GB2364862A

GB2364862A - Data packet flow control between RNC and node B

Info

Publication number: GB2364862A
Application number: GB0107297A
Authority: GB
Inventors: Raul Soederstroem; Mikael Jan Are Winberg; Johan Torsner; Osmo Pulkkinen
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2001-03-23
Filing date: 2001-03-23
Publication date: 2002-02-06
Anticipated expiration: 2021-03-23
Also published as: WO2002084952A1; GB2364862B; GB0107297D0; TW552816B

Abstract

The flow of data packets, destined for User Equipment (UE), is controlled between a Radio Network Controller (RNC) and a Node B of a UMTS Terrestrial Radio Access Network (UTRAN), where it is sought to synchronise the Node B and the RNC in order to allow data packets sent from the RNC to arrive at the Node B in time to be included in the allocated physical channel frames. A data packet from a MAC entity of the RNC is sent to the Node B, whilst retaining a copy of the sent data packet at the transmitting MAC entity, sending a notification from the Node B to the transmitting MAC entity, which notification indicates whether the sent data packet was transmitted to the UE on the allocated physical channel frame, and in the event that the sent data packet was transmitted on the allocated physical channel frame, sending a new data packet to the Node B from said MAC entity and discarding the original data packet stored at the MAC entity, and, in the event that the sent data packet was not transmitted on the allocated physical channel frame, resending the original data to the Node B from said MAC entity.

Description

<Desc/Clms Page number 1> RADIO ACCESS NETWORKS Field of the Invention The present invention relates to reducing or eliminating data loss in the radio access network of a mobile telecommunication system.

Background to the Invention The European Telecommunications Standardisation Institute (ETSI) is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as the Universal Mobile Telecommunications System (UMTS). This third generation standard is also often referred to as 3GPP.

Figure I illustrates schematically a UMTS network .1 which comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UTRAN) 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Base Stations (BSs) 5. BSs are sometimes referred to as Node Bs. Each Node B 5 is responsible for a given geographical cell and the controlling RNC 4 is responsible for routing user and signalling data between that Node B 5 and the core network 2. All of the RNCs are coupled to one another (the interface between RNC being identified as the Iur interface). A general outline of the UTRAN 3 is given in Technical Specification TS 25.401 V2.0.0 (1999-09) of the 3rd Generation Partnership Project, ETSI.

User and signalling data is carried between an RNC and a mobile terminal (referred to in UTRAN as User Equipment (UE)) using Radio Access Bearers (RABs). Typically, a UE is allocated one or more Radio Access Bearers (RABs) each of which is capable of carrying a flow of user or signalling data, and each RAB is mapped onto a corresponding logical channel. At the RNC, each RAB is dealt with by a corresponding Radio Link Controller (RLC) entity. In particular, data for a given RAB is temporarily held in an RLC buffer. At a Media Access Control (MAC) layer, data packets (i.e. RLC Protocol Data Units (PDUs)) are extracted from the RLC buffers, and processed so as to

map a set of logical channels onto a transport channel, of which there are two types: a 11common" transport channel type which is shared by different mobile terminals, and a "dedicated" transport channel type which is allocated to a single mobile terminal. There exist eight common transport channel types, one of which is a downlink shared channel (DSCH). Two other common channel types are the Forward Access Channel (FACH) and the Random Access Channel (RACH). There are three dedicated channel types of which one type is the Dedicated Channel (DCH). Various protocol layers present at the RNC and Node B are illustrated in Figure 2.

Considering further the MAC layer, present at the RNC and the UE, this provides for a MAC-c entity and a MAC-d entity, each of which is responsible for constructing MAC PDUs from RLC PDUs extracted from the RLC buffers. A frame of a traffic channel can contain one or several MAC PDUs. The MAC-d entity is responsible for coordinating the transport format combinations and transport formats for any DCHs which a given UE is using. The MAC-c entity handles the transport format combinations and transport formats for any common transport channels allocated to a given set of UEs. The MAC-d entity is responsible for executing channel switches between transport channel types for a UE upon receipt of orders from the RRC. The RRC is responsible for connecting the MAC-d and MAC-c entities to enable the former to communicate switching decisions to the MAC-c entity.

At the physical layer, which is implemented at the Node Bs, DCHs constructed by a MAC-d entity are mapped onto a Dedicated Physical Data Channel (DPDCH) for transmission over the air interface to a UE. At the MAC-c entity, the CCHs, FACH and RACH are mapped onto a Secondary Common Control Physical Channel (S-CCPCH) and a Physical Random Access Channel (PRACH) respectively, whilst DSCHs are mapped onto a Physical Downlink Shared Channel (PDSCH). More particularly, each frame of a traffic channel (dedicated or common) is mapped onto a corresponding frame of a physical channel at the Node B. In order to efficiently use resources at the Node B, the aim of an RNC is to deliver each traffic channel frame to the Node B, just in time for it to be incorporated into the appropriate frame of the designated physical channel (this avoids the need for large data buffers at the Node B). To achieve this, the RNC maintains an offset value for each Node B, the offset value approximating the offset

between the internal clocks of the Node B and the RNC. Using the offset, the RNC calculates when a traffic channel fi-ame should be sent to the Node B in order for it to arrive just in time for it to be included in the appropriate physical channel frame.

For a number of reasons, the offset value for a Node B may be wrong. If the error is severe, a traffic channel frame may arrive at the Node B too late for it to be included into the appropriate physical channel frame. In this case the received traffic channel fi-ame and the PDUs it contains are discarded by the Node B, increasing the Bit Error Rate (BER). The Node B will advise the RNC to correct the offset using TIMING ADJUSTMENT messages. However, the MAC layer does not concern itself with the resending; of data discarded by the Node B. Rather, it obtains new RLC PDU(s) from the RLC buffers, constructs new MAC PDUs, and transmits these to the Node B in a traffic channel frame. Error correction is the responsibility of higher layers (e.g. RLC). Following the establishment of a user connection between a UE and the network, the network may decide to switch the connection from one channel type to another channel type. During channel switching, the error in the offset can become particularly severe and the BER rises accordingly. This also occurs whilst handing over radio links between Node Bs.

Statement of the Invention It is an object of the present invention to overcome problems caused by a failure to adequately synchronise an RNC with a Node B. This and other objects are achieved at least in part by providing for the resending of data from the transmitting RNC MAC entity in the event that data previously sent to the Node B was received too late to be included in the allocated physical channel frame.

According to a first aspect of the present invention there is provided a method of controlling the flow of data packets, destined for User Equipment (UE), between a Radio Network Controller (RNC) and a Node B of a UMTS Terrestrial Radio Access Network (UTRAN), where it is sought to synchronise the Node B and the RNC in order

to allow MAC PDUs sent from the RNC to arrive at the Node B in time to be included in the allocated physical channel frames, the method comprising: sending a MAC PDU from a MAC entity of the RNC to the Node B, whilst retaining a copy of the sent MAC PDU at the transmitting MAC entity; and in the event that the Node B is unable to transmit the sent MAC PDU on the allocated physical channel frame, sending a notification from the Node B to the transmitting MAC entity and resending the MAC PDU to the Node B from said MAC entity.

Embodiments of the present invention reduce the bit error rate (BER) resulting from a severe loss of synchronisation between an RNC and a transmitting Node B, by buffering a sent MAC PDU in the transmitting MAC entity whereby MAC PDUs which are received by the Node B too late to be included in the allocated physical channel frame can be resent.

A MAC PDU is sent to the Node B in a traffic channel frame. The frame may contain several other MAC PDUs. Preferably, said notification is a TRVHNG ADJUSTMENT message of the frame handling protocol, and relates to all of the MAC PDUs of a given traffic channel frame.

The transmitting MAC entity may be a.-MAC-c entity in which case the MAC PDU is sent to the Node B for inclusion in a S-CCPCH, or PDSCH frame, or a MAC-d entity in which case the MAC PDU is sent for inclusion in a DPDCH frame.

Preferably, the sending MAC entity stores a copy of the MAC PDUs contained in one sent traffic channel frame in its buffer for possible resending.

The present invention is applicable in particular to reducing the BER during a channel switch, e.g. a common traffic channel (e.g. FACH, RACH) to a dedicated channel (DCH), or during a handover.

It will be understood that the transmitting MAC entity receives data, upon request, from a RLC entity of the RNC. Data to be sent to the UE is stored in one or more buffers of

the RLC entity and is extracted therefrom by the transmitting MAC entity. The transmitting MAC entity has a buffer associated with it and in which is stored the sent MAC PDU(s).

According to a second aspect of the present invention there is provided a Radio Network Controller for use in a UMTS Terrestrial Radio Access Network (UTRAN) of a UMTS system, the RNC comprising a MAC entity arranged in use to send MAC PDUs to a Node B of the UTRAN for transmission to a peer MAC entity of User Equipment (UE) over the air interface, the transmitting MAC entity having means for storing a copy of a sent MAC PDU, and means for resending that MAC PDU in the event that the MAC entity receives a notification from the Node B that the MAC PDU was not received in time for inclusion in an allocated physical frame.

Brief Description of the Drawings Figure 1 illustrates schematically a UMTS network comprising a core network and a UTRAN; Figure 2 illustrates protocol layers present at an RNC and Node B of the UTRAN of Figure 1; and Figure 3 is a flow diagram illustrating a process for reducing data loss in the UTRAN of Figure 1.

Detailed Description of a Preferred Embodiment The general structure of a UMTS network has been described above with reference to the schematic drawing of Figure 1. Various protocol layers have also been described with reference to Figure 2.

Considering the transfer of data in the downlink direction, signalling and user data packets destined for the UE are received at the RLC entity of the serving RNC and are placed in allocated RLC buffers. The RLC entity is (logically) coupled to a MAC-d entity. It is the RRC entity which decides whether data traffic destined for a given UE is to be transmitted on a Forward Access Channel (FACH), Downlink Shared Channel

(DSCH), or a Dedicated Channel (DCH). The MAC-d entity is responsible for executing a channel switch.

Assuming that a LIE has been allocated a DPDCH, the MAC-d entity of the serving RNC maps logical channels (either a Dedicated Control CHannel (DCCH) or a Dedicated traffic CHannel (DTCH)) onto DCHs. This requires the MAC-d entity to extract RLC SDUs from the appropriate RLC buffers, and construct MAC-d Service Data Units (SDUs) comprising a payload section containing logical channel data and a MAC header containing amongst other things a logical channel identifier. A DCH frame is assembled using several MAC PDUs and is passed (via the UTRAN Iur and lub Interface Data Transport & Transport Signalling for DCH Data Streams Frame Protocol entity) to the Layer I entity at the Node B. At the Layer I entity, the DCHs are multiplexed onto a single Dedicated Physical Channel (DPCH) for transmission over the Iub interface. This involves placing the DCH frames into appropriate frames of the physical charmel (in practice, the DCH frames are first segmented or concatenated, coded, and interleaved).

Where a UE has been allocated space on a Secondary Common Control Physical Channel (S-CCPCH), it is the MAC-d entity which extracts RLC SDUs from the RLC buffers in order to send then to the MAC-c entity which maps the logical channels to a CCH or a DSCH. A CCH or DSCH is assembled from several MAC-c PDUs and is passed to the Layer I entity at the Node B (again via the UTRAN lur and Iub Interface Data Transport & Transport Signalling for DCH Data Streams Frame Protocol entity) for multiplexing onto a Common Physical Channel (CPCH).

In order to minimise the complexity of the frame handling protocol used over the lur interface (between the RNC and the Node B), the transmitting entities maintain a clock offset measurement from which they can estimate when a traffic channel frame must be sent in order for it to be included in the allocated physical channel frame. So long as the error in this offset is small, traffic channel frames will be received in time by the Node B and will be sent to the UE. However, if the error becomes significant, traffic channel fi-ames may arrive too late at the Node B and may be discarded without being sent. In order to reduce the error, TIMING ADJUSTMENT messages are sent using the frame

handling protocol over the Iur interface. A TIMING ADJUSTMENT message contains the identity of the first MAC PDU (or rather transport block) of the late traffic channel frame, as well as the total number of MAC PDUs in that frame. The MAC entities use the information contained in these messages to correct the clock offsets.

In order to prevent the complete loss of sent data in the event that the error in the offset is large (and a data packet is discarded by the Node B), the sending MAC entity (MAC- c or MAC-d) of the RNC temporarily retains a copy of each sent MAC PDU in a MAC buffer. This usually involves retaining a copy of all of the MAC PDUs contained in a sent traffic channel frame. If a TIMING ADJUSTMENT message is received after the sending of a traffic channel frame, the sending entity determines that the traffic channel frame was received too late to be sent over the Iur interface. However, as the PDUs are still stored by the transmitting MAC entity, the MAC entity is able to resend the PDUs to the Node B following adjustment of the time offset. This process can be repeated several times until the MAC entity is "synchronised" to the Node B.

Figure 3 is a flow diagram further illustrating this synchronisation process.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiment without departing from the scope of the present invention.

Claims

Claims 1. A method of controlling the flow of data packets, destined for User Equipment (LJE), between a Radio Network Controller (RNC) and a Node B of a UMTS Terrestrial Radio Access Network (UTRAN), where it is sought to synchronise the Node B and the RNC in order to allow MAC PDUs sent from the RNC to arrive at the Node B in time to be included in the allocated physical channel frames, the method comprising: sending a MAC PDU from a MAC entity of the RNC to the Node B, whilst retaining a copy of the sent MAC PDU at the transmitting MAC entity; and in the event that the Node B is unable to transmit the sent NIAC PDU on the allocated physical channel frame, sending a notification from the Node B to the transmitting MAC entity and resending the MAC PDU to the Node B from said MAC entity.
2. A method according to claim 1, wherein said MAC PDU is sent to the Node B in a traffic charmel frame along with one or more other MAC PDUs, with a copy of all of these PDUs being retained at the sending MAC entity.
3. A method according to claim 2, wherein said notification is a TIMING ADJUSTMENT message of the frame handling protocol, and receipt of this notification at the sending MAC entity causes all of the MAC PDUs contained in the frame to which the TIMING ADJUSTMENT message relates, to be resent to the Node B.
4. A method according to any one of the preceding claims, wherein the transmitting MAC entity is a MAC-c entity in which case the MAC PDU is sent to the Node B for inclusion in a S-CCPCH, PRACH, or PDSCH frame, or a MAC-d entity in which case the MAC PDU is sent for inclusion in a DPDCH frame.
5. A method according to any one of the preceding claims, wherein the transmitting MAC entity receives data, upon request, from one or more buffers of an RLC entity, and the transmitting MAC entity has a buffer associated with it and in which is stored the sent MAC PDU.

<Desc/Clms Page number 9>
6. A Radio Network Controller for use in a LTMTS Terrestrial Radio Access Network (UTRAN) of a UMTS system, the RNC comprising a MAC entity arranged in use to send MAC PDUs to a Node B of the UTRAN for transmission to a peer MAC entity of User Equipment (UE) over the air interface, the transmitting MAC entity having means for storing a copy of a sent MAC PDU, and means for resending that MAC PDU in the event that the MAC entity receives a notification from the Node B that the MAC PDU was not received in time for inclusion in an allocated physical frame.