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Error propagation from the relay to the destination is a major drawback of digital relaying protocols that substantially diminishes the cooperative diversity gain of the system. Appropriately designed selection relaying schemes can effectively mitigate this phenomenon and improve the diversity order, especially when the source-relay channel state information (CSI) is not available at the destination. In this study, the authors investigate the SNR-based selection relaying scheme in a multi-relay cooperative network (with R potential relay nodes) and examine the impact of the error propagation on the end-to-end performance of the system. The authors derive closed-form expressions for the end-to-end symbol error probability (SEP) of the system for both identically distributed and non-identically distributed Rayleigh fading environments. In the analysis, the authors consider M-ary phase shift keying (M-PSK), M-ary pulse amplitude modulation (M-PAM) and M-ary quadrature amplitude modulation (M-QAM) schemes. They also propose a class of geometry-independent thresholds that effectively mitigate the error propagation and provide full diversity of order R+1. Furthermore, for the special case of single-relay network, the authors derive closed-form expressions for the optimal thresholds that minimise the end-to-end SEPs. [ABSTRACT FROM AUTHOR]

In this study, the performances analysis of cooperative-diversity networks using adaptive decode-and-forward (DF) relaying over independent non-identical flat Nakagami-m fading channels is investigated. The authors derive exact and approximate closed-form expressions for the error probability, outage probability and average channel capacity, and analyse their dependence on the channel parameters. In adaptive DF relaying, among M relays that can participate, only C relays (C≤M), with good channels to the source, DF (retransmit) the source information to the destination. Then, the destination combines the direct and the indirect signals using maximum ratio combining technique. The authors derive a closed-form expression for the the moment generating function of the total signal-to-noise ratio (SNR) at the destination node. Then, the authors find an exact closed-form expression for the probability density function (PDF) of the total SNR at the destination. This PDF is used to derive the exact closed-form expressions of the performance metrics. Such closed-form solutions are highly desirable because they allow for rapid and efficient evaluation of system performance. Since exact expressions are complicated, the authors have introduced simple approximate expressions for the error and outage probabilities where these expressions explicitly show the diversity order. Computer simulations are used to validate our analytical results. Results show the significant performance improvement because of the use of the adaptive DF cooperative diversity. Also, results indicate that increasing the number of relays will not always decrease the outage probability. [ABSTRACT FROM AUTHOR]

The system performance of the opportunistic relaying technique in amplify-and-forward cooperative systems with adaptive transmission over Rayleigh fading channels is analysed. Two rate-adaptive modulation techniques, continuous-rate and discrete-rate adaptive techniques, are considered. Closed-form expressions are derived for the average spectral efficiency, outage probability and the average bit error rate for two cases: independent and identically distributed (i.i.d.) fading links and independent and non-identically distributed (i.ni.d.) links. Using numerical evaluations, the performances of different cases are compared showing the advantages of each adaptive modulation technique in conjunction with the opportunistic relaying in a cooperative communication over both i.i.d. and i.ni.d. fading links. [ABSTRACT FROM AUTHOR]

In this study, the authors investigate the error rate performance of amplify-and-forward relaying over N*Nakagami fading channels. This is a recently introduced channel model that involves the product of N Nakagami-m-distributed random variables. Employing the moment generating function approach, the authors derive symbol error rate expressions for a single-relay system under instantaneous power scaling (IPS) and average power scaling (APS) factors at the relay node, that is, variable and fixed gains. The results achieved by the authors demonstrate that the achievable diversity order is a function of Nakagami fading parameter (m) and degree of cascading (N). An identical diversity order is obtained under both scaling factors when the relay is close to the destination. When the relay is close to the source, IPS becomes advantageous over APS. Monte-Carlo simulations are further provided to confirm the analytical results. [ABSTRACT FROM AUTHOR]

Despite their attractive features, free-space optical (FSO) communications suffer from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, multiple input multiple output (MIMO) and cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in radio frequency (RF) communications have greatly been investigated, there have not been remarkable researches on cooperative transmissions in FSO communications. In this study, the authors consider a relay-assisted FSO communication and investigate the outage performance of two well-known cooperative protocols, namely amplify-and-forward (AF) and decode-and-forward (DF), for this communication system. By a new definition of the order of diversity, the authors derive an analytical expression for the performance comparison of different cooperative and non-cooperative protocols. The authors also evaluate the performance of a FSO communication with two transmitter telescopes (MIMO systems) and compare its outage probability with those of cooperation strategies. The authors investigate the best place for the relay for both the AF and DF strategies and show that relay-assisted techniques can be used for FSO communication to achieve spatial diversity. Their performance can surpass that of the two-transmitter case by properly choosing the location of the relay. [ABSTRACT FROM AUTHOR]

The IEEE 802.11e standard is specified to support quality-of-service in wireless local area networks, and different contention parameters are designated to each type of service. This developed model is presented to analyse the scheduling and the contention between packets with different priorities, where the new features of the enhanced distributed channel access such as virtual collision, backoff, minimum contention window and different arbitration inter-frame spaces are taken into account. Based on the model, the delay performance of differentiated service traffic is analysed and a recursive method is proposed, which is capable of calculating the mean access delay. Simulations show that the model and the analysis provide an insight into the protocol and the effects of different parameters on the performance. [ABSTRACT FROM AUTHOR]

As extensions in the emerging 802.11e for quality-of-service provisioning, burst transmission and the acknowledgment aggregation are the two important operations to improve the channel efficiency of IEEE 802.11-based wireless local area networks (WLANs). However, only a few works have been done on these operations, and usually assumed the networks to be operated under saturated traffic conditions and error-free channels. In practice, the assumptions may not be valid because real-time traffic with proper rate control will not saturate the networks and the channel is generally error-prone. Thus, the authors consider two new methods resulted from these operations and analyse their performance under unsaturated and error-prone WLANs, with a Markov chain model. The results show that the new methods generally have better throughput than the conventional IEEE 802.11 medium access control (MAC) in the WLANs. [ABSTRACT FROM AUTHOR]

A new family of two-dimensional (2-D) codes constructed by combining frequency-hop and time-spreading codes is presented. The proposed codes are designed to overcome the limitation imposed on the maximum permissible number of simultaneous users in optical code-division multiple-access (OCDMA) systems. To increase the data transmission rate, the 2-D codes employ a M-ary signalling scheme. The proposed family of codes has the favourable characteristics of a zero autocorrelation constraint and a cross-correlation constraint of one. The performance of the 2-D code is analysed in terms of the bit error rate (BER) and is compared with that of a conventional OCDMA system using optical orthogonal code (OOC). The numerical and simulation results reveal that the proposed system outperforms conventional OCDMA systems both in terms of the BER and the data transmission rate. In addition, the system capacity is sufficient to ensure reliable communication (BER≤10-9) in local area networks. [ABSTRACT FROM AUTHOR]

A two-hop cooperative communication network with full-duplex relaying and relay selection is addressed. Closed-form expressions for average capacity and symbol error rate are derived in an independent and identically distributed Rayleigh fading environment. Using these expressions, performance analysis and the impact of interference between relay output and input are provided. [ABSTRACT FROM AUTHOR]

A high-gain dual-polarised planar etched slot array for use in wireless local area networks (WLAN) is presented. Two sets of four collinear slots are used as radiators for each polarisation. Each set of four collinear slots is constructed as a centre-fed long slotline, with etched crossovers to split the slotline into four resonant and in-phase half-wavelength slots. A reflector etched on a separate substrate spaced a quarter-wavelength away from the slot array ensures unidirectional radiation. The final antenna achieved a gain of 14.5 dBi, a return loss better than 10 dB, and port isolation in excess of 30 dB over the 2.4–2.484 GHz WLAN frequency band. [ABSTRACT FROM AUTHOR]

In the presence of multiple primary users (PUs), the exact outage probability of partial amplify-and-forward (AF) relay selection in the spectrum-sharing relay networks is investigated. To gain more insight, a simple and accurate outage probability approximation is also derived in the high signal-to-noise ratio (SNR) region. It is shown that, at high SNR, the outage probability is affected only by the channel gain ratio between relay-destination links to relay-PU links, regardless of the number of relays. Moreover, when more PUs are present, it causes outage performance degradation on the secondary system. Finally, simulation results are presented to verify the analysis. [ABSTRACT FROM AUTHOR]

In distributed speech recognition applications, the frame rate of observation may be reduced to match a resource limited front end device. A method to adapt the transition probabilities of hidden Markov models to match the frame rate of the observation is proposed. Experimental results on connected digit recognition show that the recognition rates of the proposed adapted system using half frame rate data were comparable to those of the original system using full frame rate data. [ABSTRACT FROM AUTHOR]

Mode selection algorithms for spatiotemporal concealment of missing macroblocks (MBs) in a corrupted video stream are presented. The proposed algorithms employ the motion compensated temporal activity in the neighbourhood of each missing MB for switching between concealment methods. Results show gains of up to 4.6 dB for a 10% packet error rate relative to other mode selection methods. [ABSTRACT FROM AUTHOR]

A modular method which derives a scalable construction of a group key exchange (GKE) scheme from a two-party key exchange (KE) one is presented. By combining the method presented with the compiler by Katz and Yung a scalable compiler can be obtained that transforms any two-party KE protocol secure against a passive eavesdropper to an authenticated GKE one secure against a stronger active adversary. [ABSTRACT FROM AUTHOR]

The article focuses on some proposals for the use of real-time Ethernet in industries. Ethernet is starting to replace the industrial fieldbuses that were developed in the 1990s. There are now three proposals for real-time Ethernet. Two are from German companies: Siemens (PROFInet v3) and Beckhoff (EtherCAT ) and the third is from B&R Automation (Powerlink), based near Salzburg. All three rely on the basic Ethernet frame or "telegram" for communication. No high level protocol like TCP/IP is used, although Ethernet and TCP/IP may co-exist on the same cable.