Transmission control in a relay network

Field of the invention

The present invention relates to communication networks employing relays. Background of the invention

The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.

A wireless communication network refers to a communication network that allows access to communication services over a radio interface. Services of a wireless network are accessible to user equipment (UE) when UE is in the radio signal coverage area of the wireless network. For example, in the case of a 3G network, when UE is in the radio signal coverage area of a base station (BS) , it is able to communicate with at least one BS of the wireless network.

Wireless network radio signal coverage can be extended using re- lays. A relay can be, for example, a decode-and-forward (DF) or an amplify- and-forward (AF) relay. An AF relay amplifies a received analog signal, and thus also amplifies the noise received with the actual content of the signal. A DF relay may apply digital operations, such as decoding and coding, on a received signal. Typically a DF relay regenerates the signal and transmits the re- generated signal forward. Due to the enhancing transmission control measures that are possible during the regeneration stage, a DF relay typically provides better signal quality that an AF relay. The use of DF relays for extending the coverage of a single base station has been noted to increase the capacity usage of the single base station and improve the signal quality received by UE. There exists several methods for controlling the transmission of data blocks, packets and frames in the air interface. Such methods include, for example, Automatic Repeat-reQuest (ARQ) and Hybrid-ARQ (HARQ), which are specified for example in document E-UTRAN (Evolved Universal Terrestrial Radio Access) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access

(E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E- UTRAN); Overall description; Stage 2 (Release 8). The referred E-UTRAN specification and the related 3rd Generation Partnership Project (3GPP) specifications define operations, functions and nodes of a communications network and are below referred to simply as E-UTRAN. In E-UTRAN, ARQ is used for controlling Radio Link Control (RLC) sublayer transmissions and HARQ is used for controlling transmissions on Medium Access Control (MAC) and Physical (PHY) sublayers.

ARQ operates by adding error detection information, such as check- sum bits, to the transmitted data and waiting for acknowledgements to the transmitted data for a defined time period. When a message that is controlled with ARQ is transmitted, an acknowledgement is expected to be received within a defined time period. If an acknowledgement is not received during the time period, ARQ makes a decision to retransmit the message. A likely reason for missing an acknowledgement is likely that an error is detected at the receiving end, typically using error detection bits in the received data. The retransmitted message can be the whole message or only a segment of the message. Particularly in E-UTRAN it is possible to retransmit a whole RLC PDU or a segment thereof. In addition to a time period used for determining a failure, a status report message can be transmitted to indicate a failure or success of a transmission.

HARQ in E-UTRAN operates to retransmit transport blocks (TB). A HARQ type used in E-UTRAN is N-process stop-and-wait, where new transport blocks are not transmitted for a single HARQ process before an acknowl- edgement message is received for the previous transport block. Meanwhile, another HARQ process can transmit data on the communication channel. This means that there can be N parallel HARQ processes active at any time, N being an integer. In HARQ, the transmitted data is encoded using forward error correction (FEC) ₁ which increases the probability of correctly receiving the transmitted data, with the expense of increased overhead in the transmission. With HARQ incremental redundancy may be used to improve efficiency of the HARQ process. The necessary retransmissions are combined with the first transmission in the receiver to achieve a successfully received data block. In E-UTRAN the HARQ process can have a limit for the maximum number of re- transmissions allowed for data blocks. When this maximum number of retransmissions is reached, a local negative acknowledgement (Local NACK) is

sent from MAC to RLC to indicate a failure in the HARQ process to ARQ. In a successful case a local acknowledgement (Local ACK) is sent.

In E-UTRAN, ARQ retransmissions are based on RLC status reports and HARQ/ARQ interactions (cf. the aforementioned Local NACK and Local ACK indications). If the HARQ process detects a failed delivery of TB or MAC protocol data unit (PDU) ₁ the relevant ARQ transmitting entities are notified to start potential retransmissions and re-segmentation. Thus HARQ operating on a MAC layer can notify a higher protocol layer ARQ process about the HARQ status. In light of this, BS can use the received information to initiate retransmissions on higher protocol layers faster than if it relied on only on the status information received from the peer higher protocol layers.

A further advantage of the HARQ/ARQ interaction is that retransmissions by the ARQ process on the RLC layer enable efficient use of transmission resources as the retransmitted RLC PDUs may be multiplexed to- gether with other RLC PDUs in lower protocol layers, thus allowing also other data to be transmitted along with the retransmitted data.

When DF-type of relays are introduced in E-UTRAN, error control processes such as HARQ and ARQ no longer take place directly between UE and BS as the DF relay decodes TBs received from BS and codes new TBs to be transmitted to UEs or other relays on a relay link. The transmission status of a data block on the relay link is not visible to the ARQ process in BS controlling the transmission between BS and UE. This eliminates any possibility to apply HARQ/ARQ interaction as a basis for ARQ retransmissions in network configurations that apply DF-type relays. Brief description of the invention

An object of the present invention is to provide a method and an apparatus for implementing the method so as to overcome the above problems. The object of the invention is achieved by a method, apparatus, a computer program distribution medium and a system, which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention enables use of status information from transmissions of transport blocks in functions or procedures of higher layer protocols, even in network configurations that comprise relay stations.

Brief description of the drawings

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which Figure 1 shows an exemplary communication system employing relays;

Figure 2 shows an apparatus in accordance with an embodiment of the invention;

Figure 3 shows protocol stacks according to an embodiment of the invention;

Figure 4 shows data structures according to an embodiment of the invention;

Figure 5 shows a flow chart illustrating the operation of one embodiment of the invention; Figure 6 shows a flow chart illustrating the operation of one embodiment of the invention;

Figure 7 shows a signalling chart according to an embodiment of the invention.

Detailed description of the invention The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiments), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

The present invention relates to using relays in a wireless communication network. A wireless communication network conventionally comprises network nodes that provide wireless access to apparatuses that are configured to operate in the network. In cellular communication systems, for example third generation systems such as UMTS (Universal Mobile Telecommunications System), GSM (Global System for Mobile Communications) or WiMAX (Worldwide Interoperability for Microwave Access), a network node that can provide wireless access is generally called a base station or node B. In wireless local area networks as defined by (IEEE) 802.11 family of standards they are generally called access points. These network nodes are

able to transmit and receive wireless signals and thus establish wireless connections for the access. A wireless connection may be implemented, for example, with a wireless transceiver that operates according to any of the above technologies or with any other suitable standard/non-standard wireless communication means. Each network node provides radio signal coverage, and the wireless communication network provides radio signal coverage in the combined coverage area of the network nodes.

The apparatus for accessing the wireless network may be a piece of equipment or a device that is arranged to associate the user terminal and its user with a subscription, and enables the user to interact with a communications system. The user terminal may be any terminal capable of receiving information from and/or transmitting information to the network. Examples of the user terminal include a personal computer, a game console, a laptop, a notebook, a personal digital assistant, a mobile station and a mobile phone. In the following the invention is described employing the context and terminology of an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) as defined in 3GPP TS 36.300 V8.1.0 (2007-06), although the invention may be applied to other networks and technologies. Although, in E- UTRAN specifications the network node for providing wireless access is referred to as eNB (E-UTRAN Node-B), it is hereinafter referred to as a base station (BS). The apparatus for the user for accessing the E-UTRAN network is referred to as UE (user equipment).

Figure 1 shows an embodied E-UTRAN communication system. The system comprises a network and UE. The network provides wireless con- nections to UE (122, 124, 126) by radio signal coverage 150, 160 through base stations (BSs) 102, 104, which are connected to a gateway (GW) 106 through a backhaul connection. The network 100 comprises also relay nodes (RN) 110, 112, 114, which can extend the radio signal coverage of the base stations by relaying the communications between the base stations and user equipment (UE) that conventionally communicate directly with BSs.

In Figure 1, the connections between RNs and UEs are depicted with arrows. As shown in Figure 1, RNs 110, 114 and UEs 120, 126 that reside within the coverage area of BSs 102, 104 can receive and transmit signals with them. In the present embodiment of the invention, the connections of RNs to BSs or other RNs are wireless, but it is clear that any type of wireless or wired connection may be applied when exchanging information between BSs and

RNs. UEs 122, 124 and RN 112 that reside outside the coverage areas 150, 160 are connected to BS 102 via RN 110. Therefore, the coverage areas 150, 160 of BSs become extended by means of RN 110. RN 112 is connected to BS 102 via RN 110. UE 126 is connected to the network via RN 114. GW provides access to other networks 108 such as the Internet or other access networks, for example GSM, WLAN, WiMAX communications networks. The other networks 108 may provide to UE, for example, web-based services that reside on application servers.

There are several reasons for employing RNs in the wireless com- munication. Primarily, RNs may be used to provide radio signal coverage beyond the coverage area of the base station. For example, in Figure 1, RNs 110, 112 route traffic between BS 102 and UEs 122 and 124. In some cases RNs are used to provide radio signal coverage also within the coverage area of BSs. RNs can route traffic to BS from a considerably wider area than the base station, which means that the traffic handling capacity available in a single BS can be taken in to use more efficiently by using RNs. RNs are less complicated elements than BSs and thus their deployment is more cost efficient than use of fully-fledged BSs.

RNs may also be applied to improve the quality of the radio signal in areas within the coverage area where the radio signal quality from BSs is typically low due to radio signal attenuation, fading and shadowing, to name a few. In Figure 1 this is illustrated by RN 114 providing connectivity in the coverage area of BS 104 to UE 126. UEs connected to BSs through RNs can achieve higher effective throughputs because they are closer to RNs than BS and thus benefit from the higher quality radio signal. RNs can also be used to reduce the transmission power needed in BSs, as the low radio signal quality areas are covered by RNs. Also the radio signal received in RN from UE may be better quality than a radio signal received from UE at BS. UE communicating with BS via RN may enable UE to use lower transmission power. RNs may also be chained to extend the radio signal coverage for example to remote areas or several storeys underground. This is illustrated in Figure 1 by means of a chain of RNs 112, 110 that connect UE 124 to BS 102.

Figure 2 shows a block diagram of an apparatus according to an embodiment of the invention. In an embodiment of the invention the apparatus 200 may operate as BS 102, 104 or as RN 110, 112, 114 of a wireless network. Although the apparatus has been depicted as one entity, different mod-

ules and memory may be implemented in one or more physical or logical entities.

As BS, the apparatus is capable of providing radio signal coverage on a geographical area and provides wireless transmission and reception of messages via the wireless network. The apparatus 200 may also be responsible for allocating resources to the traffic it serves. These resources may comprise physical resources such as a unit within the apparatus 200, for example, an antenna or a computer unit. The resources may also comprise radio resources for a radio channel for the apparatus to operate, such as frequency or time slots or codes or any combination of the previous.

As RN, for example a DF relay, the apparatus decodes the received data from BS and codes it for further transmission. Thus the apparatus receives data to be relayed, decodes the received data, prepares a new data block to be relayed and transmits the data to the recipient. In the following de- scription the naming of a source link and a relay link are used to describe the operations of a relay. A source link is a communications link or channel that RN receives data from. Thus, in downlink data transmission, RN receives data from a source element, such as BS or another RN on a source link. A relay link is a communications link or a channel, where RN transmits data to. Thus, in downlink data transmission, RN transmits data to a destination element, such as UE or another RN on relay link. Preparations for a new data block to be relayed may comprise, for example, scheduling the data blocks to be relayed, determining the physical layer parameters, such as transmission power, modulation and coding to be used in the physical layer, allocating resources to the data block to be relayed, starting a transmission control process for the data block to be relayed and storing a mapping of the data block received for relaying and the relayed data block, prioritising transmission of data block to be relayed for example to specific UEs and forming the data block to be relayed based on one or more data blocks received for relaying. Thus the relayed data block may be derived from the one or more data blocks received for relaying. Forming the data block to be relayed may further comprise identifying UE- or RN-specific data received for relaying, and forming the data block to be relayed based on the specific data.

The apparatus 200 comprises communication means such as a transceiver (Tx/Rx) functionality that is used for receiving and transmitting traffic, such as data blocks. The Tx/Rx functionality can be implemented in a

separate unit 202. The implementation of Tx/Rx unit 202 may include a processor executing computer program instructions which are stored on a computer readable medium, or the Tx/Rx 202 may be implemented using separate units for different functions needed to achieve the desired operation of the Tx/Rx unit, such as transmission and reception of data blocks and preparing data blocks to be transmitted. The Tx/Rx unit is in an embodiment of the invention configured to operate on data blocks according to predefined protocols, such as E-UTRAN protocols. The apparatus 200 may also comprise more than one Tx/Rx units 202, which enable the apparatus to operate more efficiently. The apparatus 200 also comprises a transmission control unit 204 that is used to monitor and control transmissions of data blocks formed and transmitted by the Tx/Rx unit 202 to one or more recipient network nodes, such as UE, RN or BS. The transmission control unit may have several parallel transmission control processes or entities responsible for transmission control that operate on different protocol layers.

The transmission control unit may be configured to store transmitted data blocks until a successful transmission is determined. Such determinations can be made based on, for example, ACK messages received for transmitted data blocks or based on timers. In an embodiment of the invention the trans- mission control unit can discard data blocks that cannot be retransmitted. This may be due to that data blocks do not have active transmission control processes such as ARQ or HARQ processes, there are no resources to start retransmissions or a buffer used in the transmission control unit for queuing data blocks for retransmission is full. The transmission control unit can also be con- figured to discard a transmitted data block when the transmission control process for the data block ends.

The apparatus 200 also comprises a mapping unit 206. In BS ₁ mapping unit maintains information that facilitates associating transmission control process status information received from a relay link with transmission control process of a logical channel. Thus the mapping unit may be configured to map transmission control process status information received from a relay link to a data block transmitted by BS. The mapping unit according to E-UTRAN may be configured to map the transmission control process status information received in a status information message such as Local NACK, to a logical channel data unit such as RLC PDU. The mapping unit may also identify the RLC SDU related to the RLC PDU to determine RLC PDUs containing RLC

SDU segments, In an embodiment of the invention, a BS mapping unit maintains information on associated UEs and RNs. In an E-UTRAN embodiment of the present invention, UEs can be identified using C-RNTI (cell specific radio network temporary identifier). The identifier for RN may also be C-RNTI that is known to BS to be associated with RN. Thus the mapping unit 206 in BS according to E-UTRAN maintains a mapping between identifiers of UEs and RN, for example a mapping between UE and RN C-RNTIs. In addition the mapping unit maintains information needed for operations of E-UTRAN, such as information on RLC PDU and MAC PDUs carrying RLC PDUs. In RN, mapping unit maintains information that identifies data blocks transmitted on the relay link. In RN, mapping unit 206 also enables mapping of the data blocks transmitted on the relay link with the data blocks received for relaying on the source link. In order to allow this, the RN mapping 206 unit advantageously stores identifying information about received data blocks and the decoded and received data blocks, thus source identifying information (SII). Identifying information about data blocks formed for relaying, thus relayed data identifying information (RDII), is also stored to associate the relayed data blocks with the data blocks received for relaying. Thus the source data blocks for the relayed data blocks can be identified. In an embodiment of the inven- tion, the RN mapping unit also maintains routing information based on UE and RN identifiers. Thus, the RN mapping unit 206 has information about UEs that can be reached using direct communications and UEs that can be reached via other RNs. UE and RN identifiers may be stored in the RN mapping unit to enable the routing. The mapping unit may also store information about the origi- nator of the received data blocks for relaying and the destination of the relayed data blocks. In E-UTRAN embodiment of the invention, the RNs and UEs can be identified by C-RNTIs

In RN, Tx/Rx unit 202 may be configured to form a status information message to be transmitted on the source link. The RN transmission control unit 204 generates status information for a data block transmitted on the relay link and provides this to the RN Tx/Rx unit. The RN Tx/Rx unit forms the status information message from the status information obtained from the RN transmission control unit and SII and RDII obtained from the mapping unit. After forming the status information message, the RN Tx/Rx unit may prepare the status information message to be transmitted on the source link.

Figure 3 illustrates protocol stacks 310, 320, 330 arranged in UE

122, RN 110 and BS 102, respectively, for communication according to E- UTRAN. In order to achieve the desired effect, conventional E-UTRAN protocols are supplemented by introducing MAC-r protocol layer 340 to RN and BS. MAC-r denotes here a MAC (medium access control) protocol entity modified to support RNs, and it may be used for communication between RN and BS and also between RN and RN. This means that protocol stack of UE 310 does not have to be changed because of introducing RN into E-UTRAN and RN remains transparent to UE.

As is apparent from Figure 3, MAC-r may be configured to form MAC-r PDUs, which are transmitted in the TBs of the PHY layer of E-UTRAN. MAC-r PDUs are transmitted on a shared transport channel (SCH-r) between RN and BS. Between RN and UE, MAC PDUs may be transmitted on a dedicated channel as defined in E-UTRAN. Data destined to several UEs may be multiplexed into one MAC-r PDU. MAC-r PDU may be formed based on RLC PDUs or MAC PDUs. Figure 3 shows also the transmission control processes 340, 350 applicable in E-UTRAN with relay extension. ARQ 350 is used for controlling the layers RLC and those above, and HARQ 370 is used for the layers MAC, MAC-r 340 and PHY (physical). Thus the outcome of the HARQ process between UE and RN relating to MAC PDU may be associated with SCH-r and MAC-r PDU between BS and RN through MAC and MAC-r interaction. The MAC layer in RN may indicate the MAC-r layer in RN, the outcome of HARQ process between UE and RN. If the HARQ process for MAC PDU fails, MAC in RN may identify the failed MAC PDU to the MAC-r using RDII. MAC-r can then identify the MAC-r PDU and SCH-r TB associated with the failed MAC PDU from the SM associated with RDII. Based on the MAC and MAC-r layer SIl and RDII, a status information message can be transmitted using MAC-r on the source link, for example to BS. The status information message may comprise a Local NACK message including the associated SII and RDII and the outcome of the HARQ process. Using the information received in Local NACK, BS may control the ARQ process of RLC PDU identifiable from the SII and RDM received in Local NACK. The controlling may include for example initiating retransmission of RLC PDU.. In an embodiment of the invention BS receives Local NACKs from several RNs. In another embodiment of the invention BS receives a Local NACK from RN and a Local NACK from conventional HARQ and ARQ interaction according to E-UTRAN. Thus, by introducing RN into E-UTRAN as illustrated in Figure 3, BS can use

the received transmission control process status information, such as Local NACK, to initiate retransmissions on higher protocol layers faster than relying on only the status information received from the peer higher protocol layers, such as status information from the UE RLC layer, where the status informa- tionmay be for example ARQ ACK or NACK as in E-UTRAN currently. In other transmission modes of the higher protocol layers, in which retransmission or ARQ is not applied, BS may use the received status information to discard any remaining segments) of packet(s) which had a segment sent in a failed transmission control process, such as a HARQ process. Thus, BS may use the re- ceived status information, such as Local NACK, comprising status information for the HARQ process on the relay link to determine affected RLC PDU using the information received in the status information message and further determine if the affected RLC PDU contains data from a segmented RLC SDU. If RLC PDU contains data such as a segment of RLC SDU, further segments in RLC PDUs may be discarded from the memory of BS and ARQ processes active for the discarded RLC PDUs may be terminated.

Figure 4 shows an exemplary data structure applicable in the protocol stack presented in Figure 3. According to the present embodiment of the invention, a conventional MAC layer of the current E-UTRAN is modified to MAC-r that enables communication between BS and RN on a shared channel (SCH-r). Figure 4 presents a transport block 400, which is a physical layer data structure that may carry one or more MAC-r PDUs 410 on a SCH-r transport channel between BS and RN. MAC-r PDU 412 has a header and payload part comprising MAC-r SDU 414. A MAC header 420 comprises logical channel identity (LCID) and optionally a sequence number (SN). The MAC header 420 may also comprise other fields needed to decode the MAC-r PDU or to identify the content of the MAC-r PDU, such as fields containing other LCIDs or SNs. MAC-r SDU may comprise UE specific MAC PDUs as specified in E-UTRAN. According to an alternative embodiment of the invention, MAC-r SDU is formed directly of one or more RLC PDUs, thus MAC-r SDU does not comprise MAC PDU. In such an embodiment the RLC PDUs or contents of MAC PDUs arranged in MAC-r PDU are selected in BS or RN based on the MAC-r PDU destination RN. This means that MAC-r PDUs are formed to comprise data destined to UEs communicating via destination RN of MAC-r PDU. Accordingly, BS may be configured to use MAC-r for communications with RN, and E-UTRAN MAC for communications with UE. The interac-

tion between ARQ and the HARQ processes in BS is enabled in MAC-r. Thus, the transmission status of a data block, such as MAC PDU, controlled by HARQ process on a relay link may be used to control the ARQ process of RLC PDU in BS. The transmission status of MAC PDU may be received as Local NACK in a MAC-r level message from the relay link comprising information identifying MAC-r PDU or SCH-r TB and the affected MAC PDU on the relay link. Thus the Local NACK comprises SII and RDM along with status information such as a failed transmission. SII and RDII in Local NACK is used to inform one or more ARQ processes in BS to enable retransmissions based on HARQ information as currently in E-UTRAN, where the ARQ entities are notified about a failed delivery of a TB. If MAC-r applies the HARQ process on direct transmissions, for example between BS and RN, Local NACK identifies HARQ process status information to the ARQ process as is defined in E- UTRAN. On the other hand, RN may be configured to apply HARQ on MAC

PDUs and MAC-r PDUs transmitted by RN. Also RN may be configured to operate according to a HARQ scheme for the received MAC and MAC-r PDUs, if HARQ is applied to them. The mapping unit of RN stores SII and RDM for received and transmitted TBs, or parts thereof. The transmission control unit of RN may be configured to obtain identifying information from the mapping unit. The SII and RDII may comprise sequence numbers (SN), logical channel identifiers (LCID), time stamps of reception or transmission, transmission control process identifiers, a source identifier identifying the originator of the received TB and/or a destination identifier identifying the destination of the transmitted TB.

The RN transmission control unit may be configured to obtain RDII from the RN mapping unit, when a transmission control process such as the HARQ process fails for a data block. Thus, if the HARQ process for MAC PDU fails, the transmission control unit obtains the MAC PDU associated SII to identify the source data block of the failed MAC PDU. The RN Tx/Rx unit may be configured to prepare and transmit a status information message identifying the failed transmission using the SII and RDII obtained from the mapping unit to identify the source data block. The status information such as Local NACK, is transmitted in a MAC-r level message to the originator of the source data block. The originator may be BS or another RN or UE. The status information, such as Local NACK, may contain all SII and RDII RN has for a failed MAC

PDU.

As discussed above, the channel between BS and RN ₁ which act as MAC-r peer entities, is a shared channel (SCH-r), which enables combined delivery of data destined to multiple UEs. Communications between RN and BS on a shared channel (SCH-) is handled by the MAC-r protocol entity, and communications between UE and RN is handled by the MAC layer according to E-UTRAN. Accordingly, RN is responsbile for receiving the TBs from BS or from another RN and of relaying them to UEs. The Tx/Rx unit in RN may be configured to receive TB on SCH-r. Each SCH-r TB comprises one or more MAC-r PDUs controlled by the HARQ process. The mapping unit of RN may be configured to store SII of received SCH-r TBs and MAC-r PDUs. The source identifying information may be a SCH-r TB reception timestamp, MAC-r HARQ process identifier, MAC-r PDU sequence number (SN), C-RNT] (cell- specific radio network temporary identifier) and/or an identifier for the originator of TB such as an identifier identifying BS or RN. The HARQ process identifier may be a number from 0 to N-1 in an N-process HARQ. This number may be communicated explicitly in PHY layer control signaling or implicitly in for example synchronous HARQ, in which each HARQ process has a pre-assigned slot. The RN Tx/Rx unit may be configured to prepare TBs to be trans- mitted towards UEs by using the data received and identified on SCH-r TB. The RN Tx/Rx unit identifies from the received data blocks, such as MAC-r PDUs, data destined to a certain UE using, for example, C-RNTI, and prepares TB comprising UE-specific MAC PDUs formed from the received MAC-r PDUs to be transmitted to UE. The RN transmission control unit may be configured to apply the HARQ process to MAC PDUs formed by the Tx/Rx unit. The RN mapping unit may be configured to store RDII, such as HARQ process identifier, SN ,LCID and a destination identifier such as C-RNTI, and associate those with the source identifying information in the mapping unit, for MAC PDUs transmitted to UE from RN in TBs. Thus a relationship between data received for relaying and the transmitted data can be established. The Tx/Rx unit is further configured to transmit the prepared TB to specific UE.

RN may also relay data to another RN. In that case the data to be transmitted in TB to another RN is determined based on routing information stored in the mapping unit about UEs being reachable via certain RN. Thus the mapping between UE and RN identifiers, such as C-RNTIs stored in the RN mapping unit, is used in RN Tx/Rx unit to identify data destined to a specific

RN and to prepare TBs on the basis of the identified data. In that case RN mapping unit stores MAC-r PDU SN, the associated HARQ process identity and a possible C-RNTI from the MAC-r PDUs as RDII.

Figure 5 is a flow chart that describes the steps of an operation in an apparatus according to an embodiment of the invention. The apparatus may be an E-UTRAN BS of Figures 1 or 2, configured to operate in a wireless network, where UEs can communicate with BSs and also with RNs. The flow chart begins at 500. In 502 a data block is prepared for transmission. First, data destined to UEs communicating with BS is received at BS. Then a data block is prepared from the received data to be transmitted to UE via RN. For the prepared data block, a transmission control process is started in BS in 504. In the present embodiment of the invention the transmission control process comprises an ARQ process. The ARQ process controls the transmission from BS to the recipient network node or element, in this case UE. UE is responsi- ble for transmitting acknowledgements (ACK), negative acknowledgement (NACK) or status report messages for the received data block, according to the ARQ process.

In 506 the prepared data block is transmitted to RN. In the transmission of the data block various techniques may be used for generating the radio signal without deviating from the scope of protection. The data block may be transmitted to RN within another data block. Therefore, data destined to several UEs can be transmitted to RN in one combined data block. According to an embodiment of the invention applied in E-UTRAN, the ARQ process is started for a UE specific RLC PDU related to a logical channel and thus identi- fled with LCID in 504. Using UE C-RNTIs, BS identifies which UEs are reachable via certain RN and forms TB comprising data destined to the identified UEs to be transmitted to RN on SCH-r. According to an embodiment of the invention the ARQ process is started for UE-specific RLC PDUs related to certain LCID. BS forms one or more UE-specific MAC PDUs based on the RLC PDUs. UE-specific MAC PDUs of several UEs are then packed into MAC-r PDU and SCH-r TB to be transmitted to RN. BS can also apply transmission control, such as HARQ, on the formed TB that is transmitted to RN in 506.

In 508 BS receives status information from RN about the transmission of a data block from RN towards the final recipient. On the transmission path towards the final recipient the data block may take routes through one or more RNs. At least one of RN on the transmission path employs transmission

control on the relay link. As already mentioned earlier, there may be various transmission control techniques may be employed in transmission control processes. In the present embodiment of the invention the status information received from a transmission control process is information about whether there has been a successful or failed transmission of a data block or a part thereof on the transmission path via at least one RN. This information may be communicated in 508 to BS via ACK or NACK messages, Local NACK or via other control signalling, such as a status control message.

In an embodiment of the invention in E-UTRAN, BS receives infor- mation about a failed transmission of a data block in 508. Such information may be a Local NACK message from HARQ process. The Local NACK identifies the failed data block using SII and RDM. Thus the failed data block is associated with a data block transmitted by BS. Referring back to Figure 4, in the E- UTRAN embodiment of the invention, in 508 BS receives information identify- ing MAC-r PDU and SCH-r TB that relate to the data block that failed in the HARQ process in RN. Thus, BS can identify the failed UE specific MAC PDU on the relay link using SII and RDM received in Local NACK.

In an embodiment of the invention the MAC PDU may be identified in BS from SN and LCID of the MAC PDU. In another embodiment of the in- vention, LCID of the MAC PDU is used together with SCH-r TB or MAC-r PDU identifying information, such as a HARQ process identity, receiving time stamp of SCH-r TB in RN, and SN of MAC-r PDU.

In 510 the ARQ process uses the received information to determine whether a retransmission is needed or not. When a status information mes- sage comprising Local NACK for a data block is received and it indicates a failed transmission in 508, the ARQ process determines in 510 that a retransmission is needed and orders a retransmission of the data block received in 506. When status information, such as an ACK message, is received in 508, the ARQ process determines that no retransmission is needed and the process ends in 514. In the E-UTRAN embodiment of the invention, in 410 ARQ process uses the received HARQ status information identifying relevant data blocks to determine whether a retransmission of those data blocks is needed or not. If the HARQ status information received is Local NACK, the received SII and RDlI are used to determine which RLC PDU controlled by the ARQ process needs retransmission.

In an embodiment of the invention also timers may be used to iden-

tify whether the transmission control processes, such as HARQ, on the transmission path via at least RN have been successful. However, the use of explicit signalling such as ACK, NACK or other control signal facilitates earlier start of retransmissions in the ARQ process. As explained earlier, convention- ally in ARQ the procedure is that an ACK message is waited for until a timer period and only after that retransmission is started. The time period reserved for waiting the ACK may be difficult to configure or become very long, as the number of RNs on the transmission path increases. Therefore, by introducing the transmission control process status information feedback as in Local NACK or in HARQ status control messages from a relay link on the transmission path facilitates earlier ARQ retransmission, which leads to more efficient transmission of data blocks, due to reduced delays in initiating the retransmissions.

According to an embodiment of the invention BS may control the transmission control process, such as the ARQ process, of a relayed data block. BS receives transmission control process status information from RN relaying the data block. The transmission control process status information received from RN may be HARQ status information. Based on the received status information, BS can initiate retransmission of at least part of the data block controlled by the ARQ process. In an embodiment of the invention a BS operates according to Figure 5 in E-UTRAN and has a protocol stack according to Figure 4. BS may be configured to form UE specific RLC PDUs identified by LCIDs based on RLC SDUs in 502. In 504 BS initiates ARQ processes for the formed RLC PDUs. BS determines, based on UE C-RNTIs, the UEs that are reachable via certain RN. Also RNs may be identified by C-RNTIs. Thus BS can use C-RNTI information stored in BS to form MAC-r PDU that carries MAC PDUs to UEs reachable via RN. In 506 BS forms UE-specific MAC PDUs on the basis of the RLC PDUs and packs them into MAC-r PDU to be transmitted on SCH-r TB to RN. In 508 BS receives from RN Local NACK with SIl and RDII information, identifying the failed MAC PDU on the relay link. In 510 BS may determine the affected RLC PDU on the basis of the SlI and RDII information. If the affected RLC PDU is controlled by the ARQ process in BS, the ARQ process may prepare retransmission of RLC PDU in 506. If the transmission of the affected RLC PDU is not controlled by ARQ ₁ the information received in Local NACK may be used for controlling transmissions of other RLC PDUs. For example, BS may determine using the identifying information received in Local NACK

whether RLC SDU of the affected RLC PDU was fed into several RLC PDUs. If it was, and a Local NACK was received for RLC PDU carrying the first segment of a segmented RLC SDU, further RLC SDU segments are discarded from BS. Thus unnecessary RLC PDUs are not transmitted to RN, because a failure information for the first segment of RLC SDU can be used to discard the further segments if no ARQ is applied, in case RLC PDUs do not need retransmission, for example if BS receives Local ACK identifying a successful transmission of RLC PDU, the process ends in 514.

Figure 6 presents a flow chart describing the operation of a further apparatus according to an embodiment of the invention. The apparatus may be RN 110, 112, 114 configured to operate in a wireless network where RNs may communicate with BS 102, 104 and provide UEs access to the network by relaying the messages between UEs 126, 122, 124 and BSs. An example configuration of RN according to the embodiment is described in Figure 2. The process described in Figure 6 begins at 600. In 602 RN receives from BS a transmission, which is to be relayed. At RN the transmission from BS is decoded. A data block is prepared for relaying on the basis of the received data block 604. In 606, a transmission control process is applied to the prepared data block. In the present embodiment of the invention the transmission control process is a HARQ process. The prepared data block is transmitted, and the HARQ process monitors 608 the status of the transmission, such as success or failure. The failure can be determined using for example a counter, counting the number of retransmissions done by the HARQ process. If a maximum number of retransmissions is reached, the HARQ proc- ess outputs an indication of a failed transmission of the prepared data block, such as NACK. In a successful case the maximum number of retransmissions for a prepared data block is not received and the HARQ process output is a success, such as ACK.

When NACK is determined the prepared data block causing the NACK is identified and mapped 612 to the original transmission received from BS. In 614 status information identifying the original transmission received from BS may be transmitted to BS with the information of the failed HARQ process. After transmitting information to BS and in case HARQ process is successful when ACK is determined 608, the process of Figure 6 ends in 610. RN operating in E-UTRAN has a protocol stack as presented in Figure 3. This means that MAC-r protocol entity is used in communications be-

tween RNs and BS on a shared channel (SCH-r). The process starts in 600. In 610 RN receives SCH-r TB to be relayed. RN stores identifying information of received SCH-r TBs and MAC-r PDUs, thus SII. In 604 RN prepares TB to be transmitted to UE or alternatively to another RN. RN identifies from the re- ceived TBs data destined to specific UE using for example C-RNTIs of UE. Then RN forms UE-specific TB comprising one or more MAC PDUs based on the received data.

Alternatively, in 604 TB may be prepared to be transmitted to another RN. In that case RN has to identify data destined to UEs reachable via specific RN. RN can use mapping between RNs and UEs, which may be a mapping between RN C-RNTIs and UE C-RNTIs, to identify which UEs are reachable via specific RN.

Thus RN forms TB to be transmitted to specific RN on SCH-r, the TB comprising one or more MAC-r PDUs and data destined to UEs reachable via the specific RN. In 606 RN starts the HARQ process for the prepared MAC or MAC-r PDUs in the prepared TB. RN stores RDM about the prepared TB, and associates RDiI with SII. RN transmits the prepared TB and monitors the status of the transmission in 608 to determine the HARQ process output. In successful transmission the HARQ process determines ACK and the process ends in 610. If the HARQ process determines that the transmission process has failed, e.g. because the limit of maximum number of retransmissions has been reached, the HARQ process determines NACK. In 612 RN uses the stored SII and RDM to determine identifying information for the MAC or MAC-r PDU failed in transmission. In an embodiment of the invention, where trans- mission of the failed MAC PDU has failed in the HARQ process, LCID of the failed MAC PDU may be used together with SCH-r TB or MAC-r PDU identifying information to identify the failed MAC PDU. Alternatively, the couple of the LCID and SN of the MAC PDU controlled by the failed HARQ process may be used to identify the MAC PDU to the originator of the data. The SCH-r TB iden- tifying information may be a reception timestamp and/or a HARQ process identity related to the SCH-r TB, if HARQ is used on SCH-r to control transmission of MAC-r PDUs. The MAC-r identifying information may be a HARQ process identity or SN of MAC-r PDU. In 614 RN transmits HARQ status information and identifying information identifying the failed MAC PDU in a status informa- tion message. The message may be Local NACK or Local ACK comprising identifying information identifying the failed MAC PDU. In 610 the process

ends.

Figure 7 presents a signal flow according to a further embodiment of the invention. In Figure 7 a message flow between BS, UE and RN represents a situation, where RN extends the radio signal coverage of a network and a data block is transmitted from BS to UE. The transmission process between BS and UE is controlled by the ARQ process started 710 in BS. In the relay link between RN and UE, the transmission process is controlled by the HARQ process started in RN in 720 and 760.

In 710 BS forms a data block A to be transmitted to RN. Data block A may be formed of one or more on data blocks controlled by the ARQ process. The data block A transmitted to RN in 712 may comprise data destined to several UEs. RN receives the data block A and decodes it to prepare a new block of data, data block B, to be transmitted on the relay link between RN and UE. RN starts the HARQ process in 720 for the prepared data block B and transmits data block B in 722. There may be several HARQ processes started for data block B.

In 724 UE receives and decodes data block B. UE determines whether data block B was correctly received and whether reception of further segments of data block B should be expected. In the exemplary situation of Figure 7, UE has failed to decode data block B controlled by the HARQ transmission control process. Therefore, UE transmits NACK to the HARQ process of RN in 724 to indicate the failure of the reception of data block B.

In 730 RN determines the outcome of the HARQ process for data block B, in 720. Prior to determining the outcome of the HARQ process, the HARQ process may have performed a series of retransmissions of data block B according to the HARQ scheme. Therefore several NACKs may have been received from UE or determined due to timer expiration in RN. The number of retransmissions are implementation-specific and not shown in the signalling flow. Based on the determination of the outcome of the HARQ process, the status information on the relaying of data block B is transmitted to BS in 732. In the present embodiment of the invention, HARQ status information is transmitted to BS only if the HARQ process has failed, fn that case the status information transmitted to BS is a message indicating a failure such as a NACK message transmitted in 732. As the HARQ process for data block B has failed in transmitting data block B to UE, in 750 the HARQ process is ended

and data block B is discarded from the memory of RN.

In 740 the ARQ process control in BS may make decisions based on the received HARQ process status information received from the one or more relay links in which data block A has been transmitted. If the message in 732 indicates a failure in the relay link, the ARQ process control may decide that a retransmission of the data block A 712 or a part thereof should be initiated. In 742 the data block A or a part thereof is retransmitted in data block C from BS to RN.

RN receives the ARQ retransmission data block C and decodes it to prepare a new block of data based on the retransmission, data block D, to be transmitted on the relay link between RN and UE. RN starts HARQ process 760 for the prepared data block D and transmits data block D in 762. In 764 HARQ ACK is transmitted from UE to RN as UE has successfully decoded data block C controlled by the HARQ process. In 770 RN determines the HARQ process status of data block D transmitted in 762 based on the HARQ process started in 760. The HARQ process may have performed a series of retransmissions for data block D according to the HARQ scheme. Therefore several NACKs may have been received from UE when the status of the HARQ process was determined in 762. The number of retransmissions are im- plementation-specific and not shown in the signalling flow. In 770 it is determined whether the transmission of data block D has succeeded or not. In this case the successful transmission of data block D in 762 is determined. Therefore a successful HARQ process and status information is not transmitted to BS, and the HARQ process for the data block D ends in 680. In 782 UE receives and decodes data block D. UE determines whether data block D was correctly received and whether reception of further segments of data block D should be expected. In 784 UE transmits a message indicating a successful ARQ process, for example a status control message. In one embodiment the message is an ARQ ACK message to BS to be relayed via relay. ARQ ACK is transmitted after determining that all the data has been received.

In 790 the ARQ process ends, when the part of data block A controlled by the ARQ process started in 710 is acknowledged as received by UE by transmitting in 784 ARQ ACK, which is relayed to BS in ARQ ACK, in 786. In an embodiment of the invention the HARQ process status information is communicated to BS as in 732. If BS does not have an existing ARQ

process for data block A, data block A may be removed from the memory of BS. The message received in 732 may be a status control message indicating a failed transmission.

In an embodiment of the invention, Figure 7 represents a signal flow in E-UTRAn network employing RN. Thus, RN 1 10 and BS 102 employ protocol stacks according to Figure 3.

In 710 BS prepares TB to be transmitted to RN containing data destined to one or more UEs and starts an ARQ process for one or more RLC PDUs in TB. Thus RN identifies UEs, for example based on C-RNTIs, being reachable via RN and prepares TB comprising data to the identified UEs.

In 712 the prepared transport block is transmitted to RN on SCH-r. RN receives TB on SCH-r and decodes TB to MAC-r layer PDUs. RN stores SII for the received TB and MAC-r PDUs. Next, RN forms a transport block to be transmitted to UE based on the one or more received transport blocks from BS. In 720, RN starts a HARQ process for MAC-PDUs in the transport block to be transmitted to UE. RN maintains mapping between the received transport blocks received from BS or a part thereof and the MAC PDUs and transport blocks transmitted to UE. Thus, RN stores the association between SII and RDM. In 722 the formed transport block is transmitted to UE. In 724 UE decodes the received transport block and determines whether the reception was successful or not. In 726 UE transmits NACK as it failed to decode correctly MAC PDU received in the transport block and controlled by the HARQ process. In 730 RN determines outcome for HARQ process identified in HARQ NACK transmitted by UE. As explained already earlier for operation 730, there may have been several NACKs received from UE and retransmissions made related to a specific HARQ process. HARQ NACK received in 726 identifies MAC PDU whose reception at UE failed.

The failed MAC PDU identification information, RDM, such as SN and LCID from MAC PDU header in E-UTRAN, are mapped in RN to the SII identifying the message received at RN in 712.

In 732 HARQ process status information is transmitted to BS with the mapped SII and RDII that identify the affected MAC-r PDU and the UE- specific MAC PDU(s) inside the affected MAC-r PDU. The status information may be comprised in a Local NACK MAC-r level message carrying the SII and RDM.

In 740 the ARQ process in BS may decide, based on the received

status information, such as Local NACK, that retransmission is needed. The ARQ process identifies the affected RCLS PDUs based on the SiI and RDM and prepares a new transport block to be transmitted to RN, the block comprising at least the RLC PDUs that were identified based on the status information received in 732.

In 742 the transport block is transmitted to RN. RN receives the transport block and decodes the transport block to MAC-r layer PDUs. RN stores SII for the received TB as after reception of TB transmitted in 712. Next, RN forms a transport block to be transmitted to UE based on the one or more received transport blocks from BS. In 760, RN starts the HARQ process for MAC PDUs in the transport block to be transmitted to UE. RN maintains mapping between SII and RDM. In 762 the formed transport block is transmitted to UE. In 782 UE decodes the received transport block.

In 764 UE transmits HARQ ACK as it has successfully decoded MAC PDU received in the transport block and controlled by the HARQ process.

In 770 RN determines that the HARQ process has been successful for MAC PDU transmitted in 762. Therefore the HARQ process for MAC PDU identified by HARQ ACK received in 764 ends in 780. In 784 an ARQ-level ACK is transmitted from UE to BS as UE has successfully received RLC PDU controlled by the ARQ process. Here UE transmits an RLC level message, ARQ ACK in 684, which is relayed by RN to BS in 786.

In 790 the ARQ process controlling RLC PDU ends as ARQ ACK is received which indicates a successful transmission.

The steps/points, signaling messages and related functions described above in Figures 5, 6, and 7 are in no absolute chronological order, and some of the steps/points may be performed simultaneously or in an order differing from the given one. Other functions may also be executed between the steps/points or within the steps/points and other signaling messages transmitted between the illustrated messages. Some of the steps/points or part of the steps/points may also be left out or replaced by a corresponding step/point or part of the step/point. The transmission control and mapping unit operations illustrate a procedure that may be implemented in one or more physical or logical entities. The signaling messages are only exemplary and may even comprise several separate messages for transmitting the same

information. In addition, the messages may also contain other information.

An embodiment provides a computer program embodied on a computer readable medium, comprising program instructions which, when loaded into an electronic apparatus 200, constitute the transceiver unit 202, transmission control unit 204, or mapping unit 206 described earlier.

The computer program may be in a source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. Depending on the processing power needed, the computer program may be executed in a single electronic digital computer or it may be distributed amongst a number of computers.

In another aspect, the invention provides a computer program product readable by a computer and encoding a computer program of instructions for executing a computer process.

The computer program product may include a computer readable medium, a program storage medium, a record medium, a computer readable memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunications signal, and/or a computer readable compressed software package.

The transceiver unit 202, transmission control unit 204, and the mapping units 206 may also be implemented as one or more integrated circuits, such as application-specific integrated circuits ASIC. Other hardware embodiments are also feasible, such as a circuit built of separate logic components. A hybrid of these different implementations is also feasible. When selecting the method of implementation, a person skilled in the art will consider the requirements set for the size and power consumption of the apparatuses

200, necessary processing capacity, production costs, and production volumes, for example.

For example, the transceiver unit 202, the transmission control unit 204 and mapping unit 206 may be a software application, or a module, or a unit configured as arithmetic operation, or as a program (including an added or updated software routine), executed by an operation processor. Programs, also called program products, including software routines, applets and macros, may be stored in any apparatus-readable data storage medium and they in-

elude program instructions to perform particular tasks. All modifications and configurations required for implementing functionality of an embodiment may be performed as routines, which may be implemented as added or updated software routines, application circuits (ASIC) and/or programmable circuits. Further, software routines may be downloaded into an apparatus. The apparatus, such as a server, or a corresponding server component, or a user terminal may be configured as a computer or a microprocessor, such as single-chip computer element, including at least a memory for providing storage area used for arithmetic operation and an operation processor for executing the arithmetic operation. An example of the operation processor includes a central processing unit. The memory may be removable memory detachably connected to the apparatus.

It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.