WO2010030477A1

WO2010030477A1 - Retransmission technique for a broadcast communication network

Info

Publication number: WO2010030477A1
Application number: PCT/US2009/054386
Authority: WO
Inventors: Naveen Arulselvan; Suresh Kalyanasundaram
Original assignee: Motorola, Inc.
Priority date: 2008-09-10
Filing date: 2009-08-20
Publication date: 2010-03-18

Abstract

A system and method for retransmitting data in a broadcast communication network includes a first step (300) of establishing packet transmissions arranged into transmission and retransmission intervals, and a repair-packet generation rule wherein repair packets are derived from packet data of the transmission interval XOR'ed in a permuted sequence. A next step (302) includes broadcasting the packet data in a sequence of N packets in the transmission interval. A next step (304) includes sending R repair packets in the retransmission interval. A next step (306) includes receiving the broadcast and repair packets, with a broadcast packet received in error. A next step (308) includes recovering the error packet from one of the repair packets.

Description

RETRANSMISSION TECHNIQUE FOR A BROADCAST COMMUNICATION NETWORK

FIELD OF THE INVENTION

The present invention relates generally to wireless communication networks, and in particular, a network and method to provide a retransmission of data in a communication network.

BACKGROUND OF THE INVENTION

Multimedia and group communications are becoming more important aspects of telecommunication networks, and the demand for such services will continue to increase. For instance, there are presently many different systems and networks driving 3GPP/3GPP2/IEEE to provide group communication and efficient broadcast support in a growing popularity of applications in next generation wireless access network such as LTE, UMB, HSDPA, DO-A, and 802.16x, etc. Mobile broadcast services such as mobile television, live sports/ news-casting are expected to be popular in 4G networks

Accordingly, a suite of protocols has been developed for use in broadcast communications. These protocols are used to control multimedia broadcast and multicast service (MBMS) communication sessions that include data streams such as audio (voice), video, text messaging, and internet protocols, for example between, or to, user equipment (also referred to as subscriber stations or mobile stations) in a communication network.

A broadcast group communication has the efficiency of delivering one set of informational streams to multiple UEs depending upon the broadcast radio link characteristics and capabilities of the UEs in the call. This makes MBMS very efficient on the downlink. Users subscribed to a broadcast service, may be distributed throughout the cell and as a result experience varying channel conditions. Conservative broadcast coding schemes which cater to the users with worst channel condition will invariably affect the overall system throughput. For example, an evolved Node B (eNB) or base station, knowing the minimum capabilities of the user equipment (UE) of mobile nodes it serves, can broadcast a current operating modulation and coding scheme (MCS) to the UEs in its cell directing the UEs to use that MCS to decode the MBMS service provided by the eNB. However, the MBMS system has limited bandwidth available for feedback information from all of the UEs, i.e. only a few dedicated uplink feedback control channels are provided, which can not accommodate feedback from all the UEs.

Retransmission schemes have been suggested to improve reliability in broadcast transmissions. However, these schemes rely on uplink feedback and have various implementation issues. One issue is the provisioning of an uplink feedback channel in broadcast settings which enables users to send information about the packet delivery status. However, this technique suffers from the lack of uplink bandwidth as previously described. Another issue is devising a strategy to minimize feedback overhead while collecting information representative of all users in the cell. This also suffers from limited feedback bandwidth. Therefore, a need exists to provide a retransmission technique for broadcast data stream delivery in a communication network to enable efficient error correction for a call. It would also be of benefit to provide this error correction without using any uplink resources.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims. However, other features of the invention will become more apparent and the invention will be best understood by referring to the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a simplified block diagram of a network, in accordance with the present invention;

FIG. 2 is a chart illustrating potential error correction, in accordance with the present invention;

FIG. 3 illustrates a method, in accordance with the present invention;

FIG. 4 shows a graph of an improvement provided by the present invention; and

FIG. 5 shows another graph of an improvement provided by the present invention.

Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a system and method for providing a retransmission technique for broadcast data stream delivery in a communication network to enable efficient error correction for a call. The present invention provides this error correction benefit without using any uplink resources. In particular, the present invention provides a retransmission scheme based on network-coding for MBMS services that enhances the transmission reliability. The evolved NodeB (eNodeB or eNB) transmits N data packets at a time over the broadcast channel, followed by R repair packets in a repair interval. The repair interval consists of R repair slots, which can be slots in time and/or frequency. In every repair slot, the eNodeB transmits a pre-determined combination of the TV packets transmitted earlier. Each of the TV packets goes out exactly once in the repair interval. Users can correct their erroneous packets in a memory-less manner using these repair packets, as will be detailed below.

The wireless access network is designed to support the delivery of a broadcast stream to a group of user equipment (UE) nodes through a serving base station. Preferably, this is accomplished in a secure manner supporting confidentiality, authentication, and integrity of the multicast data stream. Although described herein in a 4G Long Term Evolution (LTE) embodiment, it should be noted that the present invention is also applicable to other communication technologies including, but not limited to, High Speed Downlink Packet Access (HSDPA) and WiMAX. It is possible to implement the present invention in a separate entity, or as part of a Radio Network Controller (RNC), or a base station (eNB) as is described for the example herein.

FIG. 1 illustrates a multicast communication used as an example of the present invention described herein. An evolved Node B (eNB) 106 or base station serves a plurality of various user equipment (UE) 100, 102, 104 in the cell site and sends broadcast communications 108 to the UEs. The base station 106 can also send a group management message 110 containing data transfer parameters and rules to the UEs such as to make the UEs aware of the network coding being used in the broadcast. The network may not have enough dedicated uplink resources, if at all, to receive feedback from all of the UEs 100, 102, 104.

The present invention provides a technique which works effectively in the absence of an uplink feedback channel or any uplink feedback. In particular, the present invention is based on network coding/fountain coding and exploits the fact that error packets tend to be distributed among the various cell UEs. The network coding includes systematic codes in which the original packets are transmitted followed by repair packets which are pre-determined combinations of the packets already transmitted, in accordance with the present invention.

In operation, if UE A receives packet Pj in error and P₂ correctly, while UE B receives packet P₂ in error and packet P₁ correctly, transmitting one repair packet of Pi XOR'ed with P₂ will enable both users to decode their respective error packets correctly. For example, eNodeB can transmit JV packets given by Q= (Qi, Q₂, ...., Q_NJ- After the TV packets are transmitted, there is a repair interval of R packets (for some value of R less than N). A pre-determined fixed permutation rule (PR) maps the original sequence Q to P where the original data packets are permuted and then XOR'ed. The TV-packet permuted sequence P is sent out by the eNodeB as follows:

P ® P₂ @ ....@ P_N . _ ₄

¹ — as the first repair packet,

^PM , ^PM ₂ ^P2K as the second packet and so on till,

R R R

p (R-UN₊₁ ^{@ f}^-iw₊₂ ® ^{Φ p}N in the R^th repair packet

R R

The present invention offers several advantages; a) decoding at the UE is memory-less, which results in greatly reduced complexity on the UE side, as there is no need to buffer repair packets, b) there is low over-the-air overhead, where parameters such as the number of transmitted packets N, repair interval packets R, and the permutation rule PR can be transmitted just once over the broadcast control channel (BCCH), c) like in any systematic code, good users do not have to wait till the last encoded packet and can decode quicker, d) provides significant performance improvement compared to schemes with no network coding, e) scores higher over a conventional code's performance when file sizes are small or when streaming applications demand a short playback buffer, and f) can be flexibly implemented in the eNodeB.

The performance of the above technique degrades slightly in case error rates are very high or the retransmission interval is tight. Therefore, a variation to the basic scheme is suggested then, wherein d (a random number of) packets are dropped from the original JV packets for the repair interval transmissions. The truncated packet sequence Q^-_d is permuted as before to obtain the retransmission sequence P^-_d- This is transmitted in R repair slots, (N-d)/R packets at a time.

Referring to FIG. 2, a chart is shown demonstrating errors that are correctable by the present invention. Given ten UEs to which one hundred packets are to be sent, the repair interval includes five repair slots. For ease of illustration, identity permutation was applied. The eNodeB XOR' s groups of packet numbers 1-20, 21-40, 41-60, 61-80, 81-100 together into five repair packets respectively, which are sent in the repair interval. A user can recover a packet received in error from one of the repair slots by knowing from the permutation rule that defines which slot contains the packet received in error. For example, UE A has received the eighteenth packet (out of one hundred sent in the transmission interval) in error. UE A knows that there are five repair slots in the repair interval, and that the eighteenth packet was XOR'ed in the first repair packet from the permutation rule. UE A can then XOR the correctly received packets 1-17 and 19-20 with that first repair packet to recover the correct eighteenth packet, as long as no other packets used in that repair packet were received in error by the receiver. Therefore, UE A will not be able to recover packets 23 and 26 received in error by using the second repair packet. As a result, UE A will only be able to correct three out of its five received error packets, as shown.

The right-hand column shows the number of errors that can be corrected among the total number of errors reported by all the users. If two or more packets in the same group are in error, they cannot be corrected. This example shows that for the forty- five received in error, the present invention will correct twenty errors in all using only five repair slots. FIG. 3 illustrates a method for retransmitting data in a broadcast communication network, in accordance with the present invention, which includes a first step 300 of establishing parameters for packet transmissions arranged into a sequence of N packets in a transmission interval followed a sequence of R repair packets in a retransmission interval, where R is less than N. The parameters including a permutation rule wherein repair packets are derived from the N packets by dividing the N packets into R groups and XOR'ing (i.e. modulo-2 adding) together the packets in each group to define respective repair packets in the retransmission interval. Each repair packet is assigned to respective packet slots in the retransmission interval. The number of original data packets of the transmission interval permuted in each repair packet of the retransmission interval is N/R.

In particular, the repair packets define a permuted sequence of the original data packets in the retransmission interval, where a permuted sequence in a first packet of the retransmission interval is P © P₂ © ....© P^

R

a permuted sequence in a second packet of the retransmission interval is

Φ

K +1 K ^■+2 R R R and a permuted sequence in the R^th packet of the retransmission interval is

P ® p Θ Θ P

( R-V) N ( R-V) N N

R ⁺ R ⁺

Preferably, the permutation rule is fixed for the network. A next step (302) includes broadcasting the TV packets to a plurality of receivers in the transmission interval.

A next step (304) includes sending (i.e. broadcasting) the R repair packets to the plurality of receivers in the retransmission interval after the TV packets have been broadcast. In one option, a random linear combination of N/R packets can be sent, where the N/R packets are selected from the original /Vpackets for each retransmission. A random linear combination of these N/R packets are sent over the broadcast channel. In this case, an individual user may need to wait until all retransmission packets are received before starting to decode and perform a matrix inversion, which increases the UE-side complexity and header overhead.

In another optional embodiment, the broadcasting step includes broadcasting a truncated packet sequence ofN-d packets, where a random number of packets, d, are dropped from the original /Vpackets, and wherein the establishing step includes permuting the truncated packet sequence Q^_d as before to obtain the retransmission sequence iV_ώ which is transmitted in R repair packets, (N-d)/R transmitted packets in each repair packet. In this embodiment, UE complexity is still low, but header size of the network coded packet increases.

A next step (306) includes receiving the broadcast packets and repair packets by the receivers, wherein one or more of the broadcast packets may be received in error.

A next step (308) includes recovering the error packet from one of the repair packets by associating the position of the error packet in the transmission interval with the respective position of that packet in the repair packet group in the retransmission interval, and XOR'ing the correctly received packets in that group with that repair packet, as long as no other packets used in the repair packet of that group were received in error by the receiver.

Example To illustrate the improved performance provided by the present invention, a computer simulation was performed using an arbitrary permutation rule. Referring to FIG. 4, the simulation results show that up to 80% of transmission errors can be corrected using the memoryless technique of the present invention, as compared to only 20% correction using a traditional, no network coding approach. Note that the scheme works equally well when users have differing packet error rates. Performance degrades slightly when the error rates are very high or retransmission interval is tight. In this event, the truncated sequence version or the random linear code version can be used, as described above and as illustrated in FIG. 5.

It should be recognized that the diagrams herein are simplified for purposes of illustrating the present invention. However, those of ordinary skill in the art will realize that many other network entities and processes may be part of the communication system, which have not been shown for the sake of simplicity. For example, the present invention can be incorporated in one or more of a radio network controller, base station (eNB), session controller, a group database manager, a registration manager, an application layer router, a group entity manager, a broadcast and unicast address manager, a policy manager, a flow controller, a media manager, and a bandwidth manager, among others, all of which are known in the art. It should be appreciated that the above described entities can be integrated in the same physical or logical network element or provided as distributed or individual physical or logical network elements.

The present invention can be implemented in the eNB, and UEs should be able to distinguish between the original packets and repair packets which are network- coded. Legacy UEs will simply ignore the repair packets. It would also be of benefit for the eNodeB to transmit network coding parameters: transmission interval, repair interval, and permutation sequence. This ensures that the header overhead in repair packets is kept to a minimum.

Advantageously, the present invention is effective in the absence of uplink feedback (e.g. ACK/NACK) and provides packet decoding without using memory. In addition, the present invention provides low UE complexity: packets can be corrected after each and every retransmission slot. Further the present invention provides low overhead: the permutation sequence PR can be fixed and conveyed to the users statically. Over-the-air header length is low as a result. The permutation sequence can be chosen such that burst errors can be corrected.

It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions by persons skilled in the field of the invention as set forth above except where specific meanings have otherwise been set forth herein. The sequences and methods shown and described herein can be carried out in a different order than those described. The particular sequences, functions, and operations depicted in the drawings are merely illustrative of one or more embodiments of the invention, and other implementations will be apparent to those of ordinary skill in the art. The drawings are intended to illustrate various implementations of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. Any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

Claims

CLAIMSWhat is claimed is:

1. A method for retransmitting data in a broadcast communication network, the method comprising the steps of: establishing parameters for packet transmissions arranged into a transmission interval followed by a retransmission interval, the parameters including a permutation rule wherein repair packets are derived from packet data to be broadcast by permuting the data packets and XOR' ing them into a permuted sequence; broadcasting the packet data to a plurality of receivers in a sequence of TV packets in the transmission interval; sending the repair packets to the plurality of receivers in R repair packet slots in the retransmission interval after the TV packets have been broadcast; receiving the broadcast packets and R repair packets by the receivers, wherein one of the broadcast packets is received in error; and recovering the error packet from one of the repair packets.

2. The method of claim 1, wherein the sending and receiving steps include a different permuted sequence in each repair packet of the retransmission interval.

3. The method of claim 2, wherein the permuted sequence of an R^th repair packet in the retransmission interval is

P ® p Θ Θ P r/?-i)N _j (R-UN ₂ N

R ⁺ R ⁺

4. The method of claim 2, wherein a permuted sequence of a first packet of the retransmission interval is

P @P, >P,

a permuted sequence of a second packet of the retransmission interval is

P ®p Θ Θp

and a permuted sequence of an R^th packet of the retransmission interval is

P Θ p Θ Θp

(R-X)N (R-UN

^■+i ^■+2 N R R

5. The method of claim 1 , wherein the establishing step establishes a fixed permutation rule for the network.

6. The method of claim 1 , wherein the number of packets permuted in each repair packet of the retransmission interval is N/R and a random linear combination of the repair packets are sent in the sending step.

7. The method of claim 1, wherein the establishing step includes permuting a truncated packet sequence oiN-d packets, where a random number of packets, d, are dropped from the original TV packets and XOR' ing the truncated packet sequence to obtain the retransmission sequence P^-_d, which is transmitted in R repair packets, (N- d)/R transmitted packets in each repair packet.

8. A method for retransmitting data in a broadcast communication network, the method comprising the steps of: establishing parameters for packet transmissions arranged into a sequence of TV packets in a transmission interval followed by a sequence of R repair packets in a retransmission interval, the parameters including a permutation rule wherein the R repair packets are derived from the TV packets by dividing the TV packets into R groups and XOR'ing together the packets in each group to define respective repair packets in the retransmission interval; broadcasting the TV packets to a plurality of receivers in the transmission interval; sending the R repair packets to the plurality of receivers in the retransmission interval after the TV packets have been broadcast; receiving the broadcast packets and repair packets by the receivers, wherein one of the broadcast packets is received in error; and recovering the error packet from one of the repair packets of the permuted sequence by associating the position of the error packet in the transmission interval with the respective position of that packet in the repair packet group in the retransmission interval, and XOR'ing the correctly received packets in that group with that repair packet.

9. The method of claim 8, wherein the recovering step includes recovering the error packet as long as no other packets used in the transmitted packet group of that repair packet were received in error by the receiver.

10. A system for retransmitting data in a broadcast communication network, the system comprising: a network controller that is operable to establish parameters for packet transmissions arranged into a transmission interval followed by a retransmission interval, the parameters including a permutation rule wherein repair packets are derived from packet data XOR'ed in a permuted sequence; a base station that is operable to broadcast the packet data to a plurality of receivers in a sequence of TV packets in the transmission interval and send the permuted sequence of R repair packets to the plurality of receivers in the retransmission interval after the TV packets have been broadcast; and user equipment that is operable to receive the broadcast packets and R repair packets by the receivers, wherein one of the broadcast packets is received in error, and recover the error packet from one of the repair packets.