GB2554661A

GB2554661A - HARQ in 5G wireless communication

Info

Publication number: GB2554661A
Application number: GB1616682.9A
Authority: GB
Inventors: Muller Julien; Bucknell Paul
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 2016-09-30
Filing date: 2016-09-30
Publication date: 2018-04-11
Also published as: GB201616682D0

Abstract

In a wireless communication system comprising first 100, second 200 and third 300 stations, where data is transmitted from the third station to the first via the second with use of a retransmission protocol, the third station determines whether to buffer data for possible retransmission itself or at the second station. The first station may be user equipment UE, whilst the second and third stations may be a distributed unit DU and a central unit CU respectively. The retransmission protocol may comprise HARQ. The buffering determination may be made according to latency or load on the fronthaul link, available computing resources at the second station, or the ACK/NACK rate or number of CRC errors in the HARQ processes. The third station may notify the second of its determination by transmitting first extension information. Second extension information, indicating where the data is buffered, may be sent to the first station and included in an acknowledgment that the data has been received.

Description

(71) Applicant(s):

Fujitsu Limited (Incorporated in Japan)

1-1 Kamikodanaka 4-chome, Nakahara-ku, Kawasakishi, Kanagawa 211-8588, Japan (72) Inventor(s):

Julien Muller Paul Bucknell (74) Agent and/or Address for Service:

Haseltine Lake LLP

5th Floor Lincoln House, 300 High Holborn, LONDON, WC1V 7JH, United Kingdom (51) INT CL:

H04W 28/14 (2009.01) H04L 1/18 (2006.01) (56) Documents Cited:

US 20120236782 A1 US 20080062911 A1

US 20070264932 A1

XP051127546; How many splits in Function Split options and principles; 3GPP Draft; R3-161708; 2016-08-21 (58) Field of Search:

INT CL H04L, H04W Other: EPODOC, WPI (54) Title of the Invention: HARQ in 5G wireless communication

Abstract Title: Determination of where to buffer in a wireless relay (57) In a wireless communication system comprising first 100, second 200 and third 300 stations, where data is transmitted from the third station to the first via the second with use of a retransmission protocol, the third station determines whether to buffer data for possible retransmission itself or at the second station. The first station may be user equipment UE, whilst the second and third stations may be a distributed unit DU and a central unit CU respectively. The retransmission protocol may comprise HARQ. The buffering determination may be made according to latency or load on the fronthaul link, available computing resources at the second station, or the ACK/ NACK rate or number of CRC errors in the HARQ processes. The third station may notify the second of its determination by transmitting first extension information. Second extension information, indicating where the data is buffered, may be sent to the first station and included in an acknowledgment that the data has been received.

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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HG. 13

HARQ in 5G Wireless Communication

F.j^jd.ff.(..tbe.jnvention

This invention generally relates to wireless communications systems and methods and especially, but no? necessarily exclusively, to systems and methods transmitting data using Hybrid Automatic Repeat reQuest (HARQ), applicable io future base station architectures. Toe present invention is of particular relevance to downlink (I3L) HARQ in which data is transmitted on the DL wifi? HARQ Information and is acknowledged by an ACK or HACK transmitted on the uplink (UL).

Wireless communication standards such as LTE (3GPR Long-Term Evolution) are based on the OSi model, which divides the complex tasks of wireless communications into a set of protocols arranged in a series of layers. Layers in the OSI model are ordered from lowest ievel to highest. Together, these layers form a protocol stack”. The I..TP protocol stack contains the following layers from lowest to highest as follows: I, PHY layer, 2. Layer 2 (which includes MAC, RLC, and PDGP) and 3. Layer 3 (includes RRG), Various ’channels” are defined within these protocol layers as explained below.

it is expected that future wireless communication standards, including those currently under discussion under the umbrella term ’TXT', will likewise follow the OSI modal. For assistance in understanding the invention to be described, some relevant aspects of LTE will be briefly outlined, it being understood that the present invention is In no way limited to LTE.

in LTE, data from Layer 2 (or MAC) given to the PHY (in the form of one or more Layer 2 packets) is referred to as a transport block (TB). In the simplest case, namely a single antenna transmission mode, one transport, block is generated tor each TTi (Transmission Time Interval). The transport block size is decided by the number of Physical Resource Blocks (NPRB) and the MGS (Modulation and Ceding Scheme). A

CRC (cyclic redundancy check) in the form of parity bits is appended fo each transport block to allow error detection. The receiver can then request retransmission using Automatic Repeat reQuest (ARQ) or Hybrid ARQ (HARQ), to the transmitter in case errors are detected (in the case oi HARQ, if a TB cannot be decoded).

A difference between the two schemes is that HARQ (which ie a combination of ARQ .end some Forward Error Coding (FFC)) takes piece in the lowest sublayer of layers and the ARQ is in the RLC (one sub-layer higher).

If a TB was received with errors a NACK (Negative ACKnowledgment) bit is sent to the transmitter to Indicate this error. For a TB successfully decoded by the receiver an ACK (Acknowledgement) bit is sent to the transmitter. The transmitter will either retransmit a TB or send a new TB depending on the received control signal. Usually when the transmitter receives an ACK, a new TB is generated and transmitted, and when a NACK is received a re-transmission of the previously transmitted TB is transmuted to tire receiver, until the configured maximum number of retransmission Is reached. To allow retransmission, the TB is cached in a HARQ sending buffer, only being discarded when an ACK for that TB is received.

Typically HARQ Is performed In the DL and UL by multiple HARQ processes (step and wait HARQ time slots acting as transmission opportunities for transport blocks) in a HARQ entity (an entity with HARQ enabled). That Is, the HARQ process for one TB can start before the HARQ process for the previous TB is complete. The use of multiple HARQ processes allows the transmission of data to be continuous and not stop whilst the transmitter is awaiting the transmission of ACK or NACK from the receiver. In the case of Frequency Division Duplex, FDD, transmission, TBs can be sent in 8 consecutive time slots without having io wait for the ACK/NACK signal to be sent back from the receiver to the transmitter. Tits receiver can transmit NACK a number of times for the same TB up to a maximum number of transmissions.

In general, HARQ schemes can be categorised as either synchronous or asynchronous In their timing relationship between first and re-transmissions, In a synchronous scheme, the re-transmissions of data packets which have beer? NACKed occur at a pre-defined timing relative to the initial transmission. The advantage of using this predefined timing offset, is that there is no need to signal to the receiver such control information as an HARQ process number. Soot'! an HARQ process number is obtained from the timing of f’ne received date packets. In en asynchronous scheme, a retransmission can occur at any time relative to the first transmission, in this case the

HARQ precess number will be required so as to identify which HARQ process the retransmission is related to. In t..TE synchronous HARQ Is used for the UL and asynchronous HARQ is used for the DL.

As already mentioned, in order that data can be transported across the LTE radio 10 interface, various “channels” are defined at different protocol layers in LTE. Each channel at a given layer receives data from the higher iayerfs) within the LTE protocol structure and packages the data In a manner understood by the next lower layer.

Different channels are used to segregate different types of data, allowing the data to be transported across the radio access network in on orderly fashion,

There are three categories info which the various channels may be grouped.

Physical channels: These are transmission channels that carry user data and controi messages.

Transport channels; The physical layer transport channels offer information transfer to Medium Access Controi (MAC) and higher layers.

Logical channels: Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure.

For present purposes, the Physical and Transport channels are of most importance, and the following channels are of particular relevance.

Physical Downlink Controi Channel (PDCCH): PDCCH is used to carry control signalling from the network to a UE, and in particular scheduling information of different types. The PDCCH contains a message known as the Downlink Control Information, DCi which carries the control information for a particular UE or group of UEs.

Physical Downlink Shared Channel (PDSCHV. PDSCH is used to carry user data on he downlink, including the above mentioned TBs in DL HARQ. as well as some control signalling.

Physical Hybrid ARQ Indicator Channel (PHICH): PHICH carries the above mentioned 5 ACK and MACK bits used In HARQ, The PHICH Is transmitted within the control region of the subframe and is typically only transmitted within the first symbol, if the radio link is poor, then the PHICH is extended to a number symbols for robustness.

Physical Uplink Control Channel (PUCCH): PUCCH provides the means for a UE to send control signalling. such as SRs, Scheduling Requests, to the network,

Physical Uplink. Shared Channel (PUSCH): This physical channel found on the LTE uplink is the Uplink counterpart of PDSCH.

Trgosp.Qfi.g.b.g.n.og.js

Physical layer transport channels offer information transfer to medium access control (MAC) and higher layers. The following will be mentioned below:

io Downlink Shared Channel (DL-SCH): DL-SCH is the main channel for downlink data transfer, if is used by many logical channels.

Uplink Shared Channel (UL-SCH): ULSCH is the main channel for uplink data transfer. If Is used by many logical channels.

According to TS 36,300 (1), the HARQ within the MAC sublayer has the following characteristics;

HARQ irensnsdx and narattsiniis imnspont blacks;

/«she downlink;

- N-proc&x Slop-And-ii'ait;

~ Asynchronous adapsSve HARQ;

Uplink ACK/NAKs in roxpciwe m downlink (rebsanxndxxions on xea( on PUCCH or PUSCH;

- PDCCH slgnoH She HARQ process number end if it is a Sranssnlxxion <»· !vir«nso>i:>>ion:

- Peircnsnihodiins arc always .whi-dobol dirough PDCC.H.

bi she isp/inh;

- H~process Stop-rfetf-ii'air;

- EwtcltroMMS HARQ;

- MfXifJSiifif number of retransmissions configured per UE (as opposed ίο per radio bearer);

Hownlitd: rtCK/NA&s in response ίο uphnk (re)transinsssio»s are sent on FHiCH;

§ - HA R.Q operation in mdink is governed be the foitowing princitdes (summarised in Table 9, I1);

it Regerdiess of die content of the HARQ. feedback tACK or HACK}, when « PDCCH for the UE is correctly received. the UE follows mhcl She PDCCH asks she UE so do i.e. perform u transmission ora retransmission (referred to as adaptive retransmission);

2) i’/hen no RDCCH addressed ία the C-RHTI of the UE is detected. the fiARQ feedback dictates hose the UE performs retransmissions·'

- HACK. the UE performs a non-adopthe retransmission i.e. a retransmission on die same uplink rexosirce os prewow.s'h’ used by die same process;

A CAL· the UE does not perform any UL (re)transmission and keeps dse data ia the 15 HARQ buffet; .4 PDCCH is then repaired to perform a retransmission i.e. a isonodaptive retransmission cannot follow, in she sideiink:

- He HARQ feedback;

- Retransmissions are always performed in a pre-defined^ configured number.

2Q .Mea.omemem gaps are of higher priority dam HARQ retrunemeisio.ns - whenever an HARQ retransmission ctdiides with a measurement gap, the HARQ) retransmission does not take place.

Table 1: UL HARQ Operation (Table 3,1-1 In TS 30,330)

MAKQ fssd&gefc seen &y toe UE	pdcch ssen fey toe UE	HE bebsvlaar
ACKorNACK	Mew Transmissoe	Hew transmission according to PDCCH
ACKw&WK ϊ......................,..................	-----------------------------------	Reltatistnisslort according to P&CGH (sdupiUo nUf&rtsmtssionJ
l L· L· T-L· L· L· L·: > XL·/ L· L· : ACK	Mens	Ho (rftdanmriissinn. hasp defy in HARQ buffer and a PUCCH is required to resume rettansmissions
HACK.	N&ns	Hoti-adapiivo retransmission

Thus, HARQ is controlled in the MAC layer but the HARQ retransmission occurs at the PHY layer. At the PHY level, PHiCH is used to carry HARQ in the downlink direction tor the received uplink data, and RUSCH/PUCCH are used to carry HARQ feedback In the uplink direction for the received downlink data. Figure 1 shows a simplified call-flow of thia mechanism.

As shown in Fig. 1, the uplink process alerts with a UE 10 transmuting user date via

PUSCH to the eNB 20. in accordance with the HARQ protocol, the eNB 20 responds with ACK or NACK using PHICH, depending on whether or not the eN3 20 was abie to success!oily decode the user data. The UE10 determines if a NACK was received from the eNB 20, if so. the UE 10 retransmits the user data. Likewise on the downlink, the eNB 20 transmits user data on PDSCH and the UE 10 responds on PUCCH or

PUSCH with ACK or NACK depending on whether it could decode the data. The eNB £0 checks whether a NACK was received and if so, resends the user data on PDSCH.

Meanwhile, the transport channels DL-SCH and UL-SCH also support HARQ, Within these channels each Transpod Block is associated with its HARQ information in the following manner (taken from 3GPP TS 36.321: ’’Medium Access Control (MAC) protocol specification, hereby incorporated by reference;

HARQ information; HARQ information for DL-SCH or for UL-SCH transmissions consists of Hc-.v Data indicator (NDlf Tronspon Block (TR) sicn. For DL-SCH transmissions and for asynchronous UL HARO, the HARQ information also includes HARQ process ID. For UL-SCH transmission dh* HARO information also includes Redundancy Version t'RV). in case of spatial multiplexing on DL-SCH tin.’. HARQ information comprises a set cfHDl and TA size for cadi transport block.

in FDD and lor a given DL HARQ entity, 8 HAP.Q processes ere used, in TUB the maximum number of DL HARQ processes depends on the TDD configuration (the following Table 2 taken from 3GPP TS 36.213). Each HARQ process requires a separate soft buffer in the receiver.

Table 2: Maximum number of DL HARQ processes for TDD (Table 7-1 in TS 36.213)

Thus. HARQ involves processes at different layers in the protocol stack.

Figure 2 shows the user plane protocol stack for LTF. As can be seen, there ere peer entitles In the UE 10 and eNB 20 tor each of the PHY, MAC, RLC and PDQF protocol layers, in order for HARQ to function correctly, the thne taken for the receiver to process the incoming TB, to transmit ACK/NACK as appropriate end for this to be received at the transmitter, becomes a critical Issue.

in the case ef DL HARQ ), one factor of importance is the functional split between different entitles in the network; in other words, which entities are responsible for the processing in specific protocol layers.

Traditionally, the protocol stack shown in Figure 2 is implemented in the form shown in Figure 3(e), in which the base station 20 (eNB) defines a ceil for wireless communication by UEs 10. and provides all the functionality of the protocol layers shown at the right-hand side in Figure 2, In this architecture, the bese station consists of two functional units, a radio or RF unit (radio head), and a baseband unit (BBU). The baseband unit (BBU) connects to the network via the backhaul, which is typically an Ethernet circuit. The function of the baseband unit is to translate the data stream coming from the network into a form that is suitable for transmission over ihe air, and in the other direction, to take the data stream from the radio units, and transform it info a form suitable for transport back to the network. The radio unit transmits and receives the carrier signs! that is transmitted over the air to the UE 10. As indicated in Figure 3(e), normally both parts of base stetien 20 would be located at the foot of a tower with the antennes on top of the tower, the radio unit being connected via coaxial RF cables to the antennas. This arrangement has some drawbacks since the RF cables are bulky, expensive, and cause power loss.

However various alternative architectures have been proposed, and in some of these the functionality traditionally implemented in the base station 20 is divided among two or more physicaliy-distlnct entities. For example, Figure 3(b) shows an arrangement which has come into use recently, in which the RF unit is split from the baseband unit (BBU) 22 and housed in a Remote Radio Head (RRH) 24 - also sometimes called

Remote Radio Unit or RRLL in this arrangement the baseband signal is digitized using any of a number of available protocols including CRRI (Common Public Radio interface) arid Open Base Station Architecture initiative (O8SA1), and transmitted over fiber optic cable 23 to the RRH 24 mounted on the tower. This avoids the long runs of coaxial cable and the associated RF losses.

This concept can be taken further because by use of CRRI, OBBAI or simitar protocols there is no longer a need for the RRHs to be physically close to the baseband unit, in fact they can ba separated by several kilometres. Figure 3(c) illustrates a so-called Centralized Radio Access Network (C-RAN) in which a plurality of RRHs 24, located in different locations and providing respective cells covering a geographical area, are served by a common BBU pool 26 in a centralized location, with communication links 23 from the BBU pool to each RRH 24, The farm '’pool” indicates that there is no need for these BBUs to be distinct hardware units; rather they oar) be virtualised within one hardware unit 28 or even placed In the cloud”.

Figure 3(d) shows a variant of Figure 3(c) in which part of the baseband processing is moved to the RRH 28. This is related to (lie idea of the Distributed Unit (DU) arid Centralised Unit (CU) referred to below. In boll) cases, the connection from the 8BU pool to each RRH, typically via optical fiber, is referred to as frorsthaui (FH) to differentiate if from the traditional “backhaul¹' which connects the baseband unit to the network. In the case of Figure 3(d), the separate fiber connections 23 from (1)0 BBU pool to each RRH are replaced by a ironthaui network 27. in some cases, it may be possible to replace optical fibers by a microwave link.

For a C-RAN architecture to perform well It is important that the fronthauli network is designed to keep the ironthaui latency as low as possible. One proposed guideline is to design the network so that the one-way delay is below 75 microseconds. Light travels approximately 1 kilometre In 5 microseconds, so 15 km of fiber would produce a one way latency of 75 microseconds. Transpod equipment that injects extra date into the stream will add additional delay.

A BBU must process many signals to and iron? the RRHs and UEs. tf the BBU end RRH are co-loeated then there is tittle delay in sending signals between the two. If the radio head is placed 5 kilometres away, the BBU must wait et least 25 microseconds tor the signal to reach the radio head, and 25 microseconds for the response signet to come back. So every communication exchange will take 50 microseconds longer then if would take if the units were co located. As a result, the efficiency of the baseband unit is reduced because tasks now fake longer to execute. Heavy traffic loads, wltft a large amount of signalling over a network with high latency may result in performance degradation.

Many possible configurations am under discussion far 5G and it is possible that a combination of different architectures may be combined in one and the same network. However, the problem arises of how to implement DL HARQ in these kinds of architecture.

According to a first aspect of the present invention there is provided a wireless communication system comprising first, second and third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the third station is arranged to make a determination of where to buffer data for possible retransmission, in accordance with which determination the data is buffered in at least one of the second station and the third station.

Thus, for a specific transmission of data to the first station, the third station decides whether to buffer that data itself, or to instruct the second station to buffer the data.

in a preferred form of the system, the second station is linked via fronthaui to the third station. Whilst a “wired fronthaui would be typical, a wireless fronthaui is also possible.

Preferably the first station referred to above (arid subsequently) is a terminal sucb as a mobile station, user equipment (UE), Machine-Type Communication (MTC) device or the like.

Tbs second station may be a Distributed Unit, DU, and the third station may be a Cenfral-sed Unit, CU in the sense in which these terms are employed in current 3GPP Release 14 discussion documents,

Preferably the retransmission protocol comprises Hybrid Automatic Repeat reQuest, HARQ, and data is provided to the first station with use of one or more HARQ processes.

in one embodiment (“per-service” embodiment), the above mentioned data buffered for

1o possible retransmission may be data of one or more specific services or slices provided to the first station via the second station.

in another, ^;iper-UE^,: embodiment, any date being transmitted to the first station vie tire second station is buffered.

to a third, “per-DU” embodiment, one or more other stations are wirelessly linked to the second station, and the buffered data includes data being transmitted to any of the stations wirelessly linked to the second station.

Two or more of the above embodiments may be combined in the same third station.

As already mentioned the third station makes a determination where to buffer data for possible retransmission. This determination can be made using any one or more of the following criteria:

® latency of the fronthaui link:

» load on the fronthaui link:

® amount of memory available in the second station;

« computing resources available in the second station;

* an ACK/NACK rate achieved by the HARQ processes; and * a number of GRG errors found during the HARQ precesses.

The wireless link between the first stetion and the second station may have a Chennai Quality indicator, CQI, and the system may provide et least one service or slice io the first station, in which case the criteria may further include:

« the CQi of the wireless link; and/or « the kind of service or slice provided to the first station.

Preferably the third station Is further arranged to notify the second station ef its determination by transmitting first extension information of the retransmission protocol, the first extension information informing the second station whether or not to buffer the data tor retransmission.

The second station may be arranged to transmit second extension information of the retransmission protocol, indicating whether transmitted data is buffered In the second station or in Ihe third station, in this case, preferably, the first station is arranged to Include the second extension information in an acknowledgement of the transmitted data.

According to a second aspect of the present Invention, there is provided a third station In a -wireless communication system comprising first, secono and third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the third station is arranged te make a determination of where to buffer data for possible retransmission, in accordance with which determination the data is buffered in the second station or the third station.

The above third station is, for example, a Centralised Unit (CU) as referred to elsewhere in this specification, and oars have any of the features of the third station referred te above with respect to the wireless communication system of the invention.

According to a third aspect of the present invention, there is provided a second station in a wireless communication system comprising first, second and third stations, the first station Wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the second station is arranged to receive From the third station an instruction of whether to hotter the data for possible retransmission.

This second station may bo a Distributed Unit (DU) as referred to elsewhere in this specification, and can have any of the features of the second station referred to above with respect to the wireless communication system of the invention.

According to a fourth aspect of the present invention, there is provided a first station in a wireless communication system comprising Hrst, second and third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the first, station is arranged to receive, from the second station and in addition to the transmitted data, extension information of the retransmission protocol indicating whether the transmitted data is buffered in the second station or in the third station, and Is arranged to include the extension information in an acknowledgement returned to the second station.

The above hrst station may be a terminal such as a mobile station, user equipment (UE), Machine-Type Communication (MTC) device or the like.

A.ccording to a fifth aspect of the present invention, there Is provided a wireless communication method in a wireless communication system comprising fast, second end third stations, the first station wirelessly linked to the second station, the second station linked to the third station, the method comprising;

transmitting data to the first station via the second station with use of a retransmission protocol, wherein the third station makes a determination of where to buffer date for possible retransmission, and the data is buffered in the second station or the third station in accordance with the determination.

According to e further aspect of the present invention there is provided a wireless communication system in which a terminal has a wireless link for communication with a Distributed Unit, the Distributed Unit has a fronthaul link for communication with a Central Unit, and wherein communication over the wireless link from the Distributed

Unit to the terminal is conducted using one or more Hybrid Automatic Repeat reQuest, HARQ, processes, the Central Unit comprising a HARQ manager arranged to determine the location of a HARQ retransmission buffer as one of;

in the Central Unit; and

W in the Distributed Unit.

According to a final aspect, Invention embodiments provide a computer program which when downloaded onto an apparatus causes it to become any one of the first, second and third stations detailed above or which 'when executed on a computing device of a telecommunications apparatus carries cut a method as defined above.

Thus, embodiments cf the present invention provide a method to dynamically control and deitne the location and the behaviour of the HARQ sending buffer when the Rase Station is physically split info two different entities which are physically separated, in a RAN centralized architecture (i.e, functional split between a Centralized Unit (CU) for the upper layers and a Distributed Unit (DU) lor the lower layers, connected via a fronthaul link), the location of the HARQ buffer (i.e, in the CU or the DU) is critical for the performance (e.g, latency) and efficiency (e.g, minimum fronthaul trailic date rate).

The optimal location will he impacted by different criteria (e.g, traffic, radio link quality). This Invention proposes a method to select Ute optimal location of the HARQ buffer and the signalling needed to control this new feature.

Features detailed above with respect to any different aspects may be combined with any or ail the features of the other aspects since they rater to the same invention.

Brief

Preferred features of the preseat invention and comparative examples will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a conventional HARQ procedure;

Figure 2 illustrates a protocol stack io a conventional UE and eNB;

Figures 3(a) to (d) show various possible architectures for the functional split between

W a BBU and RRH:

Figure 4 shows the functional split between a BBU and RRH in protocol terms;

Figure 5 shows possible functional splits under discussion in 3GPP;

F-gure 6 shows a HARQ process conducted in an embodiment of the present invention between a UE, a Distributed Unit (DU; and a Central Unit (CU) in a case where a

HARQ buffer is located In the DU:

Figure 7 shows a HARQ process conducted in an embodiment of the present invention between a UE, a Distributed Unit (DU) and a Central Unit (CU) in a case where a HARQ buffer is located in the CU:

Figure 8 is a flowchart of processing steps in a DU in an embodiment of the present 20 invention;

Figure 9 is a flowchart of processing steps in a CU in an embodiment of the present invention;

Figures 10(a) and (b) ate flowcharts of processing steps in a UE in an embodiment of the present invention;

Figure 11 is a schematic block diagram of a UE which may be employed in the present invention;

Figure 12 is a schematic block diagram of a DU which may he employed in the present invention; and

Figure 13 is a schematic block diagram of a computing device which may be employed 30 as a CU in the present invention.

Detailed

Embodiments of the present invention will now be described with respect to the accompsnysrtg drawings.

As already mentioned, the logical architecture of the user plane protocol stack depicted in Figure 2 can be implemented in different physical entities, communicating for example via CPRI and separated by op to tens of kilometres. The DPR! specifications define the interface between the RF pert of the eNB end the PHY layer. Therefore, as mentioned, the eNB can be split Into two different entities, known as RRK (Remote Radio Head), for the node containing the RF parts, and BBU (BaseBand Unit), for the node containing the PHY, MAC, RID and PDCP layers (for User Pfane), tor example as shown in Figure 3 (c) and (d).

in current discussions, split entitles of such a kind are referred to as a Central Unit CU (corresponding for example to the BBU Pool of Figure 3(d)), and Distributed Unit DU (corresponding for example to the RRH having seme BB functionality in Figure 3(d)).

Thus, in the RAN centralized architecture there can be a functional spilt between a Centralized Unit (CU) for the upper layers end a Distributed Unit (DU) for the lower layers, which are connected via a fronthaut link. But different splits are foreseen and are being currently discussed in 3GPP for NR (i.e. New Radio, else called the 50 Radio Access Network), The following objective is extracted from the Release-14 Study Item Description:

• Study the feasibility of different options of splitting the architecture into a ‘Central Unit” and a “Distributed Unify with potential interface in between, inelading transport, configuration and other required functional interactions between these nodes [RAN3, BANS]

Figure 5, described below, shows the possible split options in 3GPP (sea 3GPP RAN WG3 TS 38,801 70,4.0 (2018-081). Figure 5 shows, in greater detail than Figure 4, the protocol foyers to be implemented. Each vertical arrow denotes en alternative split option, as follows:

Option 1

RRC is in the Centra) Unit, PDCP, RLC, MAC, physical layer and RF ere in the Distributed Unit,

Option 2

RRC, PDCP are in the Central Unit, RLC, MAC, physical layer and RF are in the Distributed Unit,

Option 3 [intra RLC split)

Low RLC (partial function of RLC), MAC, physical layer and RF are in Distributed Unit, PDCP and high RLC (the other partial function oi RLC) are in the Central Unit,

Option 4 (RLC-MAC spilt)

MAC, physical layer and RF are in Distributed Unit. PDCP and RLC are in the Central Unit

Option 5 (intra MAC split)

RF, physical layer and part of the MAC layer (e.g. HARQ) are in the Distributed Unit. Upper layer is in the Central Unit.

Option 6 (MAC-PHY split)

Physical layer and RF are In the Distributed Unit. Upper layers are in the Central Unit.

Option 7 (intra PHY split)

Pert of physical layer function and RF are in the Distributed Unit. Upper layers are in the Central Unit.

Option 8 (PHY-RF split)

in case of an intra-MAC spilt (option 5 on Figure 5). the HARQ function could be situated either in the Centre! Unit or in the Distributed Unit Both options have some advantages and disadvantages:

fi) HARQ Buffer in CU:

« Pros:

More memory and computing resources available for HARQ function

Less memory and computing resources are needed at DU side information from HARQ retransmission can improve the scheduling function * Cons

FH bandwidth needed for retransmission

Higher latency

Higher FH latency requirement (ii)HARQ Suffer in DU:

® Pros:

Lower latency

Lower FH bendwidih o Lower FH latency requirement * Cons;

c More memory and computing resources needed in both cases, signalling to control the HARQ function end the retransmission Is needed on end using the fronthaul link, instead of having a unique buffer located at CU or CU, embodiments of the present invention employ a dynamic buffer scheme, where Transmission Slock (TB) buffers for HARO function are located In both CU and DU.

The above principle will now be described in more detail, it Is assumed that each UE is connected to one DU, that generally a plurality of UEs connect to the same DU, and (in a similar manner to that shown in Figure 3(c) or <d)> that a plurality of DUs are linked by fronthaul to the same GU.

The following definitions will be used:

- HARD function; The full HARD process functional entity, if can be situated in the

CU, the DU, or in both

- HARQ caching function: Always situated In the CU, it will decide where the TB needs to be cached

- HARQ information: As described in TS 38.321 3.1, “Medium Access Control (MAC) protocol specification”

Figure 6 shows a HARD process (HARO functional call flow) conducted in an embodiment of the present invention between a UE 100, a Distributed Unit (DU) 200 and a Central Unit (CU) 300 in a case where a retransmission buffer is located in the DU, and Figure '7 shows the corresponding call flow in the case where the buffer is located in the CU. in a dynamic scheme as proposed in embodiments of the present invention, it is necessary for the DU and CU to be able to keep track of where the buffer is located currently, so that each node knows if It needs to store data for possible retransmission, in order for the node receiving the TS to know if it needs to cache data locally, embodiments of the present invention introduce a new' information bit in the HARD information. This information bit, called the “caching bit” is sot to 1 when the HARQ caching function fakes the decision to cache the TB in the next node (hero the DU),

Thus in Figure 6 for example, the process begins with the CU 300 sending a transport block TB, intended for UE 100, to the DtJ 200 together with the Caching bit notifying that DU 200 that the DU is to be responsible for caching of this data. Alternatively, as shown in the initial step of Figure 7 the Caching bit is set to ”0” denoting that for this TB, the caching will he done at the CU 300. As explained beiew, this notification need not be the same for all dote (all TBs) received by the DU.

Embodiments of the present invention further introduce a new field called buffer fD”. This field will be set by the last network node (here the DU) before sending the TB to the UE.

Thus, in Figure 6, when the DU 200 forwards to UE 100 a TB received from the CU 300, it sets the Buffer ID field to indicate that the TB is being buffered in the DtJ 200 in this instance. The DU maintains a separate buffer for each UE. (each HARQ process) in respect of which a caching bit “Τ’ was received. Meanwhile in Figure 7, Ore corresponding transmission from DU 200 has Buffer ID ~ CU.

As shown in both Figures 6 and 7, the UE 100 copies this field when sending its ACK/NACK response, but without interpreting it. This will help the receiving node (hers the DU) to know which buffer has been used, and therefore decide its behaviour concerning this ACK/NACK response. That is, in the case of Figure 6 the DU 200 itself must act upon the ACK/NACK information, managing the subsequent process and informing the CU 300 of the eventual successful transmission; whilst in the case of Figure 1 the DU merely forwards the ACK/NACK to the CU 300 without taking any further part in the HARQ process.

Assuming that any given UE is connected to only one DU. it is sufficient for this buffer ID to be s single bit. The specific DU which transmitted the TB is identifiable to the CU using information from other layers, such as a transport MAC address.

The full behaviour of the HARQ function, for each element of the network, including the use of the “caching bit” and the buffer ID” es described in Figures 3, 9, and 10,

The DU 200 process depicted in the flowchart of Figure 8 begins in step S202 with the DU receiving a TB from CU 300. At 5204 the DU checks whether the value of the caching bit contained in the TB is “1”, If it is, this indicates that the DU needs to cache this TB, and the DU does sc in step S205, adding its own buffer ID to the IB prior to transmission. On the other hand, if the sashing bit, is set te 0, there is no need for the DU to store the TB, and the DU merely adds the buffer ID of CU 300 to the TB. In both cases, the flow then proceeds to S210 where the caching bit is removed from the TB, end the TB is fofwarded to the UB10G.

in step B212 the DU 200 receives the ACK/NACK response from UE100, in S214 the DU examines the buffer ID contained in the ACK/NACK response, if the buffer ID is that of the DU 200 (S214, Yes) the flow proceeds to S218 where the DU checks if ACK or NACK is indicated, if ACK (S216, Yes) the DU forwards the ACK to CU 300, and removes the TB from the relevant buffer since the TB has been transmitted successfully. On the other hand, if a MACK is received (S216, No) the DU 200 retransmits the TB to UE 100 and the flow returns to step S212. Meanwhile, if the result of S214 was that the buffer ID is net that of the DU (In other words, identifies the CU) then the DU simply forwards the ACK/NACK information to CU 300 without examining it (S218).

Figure 9 shows the corresponding flow in the CU 300, it should be noted that the CU carries out the process of Figure S separately for each DU connected to it (and possibly with finer granularity still, see below), so that for some DUs the CU would act as the buffer and others the DU would handle it.

Beginning at S302, the DU decides (in a manner described below) where the HARQ buffer should be located, in other words where data should be buffered for possible retransmission, This step may be triggered periodically, in response to the some change in capabilities of the DU (see below), or even every time a TB is prepared for transmission.

In S304, ii the decision is to locate the buffer in the CU (S304, Yes) then CU 300 stores the TB Itself and adds a caching bit with value “0^s te the TB, On the other hand if the decision is to buffer the TB in the DU 230, then the CU sets the caching bit in the TB to “1”, without itself storing the TB for possible retransmission, in both oases, flowproceeds to S310 where the TB is transmitted to the DU over the FR

In Step S312 the CU 300 receives the ACK/NACK response from the DU (sae Figure 8, steps S210 and S218). Step S313 is to examine tire buffer ID contained in the

ACK/NACK information, If this Indicates the buffer ID of the CU, (S314, Yes), the flow proceeds to 3316; otherwise (3314, No), this implies that the DU is the buffer for the TB, ana the DU will manege the ACK/NACK process; thus, the CU can wait to receive the next TB which is ready to be transmitted to the UE (3318).

in 3318, it Is checked whether ACK or HACK was received, if ACK (S316, Yes) then the CU 300 knows that the UE successfully received the TB, which ears therefore be removed from the buffer in S320. On the other hand if NACK was received, then since the CU is handling the retransmission in this instance, the CU retransmits the TB to the DU in S322, end the flow returns to 3812 to await the ACK/NACK response for the retransmitted TB,

Turning now to Figure 10(a), showing the corresponding operation steps in the UE 100, the precess starts in S102 with the UE receiving a TB from the DU2Q0. in 3104 the UE 100 determines whether il could decode the TB or not (based on whether the CRC check is successful), if so, (3104, Yes) the flew proceeds to 3106 where the UE 100 examines the buffer ID contained in the TB. In the case that this ID is that of the CU (3106, Yes), the UE sends ACK with this buffer ID back to the DU 200 in 3110. in the case that this ID is ihai oi the DU (S106, No), the UE sends ACK with this buffer ID back to the DU 200 in Si 12, if. veil be noted that Ibis ACK/NACK information is different from conventional ACK/NACK because it is extended by addition of tire buffer ID.

Meanwhile, if the outcome of 3164 was that the CRC check failed, in other words that the TB did net decode correctly, flow proceeds to 3108, S114 and 3116 where the UE returns NACK with the appropriate buffer ID, assuming that this can be identified from the decoded data.

However, it Is possible ihai the UE cannot recover the Buffer ID from the TB, in that case, the UE can simply take no action (i.e. not send ACK/NACK). This will cause the DU er CU to retransmission lime the TB (along with ilia buffer ID) after e time delay, Alternatively, the UE may be configured with a default behaviour io follow in this case, such as always to send the ACK/NACK io the CU. The CU, if it receives such an ACK/NACK can forward it to the DU if it Is not recognised by the CU.

ii should be noted that steps 3106 ie 3116 ere optional, in an alternative operation flow shown in Figure 10(b), these steps are replaced by an alternative steps 3120 end

S122 in which, after the GRC check in SI04, the US simply sends to DU 200 the ACK or NACK including the buffer iD copied from the IB, without examining the buffer ID .

As will be apparent, from the above description, embodiments of the present invention involve the CU 300 taking decisions with respect te the location of HARQ buffering for each DU and possibly each UE. .A HARQ caching function, based in the CU, is provided for making these decisions. For each TR built by the upper MAC-layer, the HARQ caching function wifi then decide in which node the TB should be cached, and therefore set the caching bit to the right value before sending it to the next node.

in order to take the best decision concerning the cache location for each TP, the cache size end the number of HARQ processes, the following criteria will be used by the HARQ caching function:

Number of CRC errors et PHY layer

For example If the number of CRC errors (i.e. the number of NACK received by the HARQ function) reaches a certain threshold, the HARQ caching function can decide to cache the current TB and the foilowing ones in the DU in order to reduce the signalling on the FH.

Latency end load on FH

For example -f the latency on the FH link reaches a certain limit where the performance is degraded, the HARQ caching function can decide to cache the next TBs in the DU,

To determine that ihe performance Is degraded, the CU can use the Transport Network Layer (TNI) monitoring process; for example the CU may receive (or detect) an SNMP message when the TNI., buffer Is over 50%, SNMP stands for Simple Network Management Protocol. Alternatively the CU may detect that the latency (monitored by e ping every second) is over a configured (e.g, QAM) threshold. This SNMP trap should be sent to the CU, which will update its buffering policy

Service / Slice

A given UE cars access different kind of EndTo-End (E2E) services from the same network infrastructure, such as video streaming or voice cells. A slice is a new concept which will be introduced In future NR specifications. The current definition of a network she© in the 3GPP RAN WG3 study TR 38.801 v0.4,0 “Study on Now Radio Access Technology; Radio Access architecture and interfaces is;

A Network Slice is a network created by the operator customized to provide cc- optimized solution for a specific nsarket scenario which demands specific requirements with end to end scope ns desc.fifeed in TR 23.799.

A slice cart be built to support one or mere specific sendees.

Each Nice or service cart have different requirements for the HARQ function. For example Ultra-Reliable and Low Latency Communications (URILQ) services or slices need very quick retransmissions in order to reach their KPis, in that case the HARQ caching function will be deployed in the DU no matter what the other criteria,

DU memory and CPU

The memory and the CPU resources In the DU will be very limited compare to CU resources. The HARQ caching function will decide the best location for the HARQ buffer according to available resources in both nodes. The CU can be notified of the

DU status such as free memory either periodically, or on an event basis (e.g. when free memory fells below a configurable threshold),

ACK/NACK rate

The overall ACK/NACK rate has an impact on upper layers performance, such as TCP throughput. Improving the ACK/NACK rate by moving the HARQ buffer to the DU (in case of overloaded FH) or to the CU (in case of memory or computing resources shortage in the DUt can improve overall Quality of Experience (QoE), Therefore the HARQ buffer function can take the ACK/NACK rate into consideration to select the best location for the HARQ buffers.

CQI

For a given UE, the overall performance will depend on the efficiency of the retransmission methods. For example a UE at cell-edge will need a lot more retransmission than a UE with better coverage. This will Impact the overall system (e.g, FH load), in that case the HARQ caching function can decide to use CU or DU caching for a given UE according to its QQL

As wtii be apparent from the above list of criteria, the HARQ caching function can take info account a range of factors when deciding the optimum location for the Puffer.

Some of these factors relate to a DU, some to a UE, and some to individual services provided to a UE.

Accordingly, the principle of the invention can he applied at any or ali of the following levels'.

fit “per-DU”: in other words to ali TBs transmitted to a given DU. et least for a certain time period;

(ii) “per-UE”; to ail TBs transmitted to a DU and intended for a specific UE; end (iii) “per-service”: to TBs transmitted to a DU, intended for a specific UE and furthermore relating to a specific or slice provided to that UE.

it should he noted that the same CU may operate at more than one level at once. For example, the seme CU may handle a certain DU on a ‘per-DU “ basis, with all TBs sent to a certain DU using the same decision process, whilst in the case of another DU connected to that CU, the CU may apply the decision process on a “per~UE basis to data for individual UEs, and/or “per-service to particular services being provided to one or more UEs. In other words '’per-DU”, “per-UE” and “per-service can coexist for the same CU, with rule priorities, with the penDU* rule having the highest priority.

The HARQ caching function can apply rule priorities to manage this multi-level approach, with rule priorities. For example:

o Every UE using Narrow Band ioT (NB-toT) will use CU HARQ buffer o if above rule doesn’t apply (-,e, other service) then the decision will be taken according to UE-related criteria

With respect to the above mentioned criteria employed by the HARQ caching function, ali of them can apply to each of these decision levels, with the difference that in case of “per-DU” or “per-service”, the foliowing criteria should be averaged with the values of all the UEs using the same DU or the same service;

o Number of GRC errors at PHY layer

ACK/NACK rate o CCS

Figure 11 is a block diagram illustrating an example of a UE 100 to which the present invention may be applied. The UE 100 may include eny type of device which may be used in e wireless communication system described above and may Include cellular (or cell) phones {including smartphones), personal digital assistants (PDAs) with mobile communication capabilities, laptops or computer systems with mobile communication components, and/or any device that Is operable to communicate wirelessly. The UE 100 inciudes irsnsmitter/receiver unit(s) 804 connected fo at least one enterma 802 (together defining a communication unit) and a controller 808 having access to memory in the form of a storage medium 808. The controller 806 may be, tor example, a microprocessor, digital signal processor (DSP), application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or other logic circuitry programmed or otherwise configured to perform the various functions described above, in particuier to perform the steps shown in the flowchart of Figure 10, For example, the various functions described above may be embodied in the form of a computer program stored in the storage medium 808 and executed by the controller 808. The transmissioh/reoeption unit 804 is arranged, under control of the controller 808, to receive TBs and control messages from the DU 200 and send ACK/NACK end so forth as discussed previously.

Figure 12 is a block diagram illustrating an example of equipment suitable for use as a DU 200, it inciudes transmiiter/reoeiver uniifs} 904 connected to at least one antenna 902 (together defining a communication unit) and a controller 908. The controller may be, for example, a microprocessor, DSP, ASIC, FPGA, or other logic circuitry programmed or otherwise configured fo perform the various functions described above, in pedicular the steps in the flowchart of Figure 8. For example, the various functions described above may be embodied In the form of a computer program stored in the storage medium 908 end executed by the controller 908, The irensmission/receptson unit 904 is responsible for transmission of TBs, control messages and so on under control of the controller 908.

As Will be apparent from the above description, although it is necessary for each UE end DU to have wireless communication capabilities, there is no such requirement on the CU 308, which can take the form of a general-purpose computer. Figure 13 is a block diagram of a computing device which may be used as a CU as referred to above in order to implement a method of en embodiment. The computing device 300 comprises a computer processing unit (CPU) 993, memory, such as Random Access Memory (RAM) 990, end storage, such ss a hard disk, 990, The computing device else includes a network adapter 999 for communication with other such computing devices oi embodiments. For example, an embodiment may be composed of a network cf such computing devices. Optionally, the computing device also includes Read Only Memory 994, one or more input mechanisms such as keyboard end mouse 998, and a display unit such as one or more monitors 997. The components are connectable to one another via e bus 992. The CPU 993 is configured to control the computing device and execute processing operations, such as the steps In the flowchart shown in Figure 9. The RAM 995 stores data being reed end written by the CPU 993. The storage unit 996 may be, for example, a non- volatile storage unit, and is configured to store date.

The CPU 993 may include one or more general -purpose processing devices such as a microprocessor, centre; precessing unit, or the like, The processor may include a complex instruction set computing (CISC) microprocessor, reduced Instruction set computing (RISC) microprocessor, very long instruction word (VLiW) microprocessor, or a processor implementing other instruction sets or processors Implementing a combination of instruction sets. The processor may also include one or more specialpurpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or file like, to one or more embodiments, a processor is configured to execute instructions for performing the operations and steps discussed herein.

The storage unit 995 may include a computer readable medium, which term may refer to a singie medium or multiple media (e.g., a centralized or distributed database and/or associated caches and servers) configured to cany computer-executable instructions or have data structures stored thereon. Computer-executable instructions may include, for example, instructions and data accessible by and causing a general purpose computer, special purpose computer, or special purpose precessing device (e.g,, one or more processors) to perform one or more functions or operations.

Thus, the term ’’computer-readable storage medium” may also include any medium that is capable of storing, encoding or carrying a set ef instructions for execution by the machine and that cause the machine to perform any one or more et the methods of the present disclosure. The term “computer-readable storage medium may accordingly be taken to include, but not be limited to_: solid-state memories, optical media and magnetic media. By way of example, and not limitation, such computer-readable media may include non-transitory computer-readable storage media, including Random Access Memory (RAM), Read-Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Compact Disc Read-Only Memory (CD-ROM) or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory devices (e g., solid stale memory devices).

The display unit 997 displays a representation of data stored by the computing device and displays a cursor and dialog boxes end screens enabling interaction between a user and the programs and beta stored on the computing device. The input mechanisms 998 enable a user to input data and instructions to the computing device.

The network adapter (network t/F) 998 is connected to a network, such as a highspeed LAN or the internet, and is connectable to ether such computing devices via the network, The network adapter 999 controls date input'output from/to other apparatus via the network. Other peripheral devices such as microphone, speakers, printer, power supply unit, ten, case, scanner, trackball etc may be included in the computing device 300,

The above mentioned HARQ caching function can be implemented on a computing device such as that illustrated in Figure 13, Such a computing device need not have every component illustrated in Figure 13, and may be composed of e subset of those components. A CU embodying the present invention may take the form of a single computing device 300 in communication with! one or more other computing devices via a network. The computing device may be a data storage itself storing at least a portion of the objects. A computation node or staging node embodying the present invention may be carried out by a plurality of computing devices operating in cooperation with one another. One or more of ihe plurality of computing devices may be a data storage server storing at least a portion of the objects.

Any of the embodiments and variations mentioned above may be combined in die same system, Whilst the above description bas been made with respect to LTE and LTE-A, the present invention may have application to other kinds of wireless communication system eiso. Accordingly, references in the claims to terminal” are intended to cover any kind of subscriber station, mobile device, MIC device and the like end are not restricted to the UE of LTE.

Tbe invention also provides a computer program or a computer program product for carrying out any of the methods described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein.

A computer program embodying the invention may be stored on a computer-readable medium, or It may, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it may be in any other form.

Industrial Applicability

The fields of application of this invention would include ail wireless communications systems where HARQ protocols or other re-transmission protocols are used.

Claims

CLAIMS;

1. A wireless communication system comprising first, second end third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a

5 retransmission protocol, wherein the third station is arranged to make a determination of where to buffer data for possible retransmission, in accordance with which the data is buffered in et least one of the second station and the third station.

10
2. The wireless communication system according to claim 1 wherein the second station is linked via fronfheui to the third station.
3. The wireless communication system according to claim 1 or 2 wherein the first station is a terminal.
4. The wireless communication system according to any preceding claim wherein the second station is a Distributed Unit, DU, and the third station is a Central Unit, CU as referred to in 3GRP Release 14 discussion documents.

20 5. The wireless communication system according to any preceding claim 'wherein the retransmission protocol comprises Hybrid Automatic Repeat reQuest, HARQ, and wherein data is provided to the first station with use of one or more HARQ processes.

6, The wireless communication system according to any preceding claim wherein the buffered data is data of one or more specific services or slices provided to the first station via the second station.

7. The wireless communication system according to any preceding claim wherein any dele being transmitted to the first station vie the second station Is buffered in accordance with the determination.

8. The wireless communication system according to any preceding claim wherein one or mors other stations are wirelessly linked to the second station, end the buffered data includes data being transmitted to any of the stations wirelessly linked to the second station,

S. The wireless communication system according to claim 5 when appended to cieim 2, wherein the third station Is arranged to make the determination on the basis of at least one of the following criteria:

latency of the fronthaui link;

load on the fronthaui link;

amount of memory available In the second station;

computing resources available in the second station;

an AQK/'NACK rate achieved by the HARQ precesses; and a number of CRC errors found during the HARQ processes.

10. The wireless communication system according to claim 9 wherein the wireless link between the first station and the second station has a Channel Quality indicator, CQL the system provides a service or slice to the first station, and wherein the criteria further include:

the CQi of the wireless link; and/or the kind of service or slice provided to the first station.

11. The wireless communication system according to any preceding claim wherein the third station is further arranged to notify the second station of its determination by transmitting flisf extension information of the retransmission protocol, the first extension information informing the second station whether or not to butter the data for retransmission.
5 12. The wireless communication system according to any preceding claim wherein the second station is arranged to transmit second extension Information of the retransmission protocol, indicating whether that transmitted data is buffered in the second station or in the third station.

13. The wireless communication system according to cieirn 12 wherein the first stations is arranged to include the second extension information in an acknowledgement of the transmitted data.

14. A third station in a wireless communication system comprising first, second and third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein

20 the third station is arranged to make a determination of where to buffer data for possible retransmission, in accordance with which the data is buffered in et least one of the second station and the third station.

25 15. A second station in a wireless communication system comprising first, second and third stations, the first station wirelessly linked to the second station, the second station linked to the third stetion. data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the second station is arranged to receive from the third station an instruction of

30 whether to buffer the deta for possible retransmission.

16. A first station in a wireless communication system comprising first, second end third stations, the first station wirelessly linked to the second station, the second station linked to the third station, data being transmitted to the first station via the second station with use of a retransmission protocol, wherein the first station is arranged to receive, from the second station and in addition to toe transmitted data, extension information of the retransmission protocol indicating

5 whether the transmitted data is buffered in the second station or in the third station, and to include the extension information In an acknowledgement returned to the second station.
10 17. A wireless communication method In a wireless communication system comprising first, second and third stations, the first stetson wirelessly linked to the second station, the second station linked to the third station, the method comprising;

transmitting data to tbe first station via the second station with use of a retransmission protocol, wherein the third station makes a determination of where to
15 buffer data for possible retransmission, and the data is buffered in the second station or in the third station in accordance with the determination.

f 8. Software in the form of computer-readable instructions which, when executed by
20 a processor In a computing device, cause the computing device to be the first, second or third station referred to trr any of the preceding claims.

Intellectual

Property

Office

Application No: Claims searched:

GB1616682.9

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