Node b and method for prioritization of retransmission of protocol data units to assist radio-link-control retransmission

Description of equivalent WO 03096617 A2

[0001] SYSTEM AND METHOD FOR PRIORITIZATION

OF RETRANSMISSION OF PROTOCOL DATA UNITS TO ASSIST RADIO-LINK-CONTROL RETRANSMISSION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of wireless communications. More specifically, the present invention relates to a system and method for prioritizing the retransmission of protocol data units (PDUs) to assist radio link control (RLC) layer retransmission.

[0004] BACKGROUND

[0005] In third generation (3G) cellular systems for Frequency Division Duplex (FDD) and Time Division Duplex (TDD), there are retransmission mechanisms in the Acknowledgement Mode of the Radio Link Control (RLC) layer to achieve high reliability of end-to-end data transmissions. The RLC layer is a peer entity in both the Radio Network Controller (RNC) and the User Equipment (UE).

[0006] A block diagram of a UMTS Terrestrial Radio Access Network (UTRAN) MAC-hs layer architecture is illustrated in Figure 1, and a block diagram of the user equipment (UE) MAC hs architecture is shown in Figure 2. The architecture shown in Figures 1 and 2 is described in detail co-pending U.S. patent application number 10/270,822 filed on October 15, 2002 which is assigned to the present assignee. The UTRAN MAC-hs 30 shown in Figure 1 comprises a transport format resource indicator (TFRI) selector 31, a scheduling and prioritization entity 32, a plurality of Hybrid Automatic Repeat (H-ARQ) processors 33a, 33b, a flow controller 34 and a priority class and transmission sequence number (TSN) setting entity 35.

[0007] The UE MAC-hs 40 comprises an H-ARQ processor 41. As will be explained with reference to both Figures 1 and 2, the H-ARQ processors 33a, 33b in the UTRAN MAC-hs 30 and the H-ARQ processor 41 in the UE MAC-hs 40 work together to process blocks of data. [0008] The H-ARQ processors 33a, 33b in the UTRAN MAC-hs 30 handle aU of the tasks that are required for the H-ARQ process to generate transmissions and retransmissions for any transmission that is in error. The H-ARQ processor 41 in the UE MAC-hs 40 is responsible for generating an acknowledgement (ACK) to indicate a successful transmission, and for generating a negative acknowledgement (NACK) to indicate a failed transmission. The H-ARQ processors 33a, 33b and 41 process sequential data streams for each user data flow.

[0009] As will be described in further detail hereinafter, blocks of data received on each user data flow are assigned to H-ARQ processors 33a, 33b. Each H-ARQ processor 33a, 33b initiates a transmission, and in the case of an error, the H-ARQ processor 41 requests a retransmission. On subsequent transmissions, the modulation and coding rate may be changed in order to ensure a successful transmission. The data block to be retransmitted and any new transmissions to the UE are provided by the scheduling and prioritization entity 32 to the H-ARQ entities 33a, 33b.

[00010] The scheduling and prioritization entity 32 functions as radio resource manager and determines transmission latency in order to support the required QoS. Based on the outputs of the H-ARQ processors 33a, 33b and the priority of a new data block being transmitted, the scheduling and prioritization entity 32 forwards the data block to the TFRI selector 31.

[00011] The TFRI selector 31, coupled to the scheduling and prioritization entity 32, receives the data block to be transmitted and selects an appropriate dynamic transport format for the data block to be transmitted. With respect to H-ARQ transmissions and retransmissions, the TFRI selector 31 determines modulation and coding.

[00012] It is highly desirable for the retransmitted data blocks to arrive at the RLC entity of the receiving side (i.e., the UE) as soon as possible for several reasons. First, the missed data block will prevent subsequent data blocks from being forwarded to higher layers, due to the requirement of in-sequence delivery. Second, the buffer of the UE needs to be sized large enough to accommodate the latency of retransmissions while still maintaining effective data rates. The longer the latency is, the larger the UE buffer size has to be to allow for the UE to buffer both the data blocks that are held up and continuous data receptions until the correct sequence data block is forwarded to higher layers. The larger buffer size results in increased hardware costs for UEs. This is very undesirable. [00013] Referring to Figure 3, a simplified flow diagram of the data flow between a Node B (shown at the bottom of Figure 3) and a UE (shown at the top of Figure 3) is shown. PDUs from higher level processing are scheduled and may be multiplexed into one data block. A data block can only contain PDUs of higher layers of the same priority. A unique TSN is assigned to each data block by the scheduler. The higher layers may provide a plurality of streams of different priorities of PDUs, each priority having a sequence of TSNs. The scheduler then dispatches the data blocks to the plurality of H-ARQ processors P1B-P5B. Each H-ARQ processor P1B-P5B is responsible for processing a single data block at a time. For example, as shown in Figure 3, the Priority 1 PDUs comprise a sequence illustrated as B1I-B1N. Likewise, the Priority 2 PDUs are sequenced from B2I-B2N and the Priority 3 PDUs are sequenced from B3I-B3N. These PDUs are scheduled (and may be multiplexed) and affixed a TSN by the common scheduler. For purposes of describing the invention, it is assumed that one PDU equals one data block. After a data block is scheduled to be processed by a particular processor P1B-P5B, each data block is associated with a processor identifier, which identifies the processor P1B-P5B that processes the data block. [00014] The data blocks are then input into the scheduled Node B H-ARQ processors P1B-P5B which receive and process each data block. Each Node B H- ARQ processor P1B-P5B corresponds to an H-ARQ processor P1TJE-P5TJE within the UE. Accordingly, the first H-ARQ processor P1B in the Node B communicates with the first H-ARQ processor PIUE in the UE. Likewise, the second H-ARQ processor P2B in the Node B communicates with the second H-ARQ processor P2TJE in the UE, and so on for the remaining H-ARQ processors P3B-P5B in the Node B and their counterpart H-ARQ processors P3TJB-P5TJE respectively within the UE. The H-ARQ processes are timely multiplexed onto the air interface and there is only one transmission of an H-ARQ on the air interface at one time. [00015] For example, taking the first pair of communicating H-ARQ processors PIB and PIUE, the H-ARQ processor PIB processes a data block, for example Bli, and forwards it for multiplexing and transmitting it over the air interface. When this data block Bli is received by the first H-ARQ processor PIUE, the processor PIUE determines whether or not it was received without error. If the data block Bli was received without error, the first H-ARQ processor PIUE transmits an ACK to indicate to the transmitting H-ARQ processor PIB that it has been successfully received. On the contrary, if there is an error in the received data block Bli, the receiving H-ARQ processor PIUE transmits a NACK to the transmitting H-ARQ processor PIB. This process continues until the transmitting processor PIB receives an ACK for the data block Bli. Once an ACK is received, that processor PIB is "released" for processing another data block. The scheduler will assign the processor PIB another data block if available, and can choose to retransmit or start a new transmission at any time. [00016] Once the receiving H-ARQ processors P1TJE-P5UE process each data block, they are forwarded to the reordering buffers Ri, R₂, R3 based on their priority; one reordering buffer for each priority level of data. For example, Priority 1 data blocks B1I-B1N will be received and reordered in the Priority l reordering buffer Ri; Priority 2 data blocks B2I-B2N will be received and reordered in the Priority 2 reordering buffer R₂; and the Priority 3 data blocks B3I-B3N will be received and reordered by the Priority 3 reordering buffer R3. [00017] Due to the pre-processing of the data blocks by the receiving H-ARQ processors P1UE-P5UE and the ACK/NACK acknowledgement procedure, the data blocks are often received in an order that is not sequential with respect to their TSNs. The reordering buffers R1-R3 receive the out-of-sequence data blocks and attempt to reorder the data blocks in a sequential manner prior to forwarding onto the RLC layer. For example, the Priority 1 reordering buffer Ri receives and reorders the first four Priority 1 data blocks Blι-Bl . As the data blocks are received and reordered, they will be passed to the RLC layer. [00018] On the receiving side, the UE MAC-hs, (which has been graphically illustrated as MAC-hs control), reads the H-ARQ processor ID, whether it is sent on a control channel such as the HS-SCCH or whether the data block has been tagged, to determine which H-ARQ processor P1TJE-P5TJE has been used. If the UE receives another data block to be processed by the same H-ARQ processor PITJE- P5UE, the UE knows that that particular H-ARQ processor P1TJE-P5TJE has been released regardless of whether or not the previous data block processed by that H-ARQ processor P1UE-P5TJE has been successfully received or not. [00019] Figure 4 is an example of a prior art system including an RNC, a Node B, a UE and their associated buffers. This example assumes that the UE is the receiving entity and the Node B is the transmitting entity. In this prior art system, a PDU with SN = 3 is not received successfully by the UE. Therefore, the RLC in the UE requests its peer RLC layer in the RNC for a retransmission. Meanwhile, the PDUs with SNs = 6-9 are buffered in the Node B, and PDUs with SNs = 4 and 5 are buffered in the UE. It should be noted that although Figure 4 shows only several PDUs being buffered, in reality many more PDUs (such as 100 or more) and PDUs from other RLC entities may be buffered. [00020] As shown in Figure 5, if a retransmission of the PDU with SN = 3 is required, it must wait at the end of the queue in the Node B buffer, and will be transmitted only after the PDUs with SNs = 6-9 are transmitted. The PDUs in the UE cannot be forwarded to the upper layers until all PDUs are received in sequence.

[00021] In this case, the PDU with SN = 3 stalls the forwarding of subsequent PDUs to higher layers, (i.e. SNs = 4-9), assuming all the PDUs are transmitted successfully. Again, it should be noted that this example only reflects 11 PDUs, whereas in normal operation hundreds of PDUs maybe scheduled in advance of retransmitted data PDUs, which further aggravates transmission latency and data buffering issues.

[00022] It would be desirable to have a system and method whereby the retransmitted data can avoid the delays due to congestion in the transmission buffers. [00023] SUMMARY

[00024] The present invention is a system and method for transferring data in a wireless communication system. A plurality of data blocks are received and temporarily stored in a first memory. The plurality of data blocks are then transmitted. A determination is then made as to whether each of the transmitted data blocks was received successfully or needs to be retransmitted because the data block was not received successfully. Each of the transmitted data blocks that needs to be retransmitted is marked and stored in a second memory having a higher priority than the first memory. The marked data blocks stored in the second memory are transmitted before transmitting data blocks stored in the first memory.

[00025] Each marked data block may include a common channel priority indicator (CmCH-Pi). The CmCH-Pi of the marked data block is read and used to determine which of a plurality of memories to place the marked data block in based on the CmCH-Pi.

[00026] In accordance with one preferred embodiment of the present invention, a wireless communication system for transferring data includes a UE, a Node B in communication with the UE and a radio network controller (RNC) in communication with the Node B and the UE. The RNC transmits a plurality of data blocks to the UE via the Node B. The UE sends a status report to the RNC. The report indicates whether each of the transmitted data blocks was received successfully by the UE or needs to be retransmitted because the data block was not received successfully by the UE. The RNC marks each of the data blocks that needs to be retransmitted and sends the marked data blocks to the Node B. The Node B receives, temporarily stores and prioritizes transmission of the marked data blocks over other data blocks previously received and stored in Node B. The Node B transmits the marked data blocks to the UE before the other data blocks. [00027] BRIEF DESCRIPTION OF THE DRAWINGS

[00028] A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:

[00029] Figure 1 is a UTRAN MAC-hs.

[00030] Figure 2 is a prior art UE MAC-hs.

[00031] Figure 3 is a block diagram of the data flow between a Node B and a

UE.

[00032] Figure 4 is a diagram of the RLC layer exhibiting a missed PDU transmission.

[00033] Figure 5 is a diagram of retransmission by the RLC layer of the missed

PDU transmission.

[00034] Figure 6 is a signal diagram of a method of prioritizing retransmissions in accordance with the present invention.

[00035] Figure 7 is a block diagram of the data flow between a Node B and a

UE, whereby retransmissions are assigned to a higher priority queue.

[00036] Figure 8 is a block diagram of the data flow of a DSCH transmission scheduling PDUs with CmCH-Pi indications.

[00037] Figures 9 and 10 are diagrams of retransmission by the RLC layer of a missed PDU transmission in accordance with the present invention.

[00038] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [00039] The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. [00040] In describing the present invention, reference may be made to the terminology "buffer" and "memory." It is intended that these terms are equivalent, and are used to indicate a plurality of data blocks or PDUs in a successive queue.

[00041] In order to reduce the latency of an RLC layer retransmission, the present invention prioritizes a retransmission of a PDU over a subsequent PDU in the buffer of an intermediate node, such as a Node B for example. [00042] In the downlink direction (data transmissions from serving RNC (SRNC) to UE), one source of the latency of the retransmissions is generated in applications that buffer in the UTRAN outside of the SRNC. For example, buffering for an application could occur in the Controlling RNC (CRNC) or in the Node B. In several applications, the RNC RLC sends the PDU to the MAC-d in the RNC which creates an MAC-d PDU which is sent to the CRNC and then Node B (note that in the case that a UE has not moved out of the cell coverage of the SRNC, the CRNC will be the same RNC, and therefore, any messages sent are internal. When the UE has moved out of cell coverage of the SRNC and the new CRNC is known as the Drift RNC (DRNC). For simplification, in both cases this RNC will be referred to as a CRNC).

[00043] Since the MAC-d PDU contains exactly 1 RLC PDU (plus other potential MAC information), a MAC-d PDU can be considered equivalent to a RLC PDU. Although, discussion of PDUs in the CRNC or the Node B in the present application refers to MAC-d PDUs (not RLC PDUs), they can be considered equivalent for the purpose of the present invention and the term PDU will be used hereinafter to refer to both.

[00044] To allow for continuous data flow, the PDUs from the RNC RLC are usually queued in buffers of the CRNC or Node B for a while, before they are transmitted to the UE and thus the peer RLC. As will be described in detail hereinafter, the presently inventive method of retransmitting data at a higher priority bypasses the buffering/queuing of data in the UTRAN. [00045] One embodiment of the present invention is the RLC retransmissions from the Radio Network Controller (RNC) to the User Equipment (UE) of a system employing High Speed Downlink Packet Access (HSDPA). A method 100 for reducing the latency of retransmissions in accordance with the present invention is depicted in Figure 6. Figure 6 shows the communications between an RNC 102, a Node B 104 and a UE 106.

[00046] The RLC layer in the UE 106 generates a Status Report PDU (step 108) which indicates the status of received, (i.e., successfully transmitted), or missing PDUs, (i.e., unsuccessfully transmitted). This status report PDU is transmitted (step 110) to the RNC 102. Once the RLC layer in the RNC 102 receives the Status Report PDU from its peer entity in the UE 106, the RNC 102 prepares the retransmission of the missed PDU (step 112). [00047] The present invention implements a method to enable the Node B to distinguish the retransmitted PDU from other PDUs. In a first embodiment, the RNC 102 marks the retransmitted PDU by using a field of bits on its Frame Protocol (FP) overheads. The retransmitted PDU includes a CmCH-Pi which is updated (or increased) every time the PDU is sent (step 114) from the RNC 102 to the Node B 104. This permits the Node B 104 to track the number of times the PDU is sent and, therefore, identify the proper queue in which to place the PDU. Preferably, the CmCH-Pi is typically set and updated at the RNC 102. However, this function may also be performed at the Node B 104. The Node B 104 reads the CmCH-Pi and determines the proper priority queue for the PDU (steps 116). The Node B 104 transmission scheduler services the higher priority queues in advance of lower priority queues. The Node B 104 places the PDU to be retransmitted in a buffer having a higher priority than it originally had when the PDU was originally transmitted as a result of the setting of the CmCH-Pi by the RNC 102.

[00048] The PDU is then retransmitted (step 118) in a buffer (i.e., memory) having a higher priority than the priority of the original transmission. Other transmissions for this UE may be buffered in Node B 104 lower priority transmission queue at the time of the PDU retransmission. The setting of the increased CmCH-Pi for retransmitted PDUs results in transmission scheduling in advance of other PDUs previously received and buffered in Node B 104. [00049] Referring to Figure 7, retransmissions are assigned to a higher priority queue so that they supercede transmission of other data blocks which originate from the same "original" transmission buffer. Once the receiving H-ARQ processors P1UE-P5TJE process each data block, they are forwarded to the reordering buffers Ri, R2, R3 based on their priority; one reordering buffer for each priority level of data. For example, reordering buffer R2 reorders data blocks B2ι, B2₂ and B2 . Reordering buffer R3 reorders data blocks B33, B3 and B3β. A data block ("X") is missing between the data blocks B2₂ and B2 . An additional data block ("X") is missing between the data blocks B3 and B3δ. Thus, expected data blocks B2₃ and B3s are not received, e.g., due to a NACK message being misinterpreted as being an ACK message.

[00050] The missing data blocks are then retransmitted. Normally, the data block B23 would have been placed in the Priority 2 transmission buffer. However, since the data block B2₃ was missed and had to be retransmitted, the data block B23 is placed in a higher priority transmission buffer, (in this case the Priority 1 transmission buffer), and thus is sent earlier than if it were placed in the Priority 2 or 3 transmission buffers. Likewise, the data block B3s would have normally been placed in the Priority 3 transmission buffer. However, since the data block B3δ was missed and had to be retransmitted, the data block B3δ is placed in either the Priority 1 or Priority 2 transmission buffer so that it is transmitted earlier than if it had been placed in the Priority 3 transmission buffer. [00051] Upon reception of PDUs in the Node B, the CmCH-PI is used to determine the priority queue Bln-B3n. The scheduler services the higher priority queues first and assign transmissions to transmitting H-ARQ processors P1B-P5B, Upon successful transmission to the UE, the receiving H-ARQ processors PIUE -P5UE forward the retransmitted PDUs to the RLC layer. [00052] This procedure may also be applied for a DSCH system, except that the intermediate node is the CRNC instead of the Node B. Referring to Figure 8, PDUs 805 with CmCH-Pi indications are given priority by a prioritization entity 810 and are scheduled for transmission by the MAC-sh in the CRNC. The MAC- sh maintains multiple priority queues 815A, 815B, and a DSCH transmission scheduler 820 determines which PDU 805 is to be transmitted based on the priority of that data. Therefore, by setting increased CmCH-Pi for DSCH retransmissions, these transmissions will be serviced in advance of other data for the UE. This is similar to the HS-DSCH case where the Node B MAC-hs entity schedules transmissions.

[00053] Referring to Figure 9, a system is shown in accordance with the present invention implementing the prioritization method of Figure 6. After the RLC layer in the UE transmits a status report PDU to the RLC layer in the RNC indicating that the PDU with SN = 3 has not been successfully received, the RNC sends a retransmission of the PDU with SN = 3. The PDU will be prioritized over other PDUs in the buffer of the intermediate node by placement within a higher priority buffer. It should be noted that although only 11 PDUs are shown, in actuality, there may be hundreds of queued PDUs.

[00054] The benefits of the present invention can be seen with reference to Figure 10, which depicts the result of the prioritization function in the receiving buffer. The retransmitted PDU with SN = 3 arrives at the receiving buffer, and the in-sequence PDUs with SN = 3 to 5 can be forwarded to the higher layer much more quickly than the prior art scenario depicted in Figure 5. [00055] While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.

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