HANDOVER METHOD AND APPARATUS IN A WIRELESS TELECOMMUNICATIONS NETWORK
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for handover in a wireless telecommunications network, and more particularly, but not exclusively, to a method and apparatus implemented in accordance with the 3rd Generation Partnership Project (3GPP) evolved Universal Terrestrial Radio Access Network (E-UTRAN) and evolved Universal Terrestrial Radio Access (E-UTRA) specifications.
BACKGROUND OF THE INVENTION
Currently, 3GPP is considering development of E-UTRA and E-UTRAN as set out in the technical specification 3GPP TS 36.300 v 8.1.0 (2007- 06), incorporated herein by way of reference, and related documents. 3GPP Long Term Evolution (LTE) aims to enhance the Universal Mobile Telecommunications System (UMTS) standard, for example, by improving efficiency and services.
In E-UTRAN, user equipment (UE) communicates with a network node, NodeB (eNB), with data being sent on radio bearers (RBs) over a radio link between them. The eNB interfaces with a Mobile Management Entity/System Architecture Evolution Gateway (MME/SAE GW) via an interface designated as SI. An E-UTRAN network includes a plurality of eNBs and MME/SAE GWs.
In LTE, all the Radio Access Network (RAN) functions are integrated in each node, eNB. Downlink user data, that is Intemet Protocol (IP) packets, are transmitted from the SAE GW to the eNB. As the UE is handed over from a first, source, eNB to a second, target, eNB, the SAE GW is updated with the second eNB address and the SAE GW starts to send data to that target eNB.
However, to avoid data loss, any data that is already buffered in the source eNB must be forwarded to the target eNB. Also, data that has been sent to the source eNB during the handover (HO) procedure, before the SAE GW is updated with the new eNB address, is also forwarded by the source eNB to the target eNB.
To preserve the order of packets sent to the UE, the target eNB must strive to send data over the radio in the same order as sent by the SAE GW. That is, firstly

data buffered by the eNB is sent to the target eNB, followed by data in transit from the S AE GW during the HO process, and only when these have all been sent should the target eNB send to the UE fresh data that it receives directly from the SAE GW.
The message flow for the HO process applied to a UE 1 is shown in Figure 1 which illustrates a network including a source eNB 2, a target eNB 3 and an MME/SAE GW 4. When the source eNB 2 makes a handover decision based on measurement reports from the UE 1, it sends a Handover Request message to the target eNB 3. At the Admission Confrol step 5, the target eNB 3 configures the required resources and sends a Handover Request Acknowledge message to the source eNB 2. Following the handover command from the source eNB 2 to the UE 1, the UE 1 detaches from the old cell and synchronises to the new cell associated with the target eNB 3. Also, data packets buffered at the source eNB 2 and any in transit are forwarded to the target eNB 3 over an interface designated X2. Following the handover confirm message at step 10 from the UE 1 to the target eNB 3, a handover completion message is sent to the MME/SAE GW 4 by the target eNB 3. Data packets from the source eNB 2 continue to be delivered to the target eNB 3. The target eNB can then send fresh data arriving over SI from MME/SAE GW once all the forwarded data from source eNB 2 has been received by it.
Forwarded data from the source eNB to the target eNB may take two forms for LTE: forwarding of data buffered at the source eNB and forwarding of freshly arriving packets at the source eNB over SI. Lossless HO requires that both these types of data are forwarded from the source to the target eNB. 3GPP RAN2 has discussed and agreed the data forwarding principles to provide lossless inter-eNB handover. However, the conditions when lossless HO should be applied have not been discussed so far.
It cannot be assumed that all data received at the source eNB before the HO Command is delivered to the UE is considered "buffered" data and has accordingly been allocated Packet Data Convergence Protocol (PDCP) sequence numbers (SNs). Incoming packets may be provided with a PDCP SN as soon as they are received by the eNB or, alternatively, a PDCP SN may be provided just before delivery to the UE

in near real time. Any packets that have not been actually sent to the UE can be treated as fresh data over SI as long the last used PDCP SN is used correctly.
It was also agreed by 3GPP RAN 2 that forwarding of buffered packets should be carried out based on radio link control (RLC) status reports. RLC status reports are provided where the RLC acknowledged mode (RLC-AM) is selected for a radio bearer. The acknowledged mode (AM) provides enhanced reliability as it allows retransmission of incorrect or lost data packets, and is suitable for non-real time services, for example.
In RLC unacknowledged mode (RLC-UM), no retransmission protocol is used and there are no RLC status reports, and thus data forwarding is not envisaged for RLC-UM. The imacknowledged mode may be used, for example, for certain radio resource control (RRC) signalling and voice over IP (VoIP). The choice as to which RLC mode to adopt is based on the residual bit error rate (BER) required for the bearer and the Hybrid Automatic Repeat Request (HARQ) operating point.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the invention, a method for handover of a mobile terminal from a source node to a target node in a wireless telecommunications network includes the step of: making a determination as to whether or not to forward data from the source node to the target node, and the determination for each radio bearer being made independently of which radio link control, RLC, mode it supports. Thus, in a method in accordance with the invention, data forwarding may be selected irrespective of whether RLC-UM or RLC-AM bearers are involved and may be chosen per radio bearer. This provides flexibiUty independent of RLC mode. The method is appHcable to networks implemented in accordance with Long Term Evolution, LTE, standards, but may also be used in other types of network.
In a method in accordance with the invention, the choice of whether or not to implement data forwarding may be determined with reference to the Quality of Service (QoS) requirements for the bearer and on performance of the last mile, such as cost and/or speed, for example. Data forwarding could be chosen per radio bearer taking into account the QoS requirements of the bearer, but is not essential since the

reasons for choosing to forward data may, for example, include instead, or additionally, the amount of SI and X2 delay. For example, if the last mile topology for X2 is up to the SI anchoring point, forwarding data will be costly for the operator and slow. Operators may decide not to use data forwarding for these cases. Further, the mobility rate of the UE may also impact the decision about whether to forward data or not. It may not be worthwhile to implement data forwarding for a slow moving UE. Thus, even for RLC-AM, data forwarding may be avoided for situations such as these.
If the SI link or MME S-GW link is slow, the path switch could be delayed significantly and it may thus be desirable to forward packets considered to be firesh data, that is, without associated SNs, received by the source node over SI, even where the mode is RLC-UM.
According to another method in accordance with the invention, when it is determined that data forwarding is not used, the Packet Data Convergence Protocol (PDCP) sequence numbers (SNs) are reset, irrespective of which RLC mode is used.
A method in accordance v^th the invention may include providing an instruction to the mobile terminal for each bearer as to whether a bearer is subject to data forwarding or not. This may then be used by the mobile terminal to handle the buffered packets and PDCP SNs. The instruction maybe included, for example, as part of the Handover command or as a separate message.
Previously, for RLC-AM, the PDCP sequence number was continued in the target node after the HO. This was considered essential for re-sequencing and identifying packet duplicates over the radio. For this reason, the last used PDCP SN of the buffered packets and the last used PDCP SN were transferred to the target node by the source node. In a method in accordance with the invention, in contrast, when it is determined not to forward data to the target node, the PDCP sequence number is re¬started with a known value and the last used PDCP SN is not transferred to the target node. Where the X2 interface between the source and target nodes is slow compared to the SI interface, transferring the PDCP SN information to the target node could increase the interruption time, since the target node must wait for this SN before

sending any fresh data received directly by it over SI. By re-starting the PDCP SN, this potential interruption can be avoided.
According to another method in accordance with the invention, for RLC-UM there is no need to transfer the PDCP SN to the target node and it is simply re-started.
According to a method in accordance with the invention, since the mobile terminal must know if the PDCP should be reset or continued, a mechanism for signalling this to the mobile terminal is included. Where the PDCP SN is arranged to restart when data forwarding is not implemented, the mobile terminal may be signalled that data forwarding is not being used, so as to alert it that PDCP should be reset, whatever RLC mode is being used. Also, indicating to a mobile terminal that a bearer is not subject to data forwarding allows the mobile terminal to deHver any buffered packets immediately to the upper layers. The mobile terminal can also re-initiahse the PDCP SN to a known value.
According to a second aspect of the invention a method for handover of a mobile terminal from a source node to a target node in a wireless telecommunications network, includes the step of: forwarding data from the source node to the target node for a radio bearer in RLC-UM mode.
According to a third aspect of the invention method for handover of a mobile terminal from a source node to a target node in a wireless telecommunications network, includes the step of: not forwarding data from the source node to the target node for a radio bearer in RLC-AM mode.
According to a fourth aspect of the invention, a wireless telecommunications network is arranged to implement a method in accordance with any aspect of the invention.
According to a fifth aspect of the invention, a decision making device for a wireless telecommunications network comprises: an information store; and a decision processor arranged to receive information from the information store and use it to determine whether or not to forward data from a source node to a target node during handover, the determination for each radio bearer being made independently of which radio link control, RLC, mode it supports. The information store may hold fixed data, data that is updated at intervals or continuously revised data, or combinations of these

for different types of data, and may include information such as QoS requirements, mobile terminal velocity and performance over one or more interfaces. Measurements may be provided by a mobile terminal which is undergoing handover from a source node to update the information. The decision making device may be included in a network node, for example, a decision making device may be included in each eNB in an LTE network, and the eNB comprising the source node during a handover procedure may be the one at which the determination is made. Other types of network node may comprise a decision making device in accordance with the fifth aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments and methods in accordance with the invention are now described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 schematically illustrates a network and messaging dixring handover;
Figure 2 schematically illustrates a decision making device in accordance with the invention;
Figure 3 schematically illustrates a network and messaging during handover; and
Figure 4 schematically illustrates a network and messaging dviring handover.
DETAILED DESCRIPTION
With reference to Figure 2, a decision making device 5 includes an information store 6 containing data about various network parameters and about a mobile terminal 1 which is currently in an estabUshed coimection with a source eNB 2, shown in Figure 3. A processor 7 accesses the information held in the store 6, which may include QoS data 6a, UE speed data at 6b, and interface data relating to SI and X2 at 6c, and uses it to make a determination as to whether or not data forwarding should be implemented when the mobile terminal 1 is handed over from the source eNB 2 to a target eNB 3. The processor 7 does not take into account the

RLC mode of a radio bearer when making its determination. Suitable choice of the type of data held in the store 6 and how it is appUed to the determination permits an operator to selectively choose data forwarding per bearer depending on deployment scenario and costs. For example, the operator can choose not to forward data over a costly last mile saving on transport cost. It allows the operator to choose data forwarding if the SI link is slow for the path switch and avoid packet loss and poor service.
With reference to Figure 3, for a bearer having RLC-AM, a decision is taken by device 5 not to implement data forwarding and this decision is transmitted to the source eNB 2. When the handover command at step 7 is sent to the UE 1, it includes information that data forwarding is not being implemented, so that the UE knows to reset the PDCP SN to a known value. The source eNb 2 also sends notification to the target eNB 3 that there is no data to be forwarded, shown as step 7.5 on Figure 2. On receipt of the "No Data" message, the target eNB 3 resets the PDCP SN to a known value. Although in this described case, no data forwarding is implemented, for other cases, data forwarding may be implemented for this bearer. In that event, the handover command 7 includes information that data forwarding is being used and the process continues as shown in the messaging set out in Figure 1.
With reference to Figure 4, for a bearer having RLC-UM, a decision is taken by device 5 to implement data forwarding and this decision is transmitted to the source eNB 2. When the handover command at step 7 is sent to the UE 1, it includes information that data forwarding is being implemented. In this case, the data forwarded is fresh data. An alert may be sent to target node 3 that data forwarding is to be carried out, but this may not be necessary as the target eNB 3 will be aware of this when it starts to receive the data packets. Although in this described case, data forwarding is implemented, for other cases, data forwarding may be implemented for this bearer.
The indication of data forwarding or not to the UE reduces HO interruption time because the UE can deliver any buffered packets to the upper layers immediately. It is useful for the UE to know if data forwarding is supported by the network for each bearer. If data forwarding is not used for a particular bearer, there

are two possibilities for PDCP handling - there is no specific reason to maintain sequence numbers across the HO procedure. Further, the UE can immediately dehver any buffered packets to the higher layers without waiting for any missing packets for re-ordering. It can also immediately set the PDCP SN to the known value.
In one example, for a VoIP data bearer, an operator might choose to use RLC-UM because the HARQ residual error is considered good enough and the additional delay introduced by RLC-AM would create additional end-to-end delay. If this bearei is not subject to data forwarding, any packets in transit towards the source eNB will be lost. With this option, the operator has the flexibility to choose to do data forwarding for this bearer. If the SI path switch delay is long, it can result in more packets arriving at the source eNB and lost as they are not forwarded. With the proposed solution, specific deployments of VoIP flows can be subject to forwarding.
The other motivation is to not forward TCP flows using RLC-AM. If the last mile is costly or slow, then an operator might choose not to use data forwarding for this deployment. The solution allows the operator to make the right choice per bearer, per deployment scenario etc. on whether data forwarding is to be used or not.
The present invention may be embodied in other specific forms and implemented in other methods without departing fi"om its spirit or essential characteristics. The described embodiments and methods are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

CLAIMS
1. A method for handover of a mobile terminal from a source node to a target node in
a wireless telecommunications network, including the step of:
making a determination as to whether or not to forward data from the source node to the target node, and the determination for each radio bearer being made independently of which radio link control, RLC, mode it supports.
2. The method as claimed in claim 1 and including using performance of the last mile in making the determination as to whether or not to forward the data.
3. The method as claimed in claim 1 or 2 and including using the mobility rate of the mobile terminal in making the determination as to whether or not to forward the data.
4. The method as claimed in claim 1,2 or 3 and, when it is determined that data forwarding is not used, resetting the Packet Data Convergence Protocol (PDCP) sequence numbers (SNs).
5. The method as claimed in claim 1,2, 3 or 4 and providing an instruction to the mobile terminal for each bearer on whether a bearer is subject to data forwarding or not.
6. The method as claimed in claim 5 and wherein the instruction is included as part of a Handover command message.
7. The method as claimed in any preceding claim and, when it is determined not to forward data to the target node, re-starting the PDCP sequence number with a known value and not fransferring the last used PDCP SN to the target node.
8. The method as claimed in any preceding claim and implemented in accordance with Long Term Evolution, LTE, standards.

9. A method for handover of a mobile terminal from a source node to a target node in
a wireless telecommunications network, including the step of: forwarding data from
the source node to the target node for a radio bearer in RLC-UM mode.
10. A method for handover of a mobile terminal from a source node to a target node in a wireless telecommunications network, including the step of: not forwarding data from the source node to the target node for a radio bearer in RLC-AM mode.
11. A wireless telecommunications network arranged to implement the method as claimed in any preceding claim.
12. A decision making device for a wireless telecommunications network comprising:
an information store; and a decision processor arranged to receive information from
the information store and use it to determine whether or not to forward data from a
source node to a target node during handover, the determination for each radio bearer
being made independently of which radio link control, RLC, mode it supports