George C. Alexandropoulos

Cognitive Amplify-and-Forward Relaying with Best Relay Selection in Non-Identical Rayleigh Fading

This paper investigates several important performance metrics of cognitive amplify-and-forward (AF) relay networks with a best relay selection strategy and subject to non-identical Rayleigh fading. In particular, assuming a spectrum sharing environment consists of one secondary user (SU) source, K SU relays, one SU destination, and one primary user (PU) receiver, closed-form expressions for the outage probability (OP), average symbol error probability (SEP), and ergodic capacity of the SU network are derived. The correctness of the proposed analysis is corroborated via Monte Carlo simulations and readily allows us to evaluate the impact of the key system parameters on the end-to-end performance. An asymptotic analysis is also carried out and reveals that the diversity gain is defined by the number of relays pertaining to the SU network (i.e., K), being therefore not affected by the interference power constraint of the PU network.

Orthogonal Space–Time Block Codes With CSI-Assisted Amplify-and-Forward Relaying in Correlated Nakagami-

In this paper, the performance of multiple-input multiple-output (MIMO) dual-hop amplify-and-forward (AF) relay systems using orthogonal space-time block codes (OSTBCs) over arbitrarily correlated Nakagami- m fading channels is analyzed. In particular, closed-form expressions for the end-to-end outage probability (OP) and the symbol error probability (SEP) with arbitrary number of transceiver antennas and general correlation matrices are derived. Their mathematically tractable forms readily enable us to evaluate the performance of MIMO AF relay systems that utilize OSTBCs. For sufficiently high signal-to-noise ratios (SNRs), asymptotically tight approximations for the OP and SEP are also attained, which reveal insights into the effects of fading parameters and antenna correlation on the systems performance. Furthermore, we prove that the correlation has no impact on the achievable diversity gain, which is equal to the minimum of the sum of fading parameters between the two hops. Selected numerically evaluated results are presented, showing an excellent agreement between the proposed analysis and equivalent Monte Carlo simulations.

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Multicell Cooperative Processing (MCP) has been identified as a key technology to underpin future 4G wireless systems. In MCP enabled systems, the Base Stations (BSs) of the network form groups that jointly process user signals in order to reduce inter-cell interference and boost performance. Although MCP can lead to significant gains, it is also accompanied by overheads that are proportional to the number of BSs that are grouped together, called the cluster size. Thus it is crucial that BS clusters are of a limited size and formed in an intelligent way for achieving high performance. In this paper we present different dynamic clustering approaches for forming BS clusters of limited size. BS clustering is no longer based on geographical criteria, e.g. proximity-based static clustering, but on the channel conditions that users experience to different BSs. Dynamic clustering algorithms capture the effect of changing channel conditions and thus exploit the inherent macrodiversity of multicell wireless systems. More specifically, we propose a dynamic clustering scheme that provides significant gains while being resilient to inaccurate information on the state of wireless channels.

Fading Channels

The performance of dual-hop Decode-and-Forward relaying with relay selection (RS) is analyzed over Nakagami-m fading channels. Assuming that the direct source-to-destination link is active, closed-form expressions for the moment generating and the cumulative distribution functions of a RS-based cooperation scheme that utilizes maximal-ratio diversity at the destination are derived. These expressions are used to obtain the outage probability (OP) and average symbol error probability (ASEP) of this pure RS scheme as well as of a rate-selective one that utilizes RS only when it provides higher achievable rate than that of the direct transmission. Numerically evaluated results, verified by computer simulations, show that, although, in terms of ASEP, relaying is always beneficial, in terms of OP, it should be disabled whenever the direct link is strong.

The value of dynamic clustering of base stations for future wireless networks

New results for the multichannel Nakagami-m fading model with an arbitrary correlation matrix are presented in this paper. By using an efficient tridiagonalization method based on Householder matrices, the inverse of the Gaussian correlation matrix is transformed to tridiagonal, managing to derive a closed-form union upper bound for the joint Nakagami-m probability density function and an exact analytical expression for the moment generating function of the sum of identically distributed gamma random variables. Our analysis considers an arbitrary correlation structure, which includes as special cases the exponential, constant, circular, and linear correlation ones. Based on the proposed mathematical analysis, we obtain a tight union upper bound for the outage probability of multibranch selection diversity receivers as well as exact analytical expressions for the outage and the average error probability of multibranch maximal-ratio diversity receivers. Our analysis is verified by comparing numerically evaluated with extensive computer simulation performance evaluation results, showing the usefulness of the proposed approach.

Performance Analysis of Cooperative Networks With Relay Selection Over Nakagami-

This paper investigates the end-to-end performance of a dual-hop amplify-and-forward (AF) relaying communication system where the source-to-relay and the relay-to-destination channels are subject to different fading conditions. The relay is assumed to either possess perfect channel state information (CSI) or have a fixed gain. We consider the case where the one hops channel is subject to η - μ fading, whereas the other hops channel is subject to κ- μ fading. This mixed fading propagation channel is capable of accurately modeling various practical dual-hop transmissions. Examples of such environments are encountered in micro-/macrocellular systems and/or hybrid satellite/terrestrial wireless communication systems, where typically, only the one hops channel has a line-of-sight (LOS) component. For both CSI-assisted and fixed-gain relaying and for integer-valued fading parameters, exact analytical expressions in the form of rapidly convergent infinite series for the outage probability (OP) and average bit error probability (ABEP) of several modulation schemes are derived. Moreover, for CSI-assisted relaying and arbitrary-valued fading parameters, closed-form lower bounds [tight for high values of the signal-to-noise ratio (SNR)] for the OP and ABEP performance are obtained. The analysis is also substantiated by obtaining previously published equivalent performance expressions as special cases of our generic fading models, namely, those available for Nakagami- m and Rice fading channels. In addition, the derived analytical expressions have been numerically evaluated, and the performance evaluation results have been further validated by comparing them with equivalent results that have been obtained by means of Monte Carlo computer simulations.

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The delay constraints imposed by future wireless applications require a suitable metric for assessing their impact on the overall system performance. Since the classical Shannons ergodic capacity fails to do so, the so-called effective rate was recently established as a rigorous alternative. While prior relevant works have improved our knowledge on the effective rate characterization of communication systems, an analytical framework encompassing several fading models of interest is not yet available. In this paper, we pursue a detailed effective rate analysis of Nakagami-m, Rician and generalized-K multiple-input single-output (MISO) fading channels by deriving new, analytical expressions for their exact effective rate. Moreover, we consider the asymptotically low and high signal-to-noise regimes, for which tractable, closed-form effective rate expressions are presented. These results enable us to draw useful conclusions about the impact of system parameters on the effective rate of different MISO fading channels. All the theoretical expressions are validated via Monte-Carlo simulations.

Fading Channels

The performance of switch-and-examine diversity (SED) over L arbitrarily correlated and not necessarily identically distributed Nakagami-m fading channels is studied. Analytical expressions for the distribution of the SED output signal-to-noise ratio (SNR) are obtained for the constant correlation model. For the most general case of arbitrary correlation, by assuming half-integer or integer values for the fading parameter m, analytical expressions for the distribution of the output SNR with L ¿ 3 are derived. Moreover, for L > 3, analytical approximations for the output SNR are presented. The derived expressions are used to study the outage and average symbol error probability of SED receivers. Performance results obtained by numerical evaluation and verified by means of computer simulations show that the performance of the receivers under consideration is degraded with increasing branch correlation. Nevertheless, SED receivers outperform uncorrelated switch-and-stay diversity receivers, even when they operate under high branch correlation.

New results for the multivariate Nakagami-m fading model with arbitrary correlation matrix and applications

Although relaying can be very beneficial for wireless systems, understanding which relaying schemes can achieve specific performance objectives under realistic fading is crucial. In this paper we present a general framework for modeling and evaluating the performance of dual-hop decode-and-forward (DF) relaying schemes over independent and not necessarily identically distributed (INID) Nakagami-m fading channels. We obtain closed-form expressions for the statistics of the instantaneous output signal-to-noise ratio of repetitive transmission with selection diversity. Furthermore, we present a unified statistical overview of other three significant relaying schemes with DF, one based on repetitive transmission with maximal-ratio diversity and the other two based on relay selection (RS). To compare the considered schemes, we present closed-form and analytical expressions for the outage probability and the average symbol error probability under various modulation methods, respectively. Importantly, it is shown that when the channel state information for RS is perfect, RS-based schemes always outperform repetitive ones. Furthermore, when the direct link between the source and the destination nodes is sufficiently strong, relaying may not result in any gains, and it should be switched off.

Performance Analysis of Dual-Hop AF Relaying Systems over Mixed

This paper provides an analytic performance evaluation of the bit error rate (BER) of underlay decode-and-forward cognitive networks with best relay selection over Rayleigh multipath fading channels. A generalized BER expression valid for arbitrary operational parameters is firstly presented in the form of a single integral, which is then employed for determining the diversity order and coding gain for different best relay selection scenarios. Furthermore, a novel and highly accurate closed-form approximate BER expression is derived for the specific case where relays are located relatively close to each other. The presented results are rather convenient to handle both analytically and numerically, while they are shown to be in good agreement with results from respective computer simulations. In addition, it is shown that as in the case of conventional relaying networks, the behaviour of underlay relaying cognitive networks with best relay selection depends significantly on the number of involved relays.