Michael R. Frater

A cell-loss concealment technique for MPEG-2 coded video

Audio-visual and other multimedia services are seen as important sources of traffic for future telecommunication networks, including wireless networks. A major drawback with some wireless networks is that they introduce a significant number of transmission errors into the digital bitstream. For video, such errors can have the effect of degrading the quality of service to the point where it is unusable. We introduce a technique that allows for the concealment of the impact of these errors. Our work is based on MPEG-2 encoded video transmitted over a wireless network whose data structures are similar to those of asynchronous transfer mode (ATM) networks. Our simulations include the impact of the MPEG-2 systems layer and cover cell-loss rates up to 5%. This is substantially higher than those that have been discussed in the literature up to this time. We demonstrate that our new approach can significantly increase received video quality, but at the cost of a considerable computational overhead. We then extend our technique to allow for higher computational efficiency and demonstrate that a significant quality improvement is still possible.

A Propagation-delay-tolerant Collision Avoidance Protocol for Underwater Acoustic Sensor Networks

Underwater acoustic sensor networks can be employed by a vast range of applications, retrieving accurate and up-to-date information from underneath the water surface. Although widely used by radios in terrestrial sensor networks, radio frequencies do not propagate well underwater. Acoustic channels are therefore employed as an alternative to support long-distance and low-power communication underwater, even though such channels suffer from long propagation delay and very limited bandwidth. In this paper, we investigate the impact of the large propagation delay on the throughput of selected classical MAC protocols and their variants, and show that protocols need to be revised to accommodate large propagation delay in order to achieve good throughput. We then introduce a propagation-delay-tolerant collision avoidance protocol named PCAP and show that by taking into account the propagation delay, PCAP offers higher throughput than the protocols that are widely used by conventional wireless communication networks.

Design of a Propagation-Delay-Tolerant MAC Protocol for Underwater Acoustic Sensor Networks

Underwater acoustic sensor networks (UASNs) can be employed in a vast range of applications, retrieving accurate and up-to-date information from underneath the oceans surface. Although widely used by terrestrial sensor networks, radio frequencies (RFs) do not propagate well underwater. Therefore, acoustic channels are employed as an alternative to support long-distance and low-power communication in underwater sensor networks even though acoustic signals suffer from long propagation delay and have very limited bandwidth. In this paper, we introduce an adaptive propagation-delay-tolerant collision-avoidance protocol (APCAP) for the media access control (MAC) sublayer of UASN. The protocol includes an improved handshaking mechanism that improves efficiency and throughput in UASN where there is a large propagation delay. The mechanism guarantees nodes that can potentially interfere with a forthcoming transmission are properly informed. It also allows a node to utilize its idle time while waiting for messages to propagate, which is otherwise wasted by most existing MAC protocols. The simulation results indicate that where employed by UASN, APCAP exhibits good performance and outperforms the other MAC protocols examined in this paper.

Optimally efficient estimation of the statistics of rare events in queueing networks

Because of their rarity, the estimation of the statistics of buffer overflows in networks of queues by direct simulation is very costly. An asymptotically optimal (as the overflow recurrence time becomes large) scheme has been proposed by others, using importance sampling. Two aspects of this scheme are addressed. First, in the existing approach, a numerical minimization is required to generate the simulation network. An equivalent analytic minimization is described. A simple procedure for constructing the optimal simulation network is included. Second, it is shown that the average behaviour of the simulation system is the same as the average behavior of the original network in the period leading up to a buffer overflow. For a sufficiently large buffer size, the optimal simulation system depends only on the statistics of the service rate of one queue (that of the least serviced buffer) and the arrival process, assuming that no two service rates are actually equal, and does not depend in any way on the statistics of the service rates of buffers other than the one dominating the overflow statics. >

A new statistical model for traffic generated by VBR coders for television on the broadband ISDN

Many techniques have been proposed for modeling source rates generated by variable bit rate video coders. However, little work has been done to verify these models, especially in the area of measuring the quality of their predictions for network performance and the grade of service experienced by a customer. In this paper, we summarize the previous work in this area, highlight some of the difficulties in effectively verifying these models, and suggest methods by which source rate models for VBR video might be more effectively verified in the future. In addition to studying the problem of model verification, a new model is proposed. Using this model, a video sequence is characterized by four parameters. As in previous cases, the number of bits generated by the coder for a sequence of video frames is modelled. The new model differs from previous models in that this sequence is not Markovian. >

MPEG 2 video services for wireless ATM networks

Audio-visual and other multimedia services are seen as an important source of traffic for future telecommunications networks, including wireless networks. We examine the impact of the properties of a 50 Mb/s asynchronous transfer mode (ATM)-based wireless local-area network (WLAN) on Moving Picture Experts Group phase 2 (MPEG 2) compressed video traffic, with emphasis on the networks error characteristics. The paper includes a description of the WLAN system used and its loss characteristics, a brief discussion of relevant aspects of the MPEG 2 standards and the associated error resilience techniques for minimizing the effect of transmission errors, and a description of the method by which the video data is organized for transmission on the network. We show results on the effect of cell loss due to transmission errors on the quality of the decoded video at the receiver, and demonstrate how error resilience techniques in both the systems and video layers of MPEG 2 can be used to improve the quality of service. Situations where up to 1% of the data is lost due to network transmission errors are examined. Most important among the findings are that error resilience experiments that do not take into account the effect of the MPEG 2 systems layer will tend to significantly overestimate the quality of received video, and that the error resilience techniques provided within the MPEG 2 standard are not sufficient to provide acceptable quality with acceptable overheads, but that this quality can be significantly increased by the addition of a small number of simple techniques.

Windowed Image Registration for Robust Mosaicing of Scenes with Large Background Occlusions

We propose an enhanced window-based approach to local image registration for robust video mosaicing in scenes with arbitrarily moving foreground objects. Unlike other approaches, we estimate accurately the image transformation without any pre-segmentation even if large background regions are occluded. We apply a windowed hierarchical frame-to-frame registration based on image pyramid decomposition. In the lowest resolution level phase correlation for initial parameter estimation is used while in the next levels robust Newton-based energy minimization of the compensated image mean-squared error is conducted. To overcome the degradation error caused by spatial image interpolation due to the warping process, i.e. aliasing effects from under-sampling, final pixel values are assigned in an up-sampled image domain using a Daubechies bi-orthogonal synthesis filter. Experimental results show the excellent performance of the method compared to recently published methods. The image registration is sufficiently accurate to allow open-loop parameter accumulation for long-term motion estimation.

Electromagnetic communications within swarms of autonomous underwater vehicles

Autonomous underwater vehicles (AUVs) have great value as platforms for sensors. While traditional AUVs have been large platforms that tend to operate alone or in very small groups, there has been recent interest in using very large numbers of small AUVs in swarms. Such swarms offer the potential to increase sensor density within the same, or lower, system cost. In this paper, we compare via simulation the network throughput obtained in such a swarm using electromagnetic and acoustic signals. The results of these simulations suggest that, for the same raw channel bit rate, much higher throughputs (by up to more than one order of magnitude) can be obtained using electromagnetic signals than can be obtained using acoustic signals, and that careful choice of parameters such as carrier frequency can help optimize performance.

A high data-rate, software-defined underwater acoustic modem

Most underwater acoustic modems offer only low data rates. This is largely because they operate at low frequency, which limits the channel bandwidth available, and hence the symbol rate. The low frequency acoustic channel suffers from substantial multipath and doppler effects, which constrain the signal quality at the receiver. As a result only 1 or 2 bits per symbol are achieved, with the effective data rate further reduced by error control coding. High frequency acoustic signals are heavily attenuated in water, severely constraining the range of high frequency links. High frequency signals however offer substantially greater signal bandwidth, and probably improved channel quality which guides our design choice of a high frequency acoustic modem for underwater communication. Contemporary Field Programmable Gate Arrays (FPGAs) can provide good system functionality at low cost and with the flexibility to perform rapid testing and development of communication algorithms. They may also be competitive in production systems. In this paper we describe current progress in development of a high frequency, high data-rate modem which is implemented entirely in FPGA. This differs from most existing modems which are based on DSP processors. Being software defined, the modem is flexible because the parameters can be reconfigured with relative ease, minimising the cost of rework as the design evolves. This modem will not only demonstrate the feasibility of high frequency FPGA based modems, but will also be a valuable tool to provide a better understanding of the high frequency acoustic channel, and demonstrate the utility of absorption to enhance channel re-use rates in underwater acoustic networks. The modulator has been implemented in the FPGA, to produce laboratory and open water tests that conform to modelling. The demodulator has been implemented in Matlab, and recovers the carrier, code synchronisation and data from recordings of both laboratory and open water tests. Coding of the demodulator into the FPGA is currently in progress.

An Efficient Mode Selection Prior to the Actual Encoding for H.264/AVC Encoder

Many video compression algorithms require decisions to be made to select between different coding modes. In the case of H.264, this includes decisions about whether or not motion compensation is used, and the block size to be used for motion compensation. It has been proposed that constrained optimization techniques, such as the method of Lagrange multipliers, can be used to trade off between the quality of the compressed video and the bit rate generated. In this paper, we show that in many cases of practical interest, very similar results can be achieved with much simpler optimizations. Mode selection by simply minimizing the distortion with motion vectors and header information produces very similar performance to the full constrained optimization, while it reduces the mode selection and over all encoding time by 31% and 12%, respectively. The proposed approach can be applied together with fast motion search algorithms and the mode filtering algorithms for further speed up.