GB2383499A - Method of transmitting data in a communications system - Google Patents

Abstract

Data packets A-C for transmitting to a communications device 2 are held in buffers 14,15 and 16 located in base stations 3, 4 and 5. Each base station being within range of the communications device 2. Said device 2 detects a base station having the strongest radio link and requests transmission of a first packet A. If correctly received, the communications device 2 sends an acknowledgement to a network controller 9 via said selected base station. On receipt of the acknowledgement, the controller 9 sends another packet D to each base station 3, 4 and 5, whereupon receipt of this packet, the first packet A is deleted from the buffers 14, 15 and 16. The invention has the advantage of avoiding the build-up in a base station of data packets which have already been transmitted by another base station. The invention is applicable to a CDMA system such as UMTS.

Description

<Desc/Clms Page number 1>

PACKET DATA TRANSMISSION IN A COMMUNICATION SYSTEM This invention relates to the transmission of data packets in communication systems and is particularly applicable to the so-called third generation systems 3G (UMTS-Universal Mobile Telecommunications System).

In the field of this invention it is known that data, in the form of data packets, are transmitted across a digital communication network or system which consists of a number of individual links. Protocols are used to organise the transmission of data by means of a hierarchy of protocol layers. The hierarchy of layers typically extends from a physical layer (which dictates the manner in which individual bits are transmitted), through to an application layer, which determines, for example, how two high-level computer programs interact with each other.

An example of an intermediate layer is a link layer, which controls the transmission of data across a single organisational link in the network. The single organisational link at the link layer will often however correspond to multiple physical links in the physical layer. For example, in a cellular radio communication system according to the Universal Mobile Telephone Standard (UMTS), at the link layer a single organisational link exists between a Radio Network Controller (RNC), and a user equipment (UE), such as a mobile telephone, whereas in the physical layer the physical connection is implemented by a first physical link from the RNC to an intermediate physical entity, namely a Node-B (UMTS terminology for a base station) and a second physical link from the Node-B to the UE. In the above example, the RNC may be known as the transmit-end (Tx-end) of the link layer, and the UE as the receive-end (Rx-end).

Within the link layer, a mechanism called Automatic Repeat Request (ARQ) provides an error control mechanism for data transmission that allows the Rx-end to periodically advise the Tx-end as to whether data packets have been received without error or not. This allows the Tx-end to retransmit any packets that were transmitted in error in the previous period.

<Desc/Clms Page number 2>

The message sent by the Rx-end is known as an acknowledgement/negative acknowledgement (Ack/Nack). The Ack/Nack message contains the Ack/Nack state of the previous transmitted packet data units (PDU), also termed data packets or blocks, sent to the Rx-end by the Tx-end. On receiving the Ack/Nack message the Tx-end is able to retransmit those packets that were reported as being received in error (Nacked) by the Rx-end; typically the oldest Nacked PDU is retransmitted first.

The high speed downlink packet access (HSDPA) mode of operation proposed in UMTS standard release 5, permits packets to be sent to a UE from all node Bs within its range. Packets destined for a particular UE are transmitted from an RNC to the node B's under its control. The packets are placed in queues in buffers at each node B. The UE monitors the radio links it has with each node B, selects the best one and receives the packets over that link. In another variant (macro diversity), all node B's transmit their queued packets to the UE.

When a packet is received without error, an acknowledgement message is transmitted by the UE. However, if not all the node B's see the acknowledgement, queues of packets, which have already been received successfully, can build up in one or more node B's. This is a waste of resource. Hence, a mechanism for "flushing"queues of packets would be advantageous.

According to the present invention, a method for transmitting data packets from a controller to a user equipment in a communications system includes the steps of; sending, from the controller to a plurality of base stations, a first data packet, at each of said plurality of base stations, storing the first data packet in a buffer, transmitting from at least one of the plurality of base stations the first data packet to the user equipment, if the first data packet is received correctly at the user equipment, then at the controller, sending, to the plurality of base stations, a second packet, and at each base station, on receipt of the second packet, deleting the first packet from the buffer.

<Desc/Clms Page number 3>

Hence, packets stored in buffers in each base station (node B) can be deleted on receipt of a new packet for transmission. This can be done because the receipt of a new packet implies that a packet at the front of the buffer's queue has been received correctly and acknowledged by the user equipment (possibly via some other base station).

The invention can also be applied to"windowing"where a block of packets is acknowledged by the same acknowledgement message. In this case, an RNC can then send a second block to each base station/node B whereupon the block at the front of each buffer's queue in each base station is deleted.

Advantageously, no additional acknowledgement instruction needs to be sent from an RNC to a base station/node B in order to prompt a base station/node B to delete a packet in its buffer; just a new packet.

In a preferred embodiment, the user equipment monitors the radio links that it has with each base station (node B) and requests transmission of packets from the base station (node B) with the best link. The other base stations (which are also within range of the user equipment and have the same packets stored in their buffers) do not transmit their data packets. This has the advantage of lessening interference levels within the communications system.

An embodiment of the present invention will now be described, by way of example only, with reference to the drawings, Figures 1 to 4 which show in schematic form block diagrams of components comprising a communications system capable of supporting the transmission of data packets performed in accordance with the invention.

With reference to Figure 1, the communication system is a cellular communications system 1. A user equipment (UE) 2, e. g. a mobile telephone, for use by an end user, is coupled with three base stations known in UMTS as a node B 3,4, 5 via radio links 6,7, 8 respectively. Each node B is coupled to a radio network controller

<Desc/Clms Page number 4>

(RNC) 9 via physical links (e. g. a land line) 10, 11, 12. The RNC 9, node Bs 3,4, 5 and UE 2 are all provided with buffers 13 - 17 respectively, for storing data packets.

In this example, the user equipment 2 is within range of all three node Bs 3,4, 5 but from its measurements of received pilot signals, it determines that the link 6 to the node B 3is the strongest. Subsequently, it requests the node B 3 to transmit any data packets that are stored in its buffer and destined for the particular UE 2.

In the initial state shown in Figure 1, three packets A, B, C have been distributed from the RNC 9 to the node Bs 3,4, 5 and none have yet been sent to UE 2. The RNC 9 has three more packets D, E, F ready to send when an acknowledgment is received.

Figure 2 shows transference of packet A. The UE 2 has selected node B 3 as having the strongest radio link and packet A is transferred but not deleted from any node B buffer 14,15, 16. On successful receipt, the UE 2 sends an acknowledgement "ACK" which is received by the node B 3 and relayed over link 10 to the RNC 9.

With reference to Figure 3, on receipt of the acknowledgement from the UE 2, the RNC 9 can now send the next packet (packet D). This is received by all three node Bs and placed in their buffers 14,15, 16.

Figure 4 illustrates deletion of packet A from all the node B buffers on receipt of packet D. It does not matter which node B was used to transmit the packet over the air. The packet A was not deleted from any node B buffer on transmission, but on receipt of a new packet D from the RNC 9, it can be deleted. The actions at each node B are independent of whether they are transmitting a packet or not.

If the UE 2 later detects that another node B, 4 or 5, provides the strongest radio link, then it can request transmission of subsequent packets from this newly soidentified node B. As all node Bs are adjusting the contents of their buffers in the

<Desc/Clms Page number 5>

same fashion and in synchronism (on receipt of a new packet from the RNC 9), handing over from one node B to another will be seamless.

An apparatus for implementing the above arrangement and performing the method steps may be provided by adapting conventional apparatus and/or providing additional modules. In particular, additional apparatus may be provided within the node B. The apparatus may be in the form of hardware, fimware or software or a combination of these. The apparatus may comprise one or more processes for implementing instructions and using data stored in a storage medium such as a computer disk or programmable read-only memory (PROM).

Claims (4)

1. CLAIMS 1. A method for transmitting data packets from a controller to a user equipment in a communications system including the steps of; sending, from the controller to a plurality of base stations a first data packet, at each of said plurality of base stations, storing the first data packet in a buffer, transmitting from at least one of the plurality of base stations, the first data packet to the user equipment, if the first data packet is received correctly at the user equipment, then at the controller, sending, to the plurality of base stations, a second packet, and at each base station, on receipt of the second packet, deleting the first packet from the buffer.
2. 2. A method as claimed in Claim 1 and including the further step of in the user equipment, identifying the base station which provides the best radio link to the user equipment, and requesting transmission of the first data packet from said so-identified base station.
3. 3. A storage medium storing a processor-implementable instructions for controlling a processor to carry out the method of either of Claims 1 to 2.
4. 4. A method for the transmission of data packets substantially as hereinbefore described with reference to the drawings.