WO2007148138A1

WO2007148138A1 - Packet retransmission

Info

Publication number: WO2007148138A1
Application number: PCT/GB2007/050352
Authority: WO
Inventors: David Randall
Original assignee: Siemens Aktiengesellschaft
Priority date: 2006-06-23
Filing date: 2007-06-22
Publication date: 2007-12-27

Abstract

A method of packet retransmission in a communications system (1) comprising (5) a plurality of user devices (M1, M2, M3, M4) and at least one network device (B1) comprises broadcasting a multimedia broadcast multicast service transmission (5, 6, 7) to the plurality of user devices. One or more of the user devices (M3, M4) which failed to receive the complete transmission correctly send a request for retransmission (8) to the network device and the network device retransmits (10) requested packets of the (10) original transmission on a different transmission channel to the original broadcast channel.

Description

PACKET RETRANSMISSION

This invention relates to a method of packet retransmission, in particular for universal mobile telecommunications system (UMTS). In cellular radio systems such as UMTS and global system for a mobile communications (GSM) there is a service class known as multimedia broadcast multicast service (MBMS) whereby information is transmitted to multiple user equipments (UEs), or user devices, using a common (broadcast) downlink channel. This form of transmission is referred to in UMTS as a point-to-multipoint channel. In UMTS, MBMS services can also be transmitted by way of a data channel that is dedicated to a particular UE, known as point-to-point (ptp) transfer.

When data is transmitted in a point-to-multipoint channel, the UEs that are receiving it can independently fail to correctly decode particular data blocks, sometimes known as PDUs, or transport blocks. There are a number of well-known methods for recovering from packet loss. One method is for an outer forward error correcting (FEC) code to be applied across multiple packets. The error correction capabilities of the FEC code can repair the packet loss in many circumstances. A second method is known as automatic repeat request (ARQ), whereby UEs request from the transmitting entity that packets that were not received should be repeated. ARQ has been applied to MBMS in GSM. In the GSM case, mobile stations

(MS) that are receiving the service can be allocated an identifier that the network uses to poll the MS and in response the MS indicates the identities of packets, within a window, or range, of packet sequence numbers that it has failed to receive. The MS responds in a defined transmission interval associated with the poll. After collection of loss reports from multiple MSs, the identified packets can then be retransmitted by the transmitting entity. Retransmissions are made within the same transmission channel that was used for the original packet transmissions. The identities that the network assigns to the MS for the purpose of polling can be changed, or removed by the network. Up to sixteen MSs can be assigned identities at one time and MSs can request assignment of an identity after receiving notification on the MBMS control channel at session start, or when they enter a cell.

However, using the same transmission channel for retransmission increases complexity because the retransmission needs to be inserted into a common packet stream and the carrier capacity varies according to the level of demand for retransmission.

In accordance with the present invention, a method of packet retransmission in a communications system, the system comprising a plurality of user devices and at least one network device, comprises broadcasting a multimedia broadcast multicast service transmission to the plurality of user devices; sending a request for retransmission to the network device from one or more of the user devices which failed to receive the complete transmission correctly; and retransmitting requested packets of the original transmission on a different transmission channel to the original broadcast channel. In the present invention, the retransmissions are made on a different logical, transport or physical channel to the initial point-to-multipoint transmission on which the packets were first transmitted, thereby avoiding the problems encountered in existing GSM systems.

Preferably, the broadcast channel is an MBMS point to multipoint bearer. Preferably, the different transmission channel is a different logical, transport or physical channel to the broadcast channel.

Preferably, only certain predetermined categories of user device are permitted to request retransmission.

Preferably, the network device allocates user devices to the category of user device permitted to request retransmission, using signal strength and interference data reported by each user device.

Preferably, the network device sets a threshold for data reported by each user device, above which retransmission requests are not enabled.

Preferably, a proportion of the user devices below the threshold set by the network device make retransmission requests on periodically assigned dedicated resources.

Preferably, user devices make retransmission requests on contention access resources.

Preferably, the retransmissions are broadcast to a group comprising those user devices which have made a request for retransmission.

Alternatively, the retransmissions are made to individual user devices using hybrid automatic repeat request (HARQ). An example of a method of packet retransmission in accordance with the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a cellular radio system in which UEs are receiving an MBMS transmission and make retransmission requests in a method according to the present invention;

Figure 2 shows an information flow diagram for an MBMS data transfer mechanism incorporating retransmission in a method according to the present invention: and, Figure 3 illustrates a signalling procedure to join a retransmission request reporting group for the method of the present invention.

This invention relates to the channel used for making retransmissions and the method used to select a user equipment to report packet loss in an ARQ system, or HARQ system that repairs packet loss in an MBMS transmission system. It applies in particular to 3^rd generation partnership project (3GPP) cellular radio systems.

In the present invention retransmissions are made on a different transmission channel to the initial point-to-multipoint transmission. UEs which are receiving a service provide feedback in response to the transmission by indicating to the transmitting entity that they would like to receive a retransmission of particular packet or packets, also known as transport blocks, which they did not receive correctly. This indication may take the form of a negative acknowledge (NACK). Optionally, UEs which did receive the transmission correctly also provided feedback, by sending an acknowledge (ACK), so indicating that they do not require any retransmission. The packets that are needed can be identified by packet sequence numbers, or transport block sequence numbers, or by the radio frame sequence numbers of the radio frames in which the first transmission should have been received, which are applied at the transmitter. These can then be retransmitted by the transmitting entity. In one embodiment, only certain UEs, perhaps even none, are enabled to request retransmission. In another embodiment, all UEs are enabled to request retransmission. For any of these cases, the option of sending an ACK when receipt is successful may be set or not. The UEs that are enabled to request the retransmission of packets send their requests to the transmitter, either using dedicated (scheduled) resources, or on access resources shared by contention e.g. random access channel (RACH). For example, in the case of UMTS long term evolution (LTE) the retransmissions may be made using the downlink shared channel (DL-SCH); either in a broadcast mode without hybrid ARQ (HARQ) to all interested UEs, possibly using a group identifier; or individually to each UE using HARQ. When retransmissions are received by the UE, they may be combined in the physical layer with the corresponding transport block received with the first transmission as in HARQ, or alternatively, the first transmission for the block may be discarded and the content of the retransmission combined with packets received in the first transmission at a higher layer, e.g. at layer 2. Fig. 1 shows a cellular environment 1, consisting of a base station, Bl otherwise referred to in relation to UMTS LTE as eNB and a number of user devices illustrated here as mobile stations Ml to M4 which are otherwise known as UEs in the context of UMTS, or UMTS LTE. The base station Bl transmits 2 an MBMS service to all of the MSs. M3 and M4 transmit retransmission requests 3, 4 to the base station Bl whilst Ml and M2 do not.

The original MBMS transmission 2 is made by a point-to-multipoint bearer i.e. one that is being transmitted in a broadcast form so that it can be received by multiple UEs Ml, M2, M3, M4 simultaneously, rather than one that is transmitted in a form that is specific to a UE i.e. point-to-point. The transmission may be made in a single cell without co-ordination with the transmissions made for the same service or session in neighbour cells (not shown). Alternatively, the transmission format of the original transmission 2 may be designed so that identical synchronised transmissions are made in a group of cells so that a UE receiving both a serving and one or more neighbour cells can combine the several transmissions to enhance the received signal quality. In a UMTS network this can be done by an MBMS technique, known as soft combining and in a UMTS LTE network, this is done as a single frequency network (SFN) transmission.

One or more UEs request retransmissions 3, 4 of packets that they failed to receive from a single (serving) cell and the serving cell arranges for these packets to be resent. The serving cell retransmits the requested packets using a different logical channel and/or transport channel and/or physical channel to that which was used to transmit the packets for the first time. Fig. 2 shows a signalling exchange whereby a transmitter NB transfers data to a UE, the UE indicates that packets have been detected as missing and the transmitter resends the missing packets to the UE. The transmitter NB sends MBMS data 5, 6, 7 on one channel to the UE. The UE fails to receive some of this data correctly, so sends a retransmission request 8 to NB on a different channel. Further MBMS data 9 continues to be sent to the UE on the first channel and at a suitable point, the retransmitted MBMS data 10 is sent on a yet another channel. The transmitter continues to send original MBMS data 11, as required.

In the present invention, where the original transmissions are SFN, or soft combined and as a result the transmissions in all cells within a cell group are substantially identical, then by making the retransmissions on a separate channel from that which is used for the original transmission, the complexity of inserting retransmissions, which are specific to a cell, into a packet stream that is common to several cells, is avoided. This is particularly true if the initial common transmission is formatted in a separate node e.g. radio network controller (RNC), to the node in which the retransmission is made e.g. Node B. Furthermore, the primary carrier carrying the first transmission can be maintained at a constant data rate, unchanged by the variable capacity that is required by the retransmissions if they are not sent on a different carrier. Also, since the retransmissions are made to a single UE, or a group of UEs that is smaller than that to which the original transmission was made, then better error protection, via error correcting coding or HARQ, possibly combined with a lower transmission power, can be used for the retransmissions. For example, the retransmissions can be made on a bearer that is dedicated to the UE, so that HARQ and power control techniques can be applied. Alternatively, where the transmission is to be transmitted to a group of UEs, more powerful coding may be applied.

As an example of the application specifically to UMTS LTE, the first transmission may be made via an MBMS point to multipoint traffic channel (MTCH) logical channel mapped to a multicast channel (MCH) transport channel that is, in turn, mapped to a set of physical resources. The first transmission may be SFN or non-SFN, and the retransmissions can be made by a second MTCH mapped to an MCH, or to the downlink synchronisation channel (DL-SCH), which is mapped to a different set of physical resources to those used for the first transmission. This can be made accessible to any UE that chooses to receive the transmission and may contain the aggregate of all packets that were requested by all UEs that requested retransmissions, or each requested packet/transport block may be sent separately to enable combining at the physical layer. Alternatively, the retransmissions may be made separately to each UE that requested them, using a dedicated traffic channel (DTCH) logical channel mapped to the DL-SCH transport channel. These point-to-point transmissions make use of HARQ.

UEs can be subdivided into a group that can request retransmissions, for example using ARQ and a group that cannot request retransmissions, or can only request on contention access channels. The UEs that are enabled to request retransmissions can be selected by the network based on their signal strength and interference environment, possibly identified via measurement reports made by the UE. For example, UEs on the cell boundary may be enabled to make retransmission requests. In the variation where UEs are permitted (enabled/ disabled by network control signalling) to make retransmission requests on contention access resources e.g. RACH, a fraction of the UEs in a cell may be enabled to make retransmission requests via dedicated resources e.g. periodically assigned resources, if the UEs are in poor radio conditions. By this means load on the RACH is reduced. Requests for retransmissions can be made periodically throughout the MBMS transmission or, for example in the case of a transmission of short duration, at the end of the transmission.

The action of UEs making uplink retransmission requests consumes uplink resources and so it is desirable that the load on uplink resources is managed. For those UEs that are located close to a transmitter, the signal to interference level may be good enough for the packet loss rate to be low. For these UEs, disallowing retransmission requests, or alternatively permitting their transmission on a contention access resource, such as RACH may be sufficient.

For UEs which are located in poor signal to interference environments, such as might exist on a cell boundary, the rate at which packet loss occurs is likely to be much higher. In these circumstances, it can be more efficient for the use of uplink resources if defined uplink resources are assigned to each UE, so that their uplink transmissions can be controlled. For example, a UE reports a number of packet identities in one transmission, or as alternative to dedicated resources, these UEs may make use of contention access uplink resources e.g. RACH, provided that the number of UEs that can report by this mechanism is limited in number.

Therefore, it is preferable that the total number of UEs in a cell that are receiving an MBMS transmission and can make retransmission requests is separated into two groups, one that is not permitted to make retransmission requests and one that is permitted to make retransmission requests, either by contention access e.g. RACH or UE dedicated resources depending upon implementation. A second option is that the first group is permitted to make retransmission requests by contention access resources and the second group by dedicated resources. The subdivision of UEs into the two groups is controlled by control signalling transmitted by the serving eNB.

In one example the control signalling, transmitted for example on the MBMS control channel (MCCH), identifies a threshold parameter, for example path loss measured on the cell pilot, or signal to interference ratio, or MBMS transmission packet loss rate. If the threshold is crossed for a UE, this enables the UE to request to join the second group which can make retransmission requests. The network then assigns the UE uplink resources and/or an MBMS specific identifier that enables retransmission requests to be made. By default, a UE is in the first group, which is not permitted to make retransmission requests. This embodiment is applicable to the cases of UEs that are in the LTE radio resource control (RRC)-IdIe state or the LTE RRC-Connected state.

In a second example, the network decides to include a UE in the first, or second group above based on measurement reports that the UE makes. Based on measurements of path loss, or signal to interference ratio, the network (eNB) chooses to assign a UE to the first group, or the second group based on these measurement reports. This embodiment is applicable to UEs that are in the LTE RRC-Connected state.

In both of these embodiments, the network may remove UEs from the second group, signalling to them that they are released from the second group, if one of the following occurs: reports of path loss, measured on a cell transmission such as a pilot, made by the UE to the network show the path loss has fallen below a threshold; reports of signal to interference measurements rise above a threshold; the number of retransmission requests made by the UE in a time period falls below a threshold. Alternatively, these rules are applied in the UE and the UE requests release when they occur. Fig. 3 shows a signalling exchange relating to retransmission request reporting groups. A network device NB sends MBMS retransmission join criteria 12 which set out rules regarding whether a UE should apply to join the retransmission reporting group for a service, the UE requests to join by sending an MBMS retransmission join request 13 and is assigned parameters by the network device which enables the UE when it receives the join grant 14 to transmit.

Claims

1. A method of packet retransmission in a communications system, the system comprising a plurality of user devices and at least one network device, the method comprising broadcasting a multimedia broadcast multicast service transmission to the plurality of user devices; sending a request for retransmission to the network device from one or more of the user devices which failed to receive the complete transmission correctly; and retransmitting requested packets of the original transmission on a different transmission channel to the original broadcast channel.

2. A method according to claim 1, wherein the broadcast channel is an MBMS point to multipoint bearer.

3. A method according to claim 1 or claim 2, wherein the different transmission channel is a different logical, transport or physical channel to the broadcast channel.

4. A method according to any preceding claim, wherein only certain predetermined categories of user device are permitted to request retransmission.

5. A method according to claim 4, wherein the network device allocates user devices to the category of user device permitted to request retransmission, using signal strength and interference data reported by each user device.

6. A method according to claim 5, wherein the network device sets a threshold for data reported by each user device, above which retransmission requests are not enabled.

7. A method according to claim 6, wherein a proportion of the user devices below the threshold set by the network device make retransmission requests on periodically assigned dedicated resources.

8. A method according to any preceding claim, wherein user devices make retransmission requests on contention access resources.

9. A method according to any preceding claim, wherein the retransmissions are broadcast to a group comprising those user devices which have made a request for retransmission.

10. A method according to any of claims 1 to 8, wherein the retransmissions are made to individual user devices using hybrid automatic repeat request.