20072384
A Backward-Compatible Solution for Next Generation DVB-C System
In this paper, we propose hierarchical modulation as a backward compatible solution for next generation digital video broadcasting over cables (DVB-C2). Without modifying the architecture of legacy receivers, supplementary data can be transmitted via an added layer superimposed onto the legacy layer, and the resultant increase of transmission power is only a small fraction of that used by the legacy system. To obtain the upper bounds for the achievable throughputs, the modulation constrained capacities are derived for this layered transmission scheme. We formulate the hierarchical constellation design as a nonlinear optimization problem and then solve it through a simplified numerical method, which searches for the appropriate power allocation ratio in one-dimension space. Finally, to examine the inter-operability of channel encoder/decoder between DVB-S2 and DVB-C2, we investigate the error performance of the proposed scheme employing the LDPC codes given by DVB-S2 standard.
7527642
Spectrum Sensing for OFDM Systems Employing Pilot Tones and Application to DVB-T OFDM
Spectrum sensing is one of the core technologies of cognitive radio (CR) systems which provide a viable solution to the problem of sparsity of wireless spectrum. Nowadays, OFDM techniques are adopted by many existing or progressing wireless communication standards. Thus, a robust spectrum sensing algorithm for OFDM modulated signals is highly desired to implement CR when the primary signal uses OFDM modulation. Motivated by this demand, a time-domain symbol cross-correlation based spectrum sensing algorithm (TDSC method) is presented in this paper. The algorithm makes use of the property that the mean of the TDSC of two OFDM symbols is not zero if they have embedded the same frequency-domain pilot tones. The statistical behavior of the proposed spectrum sensor is explicitly analyzed and a theoretical lower bound on the misdetection probability is derived in this paper. An intuitive spectrum sensing method which utilizes the cyclic prefix nature of the OFDM modulated signals (CP method) is also described in this paper as a reference for comparison. Finally, we use the DVB-T Standard [8] as an application example to illustrate the proposed spectrum sensing algorithm. The simulated channel environments are the Ricean and Rayleigh channels defined in the DVB-T Standard and the additive white Gaussian noise (AWGN) channel. Four CP ratios defined in the DVB-T Standard are simulated for the probability of false alarm equaling 0.01 and the sensing time set equal to 50 ms. Simulation results show that the TDSC method can achieve a misdetection probability of 0.1 when the SNR equals -20.5 dB for all four CP ratios. The TDSC method outperforms the CP method in all cases. Furthermore, the proposed method has approximately the same detection performance for different CP ratios while the detection performance of the CP method degrades dramatically when the CP ratio becomes small. Results also reveal that the simulated performance is very close to the theoretical lower bound indicating that the lower bound can be used as a good prediction of performance.
8940979
Signature Based Spectrum Sensing Algorithms for IEEE 802.22 WRAN
Spectral sensing for the presence of ATSC DTV signals in VHF/UHF TV bands under a very low SNR environment is one of the core technologies in IEEE 802.22 WRAN. There are multiple signatures embedded in the ATSC DTV signals and these signatures can be utilized to perform spectrum sensing. Signature-based spectrum sensing algorithms have an advantage in that when the signals other than noise are detected, we are almost sure that they are signals we want to detect. Thus, in this paper, several signature based spectrum sensing algorithms are presented. The false alarm rate for detectors employing different algorithms is analytically derived. The operating characteristic curves for different detectors are determined from computer simulations using ATSC A/74 DTV signal captures as a test database.
8471073
Spectrum Sensing for DMB-T Systems Using PN Frame Headers
Spectrum sensing for the China digital television terrestrial broadcasting system is considered in this paper. The spectrum sensing algorithms utilize PN sequences embedded in the frame headers to perform spectrum sensing. A theoretical lower bound on the misdetection probability for each spectrum sensor presented in this paper is derived. The performances of the proposed spectrum sensing algorithms are demonstrated by computer simulation for the multipath Rayleigh fading and steady state multipath Rayleigh fading channels. Simulation results show that the misdetection probability evaluated by computer simulations is close to the lower bound on the misdetection probability for a steady state multipath Rayleigh channel. When the probability of false alarm is 0.01 and a 50 ms of sensing time is used, a misdetection probability equal to 0.1 is achieved when the signal to noise power ratio is -16 dB, -18.8 dB, and -18 dB for modes 1, 2, and 3 in the multipath Rayleigh fading channel, respectively.
6724826
Spectrum Sensing Using Cyclostationary Properties and Application to IEEE 802.22 WRAN
Spectrum sensing in a very low SNR environment (less than -20 dB) is considered in this paper. We make use of the noise rejection property of the cyclostationary spectrum. The sensing algorithms are based on measurement of the cyclic spectrum of the received signals. The statistics of the cyclic spectrum of the stationary white Gaussian process are fully analyzed for three measurement methods of the cyclic spectrum. The application to IEEE 802.22 WRAN is presented and the probability of false alarm is analytically derived. The operating characteristic curves for the sensing algorithms are determined from computer simulations using ATSC A/74 DTV signal captures as a test database.
7918052
Fast Identification of Error-Prone Patterns for LDPC Codes under Message Passing Decoding
Although density evolution accurately predicts the asymptotic performance of LDPC codes, it provides limited information for codes of short or moderate block length. For this reason, combinatorial methods have been employed to analyze the performance of finite-length LDPC codes under message passing decoding. The error floor phenomenon is a key issue of finite-length effects, which is caused largely by poorly connected subgraphs residing in the LPDC codes structure. Because of the unequal noise immunity of different bit levels, error floors become more pronounced for LDPC-coded high-order modulation. Nevertheless, the enumeration of error-prone patterns that lead to error floors has been proven to be NP-hard, which motivates us to find substitute metrics with manageable complexity. Considering trapping sets always comprise loops of low extrinsic message degree (EMD) and the connectivity of a loop with the residual graph can be characterized by its approximate EMD (ACE), in this paper we propose a fast method to identify loops with low ACE for a given code. In light of this, the vulnerable variable nodes residing on low-ACE loops are judiciously mapped to bit levels benefitting from stronger error protection. As a result, the error floor can be significantly improved at the cost of a minor increase of hardware complexity.
15993818
Illumination transfer using homomorphic wavelet filtering and its application to light-insensitive face recognition
In this paper, we propose a novel homomorphic wavelet filtering based illumination transfer technique to change the dominant lighting of one face image (source face image) to another face image (reference face image ). Specifically, in the proposed method, based on the ldquoreflectance-illuminationrdquo imaging model, we first obtain an approximate estimate of the illumination component of the face image through a wavelet-based Multiresolution Analysis (MRA) in the logarithm domain of the input image. Then, a homomorphic filtering procedure is applied to improve the accuracy of the illumination component estimation. Finally, the source face image is re-lighted by substituting the estimated illumination component of the reference image for that of the source image. The proposed method is entirely an image processing based method without any 3D geometry modeling steps, so it is simple but effective. The method is also applied easily to illumination invariant face recognition by transferring a standard illumination to all the face images. Experimental results show that our method is quite effective for both illumination transfer and illumination-insensitive face recognition.
15256046
Distributed Video Coding with Spatial Correlation Exploited Only at the Decoder
A new pixel-domain distributed video coding (DVC) scheme is proposed in this paper, in which both the temporal and the spatial correlations are exploited only at the decoder. A video is treated as a collection of data correlated in temporal and spatial directions. Besides splitting a video into frames at different time instants, a frame is further split by spatially sub-sampling. Each yielded part is then encoded individually. At the decoder, the side information signals are from both adjacent frames and the spatially decoupled signals. To utilize these multiple side information signals, a new probability model is proposed, in which the transitional probability is calculated from the conditional probabilities on the multiple side information signals. The coding efficiency is enhanced by further removing the spatial redundancy, while the encoding complexity remains the same as the previous pixel-domain DVC techniques that only consider the temporal correlation
17617120
Robust real-time transmission of scalable multimedia for heterogeneous client bandwidths
For robust real-time transmission of scalable image and video data over packet-loss networks, a commonly used approach is FEC-based multiple description coding, which protects a scalable bitstream with a fixed number of packets of equal length. In this paper, by considering the problem of applying this approach of multicasting a source to a collection of clients with heterogeneous bandwidths, we propose a novel technique that changes packet length but fixes packet number. In one scenario where different clients access the server via separate links, we study the sensitivity of an optimal solution to the change of packet length, and propose a local search procedure which refines the already computed optimal solutions for other bandwidths. Compared to the scheme that computes an optimal solution individually for each bandwidth, this procedure can achieve comparable performance, however with quite lower time complexity, thus can be used in real-time applications. In another scenario where many clients share a bottleneck link, we present an embedded packetization framework for layered multiple description coding, in which even simple methods with low complexities can achieve good performance. We also propose a local search algorithm to optimize the weighted average performance in case of two layers, and a fast heuristic algorithm which can achieve very good performance tradeoff among all clients in case of more than two layers.
5790149
Curse of mis-alignment in face recognition: problem and a novel mis-alignment learning solution
In this paper, we present the rarely concerned curse of mis-alignment problem in face recognition, and propose a novel mis-alignment learning solution. Mis-alignment problem is firstly empirically investigated through systematically evaluating Fisherface's sensitivity to mis-alignment on the FERET face database by perturbing the eye coordinates, which reveals that the imprecise localization of the facial landmarks abruptly degenerates the Fisherface system. We explicitly define this problem as curse of mis-alignment to highlight its graveness. We then analyze the sources of curse of mis-alignment and group the possible solutions into three categories: invariant features, mis-alignment modeling, and alignment retuning. And then we propose a set of measurement combining the recognition rate with the alignment error distribution to evaluate the overall performance of specific face recognition approach with its robustness against the mis-alignment considered. Finally, a novel mis-alignment learning method, named E-Fisherface, is proposed to reinforce the recognizer to model the mis-alignment variations. Experimental results have impressively indicated the effectiveness of the proposed E-Fisherface in tackling the curse of mis-alignment problem.
15735389
Video indexing by motion activity maps
Motion based video indexing is an important and active research area in content-based video retrieval. It explores the dynamic characteristics of video content and provides techniques for video representation and retrieval. In this paper, a new motion-based approach is proposed, in which the image generated from motion activity is used to index video. The proposed approach firstly computes the accumulation measurement of motion activity on the grids of video frames along the time axis. Then, the computed measurement is quantized into several gray levels and a gray image is generated from all measurements on the grids. The generated image called motion activity map (MAM) reserves the motion information on different spatial locations of video. The intensity of MAM is corresponding to the magnitude of motion activity. All MAMs are organized into a hierarchical tree according to the video structure. Therefore, user can browse video by the MAMs tree. The proposed approach provides a hierarchical, coarse to fine view of video and thus makes interactive video retrieval more simply and intuitively.