HK1115973B - User equipment and method of facilitating a high speed downlink shared channel cell change - Google Patents

Description

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

RELATED APPLICATIONS

The application is a divisional application of Chinese invention patent application with the application number of 03807731.0(PCT international application number of PCT/US03/10037) and the application date of 2003, 4 months and 2 days.

Technical Field

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

Background

The 3G Universal Terrestrial Radio Access Network (UTRAN) includes several RNCs, where each RNC may be coupled to several 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.

3G FDD and TDD systems typically use an RNC to buffer and schedule data for transmission to the UEs. However, for high speed channels in 3G cellular systems, the Node B buffers and schedules data for transmission. One of these high speed channels is for example a high speed downlink shared channel (HS-DSCH). Since data is allocated by the Node B, data to be used for transmission must be buffered in the Node B. Such a scheme results in the RNC not being able to acquire the latest status of the Node bs controlled by the Packet Data Unit (PDU). Therefore, upon handover between cells, scheduling of data transmission with cell change must be coordinated. Otherwise, after a cell change, the data needs to be resynchronized to avoid loss or duplicate transmission of the transmitted data. In 3G networks, handovers between cells are controlled by RNCs. The RNC controlling the cell containing the UE may change so data may be lost or repeated due to the handover. This problem is exacerbated because each RNC in the hierarchy is associated with several Node bs. The probability that a mobile UE needs a Node B change is much greater than it needs an RNC change due to UE cell handover.

The HS-DSCH enables high-speed data transmission using AMC and increases the probability of successful data transfer using H-ARQ. The serving HS-DSCH cell changes when the UE has to change the associated cell of the UTRAN access point that is performing the transmission and receiving of the serving HS-DSCH radio link. When improved physical channel conditions and/or physical capacity are achieved in the alternative cell, then the serving HS-SDCH cell change is invoked. Unlike other channels in a 3G network, which terminate at the RNC within the UTRAN, the HS-DSCH terminates at the Node B.

There are two types of HS-DSCH cell change. When a UE changes between two cells associated with the same Node B, then 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 for the Node bs changes. In the inter-Node B cell change, the Node B before the serving HS-DSCH cell change is referred to as a source Node B, and the Node B after the serving HS-DSCH cell change is referred to as a target Node B.

There are equivalent Radio Link Control (RLC) entities in both the RNC and the UE, providing automatic repeat request (ARQ) functionality for the transmission of data. The transmitting RLC entity signals a Sequence Number (SN) in the PDU header, which the receiving RLC entity uses to ensure that the PDU is not lost this time of transmission. If a PDU is lost during this transmission (due to out-of-order delivery of PDUs), the receiving RLC entity sends a status report PDU to inform the transmitting RLC entity that some PDUs are lost. The status report PDU is utilized to illustrate the status of data transmission. Where the status report PDU identifies the SN of the missing or received PDU. If a PDU is lost, the transmitting RLC entity retransmits a copy of the lost PDU to the receiving RLC.

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

The H-ARQ functionality in the Node B also provides retransmission of failed transmissions. While H-ARQ operation removes some failed transmissions and increases the success rate of delivering data, it is the RLC protocol layer that ultimately ensures successful delivery.

Due to the dynamic change of the propagation conditions, HS-DSCH cell change must be performed quickly to maintain the quality of service. During a serving HS-DSCH cell change, the UE may stop transmission and reception in the source cell before all PDUs currently stored in the source Node B are successfully transmitted. Because the source Node B performs scheduling and buffering of data, and because the data rate is very high (e.g., 10 Mb/sec 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 data loss is that no mechanism is provided within the UTRAN architecture to transfer the buffered data on the source Node B to the target Node B. Once the HS-DSCH cell changes, the RNC does not know how much data, if any, is lost, and the RNC does not know whether the UE successfully acknowledged the transmission, since the RNC node B data transmission schedule is not informed. Therefore, the RNC RLC must recover data when the data is buffered in the source Node B to maintain transmission without data loss when the HS-DSCH cell changes.

Currently, there are two preferred ways for the prior art system to recover the data buffered at the source Node B. After HS-DSCH cell change: 1) the RNC may explicitly request status PDUs from the UE; or 2) the RNC can start transmission only where the source cell has ceased and out of order delivery as recognized by the UE will generate status PDUs.

In the first case, where the RNC explicitly requests a status PDU by polling the UE, the RNC must first wait until the physical channel is established in the new cell. The status PDU request is then sent and received and processed by the UE. The UE generates a status PDU and sends it back to the RNC, which processes it and decides which PDUs need to be retransmitted.

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

In either case, if the data buffered in the source Node B needs to be recovered, a status PDU may be processed, but the time for the UE to correctly receive the retransmitted data is severely delayed. This is due to delaying the time the UE generates the PDU status, thereby delaying the time the RNC receives the PDU status.

If transmission is performed in the RLC acknowledged mode, data is not delivered to higher layers until in-order delivery of data can be performed. Thus, the UE is required to buffer out-of-order data until the lost PDU can be retransmitted. This not only causes delays in transmission, but also requires that the UE has a large enough memory 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.

Therefore, the prior art method of restoring the buffered data in the source Node B before transferring it to the target Node B may have unintended consequences. What is needed is a system and method for more efficiently recovering the buffered data of the source Node B in a shorter time to properly maintain the QoS requirements of the user.

Disclosure of Invention

It is an object of the present invention to provide a method and apparatus for implementing a sequence of actions in order to reduce latency and potentially prevent loss of PDU transmission during the serving HS-DSCH cell change procedure. A new criterion is introduced for the UE to generate a status report for the RLC PDU. After notifying the HS-DSCH cell change indicated by the RRC handover procedure, the UE automatically generates a PDU status report as soon as possible to more efficiently recover the buffered data of the source Node B. The PDU status may be signaled for each Acknowledged Mode (AM) RLC instance associated with the HS-DSCH transport channel.

According to one aspect of the present invention, there is provided a method for facilitating a high speed downlink shared channel, HS-DSCH, cell change performed by a user equipment, UE, the UE including a high speed medium access control, MAC-HS, device and a reordering buffer associated with the MAC-HS, the method comprising receiving, with the UE, a radio signal carrying a radio resource control, RRC, request message associated with the HS-DSCH cell change; the method comprises the following subsequent steps: clearing the reordering buffer after a wireless signal has been received; notifying, with the MAC-hs device, that the reordering buffer has been emptied; and generating status reports in response to emptying the reordering buffer, each status report individually corresponding to an acknowledged mode radio link control, AM, RLC, instance mapped to the HS-DSCH, the status reports indicating lost HS-DSCH packet data units, PDUs, for the UE.

According to another aspect of the present invention there is provided a method for facilitating a high speed downlink shared channel, HS-DSCH, cell change performed by a user equipment, UE, the method comprising: receiving, with the UE, a radio signal carrying a radio resource control, RRC, request message associated with an HS-DSCH cell change; clearing a reordering buffer associated with a MAC-hs in response to the wireless signal; and generating a status report in response to emptying the reordering buffer, the status report corresponding to an acknowledged mode radio link control, AM, RLC, instance mapped to the HS-DSCH, the status report indicating lost HS-DSCH PDUs for the UE.

Drawings

Fig. 1 is a flow chart of an efficient procedure for efficient recovery of Node B buffered data after HS-DSCH cell change in accordance with the present invention;

fig. 2 is a flow chart of an alternative method, wherein the RNC waits for a status PDU before starting the transmission of new data in the target cell.

Detailed Description

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

According to the present invention, in order to shorten the waiting time for the RNC to acquire the status of the PDU and to recover the data buffered in the source Node B, the UE automatically sends the status of the PDU to the RNC after the serving HS-DSCH cell change, following the notification of the HS-DSCH cell change indicated by the RRC procedure. The generation of status PDUs may be applied 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 Node B buffered data in accordance with the present invention is shown. The RNC identifies the need for a serving HS-DSCH cell change (step 12). Next, the Node B is notified of the serving HS-DSCH cell change (step 14). The UE is informed of the serving HS-DSCH cell change as indicated via the RRC request message (step 16). 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 in step 18, the UE automatically generates a status report indicating the status of the RLC PDU as soon as possible following notification of the HS-DSCH cell change indicated by the RRC procedure in order to shorten the delay time in recovering the data buffered in the source Node B (step 20). The UE does not wait for any of the triggers used in the prior art to generate a status PDU (e.g., the RNC requests a status PDU to be generated or the UE detects data that is not delivered in sequence).

In a UE, there are many different alternative methods that a UE may implement to trigger the generation of a PDU status report after a serving HS-DSCH cell change. Several examples are presented here. Preferably, the first option, the MAC-hs informs the RLC once its re-ordered buffer is empty. The second option is a layer 3 (L3) RRC procedure where the RRC notification RLC indicates the serving HS-DSCH cell change. Third, the physical layer may inform the RLC of the receipt of the HS-DSCH control channel in the target cell, or the physical layer may inform the RLC once HS-DSCH control is transferred to the target cell. Those skilled in the art will certainly appreciate that there are other methods to trigger the RLC PDU status message to be transmitted from the UE to the RNC. This procedure results in a shorter time for generating and transmitting the PDU status to the RNC (step 22), resulting in more efficient recovery of the buffered data of the source NodeB.

There are several alternative methods for the UE to send a status report of the PDU to the RNC, according to step 22. These methods of sending the status PDU are examples of how the PDU status is signaled from the UE to the RNC and are not the main transaction of the present invention, which involves generating the status PDU based on new criteria. Preferably, the UE generates an RLC status report for each AM RLC instance mapped to the HS-DSCH transport channel.

In a second alternative, the UE sends a PDU status report from the UE to the RNC via a first existing uplink message once the status report is obtained. For a serving cell change within the Node B (and assuming the HS-DSCH transport channels and radio bearer parameters are unchanged), this message is "physical channel reconfiguration complete" on the DCCH. This message is "transport channel reconfiguration complete" on DCCH if HS-DSCH transport channel and radio bearer parameters change and/or for serving cell change between Node bs. The PDU status may be identified in any RRC signaling message. Then, the RNC RRC entity informs the RLC of the status of the PDU to resume transmission to the target Node B.

In a third alternative, the UE transmits a status report from the UE to the RNC regarding the new L3 signaling message on the DCCH. 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 the status of the PDU to resume transmission to the target Node B. In this case, the PDU status message shown in fig. 1 may include two separate messages: "RRC complete" and "RLC state".

It should be noted that the specific format of the status report for this PDU may vary. For example, the format of the status report of the PDU may include: 1) the Sequence Number (SN) of the last successfully delivered PDU in the sequence; 2) the highest SN of a successfully received PDU; 3) the SN of a PDU that was not successfully received (i.e., a missing PDU) up to the highest SN of a successfully received PDU; 4) or a list of SNs of successfully received PDUs.

Once the RNC receives the message carrying the PDU status, the PDU status message is processed by the RNC RLC (step 24) to determine the missing PDUs. The RNC is now aware of the lost data due to the cell change and may retransmit the lost data to the UE (step 26). It should be noted that the message may be replaced in many ways and is not limited to carrying only PDU status reports.

It should also be noted that in embodiments 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 data is out of order if the buffered data on all source Node bs cannot be successfully transmitted, the UE RLC is forced to buffer the data so that the data can be delivered to higher layers in order. This requires the UE to have enough memory to store the out-of-order PDUs. After any order of data is lost, the transmission is limited to the memory capability of the UE until the lost order of data is successfully transmitted.

Referring now to the flowchart of FIG. 2, a method 40 for efficiently recovering Node B buffered data is shown in accordance with another embodiment of the present invention. The method 40 is similar to the method 10 shown in FIG. 1, and steps in FIG. 2 that are identically numbered as in FIG. 1 are identical to steps in FIG. 1. However, according to an embodiment of the invention, the method 40 comprises a new step 17, wherein the RNC suspends all downlink HS-DSCH transmissions to the UE until the PDU status message is processed in step 24. This embodiment minimizes the delay of the retransmission of the buffered data of the source Node B and limits the amount of data that has to be buffered at the UE.

In terms of this minimized delay, the Node B does not know the transmission sequence number of the RLC, and the transmission scheduling within the Node B is based on first-in-first-out (FIFO). Thus, if data is forwarded by the RNC in the target cell before the PDU status is processed, the data will be sent first. Such data queues in the Node B may result in further delays in possible retransmissions of source Node B buffered data.

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 it is not possible for the Node B to redirect the buffered HS-DSCH data to the target cell due to internal design issues, the RNC may indicate the need for generation of PDU status for both cases. The UE may also be unable to discern between a cell change between Node bs or within a Node B, which would result in the generation of PDU status for both the inter-Node B and intra-Node B cases. Transmitting status PDUs in this manner is beneficial for situations where there is no way to handover buffered data to a cell change between Node bs of a target cell or within a Node B.

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 (3)

1. A method for facilitating a high speed downlink shared channel, HS-DSCH, cell change performed by a user equipment, UE, the UE including a high speed medium access control, MAC-HS, device and a reordering buffer associated with the MAC-HS, the method comprising receiving, with the UE, a radio signal carrying a radio resource control, RRC, request message associated with the HS-DSCH cell change; the method is characterized by comprising the following subsequent steps:

clearing the reordering buffer after a wireless signal has been received;

notifying, with the MAC-hs device, that the reordering buffer has been emptied; and

generating status reports in response to emptying the reordering buffer, each status report individually corresponding to an acknowledged mode radio link control, AM, RLC, instance mapped to the HS-DSCH, the status reports indicating missing HS-DSCH packet data units, PDUs, by the UE.

2. The method of claim 1, wherein the UE automatically generates the status report.

3. The method according to claim 1 or 2, wherein the UE comprises a radio link control, RLC, apparatus that performs the generation of the status report.