Abstract Device‐to‐device (D2D) communications play a significant role in increasing the capacity of cellular networks. D2D enables direct interaction between mobile users without traversing the base station (BS) and at the same time causes interference between D2D and cellular links which can affect the system performance. To ensure efficient and reliable D2D operations within a cellular network, the related quality of service (QoS) requirement needs to be met in the design of such a system involving both D2D and cellular users in a same network. In this study, we have investigated three different transmission schemes (cellular, D2D and cooperative D2D modes) by deriving the closed‐form expressions of key QoS performance such as system reliability, achievable data rate, and energy efficiency for these three transmission schemes. Based on this establishment, we have also examined the effects of using different numbers of parallel relay branches and different number of relays within each branch on the network performance to reveal how effectively the cooperative D2D can enhance QoS in comparison with other transmission methods. The proposed approach can be optimised through adaptively selecting appropriate transmission schemes and, as a result, good trade‐offs between system reliability and efficiency can also be achieved under various environmental conditions.

In this paper, we investigate the fading characteristics in intra-vehicle channels based on a vehicle testbed and measurements which reflect the channel loss behavior contributed by attenuation and fading (large-scale and small-scale). We show that small-scale fading is dominant over other factors such as large-scale fading, which can be described with the Rician model. By applying the moment-based estimator method, we obtain the Rician K-factor that is correlated with the transmission distance given and follows the power law. We also show that the intra-vehicle channel can be described as near-Rayleigh for most of locations inside a vehicle. In addition, we derive an upper bound of the signal length based on our measurements for analyzing fading properties in vehicles.

In this paper we investigate the performance of Dedicated Short-Range Communications (DSRC) or IEEE 802. 11p/OCB transceiver gathering performance statistics and relating its usage for ensuring safety and mitigating collision avoidance in Vehicle-to-Vehicle (V2V) communications. To achieve this, we created a low-cost Software Defined Radio (SDR) based DSRC testbed. We focus on quality of service (QoS) and robustness of such a system to assess how it could be used to support road safety applications and assist cooperative awareness in smart cities. To deal with real-world scenarios, we established a testbed using SDR devices and tested in varied scenarios. Our experiments are tailored to give insights into the fundamental challenges regarding how the practical driving safety issues can be effectively addressed by different technologies such as SDR, V2V communications, sensing and their combination. The results we collected and analysed can be used to support future V2V experimentation.

In this paper, we investigate the intra-vehicle multipath channel properties in non-line-of-sight conditions based on field measurements. The test was conducted using a software-defined-radio (SDR) based testbed within a frequency range of 2 GHz to 3 GHz. The results reveals the property of the power delay profile for 49 different testing locations and shows the channel behavior across the passenger and boot compartments of the vehicle investigated. The coherence bandwidth and delay spread of the channel are also shown to vary between 8.7 MHz to 24 MHz and between 50 - 75 ns, respectively. In addition, the dependency of coherence bandwidth and delay spread on distance and location are also analyzed.

To address challenges on enabling reliable wireless communications inside vehicles, we initialize an investigation on the impact of intra-vehicle channel behavior and potential Electromagnetic Interference (EMI) that could hamper the performance required for wireless transmission in this environment. The investigation carried out over a vehicle testbed produces real-world data showing the loss performance and EMI across different compartments of the vehicle. The results reveal how the fading related loss, together with EMI at both peak and average levels contribute to the impairment of the intra-vehicle wireless channel.

In this study, the authors investigate the outage‐optimal relay strategy under outdated channel state information (CSI) in a decode‐and‐forward cooperative communication system. They first confirm mathematically that minimising the outage probability under outdated CSI is equivalent to minimising the conditional outage probability on the outdated CSI of all the decodable relays’ links. They then propose a multiple‐relay strategy with optimised transmitting power allocation (MRS‐OTPA) that minimises the conditional outage probability. It is shown that this MRS is a generalised relay approach to achieve the outage optimality under outdated CSI. To reduce the complexity, they also propose a MRS with equal transmitting power allocation (MRS‐ETPA) that achieves near‐optimal outage performance. It is proved that full spatial diversity, which has been achieved under ideal CSI, can still be achieved under outdated CSI through MRS‐OTPA and MRS‐ETPA. Finally, the outage performance and diversity order of MRS‐OTPA and MRS‐ETPA are evaluated by simulation.

The concern over the quality of delivering video streaming services in mobile wireless networks is addressed in this work. A framework that enhances the Quality of Experience (QoE) of end users through a quality driven resource allocation scheme is proposed. To play a key role, an objective no-reference quality metric, Pause Intensity (PI), is adopted to derive a resource allocation algorithm for video streaming. The framework is examined in the context of 3GPP Long Term Evolution (LTE) systems. The requirements and structure of the proposed PI-based framework are discussed, and results are compared with existing scheduling methods on fairness, efficiency and correlation (between the required and allocated data rates). Furthermore, it is shown that the proposed framework can produce a trade-off between the three parameters through the QoE-aware resource allocation process.

Video streaming via Transmission Control Protocol (TCP) networks has become a popular and highly demanded service, but its quality assessment in both objective and subjective terms has not been properly addressed. In this paper, based on statistical analysis a full analytic model of a no-reference objective metric, namely pause intensity (PI), for video quality assessment is presented. The model characterizes the video playout buffer behavior in connection with the network performance (throughput) and the video playout rate. This allows for instant quality measurement and control without requiring a reference video. PI specifically addresses the need for assessing the quality issue in terms of the continuity in the playout of TCP streaming videos, which cannot be properly measured by other objective metrics such as peak signal-to-noise-ratio, structural similarity, and buffer underrun or pause frequency. The performance of the analytical model is rigidly verified by simulation results and subjective tests using a range of video clips. It is demonstrated that PI is closely correlated with viewers' opinion scores regardless of the vastly different composition of individual elements, such as pause duration and pause frequency which jointly constitute this new quality metric. It is also shown that the correlation performance of PI is consistent and content independent.

In this paper a full analytic model for pause intensity (PI), a no-reference metric for video quality assessment, is presented. The model is built upon the video play out buffer behavior at the client side and also encompasses the characteristics of a TCP network. Video streaming via TCP produces impairments in play continuity, which are not typically reflected in current objective metrics such as PSNR and SSIM. Recently the buffer under run frequency/probability has been used to characterize the buffer behavior and as a measurement for performance optimization. But we show, using subjective testing, that under run frequency cannot reflect the viewers' quality of experience for TCP based streaming. We also demonstrate that PI is a comprehensive metric made up of a combination of phenomena observed in the play out buffer. The analytical model in this work is verified with simulations carried out on ns-2, showing that the two results are closely matched. The effectiveness of the PI metric has also been proved by subjective testing on a range of video clips, where PI values exhibit a good correlation with the viewers' opinion scores.

This work looks into video quality assessment applied to the field of telecare and proposes an alternative metric to the more traditionally used PSNR based on the requirements of such an application. We show that the Pause Intensity metric introduced in [1] is also relevant and applicable to heterogeneous networks with a wireless last hop connected to a wired TCP backbone. We demonstrate through our emulation testbed that the impairments experienced in such a network architecture are dominated by continuity based impairments rather than artifacts, such as motion drift or blockiness. We also look into the implication of using Pause Intensity as a metric in terms of the overall video latency, which is potentially problematic should the video be sent and acted upon in real-time. We conclude that Pause Intensity may be used alongside the video characteristics which have been suggested as a measure of the overall video quality.

In this paper we propose a hybrid TCP/UDP transport, specifically for H.264/AVC encoded video, as a compromise between the delay-prone TCP and the loss-prone UDP. When implementing the hybrid approach, we argue that the playback at the receiver often need not be 100% perfect, provided that a certain level of quality is assured. Reliable TCP is used to transmit and guarantee delivery of the most important packets. This allows use of additional features in the H.264/AVC standard which simultaneously provide an enhanced playback quality, in addition to a reduction in throughput. These benefits are demonstrated through experimental results using a test-bed to emulate the hybrid proposal. We compare the proposed system with other protection methods, such as FEC, and in one case show that for the same bandwidth overhead, FEC is unable to match the performance of the hybrid system in terms of playback quality. Furthermore, we measure the delay associated with our approach, and examine its potential for use as an alternative to the conventional methods of transporting video by either TCP or UDP alone.

In this paper, we investigate the hop distance optimization problem in ad hoc networks where cooperative multi-input-single-output (MISO) is adopted to improve the energy efficiency of the network. We first establish the energy model of multihop cooperative MISO transmission. Based on the model, the energy consumption per bit of the network with high node density is minimized numerically by finding an optimal hop distance, and, to get the global minimum energy consumption, both hop distance and the number of cooperating nodes around each relay node for multihop transmission are jointly optimized. We also compare the performance between multihop cooperative MISO transmission and single-input-single-output (SISO) transmission, under the same network condition (high node density). We show that cooperative MISO transmission could be energy-inefficient compared with SISO transmission when the path-loss exponent becomes high. We then extend our investigation to the networks with varied node densities and show the effectiveness of the joint optimization method in this scenario using simulation results. It is shown that the optimal results depend on network conditions such as node density and path-loss exponent, and the simulation results are closely matched to those obtained using the numerical models for high node density cases.

Erasure control coding has been exploited in communication networks with an aim to improve the end-to-end performance of data delivery across the network. To address the concerns over the strengths and constraints of erasure coding schemes in this application, we examine the performance limits of two erasure control coding strategies, forward erasure recovery and adaptive erasure recovery. Our investigation shows that the throughput of a network using an (n, k) forward erasure control code is capped by r =k/n when the packet loss rate p ≤(te/n) and by k(1-p)/(n-te) when p >; (te/n), where teis the erasure control capability of the code. It also shows that the lower bound of the residual loss rate of such a network is (np-te) /(n-te) for (te/n) <; p ≤ 1. Especially, if the code used is maximum distance separable, the Shannon capacity of the erasure channel, i.e. 1-p, can be achieved and the residual loss rate is lower bounded by (p+r-1)/r, for (1-r)<; p ≤ 1. To address the requirements in real-time applications, we also investigate the service completion time of different schemes. It is revealed that the latency of the forward erasure recovery scheme is fractionally higher than that of the scheme without erasure control coding or retransmission mechanisms (using UDP), but much lower than that of the adaptive erasure scheme when the packet loss rate is high. Results on comparisons between the two erasure control schemes exhibit their advantages as well as disadvantages in the role of delivering end-to-end services. To show the impact of the bounds derived on the end-to-end performance of a TCP/IP network, a case study is provided to demonstrate how erasure control coding could be used to maximize the performance of practical systems.

Abstract Communication through relay channels in wireless sensor networks can create diversity and consequently improve the robustness of data transmission for ubiquitous computing and networking applications. In this paper, we investigate the performances of relay channels in terms of diversity gain and throughput via both experimental research and theoretical analysis. Two relaying algorithms, dynamic relaying and fixed relaying, are utilised and tested to find out what the relay channels can contribute to system performances. The tests are based on a wireless relay sensor network comprising a source node, a destination node and a couple of relay nodes, and carried out in an indoor environment with rare movement of objects nearby. The tests confirm, in line with the analytical results, that more relay nodes lead to higher diversity gain in the network. The test results also show that the data throughput between the source node and the destination node is enhanced by the presence of the relay nodes. Energy consumption in association with the relaying strategy is also analysed. Copyright © 2009 John Wiley & Sons, Ltd.

The multiple-input multiple-output (MIMO) technique can be used to improve the performance of ad hoc networks. Various medium access control (MAC) protocols with multiple contention slots have been proposed to exploit spatial multiplexing for increasing the transport throughput of MIMO ad hoc networks. However, the existence of multiple request-to-send/clear-to-send (RTS/CTS) contention slots represents a severe overhead that limits the improvement on transport throughput achieved by spatial multiplexing. In addition, when the number of contention slots is fixed, the efficiency of RTS/CTS contention is affected by the transmitting power of network nodes. In this study, a joint optimisation scheme on both transmitting power and contention slots number for maximising the transport throughput is presented. This includes the establishment of an analytical model of a simplified MAC protocol with multiple contention slots, the derivation of transport throughput as a function of both transmitting power and the number of contention slots, and the optimisation process based on the transport throughput formula derived. The analytical results obtained, verified by simulation, show that much higher transport throughput can be achieved using the joint optimisation scheme proposed, compared with the non-optimised cases and the results previously reported.

In this paper, we are concerned with robust H.264/AVC video transmission over lossy packet networks and present a hierarchical unequal packet loss protection (HULP) scheme in a transmission system that efficiently combines erasure coding, H.264/AVC error resilience techniques and importance measures in video coding schemes. The importance of the video stream packets is distinguished by three criteria: the frame sequence number in a group of pictures (GOP), the per-frame bitrate and the H.264/AVC data partition type. Using a fixed amount of redundancy, more important packets of a video stream are protected with a more powerful erasure code than the less important packets. We demonstrate the effectiveness of this approach by system implementation and performance evaluation. In the presence of packet loss, we show that the received video quality, as measured by PSNR, is significantly improved when the HULP scheme is used. More importantly, in our experiments, HULP can achieve higher PSNR values and better user perceived quality, but requiring less redundancy, than equal loss protection (ELP) schemes.

Although a number of investigations have been conducted using IEEE 802.11e enabled networks to stream class differentiated video, very few reports are available based on a real testbed. In our work, we set up a wireless testbed for H.264 video streaming through assigning the partitioned video packets onto the DCF MAC layer and different access classes of the EDCA MAC layer. We investigate three assignment schemes: 1) DCF is used and all the traffic is treated equally; 2) video traffic is assigned to each of the access classes in turn; and 3) the packets are assigned according to their importance and the class priority. In addition to the video stream we introduce TCP traffic from three clients in the best effort class. We show that video quality can be improved through properly assigning packets to wireless access classes compared to the standard best effort scheme. Importantly, we show, based on our testbed results, that the single class assignment can achieve better performance than the multi-class assignment suggested by other researchers. Finally we show that virtual contention between traffic classes at the access point is an important issue to address.

In this paper we investigate the possible benefits of a caching scheme for video encoded using the data partitioning option from the H.264 standard. We propose a method to measure the importance of a partition based on its contribution to the peak signal-to-noise ratio (PSNR) on all affected frames, and we term this measurement as PSNR profit. We calculate the average PSNR profit for each frame and partition type of real H.264 video footage. We then apply the Knapsack model to the storage optimisation of the partitioned video in terms of the total profit on a caching server. We show that the optimized cache (OC) outperforms the conventional cache (CC) when the cache reaches its maximum capacity and new videos continue to arrive. Under these circumstances, optimal caching aims to keep as many the most important partitions as possible, while conventional caching treats all the partitions equally. As a consequence, the server profit of an OC will continue to rise as the number of videos in the system increases, compared to the unchanged profit of a CC; and the client profit (relating to the received video quality) will reduce at a slower rate than that of the CC option. We also show that optimal caching can provide benefit even in a lossy environment because in this scenario the client profit is degraded more gracefully than that in the un-optimized scheme. When the optimized caches are formed as a caching network, it can be ensured that all the partitions of each video will remain somewhere in the network, instead of being deleted entirely as happened in the case where a sole cache is used. The required number of the servers or the proportion of space allocated for long term storage of videos in the network can then be determined.

In this paper we investigate the effect of path diversity on the packet loss performance through simulations and experiments using three overlay networks created on top of the PlanetLab testbed. The overlay network consists of a number of relay nodes that are positioned around the Internet. In each of the overlay networks UDP packets are streamed from a client to a destination for a period of 50 days, across up to six paths with a static packet dispatching algorithm. From the results obtained, we can see that increasing the number of paths could increase as well as decrease the packet loss probability, depending on the network environment and the structure of the multiple paths created. We have also investigated the correlation in the underlying network and demonstrated the performance of the metric defined.

In this paper, a robust H.264/AVC video transmission system over lossy packet network is investigated and implemented. The H.264/AVC standard has defined a new data partition scheme which can be used to perform unequal loss protection (ULP) in video transmission systems. However, few actual implementation and experiments which integrate data partition and loss protection algorithms have been reported. Based on the H.264/AVC data partitioning mechanism, we implement three loss protection schemes using Reed-Solomon and XOR parity codes, namely equal loss protection (ELP), ULP and partition A protection only (PAP). The performance of each scheme is evaluated under different network environments. Experimental results and performance analysis show that ELP has higher peak signal-to-noise ratio (PSNR) values than ULP and PAP, but PAP requires the least redundancy and offers the best tradeoffs between PSNR values and redundancy among all the three schemes.

We investigate the performance of an experimental wireless relay sensor network comprising a source node, a destination node and dedicated relay nodes. Two relaying algorithms, dynamic relaying and fixed relaying, are tested to find out to what extent the relaying algorithms could bring any benefit to a wireless sensor network. A simple scheme is applied at transmit nodes to eliminate the need for synchronization. The tests are carried out in an indoor environment with rare movement of objects. The results show that the network performance can be significantly improved by the presence of relay nodes if data are not protected by a repetition code. However, if repetition coding is deployed the performance difference between the networks with and without relaying is very small. Moreover, it is shown that a single relay node using the fixed relaying algorithm can offer the best trade-off between performance improvement and energy consumption.

Two product array codes are used to construct the$(24, 12, 8)$binary Golay code through the direct sum operation. This construction provides a systematic way to find proper$(8, 4, 4)$linear block component codes for generating the Golay code, and it generates and extends previously existing methods that use a similar construction framework. The code constructed is simple to decode.