| Summary: | 碩士 === 國立交通大學 === 電信研究所 === 85 === 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.
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