WO2008023204A1

WO2008023204A1 - Method of handover

Info

Publication number: WO2008023204A1
Application number: PCT/GB2007/050502
Authority: WO
Inventors: David Randall
Original assignee: Nokia Siemens Networks Gmbh & Co. Kg
Priority date: 2006-08-22
Filing date: 2007-08-21
Publication date: 2008-02-28

Abstract

A method of handover in a communication system comprising a base station (2, 3) and a user device (1) comprises determining in a serving base station (2) that a handover is required and instructing the user device (1) to handover to a target base station (3). A Handover Command (5) is sent from the serving base station (2) to the user device (1). The Handover Command includes one or more conditions for the handover to take place and data transfer between the user device (1) and the serving base station (2) continues until one or more of the conditions in the Handover Command has been met.

Description

METHOD OF HANDOVER

This invention relates to a method of handover, in particular for cellular communications systems, such as universal mobile telecommunications system (UMTS) long term evolution (LTE),

In UMTS LTE, it is desirable that radio capacity is not wasted when a handover is executed. For the particular case where, if at the time of handover execution, there is a partial transmission of internet protocol (IP) packets, then the capacity used to partially transmit the packet will be lost. However, it is also desirable that the handover is executed as quickly as possible.

One method of combining the two requirements is for the Handover Command to be executed when any ongoing transfer of an IP packet between a base station, or user equipment (UE) and a base station, or eNB, in the uplink and/or downlink, is complete. However, due to use of hybrid automatic repeat request (HARQ), which has a variable transmission time, and dynamic assignment of radio resources to the UE by the base station, or eNB, it is difficult for the radio resource controller (RRC) in the serving base station controlling the handover to predict when the packet transfer will be complete and, furthermore, the delay in completing the transfer of the packet could well be long enough for radio conditions to degrade, such that the handover command is not received by the UE.

In accordance with the present invention, a method of handover in a communication system comprising a base station and a user device comprises determining in a serving base station that a handover is required and instructing the user device to handover to a target base station; wherein a Handover Command is sent from the serving base station to the user device; wherein the Handover Command includes one or more conditions for the handover to take place; and wherein data transfer between the user device and the serving base station continues until one or more of the conditions in the Handover Command has been met.

The present invention addresses the problem of wasted resources when a handover command occurs during transmission of IP packets, by setting criteria for the timing of the handover to take place, so that rather than always execute the handover immediately after receiving the Handover Command, the data transfer is able to continue, avoiding wastage of resources. In many cases this means that the data transfer continues until complete, which is highly desirable in terms of efficient use of radio capacity.

Preferably, the conditions include at least one of elapsed time to handover, specified time of execution of handover; receipt of a data transfer complete indication; and immediate action flag.

There are various criteria which can be used to set the time at which the handover occurs, in some cases relating to some other event having occurred first, such as data transfer being complete, in others causing it to happen immediately, after expiry of a certain time, or at a specified absolute time. Preferably, the time of execution of handover, or the elapsed time to handover take precedence over receipt of a data transfer complete indication.

If more than one condition is set, e.g. after a certain time has elapsed and on completion of data transfer, then the elapse or setting of time overrides the absence of a completion of data transfer notification. Thus, if no data transfer complete indication is received before either time is reached, the handover goes ahead anyway.

Preferably, the data transfer complete indication is provided through downlink signalling.

This can be implicit, i.e. if the downlink grant goes to zero, then this is deemed to be an indication that data transfer is complete. The handover method applies to systems which carry out handover in which a

Handover Command is sent from a base station to a user device, but preferably, the system is universal mobile telecommunications system (UMTS).

An example of a method of handover in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a message sequence chart illustrating a first example of the method of the present invention; and,

Figure 2 is a message sequence chart illustrating a second example of the method of the present invention.

Fig. 1 illustrates the basic message steps involved in the method of the present invention. A UE 1 is within range of a serving evolved node B (eNB) 2 and a target eNB 3. Data transfer 4 is taking place between the serving eNB and the UE. An RRC of the serving eNB determines that a handover is required and prepares the target eNB 3, then issues a Handover Command 5 to the UE 1. The Handover Command may include a specific time at which it must be executed. In this example, the UE continues 6 to transmit data to the current, serving eNB 2 and to receive data from the current eNB until the UE receives an indication 7 that data transfer is complete, in this case a medium access control (MAC) instruction. The data transfer complete is triggered by the eNB determining that data transfer in the downlink and/or the uplink has fulfilled particular criteria e.g. partial transmission of IP Packets (SDUs) is complete. On receiving the MAC command the UE executes a handover 8 to the target cell. The indication does not have to be a MAC command. An alternative is to use an RRC issued command with the same meaning. If the UE fails to receive the MAC instruction before the specified execution time contained in the Handover Command 5 then the UE 1 executes the handover 8 at the specified time and aborts any ongoing data transfer. Alternatively, if there is no execution time included in the handover command, the UE can execute the handover at a pre-defined time interval after receiving the handover command. A variation is to include an 'ignore data transfer' flag in the Handover Command which controls whether the UE ignores data transfer and executes the handover at a signalled time, or immediately, depending upon what was signalled in the Handover Command.

In another example, shown in Fig. 2, the RRC of the serving eNB 2 determines that the handover is required, prepares the target eNB 3 and issues the Handover Command 5 to the UE 1. The Handover Command may include an execution time. The UE continues to receive data from the current, serving eNB until the UE detects 9 that it is no longer being scheduled data reception events in the downlink. Detection of this may be based on a failure to receive resource allocations in one, or more than one, consecutive scheduling events. When the UE detects this failure, it executes the handover 8.

If the UE is in the process of transmitting data in the uplink then this transmission is aborted on executing the handover. Alternatively, the UE may continue to transmit in the uplink for a period of time specified in control signalling received previously, except that the UE 1 must stop when the transmission of service data units (SDUs) whose transmission was in progress when the timer started is complete. Alternatively, the UE may continue to transmit in the uplink provided that the execution time contained in the handover command has not occurred and provided that transmission of any SDUs whose transmission was in progress when the end of downlink scheduling was detected has not been completed. If the execution time occurs before the above conditions are met then the UE executes the handover.

Claims

1. A method of handover in a communication system comprising a base station and a user device, the method comprising determining in a serving base station that a handover is required and instructing the user device to handover to a target base station; wherein a Handover Command is sent from the serving base station to the user device; wherein the Handover Command includes one or more conditions for the handover to take place; and wherein data transfer between the user device and the serving base station continues until one or more of the conditions in the Handover Command has been met.

2. A method according to claim 1, wherein the conditions include at least one of elapsed time to handover, specified time of execution of handover; receipt of a data transfer complete indication; and immediate action flag.

3. A method according to claim 2, wherein the time of execution of handover, or the elapsed time to handover take precedence over receipt of a data transfer complete indication.

4. A method according to claim 2 or claim 3, wherein the data transfer complete indication is provided through downlink signalling.

5. A method according to any preceding claim, wherein the system is universal mobile telecommunications system.