Distributed video coding for wireless video sensor networks: a review of the state-of-the-art architectures <s> Fig. 1 WVSN architecture <s> With the introduction of the H.264/AVC video coding standard, significant improvements have recently been demonstrated in video compression capability. The Joint Video Team of the ITU-T VCEG and the ISO/IEC MPEG has now also standardized a Scalable Video Coding (SVC) extension of the H.264/AVC standard. SVC enables the transmission and decoding of partial bit streams to provide video services with lower temporal or spatial resolutions or reduced fidelity while retaining a reconstruction quality that is high relative to the rate of the partial bit streams. Hence, SVC provides functionalities such as graceful degradation in lossy transmission environments as well as bit rate, format, and power adaptation. These functionalities provide enhancements to transmission and storage applications. SVC has achieved significant improvements in coding efficiency with an increased degree of supported scalability relative to the scalable profiles of prior video coding standards. This paper provides an overview of the basic concepts for extending H.264/AVC towards SVC. Moreover, the basic tools for providing temporal, spatial, and quality scalability are described in detail and experimentally analyzed regarding their efficiency and complexity. <s> BIB001 </s> Distributed video coding for wireless video sensor networks: a review of the state-of-the-art architectures <s> Fig. 1 WVSN architecture <s> This paper surveys recent trends and perspectives in distributed video coding. More specifically, the status and potential benefits of distributed video coding in terms of coding efficiency, complexity, error resilience, and scalability are reviewed. Multiview video and applications beyond coding are also considered. In addition, recent contributions in these areas, more thoroughly explored in the papers of the present special issue, are also described. <s> BIB002
A video sequence is a time-indexed set of frames (images) which contains a high degree of temporal redundancy among adjacent frames. Each frame is divided into subblocks for coding purpose. Conventional video coding employs two primary coding modes BIB001 ): • Inter-frame coding mode Compression in inter-frame coding mode exploits not only temporal but also spatial correlation among video frames and performs high complexity motion estimation and compensation operations to predict the best matching block for the block under reconstruction. Only the residue between the given block and the corresponding predictor is encoded and transmitted. Therefore, compression efficiency of inter-frame coding mode is very high at the expense of higher computational complexity and less robustness against packet losses. • Intra-frame coding mode In contrast, intra-frame coding mode only exploits the spatial correlation for encoding a block in a given frame. Therefore, the encoding complexity and compression efficiency is lower than inter-frame coding mode. However, intra-frame coding mode does not depend on adjacent frames and is more robust against packet losses since it treats each frame as a still image and encodes it separately without exploiting dependencies between adjacent frames. Irrespective of the coding modes, the PVC architecture has two significant drawbacks (Girod et al. 2005;  ): • Rigid allocation of functionality PVC enforces rigid allocation of functionality between encoder (complex) and decoder (simple), with the complex motion search operation dominating the overall encoder's complexity. • Drift error Since no real-time synchronization channel exists between PVC encoder and decoder, there is a higher probability of prediction mismatch in uncontrolled environment such as communication over wireless/radio links which are more prone to channel errors. As opposed to conventional video coding that follows joint-encoding/independentdecoding configuration, DVC architecture follows an independent-encoding/jointdecoding configuration which is more feasible for video applications in WVSNs. DVC reverses the conventional structure of the video codec by exploiting source statistics at the decoder. The theory of DVC stipulates that the source statistics of two or more correlated video sequences can (wholly or partially) be exploited at decoder given that the bitstreams received from multiple encoders performs joint decoding via analyzing statistical correlation between them. It should also be noted that the term distributed in DVC originally refers to the encoding mode rather than the encoder's physical location (Girod et al. 2005; BIB002 ). The shift of complexity in DVC is accomplished by assigning to the decoder the responsibility for generating the prediction, and relieving the encoder from such complex and computation intensive task. However, the encoder still possesses the ability to select the best prediction based on a comparison with the original frame to be coded. On the other hand, the decoder is unable to perform such comparison since it has only access to already decoded information and not the original frame, which complicates the decoder's task to estimate a precise motion field as compared to conventional predictive video coding. To reliably deliver high quality video to the sink server (decoder) in energy-constrained WVSN, the use of energy-efficient video coding scheme, rate-control and QoS management algorithms is critical. With a competitive compression performance and lower energy consumption, DVC is a promising alternative to traditional PVC architectures which are not suitable for WVSNs. In this paper, a comprehensive review of the state-of-the-art on DVC based architectures appropriate for WVSN is presented for the first time. The review includes a comparative discussion of several well-known architectures in literature with a focus on their functional aspects, performance comparison and suggests a number of possible enhancements to the design of these architectures. The rest of the paper is organized as follows: "Wireless video sensor network: challenges and issues" presents a general overview of WVSN and its current challenges. "Information theory of PVC and DVC" presents the information theory underpinning the conventional (PVC) and DVC architectures, "DVC in wireless sensor networks" discusses the significance of DVC architecture in the context of WSNs, "DVC architectures" overviews representative DVC architectures, "Comparison and analysis" compares and analyzes key differences between the architectures discussed in "DVC architectures", and finally "Conclusion" concludes the paper.