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The two main coding theorems of classical information
theory determine the fundamental performance limits for data compression
(source coding theorem) and transmission rate of communication (channel
coding theorem). The "separation theorem" is another fundamental
theory stating that the performance of the optimum source-channel coding
scheme can be achieved by optimizing the source coding and the channel coding
separately. Unfortunately, this theorem does not hold any more, by adding the
realistic constraints on, such as bandwidth, power, delay and complexity. And
joint source-channel coding (JSCC) is investigated to attain the optimum
performance. The advances achieved in cross-layer design at the
lower layers (PHY and DLL) can be further enhanced by taking into account
source characteristics and requirements allowing existing networks to provide
optimal time-varying Quality of Service (QoS). This issue is especially
important for video/multimedia applications over wireless or heterogeneous
channels, which are essentially delay-sensitive, bandwidth-intense and
loss-tolerant. We present an adaptive video transmission scheme with
optimal bandwidth allocation over wired and wireless CDMA networks. The
transmission channel is a tandem channel which models both packet erasures
and burst bit errors due to noise, fading and interference. We derive the
statistics of the received signal and a theoretical bound on the block drop
rate at the receiver. Based on these results, an algorithm for optimal
bandwidth allocation among source coding, channel coding and spreading is
derived at the packet level, which incorporates the effects of both the
changing channel characteristics and the source content. Detailed simulations
are done to evaluate the performance of the systems. The sensitivity of the
system to estimation error is also analyzed. Another fundamental tradeoff in the cross-layer design
of a communications system that we study is how to optimize the system when
there is a delay constraint imposed by the applications, i.e., the issue of
delay allocation. The key elements in this tradeoff are the queuing delay in
the source encoder output buffer, and the delay caused by the interleaver,
and the delay caused by iterative channel decoding. We formulate the delay
partitioning problem mathematically, and end up with a relationship among
these three main delay components. In particular, we study how the tradeoff
will be affected by the motion of the video content, the rate of variation of
the channel, the delay budget and the channel bit rate. |
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Progress in the domain of motion compensation will be
vital to achieve higher compression gains for coding of video sequences. And
it will benefit greatly from the development of algorithms for optimal
intra/inter mode switching within a rate-distortion framework. We propose an algorithm to optimally switch between
intra-coding and inter-coding modes, for a video coder that operates on a packet-switched
network with fixed-length packets. Different re-synchronization schemes are
considered and compared. This optimal mode selection algorithm is integrated
with an efficient channel encoder. The system performance is both analyzed
and simulated, where the channel is assumed to have bit errors (due to noise
and fading on the wireless portion of the channel) and packet erasures (due
to congestion on the wired portion). The framework is further extended to
operate on a time varying wireless Internet channel with feedback information
from the receiver. Both instantaneous feedback and delayed feedback are
evaluated, and an improved method of refined distortion estimation for
encoding is presented and evaluated. |
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Multi-carrier (MC) communications uses a form of FDM
technology, by splitting a signal into a number of signals, modulating each
of these new signals over its own frequency channel, multiplexing these
different frequency channels together in an FDM manner. Amongst various MC
radio transmission systems, orthogonal frequency division multiple access
(OFDMA) and MC CDMA are the two most important methods for the cellular
wireless telecommunication industry. Both OFDMA and MC-CDMA use the ideas of
well-known OFDM and CDMA systems, and both of them are taken into account as
parts of the UMTS standard. Moreover, these multiple access techniques are being
studied as the most likely options for the radio interface of future We study the channel estimation algorithms for OFDMA
and MC CDMA systems, which are important in any OFDM systems for demodulation
and decoding. In general, the fading channel of an OFDM system can be viewed
as a two-dimensional (2D) lattice in time and frequency plane, which is
sampled at pilot positions and the channel characteristics between pilots are
estimated by interpolation. The art in designing channel estimators is to
solve this problem with good tradeoff between complexity and performance. In particular, we investigate WiMAX
(Worldwide Interoperability for Microwave Access) systems, which are
OFDMA-based systems standardized by the IEEE 802.16e/D10. The dynamic
resource allocation scheme is presented. And the possible channel estimation
algorithms are proposed and compared. |
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The Multimedia over Coax Alliance (MoCA) technology
will allow a new generation of digital multimedia devices to simultaneously
serve high bandwidth video and broadband data, in conjunction with the
existing analog and digital cable and satellite services currently on the
cable. Our work include to define the
interface specifications for a digital transport system for multimedia
content over coaxial cable, to identify and specify the functional
implementation of the physical layer (PHY), and to partition the MAC and PHY
algorithms between VLSI and firmware groups. |
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Available upon request only. |
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