基於第二代動態影像壓縮技術之無線多碼是分碼多工非同步傳輸網路架構設計

Abstract

本論文的研究內容著重在建立基於第二代動態影像壓縮(MPEG2)技術之無 線展頻分碼多工非同步傳輸(ATM)網路系統。MPEG-2視訊服務所要求的位 元速率極大(64kbps~6Mbps)而且還是時變的(VBR,variable bit rate)。 因此此一特性極不利於在有限頻寬的無線網路上進行視訊多人擷取與傳送 。為了解決以上的困難本論文採用多碼式分碼多工展頻通訊系統。 In this paper, we present an ATM-based multi-code CDMA transport architecture for theMotion Picture Experts Group phase 2 (MPEG 2 ) compressed video servicesover a bandlimited mobile channel. Such services allow users to share novel MPEG2 videoapplications without any geographical restrictions. The ATM technique is especially well suited for variable bit rate (VBR) MPEG2 video because of its ability to allocate bandwidth on demand to these services. However, since the mobile radio has a limited channel capacity, the overall capacity of the traditional ATM-based SS-CDMA system may not be sufficient to accommodate the MPEG2 video services requested by the multiple mobile users simultaneously.To tackle this difficulty, a multi- code CDMA technique is proposed to provide VBR MPEG2 video services by varying the number of spreading codes assigned to each MPEG in order to meet its dynamic throughput requirement. Meanwhile, these extra spreading codes would also create the interference to MPEG videos and degrade their picture quality. Hence, a cost-effectivedynamic spreading code assignment mechanism has then been proposed to dynamically assign appropriate number of codes to each MPEG for achieving both the maximum resource utilizationand the best picture quality for multiple MPEG2 video services via the mobile radio channel. However, both the fading and interference on SS-CDMA radio channels tend to cause significant transmission error and MPEG2 bit streams are very vulnerable to these errors. Powerful forward error correction(FEC) codes are therefore necessary to protect the video dataso that it can be successfully transmitted at acceptable signal power levels. Two separate FEC code schemes are applied to the header and payload of a ATM cell containing MPEG2 video data, respectively. The ATM cell header is protected by a relatively powerful FEC code to ensure correct delivery and low cell loss rate (CLR). On the other hand, the ATM cell payload is encoded for varying degrees of error protection according to the priority of the payload data in MPEG2 videos. An adaptive FEC code combining scheme is proposed to provide the good protection for payload data with the maximizationof its code rate in order to minimize the extra bandwidth for FEC overhead. Simulation results would show that our FEC code combining scheme is able to ensure the average number of ATM cell losses in each typical MPEG2 picture frame be in the order of O(1). Finally, both the multi-resolution multi-directional interpolation and iterative pattern matching methods have been proposed to conceal such cell losses by taking accountof spatial and temporal correlation in the MPEG2 video sequence since the impact of these cell losses on decoded video quality can be quite serious for MPEG video. Two typical MPEG2 test sequences are conducted to evaluate the effectiveness of our system.