HK1074321B - User equipment and method for assisting high speed downlink shared channel cell change - Google Patents

Description

User equipment and method for facilitating high speed downlink shared channel cell change

Technical Field

The present invention relates to the field of wireless communications. More particularly, the present invention relates to efficiently recovering buffered data after handover in a wireless system that distributes data from an intermediate point. A third generation (3G) system that includes a Remote Network Controller (RNC) coupled to one or more node bs, which in turn are wirelessly coupled to a plurality of User Equipments (UEs), and that utilizes Adaptive Modulation and Coding (AMC) and hybrid automatic repeat request (H-ARQ) techniques, is but one example of such a system.

Background

A third generation (3G) Universal Terrestrial Radio Access Network (UTRAN) includes a number of RNCs, where each RNC may be coupled to a number of node bs. A node B is an entity that includes one or more base stations, where each base station manages traffic for one or more cells.

Both 3G Frequency Division Duplex (FDD) and Time Division Duplex (TDD) systems typically utilize the RNC to buffer and schedule (schedule) data for transmission to the UE. However, for the high speed channel of the 3G cellular system, the data to be transmitted is buffered and scheduled by the node B. One of the high speed channels is, for example, the high speed downlink shared channel (HS-DSCH). Since the data is distributed by the node B, the data to be transmitted must be buffered in the node B. This structure results in the RNC not taking up the latest state that the node B has control of packet data cell (PDU) transmission. Thus, upon handover between cells, the scheduling of the data transmission must be coordinated with respect to the cell change. Otherwise, the data transmission must be resynchronized after the cell change to avoid "dropping" or "duplicate transmission" of the transmitted data. In 3G networks, handovers between cells are controlled by the RNC. Since the RNC controlling the cell containing the UE changes, data may be missed or repeated due to the handover. This problem is exacerbated by the fact that each RNC in the architecture is associated with several node bs. The probability that a mobile UE requires a node B change is much greater than it requires an RNC change due to a handover of the UE cell.

The HS-DSCH utilizes AMC to enable high-speed transmission of data and H-ARQ to increase the likelihood of successful delivery of data. When the UE needs to change the associated cell of the UTRAN access point that is transmitting and receiving the serving HS-DSCH radio link, it results in a serving HS-DSCH cell change. When the replacement cell becomes aware of the improvement in physical channel conditions and/or physical capacity, it causes the serving HS-SDCH cell to change. Unlike other channels in the 3G network, the HS-DSCH is interrupted on the node B instead of the RNC within the UTRAN.

There are two HS-DSCH cell change types. When the UE changes between two cells associated with the same node B, the serving HS-DSCH cell within the node B changes. When the UE changes between two cells associated with different node bs, then the serving HS-DSCH cell changes between one node B. For inter-node B cell change, the node B prior to the serving HS-DSCH cell change is referred to as the source node B; and the node B after the serving HS-DSCH cell change is referred to as the target node B.

There is a same-layer Radio Link Control (RLC) entity in both the RNC and the UE, providing an automatic repeat request (ARQ) function for transmitting data. The transmitting RLC entity signals a Sequence Number (SN) in the PDU header, which the receiving RLC entity uses to ensure that no PDUs are missed during the transmission. If several PDUs are missed during the transmission (due to out-of-order delivery of PDUs), the receiving RLC entity transmits a PDU status report to inform the transmitting RLC entity that "some PDUs were missed". The PDU status report is used to indicate the status of the data transmission. Wherein the PDU status report identifies the SN of the missing or received PDU. If a PDU is missed, the transmitting RLC entity retransmits a copy of the missed PDU to the receiving RLC.

The transmitting RLC entity may also poll the receiving RLC entity for PDU status reports; or may generate status reports periodically. The polling function provides a mechanism for the transmitting RLC entity to request the status of the PDU transmission.

The H-ARQ functionality of the node B also provides the functionality to retransmit failed transmissions. While the H-ARQ operation removes some of the failed transmissions and increases the probability of successful delivery of data, it is the RLC protocol layer that ultimately ensures successful delivery.

Due to the dynamic change of the propagation conditions, the HS-DSCH cell change has to be performed quickly to maintain the quality of service. During the serving HS-DSCH cell change, the UE may stop transmission and reception in the source cell until all PDUs currently stored in the source node-B have been transmitted. Since the source node-B schedules and buffers the data, and since the data transmission rate is very high (e.g., one million bits/second or higher), a large amount of data buffered in the source node-B may be lost when the UE performs a serving HS-DSCH cell change (particularly, inter-node-B handover). One reason for this is that no mechanism is provided within the UTRAN architecture for transferring buffered data at the source node B to the target node B. Once the HS-DSCH cell change is provided, the RNC does not know how much data was missed (if any) because the RNC is not informed of the "node B data transmission schedule", and the RNC does not know whether the UE successfully resumed acknowledging the transmission. So once the HS-DSCH cell change is provided, if there is still data buffered in the source node B, the RNC RLC has to recover the data to keep transmitting without missing the data.

Currently, there are two preferred ways in prior art systems to recover the buffered data at the source node-B. I.e. after the HS-DSCH cell change: 1) the RNC can explicitly request PDU status from the UE; or 2) the RNC can start transmission only from where it left off in the source cell and the UE will generate the PDU status through its perceived out-of-order delivery.

In the first case, the RNC explicitly requests PDU status by polling the UE, the RNC has to wait until the physical channel is set up in the new cell. Then, the RNC transmits the PDU status request, and the UE receives and processes the request. The UE generates the PDU status and returns the PDU status to the RNC, which processes the PDU status and determines which PDUs need to be retransmitted.

In the second case, the RNC only starts transmitting PDUs from where it left off in the source cell, and the UE recognizes the out-of-order delivery of data and generates a PDU status back to the RNC. The RNC handles the PDU status and knows which PDUs to retransmit.

In either case, the PDU status is processed if the buffered data in the source node-B needs to be restored; however, the UE is severely delayed in its time to properly receive the retransmitted data. This is because the time for the UE to generate the PDU status is delayed, thereby delaying the time for the RNC to receive the PDU status.

If transmission is performed in the RLC recovery acknowledged mode, data is not delivered to a higher layer until the data can be delivered sequentially. If so, the UE may be required to buffer the out-of-order data until such time as the missing PDU can be retransmitted. This not only delays the transmission, but also requires that the UE have a memory large enough to buffer data until the data stored in the source node-B can be successfully delivered. Otherwise the effective data transfer rate is reduced, thereby affecting the quality of service. This is an undesirable design constraint because memory is expensive.

Thus, the prior art method of restoring the buffered data of the source node B prior to transferring the buffered data to the target node B has extremely undesirable consequences. It would be desirable to have a system and method that more efficiently recovers the buffered data of the source node-B in a shorter amount of time to properly maintain the quality of service requirements of the user.

Disclosure of Invention

The present invention is a method and system that performs a series of actions to facilitate reducing the latency and potentially preventing loss of PDUs during transmission during the serving HS-DSCH cell change procedure. New criteria are introduced for the UE to generate the RLC PDU status report. After notifying the HS-DSCH cell change indicated by the RRC procedure, the UE autonomously generates the PDU status report as soon as possible to more efficiently recover the source node B's buffered data. A PDU status may be signaled for each Acknowledged Mode (AM) RLC instance (instance) associated with the HS-DSCH transport channel.

According to a first aspect of the present invention, there is provided a user equipment for facilitating a high speed downlink shared channel cell change, the user equipment comprising: means for receiving a radio signal carrying a radio resource control request message related to the high speed downlink shared channel cell change; means for clearing a reordering buffer associated with high speed medium access control in response to receiving the wireless signal; and means for providing information that a reordering buffer has been flushed, and generating status reports in response to said information, each said status report corresponding to a respective acknowledged mode radio link control instance mapped to said high speed downlink shared channel, said status reports indicating missed high speed downlink shared channel packet data units for said user equipment.

According to a second aspect of the present invention, there is provided a method for use in a user equipment of facilitating a high speed downlink shared channel cell change, the method comprising: receiving a radio signal carrying a radio resource control request message related to the high speed downlink shared channel cell change; clearing a reordering buffer associated with high speed medium access control in response to receiving the wireless signal; and providing information that a reordering buffer has been flushed, and in response to the information, generating status reports, each corresponding to a respective acknowledged mode radio link control instance mapped to the high speed downlink shared channel, the status reports indicating missed high speed downlink shared channel packet data units for the user equipment.

Drawings

Fig. 1 is a flowchart of an efficient procedure for efficiently recovering buffered data of a node B after a HS-DSCH cell change in accordance with the present invention.

Figure 2 is a flow chart of another method for the RNC to wait until the PDU status before starting to transmit a new data in the target cell.

Detailed Description

Preferred embodiments of the present invention will be described with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout.

According to the present invention, in order to shorten the waiting time for the RNC to acquire the PDU status and recover the buffered data of the source node B after the serving HS-DSCH cell is changed, the UE autonomously transmits the PDU status to the RNC after notifying the HS-DSCH cell change indicated by the RRC procedure. The generated PDU status may be used on each AM RLC instance associated with the HS-DSCH transport channel.

Referring to the flow chart of fig. 1, a method 10 for efficiently recovering buffered data of a node B in accordance with the present invention is illustrated. The RNC recognizes that the HS-DSCH cell change needs to be provided (step 12). The node B is then informed of the "serving HS-DSCH cell change" (step 14). The UE is notified of the "serving HS-DSCH cell change" (step 16), as indicated via the RRC request message. Note that it is also possible to invoke step 16 before step 14 without any adverse consequences.

Once the UE receives the RRC request message (step 18), the UE generates a status report indicating the status of the RLC PDU as soon as possible after being informed of the HS-DSCH cell change indicated by the RRC procedure (step 20) to shorten the delay time in recovering the buffered data of the source node B. The UE does not wait for any prior art trigger to generate the PDU status (e.g., the RNC requests PDU status generation, or the UE detects out-of-order delivery of data).

The UE may perform a number of different alternative methods to trigger generation of a PDU status report after a serving HS-DSCH cell change. However, several embodiments are presented herein. Preferably, as a first option, the high speed medium access control (MAC-hs) notifies the RLC as soon as its reordered buffers are cleared. The second option is for the RRC to inform the RLC "layer 3 (L3) RRC procedure" indicating the serving HS-DSCH cell change. Third, the physical layer may inform the RLC "the HS-DSCH control channel in the target cell is received"; or the physical layer may inform the RLC once HS-DSCH control transitions to the target cell. Those skilled in the art will certainly appreciate that other methods may be used to trigger the RLC PDU status message to be sent from the UE to the RNC. Due to this procedure, the PDU status is generated and transmitted to the RNC in a shorter time (step 22), resulting in more efficient recovery of the buffered data of the source node B.

There are several options for the UE to transmit the PDU status report to the RNC, according to step 22. The method of transmitting the PDU status is an embodiment (not a priority of the present invention) of how the UE can signal the PDU status to the RNC, which is to say that the PDU status is generated as soon as there is a new criterion. Preferably, the UE generates an RLC status report for each AMRLC instance mapped on the HS-DSCH transport channel.

In a second option, the PDU status report is transmitted from the UE to the RNC via the first current uplink message as soon as the UE gets the PDU status report. For intra-node B serving cell change (and assuming that the HS-DSCH transport channel and radio bearer parameters are unchanged), the message on the DCCH is "physical channel reconfiguration complete". The message on the DCCH is "transport channel reconfiguration complete" if the HS-DSCH transport channel and radio bearer parameters change, and/or in the case of an inter-node B serving cell change. The PDU status may be identified in any RRC signaling message. The RNC RRC entity then informs the RLC of the status of the PDU to resume transmission to the target node B.

In a third option, the UE transmits the status report on the new L3 signaling message on the DCCH from the UE to the RNC. The new message is transmitted from a Radio Resource Control (RRC) layer of the UE to an RRC layer of the RNC. Then, the RNC notifies the RLC layer of "status of the PDU" to resume transmission to the target node B. As such, the PDU status message shown in this fig. 1 may include two different messages "RRC" and "RLC status".

Note that the particular format of the PDU status report may vary. For example, the PDU status report format may include: 1) the Sequence Number (SN) of the last successfully delivered PDU in sequence; 2) the highest SN of the successfully received PDUs; 3) the sequence number SN of the unsuccessfully received PDU (i.e., missing PDU), up to the highest sequence number SN of the successfully received PDU; 4) or a list of sequence numbers that indicate successful receipt of the PDUs.

Once the RNC receives the message with the PDU status attached, the RNC RLC processes the PDU status message (step 24) to determine the missing PDUs. The RNC is now aware of the data missed due to the cell change and may retransmit the missed data to the UE (step 26). Note that the message may be many and is not necessarily limited to "accompanied by the PDU status report".

It is also noted that in the embodiment of the present invention, the RNC may continue to forward data in the target cell for transmission to the UE between steps 16 and 24. Since the order of all buffered data at the source node-B is out of order if it is not successfully transmitted, the UE RLC is forced to buffer the data so that the data can be delivered to higher layers in sequence. This requires that the UE have a memory sufficient to store the out-of-order PDUs. After any sequential data is missed, the transmission is limited to the memory capability of the UE until the missed sequential data is successfully transmitted.

Referring now to the flowchart of fig. 2, a method 40 for efficiently recovering node B buffered data in accordance with another embodiment of the present invention is illustrated. The method 40 is similar to the method 10 shown in fig. 1, and the steps described in fig. 2 are labeled identically to fig. 1. However, according to this embodiment of the invention, the method 40 includes a new step 17 in which the RNC stops all downlink HS-DSCH serving to the UE until the PDU status message is processed (step 24). This embodiment minimizes the delay in retransmitting the buffered data from the source node-B and limits the amount of data that must be buffered on the UE.

For the minimization of the delay time, the node B does not know the RLC transmission sequence number, and the transmission scheduling in the node B is based on a first-in-first-out (FIFO). Therefore, if the RNC of the target cell forwards data before the PDU status is processed, the data is transmitted first. The data queued in node B may potentially take longer to retransmit the buffered data for the source node B.

The present invention is applicable to HS-DSCH cell change in both inter-node B cell change and intra-node B cell change. Since in the intra-node B case (case), the node B cannot redirect the buffered HS-DSCH data to the target cell due to internal design issues, the RNC may indicate a need to generate a PDU status for both cases. The UE may also be unable to distinguish between an inter-node B cell change and an intra-node B cell change, thus also resulting in PDU status being generated for both inter-node B and intra-node B instances. It is beneficial to transmit the PDU status in this manner in the event of an inter-or intra-node B cell change that the buffered data cannot be transferred to the target cell.

Although the present invention has been described in detail, it is to be understood that the invention is not limited thereto and that various changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (2)

1. A user equipment for facilitating a high speed downlink shared channel cell change, the user equipment comprising:

means for receiving a radio signal carrying a radio resource control request message related to the high speed downlink shared channel cell change;

means for clearing a reordering buffer associated with high speed medium access control in response to receiving the wireless signal; and

means for providing information that a reordering buffer has been flushed, and in response to the information, generating status reports, each corresponding to a respective acknowledged mode radio link control instance mapped to the high speed downlink shared channel, the status reports indicating a missed high speed downlink shared channel packet data unit for the user equipment.

2. A method for use in a user equipment for facilitating a high speed downlink shared channel cell change, the method comprising:

receiving a radio signal carrying a radio resource control request message related to the high speed downlink shared channel cell change;

clearing a reordering buffer associated with high speed medium access control in response to receiving the wireless signal; and

providing information that a reordering buffer has been flushed, and in response to the information, generating status reports, each corresponding to a respective acknowledged mode radio link control instance mapped to the high speed downlink shared channel, the status reports indicating missed high speed downlink shared channel packet data units for the user equipment.