This work introduces a multiple-description product code which aims at optimally generating multiple, equally-important wavelet image descriptions from an image encoded by the popular SPIHT image coder. Because the SPIHT image coder is highly sensitive to errors, forward error correction is used to protect the image against bit errors occurring in the channel. The error-correction code is a concatenated channel code including a row (outer) code based on RCPC codes with CRC error detection and a source-channel column (inner) code consisting of the scalable SPIHT image coder and an optimized array of unequal protection Reed-Solomon erasure-correction codes. By matching the unequal protection codes to the embedded source bitstream using our simple, fast optimizer, we maximize expected image quality and provide for graceful degradation of the received image during fades.To achieve unequal protection, each packet is split into many Reed-Solomon symbols. The i th symbol in each packet forms an (n, k) Reed-Solomon code or "column". A fast, nearly-optimal optimizer, based on Lagrange multipliers and optimal to within convex hull and discretization approximations, chooses k for each Reed-Solomon "column" to minimize the expected mean-squared error at the receiver.We validated our use of this structure by evaluating its performance in the context of transmitting images over a wireless fading channel. The performance of this scheme was evaluated by simulating the transmission of the Lena image over a Clarke flat-fading channel with an average SNR of 10 dB and a normalized Doppler frequency of 10 −5 Hz. Our implementation of the Sherwood and Zeger "UEP2" unequalprotection code published at ICIP 1998 achieves a mean PSNR of 27.75 dB (28.36 dB peak) at a bit rate of 0.237 bpp. Using the same channel and packet-loss data, the RCPC/CRC+MDFEC scheme using the efficient Lagrange optimizer achieves an expected PSNR of 27.90 dB (28.84 dB peak) By substituting a rate 2/3 RCPC code for Sherwood and Zeger's rate 1/2 code, we can achieve an expected PSNR at the receiver of 28.38 dB (29.75 dB peak), a 0.63 dB improvement over the "UEP2" code.In addition to its high performance compared to other techniques for sending images over wireless channels, this packetization scheme and optimizer is also ideally suited to hybrid packet-network and wireless channels. Because the optimization is based only on an end-to-end packet loss distribution, such a hybrid network can be evaluated easily without considering which exact packets arrived intact. And since all packets are equally important, no concept of packet priority is required.The complete paper can be downloaded from http://www.ifp.uiuc.edu/~sachs.

In this paper, we address the problem of real-time video streaming over wireless LANs for both unicast and multicast transmission. The wireless channel is modeled as a packet-erasure channel at the IP level. For the unicast scenario, we describe a novel hybrid Automatic Repeat reQuest (ARQ) algorithm that efficiently combines forward error control (FEC) coding with the ARQ protocol. For the multiple-users scenario, we formulate the problem of real-time video multicast as an optimization of a maximum regret cost function across the multicast user space. The proposed solution efficiently combines progressive source coding with FEC coding. We present a theoretical analysis of the unicast and multicast cases, as well as experimental results that demonstrate the performance advantages of the proposed algorithms over existing methods.

Abstract:Energy efficiency has become a primary design criterion for mobile multimedia devices. Prior work has proposed saving energy through coordinated adaptation in multiple system layers, in response to changing application demands and system resources. The scope and frequency of adaptation pose a fundamental conflict in such systems. The Illinois GRACE project addresses this conflict through a hierarchical solution which combines (1) infrequent (expensive) global adaptation that optimizes energy for all applications in the system and (2) frequent (cheap) per-application (or per-app) adaptation that optimizes for a single application at a time. This paper demonstrates the benefits of the hierarchical adaptation through a second-generation prototype, GRACE-2. Specifically, it shows that in a network bandwidth constrained environment, per-app application adaptation yields significant energy benefits over and above global adaptation.

Traditionally, video encoders have been designed assuming that the more redundancy is removed, the better the encoder. However, on current laptops, reducing the compression efficiency of the video encoder by reducing the number of instructions used to perform compression can actually reduce the total energy used to encode and transmit a sequence. The correct balance between computation and compression efficiency may change dynamically, motivating adaptive encoders. At the same time, recent generalpurpose processors also employ energy-driven adaptations. For best gains, the adaptations in the hardware and application layers must be coordinated. From a system design viewpoint, this coordination must happen through minimal, well-defined interfaces.This paper develops (I) an adaptive video encoder for generalpurpose processors that trades computational complexity for compression efficiency to minimize total system energy, and (2) a method for determining the best configuration for such an encoder when running on a processor that is also adaptive. Our adaptive processor employs recent energy saving techniques of dynamic voltage and frequency scaling and architectural adaptation. Using a detailed simulator, we show that our cross-layer adaptive application algorithm reduces energy significantly, when employed on a fixed or adaptive processor.