Marie Zwingelstein-Colin

Reduced complexity H.264/AVC transrating based on frequency selectivity for high-definition streams

In this paper, we present a complete transrating architecture for H.264/AVC high-definition video bitstreams. Based on frequency-selective filtering, this architecture has a minimized complexity and is drift-error-free for intra-coded pictures. Moreover, both Video Quality Metric (VQM) experiments and subjective analysis show that the algorithm gives satisfactory results in terms of visual quality. Our solution introduces blurring only compared to conventional requantization transrating, which also introduces other artifacts as distortion. We propose an easy and efficient bitrate estimation model controlled by transrating parameters.

Cognitive cooperation for the downlink of frequency reuse small cells

We develop and analyze a space time coded cooperative diversity protocol that achieves improved quality of service for mobile users in the downlink of small cells. The protocol, called cooperative frequency reuse (CFR), leverages the cellular frequency reuse concept to create space and frequency diversity among pairs of adjacent base stations. The CFR protocol, which is consistent with the half-duplex mode assumption, consists of two phases. During the first phase, each base station transmits symbols to it own user on its dedicated frequency band, while listening to the symbols intended to the users of the other base station on another frequency band. Cognitive cooperation is implemented in the second phase. Each base station transmits on the two frequency bands to the scheduled users in both base stations, by means of an appropriately chosen space time code. We construct a full rate distributed space time code, based on the Golden code, for cooperation between the two adjacent base stations. We discuss the performance of the proposed protocol in terms of bit error rate and probability of outage. Simulation results show that the proposed CFR protocol yields considerable improvement over traditional direct transmission frequency reuse strategies.

An SVD-Aided Efficient Bit-Loading Algorithm for MIMO Transmission Over Spatially Correlated Channels

This paper proposes an efficient bit-loading algorithm for Multiple-Input-Multiple-Output transmission over spatially correlated channels with perfect Channel State Information at both link ends. Based on water-filling principle, the algorithm does not assume the well-known gap approximation and accounts for the spatial correlation effects through the initial bit allocation. The result is the optimal distribution of bits and power over the eigenmodes for multiple antenna systems. We report the performance evaluations in the presence of spatial correlation in order to illustrate the efficiency and the robustness of our bit-loading algorithm in realistic environments. The simulation results demonstrate that our algorithm leads to significant improvements. In addition, these results show that our algorithm achieves the maximal diversity order, as well as exhibiting a high extra coding gain.

Random Matrix Theory Based Resource Allocation in Correlated MIMO Systems with ARQ Feedback

We consider resource allocation under partial feedback in a spatially correlated MIMO link, when the ARQ protocol is implemented at the MAC layer. We propose a design framework, which makes use of results from random matrix theory (RMT), to find the rate as well as the input covariance matrix that maximize the long term goodput. We consider partial feedback in terms of positive/negative acknowledgment bits (ACK/NAK), which comes essentially for free since they are always present in the signaling of the upper layers. We provide explicit expressions of the long term goodput, which, in association with a RMT based approximation of the mutual information enable us to optimize the resource allocation problem. Interestingly, the simulations show that the asymptotic optimization analysis is still valid for MIMO sizes as small as 2×2.

An improved adaptive modulation scheme for closed-loop MIMO transmission in tunnel environments

This paper proposes a new adaptive modulation (AM) scheme for closed-loop MIMO transmission over wireless correlated channels. Two correlation-based channel models are considered to illustrate the interest of the proposed solution in realistic environments representative of transportation systems. Several simulation results for different transmission scenarios are presented and discussed. It is shown that the proposed AM approach outperforms several precoding schemes and provides full diversity order.

On modeling the downlink inter-cell interference power in wireless cellular networks

In this paper, we analyse the interference power in a typical wireless cellular network. We provide an analytical approximation for the probability density function of the interference power, which does not depend on the users position, and which is validated by simulation. The key idea of the approximation results from our observation that the variations of the pathloss between a user and an interfering base station (due to the variations of the users position within its cell) are negligible in comparison with the multipath fading variations. The results obtained in this paper may be used to greatly simplify the analysis of some network optimization problems (inter-cell interference coordination, capacity optimization…), as well as system level simulations of wireless cellular networks.

Comparison between H.264/AVC intra transrating schemes based on frequency position and frequency level selectivity

In this paper, we propose to compare two different H.264/AVC intra transrating architectures based on frequency position selectivity and frequency level selectivity respectively, using intra high-definition video bitstreams. We evaluate these solutions in terms of bitrate reduction and visual quality using both PSNR and Video Quality Metric (VQM) experiments. Simulation results show that the frequency position selectivity has a slight lower computing complexity while providing better or similar visual quality than frequency level selectivity.

Multiple description coding and scalable video coding combined with multiple input multiple output techniques: two strategies to enhance train to wayside video transmissions in tunnels

Video monitoring applications for underground rely on wireless train-to-wayside communication systems which require high data rate as well as high Quality of Service (QoS) level. In order to satisfy both constraints we propose a combined source and channel coding approach in the context of MIMO (Multiple Input Multiple Output) video transmission. In the present case, MIMO transmission is based on the PHY layer of IEEE 802.11n Wi-Fi standard currently deployed in a railway tunnels. Two different strategies are studied: first, the association between Multiple Description Coding (MDC) and a STBC (Space Time Block Code) MIMO scheme is considered when no channel information is available at transmitter side. In the case when perfect channel information is available at transmitter side (CSIT), a Singular Value Decomposition of the MIMO channel is possible. This transmission scheme is then associated with scalable video coding, which consists here in the separation of the scene into different Regions Of Interest (ROI). The creation of the regions of interest is based on the Flexible Macroblock Ordering (FMO) technique introduced in the new H.264/AVC compression standard. The stream associated to the area with the maximal perceptual relevance is transmitted on the eigen-channel with the higher gain. Consequently, this strategy which provides unequal protection against channel errors, allows guaranteeing better robustness and acceptable reconstructed video quality at the control-centre. The two different strategies of transmission have been evaluated thanks to realistic simulations. Two antenna configurations representative of real cases encountered in railway tunnels are considered. The channel model is generated by using the correlation based Kronecker model obtained by computing the channel matrix with a 3D ray tracing tool. Simulation results show that the two proposed solutions allow enhancing the reconstructed video quality compared to conventional transmission schemes with no increase of the transmitted power and of the number of radio access points along the infrastructure, even in tunnels in presence of spatial correlation.