Abstract:
An adaptive and scalable packet error correction apparatus and method in a wireless multicast network is provided. Each retransmission request from a receiver contains a round number and the number of repairs sent in that round. At each receiver, there are two counters for counting the rounds sent out on the network and the number of repairs that have been required. A receiver on the wireless multicast network listens to the ARQ requests sent by other receivers to update the two counters and determines whether its request should be suppressed or be sent out.

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/CN2007/000710, filed Mar. 6, 2007, which was published in accordance with PCT Article 21(2) on Sep. 12, 2008 in English. 
     FIELD OF THE INVENTION 
     This invention relates to data transmission, and particularly relates to an adaptive and scalable packet error correction apparatus and method. 
     BACKGROUND OF THE INVENTION 
     Using wireless network to perform data communication, especially to perform video and/or audio streaming is a prevailing application nowadays. Some wireless systems, for example WLAN, can distribute video streams from a sender to multiple receivers in a multicast area. 
       FIG. 1  is a diagram showing a conventional multicast wireless system. The system is used to transmit video streams from at least one sender  10  to multiple receivers  20 . Video packets are generated by a video server (not shown in  FIG. 1 ) at the sender and are transmitted to the receivers. When the receivers receive the video packets, they decode these video packets. If packet loss occurs in received video packets at a receiver, the decoder at the receiver will not be able to decode the video packets. Thus some error correction methods are needed. Forward Error Correction (FEC) and Automatic Repeat Request (ARQ) are two basic error correction methods. 
     Forward Error Correction (FEC) is a method with some redundant packets being sent to a receiver besides the source data packets, so that the packet loss can be recovered by the redundant packets at the receiver. In a kind of FEC methods, some proactive repairs (redundant packets) are included in a stream of data packets. The proactive repairs are sent before it is known whether the transmission of the repairs is necessary. In FEC, the packet sender introduces N-K repairs for every K source packets to make N packets. When the receiver receives at least K out of the N packets, it can recover all the K source packets. In FEC method, the number of lost source packets has to be compensated by the receipt of at least the same number of repairs in order to recover all of the source packets. 
     If the repairs are insufficient to reliably decode the source data at a receiver, the receiver will make a request for retransmission of additional repairs. This request is called Automatic Repeat Request (ARQ). The ARQs are continuously made until the data packets are decoded successfully. 
     A combination of FEC and ARQ, named hybrid FEC/ARQ method, has been extensively studied in the past decades. An example is shown in “A Study of Proactive Hybrid FEC/ARQ and Scalable Feedback Techniques for Reliable, Real-Time Multicast, which appears in Computer Communication, March 2001, written by Dan Rubenstein et al. The paper discloses a hybrid FEC/ARQ method used in multicast network, in which the sender chooses a proactivity factor to adjust the proactive repairs according to network conditions. If the sender is being heavily inundated with feedback, then it should increase the proactivity factor to add more proactive repairs. If the sender is receiving little feedback, then it should reduce the proactivity factor to reduce the number of the proactive repairs. 
     Each ARQ/NAK ((i.e. Negative Acknowledgement, notifying the sender of a packet loss), ARQ is an equivalent of NAK) from a receiver contains the number of repairs n r  that the receiver requires for transmission, as well as a NAK round, N r . The NAK round indicates the current “round” of the block transmission as perceived by the receiver. When the sender transmits repairs, a NAK round number, N 8 , is contained in its packet, corresponding to the largest NAK round it has received from any receiver for that block. 
     Three ways can be used by the sender to respond to a NAK requesting n r  repairs that has just arrived with round number N r  from the receiver. One of the methods used by the sender to respond to a NAK from a client is called round-current. In this method, the sender maintains a count, n, of the total number of repairs transmitted whose round number equals the current sender round number, N s . For the arriving NAK, if N r &lt;N s , then the NAK is ignored. If N r =N s , max(0, n r -n) packets are transmitted, and n is set to max(n, n r ). If N r &gt;N s , n is set to n r , N s =N r , and n r  packets are transmitted. 
     A receiver&#39;s NAK round number, N r , is initially 1. After sending a NAK, it increments its NAK counter by 1, N r =N r +1, for example. Upon receiving a packet from the sender with a NAK round number larger than or equal to its own current value, the receiver updates its NAK round number to the sender&#39;s current value plus one (if N s ≧N r , let N r =N s +1). 
     However, as shown in the paper, some redundant requests are sent unnecessarily in the above method. For example, when there are two receivers, A and B, in which receiver A has a smaller round trip time than that of receiver B. When both receivers send NAKs with round number  1 , with B requesting n b  repairs and A requesting a smaller number of repairs n a . Assume that the sender gets the request for n a  repairs first, and multicasts out n a  repairs. If some of these repairs are lost on the path to receiver A, then A will request an additional repairs with round number  2 , prior to the arrival (or its knowledge) of the n b -n a  additional repairs being transmitted with round number  1 . Hence, the repairs requested in round  2  are redundant when n b -n a  repairs can recover the remaining lost packets, but because the round number in the NAK is larger, all packets are transmitted. 
     In addition, in this method, all receivers are coordinated by the NAK round number N s  and the repairs information from the sender. If a receiver sends its request with a round number and the number of repairs needed, other receivers won&#39;t know this information until the procedure from the receiver to the sender and then from the sender to them finishes. Since this procedure takes quite a lot of time, some senders may not get the information sent by other receivers timely and may send some redundant requests. 
     SUMMARY OF THE INVENTION 
     In one aspect, a packet error correction method at a receiver on a multicast network is provided. The method includes steps of listening to other receivers to detect retransmission requests sent by other receivers; determining whether the retransmission request at the receiver should be suppressed or be sent out as a function of the cumulation of a parameter of the detected retransmission requests; and sending out the retransmission request if the retransmission request is not suppressed. 
     According to an embodiment, each retransmission request from the receiver contains information showing the number of current round of retransmission request sent out on the network and showing how many repairs the receiver requests in the current round of retransmission request. 
     According to another embodiment, the cumulation of a parameter of the detected retransmission requests is cumulating the number of repairs that have been requested by the detected retransmission requests. 
     In still another embodiment, the receiver further keeps the largest round number of the detected retransmission requests. 
     Advantageously, the receiver adds the number of repairs contained in the information of a detected retransmission request to the accumulation and updates the kept round number at the receiver when the detected retransmission request contains a larger round number than the round number kept at the receiver. 
     In a preferable embodiment, the receiver sends out its retransmission request when the number of repairs the receiver needs is larger than the number of repairs cumulated. 
     In a further embodiment, when the receiver sends out the retransmission request, the round number kept at the receiver is increased by 1 and the increased round number is contained in the retransmission request. 
     In another embodiment, when the receiver sends out the retransmission request, the number of repairs requested is the difference between the number of the accumulated repairs and the number of repairs the receiver needs, and the difference is added to the accumulation. 
     According to still another embodiment, when the receiver detects packet loss, it will wait a delay period before sending a retransmission request out, and the delay period is preferably inversely ratio to the number of repairs that the receiver needs. 
     In another aspect, there describes a receiver, for receiving the data packets from a sender on the multicast network. The receiver includes a retransmission request detector, detecting the retransmission requests sent by other receivers; a retransmission request processor, determining whether an retransmission request should be suppressed or be sent out as a function of the cumulation of a parameter of the detected retransmission requests; and a retransmission request transmitter, sending out the retransmission request if the retransmission request is not suppressed. 
     According to an embodiment, each retransmission request from the receiver contains information showing the number of current round of retransmission request sent out on the network and showing how many repairs the receiver requests in the current round of retransmission request. 
     According to another embodiment, the cumulation of a parameter of the detected retransmission requests is cumulating the number of repairs that have been requested by the detected retransmission requests. 
     According to a further embodiment, the receiver further includes a repairs counting unit, cumulating the number of repairs that have been requested by the detected retransmission requests. 
     In another embodiment, the receiver further includes a round counting unit, keeping the largest round number of the detected retransmission requests. 
     In an embodiment, the round counting unit updates the counting of the number of rounds when the round count kept in the round counting unit is smaller than the round number contained in the detected request. 
     In another embodiment, the receiver sends out its retransmission request when the number of repairs the receiver needs is larger than the number of repairs cumulated. 
     In still another embodiment, when the receiver sends out the retransmission request, the round number kept in the round counting unit is increased by 1 and the increased round number is contained in the retransmission request. 
     Advantageously, when the receiver sends out the retransmission request, the number of repairs requested is the difference between the number of the accumulated repairs and the number of repairs the receiver needs, and this difference is added to the accumulation. 
     In a preferable embodiment, the receiver further includes a delay period timer, timing a delay period which will be waited before the receiver sends out a retransmission request. 
     In a further embodiment, the delay period is inversely ratio to the number of repairs that the receiver needs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will be apparent through the description of a non-limiting embodiment of the invention, which will be illustrated with the help of the accompanying drawings. 
         FIG. 1  is a diagram showing a conventional multicast wireless system; 
         FIG. 2  is a block diagram showing the components of the receiver according to an embodiment. 
         FIG. 3  is a flowchart showing the round number updating procedure at the packet receivers. 
         FIG. 4  is a flow chart showing the process when packet loss occurs during the buffering period. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The exemplifications set out herein illustrated preferred embodiments of the invention, and such exemplifications should not be construed as limiting the scope of the invention in any manner. 
     Herein are some illustrating embodiments to explain the methods, processes and devices dealing with video streaming at packet sender and packet receiver respectively. 
     As known by those skilled in the art, in some real-time video applications, such as on-line movies, for a block of video packets transmitted to a receiver, these video packets are buffered at the receiver for a period of time BT before they are sent to the decoder. During the buffering period BT, if there is no packet loss at the receiver, the video packets can be successfully decoded by the decoder after the buffering period BT ends. Whenever the receiver detects a number of packets lost during the buffering period BT, it will send ARQs to the sender for additional repairs retransmission until the video packets are recovered or until the buffering period expires. 
       FIG. 2  shows a receiver according to the present embodiment. As shown in  FIG. 2 , each receiver  200  includes a packet receiving unit  21 , receiving video packets together with some proactive repairs to recover the packet loss, and receiving retransmitted repairs from the sender when the packet loss can&#39;t be recovered by the proactive repairs, additional repairs being requested by and retransmitted to the receiver  200 ; a packet loss analyzer  22 , analyzing how many packets are lost according to the received packets; an ARQ detector  25 , detecting the ARQs sent by other receivers; a round counting unit  24 , counting the number of rounds that have been launched; a repairs counting unit  26 , counting the number of repairs sent in all rounds launched; an ARQ processor  23 , determining whether an ARQ should be sent out or be suppressed based on the detected ARQs sent by other receivers and the packet loss at the receiver  200 ; an ARQ transmitter  29 , broadcasting the ARQ over the network; a buffering period timer  28 , timing the buffering period BT; a delay period timer  27 , timing the delay period waited before an ARQ is sent out. 
     Now consider  FIG. 3 .  FIG. 3  is a flowchart showing the round number updating procedure at the packet receivers. 
     Each ARQ A i  from a receiver C i  contains the number of repairs that the receiver C i  requires for transmission, n i , as well as an ARQ round number, N i , which indicates the current round of the block transmission. 
     For a block of video packets transmitted to the receiver C i , the buffing period timer  28  first sets the buffering period BT. The method of setting the period BT is well known in the prior art, which will not be described herein. There is an ARQ round counter N i ′ and a repairs counter n i ′ at the receiver C i . The ARQ round counter N i ′ is used to count how many rounds of ARQs have been sent out, and the repairs counter n i ′ is used to count the number of repairs sent in all rounds launched. The round counter is initially set to 0 at the round counting unit  24 , and the repairs counter n i ′ is also initialized to 0 at the repairs counting unit  26 . During the BT period, the receiver C i  keeps listening to the ARQs from other receivers to update the round counter N i ′ and the repairs counter n i ′. For clarity of the illustration, the round counter and the repairs counter updating processes and the packet loss handling process are described separately below. 
     The round counter and the repairs counter updating processes are introduced first. Combine  FIG. 2  with  FIG. 3 . The buffing period timer  28  sets the buffering period BT in step  310 . In step  320 , the ARQ round counter N i ′ is initially set 0 at the round counting unit  24 , and the repairs counter n i ′ is also initialized to 0 at the repairs counting unit  26 . 
     In step  330 , the ARQ detector  25  at the receiver C, listens to other receivers on the wireless network during the buffering period BT. Then it is determined in step  340  whether the receiver C i  has heard any other ARQ A j (N j , n j ) from other receivers. If no (N), the process goes to step  370  to detect whether the BT expires. If it is “Y” in step  340 , it is further determined whether N j  is larger than N i ′ in step  350 . This step is used to determine whether another round of request has been sent out after the round N i ′, and whether the round counter N i ′ and the repairs counter n i ′ at the receiver C i  need to be updated at the round counting unit  24  and the repairs counting unit  26  respectively. If N j  is larger than N i ′, i.e. “Y” in step  350 , the round counting unit  24  at the receiver C i  will update the round counter N i ′ according to N j , and the repairs counting unit  28  sets n i ′=n j +n i ′ in step  360 . That is, another round of ARQ has been sent out, and the total number of the rounds of ARQs launched by all receivers are N j  and the number of the repairs requested by all receivers is n i ′=n j =n i ′ now. If no, the request A j  is ignored, and the process goes to step  370 . Because when N i ′ is larger than or equal to N j , it means that the receiver C j  may have missed some receivers sending their requests before it sends the request A j . So sometimes the packet loss at the receiver C j  may be recovered by the repairs sent by the receivers unheard by receiver C j . To avoid such a case occurring, this kind of requests is ignored at the receiver. And though the request A j  has been sent out by receiver C j , the request may also be ignored at the sender (when N j  is smaller than N i ′). In step  370  the receiver C i  will detect whether the buffering period BT expires. If the buffering period expires, i.e. “Y” in step  370 , it means that the block of video packets needs to be sent to the decoder. So the procedure then ends at the step  380 . Otherwise the process turns to step  330  to continue detecting the ARQs sent on the network. 
     In this way, with the updating process, each receiver can know the situation of ARQs sent by other receivers promptly, knowing how many rounds of ARQs have been performed and how many ARQs have been sent out. 
     While the receiver C i  listens to other receivers to update the round counter N i ′ and the repairs counter n i ′, it also detects whether the packet loss happens to its block of video packets and deals with the packet loss accordingly. 
       FIG. 4  is a flow chart showing the process when packet loss occurs. As shown in  FIG. 4 , the buffering period BT is set in step  400 , and the round counter N i ′ and the repairs counter n i ′ are initialized to 0 in step  410 , which are the same with that shown in step  310  and  320  of  FIG. 3 . During the buffering period BT, it is determined whether some packets are lost in step  420 . If there is no packet loss, i.e. “N” in step  420 , the process goes to step  530 . When the packet loss occurs at the receiver C i , i.e. “Y” in step  420 , the ARQ processor  23  at the receiver C i  will determine the number of repairs needing to be retransmitted, n i . Before sending out the ARQ, the receiver C i  needs to wait a delay period T i . This delay period T i  is set by the delay period timer  27  in step  430 . The delay period T i  is smaller than the buffering period BT and adaptively set according to, preferably inversely ratio to, the number of repairs the receiver needs, n i , so that the ARQ requiring larger number of repairs can be sent out earlier. Thus some redundant requests can be suppressed. Because even if the receivers with less lost video packets haven&#39;t sent out their requests to the sender, the retransmission with larger number of repairs requested by the receivers with more lost packets are enough to recover the lost packets at those receivers with less lost packets. 
     In step  440 , it is determined whether the receiver C i  has received some retransmitted repairs. If “Y”, the receiver C i  will determine how many lost packets have been recovered and update the number of repairs n i . Then it is determined whether the request A i  should be suppressed or not. This decision is made based on the comparison of the number of repairs n i  that the receiver C i  needs with the repairs counter n i ′ at the repairs counting unit  26  in step  460 . If the number of repairs that the receiver C i  needs, n i , is not larger than the lately updated repairs counter n i ′ at the repairs counting unit  26 , i.e. “N” in step  460 , the ARQ processor  23  will suppress its own ARQ A i  in step  490  and wait for repairs retransmission in step  520 , because in such a case, the ARQs sent with the number of repairs requested are enough to recover the packet loss at the receiver C i . So the request A i  sent by receiver C i  is not necessary. In step  470 , it is determined whether the delay period T i  expires. Because during the delay period T i  the repairs counter n i ′ is also kept updated according to the ARQs from other receivers. Whenever the updated repairs counter n i ′ is larger than or equals to the receiver C i  will suppress the ARQ A. The receiver C i  will continue to compare n i  with n i ′ until the delay period T i  expires. 
     If the number of repairs, n i , that the receiver C i  needs is still larger than the parameter n i ′ when the delay period T i  expires, i.e. “Y” in step  470 , the receiver C i  will need to perform a new round of ARQ. Upon the delay period T i  expires, the round counter is increased by 1 at the round counting unit  24  in step  480  and the ARQ A i  with the new round number N i ′ and the number of repairs needed to retransmit by the sender n i -n i ′ is sent out in step  500 . And then the repairs counter n i ′ is updated in step  510 , i.e. set n i ′=n i  at the repairs counting unit  26 . Because if the ARQ A i  is sent out, there will be n i  repairs have been requested. 
     Then the receiver will wait for the retransmission for a period in step  520 . It is determined in step  530  whether the buffering period BT expires. If it is “Y”, the process ends in step  550 . If it is “N”, when the receiver receives the repairs before BT expires, it will determine whether the packet loss can be recovered by the retransmitted repairs and whether another ARQ needs to be sent out in step  540 . If some of the retransmitted repairs are lost during the retransmission, the receiver will decide to perform another round of request and the process returns to step  430  to set a delay period T i  according to the number of packets still needed as described before. If the retransmitted repairs are enough to recover the packet loss at the receiver C i  and the receiver will not need another round of ARQ transmission, i.e. “N” in step  540 , the process ends in step  550 . 
     The process at the sender is performed in round-current way as proposed by Dan Rubenstein et al. The sender maintains a round N s  which is 0 initially, and maintains a count, n, of the total number of repairs transmitted whose round number equals the current sender round number, N s . For the arriving ARQ, if N i &lt;N s , then ignore the ARQ. If N i =N s , transmit max (0, n i -n) packets, and set n=max (n, n i ). If N r &gt;N S , set n=n i , N S =N 1 , and transmit n i  packets. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.