US 20070008990 A1
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.
1. A method of reducing impact of transmission errors by means of a retransmission protocol, utilizing a retransmission loop involving packet radio transmissions from user equipment to a control element connected to one or more radio base stations, wherein the user equipment radio transmissions are received at one or more radio base stations for forwarding to the control element, the base station acknowledging, positively or negatively, transmissions from the user equipment and the control element acknowledging, positively or negatively, transmissions forwarded to it.
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13. A signal format for uplink transmissions from user equipment to radio base station of a radio communications system, wherein the signal format comprises signal elements allowing radio base station reception combining and acknowledgment of successive received transmissions concerning same data in uplink direction prior to forwarding received transmissions, the signal elements comprising:
new data indicator, and
wherein the new data indicator indicates whether or not payload data of a process with identity as indicated by the process identity element has been transmitted previously.
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20. The signal format according to clam 19, wherein the process identity, new data indicator and payload elements are arranged for uplink transmission in a transmission time interval shorter than 4 milliseconds.
21. A radio network controller of a radio communications system, comprising:
receive means for receiving one or more transmissions originated in a UE in uplink direction from one or more radio base stations where the one or more transmissions in uplink direction have been pre-detected according to an ARQ protocol,
receive means, for receiving first protocol data units,
buffering means, for buffering received first protocol data units,
segmentation means, for segmenting received first protocol data units into second protocol data units,
reassemble means, for reassembling second protocol data units into service data units,
transfer means, for transferring service data units, and
reordering means, for reordering first or second protocol data units.
22. The radio network controller according to
processing means, and
the processing means being arranged to verify second protocol data units according to an error detecting code and the transmit means transmitting positive or negative acknowledgments depending on whether or not the second protocol data unit is detected to be erroneous.
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28. A radio base station, comprising:
receive means, for receiving one or more first protocol data units,
a protocol entity, for processing first protocol data units, and
transmit means, for transmitting acknowledgments and for forwarding of first protocol data units.
29. The radio base station according to
buffering means, for buffering one or more first protocol data units.
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35. A user equipment apparatus of a radio communications system, comprising:
assemble means for assembling one or more second protocol data units into one or more first protocol data units,
buffering means for buffering first protocol data units,
transmit means for transmitting first protocol data units according to an ARQ protocol,
receive means for receiving one or more acknowledgments of first protocol data units, and
receive means for receiving one or more acknowledgments of second protocol data units.
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The present invention relates to retransmissions in a communications system, and more especially it relates to a cellular mobile radio system, particularly to a Universal Mobile Telecommunications System, UMTS or WCDMA system.
Retransmission of data to or from a mobile station, MS, or user equipment, UE, is previously known. It is also known to use medium access control and radio link control layers of a UMTS protocol structure in acknowledged mode for dedicated channels.
In acknowledged mode, retransmissions are undertaken in case of detected transmission errors not recovered by forward error control. This is also called automatic repeat request, ARQ. With ARQ, retransmissions can be undertaken unless a transmitted message is (positively) acknowledged. Retransmissions could also be initiated at explicit negative acknowledgments of transmitted messages. Generally there are time limits for the respective positive and negative acknowledgements to be considered.
Within this patent application, a radio network controller, RNC, is understood as a network element including a radio resource controller. Node B is a logical node responsible for radio transmission/reception in one or more cells to/from a User Equipment. A base station, BS, is a physical entity representing Node B.
Medium access control, MAC, and radio link control, RLC, is used within radio communications systems like General Packet Radio Services, GPRS, and UMTS.
U.S. Pat. No. 5,570,367 discloses a wireless communications system arranged to transmit acknowledgement and request for retransmission messages. Data received in a microcell from an end user device is forwarded to a cell site. Data received by the cell site is transmitted to a cellular switch. A base station sends a poll message to the end user device, inquiring for the status of unacknowledged messages previously transmitted from the base station.
Also, a base station transmitter window is defined. A lower end pointer identifies a lowest numbered packet transmitted to and acknowledged by the base station. The upper end pointer identifies the highest numbered packet transmitted by the base station. Consequently, the window represents packets transmitted by the base station and unacknowledged by the end user device.
International Patent Application WO02096044 reveals a method and system of reducing or eliminating transmissions over a scarce communication link resource in a communications system by establishing an image of downlink transmissions status.
International Patent Application PCT/SE02/02186 includes a method and system for in-sequence delivery of RLC PDUs, transmitted in downlink direction, to a user equipment at handover.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, Physical Layer Procedures, 3G TS 25.301 v3.6.0, France, September 2000, specifies in chapter 5 Radio Interface Protocol Architecture of a UMTS system. There are three protocol layers:
Layer 2, L2, and layer 3, L3 are divided into Control and User Planes. Layer 2 consists of two sub-layers, RLC and MAC, for the Control Plane and four sub-layers, BMC, PDCP, RLC and MAC, for the User Plane. The acronyms BMC, PDCP, RLC and MAC denote Broadcast/Multicast Control, Packet Data Convergence Protocol, Radio Link Control and Medium Access Control respectively.
Radio Access Bearers, RABs, make available radio resources (and services) to user applications. For each mobile station there may be one or several RABs. Data flows (in the form of segments) from the RABs are passed to respective Radio Link Control, RLC, entities which amongst other tasks buffer the received data segments. There is one RLC entity for each RAB. In the RLC layer, RABs are mapped onto respective logical channels. A Medium Access Control, MAC, entity receives data transmitted in the logical channels and further maps logical channels onto a set of transport channels. In accordance with subsection 188.8.131.52 of the 3GPP technical specification MAC should support service multiplexing e.g. for RLC services to be mapped on the same transport channel. In this case identification of multiplexing is contained in the MAC protocol control information.
Transport channels are finally mapped to a single physical channel which has a total bandwidth allocated to it by the network. In frequency division duplex mode, a physical channel is defined by code, frequency and, in the uplink, relative phase (I/Q). In time division duplex mode a physical channel is defined by code, frequency, and time-slot. The DSCH (Downlink Shared Channel), e.g., is mapped onto one or several physical channels such that a specified part of the downlink resources is employed. As further described in subsection 5.2.2 of the 3GPP technical specification the L1 layer is responsible for error detection on transport channels and indication to higher layer, FEC encoding/decoding and interleaving/deinterleaving of transport channels.
PDCP provides mapping between Network PDUs (Protocol Data Units) of a network protocol, e.g. the Internet protocol, to an RLC entity. PDCP compresses and decompresses redundant Network PDU control information (header compression and decompression).
For transmissions on point-to-multipoint logical channels, BMC stores at UTRAN-side Broadcast Messages received from an RNC, calculates the required transmission rate and requests for the appropriate channel resources. It receives scheduling information from the RNC, and generates schedule messages. For transmission the messages are mapped on a point-to-multipoint logical channel. At the UE side, BMC evaluates the schedule messages and deliver Broadcast Messages to upper layer in the UE.
3G TS 25.301 also describes protocol termination, i.e. in which node of the UTRAN the radio interface protocols are terminated, or equivalently, where within UTRAN the respective protocol services are accessible.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, Physical Layer Procedures, 3G TS 25.322 v3.5.0, France, December 2000, specifies the RLC protocol. The RLC layer provides three services to the higher layers:
In subsection 184.108.40.206 an acknowledged mode entity, AM-entity, is described (see FIG. 4.4 of the 3GPP Technical Specification). In acknowledged mode automatic repeat request, ARQ, is used. The RLC sub-layer provides ARQ functionality closely coupled with the radio transmission technique used. The 3GPP technical specification also reveals various triggers for a status report to be transmitted. The receiver shall always send a status report, if it receives a polling request. There are also three status report triggers, which can be configured
For trigger 1, the receiver shall trigger transmission of a status report to the sender if a payload unit, PU, is detected to be missing. (One PU is included in one RLC PDU.) With trigger 2, a receiver triggers transmission of a status report periodically according to a timer. Finally, trigger 3 relates in short to a timer corresponding to an estimated number of received PUs before the requested PUs are received. The 3GPP Technical Specification specifies a status PDU used to report the status between two RLC AM (‘Acknowledged Mode’) entities.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, High Speed Downlink Packet Access (HSDPA), Overall Description, 3G TS 25.308 v5.3.0, France, December 2002, describes the overall support of High Speed Downlink Packet Access in UTRA. FIGS. 5.1-1 and 5.1-2 illustrate protocol architecture of HS-DSCH. Chapter 6 specifies HS-DSCH MAC architecture for the downlink.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, UTRAN Overall Description, 3G TS 25.401 v4.5.0, France, September 2002, describes the overall architecture of UTRAN, including internal interfaces and assumptions on radio and Iu interfaces. Section 11.2.5 presents the DSCH Transport channel. DSCH scheduling is performed by MAC-c/sh in the CRNC.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, Physical layer aspects of UTRA High Speed Downlink Packet Access, 3G TS 25.848 v4.0.0, France, March 2001, describes, among other things, physical layer aspect of the techniques behind the concept of high-speed downlink packet access (HSDPA). Section 6.3.1 presents a complexity analysis of a dual channel stop-and-wait protocol for downlink hybrid ARQ.
3rd Generation Partnership Project (3GPP): Technical Specification Group Radio Access Network, Physical layer aspects of UTRA High Speed Downlink Packet Access, 3G TS 25.950 v4.0.0, France, March 2001, describes several techniques for facilitating high-speed downlink packet access. Chapter 8 describes various properties of stop-and-wait hybrid ARQ.
None of the cited documents above discloses a method and system of reducing uplink retransmission delay of a radio communications system by introducing an uplink MAC ARQ layer of Node B. Further, none of the cited documents mentions a MAC PDU data indicator for soft combining control in Node B.
Cited prior art references describe retransmissions between a UE and an RNC. According to preferred embodiments of the invention, transmissions and retransmissions are partially terminated in Node B, rendering soft combining of subsequent transmissions possible. Corresponding acknowledgements (positive or negative), if any, are terminated in UE.
Current WCDMA uplink connections has a latency in an approximate range of 100-150 ms. This latency is of such an amount that for some humans it is considered annoying, e.g., when using interactive services over the connection.
Further, present WCDMA releases provide wide area coverage for uplink data rates of 384 kbps. Applications proposed or recently presented require high peak rates in the uplink.
Consequently, it is an object to reduce transmission delay and latency over a scarce resource such as a radio interface.
A further object is to generate ARQ status reports in Node B and to render possible soft combining of subsequent (re-)transmissions.
It is also an object to segment RLC PDUs into MAC PDUs suitable for transmission in one transmission time interval, that may be shorter than allowed transmission time intervals of present WCDMA releases.
Finally, it is an object to introduce a reordering entity of RNC, handling RLC-PDUs received out of order, for improved out-of-order handling at RNC.
These objects are met by the invention, which is particularly well suited for a high-speed uplink packet access channel of an evolved universal mobile telecommunications system.
Preferred embodiments of the invention, by way of examples, are described with reference to the accompanying drawings below.
With reference to
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.
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.
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.
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.
At the receiving end, error detection is also performed by layer L2 RLC of
A network layer PDU or L3 PDU can comprise several RLC PDUs, as illustrated in
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 re-orders 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).
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:
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.
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.
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
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.
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.
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.
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
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.
Positive or negative acknowledgments of second protocol data units are received by receive means 18 optionally distinguishing from receive means 17.
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.
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.