| With the advancement of broadband access technique and the growing of network available bandwidth, multimedia communication is is developing quickly, in particular video communication. IEEE 802.16, as a new kind of broadband wireless access system, is one of the most attracting technologies between the wireless access systems in the world. Because of the new physical layer technology such as OFDM/OFDMA, MIMO, AMC, AAS, and HARQ, IEEE 802.16 system get some potential international competitiveness in mobile communications market. But on account of the high bit error rates of wireless network and the real time of video services, broadband wireless access systems such as IEEE 802.16 can not provide a satisfactory quality of real-time video service. So a further study is necessary.H.264 is a new video compression standard developed by a Joint Video Team (JVT) which the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) formed. It uses the newest video encoding technology and supports toughness coding and flexible organization forms of encoded data, so H.264 compressed video streams have much better performance on error-resilience. Raw video or encoded video stream can tolerate a certain bit error, and video services prefer packet error rather than packet loss. So it is unreasonable to discard the video frames because of a little bit error, when video services transmit in the wireless channel. But if not discarding the error frames can cause seriously video errors and the decline of video playback quality, and discarding the error frames can lead to automatic retransmission happened, the system throughput reduced and the delay of real-time video increased. So how to balance the video playback quality and system throughput has become an inevitable issue.A scheme using the RTP/UDP-Lite/IP/MAC-Lite protocol stack tries to address this issue, which can enhance throughput of the system. In this system a new MAC-Lite protocol will upload all error packets to the top layer. And the UDP-Lite protocol which has replaced Length field with a Checksum Coverage field can control system fault-tolerance ability. So the system can transmit fault-tolerant video. But the Checksum Coverage field in UDP-Lite protocol only verify whether the computed checksum of a sensitive partis is zero, cannot control the error level of the entire packet. And some serious wrong packets will be uploaded to the application layer, resulting in poor video playback quality and even video decoding failure in the receiver. So this fault-tolerant transmission system requires some expansion and improvement.In this paper, we propose an IEEE 802.16d error-resilient transmission system for H.264 real-time video services. First building an IEEE 802.16d fault-tolerant transmission system, in this system BS and SS transmit H.264 real-time video services by RTP/UDP-Lite/IP protocol stack. Using cross-layer technology, PHY layer provides a packet error instruction to MAC layer in real time. According to the error instruction MAC layer decides whether the error packet will be uploading to the upper layer. It can reduce packet loss rate in MAC layer, save wireless resources and decrease transmission delay. In order to improve video quality, upper layer should repair the error video frames by error control technology. And we design a packet header detection algorithm to improve the system throughput in UDP-Lite and IP protocol. At last we study on the system throughput and video quality in the IEEE 802.16d fault-tolerant transmission system for H.264 real-time video services.The result shows that the system throughput is enhanced by video error-resilient transmission. And if selecting a suitable threshold of packet error level, video playback quality can be guaranteed. |
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