| The unpredictable and time-varying packet losses due to fading, interference, and mobility are the main challenges of video transmissions over wireless IP networks. This data loss can heavily damage the quality of received video. The prediction coding and entropy coding technologies employed in traditional video encoders speed up the propagation of channel errors along spatial and temporal directions in video sequences. This makes it more likely for compressed bit streams to be destroyed by channel errors. Many video decoders have the ability to conceal the slight errors caused by random packet loss. However, ruined video image brought by heavy burst losses are hard to be hided. Thus, it is essential to impose error control mechanism to guarantee the quality of wireless video service.Recent years, the error control schemes based on Automatic Retransmission Request (ARQ) and Forward Error Correction (FEC) are widely used to correct video errors. Although ARQ has the ability of providing a reliable transmission over wireless channels, sorts of delays will be generated. Therefore, it is not advisable to introduce ARQ mechanism to resist the channel errors in wireless video transmission. Wireless multimedia studies have revealed that the FEC mechanism on corrupted packets yields better bandwidth utilization and lower delay than ARQ. FEC is extensively adopted to increase error resilience in multimedia communications. However, FEC mechanism introduces extra bandwidth overhead. Hence, how to balance the utilization of the wireless bandwidth and the QoS of wireless video service has become an inevitable and important issue.Based on above discussion, we propose a novel channel adaptive FEC algorithm. The algorithm aims at ascertaining an optimal amount of FEC redundant packets to coordinate the conflict between the utilization ratio of wireless channel and the QoS of video transmission. In terms of the coding relationship amoung I frame, P frame and B frame, we develop an predictive model of the playable frame rate for MPEG-4 stream with the constraint of maximum sending rate. This predictive model can predict the playable frame rate of video recoverd by receiving terminal. With the predictive model of the playable frame rate, the video sender of wireless video is able to predict the playback quality of FEC–protected video stream which are transmited over wireless channel. In addition, we derived the concept of effective utilization ratio of FEC. According to the relationship between the number of FEC redundant packets and the wireless channel packet loss probability, we derive another predictive model of the effective utilization ratio of FEC which reflects the utilization of wireless bandwidth. Based on these predictive models, first we predict the quality of FEC–protected video stream and the effective utilization ratio of FEC in different packet loss probabilities firstly. Then we calculate and decide the most appropriate amount of FEC redundant packets which makes the quality of video stream and the effective utilization ratio of FEC approximate the maximum.Based on the theoretical analysis, a WLAN environment is built up in NS2 environment, the proposed channel adaptive FEC algorithm is applied in the MAC layer of wireless terminal. We design a novel mechanism to track the state of wireless network, which will detect the loss probability of wireless channels in the MAC layer of wireless terminal. In an unicast transmission, the FEC redundancy is usually added adaptively on the basis of the feedback of loss pattern. Considering the feedback will introduce much more additional delay, we use a network estimator at the sender to estimate the wireless channel conditions based on the number of ACK packets. This allows the proposed adaptive algorithm to adapt the operating parameters to the observed network conditions timely. Since the proposed adaptive FEC algorithm takes one GOP as object of study, the packet loss probability is calculated after all the video packets in one GOP are transmitted by wireless terminal.FEC protection is added into video stream according to outputs of the analytical models. Compared with the feedback pattern, this method is more accurate and with lower delay.The simulation results showed that the proposed channel adaptive FEC algorithm provides a better QoS guarantee for wireless video transmission than the algorithms which only consider the utilization ratio of FEC. Moreover, our algorithm achieves a much better effective utilization ratio of FEC than the algorithm which only considers the playable frame rate. It means that our algorithm can evidently improves the utilization ratio of wireless bandwidth. According to current network state, the proposed channel adaptive FEC algorithm has ability to adapt the amount of FEC packets to packet loss probability, and adopts unequal error protection for the video data in terms of the different significance of I, P and B frame. Although our algorithm engenders some delay, it is much shorter than the delay tolerance of video service. Hence, the delay caused by our algorithm will not impair the service quality of wireless video. It is undeniable that the collision of multi-wireless terminals has exerted a negative impact on the proposed channel adaptive FEC algorithm, however, such an undesirable consequence seems trivial thus difficult to be perceived by clients. Unfortunately, our algorithm generates some inevitable delay jitter which can not be ignored. Such demerit of the proposed channel adaptive FEC algorithm requires to be optimized.In the end, we summarize the major contributions of this paper, and put forward further improved scheme. It is essential for us to minimize the delay jitter cased by the proposed channel adaptive FEC algorithm. In addition, our algorithm only analyze the technology of motion estimation and motion compensation which is one of MPEG-4 kernel technologies. If we can detect the playback quality of MPEG-4 video steams in terms of more than one kernel technologies, the clients would obtain much better service quality of wireless video. |
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