ABSTRACT For a wireless ad hoc network to achieve fault‐tolerance, it is desired that the network is bi‐connected. This means that each pair of nodes in the network have at least two node‐disjoint paths between them, and thus, failure at any single node does not partition the network. In other words, in a bi‐connected network, there is nocut‐node(defined as a node such that the removal of it partitions the network). To make a connected but not bi‐connected network become bi‐connected, actions should be taken such that all cut‐nodes become non‐cut‐nodes. In this research, we propose to deal with cut‐nodes from a new perspective. Specifically, we first introduce a new concept ofremovable node, defined as a non‐cut‐node such that the removal of it does not generate any new cut‐node in the network. Then, we propose to move a removable node to a new location around a cut‐node. In this way, the cut‐node becomes a non‐cut‐node, that is, the failure of it does not partition the network anymore. Algorithms are provided (i) to identify removable nodes; (ii) to match cut‐nodes with a feasible set of removable nodes, in which all nodes can be simultaneously removed from the network without generating any new cut‐node in the network; and (iii) to derive the final location of a removable node such that its movement distance is the shortest and the associated cut‐node becomes a non‐cut‐node. The proposed algorithms do not guarantee the final bi‐connectivity but have the merits of a large success rate (almost 100% in the simulation), a small number of moved nodes, and a short total movement distance. In addition, the proposed algorithms are shown to be effective even when there are a large portion of fixed nodes in the network. Copyright © 2011 John Wiley & Sons, Ltd.

In 3-D video coding, texture videos and depth maps need to be jointly coded. The distortion of texture videos and depth maps can be propagated to the synthesized virtual views. Besides coding efficiency of texture videos and depth maps, joint bit allocation between texture videos and depth maps is also an important research issue in 3-D video coding. First, we present comprehensive analyses on the impacts of the compression distortion of texture videos and depth maps on the quality of the virtual views, and then derive a concise distortion model for the synthesized virtual views. Based on this model, the joint bit allocation problem is formulated as a constrained optimization problem, and is solved by using the Lagrangian multiplier method. Experimental results demonstrate the high accuracy of the derived distortion model. Meanwhile, the rate-distortion (R-D) performance of the proposed algorithm is close to those of search-based algorithms which can give the best R-D performance, while the complexity of the proposed algorithm is lower than that of search-based algorithms. Moreover, compared with the bit allocation method using fixed texture and depth bits ratio (5:1), a maximum 1.2 dB gain can be achieved by the proposed algorithm.

As zoom motion is common in video applications, a linear motion model is derived to describe zoom motion based on the analyses of camera imaging principles. Based on the motion model, a motion vector predictor for videos involving zoom motion is proposed. A rate distortion (RD) criterion is used to choose the optimal motion vector predictor between the one utilized in H.264/AVC and the one derived from the linear motion model. Experimental results demonstrate that by implementing the proposed method into Key Technology Area test platform version 2.2r1(KTA2.2r1), the maximum and average bit rate savings can be achieved as high as 7.66% and 4.90% respectively, while maintaining the same quality of reconstructed videos.

The video coding standard H.264/MPEG-4 AVC is designed based on the basic unit of macroblock (MB). In order to support the arbitrary resolution video coding when the picture does not contain an integral number of MBs, the H.264/MPEG-4 AVC encoder usually invokes a picture padding process to extend the boundaries of a picture by adding extra pixels. In this way, the video of an arbitrary resolution can be coded by an ordinary encoder and the resulting bitstream follows the H.264/MPEG-4 AVC syntax. From the perspective of coding efficiency and the concept of soft decision, we consider the issue of rate-distortion (R-D) based picture padding as an optimization problem, and present an iterative solution which integrates the optimal picture padding in the R-D optimization (RDO) process. Furthermore, to reduce the related computational burden, a heuristic solution is proposed based on comprehensive analyses on the influence of the padded pixels on the encoder performance. This solution takes the prediction errors and compression distortions of padded pixels as zero in the RDO process of the encoder, and automatically implements the concepts and techniques of template matching and spatial extrapolation to determine the padded pixels. Since it is carried out by directly utilizing the already existing algorithms of RDO, the proposed heuristic solution brings little additional computational complexity to the encoder. Based on the reference software for H.264/MPEG-4 AVC with its version of JM15.1, the effectiveness of the proposed method has been verified by extensive experiments.

Both temporal prediction and inter-view prediction are employed to improve the coding efficiency in multiview video coding. Hierarchical B pictures are usually used as the basic structure for temporal prediction. The inter-view prediction in each temporal hierarchy level brings different improvement to the entire coding efficiency. We propose a scalable prediction structure in which inter-view prediction would be disabled if the picture redundancy can be almost exploited by temporal prediction and intra prediction. In this way, time-consuming computation of disparity estimation can be saved. Experimental results show that lower encoding complexity, smaller decoded picture buffer size and better random access ability can be achieved with a slightly gain loss.

High complexity of frequency-hopping (FH)/spreadspectrum (SS) sequence is of great importance to high-security multiple-access communication systems, for it makes FH/SS sequence difficult to be analyzed. With the growing development in the design of FH/SS sequence in much wider fields, the wellknown complexity measuresiquestthe linear complexity (LC), the linear complexity profile (LCP) and the k-error linear complexity (k-error LC)iquestare widely used but not sufficient to evaluate the complexities of the sequences available, such as the cryptographical sequence and the chaotic sequence families. In this paper, a new complexity metric to evaluate the unpredictability of FH/SS sequence based on the approximate entropy (ApEn) is proposed in the view of the maximal randomness of the sequences with arbitrary length. And the theoretical bounds of the ApEn are derived from a probabilistic point of view. Simulations and analysis results show that, the proposed ApEn works effectively to discern the changing complexities of the FH/SS sequences with small number of samples, which provide superior performance over its candidates.

We propose a new SNR estimation method based on the preamble for OFDM systems in frequency selective channels. The OFDM training symbols in the preamble are equalized by the known data in frequency domain and employed to estimate the noise variance. The second order moments of the received symbols are used to estimate the signal plus noise power in the OFDM packets. The SNRs on the subchannels and the average SNR of the packets can all be estimated. Simulation results show that the proposed method is robust to frequency selectivity in wireless channels, and its performance is considerably improved compared with the available methods.

As motion of neighboring views is highly correlated in multiview video systems, the inter-view motion skip mode has been proposed to improve the efficiency of multiview video coding (MVC) by reusing the motion information of neighboring views. To fully exploit the inter-view motion correlation, a fine-granular motion matching algorithm for the motion skip mode is presented in this paper. The proposed algorithm searches for the best matching motion information in neighboring views with fine granularity, and then uses the motion information for the motion-compensated coding in the motion skip mode. The rate-distortion optimization criterion is applied to find the best matching motion information. Experimental results show that the proposed algorithm can improve the performance of the motion skip mode, and further improve the efficiency of MVC.

An end-to-end rate-distortion optimized motion estimation method for robust video coding in lossy networks is proposed. In this method the expected reconstructed distortion after transmission and the total bit rate for displaced frame difference are estimated at the encoder. The results are fed into the Lagrangian optimization at the encoder to perform motion estimation. Here the encoder automatically finds an optimized motion compensated prediction by estimating the best trade off between coding efficiency and end-to-end distortion. Computer simulations in lossy channel environments were conducted to assess the performance of the proposed method. A comparative evaluation using other conventional techniques from the literature was also conducted.

The topology of a wireless sensor network has a significant effect on the network performance. Although topology control algorithms can optimize the network topology, the resultant topology is usually changeable. In this paper, we focus on how to maintain the network connectivity while reducing the maintenance overhead as some nodes fail. A distributed topology maintenance algorithm based on the shortest path tree is proposed. In this algorithm, nodes in immediate surroundings of a faulty node are first triggered to respond, and if they couldn’t ensure that the network is connected, the other related nodes are further triggered. Simulation results show that the proposed scheme can efficiently maintain the network connectivity with significant reduction in the overhead for topology maintenance.

A novel objective no-reference metric is proposed for video quality assessment of digitally coded videos containing natural scenes. Taking account of the temporal dependency between adjacent images of the videos and characteristics of the human visual system, the spatial distortion of an image is predicted using the differences between the corresponding translational regions of high spatial complexity in two adjacent images, which are weighted according to temporal activities of the video. The overall video quality is measured by pooling the spatial distortions of all images in the video. Experiments using reconstructed video sequences indicate that the objective scores obtained by the proposed metric agree well with the subjective assessment scores.

A novel family of frequency hopping sequences based on 3DES iterated block cipher is proposed for frequency-hopping multiple-access (FHMA) communications. The design offers a class of nonlinear FH codes with high security, large linear span and a uniform spread over the entire frequency bandwidth. Moreover, FH sequences among the family are independent from each other and they perform as well as random patterns in terms of multiple access interference in anti-jamming applications. With the performance of packet error and throughput for FHMA network being derived in theory, many numerical results of the 3DES sequences are presented, comparing with those of shift register sequences and chaotic FH sequences. Efficiently implemented in field-programmable–gate-arrays (FPGA), the generator prototype of the proposed sequence has been realized and incorporated into fast FH radio.

The variable-length codes (VLCs) are very sensitive to errors, usually one bit error will cause incorrectly decoded symbols in succession. This error propagation effect makes the symbol error probability 2 to 3 orders of magnitude higher than the original bit error probability. In video communication, error propagation effect leads to that the reconstructed video perceptive quality can not be accepted even when the original bit error-probability is less than 10. In this paper, we propose a novel VLC encoding algorithm, called AVLC, which employs two types of VLC codes to encode the source symbols alternately. The maximum aposteriori probability (MAP) decoding algorithm is used when errors occurred We have obtained very good results that, under Gauss or BSC channel with bit-error probability about 10/sup -3/ the error propagation effect can be avoided effectively without any channel coding protection. This algorithm provides an effective way to guarantee the reliability of data communication, especially the video communication, against the background of high noise.

Video quality assessment in real time is a critical requirement for channel evaluation and codec optimization in mobile multimedia system. However, most proposed approaches for video quality assessment need reference sequences, which is quite impossible in real time multimedia communications especially in wireless and IP video services. This paper proposes a novel no-reference video quality assessment method. By comparison of the extracted watermark embedded in transmitted video with the copy of original watermark in the destination, we can assess all reconstructed video quality without any original reference video sequences. Simulation results conclusively demonstrate that our method offers no-reference video quality assessment with little channel overhead and can be applied to real time multimedia systems.