John G. Apostolopoulos

Divert: fine-grained path selection for wireless LANs

The performance of Wireless Local Area Networks (WLANs)often suffers from link-layer frame losses caused by noise, interference, multipath, attenuation, and user mobility. We observe that frame losses often occur in bursts and that three of the five main causes of frame losses -- multipath, attenuation, mobility--depends on the transmission path traversed between an access point (AP)and a client station.In a typical WLAN deployment, different transmission paths to a client exist in places where overlapping coverage is provided by a set of neighboring APs. Using experimental measurements and analysis on a 802.11b testbed, we show that fine-grained path selection among a set of neighboring APs can significantly reduce path-dependent losses in WLANs.We design and implement a WLAN distribution system called Divert, which supports fine-grained path selection for downlink communications, on an 802.11b testbed. Divert reduces frame losses without consuming any extra bandwidth in the wireless medium. Our experimental results show that Divert can reduce frame loss rates in realistic scenarios by as much as 26% compared to a fixed-path scheme that uses the best available transmitter.

Low-latency wireless video over 802.11 networks using path diversity

Wireless local area networks, such as 802.11b, are becoming wide-spread as they provide simple wireless connectivity and data delivery. This paper examines low-latency (conversational) video communication over 802.11b networks. The challenges to enable low-latency video include overcoming the highly variable delays, losses, and bandwidth of 802.11b wireless networks. To overcome these challenges we (1) employ the H.264/MPEG-4 advanced video coding (AVC) standard for high video compression efficiency and good resilience to losses, (2) use low-latency best-effort transport mechanisms, and (3) exploit the potential path diversity between each mobile client and multiple access points in the infrastructure, where we use multiple paths simultaneously or switch between multiple paths (site selection) as a function of channel characteristics. Our results indicate that the proposed system can provide significant benefits over conventional single access point (single path) systems.

Critically sampled wavelet representations for multidimensional signals with arbitrary regions of support

Transform/subband representations form an important element of many signal processing algorithms and applications. Until recently, representations have typically been designed for signals with convenient supports, e.g. 2-D signals with rectangular supports. However, a number of applications require representations for signals with arbitrary (non-rectangular) regions of support. We present a novel algorithm for creating critically sampled perfect reconstruction wavelet representations for signals defined over arbitrary supports. The proposed algorithm selects a subset of vectors from a convenient superset basis which under appropriate conditions provides a basis over the given arbitrary support. The algorithm can be interpreted as solving a corresponding sampling problem.

Position-dependent encoding

In typical MC-DCT compression algorithms, up to 90% of the available bit rate is used to encode the location and amplitude information of the nonzero quantized DCT coefficients. Therefore, efficient encoding of the location and amplitude information is extremely important for high quality video compression. In this paper we describe the position-dependent encoding (PDE) approach for encoding the DCT coefficients. This method attempts to exploit the inherent differences in statistical properties of the run-lengths and amplitudes as a function of position. This paper includes preliminary comparisons between the PDE scheme and conventional coding approaches. The PDE approach can be applied in any transform/subband filtering scheme for image or video compression.<<ETX>>

Coding of arbitrarily shaped regions

Region-based coding schemes may yield considerable improvements in performance as compared to block-based schemes. A fundamental problem in region-based coding is to efficiently encode the interior of each region. This paper proposes two approaches for coding the interiors of arbitrarily-shaped regions. The first is an adaptive iterative scheme and the second is a matching pursuits-type scheme. A geometric interpretation of the problem is given to provide insight into these approaches and to compare their different properties and performances. A number of examples illustrate the performance of the previous and proposed approaches.

Representing arbitrarily-shaped regions: a case study of overcomplete representations

Efficiently representing the interior of an arbitrarily-shaped region is an important problem in many applications, including object- or region-based image/video compression. This paper focuses on representing the interior as a linear combination of vectors defined on a superset basis. Two seemingly different, though highly related, problem formulations are given and a number of approaches that result are discussed and analyzed. A geometric interpretation of the problem is given to provide insight into the approaches and examine their properties. The incorporation of quantization is also briefly discussed. In addition, the chosen set of vectors corresponds to a special/structured class of overcomplete representations with O(N log/sub 2/ N)-type processing, low memory requirements, and other important properties.

Transform/subband representations for signals with arbitrarily shaped regions of support

Transform/subband representations form a basic building block for many signal processing algorithms and applications. Most of the effort has focused on developing representations for infinite-length signals, with simple extensions to finite-length 1-D and rectangular support 2-D signals. However, many signals may have arbitrary length or arbitrarily shaped (AS) regions of support (ROS). We present a novel framework for creating critically sampled perfect reconstruction transform/subband representations for AS signals. Our method selects an appropriate subset of vectors from an (easily obtained) basis for a larger (superset) signal space, in order to form a basis for the AS signal. In particular, we have developed a number of promising wavelet representations for arbitrary-length l-D signals and AS 2-D/M-D signals that provide high performance with low complexity.

Designing a video compression system for high definition television

There has been significant interest in the design of a digital television system for terrestrial HDTV broadcasting. The authors discuss various issues related to the design of a HDTV system. They examine the design of a video compression system for the efficient delivery of digital television signals. A number of considerations that arise when designing the video compression system are discussed, and a sample system which meets most of these requirements is presented. It is shown that the video compression principles and system issues can be integrated into a framework for designing a high-performance digital HDTV system.<<ETX>>

Video Compression for Digital Advanced Television Systems

Television began in 1941 with the adoption of a monochrome television standard by the National Television Systems Committee (NTSC) in the U.S. A fully compatible color television signal that fits within the same monochrome channel was adopted in 1953. Originally, television was perceived simply as a form of entertainment. Today, television in its many forms is also used for business, medicine, science, education, the military, and multi-media applications. These are a multitude of applications, with many more to come. With the various applications arise different sets of requirements. For example, accurately perceiving the beating of a human heart or the movements of a gymnast in the Olympics requires high temporal resolution, while examining museum pieces in an art class requires high spatial resolution. The advanced video systems that fulfill these requirements are called Advanced Television (ATV) Systems.

Path Diversity for Media Streaming: The Use of Multiple Description Coding

Publisher Summary This chapter provides an overview of path diversity of complementary media coding techniques, such as multiple description coding, and of their benefits and uses for improved media streaming. Path diversity is a transmission technique that sends data through two or more paths in a packet-based network. A path diversity system may use multiple paths at the same time or may perform path selection to switch among them. This is in contrast to the conventional approach where all packets are sent over a single path between sender and receiver, and this path does not vary with time under the direct or indirect control of the application. The paths may originate from single or multiple sources. This chapter provides a survey of the benefits, architectures, system design issues, and open problems associated with streaming media delivery using path diversity. The complementary media-coding techniques, such as multiple description coding, are also reviewed in the chapter.