Pierre Ferré

Throughput analysis of IEEE 802.11 and IEEE 802.11e MAC

This work focuses on the throughput analysis of the IEEE 802.11 medium access control (MAC). In the IEEE 802.11 standard, the main access scheme is called the distributed coordination function (DCF) and is the basis for the other access schemes, such as the point coordination function (PCF) and the enhanced DCF (EDCF) of the new IEEE 802.11e MAC. Since the IEEE 802.11 MAC does not support quality of service (QoS), the IEEE is currently working on the final draft of an enhanced version known as the IEEE 802.11e. In this enhanced standard, QoS and service differentiations are supported. The throughput and delay performance of the DCF/PCF of the IEEE 802.11 MAC are presented for different packet lengths and different numbers of users. Throughput performances are also detailed for the EDCF.

Robust Video Transmission Over Wireless LANs

Home wireless networks are mainly used for data transmission; however, they are now being used in video delivery applications, such as video on demand or wireless internet protocol (IP) television. Off-the-shelf technologies are inappropriate for the delivery of real-time video. In this paper, a packetization method is presented for robust H.264 video transmission over the IEEE 802.11 wireless local area network (WLAN) configured as a wireless home network. To overcome the poor throughput efficiency of the IEEE 802.11 Medium Access Control (MAC), an aggregation scheme with a recovery mechanism is deployed and evaluated via simulation. The scheme maps several IP packets (each containing a single H.264 video packet called a Network Abstraction Layer (NAL) unit) into a single larger MAC frame. Video robustness is enhanced by using small NAL units and by retrieving possible error-free IP packets from the received MAC frame. The required modifications to the legacy MAC are described. Results in terms of throughput efficiency and peak-signal-to-noise ratio (PSNR) are presented for the case of broadcast and real-time transmission applications. Compared to the legacy case, an 80% improvement in throughput efficiency is achieved for a similar PSNR video performance. For fixed physical layer resources, our system provides a 2.5-dB gain in video performance over the legacy case for a similar throughput efficiency. The proposed solution provides considerable robustness enhancement for video transmission over IEEE 802.11-based WLANs.

Distortion-Based Link Adaptation for Wireless Video Transmission

Wireless local area networks (WLANs) such as IEEE 802.11a/g utilise numerous transmission modes, each providing different throughputs and reliability levels. Most link adaptation algorithms proposed in the literature (i) maximise the error-free data throughput, (ii) do not take into account the content of the data stream, and (iii) rely strongly on the use of ARQ. Low-latency applications, such as real-time video transmission, do not permit large numbers of retransmission. In this paper, a novel link adaptation scheme is presented that improves the quality of service (QoS) for video transmission. Rather than maximising the error-free throughput, our scheme minimises the video distortion of the received sequence. With the use of simple and local rate distortion measures and end-to-end distortion models at the video encoder, the proposed scheme estimates the received video distortion at the current transmission rate, as well as on the adjacent lower and higher rates. This allows the system to select the link-speed which offers the lowest distortion and to adapt to the channel conditions. Simulation results are presented using the MPEG-4/AVC H.264 video compression standard over IEEE 802.11g. The results show that the proposed system closely follows the optimum theoretic solution.

A video error resilience redundant slices algorithm and its performance relative to other fixed redundancy schemes

In this paper, a redundant picture formation algorithm that takes into account a given redundancy rate constraint is presented for error resilient wireless video transmission without reliance on retransmissions. The algorithm assigns priorities to macroblocks (MBs) according to two suggested metrics and ranks MBs accordingly. The first metric is based on an end-to-end distortion model and aims at maximising the reduction in distortion per redundancy bit. The end-to-end distortion accounts for the effects of error propagation, mismatch between the primary and redundancy description and error concealment. MBs providing large distortion reduction for fewer bits spent are assigned a higher priority. The second metric employs the variance of the motion vectors of a macroblock and those of its neighbouring blocks. Results show that the rate distortion metric outperforms other examined metrics by up to 2dB. A comparison with other error resilience schemes, such as loss adaptive-rate distortion optimisation (LA-RDO) and cross packet forward error correction (FEC), is also presented. Results show that our approach outperforms the LA-RDO solution. Cross packet FEC provides better performance in the presence of random packet losses. However, results show that our approach can tackle bursty losses better. With bursty errors, the correction capabilities of the Reed-Solomon codes can be exceeded, and the availability of a coarse representation of the coded macroblocks, through a redundant slice scheme, can offer performance gains of up to 1dB.

Frame Delay and Loss Analysis for Video Transmission over time-correlated 802.11A/G channels

This paper presents simulation results for the transmission of unicast MAC frames over 802.11a/g. Fading channel models at various Doppler frequencies are developed to generate time- correlated SNR waveforms. These are then used together with a bit accurate MAC/PHY simulator to estimate the frame loss rate, the transmission delay, and the jitter for a steady flow of transmit frames. Time correlated channels are required to correctly simulate the bursty nature of packet loss in a wireless channel. The Doppler spread is shown to have a strong effect on the performance of the ARQ mechanism in the MAC layer. Delay is computed as the sum of the transmission delay and the accumulated queuing delay in the MAC buffer. Delay and frame loss are compared for time correlated and time uncorrelated fading channels. Compared to the slow fading case, in a fast fading channel fewer retransmissions are required and the end-to-end delay is significantly reduced. When channel conditions are poor the simulated delay and frame loss rate are seriously underestimated when time uncorrelated fading is assumed. To analyze the performance of video codecs, we show that a time correlated channel model must be combined with a dedicated 802.11a/g MAC/PHY simulation.

Robust video broadcasting over 802.11a/g in time-correlated fading channels

In order to deliver video streams efficiently over WiFi to many thousands of consumer handheld devices, broadcast protocols must be employed. In this mode of operation the received video quality can deteriorate rapidly as a result of high application layer packet loss which occurs because MAC frame retransmission cannot be used. In this paper we develop a robust video delivery solution for broadcast transmission over 802.11a/g. Using a cross-layer WiFi simulator in combination with an accurate time-correlated fading channel, the received video quality is evaluated for broadcast H.264 video sequences. Application layer cross-packet forward error correction is then used together with error concealment at the video client. Furthermore, the application of an external packet interleaver is considered. Combining a block size of two hundred packets (which introduces a 4.8 second delay) and an application layer FEC code rate of 0.75 our results demonstrate that video can be successfully broadcast over WiFi to many thousands of handheld terminals at large-scale spectator events.

Fusion Methods for Side Information Generation in Multi-View Distributed Video Coding Systems

The generation of the side information is an important part in the design of a distributed video coding (DVC) system as it directly relates to the systems rate distortion performance. In multi-view systems spatial/view inter-camera correlations can be exploited alongside temporal/motion intra camera ones for generating the side information as accurately as possible. In this paper, algorithms for fusing multiple such side information estimates, generated with temporal and view interpolation methods, are proposed. Two algorithms are suggested along with weighted fusion schemes for combining multiple methods. The results presented indicate performance improvements of up to 2 dB compared to exiting fusion approaches.

Cross-layer WLAN measurement and link analysis for low latency error resilient wireless video transmission

This work introduces a cross-layer measurement and link analysis strategy for video transport over IEEE 802.11. Field trial measurement data is presented for streamed H.264 video over ad-hoc 802.11g links. The data is used to analyze the interactions between the physical/network/transport and application layers. The development of cross layer optimized low latency error resilient video transmission schemes is discussed.

Multimedia Transmission over IEEE 802.11g WLANs: Practical Issues and Considerations

Multimedia transmission is widely available over wired networks. With the advent of low-cost WLAN devices, the wireless delivery of multimedia content is highly desirable. However, for media requiring low end-to-end latency, the use of WLAN technology introduces many significant challenges. These challenges are further enhanced if multicast/broadcast transmission is employed to serve a wide range of wireless terminals. This paper provides an understanding of the practical issues associated with WLAN multimedia transmission. A cross-layer measurement programme is performed to identify design issues for low-cost off-the-shelf WLAN multimedia systems. Problems identified include i) broadcast/multicast transmission using the slowest link-speed, ii) common link adaptation mechanisms for all clients, iii) lack of a call admission policy, and iv) irreducible PER even in good channel conditions.

Macroblock selection algorithms for error resilient H.264 video wireless transmission using redundant slices

In this paper, a redundant picture formation algorithm that takes into account a given redundancy rate constraint is presented for error resilient wireless video transmission without reliance on retransmissions. The algorithm assigns priorities to MBs according to two suggested metrics and ranks macroblocks accordingly. The first metric is based on an end-to-end distortion model and aims at maximising the reduction in distortion per redundancy bit. The end-to-end distortion accounts for the effects of error propagation, mismatch between the primary and redundancy description and error concealment. Macroblocks providing large distortion reduction for fewer bits spent are assigned a higher priority. The second metric employs the variance of the motion vectors of a macroblock and those of its neighbouring blocks. Results show that the rate distortion metric outperforms other examined metrics by up to 2dB. Moreover, gains over existing error resilience schemes, such as LA-RDO, are presented.