Abstract-Subpixel motion estimation plays an important role in compression efficiency within modern video codecs such as MPEG2, MPEG4, and H.264. Subpixel motion estimation is implemented within these standards using interpolated values at 1/2 or 1/4 pixel accuracy. Such interpolation gives a good reduction in residual energy for each predicted macroblock and, therefore, improves compression. However, this leads to a significant increase in computational complexity at the encoder. This is especially true for H.264 where the cost of an exhaustive set of macroblock segmentations need to be estimated in order to obtain an optimal mode for prediction. This paper presents a novel interpolation-free scheme for subpixel motion estimation using the result of the full pixel sum of absolute difference distribution of each motion compensated block applied to an H.264 encoder. This system produces reduced complexity motion estimation with a controllable tradeoff between compression performance and encoder speed. These methods facilitate the generation of a real time software H.264 encoder.

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. (UDP). While UDP offers a time bounded service, it does not guarantee the delivery of the video packets. To use this protocol successfully, the video codec must support strong error resilience and concealment. Furthermore, some video applications require transmission to a large number of remote terminals. Due to a lack of radio bandwidth, in these situations a dedicated point-to-point UDP-IP transmission cannot be accommodated with each remote client. Instead, Broadcast (or Multicast) transmission must be used at the WLAN. For these types of link there is absolutely no retransmission (ARQ) of errored or missing data packets (either at the WLAN MAC or the transport layer). As discussed above, wireless UDP based unicast and multicast/broadcast transmissions pose the problem of packet erasures. The lossy nature of the transmission medium can lead to unacceptable video quality at the client. As a result, it is important to consider the channel behaviour I. INTRODUCTION

This paper 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 physicaunetworkkransport and application layers. The development of cross layer optimized tow latency error resilient ddeo transmission schemes is discussed.

This paper proposes a low-complexity sub-pixel refinement to motion estimation based on full-search block matching algorithm (BMA) at integer-pixel accuracy. This algorithm eliminates the need to produce interpolated reference frames, which is may be too memory-and processor-intensive, for some real-time mobile applications. The algorithm assumes the BMA is done at pixel resolution and the (sum-of-absolute-differences) SADs of the candidate motion vector and its neighbouring vectors are available for each block. The proposed method than models the SAD distribution around the candidate motion vector and its neighbouring points. Actual minimum point at sub-pixel resolution is then computed according to the model used. 3 variations of the parabolic model are considered and simulations using the H.263 standard encoder on several test sequences reveal an improvement of 1.0 dB over integeraccuracy motion estimation. Albeit its simplicity, some test cases come close to the results obtained by actual interpolated reference frames.

The EU FP6 WCAM (Wireless Cameras and Audio-Visual Seamless Networking) project aims to study, develop and validate a wireless, seamless and secured end-to-end networked audio-visual system for video surveillance and multimedia distribution applications. This paper describes the video transmission aspects of the project, with contributions in the areas of H.264 video delivery over wireless LANs. The planned demonstrations under WCAM include the transmission of H.264 coded material over 802.11b/g networks with TCP/IP and UDP/IP being employed as the transport and network layers over unicast and multicast links. UDP based unicast and multicast transmissions pose the problem of packet erasures while TCP based transmission is associated with long delays and the need for a large jitter buffer. This paper presents measurement data that have been collected at the WCAM trial site along with analysis of the data, including characterisation of the channel conditions as well as recommendations on the optimal operating parameters for each of the above transmission scenarios (e.g. jitter buffer sizes, packet error rates, etc.). Recommendations for error resilient coding algorithms and packetisation strategies are made in order to moderate the effect of the observed packet erasures on the quality of the transmitted video. Advanced error concealment methods for masking the effects of packet erasures at the receiver/decoder are also described.