Patent Description:
In mobile telecommunications networks, there is a requirement for User Equipment (UE) to handover from one base station to another. The signaling sequence for the intra-LTE handover procedure has been described in 3GPP specification TS <NUM>.

Other relevant background art is <NPL>).

However, the specifics on resource allocations in the target cell have not yet been addressed in detail. The various acronyms applicable to <NUM> communications will of course be familiar to those skilled in the art but a glossary is appended for the benefit of lay readers.

Although for efficiency of understanding for those of skill in the art the invention will be described in detail in the context of a <NUM> system, the principles of the handover procedure can be applied to other systems, e.g. other CDMA or wireless systems in which a mobile device or User Equipment (UE) communicates with one of several other devices (corresponding to eNodeB) with the corresponding elements of the system changed as required.

The invention is set out in the independent claims appended hereto, and preferred embodiments are indicated in the dependent claims that follow.

According to one example, there is provided a method performed in a communications device comprising: transmitting uplink data to a source communications node; pausing transmission of uplink data upon receipt of a handover command from the source communications node; buffering uplink data while transmission of the uplink data is paused; synchronising with a target communications node; transmitting to the target node a handover complete message together with a buffer status report indicating an amount of buffered uplink data; and resuming the transmission of the uplink data using resources allocated by the target communications node. As the target node is informed of the buffer level at the end of the handover procedure, the target node can accurately allocate the required uplink resources that are needed by the UE.

The method can be used for both inter and intra base station handovers.

The present example also provides a method performed in a target communications node to facilitate handover of a remote communications device from a source communications node to the target communications node, the method comprising: receiving a handover complete message together with a buffer status report indicating an amount of buffered uplink data within the remote communications device; allocating resources to the remote communications device in dependence upon the received buffer status report; and receiving uplink data from the remote communications device using resources allocated by the target communications node. Preferably the allocating step performs the allocation in dependence upon service data received from the source communications node relating to the service provided to the remote device by the source communications node.

The present example also provides a communications device comprising: a transceiver for transmitting data to and receiving data from remote communications nodes; a controller operable: to transmit uplink data to a source communications node; to stop transmission of said uplink data upon receipt of a handover command from the source communications node; to buffer uplink data while transmission of the uplink data is stopped; to synchronise with a target communications node; to transmit a handover complete message together with a buffer status report indicating an amount of buffered uplink data; and to transmit uplink data using resources allocated by the target communications node.

The present example also provides a communications node comprising: a transceiver for transmitting data to and receiving data from a remote communications device; a controller operable: to receive, from a remote communications device, a handover complete message together with a buffer status report indicating an amount of buffered uplink data within the remote communications device: to allocate resources to the remote communications device in dependence upon the received buffer status report; and to receive uplink data from the remote communications device using the allocated resources.

Preferably, the controller allocates the resources in dependence upon service data received from a source communications node relating to a service provided to the remote device by the source communications node.

These and other features and aspects of the invention will become apparent from the following exemplary embodiments which are described with reference to the accompanying drawings in which:.

<FIG> schematically illustrates a mobile (cellular) telecommunications system <NUM> in which users of mobile telephones (MT) <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> can communicate with other users (not shown) via one of the base stations <NUM>-<NUM> or <NUM>-<NUM> and a telephone network <NUM>. In this embodiment, for the downlink (DL), the base stations <NUM> use an orthogonal frequency division multiple access (OFDMA) technique to transmit data to the mobile telephones <NUM> (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>); and, for the uplink (UL), the mobile telephones <NUM> use a single carrier frequency division multiple access (FDMA) technique to transmit data to the base stations <NUM> (<NUM>-<NUM>, <NUM>-<NUM>). Different sub-carriers are allocated by the base stations <NUM> to each mobile telephone <NUM> depending on the supported bandwidth of the mobile telephone <NUM> and the amount of data to be sent to/from the mobile telephone <NUM>. When a mobile telephone <NUM> moves from the cell of a source base station (e.g. base station <NUM>-<NUM>) to a target base station (e.g. base station <NUM>-<NUM>), a handover (HO) procedure (protocol) is carried out in the source and target base stations <NUM> and in the mobile telephone <NUM>, to control the handover process.

In this exemplary embodiment, the available transmission band is divided into a number of sub-bands, each of which comprises a number of contiguous sub-carriers arranged in contiguous blocks. Different mobile telephones <NUM> are allocated different resources block(s) (sub-carriers) within a sub-band at different times for transmitting their data.

<FIG> is a block diagram illustrating the main components of each of the base stations <NUM> used in this embodiment. As shown, each base station <NUM> includes a transceiver circuit <NUM> which is operable to transmit signals to and to receive signals from the mobile telephones <NUM> via one or more antenna <NUM> (using the above described sub-carriers) and which is operable to transmit signals to and to receive signals from the telephone network <NUM> via a network interface <NUM>. A controller <NUM> controls the operation of the transceiver circuit <NUM> in accordance with software stored in memory <NUM>. The software includes, among other things, an operating system <NUM>, a downlink scheduler <NUM> and a resource allocations module <NUM>. The downlink scheduler <NUM> is operable for scheduling user data packets to be transmitted by the transceiver circuit <NUM> in its communications with the mobile telephones <NUM>; and the resource allocations module <NUM> is operable to allocate frequency resources for use by the mobile telephones <NUM> for transmitting their uplink data to the base station <NUM>. The software also includes a handover module <NUM>, the operation of which will be described below.

<FIG> schematically illustrates the main components of each of the mobile telephones <NUM> shown in <FIG>. As shown, the mobile telephones <NUM> include a transceiver circuit <NUM> that is operable to transmit signals to and to receive signals from the base station <NUM> via one or more antenna <NUM>. As shown, the mobile telephone <NUM> also includes a controller <NUM> which controls the operation of the mobile telephone <NUM> and which is connected to the transceiver circuit <NUM> and to a loudspeaker <NUM>, a microphone <NUM>, a display <NUM>, and a keypad <NUM>. The controller <NUM> operates in accordance with software instructions stored within memory <NUM>. As shown, these software instructions include, among other things, an operating system <NUM>. In this embodiment, the memory also provides uplink data buffers <NUM>. The software for controlling the handover process is provided by a handover module <NUM>, the operation of which will be described below.

In the above description, both the base station <NUM> and the mobile telephones <NUM> are described, for ease of understanding, as having respective discrete handover modules which control the handover procedure when a mobile telephone <NUM> moves from a source base station to a target base station. Whilst the features may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, the handover features may be built into the overall operating system or code and so a handover module as a discrete entity may not be discernible. This is similarly true for the other software modules.

A description will now be given of the operation of the handover modules <NUM> and <NUM>. The following description will use the nomenclature used in the Long Term Evolution (LTE) of UTRAN. Therefore, the mobile telephone <NUM> that is changing base stations <NUM> will be referred to as a UE, the source base station <NUM>-<NUM> will be referred to as the source eNodeB and the target base station <NUM>-<NUM> will be referred to as the target eNodeB.

<FIG> and <FIG> illustrate the currently agreed signalling flow for the control plane for the inter eNodeB handover (HO) procedure. As shown, the sequence proceeds as follows:.

The handover execution phase begins in the UE on receiving the RRC Handover Command in step <NUM> form the source eNodeB. On receiving the Handover Command the UE stops the uplink transmission, starts buffering the uplink packets, detaches from the old cell and attempts synchronisation to the target cell in Step <NUM>.

After the UE attains uplink synchronisation, the target eNodeB responds with an uplink allocation for sending a Handover Complete message. The UE sends the Handover Complete Message in Step <NUM> which completes the HO procedure in the UE.

After the reception of the Handover Complete Message in the target eNodeB, it makes an appropriate allocation of the resources for the downlink U-plane data, based on the status of downlink buffers and the QoS parameters received from the source eNodeB (which identify the service that was being provided to the UE by the source eNodeB). The target eNodeB has to allocate uplink U-plane resources for the UE. It could do this based on a guess of the uplink buffer status within the UE and the QoS parameters. However, such uplink U-plane resource allocations within the target cell immediately after handover would be suboptimal considering that the UE was unable to perform any UL transmissions while it was trying to synchronize with the target cell. During the time that the UE can not transmit uplink data, it buffers the data in its internal buffers <NUM>. By the time that the UE is able to send uplink data to the target eNodeB, there may be a large number of uplink data packets sitting in the uplink buffers <NUM>. In order that the UE can quickly bring its buffers levels down, the target eNodeB must allocate sufficient resources in the target cell for this unlink U-plane data.

Although the handover execution procedure described above is for inter eNodeB handover scenarios, it is equally applicable for intra eNodeB handovers as far as the scheduling and resource allocation in the UL is concerned.

As an alternative to guessing the required uplink resource needs, the target eNodeB may make an initial allocation to the UE based on the last buffer status report sent by the UE to the source eNodeB and the QoS parameters. The UE can subsequently request additional uplink resources by sending an uplink buffer Status report to bring down the increased buffer levels due to the pause in the uplink transmission. However, this requires the source eNodeB to have to store the most recent uplink buffer status report for each UE and transfer it to the target eNodeB during the handover procedure.

Therefore, in the claimed embodiment, the handover procedure described above is modified slightly such that when the UE sends the HANDOVER COMPLETE message, it appends to that message the latest uplink buffer status report. The target eNodeB can then use this information to allocate accurately the required uplink resources that are needed by the UE. This modified handover procedure is illustrated in <FIG> and <FIG> and has a number of advantages over the other techniques discussed above:.

A detailed embodiment has been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiment whilst still benefiting from the inventions embodied therein. By way of illustration only a number of these alternatives and modifications will now be described.

In the above embodiment, a mobile telephone based telecommunications system was described. As those skilled in the art will appreciate, the handover techniques described in the present application can be employed in any communications system. In particular, many of these handover techniques can be used in wire or wireless based communications systems which either use electromagnetic signals or acoustic signals to carry the data. In the general case, the base stations and the mobile telephones can be considered as communications nodes or devices which communicate with each other. In intra eNodeB handover, the source and target communications nodes will be formed by respective scheduling entities within one base station. Other communications nodes or devices may include user devices such as, for example, personal digital assistants, laptop computers, web browsers, etc..

In the above embodiments, a number of software modules were described. As those skilled will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the base station or to the mobile telephone as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits. However, the use of software modules is preferred as it facilitates the updating of base station <NUM> and the mobile telephones <NUM> in order to update their functionalities.

The following is a detailed description of the way in which the present invention may be implemented in the currently proposed 3GPP LTE standard. Whilst various features are described as being essential or necessary, this may only be the case for the proposed 3GPP LTE standard, for example due. requirements imposed by the standard. These statements should not, therefore, be construed as limiting the present invention in any way.

The signalling sequence for the intra-LTE handover procedure has been captured in TS <NUM>, however the specifics on resource allocations in the target cell have not yet been addressed in detail. In this contribution we address some further details on the UL resource allocation in the target cell after Handover and handling of UL buffer status report.

Studying a "typical" signalling flow for mobility, we can see that the handover procedure consist of the following: radio conditions are changing, UE sends a measurement report, the network takes a decision and prepares the target cell, the network commands the UE to change cell, the UE reconfigures L1 and synchronizes to the target cell, data is transmitted and received in the target cell and resources in the source cell are released.

The signalling flow for the control plane that was agreed for inter eNodeB handover procedure is recapitulated and is taken as the basis for further discussion. The description from draft Stage <NUM> TS for the signalling sequence is also included.

Below is more detailed description of the intra-MME/UPE HO procedure:.

The handover execution phase begins in the UE on receiving the RRC Handover Command in step <NUM> form the source eNodeB. On receiving the Handover Command the UE shall stop the UL transmission, start buffering the UL packets, detach from the old cell and shall attempt to synchronisation to target cell in Step <NUM>.

After the UE attains UL synchronisation, eNodeB will respond with UL allocation for sending Handover Complete message. The UE shall send the Handover Complete Message in Step <NUM> which will complete the HO procedure in UE.

After the reception of Handover Complete Message in the target _ eNodeB, it shall appropriately allocate the resources for DL-U plane data based on the status of DL buffers and the QoS parameters received from source eNodeB. However for the UL-U plane data, the target eNodeB could allocate resources based on a guess of the buffer status in the UL within UE and the QoS parameters. Such UL- U plane resource allocations within the target cell immediately after HO would be suboptimal considering the fact that the UE was unable to perform any UL transmissions while it was trying to synchronize with the target cell. This can cause large amount of UL data packets to be buffered within the UE during the Hand over execution phase and the buffers levels need to be quickly brought down by allocating the sufficient amount of resources for UL- U plane data in the target cell.

Although the handover execution procedure described above is for inter eNB hand over scenarios, it is equally applicable for the intra eNB handover as far as the scheduling and resource allocation in UL is concerned.

Considering the above aspects, it is necessary that the appropriate amount of UL resources for U-plane data is allocated in the target cell immediately after handover execution phase. Possible ways of achieving this is to have either a two step approach or a one step approach described below:.

Step <NUM>: Initially UL Resources are allocated in the target cell based on the last buffer status report sent by the UE in the source cell and the QoS parameters.

Step <NUM>: UE shall subsequently request additional UL resources subsequently by sending UL buffer Status report to bring down the increased buffer levels due to pause in UL transmission.

Step <NUM>: UE sends the UL buffer status report along with the Handover Complete Message and the eNB allocates the UL resources accordingly.

On comparing the two approaches, we believe that one step approaches has significant advantages over the two step approach.

With one step approach we see the following advantages.

In this paper we take a detailed look at the handover execution phase and suggest a handling of UL Buffer status report and UL resource allocation in the target cell.

At the beginning of Handover execution phase, (i.e. on reception of Handover Command) the UE stops the UL transmissions and attempts to synchronize with the target cell. The buffers in the UE would keep accumulating the data packets till the UE receives the UL grants after the HO in the target cell. It is necessary that the Network accurately allocates the UL resources in the target cell such that increased buffer levels due to the pause in UL transmission are brought down quickly.

Claim 1:
A communications device (<NUM>) comprising:
means for receiving (<NUM>), from a source communications node (<NUM>-<NUM>), a message to perform a handover, the message including a Cell-Radio Network Temporary Identifier, C-RNTI;
means for detaching (<NUM>) from a cell of the source communications node (<NUM>-<NUM>); means for synchronizing (<NUM>) with a cell of a target communications node (<NUM>-<NUM>);
characterized by
means for transmitting (<NUM>), to the target communications node (<NUM>-<NUM>), a handover complete message together with a buffer status report including information of an amount of buffered uplink data stored in an uplink data buffer; and
means for transmitting (<NUM>), to the target communications node (<NUM>-<NUM>), uplink data using resources allocated by the target communications node (<NUM>-<NUM>) based on the buffer status report.