present invention will now be described with reference to the 3rd Generation Partnership Program (3 GPP) work on defining a future cellular communication system, which presently is called the Evolved Universal Terrestrial Radio Access (E- UTRA) or Long Term Evolution (LTE).

An example of the architecture of an E-UTRA system 100 is shown in fig. 1, and consists of two kinds of nodes, radio base stations (stationary radio transceivers for duplex communication or, as the case may be, radio transmitters for downlink only mode communication. In fig. 1, the base stations are provided with radio transceivers), in E-UTRA called eNBs (enhanced Node B) 101-103 and aGWs (access GateWay) 104-105, where the eNBs 101-103 belongs to the evolved UTRAN (E-UTRAN) 106 and the aGW belongs to the evolved packet core (EPC) 107. A user entity (UE) 108 connects to the network (E-UTRAN and EPC) by means of a radio interface, Uu interface 109. The eNB 101-103 handles communication over the radio interface in a certain coverage area, i.e., cell, which is the area wherein the radio signal is strong enough to allow a satisfactory communication with UEs within said area.

When the UE 108 moves around in the coverage area provided by the communication system, the UE 108 will move from one cell to another, and thereby an ongoing communication will be transferred (handed over) from the eNB to which the UE presently belongs, i.e., the source cell, cell 103 in fig. 1, to the cell into which the UE is entering, i.e., the target cell, e.g., cell 102 in fig. 1. This is accomplished by establishing a communication channel on the Uu interface of the target cell, and terminating the communication channel on the Uu interface of the source cell. The target cell is usually selected based on measurements performed by either of, or both the UE and the network (Node B). The handover of a communication from one base station to another can be network controlled or UE controlled or a combination thereof. In case of UE controlled handover, the target cell is not aware of the handover until contacted by the UE, when the UE contacts the target cell base station, this base station communicates with the base station controlling the source cell so as to request the source cell base station to forward information about the UE that is required in order to establish the connection with the UE and continue the ongoing communication. UE controlled handover is sometimes called a forward handover.

When the handover is network controlled, the UE can provide the network with information to assist the network in making a handover decision, for example by transmitting neighbour cell measurements. This is referred to as Mobile Assisted HandOver (MAHO), and the UE performs measurements on surrounding base stations, which measurements are transmitted to the eNB for processing by the system. In order to reduce signalling, the UE normally only measures potential target cells. The "potential" target cells are defined in a neighbour cell list, which is transmitted to UEs within range of the eNB on a radio channel.

As an alternative to the network transmitting neighbour cell lists, the neighbour cell list information can be determined in the UE, in active and/or idle mode, without the need of transmitting extensive lists of surrounding, or neighbouring, cells from each cell in the system. An example of such a system is disclosed in 3GPP document

R2-070493, "Neighbour cell list reduction", which describes a method wherein the

UE implicitly generates a neighbour cell list based on information transmitted in each cell.

In fig. 2 is shown a typical handover process, for example when UE 108 leaves the coverage area of eNB 103 and enters the coverage area of eNB 102 in fig. 1. Prior to the handover, data to the UE is communicated from aGW 105 to eNB 103 and further onto the UE 108 as indicated by arrow 211. As was mentioned above, the UE continuously measures, e.g., signal strength of surrounding base stations, which, e.g., can be stated in a neighbour cell list transmitted by the serving cell base station. In the disclosed example, the list can, for example, contain eNB 101 and 102 and the serving eNB 103. The measurement reports, in turn, can, for example, contain signal strength and/or other channel quality parameters of base stations 101 and 102, and are sent by the UE to eNB, arrow 212, either continuously or on demand (indicated by arrow 210). eNB 103 then makes a handover decision based on own and UE measurements (decision box 213 in fig. 2). In this example, eNB 102 is determined to be suitable as a new target cell.

After the decision, the eNB 103 sends a Handover Request message, arrow 214, to the eNB of the candidate target cell, eNB 102, and in decision box 215 the target cell eNB 102 allocates resources for the handover and replies to eNB 103, arrow 216, with a Handover Request Acknowledge including information describing this resource. When the serving eNB receives the Handover Request Acknowledge 216, a. Handover Command containing the information that is required for the UE in order to successfully establish a connection to the target cell, such information can, for example, include an identity of the target cell and/or communication resource information regarding the target cell communication resource(s), (e.g., the allocated resources of step 215) to be used, is transmitted to the UE 108 from eNB 103, arrow 217.

The UE 108 then changes communication resource(s) to the new allocated radio resource(s) and establishes radio contact with the target cell eNB 102, arrow 218. eNB 102 then communicates with aGW 105 in order to inform aGW about the handover, arrow 219, aGW performs the necessary path switching, 230, to transfer the data flow from eNB 103 to eNB 102 and aGW responds with an acknowledgement, arrow 220.Upon reception of the acknowledgement from the aGW, eNB 102 signals to the now former source (serving) eNB 103 that eNB 103 can release its resources for other use, and user data, arrow 221, is communicated to the UE by means of eNB 102. If, as shown in fig. I₅ the eNBs would belong to different aGWs, e.g., if the above communication were to be handed over to eNB 101 instead of eNB 102, eNB would transmit the handover complete information to aGW 104 instead of aGW 105, and in such a situation some additional path switching signalling 230 would be required to "handover" the communication from aGW 105 to aGW 104 for further transport to eNB 101.

The solution described in fig. 2, however, has some disadvantages. As mentioned, an eNB normally initiates the above described handover when measurements on the channel of the ongoing communication and/or measurements received from the UE indicate that the current connection with the UE is deteriorating, e.g. due to the UE moving out of the coverage area of the eNB.

If the deterioration is slow, which, for example often is true in rural environments where there is more or less line of sight between the UE and the antenna system of the cell, the gradual degradation is usually slow enough to not only ensure that the deterioration is in fact identified and classified as a deterioration due to the

UE moving out of the coverage area, and not a local coverage dip (for example, urban environments normally suffers from Rayleigh fading, which is characterized by small spotty areas with very bad reception), but also to ensure that a candidate target cell is identified, and a handover decision is made and safely communicated to the UE before the communication deteriorates to such extent that the communication is lost all together.

However, in dense urban environments, the deterioration is typically rapid, since the radio waves of the connection can be suddenly obstructed by a large building or other objects in the urban environment. In such situations, the deterioration can be so rapid that the handover process is not completed before the communication is lost, since the time it takes for the handover process to be completed depend, e.g., on the fact that the eNB needs to consider measurements, performed by the base station as well as the UE over some period of time in order to ensure that a handover is not triggered for short coverage dips. In addition, the UE may not be able to measure on the candidate target cells until the current cell connection already has become rather bad. This typically happens in urban areas when the UE is moving around the corner of a building (the so called around-the-comer effect). Consequently, if it takes too long time to determine that a handover is required, and to determine a suitable target cell, the current connection can have deteriorated to such extent that the handover command when transmitted is not properly received by the UE, and with regard to network controlled handover, it is essential that the UE receives the handover command so as to become aware of to which cell handover will be performed in order to avoid interruptions in the communication, which possibly leads to a noticeable break in speech, or loss of data to the UE user. Even though, as stated above, the source eNB can increase the power when transmitting the handover command in order to increase the probability for the message to be received correctly by the UE, or repeat the handover without waiting for an acknowledgment, there is still a substantial risk that also these messages to the UE are lost.

The present invention overcomes these drawbacks by a handover procedure, an exemplary embodiment of which being described in fig. 3. The main idea of the present invention is to utilize a cell other than the source (serving) cell when sending the handover command message. In the example in fig. 3, both the source cell and the target cell are used. The handover procedure shown in fig. 3 is in many aspects similar to the handover procedure of fig. 2. For example, the steps 310-313 in fig. 3 are similar to steps 210-213 in fig. 2 and are therefore not elaborated further.

In this exemplary embodiment, it is envisaged that neighbouring base stations can transmit on the same physical resources, i.e. on the same radio channels. This is, for example, true for E-UTRAN. For example, transmissions in E-UTRAN can be arranged to be transmitted in a single cell or in a group of cells, wherein the transmission can be synchronized in such a way that the UE may combine the energy from multiple transmissions on the same channel from different cells without additional receiver complexity. Such transmission is possible if identical information is transmitted from different cells in "the same" physical radio resource at the same time using the same coding scheme. Consequently, the UE need not be aware of the fact that it is one or more base stations other than the source base station that is transmitting the received information. In OFDMA, which is used in E-UTRAN, a physical radio resource is defined by a set of frequencies and a time slot. Accordingly the transmissions must correspond in frequency and time.

Reverting to fig. 3, when the source cell has made the handover decision, 313, it will send information to the target cell not only including the Handover Request, but also the channel on which the target cell shall send the Handover Command message, which corresponds to the channel on which the source cell will send the handover command. This information can be included in the existing Handover Required message, or sent in a separate message. In this way, the transmissions from source cell and target cell, as long as they are simultaneous, simply will appear as a single transmission, however with a multipath (diversity) spread. Consequently, as long as the transmissions from source cell and target cell are synchronized in a manner such that the UE may combine the energy from the transmissions, the two transmissions, indicated by arrows 317a and 317b, respectively, therefore will appear as a single transmission to the UE, however with a substantially improved probability of proper reception, since even if the communication link to the source cell has deteriorated totally, the signal quality of the signal transmitted from the target cell most likely will be more than sufficient for allowing proper reception of the handover information, whereafter the procedure can continue as described in fig. 2. With regard to the timing information on when to send the Handover Command message, either the source cell can indicate to the target cell when to send the handover command, or the target cell indicate to the source cell when to send the handover command. This information can be sent in a separate message or, preferably, be included in the Handover Request and Handover Request Ack messages.

In sum, the UE will both benefit from the normally stronger signal from the target cell, and the benefit of multipath combining gain as long as the handover command messages from the source and target cell are identical, sent on exactly the same radio channel and synchronized rather tightly (i.e., received within the

"multipath" (equalizer) receive window of the UE).

In today's cellular systems, such tight synchronization is normally not implemented, but in future cellular systems, such as E-UTRAN this will become more common.

In GSM, the channel wherein the handover command message is to be sent is known in advance by both the source cell and the UE. But in other systems, such as E- UTRAN, the channel has to be allocated just before the Handover Command message is to be sent. This is called a Downlink (DL) Allocation. Messages like the DL Allocation, and other messages associated to the sending of the handover command message, can be treated in the same way as the handover command message itself. I.e., the DL allocation can be sent simultaneously by both the source and target cell.

A further advantage of the present invention is that it has little or no impact on the UE in the sense that the UE doesn't need to take any specific action, it just behaves as if the handover command message was received from the source cell only. Above, the source and target cell send the handover command message simultaneously. As stated, this exemplary embodiment requires the two cells to be tightly synchronized. Although this can easily be achieved in networks supporting Single Frequency Network operation, e.g. the E-UTRAN system, the above problems of lost communication are equally valid in communication systems with less strict inter-base station synchronization.

In fig. 4 is disclosed an exemplary solution for such systems. When the handover decision is made, it is likely that the target cell will have better radio conditions than the source cell for communication with the UE. Therefore, it can be advantageous to only let the target cell send the Handover Command message, preferably in a manner such that the UE does not need to be aware of anything but the source cell channels. For example, this can be accomplished by the target cell transmitting the handover command on the very same physical channel that the source cell otherwise would have transmitted the command. Thereby, in the UE point of view, it will look just as if the source cell had sent the handover command. This is shown in fig. 4, which discloses a handover procedure that is identical to fig. 3 apart from the fact that only the target cell transmits the Handover Command message 517. Consequently, similar to the solution in fig. 3, the source cell indicates, 514, to the target cell which control channel the source cell is using for communication with the UE, and that is to be used for sending the Handover Command message. Similar to the above, this information can be included in the existing Handover Required message, 514, or be sent in a separate message. Consequently, the target cell behaves as a source cell since it will send the Handover Command message 517 precisely as the source would have done. Although synchronization demands are not as strict, it is still required that the target cell is synchronized to the source cell on a level that ensures that the Handover Command message is sent within any timeslot window allocated to the UE in the source cell. The solution described in fig. 4 also has the advantage that it has no impact on the UE in the sense that the UE doesn't need to take any specific action, it just behaves as if the Handover Command message was received from the source cell. In fig. 5 is shown a further embodiment according to the present invention. In this embodiment, steps 610-613 correspond to steps 210-213 in fig. 3. In step 614, however, the information transmitted to the target cell further includes an instruction for the target cell to transmit a Handover Command message if, after a certain period of time, it is discovered that the UE has failed to receive the Handover Command message transmitted from the source cell. There are several ways to discover that the handover command has failed. For example, the target cell is waiting for the Handover Confirm message, and when the message is not received within a certain time (timer controlled), this can be the trigger. In another exemplary embodiment the Handover Command message is to be acknowledged by the UE sending an ACK message to the source cell. If this ACK is not received, or if a Negative Ack (NACK) is received, this triggers the sending of Handover Command message by the target cell. In this latter case, the source cell will send a trigger message to the target cell. Further, the source cell can be arranged to transmit the waiting time (timer value) to the target cell in addition to the other information in step 614. The timer value can alternatively be fixed or pre-set by Operation and Maintenance procedures.

As an alternative to only transmitting the Handover Command by the target cell after the timer has expired, both the source cell and target cell can be arranged to transmit the Handover Command, e.g., in a synchronized manner according to fig. 3.

An even further exemplary embodiment of the present invention is to utilize a special dedicated channel with better coverage for sending the Handover Command message. For example, this channel can be dedicated to only comprise handover data, either for the said source cell or for a group of cells sharing this dedicated channel, in which case, a particular UE can be reached using a UE specific address. The UE can be informed of the details of this channel anytime during its connection with the source cell. In one embodiment, if the sending of the Handover Command message using a conventional channel fails, the network starts transmitting a similar message on this dedicated channel. For example, the UE can be arranged to start listening to this channel when it has lost its connection with the source cell. Alternatively, the handover commands can at all times be arranged to be transmitted on the dedicated channel.

The dedicated channel can be used for transmission from the target cell alone, from both the source cell and the target cell, or from one or more cells being different from said source cell and/or said target cell. For example, at the time of handover, there may be a plurality of cells having better channel quality at the UE location than the presently serving cell. In such a situation, such cells can be identified from the UE measurement data transmitted to the network, whereupon the cells can be arranged to transmit the handover command on the dedicated channel, or, in another embodiment, on the channel used by the source cell.

An advantage of using a dedicated channel is that the dedicated channel can be tuned for having better performance on the cell border between source and target cell than the normal control channel from the source cell. This can be achieved, e.g., by a simultaneous transmission from both source and target cells, e.g., transmission in a

Single Frequency Network mode.

The UE can be informed about this channel any time during the connection, and if the UE loses connection with the source cell it can listen to this channel for handover command messages. The network will, if detecting that the UE is lost, start transmitting a Handover Command message on the dedicated channel. If, after expiry of a timer, the UE can still not receive any Handover Command message, it can proceed to a call re-establishment procedure if available. In a further embodiment, cells in said communication system are arranged to transmit identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other. In this embodiment, the handover command can be arranged to include a representation of the position of the target cell in said cell pattern. The cells in said pattern can be distinguishable by a logical cell value (LCV) representing the position of a cell in said pattern, and wherein said handover command at least partly consist of a representation of a logical cell value representing the target cell. Such a cell pattern system is disclosed more in detail in, e.g., 3GPP document R2-070493, "Neighbour cell list reduction". In sum, all embodiments of the present invention have in common that the probability of the Handover Command being received correctly by the UE increases, and for at least some of the embodiments the present invention has little or no impact on the UE, i.e., no additional features has to be provided to the UE.

Although the present invention has been described in connection with an E- UTRAN system, the principles of the invention applies to cellular access systems in general, and are therefore applicable in any cellular system, such as, but not limited to,

CDMA2000, UMTS & GSM. In particular, the exemplary embodiments wherein the target cell only, or two or more cells in sequence transmits handover commands, are applicable at least for as long as these cells are capable of transmitting the handover command on a channel that the UE is capable of decoding. Further, the exemplary embodiments wherein two or more cells transmit the handover command synchronously, these embodiments are applicable at least in all systems wherein such synchronized transmission is possible.

Claims

1. Method for communicating a handover command in a handover process in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein, in said handover process, communication with a user entity in said communication is handed over from a source cell to a target cell, and wherein a handover command is transmitted to said user entity from said communication system prior to the handover, characterised in that the method comprises the step of: transmitting said handover command from at least a first radio transmitter belonging to a cell being different from said source cell.

2. Method according to claim 1, characterised in that said first radio transmitter belonging to a cell being different from said source cell is a target cell radio transmitter.

3. Method according to claim 1, characterised in that said first radio transmitter belonging to a cell being different from said source cell is a radio transmitter in a cell being different from both said source cell and said target cell.

4. Method according to claim 1, characterised in that said handover command is transmitted from at least a first radio transmitter in a first cell and at least a second radio transmitter in a second cell, said second cell being different from said first cell.

5. Method according to claim 4, characterised in that said first radio transmitter is a target cell radio transmitter.

6. Method according to claim 4, characterised in that said second radio transmitter is a source cell radio transmitter.

7. Method according to claim 4, characterised in that said second radio transmitter is a radio transmitter in a cell being different from said source cell.

8. Method according to claim 1, characterised in that said step of transmitting said handover command is executed when said user entity moves from a coverage area of said source cell to a coverage area of said target cell.

9. Method according to claim 1, characterised in that said handover command comprises information that is required in the UE when establishing a connection to the target cell.

10. Method according to claim 4, characterised in that said handover command 5 further is transmitted to said user entity from at least a third radio transmitter in a third cell.

11. Method according to claim 4, characterised in that said handover command is transmitted to said user entity from said radio transmitters in a synchronized manner.

12. Method according to claim 11, characterised in that said handover command is 10 transmitted by the radio transmitters on the same physical resource.

13. Method according to claim 1, 4 or 12, characterised in that said radio transmitter or radio transmitters transmit the handover command on a physical channel resource used by said source cell for communication with said user entity.

14. Method according to claim 1, characterised in that said radio transmitter or 15 radio transmitters transmit the handover command on at least one physical channel resource being dedicated for handover information communication.

15. Method according to claim 14, characterised in that said at least one dedicated channel resource is shared among a plurality of base stations and/or user entities.

16. Method according to claim 1, characterised in that said handover command is _0 transmitted on a physical channel resource consisting of one or more frequencies and one or more transmission time intervals.

17. Method according to claim 1, characterised in that the method further includes the step of, prior to transmitting said handover command, determining the said target cell for said handover in said communication system.

>5 18. Method according to claim 17, characterised in that said determination is performed by said source cell radio transmitter.

19. Method according to claim 1, characterised in that each of said cells in said communication system transmits identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other, said handover command including a representation of the position of the target cell in said cell pattern.

20. Method according to claim 19, characterised in that said cells in said pattern are distinguishable by a logical cell value (LCV) representing the position of a cell in said pattern, and wherein said handover command at least partly consist of a representation of a logical cell value representing the target cell.

21. Method according to claim 1, characterised in that said step is performed if a handover command transmitted from said source cell is not received by the user entity.

22. Method according to claim 1, characterised in that it further includes the step of transmitting a Downlink (DL) Allocation message to said user entity using said radio transmitter.

23. Method according to claim 1, characterised in that said radio transmitter is a radio transceiver.

24. Method according to claim 1, characterised in that said radio transmitter is a radio base station.

25. Method according to claim 24, characterised in that said radio transceiver is an eNB (enhanced Node B) in a Evolved UTRA (E-UTRA) system.

26. A cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein, in a handover process, communication with a user entity in said communication is handed over from a source cell to a target cell, and wherein a handover command is transmitted to said user entity from said communication system prior to the handover, characterised in that the system includes means for, in said handover process: transmitting said handover command from at least a first radio transmitter belonging to a cell being different from said source cell.

27. System according to claim 26, characterised in that said first radio transmitter belonging to a cell being different from said source cell is a target cell radio transmitter.

28. System according to claim 26, characterised in that said first radio transmitter belonging to a cell being different from said source cell is a radio transmitter in a cell being different from both said source cell and said target cell.

29. System according to claim 26, characterised in that it includes means for transmitting said handover command from at least a first radio transmitter in a first cell and at least a second radio transmitter in a second cell, said second cell being different from said first cell.

30. System according to claim 29, characterised in that said first radio transmitter is arranged to be a target cell radio transmitter.

31. System according to claim 29, characterised in that said second radio transmitter is arranged to be a source cell radio transmitter.

32. System according to claim 29, characterised in that said second radio transmitter is arranged to be a radio transmitter in a cell being different from said source cell.

33. System according to claim 26, characterised in that it includes means for executing said handover when said user entity moves from a coverage area of said source cell to a coverage area of said target cell.

34. System according to claim 26, characterised in that said handover command comprises information that is required in the UE when establishing a connection to the target cell.

35. System according to claim 34, characterised in that said handover command includes a representation of communication resource(s) to be used in the target cell and/or a target cell identity.

36. System according to claim 29, characterised in that said system further includes means for transmitting said handover command to said user entity from at least a third radio transmitter in a third cell.

37. System according to claim 29, characterised in that said system includes means for transmitting said handover command to said user entity from said radio transmitters in a synchronized manner.

38. System according to claim 37, characterised in that said handover command is arranged to be transmitted by the radio transmitters on the same physical resource.

39. System according to claim 26, 29 or 38, characterised in that said radio transmitter or radio transmitters is arranged to transmit the handover command on a physical . channel resource used by said source cell for communication with said user entity.

40. System according to claim 26, characterised in that said radio transmitter or radio transmitters is arranged to transmit the handover command on at least one physical channel resource being dedicated for handover information communication.

41. System according to claim 40, characterised in that said at least one dedicated channel resource is shared among a plurality of base stations and/or user entities.

42. System according to claim 26, characterised in that said handover command is arranged to be transmitted on a physical channel resource consisting of one or more frequencies and one or more transmission time intervals.

43. System according to claim 26, characterised in that it further includes means for, prior to transmitting said handover command, determining the said target cell for said handover in said communication system.

44. System according to claim 43, characterised in that said determination is performed by said source cell radio transmitter.

45. System according to claim 26, characterised in that each of said cells in said communication system are arranged to transmit identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other, said handover command including a representation of the position of the target cell in said cell pattern.

46. System according to claim 45, characterised in that said cells in said pattern are distinguishable by a logical cell value (LCV) representing the position of a cell in said pattern, and wherein said handover command at least partly consist of a representation of a logical cell value representing the target cell. .

47. System according to claim 26, characterised in that said transmission of said handover command from said first radio transmitter is performed if a handover command transmitted from said source cell is not received by the user entity.

48. System according to claim 26, characterised in that it further includes means for transmitting a Downlink (DL) Allocation message to said user entity using said radio transmitter.

49. System according to claim 26, characterised in that said radio transmitter is a radio transceiver.

50. System according to claim 26, characterised in that said radio transmitter is a radio base station.

51. System according to claim 50, characterised in that said radio transceiver is an eNB (enhanced Node B) in a Evolved UTRA (E-UTRA) system.

52. Computer program, characterised in code means, which when run in a computer causes the computer to execute the method according to any of the claims 1-25.

53. Computer program product including a computer readable medium and a computer program according to claim 52, wherein said computer program is included in the computer readable medium.

54. Computer program product according to claim 53, characterised in that said computer readable medium consists of one or more from the group: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), hard disk drive.