Method and system for retransmission

Abstract

The present invention relates to retransmissions In a communications system. A method and system of reducing uplink retransmission delay of a radio communications system by introducing an uplink MAC ARQ layer of Node B is disclosed. Further, a MAC PDU data indicator for soft combining control In Node B and RLC PDU reordering is introduced.

Claims

1. A method in a user equipment for reducing the impact of transmission errors by means of a retransmission protocol, the method comprising: assembling one or more second protocol data units into a first protocol data unit; transmitting the first protocol data unit to a first base station; transmitting the same first protocol data unit, or an identically assembled first protocol data unit, from the user equipment to at least one additional base station, wherein the first protocol data unit and the same first protocol data unit, or an identically assembled first protocol data unit, are transmitted within the same exact transmission time interval; and cancelling the first protocol data unit from a buffer storing non-acknowledged protocol data units as soon as a positive acknowledgement for the first protocol data unit is received from one or more of the base stations within a predetermined time or retransmitting the first protocol data unit if no positive acknowledgement for the first protocol data unit has been received for the first protocol data unit from any of the base stations within a predetermined time; wherein the user equipment executes a retransmission loop involving packet radio transmissions from the user equipment to a network controller connected to two or more base stations.

2. The method according to claim 1, wherein the first protocol data units are MAC PDUs.

3. The method according to claim 1, wherein the second protocol data units are RLC PDUs.

4. The method according to claim 1, further comprising receiving acknowledgments of the second protocol data units.

5. The method according to claim 1, wherein the user equipment is a user equipment of a UMTS or WCDMA system.

6. A user equipment adapted to reduce the impact of transmission errors by means of a retransmission protocol, comprising: means configured to assemble one or more second protocol data units into a first protocol data unit; means configured to transmit the first protocol data unit to a first base station and further configured to transmit the same or an identically assembled first protocol data unit from the user equipment to at least one additional base station wherein the first protocol data unit and the same, or an identically assembled, first protocol data unit are transmitted within the same exact transmission time interval; and means configured to cancel the first protocol data unit from a buffer storing non-acknowledged protocol data units as soon as a positive acknowledgement for the first protocol data unit is received from one or more of the base stations within a predetermined time; or, to retransmit the first protocol data unit if no positive acknowledgement has been received for the first protocol data unit from any of the base stations within a predetermined time; means configured to execute a retransmission loop involving packet radio transmissions from the user equipment to a network controller connected to two or more base stations.

7. The user equipment according to claim 6, wherein the first protocol data units are MAC PDUs.

8. The user equipment according to claim 6, wherein the second protocol data units are RLC PDUs.

9. The user equipment according to claim 6, wherein the user equipment is further configured to receive acknowledgments of the second protocol data units.

10. The user equipment of claim 6, wherein the user equipment is a user equipment of a UMTS or WCDMA system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 displays a layered protocol structure, according to prior art, in a radio communications system.

(2) FIG. 2 displays a layered protocol structure, according to the invention, in a radio communication system.

(3) FIG. 3 shows communication, according to the invention, between a UE and a base station involved in a connection between an RNC and the UE.

(4) FIG. 4 schematically illustrates MAC and RLC protocol layers, according to the invention, in a multilayer protocol structure.

(5) FIG. 5 gives an overview of the uplink protocol structure according to a preferred embodiment of the invention.

(6) FIG. 6 shows a MAC PDU according to the preferred embodiment of the invention.

(7) FIG. 7 illustrates transmission of data and associated control fields on separate channels, according to the invention.

(8) FIG. 8 illustrates N time multiplexed channels for N channel stop-and-wait according to the preferred embodiment of the invention.

(9) FIG. 9 displays an RNC according to the invention.

(10) FIG. 10 displays a Node B according to the invention.

(11) FIG. 11 displays a User Equipment according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(12) FIG. 2 shows a preferred layered protocol structure, a protocol stack, according to the invention. The L2 MAC layer of FIG. 1 has been extended and divided into two sub-layers, a new L2 MAC sub-layer and a new MAC-ARQ sub-layer. Essentially the new L2 MAC sub-layer corresponds to the prior art L2 MAC sub-layer of FIG. 1. The MAC-ARQ plus the MAC layer could be regarded as one single MAC layer extended to also include hybrid ARQ functionality. However, for reasons of explanation they are preferably regarded as separate sub-layers. Further, on the network side, considering them as separate protocol sub-layers physically better corresponds to the physical entities where they reside. As illustrated in FIG. 2 on the UTRAN-side (or network side) L2 MAC sub-layer is preferably located in RNC, whereas L2 MAC-ARQ sub-layer is located in Node B. Hybrid ARQ protocol combine successively received retransmissions. It is a great advantage to have the hybrid ARQ protocol close to the physical layer and, particularly, in Node B. Among the advantages achieved thereby, e.g., the roundtrip delay is reduced as compared to a location in RNC. Within this patent application the protocol layers, except for L2 MAC and L2 MAC-ARQ as just explained, correspond to those of FIG. 1.

(13) With reference to FIG. 3, Node B 1 and Node B 2 of a radio communications system are logical nodes responsible for radio transmission/reception in one or more cells to/from the User Equipment UE. BS 1 and BS 2 are physical entities representing Node B 1 and Node B 2 respectively. Node B 1 and Node B 2 terminate the air interface, called Uu interface within UMTS, between UE and respective Node B towards the radio network controller RNC. In UMTS the interface between a Node B and an RNC is called Iub interface.

(14) Preferably, all Nodes B of the radio communications system operate according to the invention for outstanding performance. However, the invention can also be used in systems also including Nodes B not operating according to the invention.

(15) FIG. 4 describes somewhat more detailed than FIG. 2 the information transfer between the sub-layers of the protocol stack. In an exemplary situation, and with reference to FIG. 3, UE communicates over a radio link associated with BS 1. Packet switched data is transmitted in protocol data units, PDUs, in both directions. Each PDU is transported on a transport channel in at least one transport block, TB, as shown in FIG. 4. Preferably there is one TB for each PDU. As described above, transmission errors on the transport channel are corrected and detected by layer L1. Each transport block, TB, in FIG. 4 can be provided an individual CRC error-detecting checksum prior to transmission on the physical channel. However, preferably a transmission unit, carrying one or more TBs, is provided only one CRC error-detecting checksum. If a transmission unit is detected to be in error on the receiving side, this is reported to the L2 MAC layer.

(16) L2 MAC layer can request retransmission of transmission units received in error. Transmission units, detected to be in error, still carry information that should not be wasted. Preferably hybrid ARQ, utilizing information available from earlier transmission(s) of a transmission unit by proper combining with the latest retransmission, is used prior to an L2 MAC layer request for retransmission.

(17) It is preferred that the Hybrid ARQ is terminated in Node B. Given L2 RLC located in RNC, the RLC layer should not be responsible of Hybrid ARQ. According to preferred embodiments of the invention, an L2 MAC-ARQ sub-layer is responsible of Hybrid ARQ. At UTRAN-side the L2 MAC-ARQ sub-layer is located in Node B.

(18) One reason for terminating the Hybrid ARQ in Node B is the reduction of roundtrip delay as compared to terminating it in RNC. Another reason is that Node B is capable of using soft combining of multiply transmitted data packets, whereas RNC generally only receives hard-quantized bits.

(19) At the receiving end, error detection is also performed by layer L2 RLC of FIG. 4. If an RLC protocol data unit, PDU, is received in error or the PDU is missing, it will be requested for retransmission at a point in time when a status report is established by the RLC layer. RLC PDUs are transferred to/from the MAC layer SDUs. The MAC SDU (Service Data Unit) possibly includes a header not included in the RLC PDU. As explained in relation to FIG. 2, according to the invention there are preferably two UTRAN MAC sub-layers, an L2 MAC sub-layer and an L2 MAC-ARQ sub-layer. In a preferred embodiment of the invention, the L2 MAC sub-layer is located in RNC and the L2 MAC-ARQ sub-layer is located in Node B. As previously described, the RLC PDUs are transferred in transport blocks, TBs, on the physical channel. In the uplink direction the L2 MAC-ARQ layer transfers TBs, possibly after combining of multiple (re-) transmissions of the respective TBs, indicated to be error-free to the L2 MAC layer.

(20) A network layer PDU or L3 PDU can comprise several RLC PDUs, as illustrated in FIG. 4. RLC PDUs are reassembled into RLC service data units, RLC SDU, prior to delivery to higher layer PDU. The L3 protocol can be, e.g., the Internet Protocol, IP. Upon reception from L3, RLC SDUs are segmented into RLC PDUs.

(21) FIG. 5 gives an overview of the uplink protocol structure according to the preferred embodiment of the invention. <<RNC>>, <<Node B 1>>, <<Node B 2>> and <<UE>> have been described in relation to FIG. 3. <<MAC-ARQ 1>> and <<MAC-ARQ 2>> are MAC-ARQ protocol layers according to the invention of Nodes B <<Node B 1>> and <<Node B 2>> respectively. In User Equipment <<UE>>, an integer number of RLC PDUs <<1>>, <<2>>, . . . , <<i>> are segmented into a MAC-layer PDU <<MAC PDU>> adapted for uplink transmission in one transmission time interval, TTI. Preferably, uplink transmissions are performed on an enhanced uplink channel with transmission time intervals shorter than transmission time intervals of prior art. Each Node B <<Node B 1>>, <<Node B 2>> (positively) acknowledges <<ARQ Status>> successfully received MAC PDUs. Optionally, it may negatively acknowledge <<ARQ Status>> unsuccessfully received MAC PDUs. The acknowledgements are transmitted in downlink direction to User Equipment <<UE>>. With only positive acknowledgements, if no acknowledgement is received within a predetermined time, <<UE>> retransmits the unacknowledged MAC PDU, otherwise <<UE>> cancels the MAC PDU from buffer storing not yet acknowledged MAC PDUs. With optional negative acknowledgements the predetermined time period becomes less critical. However, due to the risk of misinterpreting a negative acknowledgement as a positive acknowledgment, channel dependent probability of such misinterpretation needs to be considered for various channel environments for an optimum selection.

(22) MAC PDUs received by respective MAC-ARQ protocol layer <<MAC-ARQ 1>>, <<MAC-ARQ 2>> of Nodes B <<Node B 1>>, <<Node B 2>> are delivered to Radio Network Controller <<RNC>>, which reorders received MAC PDUs into sequential order. This is needed as, e.g., a single Node B may obtain successfully transmitted and received MAC PDUs out of order due to varying number of required retransmissions of different PDUs. Another exemplary reason is that at handover involving a plurality of Nodes B, different MAC PDUs of an RLC PDU are received and (re-)transmitted to RNC at different Nodes B for assembly in RNC, whereas for downlink transmissions there is only one transmitting entity (Node B) and one receiving entity (UE).

(23) A reordering entity in radio network controller <<RNC>> provides in-sequence delivery of RLC PDUs <<RLC PDUs>>, received in RNC, to RLC layer <<RLC>>. Two alternative number sequences for reordering are considered: reordering based on RLC sequence numbers for both UM and AM RLC; or special sequence numbers on MAC level.

(24) Special sequence numbers on MAC level is used for downlink HS-DSCH. However, if reordering is made in RNC number of sequences can be reduced if reordering is based on RLC sequence numbers, keeping transmission overhead at a minimum.

(25) The risk of receiving duplicate RLC PDUs also needs consideration. Elimination of duplicate RLC PDUs can be based on RLC sequence numbers as well. The elimination of duplicate RLC PDUs also reduces the negative impact of positive acknowledgements being received as negative acknowledgments in UE, if negative acknowledgments are made use of.

(26) Preferably, hybrid ARQ is used, softly combining, if more than one, successive received (re-)transmissions of each MAC PDU. An identification of process identity <<ProcessID>> and an indicator of new data <<NewData>> in addition to payload <<Payload>> may either be included in data packets <<MAC PDU>> according to the HARQ protocol as indicated in FIG. 6 or be handled separately but in association with payload data, illustrated in FIG. 7. The payload field <<Payload>> comprises an integer number of RLC PDUs. The indicator of new data <<NewData>> indicates whether or not a MAC PDU is a retransmission and is included for control of soft combining. This may be explicitly indicated, or indicated implicitly by a sequence number, which is increased for each transmission of a not earlier transmitted MAC PDU.

(27) There is need for greater reliability at transmission of process identity and new data indicator fields <<ProcessID>>, <<NewData>>, than payload. If control data is not received, corresponding payload cannot be soft-decision combined with data of other transmissions. Therefore, control fields <<ProcessID>> and <<NewData>> are transferred with higher reliability than the data channel transferring the payload. The higher reliability can be achieved by, e.g., more error resistant error control coding or increased transmission power. Preferably, control fields for process identity <<ProcessID>> and new data indicator <<NewData>> are transmitted on a control channel separate from the data channel. The control and data channels are synchronized.

(28) In addition to the control fields mentioned above there are control fields of nature well known in the art carrying information on, e.g., number of payload RLC PDUs and transport block size.

(29) A well-known (one-channel) stop-and-wait ARQ protocol does not provide sufficient throughput for most studied relevant cases. According to the invention selective repeat or N-channel stop-and-wait ARQ is preferred.

(30) For N-channel stop-and-wait, Node B needs to store soft samples of up to N different MAC PDUs for each UE. The N channels are time multiplexed as illustrated in FIG. 8. For a particular channel, no further data is transmitted until an acknowledgement is received or time for acknowledgement has elapsed according to a stop-and-wait protocol. I.e. acknowledgements are scheduled such that acknowledgement <<1st ACK>> acknowledging data transmitted in a 1.sup.st channel <<1st channel>> arrives not later than next transmission instant of 1.sup.st channel <<1st channel>>. The same holds for 2.sup.nd channel <<2nd channel>> up to N.sup.th channel <<Nth channel>>

(31) FIG. 9 displays an RNC according to the invention. Receive means 1 receives first protocol data units, e.g. MAC PDUs, transferred from one or more Nodes B. Received first protocol data units are stored by buffering means 2. The first protocol data units are segmented into second protocol data units, e.g. RLC PDUs, by segmentation means 3. Reorder means 4 operates on the segmented buffered data units and reorders the second protocol data units as need be in consecutive order according to a sequence number. For transfer to higher layers, reassemble means 5 reassembles the second protocol data units into service data units, which are transferred by transfer means 6.

(32) The radio network further comprises processing means 7, verifying second protocol data units according to an error detecting code. Acknowledgments, preferably included in status reports, of second protocol data units are transmitted by transmit means 8.

(33) FIG. 10 displays a Node B according to the invention. Node B transmits responses to UE providing information on the outcome of data transfers over the radio interface from UE to Node B. In an example situation receive means 9 receive one or more first protocol data units, e.g. MAC PDUs, the received first protocol data units are stored in buffering means 10 and processed in protocol entity 11 determining, among other things, whether the received data unit is valid codeword. Transmit means 12 accordingly acknowledges the received data unit to the sender (UE). When a multitude of transmissions are required for obtaining a valid received protocol data unit, the number of (re-)transmissions may be reduced by properly combining the transmissions. Optional combining means 13, performs such combining, preferably using soft decision information from the receive means 9.

(34) FIG. 11 displays a User Equipment, UE, according to the invention. Assemble means 14 assembles second protocol data units, e.g. RLC PDUs, to first protocol data units, e.g. MAC PDUs. The first protocol data units are buffered by buffering means 15 and transmitted one or more times, as need be, by transmit means 16. If transmission is considered successful according to, appeared or non-appeared, acknowledgements, positive or negative acknowledgements of first protocol data units are received by receive means 17.

(35) Positive or negative acknowledgments of second protocol data units are received by receive means 18 optionally distinguishing from receive means 17.

(36) A person skilled in the art readily understands that the receiver and transmitter properties of a BS or a UE are general in nature. The use of concepts such as BS, UE or RNC within this patent application is not intended to limit the invention only to devices associated with these acronyms. It concerns all devices operating correspondingly, or being obvious to adapt thereto by a person skilled in the art, in relation to the invention. As an explicit non-exclusive example the invention relates to mobile stations without a subscriber identity module, SIM, as well as user equipment including one or more SIMs. Further, protocols and layers are referred to in close relation with UMTS terminology. However, this does not exclude applicability of the invention in other systems with other protocols and layers of similar functionality.

(37) The invention is not intended to be limited only to the embodiments described in detail above. Changes and modifications may be made without departing from the invention. It covers all modifications within the scope of the following claims.