Showing papers by "Murat Uysal published in 2010"

Generalized Maximum-Likelihood Sequence Detection for Photon-Counting Free Space Optical Systems

Abstract: We investigate detection methods for on-off keying (OOK) photon-counting Free Space Optical (FSO) systems in the presence of turbulence-induced fading, assuming no channel state information at the receiver. To recover the performance loss which is associated with symbol-by-symbol detection in such a scenario, we consider sequence detection techniques, exploiting the temporal correlation of the FSO channel. Due to its high complexity in the calculation of its metric, optimal maximum likelihood sequence detection (MLSD) is infeasible for most practical purposes. Hence, we propose a suboptimal low-complexity detection rule, which is based on the generalized maximum-likelihood sequence estimation. The proposed scheme allows the detection of sequence lengths that are prohibitive for conventional MLSD, without using any kind of channel knowledge. Monte Carlo simulation results show its performance to be very close to the optimum for large sequence lengths and various fading models.

Diversity–Multiplexing Trade-Off in Coherent Free-Space Optical Systems With Multiple Receivers

Abstract: In this paper, from an information theory point of view, we investigate the performance of a coherent free-space optical (FSO) communication system with multiple receive apertures over atmospheric turbulence channels. Our study builds on a recently introduced statistical model that characterizes the combined effects of turbulence-induced wavefront distortion and amplitude fluctuation in coherent receivers with phase compensation. We investigate the link reliability as quantified by “diversity gain” and the relationship between the link reliability and the spectral efficiency as quantified by “diversity-multiplexing trade-off (DMT).” Our results provide insight into the performance mechanisms of coherent FSO systems and demonstrate significant performance gains that can be obtained through the deployment of multiple receive apertures and phase compensation techniques.

Channel estimation for amplify-and-forward relaying: Cascaded against disintegrated estimators

Abstract: The authors investigate the performance of amplify-and-forward relaying with two different pilot-symbol-assisted channel estimation methods. In the first estimation method, the cascaded channel consisting of source-to-relay and relay-to-destination links is estimated at the destination terminal. No channel estimator is required at the relay terminal. In the second estimation method, the estimation of cascaded channel is disintegrated into separate estimations of source-to-relay and relay-to-destination links which are carried out at the relay and the destination terminals, respectively. The latter method involves feed-forwarding a quantised version of the source-to-relay channel estimate to the destination terminal. Through the derivation of mean squared error (MSE) diversity gain analysis and Monte-Carlo simulations, the authors investigate the performance of amplify-and-forward relaying with these channel estimation methods. Our results demonstrate that full diversity can be achieved in the presence of channel estimation. The authors further show that cascaded channel estimator outperforms its competitor with a small number of quantisation bits. As the number of employed quantisation bits increase, disintegrated channel estimator approaches to its competitor eventually outperforming it.

Capacity of Hybrid Cognitive Radio Networks With Distributed VAAs

Abstract: A cooperative hybrid cognitive radio (CR) network is proposed to simultaneously operate on a dedicated licensed band and a secondary band. The licensed band is used for communications between a base station (BS) and mobile CR users, whereas the secondary band is used to facilitate the licensed band communication by coordinating multiple CR users to form distributed virtual antenna arrays (VAAs). The capacity of the proposed CR network is studied at both the link and system levels. At the link level (single VAA case), we present an amplify-and-forward-based cooperative signaling scheme that employs power control to prevent harmful noise propagation. The resulting virtual multiple-input-multiple-output (MIMO) link capacity is derived and compared with the real MIMO system. At the system level (multiple VAAs case), the system capacity is derived as a function of multiple parameters, including the primary user density, CR user density, primary exclusion region radius, and VAA radius. Under an average interference power constraint, the maximum system capacity is further calculated by solving an optimization problem with adjustable system parameters. Numerical studies reveal that the proposed cooperative hybrid CR network has a fundamental advantage over a pure CR network by being insensitive to the characteristics of the coexisting primary network. This merit, however, relies on a high CR user density and a wide bandwidth of the secondary band.

Iterative Near Maximum-Likelihood Sequence Detection for MIMO Optical Wireless Systems

Abstract: A major performance-limiting factor in terrestrial optical wireless (OW) systems is turbulence-induced fading. Exploiting the additional degrees of freedom in the spatial dimension, multiple laser transmitters combined with multiple receive apertures provide an effective solution for fading mitigation. Although multiple-input multiple-output (MIMO) OW systems have been extensively studied in recent years, most of these works are mainly limited to symbol-by-symbol decoding. MLSD exploits the temporal correlation of turbulence-induced fading and promises further performance gains. In this paper, we investigate MLSD for intensity-modulation/direct-detection MIMO OW systems over log-normal atmospheric turbulence channels. Even with a low-order modulation scheme such as OOK, which is typically used in OW systems, the complexity of MLSD might be prohibitive. We therefore present an iterative sequence detector based on the expectation-maximization (EM) algorithm. The complexity of the proposed algorithm is considerably less than a direct evaluation of the log-likelihood function, and it is independent of the channel's fading statistics. The Monte Carlo simulation results demonstrate that the EM-based algorithm outperforms the symbol-by-symbol decoder and achieves a performance, which lies within 0.3 dB of that of the optimal MLSD.

Performance Analysis of Cooperative Diversity Networks with Imperfect Channel Estimation

Abstract: In this paper, we investigate the effect of channel estimation error on the error rate performance of a multi-relay system with amplify-and-forward relaying. We consider orthogonal relaying and study both conventional cooperative systems (i.e., all relays participate in the relaying phase) and opportunistic cooperative systems (i.e., only the best relay participates in the relaying phase). Based on the derivation of effective signal-to-noise ratio at the receiver taking into account channel estimation error, we obtain closed-form expressions for bit error rate. Monte-Carlo simulations are further provided to confirm the analytical results.

Error Rate Performance of Network-Coded Cooperative Diversity Systems

Abstract: In this paper, we study network-coded cooperative diversity (NCCD) systems comprising multiple sources, one relay, and one destination. The relay detects the packets received from all sources and performs Galois field network coding. We propose a simple cooperative maximum-ratio combining scheme for the destination which is shown to achieve the maximum diversity gain of the system. Furthermore, we provide a mathematical framework for the asymptotic analysis of NCCD systems with M-ary modulation for high signal-to-noise ratios. Based on this framework, we derive simple and elegant closed- form expressions for the asymptotic symbol and bit error rates which provide significant insight into the impact of various system and channel parameters on performance and can be exploited for performance optimization. Simulation results confirm the accuracy of the presented analysis and show that large performance gains are possible by optimizing the power allocation in NCCD systems based on the developed analytical results.

Novel Distributed Space-Time Trellis Codes for Relay Systems over Cascaded Rayleigh Fading

Abstract: In this letter, we consider the deployment of distributed space-time trellis codes (STTCs) for user cooperation. We derive a pairwise error probability (PEP) expression for distributed STTCs over cascaded Rayleigh fading channels. Using the derived PEP expression, we determine a code design criterion and propose novel 4-/8-/16-state 4-PSK and 8-/16-state 8-PSK distributed STTCs through a systematic code search. We confirm the superiority of the proposed codes through extensive Monte-Carlo simulations.

Performance Analysis of Adaptive L-QAM for Opportunistic Decode-and-Forward Relaying

Abstract: -In this paper, we investigate the performance of constant-power adaptive L-ary quadrature modulation (L-QAM) for an opportunistic decode-and-forward (DF) relaying scheme. Specifically, we adopt adaptive five-mode L-QAM with fixed and optimum switching and derive expressions for the error rate, outage probability and spectral efficiency over both independent and identical (i.i.d.) and independent and non-identical (i.n.d.) Rayleigh fading channels.

Performance analysis of cooperative diversity networks with imperfect channel estimation over Rician fading channels

Abstract: In this paper, we examine the effect of channel estimation error on the error and outage probabilities of a multi-relay system with amplify-and-forward relaying over frequency-flat Rician fading channel. We consider orthogonal relaying and study both conventional cooperative systems (i.e., all relays participate in the relaying phase) and opportunistic cooperative systems (i.e., only the best relay participates in the relaying phase). Based on the derivation of effective signal-to-ratio (SNR) at the receiver taking into account channel estimation error, we obtain closed-form expressions for error and outage probabilities in high SNR regime. Such closed form solutions are highly desirable because they allow for rapid and efficient evaluation of system performance. Computer simulations are used to validate our analytical results.

Exact Closed-Form Error Probability Expression for Cooperative Diversity Networks with Channel Estimation Errors in Time Selective Rayleigh Fading Channels

Abstract: -In this paper, we investigate the performance of a cooperative network with adaptive decode-and-forward (DF) relaying over time-selective frequency-flat Rayleigh fading channels. In adaptive DF relaying, only a subset of the available relays with "good" channels are allowed to participate in the relaying phase. The destination combines the direct and the relayed signals using maximal ratio combining technique. Pilot-symbol-assisted modulation (PSAM) is used for the estimation of time-varying fading channel coefficients. For the system under consideration, we derive an exact closed-form expression for the average bit error rate. Such closed form solutions are highly desirable because they allow for rapid and efficient evaluation of system performance. We further present computer simulations to validate our analytical results.

DMT analysis of coherent free-space optical systems over atmospheric turbulence channels

Abstract: In this paper, we investigate the diversity-multiplexing tradeoff (DMT) performance of a coherent free-space optical (FSO) communication system with multiple receive apertures over atmospheric turbulence channels. Our study builds on the recently introduced statistical model that characterizes the combined effects of turbulence-induced wavefront distortion and amplitude fluctuation in coherent receivers with phase compensation. Our results provide insight into performance mechanisms of coherent FSO systems and demonstrate significant performance gains that can be obtained through the deployment of multiple receive apertures and phase compensation techniques.

Time synchronization in amplify-and-forward cooperative communication systems

Abstract: The synchronization problem in wireless communication systems becomes more important especially in cooperative communication systems than it is for other diversity systems because all terminals in the cooperation use their own local oscillators. In this study, a maximum likelihood based receiver is proposed as a solution to time synchronization problem in cooperative communication systems. The bit error rate performance of the proposed receiver is demonstrated by using computer simulations. Based on experimental results, we see that a considerable amount of power gain can be obtained using the proposed receiver. This power gain is increased as the time difference between signals arriving to the receiver from different terminals goes up.

Relay-assisted free-space quantum-key distribution with partial phase compensation

Abstract: We study the effect of partial phase compensation on a free-space relay-assisted quantum-key distribution system. Our analytical results demonstrate that the relay-assisted system outperforms the direct transmission even with partial phase compensation.

Design of distributed space-time trellis code for relay systems over cascaded Nakagami channels

Abstract: In this paper, error performance of a coded cooperative relay system is investigated when channels have cascaded Nakagami distribution. By using generalpairwise error probability upper bound of the system, code design criterion for the distributed space-time code is derived and using this criterion, 8-state distributed space-time trellis codes for 4-PSK and 8-PSK modulations are designed. The error performances of the new codes are evaluated by computer simulations and compared those of the corresponding codes.

Kuvvetlendir-ve-Aktar Tipi birlikli Haberleme Sistemlerinde Zaman Senkronizasyonu Time Synchronization in Amplify-and-Forward Cooperative Communication Systems

Abstract: The synchronization problem in wireless communication systems becomes more important especially in cooperative communication systems than it is for other diversity systems because all terminals in the cooperation use their own local oscillators. In this study, a maximum likelihood based receiver is proposed as a solution to time synchronization problem in cooperative communication systems. The bit error rate performance of the proposed receiver is demonstrated by using computer simulations. Based on experimental results, we see that a considerable amount of power gain can be obtained using the proposed receiver. This power gain is increased as the time difference between signals arriving to the receiver from different terminals goes up. 1. Giri�