WO2009088342A1

WO2009088342A1 - Reordering timer for a retransmission protocol

Info

Publication number: WO2009088342A1
Application number: PCT/SE2008/051379
Authority: WO
Inventors: Johan Torsner; Michael Meyer; Anna Larmo
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-01-07
Filing date: 2008-12-01
Publication date: 2009-07-16

Abstract

A retransmission controller for a communication device uses a reordering timer to supervise a retransmission protocol. The retransmission controller starts the reordering timer when a first gap comprising one or more first missing data units is detected and the timer is inactive. The retransmission controller stops the reordering timer when all of said first missing data units are received even if second missing data units corresponding to a second gap preceding the first gap are missing. If the reordering timer expires before all of the first missing data units are received, the retransmission controller sends a retransmission request to the transmitter.

Description

REORDERING TIMER FOR A RETRANSMISSION PROTOCOL

TECHNICAL FIELD

The present invention relates generally to retransmission protocols for wireless communication systems and, more particularly, to a reordering timer for a retransmission protocol to enable out of sequence delivery of data units

BACKGROUND

In communication networks implementing the Long Term Evolution (LTE) standards a retransmission protocol is implemented in both the Radio Link Control (RLC) layer and Medium Access Control (MAC) layer to make data transmission more reliable A Hybrid ARQ (HARQ) protocol is implemented in the MAC layer to reduce the error rate to approximately 1 % A second retransmission protocol is implemented at the RLC layer to correct residual errors and further reduce the error rate to a level acceptable for the service, which is typically in the range of 10 ⁶ The HARQ protocol used in HSPA and LTE causes some RLC Protocol Data Units (PDUs) to be received out of order, i e , in another order than in which they were originally transmitted by the peer entity If data is delivered to RLC out of order, it may trigger unnecessary retransmissions because the RLC will interpret gaps in the sequence numbers as missing PDUs, even if they may still be under retransmission by the underlying HARQ protocol In HSDPA, a reordering functionality is used to restore the order of the RLC PDUs at the

MAC layer i e , before they are delivered to RLC In LTE, however, the reordering has recently been specified as part of RLC The LTE reordering functionality starts a reordering timer when a missing RLC PDU is detected, i e , when the first PDU following one or more missing PDUs is received If the missing PDU has not been received when the reordering timer expires, it triggers the transmission of an RLC status report, which will include a negative acknowledgement (NACK) for the missing PDU The request for retransmission of the missing PDU may be delayed some configurable amount of time to allow for HARQ retransmissions of the PDU to be completed After this time has passed, it is assumed that HARQ has failed, and the PDU will be retransmitted by RLC If the missing PDU is received while the reordering timer is running the timer is stopped since the gap no longer exists

The specified reordering works well when there is a single gap in the sequence numbers i e , one or several consecutive PDUs are missing However, this approach has some disadvantages in the case where several gaps exist in the received sequence numbers One such problem occurs when the missing PDUs corresponding to a later gap are received before the missing PDUs corresponding to an earlier gap In this case, the reordering timer is not stopped Therefore any poll (ι e , request for transmitting a status report) received will be unnecessarily delayed until the missing PDUs are received Further, the expiry of the reordering timer may trigger a redundant request for retransmission of the missing PDUs, even if a retransmission request for the missing PDUs has already been sent.

SUMMARY The present invention provides an improved method and apparatus for supervising a retransmission protocol. Data units received out of sequence are temporarily stored while waiting for missing data units to arrive. According to one exemplary embodiment, a reordering timer is started when a first gap comprising one or more missing data packets is detected and the timer is not already running. The timer is stopped when all of the first missing data packets are received, even if one or more second missing data units corresponding to a second gap preceding said first gap are not yet received. The timer is continued when second missing data packets corresponding to a second gap preceding said first gap are received. When the timer expires, a request for retransmission of the first missing data packets is sent.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates an exemplary communication network.

Fig. 2 illustrates an exemplary protocol stack for a wireless communication device including a radio link control layer that implements a retransmission protocol.

Figs. 3-5 illustrate an exemplary method of operating a reordering timer to supervise a retransmission protocol.

Fig. 6 illustrates an exemplary user terminal.

Fig. 7 illustrates an exemplary processor for implementing a retransmission protocol. Figs. 8A and 8B are flow diagrams illustrating exemplary methods for operating a reordering timer.

DETAILED DESCRIPTION

Referring now to the drawings, the present invention will be described in the context of an exemplary mobile communication system 10 based on the Long Term Evolution (LTE) standards being developed by the Third Generation Partnership Project (3GPP). Those skilled in the art will appreciate, however, that the present invention may also be used in mobile communication systems 10 based on other standards now known or later developed. Thus, the following description should be viewed as illustrative, and not limiting.

The communication system 10, shown in Fig. 1 , comprises a transmitting terminal 20 and a receiving terminal 30. The transmitting terminal 20 transmits signals over a communication channel 40 to the receiving terminal 30. The transmitting terminal 20 may comprise a base station in an LTE network, sometimes referred to as an Evolved Node B (eNodeB), and the receiving terminal 30 may comprise a user terminal (e.g., cellular telephone, PDA, laptop computer, etc.). Alternatively, the transmitting terminal 20 may comprise a user terminal and the receiving terminal 30 may comprise a base station. It will also be appreciated that transmitting terminal 20 and receiving terminal 30 may each comprise a communication terminal 100 (Fig. 4) with a transceiver for two-way communications. The terms "transmitting" and "receiving" are thus used herein to indicate the direction of communication between two communication terminals.

Fig. 2 provides a general overview of the LTE protocol architecture 200, for communications between a transmitting terminal 20 and a receiving terminal 30 in LTE systems. The LTE protocol architecture 200 is a layered protocol stack. Each layer of the protocol stack 200 represents a set of protocols or functions needed to communicate over the communication channel 40. The protocol stack 200 in LTE includes a physical (PHY) layer 240, the medium access control (MAC) layer 230, the radio link control (RLC) layer 220, and the packet data convergence protocol (PDCP) layer 210. The protocol stack 200 is typically implemented by a specially programmed processor.

User plane data to be transmitted in the form of IP packets enters the PDCP layer 210, where the IP headers may be compressed to reduce the number of bits transmitted over the air interface. PDCP layer 210 also performs ciphering and deciphering of the IP packets for security. RLC layer 220 ensures almost error free, in sequence delivery of compressed IP packets to the PDCP layer 210 at the receiving terminal 30, which is needed for certain types of communication. At the transmitting terminal 20, RLC layer 220 segments and/or concatenates compressed IP received from the PDCP layer 210 over radio bearers to create RLC protocol data units (PDUs). At the receiving terminal 30, the received RLC PDUs are reassembled into compressed IP packets and delivered to the PDCP layer 210. The RLC layer 220 implements a retransmission protocol described in more detail below to handle retransmission of missing or erroneously received RLC PDUs. MAC layer 230 offers services to the RLC layer 220 in the form of logical channels. The MAC layer 230 maps RLC PDUs received on various logical channels from RLC layer 220 to corresponding transport channels. MAC layer 230 is responsible for uplink and downlink scheduling. MAC PDUs are fed by the MAC layer 230 to the PHY layer 240. PHY layer 240 handles coding/decoding, modulation/demodulation, interleaving, and spreading prior to transmission of one or more PHY layer PDUs. In communication systems 10 implementing the LTE standards, a retransmission protocol is implemented in both the RLC layer 220 and MAC layer 230 to make data transmission more reliable. A Hybrid ARQ (HARQ) protocol is implemented in the MAC layer 230 to reduce the error rate to approximately 1 %. A second retransmission protocol is implemented at the RLC layer 220 to correct residual errors and further reduce the error rate to a level that is needed for the service, typically in the order of approximately 10^"6. Because MAC layer 230 is not required to deliver RLC PDUs in sequence to RLC layer 220, the HARQ protocol implemented in the MAC layer 230 causes some RLC PDUs to be received out of order. Therefore, reordering of the RLC PDUs is performed in the RLC layer 220. Conventionally, gaps in the sequence numbers of received RLC PDUs would result in RLC requesting retransmission of missing RLC PDUs or PDU segments. However, in the current version of LTE, the missing data units may still be subject to retransmission by the MAC HARQ process. Therefore, to prevent unnecessary retransmission requests, a reordering timer for RLC is introduced. When a missing RLC PDU is detected, the reordering timer is started and the retransmission request is delayed until the expiration of the reordering timer. If the missing RLC PDU is received before expiration of the timer, the reordering timer is stopped.

Problems may arise in the operation of the reordering timer when there are multiple gaps in the sequence numbers of the received RLC PDUs. Fig. 3 illustrates operation of one exemplary reordering timer in the scenario where a later gap in the sequence number is filled before an earlier gap is filled. At time tθ, PDU 3 has been received and the reordering timer is started, since PDU 2 is missing. The timer expires at time t1 and a request for retransmission of PDU2 is sent. At time t2 (e.g., after the reordering timer has expired), PDUs 4 and 7 are received. The reordering timer is restarted since a new gap comprising PDUs 5 and 6 is detected. At time t3, the PDUs 5 and 6 are received. If the reordering timer is continued at t3, any poll request (e.g., request for a status report) received in PDUs with sequence numbers 5 or above will be unnecessarily delayed. Further, the expiration of the reordering timer will trigger a redundant request for retransmission of PDU 2 even if a retransmission request for PDU 2 has already been sent. To avoid problems that arise when more than one gap exists in the sequence numbers of the received RLC PDUs, the RLC layer 220 may in some embodiments of the invention remember the gap that triggered the reordering timer and stop the reordering timer when the corresponding RLC PDUs are received, even if other RLC PDUs with a lower sequence number are still missing. This alternative method of operation is illustrated in Fig. 4. At time tθ, PDU 3 has been received and the reordering timer is started, since PDU 2 is missing. The timer expires at time t1 and a request for retransmission of PDU 2 is sent. At time t2 (e.g., after the reordering timer has expired), PDUs 4 and 7 are received and the reordering timer is restarted since a new gap comprising PDUs 5, and 6 is detected. In this example, the RLC layer 220 remembers the gap that triggered the reordering timer. At time t3, the PDUs 5 and 6 are received and the reordering timer is stopped since the gap that triggered the timer is closed, even though RLC PDU 2 is still missing.

Fig. 5 illustrates operation of the exemplary reordering timer in the scenario where an earlier gap in the sequence number is filled before a later gap is filled. At time tθ, PDU 3 has been received and the reordering timer is started, since PDU 2 is missing. The timer expires at time t1 and a request for retransmission of PDU 2 is sent. At time t2 (e.g., after the reordering timer has expired), PDUs 4 and 7 are received and the reordering timer is restarted since a new gap comprising PDUs 5, and 6 is detected. In this example, the RLC layer 220 remembers the gap that triggered the reordering timer. At time t3, the PDU 2 is received, closing the earlier gap. In this case, the reordering timer is continued since the gap that triggered the timer is still open.

Fig. 6 illustrates an exemplary communication device 100 that implements the retransmission protocol as previously described. The communication device 100 may comprise a base station that implements the retransmission protocol for uplink transmissions, or a user terminal that implements the retransmission protocol for downlink transmissions. The communication terminal 100 comprises a transceiver 1 10 connected to an antenna 1 12, a processor 120, and memory 130. Transceiver 1 10 enables the communication terminal 100 to transmit signals to and receive signals from a remote terminal. The communication terminal 100 may comprise, for example, a conventional cellular transceiver operating according to the LTE standard, WCDMA standard, or other communication standard now known or later developed. The processor 120 processes the signals transmitted and received by the transceiver 1 10 and controls operation of the user terminal 100. Processor 120 includes an RLC module 122 for implementing the RLC protocols as previously described. Memory 130 stores programs and data needed for operation.

Fig. 7 illustrates the main functional elements of the RLC module 122. RLC module 122 includes a retransmission controller 124, a reordering timer 126 under the control of the retransmission controller 124, and a buffer 128 for temporarily storing RLC PDUs while reordering is performed. The retransmission controller 124 comprises the main control logic for implementing RLC layer protocols and is responsible for reordering RLC PDUs that are received out of sequence. RLC controller 124 starts the reordering timer 126 when a gap is detected and stops the reordering timer 126 when a gap is filled. If the reordering timer 126 expires before a gap is filled, retransmission controller 124 sends a retransmission request (e.g., NACK) for the missing RLC PDU to the transmitting terminal 20. Thus, the reordering timer 126 delays a retransmission request when a missing packet is detected to give time for the HARQ protocols implemented in the MAC layer 230 to deliver the out of sequence RLC PDUs.

Fig. 8A illustrates an exemplary method 400 implemented by the retransmission controller 124 when a missing gap is detected in the sequence numbers of the received RLC PDUs. When a gap is detected (block 402), the retransmission controller 124 determines whether the reordering timer is active (block 404). If the reordering timer 126 is already active, the retransmission controller terminates the procedure 400 (block 412). On the other hand, if the reordering timer 126 is not active, the retransmission controller 124 starts the reordering timer 126. If the missing RLC PDUs are not received when the timer expires (block 408), retransmission controller 124 sends a retransmission request to the peer RLC layer at the transmitting terminal 20 (block 410) and the procedure ends (block 412).

After the reordering timer 126 is started, missing RLC PDUs may be received by the RLC layer 220. Fig. 8B illustrates exemplary method 450 performed by the retransmission controller 124 to process a missing RLC PDU received by the RLC layer 220 while the reordering timer 126 is active. The procedure is triggered by receipt of a missing RLC PDU (block 452). As previously noted, the sequence numbers of the received RLC PDUs may have multiple gaps. A received RLC PDU may belong to any one of the unfilled gaps. After sending the received RLC PDU to the reordering buffer 128 (block 454), the retransmission controller 124 determines whether the received RLC PDU belongs to the gap that triggered the reordering timer 126 (block 456). The gap triggering the reordering timer 126 is referred to herein as the current gap. If the received RLC PDU belongs to an earlier gap (e.g., a gap preceding the current gap), the retransmission controller 124 continues the reordering timer 126 (block 458). If the received RLC PDU belongs to the current gap, the retransmission controller 124 next determines whether all RLC PDUs in the current gap have been received (block 460). If not, the retransmission controller 124 continues the reordering timer (block 458). If all RLC PDUs in the current gap have been received, the retransmission controller 124 stops the reordering timer (block 462) and terminates the procedure (block 464).

Claims

CLAIMSWhat is claimed is:

1. A method of controlling a reordering timer (126) for supervising a retransmission protocol, said method characterized by: starting said reordering timer (126) when a first gap comprising one or more first missing data units is detected and said timer (126) is inactive; stopping said reordering timer (126) when all of said first missing data units are received even if second missing data units corresponding to a second gap preceding said first gap are missing; and sending a retransmission request responsive to expiration of said reordering timer (126) if one or more of said first missing data units are not received.

2. The method of claim 1 further comprising continuing said reordering timer (126) when said second missing data units preceding said first missing data units are received if said reordering timer (126) is still active.

3. The method of claim 1 or claim 2 performed by a user terminal in communication with a base station.

4. The method of claim 1 or claim 2 performed by a base station in communication with a user terminal.

5. A processor (120) for a communication device (100), said processor (120) including a control module (122) for implementing a retransmission protocol and characterized in that the control module (122) is configured to: start a reordering timer (126) when a first gap comprising one or more first missing data units is detected and said timer (126) is inactive; stop said reordering timer (126) when all of said first missing data units are received even if second missing data units corresponding to a second gap preceding said first gap are missing; and send a retransmission request responsive to expiration of said reordering timer (126) if one or more of said first missing data units are not received.

6. The processor (120) of claim 5 wherein the control module (122) is further configured to continue said reordering timer (126) when second missing data units preceding said first missing data units are received if said reordering timer (126) is still active.

7. The processor (120) of claim 5 or claim 6 adapted for use in a user terminal.

8. The processor (120) of claim 5 or claim 6 adapted for use in a base station.

9. A communication device (100) comprising: a transceiver (110) for communicating with a remote terminal (20, 30); a processor (120) for processing signals transmitted and received by said transceiver (110), said processor (120) including a control module (122), characterized in that the control module (122) is configured to: start a reordering timer (126) when a first gap comprising one or more first missing data units is detected and said timer (126) is inactive; stop said reordering timer (126) when all of said first missing data units are received even if second missing data units corresponding to a second gap preceding said first gap are missing; and send a retransmission request responsive to expiration of said reordering timer (126) if one or more of said first missing data units are not received.

10. The communication device (100) of claim 9 wherein the control module (122) is further configured to continue said reordering timer (126) when second missing data units preceding said first missing data units are received if said reordering timer (126) is still active.

1 1. The communication device (100) of claim 9 or claim 10 comprising a user terminal.

12. The communication device (100) of claim 9 or claim 10 comprising a base station.