Wireless multimedia sensor networks will play a central role in the Internet of Things world, providing content-rich information for an uncountable number of monitoring and control scenarios. As more applications rely on multimedia data, security concerns gain attention, and new approaches arise to provide security for such networks. However, the usual resource constraints of processing, memory and the energy of multimedia-based sensors have brought different challenges for data encryption, which have driven the development of different security approaches. In this context, this article presents the state-of-the-art of cryptography in wireless multimedia sensor networks, surveying innovative works in this area and discussing promising research directions.

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Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions

Practically all known good constructive coding techniques for bandlimited channels, including lattice codes and various trellis-coded modulation schemes, can be characterized as coset codes. A coset code is defined by a lattice partition Lambda / Lambda ' and by a binary encoder C that selects a sequence of cosets of the lattice Lambda '. The fundamental coding gain of a coset code, as well as other important parameters such as the error coefficient, the decoding complexity, and the constellation expansion factor, are purely geometric parameters determined by C Lambda / Lambda '. The known types of coset codes, as well as a number of new classes that systematize and generalize known codes, are classified and compared in terms of these parameters. >

Coset codes. I: Introduction and geometrical classification

Gray code application in distributed video coding (DVC) has been studied in the literature and it was claimed that DVC performance could be improved by Gray code because of stronger bitplane correlation presented. However, two factors are ignored in the literature, which renders the aforementioned claim untenable. These factors are log-likelihood ratio (LLR) computation and robustness to wrongly decoded bits, which may influence the DVC performance when different bit representations are used. This paper comprehensively evaluates the performances of Gray code in different DVC schemes, such as feedback channel-based transform domain Wyner–Ziv (FC_TDWZ) video coding, parallel pixel domain Wyner–Ziv video coding, and encoder rate control-based TDWZ (ERC_TDWZ). Experimental results suggest that the Gray code does not always improve the DVC performance, although stronger bitplane correlation is presented. In particular, the performance of FC_TDWZ with Gray code is exactly the same as that with natural bit code, because the same magnitudes of LLRs are obtained. Moreover, the reconstruction quality of the WZ frames in ERC_TDWZ is improved by Gray code not because of stronger bitplane correlation but because Gray code is more robust to wrongly decoded bits induced by rate underestimation. However, these RD gains are quite limited when the rate is controlled accurately at the encoder.

Can Gray Code Improve the Performance of Distributed Video Coding

Distributed Video Coding (DVC) is a new approach in video coding which due to low computational complexity at the encoder side, has a great potential to be used in Wireless Multimedia Sensor Networks (WMSN). However, the different architecture of this codec affects the efficiency of transmission protocols and in order to efficient transmission of DVC over WMSN, it is necessary to evaluate the performance of the transmission protocols in the presence of DVC characteristics. In the view of these protocols, error control methods are important mechanisms that provide quality of service and robust multimedia communications. For this reason, we performed a comparative performance analysis for all error control schemes that consist of Automatic Repeat Request (ARQ), Forward Error Correction (FEC), Erasure Coding (EC), hybrid link layer ARQ/FEC and multi-layer hybrid error control schemes for DVC in WMSNs. These analyses are in the terms of the most importance metrics in multimedia communications over WSNs, such as objective and subjective video quality criteria, delay, energy consumption and some DVC-specific metrics. The results show the distinct behavior of DVC in the presence of channel error and can be used to propose an effective and efficient error control scheme for DVC over WMSN.

Performance evaluation of error control schemes for distributed video coding over wireless multimedia sensor networks

The quality of side information has an important impact on the performance of a distributed video coding system. At present, the generation of side information is mainly based on the translational motion model using the inter-frame correlation for motion estimation. Considering that the generated side information is prone to block effect and ghosting, as well as the situation that the nonlinear motion is not fully considered and the intra-frame correlation is not fully utilized, a side information hybrid generation algorithm based on an improvement model for the motion vector field is proposed. The side information frame to be generated for the current Wyner-Ziv frame is divided into easy-to-estimate and difficult-to-estimate macroblocks. For difficult-to-estimate macroblocks, the Horn and Schunck dense optical flow method is used to generate reliable motion vectors, for easy-to-estimate macroblocks, the block matching method is used to generate unreliable motion vectors which are modified by the proposed scheme, and then, the improved motion vectors are used for motion compensation to produce the final side information frame. Experiment results show that the quality of side information obtained by using the improved motion vector field for motion compensation has been significantly improved, thus the overall performance of the distributed video coding system has been effectively improved.

Side information hybrid generation based on improved motion vector field

Distributed video coding (DVC) is a relatively new video coding architecture originated from two fundamental theorems namely, Slepian–Wolf and Wyner–Ziv. Recent research developments have made DVC attractive for applications in the emerging domain of wireless video sensor networks (WVSNs). This paper reviews the state-of-the-art DVC architectures with a focus on understanding their opportunities and gaps in addressing the operational requirements and application needs of WVSNs.

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A. C. M. Fong

Papers

Distributed video coding for wireless video sensor networks: a review of the state-of-the-art architectures