This paper introduces a network architecture that integrates the space-air-ground integrated network with mobile edge computing (MEC) and orbital edge computing to advance sixth-generation communication systems. The proposed system employs unmanned aerial vehicles equipped with reconfigurable intelligent surfaces and satellite-based MEC to optimize resource management in complex, dynamic environments. By efficiently managing resources such as bandwidth and computational power at both base stations and low Earth orbit satellites, while making offloading decisions, the system aims to minimize utility costs while meeting stringent performance requirements. We utilize a long short-term memory (LSTM)-enhanced deep deterministic policy gradient (DDPG) algorithm to solve the formulated nonlinear programming problem, enabling dynamic and adaptive resource management. The LSTM-enhanced DDPG improves convergence speed by 44.44 % compared to conventional DDPG, significantly enhancing cost efficiency. Simulation results validate the robustness of the proposed method against state-of-the-art approaches.

This paper presents a novel low-complexity discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) scheme designed to reduce computational overhead while maintaining robust performance in wireless communication systems. The core idea is to modify the signal processing chain by merging the computations of multiple functional blocks, resulting in a significant reduction in the complexity of conventional DFT-s-OFDM implementations. A comparative analysis is conducted to evaluate the proposed scheme against traditional DFT-s-OFDM, using key metrics such as error vector magnitude (EVM) and computational complexity. Simulation results demonstrate that the proposed scheme achieves performance comparable to conventional DFT-s-OFDM in terms of EVM under various system configurations, while significantly reducing computational complexity. This makes the proposed scheme a promising candidate for next-generation wireless communication systems, particularly in uplink scenarios involving battery-powered devices.

Future 6G mobile networks are expected to provide ubiquitous communication to a wide range of mobile devices through heterogeneous networks including non-terrestrial networks (NTNs). As an important complement to terrestrial networks, unmanned aerial vehicles (UAVs) based NTNs offer benefits such as flexible deployment, mobility control, and strong line-of-sight links. In this paper, a cognitive radio (CR) assisted cooperative non-orthogonal multiple access (NOMA) UAV system is proposed and its performance is analysed. In this system, the base station uses power-domain NOMA regarding the UAVs. The UAVs act as decode-and-forward relays serving clusters of user equipments (UEs). The CR concept is applied to reduce the interference caused by the UAVs to adjacent clusters of UEs. Analytical expressions of the outage probability and sum rate of this system are derived for the case of Nakagami-m fading. Numerical results are provided to reveal the impact of system parameters on the outage probability and sum rate. Further, design options and trade-offs between orthogonal multiple access and CR assisted cooperative NOMA UAV systems are illustrated.

In this paper, a mobile virtual reality (VR) video streaming system is considered and assessed. The system comprises of a VR server, a VR center offering mobile edge computing along with a buffer, and a mobile head-mounted display (HMD). Markov-modulated processes are used to model the arrival and departure traffic at the VR center and the time-varying link quality of the wireless channel to the mobile HMD. As the Markovian arrival process at the VR center is independent of the link quality variations of the wireless channel, the VR center controls the delivery of VR video packets to the mobile HMD subject to the link quality of the wireless channel. The Markov models are used to derive expressions of queueing theoretical performance measures, i.e., throughput, queueing delay, and packet loss probability. Numerical results are provided to investigate performance trade-offs for real data traces.

The integration of terrestrial and non-terrestrial networks with mobile edge computing (MEC) and orbital edge computing (OEC) technologies is essential for advancing 6G communication networks. This paper introduces a network architecture that combines terrestrial and non-terrestrial networks by integrating drones (also known as UAV)-carried reconfigurable intelligent surfaces (RIS) and satellite-based MEC to optimize resource allocation in intelligent autonomous transportation systems (IATS). The primary objective is to minimize total system utility costs through the optimal allocation of bandwidth, computational power at the base station and low Earth orbit (LEO) satellite, and offloading decisions, all while adhering to strict performance and delay constraints. We address the complex resource optimization challenge by formulating a nonlinear programming (NLP) problem. To solve this problem, we employ long short-term memory (LSTM)-enhanced deep deterministic policy gradient (DDPG) and LSTM-enhanced twin delayed deep deterministic policy gradient (TD3) algorithms, which enable dynamic and adaptive resource management. These LSTM-enhanced algorithms improve convergence speed by 44.44% and 73.81%, respectively, compared to their conventional counterparts, while significantly enhancing cost efficiency. Our simulation results demonstrate substantial improvements in system performance, with effective resource allocation and minimal utility costs, providing a robust solution for ensuring high-quality, low-latency communication in diverse 6G IATS environments.

In this paper, we consider an Internet-of-Things network supported by several satellites and multiple cache-assisted unmanned aerial vehicles (UAVs). Due to the long-distance transmission and detrimental effects from the transmission environment, the latency can be extremely high, especially in the presence of backhaul congestion. Therefore, we formulate an optimisation problem with the aim of minimising the total network latency. To reduce the complexity of the original problem, it is divided into three sub-problems, namely, clustering ground users associated with UAVs, cache placement in UAVs (to support the network in avoiding backhaul congestion), and power allocation for satellites and UAVs. We propose a distributed optimisation method consisting of: a non-cooperative game is designed to obtain the solution to the clustering problem; a genetic algorithm, which is powerful in the scenario of many variables, is employed to obtain the optimal solution to the high-complexity caching problem; and a quick estimation technique is used for power allocation. Additionally, a centralised optimisation method is presented as a benchmark. Simulation results show that although the distributed method leads to network latency of approximately 30% higher than the centralised method, it takes significantly less time to execute and is suitable for systems requiring strict real-time computing constraints. Furthermore, the numerical results prove the efficiency of our methods compared with other conventional ones.

In this paper, we consider a downlink scenario in which a base station (BS) communicates with user equipment (UE) that move with high speed. Orthogonal time frequency space (OTFS) modulation is used to overcome the effects of high Doppler shifts inherent with high-mobility scenarios. To increase spectral efficiency, power-domain non-orthogonal multiple access (NOMA) is combined with OTFS to form an OTFS-NOMA system. The K-means clustering algorithm is deployed which partitions the coverage area of the BS into clusters such that the UEs within a cluster have a shorter distance to the respective cluster center than to the centers of the other clusters. The cluster centers act as simultaneous multiple users on which basis power-domain NOMA selects the power allocation coefficients to reduce the mutual interference among UEs. OTFS can control the resolution of the delay-Doppler domain to more accurately compensate for propagation delays and Doppler shifts. The performance of this OTFS-NOMA system with K-means clustering of the UEs is assessed in terms of sum rate. The numerical results illustrate that the successive interference cancellation of power-domain NOMA significantly improves sum rate. Additional improvements are obtained by choosing a sufficiently large size of the delay-Doppler grid of OTFS which effectively copes with the high Doppler shifts of the considered high-mobility scenario.

The performance of remotely rendered Virtual Re-ality (VR) is sensitive to temporal disturbances in communication channels. An earlier Quality of Experience (QoE) study of tem-poral impacts in the form of frame stalls has revealed difficulties with subjective disturbance ratings while performing a task in an interactive 6-degrees-of-freedom (DOF) VR environment. This study follows up on above observation by comparing QoE ratings in the presence and absence of a task. The exploratory findings show that the task-free subjective tests yield lower ratings compared to the subjective tests with task. This indicates that the participants became more sensitive to temporal impairments in the absence of a task. Also, the positive impact of reprojection on the QoE ratings decreased in the task-free environment. The simulator sickness results for individual symptoms were on similar low levels in both settings. The total score (TS) of sickness severity was higher after than before the subjective tests with task while the difference between the TS before and after the task-free subjective tests was insignificant,

Subjective 360° video quality experiments provide the ground truth for developing related systems such that acceptable quality of experience is offered to the users. To obtain statistically meaningful insights from the data recorded in quality experiments, engaging a sufficiently large number of subjects is essential. In contrast to conventional videos, little has been reported on statistical measures that may guide the choice of the number of subjects required for 360° video quality experiments. In this paper, the standard deviation of opinion score (SOS) hypothesis, measuring the diversity among the opinion scores given by a panel of subjects, is used to reveal the number of subjects needed. It is shown that the number of subjects needed varies depending on a threshold on SOS characteristics in terms of diversity among opinion scores.

Remotely rendered Virtual Reality (VR) typically has to rely upon less capable and less reliable communication channels as compared to locally rendered VR. Such communication channels pose the risk of impairing the VR experience with temporary disturbances in form of frame stalls that would not be present in local VR. However, it is not obvious to which extent the durations of temporary stalls affect the Quality of Experience (QoE) in interactive 6-degrees-of-freedom (DOF) VR, and neither to which extent present reprojection/warping techniques mitigate any adverse effects of such stalls on user perception. In this paper, subjective experiments were conducted with N =29 users to quantify the relationship between QoE and single, isolated stall durations in interactive 6-DOF remote VR when stalls either freeze the frame in place, or when frames are reprojected by the VR driver. Results indicate that given a 90 fps headset and a simple task that requires moderate head-movement; (1) short, isolated stalls of up to 4 frames go unnoticed on average, no matter whether reprojection is used or not, (2) the number of perceived stalls when using reprojection is the same or lower than the reference (no stall) in the range 1–4 frame stalls, (3) reprojection entails significantly higher user ratings for stalls ranging from 8 frames and beyond, (4) the improvement of using reprojection over freezes in terms of making more stalling events imperceptible is highest in the range 8–16 frames.

A dataset has been created from an experiment regarding the subjective Quality of Experience (QoE) in relation to stalling video feeds in interactive 6-Degrees-of-freedom Virtual Reality (6-DOF VR). The main purpose of collecting this data is to provide insight into the QoE of stalls that may occur in interactive remote-rendered 6-DOF VR. The data contains information regarding user ratings coupled with various stall lengths and for both frozen or reprojected images. Additionally, user motion and pixel changes (in the form of Mean Square Errors) in successive images in the vicinity of the stall are also stored. In summary, 29 users participated in testing, each rating 16 scenes, amounting to a total of 464 rated scenes.

In this paper, we propose and assess the performance of a downlink non-orthogonal multiple access (NOMA)based full-duplex (FD) unmanned aerial vehicle (UAV) network for disaster scenarios. The K-means algorithm is used to partition the user equipments (UEs) residing in the disaster region into a number of clusters. The UAVs are located at the cluster centers and act as decode-and-forward relays to secure coverage from an operational base station (BS) into the disaster region. The power-domain NOMA used at the BS and the UAVs operating in FD mode improve the system performance in terms of outage probability and sum rate compared to orthogonal multiple access and half-duplex mode. In particular, analytical expressions for the outage probability and sum rate are derived. Numerical results are provided to reveal the impact of system parameters on the performance of the NOMA-based FD UAV network over Nakagami-m fading which in turn illustrate design options for applications in disaster scenarios.

Abstract Emerging 5G and future 6G mobile networks are expected to cater for high mobility scenarios ranging from vehicle-to-vehicle communications to unmanned aerial vehicles and airborne platforms. Communications in this type of deployments suffer from severe Doppler shifts which require new modulation waveforms. Orthogonal time frequency space (OTFS) modulation has recently been proposed as a promising technology for coping with high Doppler channels. OTFS converts a time-varying fading channel into a time-independent channel in the two-dimensional delay-Doppler (DD) domain. The transmit symbols are multiplexed into a nearly constant channel with a complex channel gain in the DD domain. In this paper, we consider a high Doppler airborne communication network where relative mobile node speeds can be above 1200 m/s. The considered system represents a mobile ad-hoc network where the airborne mobile nodes can join or leave the network. Furthermore, each node is equipped with an antenna array that supports directed communication among mobile nodes. The Doppler shifts in this airborne communication network are in the order of 52-72 kHz and may potentially be even higher depending on the selected carrier frequency and the relative speed among the airborne platforms. As such, OTFS modulation is used in this work to efficiently compensate for the high Doppler shifts in the DD domain. In particular, a comprehensive performance assessment in terms of bit error rate (BER) is conducted to reveal the potential of OTFS modulation in dealing with such extreme transmission scenarios. The impact of physical layer parameters, number of delay-Doppler bins in the DD domain used for OTFS modulation, directed versus two-ray channels, and the combination of multiple-input multiple-output (MIMO) systems with OTFS modulation on the BER is assessed. It is shown that both OTFS modulation over a two-ray channel as well as MIMO-OTFS modulation provide a reliable airborne communication network with low BER.

The COVID-19 pandemic has induced dramatic effects in all areas of society worldwide. It has led to drastic restrictions on academic exchanges and caused research projects to be put on hold. The need to maintain social distancing has severely affected the research communities that rely on in-person studies. In particular, Quality of Experience (QoE) research relies heavily on user studies and in-person subjective tests as means of obtaining ground truths for system designs. In this position paper, we focus on the impact of COVID-19 on conducting subjective tests for digital media quality assessment. An overview of the related discussions that take place in associated research communities is provided. A number of challenges are posed in terms of hygiene, ethics, standardization, and research quality issues. Opportunities for consolidating QoE research under COVID-19 and beyond are put forward, including alternative experimental designs and adaptation of research methodologies. Numerous enablers are suggested and discussed, revealing various avenues that can be followed to assist in conducting subjective tests under pandemic conditions. We believe that this position paper will be helpful for researchers and practitioners relying on in-person studies to keep abreast about the potential measures that support coming out of the COVID-19 pandemic strong and being prepared for future endeavors.

Virtual Reality (VR) applications put high demands on software and hardware in order to enable an immersive experience for the user and avoid causing simulator sickness. As soon as networks become part of the Motion-To-Photon (MTP) path between rendering and display, there is a risk for extraordinary delays that may impair Quality of Experience (QoE). This short paper provides an overview of latency measurements and models that are applicable to the MTP path, complemented by demands on user and network levels. It specifically reports on freeze duration measurements using a commercial TPCAST wireless VR solution, and identifies a corresponding stochastic model of the freeze length distribution, which may serve as disturbance model for VR QoE studies.

Cooperative communications in which relays assist the transmission of signals from source to destination offer extended radio coverage and improved link reliable. However, transmitting signals with the help of a relay network may also open additional avenues for eavesdropper to overhear confidential information. Further, as jammers and eavesdroppers may cooperate to attack the relay network, offering secure communications becomes a challenging problem. To cope and reduce the effect of such cooperative attacks, we propose a proactive attack scheme in which the legitimate users generate jamming signals in an attempt to counteract such hostile cooperative attacks. In order to assess the security performance of the proactive attack scheme compared to a non-protection scheme, an analytical expression of the secrecy outage probability is derived. Numerical results for different system settings are provided showing that the proactive attack scheme can indeed significantly improve the security performance of the considered relay networks.

In this paper, we propose a medium access control (MAC) protocol for multi-channel cognitive cooperative radio networks (CCRNs). In this protocol, each secondary user (SU) senses for spectrum opportunities within M licensed bands of the primary users (PUs). To enhance the accuracy of spectrum sensing, we employ cooperative sequential spectrum sensing where SUs mutually exchange their sensing results. Moreover, the information obtained from cooperative spectrum sensing at the physical layer is integrated into the channel negotiation process at the MAC layer to alleviate the hidden terminal problem. Finally, the performance of the proposed MAC protocol in terms of aggregate throughput of the CCRNs is analyzed. Numerical results are provided to assess the impact of channel utilization by PUs, number of contenting CCRNs, number of licensed bands, and false alarm probability of SUs on the aggregate throughput.

In this paper, we examine a full-duplex transmission scheme for cognitive cooperative radio networks (CCRNs) to improve capacity. In this network, the secondary transmitter and secondary relay are allowed to utilize the licensed spectrum of the primary user by using underlay spectrum access. We assume that the CCRN is subject to the interference power constraint of the primary receiver and maximum transmit power limit of the secondary transmitter and secondary relay. Under these constraints, we propose an optimal power allocation policy for the secondary transmitter and the secondary relay based on average channel state information (CSI) to optimize capacity. Then, we derive an expression for the corresponding achievable capacity of the secondary network over Nakagami-m fading. Numerical results are provided for several scenarios to study the achievable capacity that can be offered by this full-duplex underlay CCRN using the proposed optimal power allocation scheme.

In this paper, we study the outage probability of a multiple relay cooperative cognitive radio network over amp;#954;-amp;#956; shadowed fading channels where the relays use the decode-and-forward (DF) protocol. The probability density function and cumulative distribution function of the total signal-to-noise ratio at the secondary receiver is derived for the case of selection combining. In particular, approximating the channels by Nakagami-m fading results in an analytical expression for the outage probability that can be expressed in terms of well-known functions. This approximation provides a good match when the parameters of the amp;#954;-amp;#956; shadowed fading channels translate to integer values of fading severity parameter $m$. Numerical results are provided to illustrate the impact of system parameters on the outage probability for amp;#954;-amp;#956; shadowed fading channels.

In this paper, we analyze the outage probability of multiple-input multiple-output cognitive cooperative radio networks (CCRNs) with multiple opportunistic amplify-and-forward relays. The CCRN applies underlay spectrum access accounting for the interference power constraint of a primary network and utilizes orthogonal space-time block coding to transmit multiple data streams across a number of antennas over several time slots. As such, the system exploits both time and space diversity to improve the transmission reliability over Nakagami-m fading. The CCRN applies opportunistic relaying in which the relay offering the highest signal-to-noise ratio at the receiver is selected to forward the transmit signal. Furthermore, selection combining is adopted at the secondary receiver to process the signal from the direct and relaying transmissions. To evaluate system performance, we derive an expression for the outage probability which is valid for an arbitrary number of antennas at the source, relays, and receiver of the CCRN. Selected numerical results are provided using Mathematica for analysis and Matlab for simulations, to reveal the effect of network parameters on the outage probability of the system.

In this paper, we consider a two-tier cellular network consisting of a primary macro cell base station (PMBS) which is overlaid by cognitive small cell base stations (CSBSs) to achieve efficient spectrum utilization. The deployment of two-tier cellular networks can provide higher capacity for the system but also causes cross-tier, intra-tier, and inter-tier interference within the cellular networks. Thus, we employ transmit and receive beamforming in the considered two-tier cellular network to mitigate interference. We first design the receive beamforming vector for a primary user (PU) such that it cancels all inter-tier interference from other PUs. Then, the transmit beamforming vectors at the secondary users (SUs) are designed to null out the cross-tier interference to the PUs. Further, the receive beam-forming vectors at the SUs are designed to mitigate the cross-tier interference from the PUs to the SUs. Finally, the transmit beamforming vector at the PMBS is designed to maximize the signal-to-interference-plus-noise ratio at the PUs. To quantify the performance of the system, we derive an expression for the channel capacity in the downlink from the CSBSs to the SUs. Numerical results are provided to reveal the effect of network parameters such as intra-tier interference distances, fading conditions, and number of antennas on the channel capacity of the SUs.

This paper presents a queueing analytical framework for the performance evaluation of the secondary user (SU) packet transmission with service differentiation in a point-to-multipoint underlay cognitive radio network. The transmit power of the SU transmitter is subject to the joint outage constraint imposed by the primary user receivers (PU-Rxs) and the SU maximum transmit power limit. The analysis considers a queueing model for secondary traffic with multiple classes, and different types of arrival and service processes under a non-preemptive priority service discipline. The SU quality of service (QoS) is characterized by a packet timeout threshold and target bit error rate. Given these settings, analytical expressions of the packet timeout probability and average transmission time are derived for opportunistic and multicast scheduling. Moreover, expressions of the average packet waiting time in the queue and the total time in the system for each class of traffic are obtained. Numerical examples are provided to illustrate the secondary network performance with respect to various parameters such as number of PU-Rxs and SU receivers, SU packet arrival process, QoS requirements, and the impact of interference from the primary network to the secondary network.

In this paper, a performance analysis for cognitive radio networks (CRNs) under the outage probability constraint of the primary user and peak transmit power constraint of the secondary user is conducted. Given an automatic repeat request protocol, analytical expressions for the packet delay and throughput of the CRN are derived. Most importantly, these expressions can be used to understand the quality of experience on web services which are assumed to be offered by the considered CRN.

Abstract This paper analyzes the ergodic capacity of secondary point‐to‐multipoint communications under the outage constraint of multiple primary user receivers (PU‐Rxs) and the secondary user (SU) maximum transmit power limit. We first derive analytical expressions of the ergodic capacity for three scheduling schemes, that is, round robin, Max‐signal‐to‐interference‐plus‐noise‐ratio (Max‐SINR) and Min‐SINR, and compare their performance. Numerical examples show that when the number of SU receivers (SU‐Rxs) increases in the presence of a single PU‐Rx and at high SINR, the Min‐SINR outperforms the Max‐SINR scheme. As the number of PU‐Rxs increases, the Max‐SINR performs better than the Min‐SINR scheduling. When the number of SU‐Rxs becomes large, the system capacity is enhanced but so is the feedback load. To exploit the Max‐SINR transmission while reducing the feedback load, we assume a threshold based on the channel quality where the SU‐Rxs with the instantaneous SINR above the threshold participate in the Max‐SINR scheduling; otherwise, an SU‐Rx is selected randomly. Then, an analytical expression of the average capacity is derived. Numerical results illustrate that the capacity with limited feedback is approximately the same as for the case of Max‐SINR with full feedback when the SU threshold for feedback condition is set to low and medium values. Copyright © 2016 John Wiley & Sons, Ltd.

In this paper, the effective capacity, a link-layer model supporting Quality of Service (QoS) metrics such as user data rate, packet loss rate and service delay, is analyzed. We consider an underlay cognitive cooperative relay network (CCRN) subject to independent non-identically distributed aH fading. An analytical expression of the effective capacity is derived by means of Fox H-functions and Meijer G-functions along with the Mellin transform of the product of two H-functions. The network performance is analyzed against the QoS exponent and the peak interference power constraint imposed by primary users.

In this paper, we consider the application of partial buffer sharing to an M/G/1/K queueing system for cognitive radio networks (CRNs). It is assumed that the CRN is subject to Nakagami-?? fading. Secondary users are allowed to utilize the licensed radio spectrum of the primary users through underlay spectrum access. A finite buffer at the secondary transmitter is partitioned into two regions, the first region serves both classes of packets while the second region serves only packets of the highest priority class. Therefore, the examined CRN can be modeled as an M/G/1/K queueing system using partial buffer sharing. An embedded Markov chain is applied to analyze the queueing behavior of the system. Utilizing the balance equations and the normalized equation, the equilibrium state distribution of the system at an arbitrary time instant can be found. This outcome is utilized to investigate the impact of queue length, arrival rates, and fading parameters on queueing performance measures such as blocking probability, throughput, mean packet transmission time, channel utilization, mean number of packets in the system, and mean packet waiting time for each class of packets.

This paper analyzes the secondary network performance under the joint constraint of the primary user (PU) peak interference power and maximum transmit power limit of the secondary user (SU). In particular, N SU transmitters (SU-Txs) communicate with an SU receiver in the presence of a primary network with a PU transmitter and multiple PU receivers. Moreover, we exploit opportunistic scheduling where the SU-Tx with the best channel condition is scheduled for transmission. Analytical expressions of the outage probability and symbol error probability of the SU are obtained considering either perfect or statistical knowledge of the PU channel gains. Furthermore, we analyze the SU throughput for delay constrained applications with two hybrid automatic repeat request protocols, namely, repetition time diversity and incremental redundancy. Numerical results are provided to assess the effect of the number of SU-Txs and number of packet retransmissions on the secondary network performance. In addition, the impact of the primary network parameters on the secondary network is investigated.

Abstract In this paper, we study the performance of multiple‐input multiple‐output cognitive amplify‐and‐forward relay networks using orthogonal space–time block coding over independent Nakagami‐m fading. It is assumed that both the direct transmission and the relaying transmission from the secondary transmitter to the secondary receiver are applicable. In order to process the received signals from these links, selection combining is adopted at the secondary receiver. To evaluate the system performance, an expression for the outage probability valid for an arbitrary number of transceiver antennas is presented. We also derive a tight approximation for the symbol error rate to quantify the error probability. In addition, the asymptotic performance in the high signal‐to‐noise ratio regime is investigated to render insights into the diversity behavior of the considered networks. To reveal the effect of network parameters on the system performance in terms of outage probability and symbol error rate, selected numerical results are presented. In particular, these results show that the performance of the system is enhanced when increasing the number of antennas at the transceivers of the secondary network. However, increasing the number of antennas at the primary receiver leads to a degradation in the secondary system performance. Copyright © 2014 John Wiley & Sons, Ltd.

In this paper, we analyze the performance of cognitive amplify-and-forward (AF) relay networks with beamforming under the peak interference power constraint of the primary user (PU). We focus on the scenario that beamforming is applied at the multi-antenna secondary transmitter and receiver. Also, the secondary relay network operates in channel state information-assisted AF mode, and the signals undergo independent Nakagami-m fading. In particular, closed-form expressions for the outage probability and symbol error rate (SER) of the considered network over Nakagami-m fading are presented. More importantly, asymptotic closed-form expressions for the outage probability and SER are derived. These tractable closed-form expressions for the network performance readily enable us to evaluate and examine the impact of network parameters on the system performance. Specifically, the impact of the number of antennas, the fading severity parameters, the channel mean powers, and the peak interference power is addressed. The asymptotic analysis manifests that the peak interference power constraint imposed on the secondary relay network has no effect on the diversity gain. However, the coding gain is affected by the fading parameters of the links from the primary receiver to the secondary relay network

Abstract In this paper, we study the performance of cognitive cooperative radio networks under the peak interference power constraints of multiple primary users (PUs). In particular, we consider a system model where the secondary user communication is assisted by multiple secondary relays (SRs) that operate in the decode‐and‐forward mode to relay the signal from a secondary transmitter to a secondary receiver. Moreover, we assume that the transmit powers of the secondary transmitter and the SRs are subject to the peak interference power constraints of multiple PUs that operate in their coverage range. Given this system setting, we first derive the cumulative distribution function of the instantaneous end‐to‐end signal‐to‐noise ratio. Then, we obtain a closed‐form expression for the outage probability and an exact expression for the symbol error probability of the considered network. These tractable formulas enable us to examine the impact of the presence of multiple PUs on the performance of the considered spectrum sharing system. Furthermore, our numerical results show that system performance is improved significantly when the number of SRs increases or the channel mean power from the secondary user to the PUs is low. Also, any increase in the number of PUs in the coverage range of the secondary transmitter or the SRs leads to degradation in system performance. Finally, Monte Carlo simulations are provided to verify the correctness of our analytical results. Copyright © 2013 John Wiley & Sons, Ltd.

This paper studies the performance of adaptive modulation and coding in a cognitive incremental decode-and-forward relaying network where a secondary source can directly communicate with a secondary destination or via an intermediate relay. To maximize transmission efficiency, a policy which flexibly switches between the relaying and direct transmission is proposed. In particular, the transmission, which gives higher average transmission efficiency, will be selected for the communication. Specifically, the direct transmission will be chosen if its instantaneous signal-to-noise ratio (SNR) is higher than one half of that of the relaying transmission. In this case, the appropriate modulation and coding scheme (MCS) of the direct transmission is selected only based on its instantaneous SNR. In the relaying transmission, since the MCS of the transmissions from the source to the relay and from the relay to the destination are implemented independently to each other, buffering of packets at the relay is necessary. To avoid buffer overflow at the relay, the MCS for the relaying transmission is selected by considering both the queue state and the respective instantaneous SNR. Finally, a finite-state Markov chain is modeled to analyze key performance indicators such as outage probability and average transmission efficiency of the cognitive relay network.

In this paper, we investigate the performance of cognitive multiple decode-and-forward relay networks under the interference power constraint of the primary receiver wherein the cognitive downlink channel is shared among multiple secondary relays and secondary receivers. In particular, only one relay and one secondary receiver which offers the highest instantaneous signal-to-noise ratio is scheduled to transmit signals. Accordingly, only one transmission route that offers the best end-to-end quality is selected for communication at a particular time instant. To quantify the system performance, we derive expressions for outage probability and symbol error rate over Nakagami-m fading with integer values of fading severity parameter m. Finally, numerical examples are provided to illustrate the effect of system parameters such as fading conditions, the number of secondary relays and secondary receivers on the secondary system performance.

In this paper, we analyze the power consumption of wireless sensor nodes with min(N,T) policy and M/G/1 queue in the presence of Nakagami-m fading. In particular, this system setting is applied to a wireless sensor node operating in a cognitive radio system as secondary user in the presence of a primary user. As such, not only the queue policy influences the power consumption but also the interference power constraint imposed on the wireless sensor node by the primary user. Thus, a queued sleep/wake-up strategy is analyzed in order to mitigate the average power consumption of a sensor node using min(N,T) policy in the context of an M/G/1 queue and a spectrum sharing environment in the presence of signal fading. Numerical examples are presented to illustrate the impact of queuing parameters and fading channel on the power consumption of a wireless sensor node.

This work investigates the end-to-end performance of randomized distributed space-time codes with complex Gaussian distribution, when employed in a wireless relay network. The relaying nodes are assumed to adopt a decode-and-forward strategy and transmissions are affected by small and large scale fading phenomena. Extremely tight, analytical approximations of the end-to-end symbol error probability and of the end-to-end outage probability are derived and successfully validated through Monte-Carlo simulation. For the high signal-to-noise ratio regime, a simple, closed-form expression for the symbol error probability is further provided.

In this study, the authors analyse the average end‐to‐end packet delay for a cognitive ad hoc network where multiple secondary nodes randomly contend for accessing the licensed bands of primary users in non‐slotted time mode. Before accessing the licensed bands, each node must perform spectrum sensing and collaboratively exchange the sensing results with other nodes of the corresponding communication as a means of improving the accuracy of spectrum sensing. Furthermore, the medium access control with collision avoidance mechanism based distributed coordination function specified by IEEE 802.11 is applied to coordinate spectrum access for this cognitive ad hoc network. To evaluate the system performance, the authors model the considered network as an open G/G/1 queuing network and utilise the method of diffusion approximation to analyse the end‐to‐end packet delay. The authors’ analysis takes into account not only the number of secondary nodes, the arrival rate of primary users and the arrival rate of secondary users but also the effect of the number of licensed bands when assessing the average end‐to‐end packet delay of the networks.

In this paper, we study a hybrid interweave-underlay spectrum access system that integrates amplify-and-forward relaying. In hybrid spectrum access, the secondary users flexibly switch between interweave and underlay schemes based on the state of the primary users. A continuous-time Markov chain is proposed to model and analyze the spectrum access mechanism of this hybrid cognitive cooperative radio network (CCRN). Utilizing the proposed Markov model, steady-state probabilities of spectrum access for the hybrid CCRN are derived. Furthermore, we assess performance in terms of outage probability, symbol error rate (SER), and outage capacity of this CCRN for Nakagami-$m$fading with integer values of fading severity parameter$m$. Numerical results are provided showing the effect of network parameters on the secondary network performance, such as the primary arrival rate, the distances from the secondary transmitters to the primary receiver, the interference power threshold of the primary receiver in underlay mode, and the average transmit signal-to-noise ratio of the secondary network in interweave mode. To show the performance improvement of the CCRN, comparisons for outage probability, SER, and capacity between the conventional underlay scheme and the hybrid scheme are presented. The numerical results show that the hybrid approach outperforms the conventional underlay spectrum access.

The performance of cognitive spectrum sharing systems with opportunistic relay selection over Nakagami-m fading is analyzed in the presence of multiple primary users (PUs). In particular, we derive an exact closed-form expression for the outage probability (OP) of the considered cognitive relay systems under the joint impact of maximal transmit power Ptat secondary transmitter and peak interference power Ipat the primary user. Our general formulas cover several specific practical scenarios, e.g., where the maximal transmit power can be neglected compared to the peak interference power. In addition, a tractable expression for the asymptotic OP is also derived and reveals important insights into the system performance. We show that the number of PUs only affects the coding gain but not the diversity gain.

In this paper, we investigate the performance of an underlay cognitive relay network over α-μ fading channels. In particular, we assume non-identical fading parameters and derive an analytical expression for the outage probability of a dual-hop underlay system subject to a peak interference power constraint. Numerical and simulation results are presented to evaluate the effect of fading parameters on the outage probability of the system.

In this paper, we consider a spectrum sharing system in which a primary transmission coexists with a secondary network composed of a secondary base station (SBS) and multiple secondary user receivers (SU-Rxs). The secondary user (SU) can access the spectrum licensed to the primary user (PU) as long as the SU does not cause any harmful interference to the PU. Among all available SU-Rxs, only the best SU-Rx with highest signal-to-interference-plus-noise ratio (SINR) is selected to receive the transmit signals from the SBS. We derive an analytical expression for the outage probability (OP) of the SU-Rx with highest SINR. Moreover, we derive an approximation of the ergodic capacity at the SU-Rx. In order to meet the requirements at the PU receiver (PU-Rx), at the same time, satisfying the quality of the SU link, an adaptive power allocation strategy is developed. The power allocation policy imposes the SU transmitter (SU-Tx) to adapt its transmit power in order to avoid harmful interference at the PU receiver (PU-Rx), with a PU predefined outage constraint.

In this paper, we investigate a downlink model for underlay cognitive radio networks (CRNs). In particular, we assume that the secondary transmitter (SU-Tx) sends common packets to secondary receivers (SU-Rx) under the outage constraint of a single primary user (PU) and the peak transmit power constraint of the SU-Tx. All channels undergo Rayleigh fading. Given these settings, an adaptive transmit power policy for the SU-Tx is considered. Moreover, the cumulative distribution function (CDF) and probability density function (PDF) for packet transmission time are derived to analyze the timeout probability, the average packet transmission time and the average time a packet spends in the system for the SU-Rx having the best channel condition. Numerical results are provided to investigate the impact of the PU transmit power and channel mean powers on secondary network performance. The results show that when the number of SU-Rx becomes large, the timeout probability and the packet average time in the system decrease to a minimum.

In this paper, we study the performance of cognitive amplify-and-forward (AF) relay networks where the secondary users opportunistically access M licensed bands of the primary users over Nakagami-m fading channels. In order to enhance the accuracy of spectrum sensing and strongly protect the primary users from being interfered by the secondary transmission, collaborative spectrum sensing is deployed among the secondary transmitter, secondary relay, and secondary receiver. In particular, an analytical expression for the capacity of the considered network is derived. Numerical results are provided to show the influence of the arrival rate of the primary users on the channel utilization of licensed bands. Finally, the impact of the number of the licensed bands, channel utilization of the primary users, false alarm probability, and transmission distances on the capacity of the considered system are investigated.

In this paper, we study the use of adaptive modulation (AM) for a dual-hop multiple-input multiple-output (MIMO) network with transmit antenna selection (TAS) at the transmitter and maximal ratio combining (MRC) at the receiver. Our system deploys incremental amplify-and-forward (AF) relaying with AM to forward the source signal. A switching policy for selecting between the relaying and the direct communications is utilized to maximize spectrum efficiency. In particular, we derive expressions for the outage probability, spectrum efficiency, and bit error rate (BER) for the considered system over Nakagami-m fading. The considered network not only takes advantage of TAS transmission such as achieving full diversity order with low transmit complexity but also inherits the beneficial feature of AM such as spectrum efficiency improvement.

In this study, the authors analyse the impact of channel estimation error (CEE), feedback delay (FD), and co‐channel interference (CCI) on the performance of multiple‐input multiple‐output (MIMO) systems deploying maximum ratio transmission (MRT). In particular, the authors derive closed‐form expressions for the ergodic capacity and the symbol error rate (SER) as well as the outage probability (OP). In addition, to reveal the effect of CEE, FD and CCI on the MIMO‐MRT system, the authors adopt more simplified and tractable formulas in terms of asymptotic expressions for the ergodic capacity, SER and OP. The analysis shows that the system performance is degraded considerably under imperfect transmission conditions such as CEE, FD and CCI. However, the authors can compensate part of this degradation by deploying MRT transmission with a large number of antennas at the transmitter and receiver. Finally, the selected examples exhibit consistency between analytical results and Monte Carlo simulations.

In this study, the authors consider distributed orthogonal space–time block coding for relay‐based channel state information‐assisted amplify‐and‐forward networks. Specifically, they show that opportunistic relaying (OR) is an optimal solution in terms of instantaneous signal‐to‐noise ratio (SNR), that is, it provides the maximum instantaneous SNR under the constraint of fixed transmit power for the relays. In particular, instead of allocating the given transmit power to all relays, letting the best relay transmit with this power is an optimal solution for maximising the received SNR. To exhibit this benefit, Monte Carlo simulations are presented showing superior performance of the OR scheme compared to equal power allocation policy for the considered relay networks. For the considered optimal scenario, the authors further derive analytical expressions for the outage probability and symbol error rate (SER) over quasi‐static independent, not necessarily identically distributed Nakagami‐ m fading channels. They further present asymptotically tight approximations for the outage probability and SER in the high SNR regime, rendering insights into the cooperative diversity behaviour. Finally, numerical results are provided to examine the effect of network parameters on the system performance of the considered network.

In this article, the authors study the effect of peak interference power constraint given by the primary receiver on the performance of multi‐hop cognitive amplify‐and‐forward (AF) relay networks. The athours assume that all involved channels are subject to independent, not necessarily identically distributed Nakagami‐ m fading and the secondary multi‐hop relay network operates in channel state information‐assisted AF mode. An analysis of the system performance in terms of outage probability and symbol error rate (SER) is presented. Accordingly, closed‐form expressions for the tightly bounded outage probability and SER are formulated which are used for quantifying the impact of the fading channels, the interference power constraint and the number of hops on system performance. More importantly, an asymptotic performance analysis, which intuitively reveals benefits of cooperative diversity of the secondary relay network, is established. The analysis shows that the diversity gain of the considered cognitive relay networks is equal to the minimum of the fading severity parameters of all relaying hops. Also, the interference power constraint imposed by the primary receiver only affects the coding gain of the secondary relay network.

Fixation density maps (FDM) created from eye tracking experiments are widely used in image processing applications. The FDM are assumed to be reliable ground truths of human visual attention and as such, one expects a high similarity between FDM created in different laboratories. So far, no studies have analyzed the degree of similarity between FDM from independent laboratories and the related impact on the applications. In this paper, we perform a thorough comparison of FDM from three independently conducted eye tracking experiments. We focus on the effect of presentation time and image content and evaluate the impact of the FDM differences on three applications: visual saliency modeling, image quality assessment, and image retargeting. It is shown that the FDM are very similar and that their impact on the applications is low. The individual experiment comparisons, however, are found to be significantly different, showing that inter-laboratory differences strongly depend on the experimental conditions of the laboratories. The FDM are publicly available to the research community.

The distributed Alamouti space-time code in two-way fixed gain amplify-and-forward (AF) relay is proposed in this paper. In particular, closed-form expressions for approximated ergodic sum-rate and exact pairwise error probability (PWEP) are derived for Nakagami-m fading channels. To reveal further insights into array and diversity gains, an asymptotic PWEP is also obtained. Finally, numerical results are provided to corroborate the proposed theoretical analysis.

We analyze the outage performance of a two-way fixed gain amplify-and-forward (AF) relay system with beam-forming, arbitrary antenna correlation, and co-channel interference (CCI). Assuming CCI at the relay, we derive the exact individual user outage probability in closed-form. Additionally, while neglecting CCI, we also investigate the system outage probability of the considered network, which is declared if any of the two users is in transmission outage. Our results indicate that in this system, the position of the relay plays an important role in determining the user as well as the system outage probability via such parameters as signal-to-noise imbalance, antenna configuration, spatial correlation, and CCI power. To render further insights into the effect of antenna correlation and CCI on the diversity and array gains, an asymptotic expression which tightly converges to exact results is also derived.

In this paper, we examine the performance of multiple-input multiple-output (MIMO) cognitive amplify-and-forward (AF) relay systems with maximum ratio transmission (MRT). In particular, closed-form expressions in terms of a tight upper bound for outage probability (OP) and symbol error rate (SER) of the system are derived when considering channel estimation error (CEE) and feedback delay (FD) in our analysis. Through our works, one can see the impact of FD and CEE on the system as well as the benefits of deploying multiple antennas at the transceivers utilizing the spatial diversity of an MRT system. Finally, we also provide a comparison between analytical results and Monte Carlo simulations for some examples to verify our work.

In this paper, we study the performance for the primary and secondary transmissions in cognitive radio networks where the amplify-and-forward (AF) secondary relay helps to transmit the signals for both the primary and secondary transmitters over independent Nakagami-m fading. First, we derive exact closed-form expressions for outage probability and symbol error rate (SER) of the primary network. Then, we derive an exact closed-form expression for outage probability and a closed-form expression of a tight upper bound for SER of the secondary network. Furthermore, we also make a comparison for the performance of the primary system with and without the help of the secondary relay. Finally, we show a good agreement between analytical results and Monte-Carlo simulations.

In this paper, the performance of a dual-hop multiple-input multiple-output (MIMO) cooperative multiple-relay network using orthogonal space-time block coding (OS-TBC) transmission is examined. The fading environment is modeled as independent, not identical distributed (i.n.i.d.) Nakagami-m fading. Also, we consider the case that each terminal in the network has multiple-antennas, the relays operate in channel state information (CSI)-assisted amplify-and-forward (AF) relaying mode, and the direct transmission from the source to the destination is applicable. In particular, we derive a closed-form expression for the moment generating function (MGF) of the instantaneous signal-to-noise ratio (SNR). Utilizing this derivation, we can analyze the SER performance of the considered system for M-ary phase shift-keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) scheme. Also, the outage performance of the considered relay networks is evaluated. We further investigate the cooperative diversity behavior by deriving asymptotic approximations for the outage probability and the SER. Finally, numerical results are provided showing very close agreement between the analytical results and the Monte-Carlo simulations.

In this letter, we investigate the effect of feedback delay on the performance of a dual-hop amplify-and-forward (AF) relay network with beamforming in the presence of multiple interferers over Rayleigh fading channels. Specifically, we derive closed-form expressions for the outage probability (OP) and the symbol error rate (SER). Furthermore, to render insights into the effect of feedback delay and interference on the network performance, asymptotic OP and SER are also presented. These asymptotic expressions are very tight in the high signal-to-noise ratio regime, readily enabling us to obtain the diversity and coding gains of the considered network.

In this paper, the effect of keyhole on the performance of multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks with orthogonal space-time block codes (OSTBCs) transmission is investigated. In particular, we analyze the asymptotic symbol error probability (SEP) performance of a downlink communication system where the amplifying processing at the relay can be implemented by either the linear or squaring approach. Our tractable asymptotic SEP expressions enable us to obtain both diversity and array gains. Our finding reveals that with condition n_S > min(n_R,n_D), the linear approach can provide the full achievable diversity gain of min(n_R,n_D) when only the second hop suffers from the keyhole effect, i.e., single keyhole effect (SKE), where n_S, n_R, and n_D are the number of antennas at source, relay, and destination, respectively. However, for the case that both the source-relay and relay-destination links experience the keyhole effect, i.e., double keyhole effect (DKE), the achievable diversity order is only one regardless of the number of antennas. In contrast, utilizing the squaring approach, the overall diversity gain can be achieved as min(n_R,n_D) for both SKE and DKE. An important observation corroborated by our studies is that for satisfying the tradeoff between performance and complexity, we should use the linear approach for SKE and the squaring approach for DKE.

Multi-keyhole bridges the gap between single keyhole and full-scattering multiple-input multiple-output (MIMO) channels. In this paper, we therefore investigate the multikeyhole effect on the MIMO amplify-and-forward (AF) relay downlink transmission with orthogonal space-time block codes. In particular, we derive the analytical symbol error rate (SER) expression for the considered system with arbitrary number of keyholes. Moreover, SER approximations in the high SNR regime for several important special scenarios of multi-keyhole channels are further derived. These asymptotic results provide important insights into the impact of system parameters on the SER performance. Our analysis is confirmed by comparing with Monte-Carlo simulations.

Cognitive relaying is a promising technique to improve spectrum utilization in mobile networks. In this paper, we consider a scenario in which a secondary transmitter communicates with a secondary receiver by the help of a secondary relay. The secondary user should adjust its transmission power in order to not cause any harmful interference to the multiple primary users operating in the licensed spectrum. In this context, we investigate the end-to-end performance of cognitive relay networks in terms of outage probability and ergodic capacity over Nakagami-m fading channels. A closed-form expression for outage probability and an approximation for ergodic capacity are derived. Furthermore, the presented numerical examples illustrate the impact of the number of primary users on the performance of cognitive relay networks.

In this paper, we investigate the degenerative effects of antenna correlation and keyhole on the performance of multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks. In particular, we considered a downlink MIMO AF relay system consisting of an nS-antenna base station S, an nR-antenna relay station R, and an nD-antenna mobile station D, in which the signal propagation originated from the mobile station suffers from keyhole and spatial correlation effects. We have derived an exact expression for the moment generation function of the instantaneous signal-to-noise ratio which enables us to analyze the symbol error probability and outage probability of the considered system. We have shown that although the mobile station is in a poor scattering environment, i.e., keyhole, the relay channel (S → R → D link) still achieves a cooperative diversity order of min(nR, nD) provided that the channel from the source (S → R link) is keyhole-free. This result is important to radio system designers since under such a severe scenario it is unnecessary to deploy a large number of antennas on the relay station (nR) but only requires nR= nDto obtain the maximum achievable diversity gain.

In this paper, we propose a cross-layer optimization approach for replication of video streaming over multihop wireless network by jointly taking into account application, network, MAC, and physical layers. Specifically, we design a middleware replication strategy layer with a stack profile, in which parameters across the hops and all considered layers are exchanged for optimal video replication. The simulation results demonstrate that our method improves the replication strategy performance in terms of user-perceived video quality. In turn, it satisfies the heterogeneous receive bandwidth constraint of users for efficiently using the network bandwidth compared to other optimal replication strategies without cross-layer approach.

We introduce a novel region-of-interest (ROI) database for natural image content, the ROI-D database. The database consists of ROI maps created from manual selections obtained in a psychophysical experiment with 20 participants. The presented stimuli were 42 photographic images taken from 3 publicly available image quality databases. In addition to the ROI selections, dominance ratings were recorded that provide further insight into the interest of the selected ROI in relation to the background. In this paper, the experiment is described, the resulting ROI database is analysed, and possible applications of the database are discussed. The ROI-D database is made freely available to the image processing research community.

In this paper, we study the symbol error rate (SER) performance of dual-hop multiple-input multiple-output (MIMO) cooperative relay networks with orthogonal space-time block coding (OSTBC) transmission over independent, not necessarily identical Nakagami-m fading channels. Our analysis considers channel state information (CSI)-assisted amplify-and-forward (AF) relaying with the availability of the direct communication from the source to the destination. In particular, we derive a closed-form expression for moment generating function (MGF) of the instantaneous signal-to-noise ratio (SNR). Utilizing this result, we can analyze the SER performance of the considered systems for M-ary phase shift-keying (M-PSK) and M-ary quadrature amplitude (M-QAM) modulations. We further investigate the cooperative diversity behavior of the considered relay systems by deriving asymptotic approximations of SER in the high SNR regime. Finally, representative numerical results are presented showing an excellent agreement with the Monte-Carlo simulations which validates our analysis.

In this paper, we analyze the performance of dual-hop channel state information (CSI)-assisted amplify-and-forward (AF) cooperative networks using orthogonal space-time block codes (OSTBCs) over independent but not identically distributed Nakagami-$m$ fading channels. Specifically, we present closed-form expressions of the outage probability (OP) and the symbol error probability (SEP). The analytical results are given in tractable forms which readily allow us to assess the performance of AF relay systems using OSTBCs. For sufficiently large signal-to-noise ratio, we obtain the asymptotic results for OP and SEP which reveal insights into the effect of fading factors on the diversity and coding gains. It has been shown that between the two hops the more severe link solely determines the diversity and coding gains. In particular, these two gains strictly depend on the fading severity parameters and the channel mean powers of the more rigorous hop, respectively. Numerical results are provided showing an excellent agreement between our analytical results and those of Monte-Carlo simulations for selected scenarios.

We analyse variations of saliency between human observers when viewing natural image content. For this purpose, we conducted an eye tracking experiment to record gaze patterns of humans when viewing natural images. The resulting gaze patterns were used to create saliency maps for each observer and image. A correlation analysis is then conducted with the aim to quantify the variability of saliency between the observers and the different images. The results are considered to be a first step towards gaining a better understanding regarding the reliability of saliency maps for natural images.

Visual attention models are typically designed based on human gaze patterns recorded through eye tracking. In this paper, two similar eye tracking experiments from independent laboratories are presented, in which humans observed natural images under task-free condition. The resulting saliency maps are analysed with respect to two criteria; the consistency between the experiments and the impact of the image presentation time. It is shown, that the saliency maps between the experiments are strongly correlated independent of presentation time. It is further revealed that the presentation time can be reduced without substantially sacrificing the accuracy of the convergent saliency map. The results provide valuable insight into the similarity of saliency maps from independent laboratories and are highly beneficial for the creation of converging saliency maps at reduced experimental time and cost.

The exact expressions for symbol error probability and outage probability of randomized distributed space-time codes (RDSTC) under Rayleigh fading are derived. The diversity gain derived in the literature uses the Chernoff bound which may not be achievable in general. Utilizing the exact expression, the achievable diversity gain of RDSTCs is validated. The analytical expressions are verified by Monte-Carlo simulations.

We analyze the performance of amplify-and-forward (AF) multiple-input multiple-output (MIMO) cooperative networks over independent but not identically distributed Nakagami-m fading channels. Specifically, we consider an AF MIMO relay network where the source deploys the transmit antenna selection scheme to communicate with the destination through the help of a relay. We derive analytical expressions for outage probability (OP) and symbol error probability (SEP). Moreover, asymptotically tight approximations for OP and SEP have been obtained to facilitate insights on how fading parameters affect the performance. Our analytical results are verified by Monte-Carlo simulations for respective scenarios.

In this paper, distortions caused by packet loss during video transmission are evaluated with respect to their perceived annoyance. In this respect, the impact of visual saliency on the level of annoyance is of particular interest, as regions and objects in a video frame are typically not of equal importance to the viewer. For this purpose, gaze patterns from a task free eye tracking experiment were utilised to identify salient regions in a number of videos. Packet loss was then introduced into the bit stream such as that the corresponding distortions appear either in a salient region or in a non-salient region. A subjective experiment was then conducted in which human observers rated the annoyance of the distortions in the videos. The outcomes show a strong tendency that distortions in a salient region are indeed perceived as much more annoying as compared to distortions in the non-salient region. The saliency of the distorted image content was further found to have a larger impact on the perceived annoyance as compared to the distortion duration. The findings of this work are considered to be of great use to improve prediction performance of video quality metrics in the context of transmission errors.

In mobile multimedia networks, different traffic classes require various levels of quality of service (QoS). In this paper, we propose a cross-layer design to provide diverse QoS provisions for integrated realtime/non-realtime traffic by jointly taking into account the physical layer, data link layer, and application layer. Specifically, at the physical layer, adaptive modulation and coding (AMC) is employed to cope with the time-varying nature of fading channels whereas impairments due to queuing effects are considered at the data link layer. At the application layer, realtime and non-realtime traffic are classified into high and low priority classes, respectively, with strict priority levels. The cross-layer scheme is characterized as a discrete Markov modulated Poisson process. Solving the steady state probability of this Markov model readily enables us to investigate several important performance metrics such as packet drop rate due to buffer overflow and packet error rate due to transmission errors and network impairments. Our proposed cross-layer design is verified by comparing the analytical results with Monte-Carlo simulations. Furthermore, it has been shown that the proposed cross-layer design can flexibly guarantee QoS requirements for each traffic class.

An analytical framework for calculation of Erlang capacity per cell of UMTS with perceptual-based power control is derived. A comparison of the capacity of the system using the proposed power control algorithm with its conventional counterpart is performed. This is supported by numerical results on Erlang capacity over a variety of system parameters for both considered power control approaches. It is shown that the use of perceptual speech quality metrics in power control results in a capacity gain of at least 10% over that of conventional UMTS.

Mean opinion scores obtained in subjective image quality experiments are widely accepted as measures of perceived visual quality. They have, however, a strong limitation regarding the reliability of the rated quality, since there is no explicit information as to whether the human observer experienced difficulties when judging image quality. We thus suggest that additional information about the observers confidence should be provided along with the actual quality measure. In this paper, we analyse two ways of obtaining this confidence measure; firstly as a confidence score given by the human observer and secondly as an indirect measure of the observers response time to provide the quality score. We reveal strong relationships of confidence scores and response times to the quality scores. We further propose a model to predict observer confidence based on the quality scores and response times.

The modelling of perceptual image quality metrics has experienced increased effort in recent years. In order to allow for model design, validation, and comparison, a number of subjective image quality databases has been made available to the research community. Most metrics that were designed using these databases assess the quality uniformly over the whole image, not taking into account stronger attention to salient regions of an image. In order to facilitate incorporation of visual attention (VA) into objective metric design we have conducted an eye tracking experiment. The experiment and its outcomes will be explained in detail in this paper. The actual gaze data recorded in the experiment is publicly available in order to facilitate and encourage VA modelling for image quality metrics.

Visual attention (VA) is an integral property of the human visual system. The deployment of a VA model can be beneficial for many image and video applications, such as, compression, transmission, and quality assessment. However, the design of a VA model is highly dependent on the subjective VA data used as ground truth and the application that the model is intended for. In this paper, we discuss two ways of obtaining subjective VA data that can subsequently be used to develop VA models; selective regions-of-interest and visual fixation patterns. The feasibility of both methods will be discussed, in particular, with respect to visual quality assessment.

An adaptive transmission scheme for cooperative communications is proposed in this paper. Communication protocols with amplify-and-forward relays based on the distributed-Alamouti space-time code (achieving spatial diversity gain) and cooperative spatial multiplexing (C-SM) (pertaining spatial multiplexing gain) are considered. Specifically, under a fix transmission rate, we use adaptively the cooperative diversity (C-DIV) and C-SM according to channel conditions. With this strategy, either C-DIV with higher order modulation or C-SM with lower order modulation can be properly selected to enhance the error rate performance. The proposed adaptive protocol is shown to provide superior performance compared to C-DIV and C-SM.

Cooperative spatial multiplexing (CSM) systems can achieve high spatial multiplexing gain in wireless relay networks since each single-antenna relay forwards simultaneously a different subset of source messages to the destination. In this paper, the performance of CSM with amplify-and-forward (AF) relays are presented. Specifically, we derive exact closed-form expressions for symbol error probability (SEP) and achievable spectral efficiency of CSM with linear receiver in dissimilar Rayleigh fading channels. Assessing the SEP performance in the high signal-to-noise regime, we further show that CSM with AF relays obtains the first order of diversity gain. Furthermore, it has been observed that CSM outperforms two well-known cooperative diversity systems, e.g., selection combining and maximal ratio combining, in term of spectral efficiency. We also perform Monte-Carlo simulations to validate our analysis.

In this paper, we investigate the performance gain of hybrid decode-amplify-forward (HDAF) relay protocol over the two conventional ones: the adaptive decode-forward (ADF) and amplify-forward (AF) in dual-hop multiple-relay networks. We analyze the performance gain in terms of the symbol error probability (SEP) in the high signal-to-noise ratio (SNR) regime. Similarly as in the single relay case [1], under a specific modulation scheme, these gains only depend on the ratio of channel mean power between the first-hop and second-hop transmission. In particular, we show that the HDAF protocol obtains a large gain, medium gain, and small gain over ADF, AF as the relays are placed close to the destination, half-way between the source and destination (symmetric case), and nearby the source, respectively. In contrast to the single-relay case in which the HDAF scheme has no benefit compared to ADF and AF as the relay is located close to the source [1], HDAF still achieves a small gain with multiple relays. Interestingly, we further show that for all cases the performance gains are saturated as the number of relays is large.

The number of spreading sequences required for direct-sequence code-division multiple-access (DS-CDMA) systems depends on the number of simultaneous users in the system. Often a sequence family provides more sequences than are required; in many cases the selection of the employed sequences is a computationally intensive task. This selection is a key consideration, as the properties of the sequences assigned affect the error performance in the system. In this paper, a branch and bound algorithm is presented to perform this selection based on two different cost functions. Numerical results are presented to demonstrate the improved performance of this algorithm over previous work.

In this paper, regional attention to structural degradations in images is analyzed to improve perceptual quality prediction performance of objective image quality metrics. Subjective experiments were conducted to identify regions-of-interest for a set of natural images. A region-selective metric design is then applied to four objective image quality metrics which were trained and validated with respect to quality prediction accuracy and generalization to unknown images. For this purpose, data is used from subjective quality experiments conducted at two independent laboratories. It is shown that the region-selective design is highly beneficial for the considered objective image quality metrics, in particular, prediction accuracy can be significantly increased.

In this paper, we consider multiobjective optimization for obtaining the feature weights associated with structural image impairments and its application to wireless imaging quality assessment. The proposed framework supports optimization with respect to finding a trade-off between metric prediction accuracy and generalization to unknown images. Evaluation of optimal trade-off solutions for two representative scenarios reveal the benefits of the proposed approach. In particular, quality prediction accuracy of an objective image metric can be strongly increased and negligible features are identified that can be discarded to save computational complexity.

In this paper, multiobjective optimization is applied to determine the optimal feature weights for a multi-resolution image quality metric. The optimization is conducted with respect to two aims; maximization of quality prediction accuracy and generalisation ability to unknown images. A thresholding of the optimal weights is applied to further reduce computational complexity of the quality metric. A detailed discussion of the trade-off between prediction performance and complexity with respect to the threshold is provided. The metric design and evaluation is supported using mean opinion scores from subjective quality experiments that we conducted in two independent laboratories. Comparison to other state of the art quality metrics reveals the ability of the proposed metric to accurately predict subjective image quality perception.

In digital transmission, images may undergo quality degradation due to lossy compression and error-prone channels. Efficient measurement tools are needed to quantify induced distortions and to predict their impact on perceived quality. In this paper, an artificial neural network (ANN) is proposed for perceptual image quality assessment. The quality prediction is based on structural image features such as blocking, blur, image activity, and intensity masking. Training and testing of the ANN is performed with reference to subjective experiments and the obtained mean opinion scores (MOS). It is shown that the proposed ANN is capable of predicting MOS over a wide range of image distortions. This applies to both cases, when reference information about the structure of the original image is available to the ANN but also in absence of this knowledge. The considered ANN would therefore be well suited for combination with link adaption techniques.

Large area (LA) sequences form a class of ternary spreading sequences which exhibit an interference free window. In addition, these sequences have low correlation properties outside this window. Work to date has concentrated on examining the parameters and performance of LA sequences with reference to only a small number of example families. In this paper we develop general conditions which an LA family must satisfy. The development of these conditions allows for the production of computationally efficient tests to determine whether a given family is an LA family. In particular, these tests can form the basis for algorithms to construct LA families, allowing for a larger number of families with the potential for higher energy efficiency than those of previous work.

This paper evaluates the potential capacity improvements in mobile radio systems due to perceptual-based power control. It builds on our previous work on the application of a Real-time Perceptual Speech Quality Metric (RTPSQM) in power control. Encouraged by the performance improvements offered with perceptual-based power control, this paper investigates on how gains in average transmitter power and SIR target translate to gains in the actual capacity in the Global System for Mobile communication (GSM) and the Universal Mobile Telecommunication System (UMTS). In particular, capacity improvements in terms of number of users per cell between 25% to 91% are obtained for perceptual-based power control in GSM. As for UMTS, the results show an increase in Erlang capacity per cell by at least 10%.

We present an a posteriori probability (APP) decoding algorithm for binary linear block codes over a Gilbert-Elliott channel (GEC) using generalized weight polynomials. The proposed approach is based on a single-sweep APP decoding technique that uses matrix multiplications. By fixing the crossover probability in the `bad' state of the GEC at fifty percent, an APP decoding decision can be reached by evaluating trivariate polynomials without the need for the computationally more expensive matrix multiplications. In this case, the GEC is described by three variables, namely, the average fade to average connection time ratio, the burst factor, and the channel reliability factor. These variables can easily be deduced from error sequence measurements and hence can be related to many practical digital communication scenarios. The polynomial approach is demonstrated using a simple example and results of computer simulations are presented.

In this paper, the validity of P.1546, in short range (typically less than 20km) terrestrial environment is analysed by comparing its three versions (P.1546, P.1546-1, and P.1546-2) against simple models and against measurement results obtained by utilising the pilot signal of a commercial CDMA mobile network in rural Australia. The results show that the latest version on average underestimates the field strength by more than 10dB for typical Australian rural areas, however it improves the error standard deviation. The causes of these effects and the suggestions for further development of the Recommendation are discussed.

An exact channel reliability metric for Nakagami-m fading when no channel state information (CSI) is available at the receiver is developed. The proposed metric is suitable for combination with iterative decoding schemes to ensure optimum performance. A low complexity cubic approximation to the exact metric, which offers almost identical performance and significantly outperforms the conventional linear approximation, is presented. The numerical results obtained using a single parity check (SPC) block turbo code (BTC), show that the conventional linear approach can result in up to a 27.5 dB penalty at BER of 10-5, compared to the case where perfect CSI is available. On the other hand, the use of the proposed exact reliability metric results in only a 1.1 dB performance loss at the same BER

Numerous spread spectrum applications require families of long spreading sequences, often used at very high chip rates. In this paper, an algebraically simple way of generating long sequences by combining shorter polyphase sequences is presented, aimed at asynchronous spread spectrum systems. The approach is motivated by the fact that polyphase sequences offer increased design options, in terms of the supported range of correlation characteristics, while combination sequences allow for simpler generation of long sequences. It leads to the definition of combination Oppermann sequences, the properties of which are investigated in this paper. Numerical results indicate that the families of these proposed combination sequences provide favourable aperiodic correlations. The presented family of combination Oppermann sequences is therefore suitable for applications that rely on rapid synchronisation and are required to provide multiple access to the system.

In this paper we introduce periodic Oppermann sequences, which constitute a special class of polyphase sequences. The properties of these sequences are presented, and indicate that periodic Oppermann sequences are suitable for combination to generate families of longer sequences. Numerical examples show that periodic Oppermann sequences can be designed for ranging or synchronisation applications or for supporting multiple access spread spectrum communication systems.

This paper presents a new normalized frequency domain approach for adaptive blind equalization for multiple-input multiple-output (MIMO) communication systems. We first develop a time domain block based algorithm that updates the equalizer coefficients once per block of data symbols. As the time domain algorithm involves infinite summations in the separation cost function, an approximation is then proposed to reduce the cost function to finite summations. As a consequence, the block based time domain algorithm can be implemented in the frequency domain to reduce the computational complexity associated with the conventional symbol-by-symbol time domain update. Furthermore, by recognizing that the signals in the frequency domain are orthogonal, it is possible to significantly improve the convergence rate by normalizing the update equations with respect to the signal power in each frequency bin. Simulation results show that the proposed algorithm can successfully equalize the received signals while maintaining low computational complexity. Moreover, the presented normalized frequency domain blind equalization algorithm for MIMO systems significantly improves the convergence rate.

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These schemes aim to utilise channel capacity better than fixed transmission schemes, by adapting signal transmission parameters, such as modulation constellation and transmit power. Adaptive modulation schemes, which adapt the modulation constellation, have gained considerable favour for exploiting time-varying channel conditions without increasing the level of cochannel interference. However, the traditional adaptive modulation schemes are designed assuming zero delay between the channel estimation and the modulation mode adaptation. In the case of non-zero delay, the transmitter would update the modulation mode based on outdated channel state information from the receiver. We investigate an adaptive quadrature amplitude modulation (AQAM) scheme, which, by way of retransmissions, allows a targeted reliability level to be met, even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated. Closed form expressions for the average number of bits per symbol (BPS) throughput and the average bit error rate (BER) of the proposed scheme are derived.

An implicit link adaptation technique based on hybrid automatic repeat request (H-ARQ) using a soft-combining algorithm is considered for image transmission over wireless channels. Instead of the conventional link-level metrics such as bit error rate and frame error rate that do not correlate well with the quality as perceived by the end user, adaptation is carried out using metrics obtained from models that take the human perception into account. In this paper, a novel reduced-reference hybrid image quality metric (RR-HIQM) is proposed and used as a criterion to terminate the retransmission request in link adaptation. Numerical results show that the use of RR-HIQM in link adaptation provides robust link performance while meeting satisfactory quality constraints.

Broadband wireless systems require powerful equalizers which can cope with long channel impulse responses and severe intersymbol interference (ISI). In this paper, a frequency domain modified constant modulus algorithm (PD-MCMA) is presented. The employed modified CMA (MCMA) is chosen to confine the phase ambiguity of the conventional CMA to a 90/spl deg/ phase shift solution. On this foundation, a block MCMA algorithm which updates the equalizer coefficients on a block-by-block basis is proposed. The advantage of this block MCMA is that it can be equivalently implemented in the frequency domain. This approach offers a large reduction in computational complexity compared to time domain algorithms. For an equalizer with 256 coefficients, the number of real multiplications required by the FD-MCMA is only 9.5% of that required by the MCMA. Furthermore, convergence of the proposed FD-MCMA is significantly improved by properly normalizing the update equations. Simulations show that the normalized FD-MCMA converges much faster than the MCMA and the FD-MCMA while maintaining approximately the same bit error rate (BER) performance.

The performance of a code division multiple access system depends on the correlation properties of the employed spreading code. An auto-correlation function with a distinct peak enables proper synchronization and suppresses intersymbol interference. Low cross-correlation values between spreading sequences are desired to suppress multiple access interference. However, these requirements contradict each other and a trade-off needs to be established. In this paper, a modified bridging method is proposed to minimize cross-correlation with auto-correlation being allowed to lie within a fixed region. This approach is applied to the design of complex spreading sequences.

The evolution of digital mobile communications along with the increase of integrated circuit complexity has resulted in frequent use of error control coding to protect information against transmission errors. Soft decision decoding methods, such as the maximum a posteriori (MAP) decoding algorithm, process soft information and aim at minimizing bit error rate. This paper investigates application of a trellis-based MAP decoding approach to JPEG image transmission in wireless channels. When used with a soft-combining technique, it is shown to improve the performance significantly. This facilitates transmission of highly error-sensitive JPEG images in mobile radio systems, even when the ratio of average bit energy to noise power spectral density is quite low.

Soft-combining techniques can be used to provide reliable communication over very noisy channels. This is achieved by repeating and then combining codewords until a desired reliability level has been obtained. The heart of the presented soft-combining approach for linear unequal error protection codes is a maximum a posteriori decoder. The decoding problem is mapped into a spectral domain with a greatly simplified computational structure. The decoder releases soft-output, which is used later on as a priori information for decoding of a retransmitted codeword. Numerical examples show that significant performance improvement can be obtained with soft-combining compared to simple soft decision decoding.

We present a method to evaluate post-decoding bit error probabilities for individual bit positions in a linear unequal error protection (LUEP) code with symbol-by-symbol MAP decoding. A matrix description of the LUEP code trellis is used to formulate the MAP decoding problem analytically in original domain. This can be illustrated by a weighted trellis with joint probabilities being the weights of the trellis. By way of a similarity transformation, the decoding problem is mapped into a spectral domain having a much simpler computational structure. Because the considered LUEP codes are simple to implement, the method may be used in applications where the decoding delay needs to be kept at a minimum.

We present simulation results for the 13 channel DS CDMA WATM LAN utilising optimized complex spreading signatures based on Walsh functions. The method to obtain those optimized spreading signatures, as well as the full set of the coefficients giving the minimum level of cross-correlation between any pair of the channels is shown. The resultant system BER as well as the distribution of errors within WATM cells is given. The obtained results indicate that with the application of a hybrid ARQ scheme with errors and erasures decoding, the number of WATM cells which would require retransmission is in the order of 0.46%.

A major challenge in the design of wireless ATM LANs is to overcome the severe degradation of channel bit error rate resulting from channel impairments such as multi-path propagations. In this paper, we propose a WATM LAN which incorporates the CDMA spread spectrum technique together with hybrid ARQ error control schemes to effectively combat multi-path effects. We present a TDMA based multiple access protocol to arbitrate access of ATM cells from mobile terminals into time-slots on multiple CDMA channels. The multiple access protocol can efficiently support a mixture of real-time, delay-sensitive services and loss-sensitive, data services.

We present a unified method to compute the exact performance of ARQ schemes for wireless networks. A wireless channel is modeled as a stochastic sequential machine. Accordingly, we propose a channel state ordered representation of the performance provided by an underlying block code. Based on proper events introduced to assess reliability, throughput, and delay performance of an ARQ scheme, we employ a discrete system model to obtain a matrix description for ARQ schemes used on wireless channels. Then, closed formulae are developed for all examined performance characteristics. Numerical results are given for a wireless ATM system.