Mohamed Gharbi

QoS considerations for DMT-based ADSL and VDSL systems

Thanks to their high bandwidth capability, asymmetric digital subscriber lines (ADSL) and very high speed DSL (VDSL) are access technologies that permit the transmission of several applications simultaneously on telephone subscriber lines. Considering that these applications may require a different quality of service (QoS), and particularly different bit error rates (BER), for transmission, this paper addresses the problem of providing simultaneously two BERs for transmission over a DMT-based ADSL or VDSL link. Both coded and uncoded systems are considered.

A postprocessing technique for block effect elimination using a perceptual distortion measure

One of the drawbacks of the discrete cosine transform (DCT) is visible block boundaries due to coarse quantization of the coefficients. In this paper, an algorithm for the reduction of blocking artifacts is presented. The proposed method allows to produce higher quality reconstructed images by adaptively filtering the video signal according to the noise visibility. A visual model is therefore defined for predicting the block edge visibility across each picture of the coded sequence. This model that accounts for the perceptual characteristics of the block distortion is described. Experimental results are presented for low bit-rate coded sequences. They show that the postfiltering operation yields significant results with enhanced visual quality.

An MIMO Random Channel Generator for Indoor Power-Line Communication

Multiple-input multiple-output (MIMO) technology has already been used in power-line communication (PLC). An accurate description of MIMO channels for PLC network topologies is of great interest and importance in the development and testing of digital communication algorithms. However, the considerable variability of PLC networks makes this task quite difficult. In this regard, a random channel generator for a single-input single-output (SISO) configuration has been presented in the literature. The procedure was applied to a set of measured channels that were interestingly divided into nine classes. In this paper, an MIMO random channel generator based on the aforementioned SISO configuration is proposed. An MIMO channel is considered as the superposition of correlated SISO channels so we attempt to describe an MIMO channel using the nine classes obtained with the SISO configuration. An additional parameter, related to the correlation factor, had to be taken into account. It is shown that when this parameter is obtained from probabilistic distribution with properly chosen standard deviations for each of the nine classes, the European MIMO PLC field measurements can be accurately reproduced by the random channel generator.

An analytical model for the intercell interference power in the downlink of wireless cellular networks

In this paper, we propose a methodology for estimating the statistics of the intercell interference power in the downlink of a multicellular network. We first establish an analytical expression for the probability law of the interference power when only Rayleigh multipath fading is considered. Next, focusing on a propagation environment where small-scale Rayleigh fading as well as large-scale effects, including attenuation with distance and lognormal shadowing, are taken into consideration, we elaborate a semi-analytical method to build up the histogram of the interference power distribution. From the results obtained for this combined small- and large-scale fading context, we then develop a statistical model for the interference power distribution. The interest of this model lies in the fact that it can be applied to a large range of values of the shadowing parameter. The proposed methods can also be easily extended to other types of networks.

A Low Complexity Image Quality Metric for Real-Time Open-Loop Transcoding Architectures

In this paper, we present an original image quality metric for open-loop transcoding architectures based on frequency selective transmission. The proposed metric computes the normalized HVS-weighted mean squared error (NWMSE) between the reference digital video compressed bitstream, and its transcoded version. This computation is performed directly from the quantized DCT coefficients of the compressed bitstream. The resulting MSE error is implicitly weighted by the HVS sensitivity by considering the MPEG perceptual coding properties. Finally, rate control and error propagation are also taken into account, in order to improve transcoded video quality assessment. Experimental results are given which show that the proposed NWMSE metric correlates well with a conventional WPSNR. This validates the proposed assumptions used for the NWMSE computation. The major advantage of the approach is its simplicity since it does not need to perform the complete decoding of the compressed bitstream. Hence, the NWMSE metric is highly suited for real-time video applications.

Extended Coverage for DSL Video Distribution Using a Quality-Oriented JSCC Architecture

This paper proposes a layered video transmission scheme to extend the coverage area for DSL video delivery. The proposed architecture is based on a joint source-channel coding approach, and combines layered video transcoding with unequal error protection using hierarchical multi-carrier transmission. Based on a new image quality metric, the developed method automatically determines the optimal system parameters in terms of end-user perceived quality. Simulation results show that the proposed quality-oriented video delivery architecture extends the coverage area for DSL video distribution significantly for the end user, while providing more robust video services.

Reduced Complexity Power Minimization Algorithm for DMT Transmission - Application to Layered Multimedia Services over DSL

This paper presents an original bit and power loading algorithm for discrete multi tone data transmission of layered multimedia services. The proposed method, based on the waterfilling principle, realizes the bit allocation directly from a non iterative LUT-based solution compared to classical loading algorithms. First, the method is described in the single layer case. Then, the method is extended for multiple services transmission. Each service is characterized by its own quality of service (QoS) requirement and the bit and power allocation minimizes the total power consumption under bit rates and bit error rates constraints. In this case, the simultaneous transmission of the multiple services is achieved using a frequency division multiplexing (FDM) scheme, and the method used to assign different layers to subchannels is detailed. Experimental results of video delivery over ADSL show that the proposed algorithm comes near the optimal solution (the increase of power is lower than 0.2 dB) whereas its computational complexity is lower than traditional loading algorithms.

Blind spectrum sensing based spatial scanning and maximum energy ratio

Cognitive radio is the fundamental component for the future generations of wireless communication. Indeed, by sensing its environment, the cognitive radio will be able to exploit unused frequencies for opportunistic wireless communications. In this paper, a new sensing method in the narrowband context is proposed for ray propagation channels by combining the spatial scanning technique with the maximum energy ratio. The performance of the proposed method is evaluated and compared with existing state of the art algorithm. Simulation results show that the proposal is an interesting approach to deal with the detection challenge under very low signal-to-noise ratio (SNR).

Ladder factorization for biorthogonal modulated filter banks with variable global delay

In this paper, a product form of the polyphase matrix of the biorthogonal perfect reconstruction (PR) cosine Modulated Filter Bank (MFB) matrix is given with an adjustable overall input-output delay. The use of the lifting scheme for the factorization yields to an efficient and minimal implementation.

Systematic Lossy Error Protection of Video based on Reduced Spatial Resolution

In this paper, a new video transmission scheme based on Wyner-Ziv coding is developed. It applies the general framework of systematic lossy source-channel coding, using a low spatial resolution version of the transmitted sequence as a Wyner-Ziv supplementary stream. This Wyner-Ziv description is used at the decoder side in order to insure error free reconstruction of the base layer, while giving access to a reduced resolution version of transmitted video. First, we describe the proposed scheme and discuss its main characteristics. Then we derive a distortion model in order to evaluate the error correction capability of the scheme. Finally, simulation results are presented to demonstrate the advantages of this method over traditional methods such as FEC, in terms of robustness to packet loss.