Majid Safari

Relay-Assisted Free-Space Optical Communication

In this paper, we present relay-assisted transmission as a powerful fading mitigation tool for free-space optical systems operating in atmospheric turbulence channels. We study both serial (i.e., multi-hop transmission) and parallel (i.e., cooperative diversity) decode-and-forward relaying. We consider an aggregated channel model which takes into account both path-loss and turbulence-induced log-normal fading. Since fading variance is distance-dependent in free-space optical systems, relay-assisted transmission takes advantage of the shorter hops and brings significant performance improvements. We have derived closed form expressions for outage probability of the relaying schemes under consideration which are further confirmed through Monte-Carlo simulations. Our outage probability analysis demonstrates that an impressive performance improvement of 18.5 dB is possible with the use of a single relay at a target outage probability of 10-6.

Do We Really Need OSTBCs for Free-Space Optical Communication with Direct Detection?

In this letter, the authors investigate spatial diversity techniques for free-space optical (FSO) links with intensity modulation and direct detection (IM/DD) over log-normal atmospheric turbulence-induced fading channels. We restrict our attention to the deployment of orthogonal space-time block codes (OSTBCs) and repetition codes both of which have been recently proposed for FSO links. Our performance analysis demonstrates that, although both schemes are able to extract full diversity, repetition codes outperform OSTBCs. The performance gap increases with the increasing number of transmit apertures. Our findings clearly point out that deployment of OSTBCs is not necessary for a FSO IM/DD link.

Cooperative diversity over log-normal fading channels: performance analysis and optimization

Although there has been a growing interest on cooperative diversity, the current literature is mainly limited to the results obtained for Rayleigh, Rician, or Nakagami fading channels. In this paper, we investigate the performance of cooperative diversity schemes over log-normal fading channels which provide an accurate channel model for indoor wireless environments. We focus on single-relay cooperative networks with amplify-and-forward relaying and consider three TDMA-based cooperation protocols: which correspond to distributed implementations of MIMO (multi-input multi-output), SIMO (single-input multi-output), and MISO (multi-input single-output) schemes. For each protocol under consideration, we derive upper bounds on pairwise error probability over log-normal channels and quantify the diversify advantages. Based on the minimization of a union bound on the bit error rate performance, we further formulate optimal power allocation schemes which demonstrate significant performance gains over their counterparts with equal power allocation.

Multi-Hop Relaying over the Atmospheric Poisson Channel: Outage Analysis and Optimization

In this paper, we study the outage behavior of a decode-and-forward multi-hop free-space optical (FSO) system over a Poisson channel degraded by atmospheric turbulence. We assume that perfect channel side information (CSI) is available at the receiver side and consider both cases of perfect CSI and no CSI at the transmitter side. We solve the outage probability minimization problem subject to a peak power constraint as well as a short- or long-term average sum power constraint. As a result, optimal power control strategies are presented for different scenarios under consideration. A sub-optimal yet low-complexity solution is further proposed under the short-term power constraint. Our results demonstrate that multi-hop relaying yields significant performance improvements which are particularly important for long-range FSO links.

Optical OFDM With Single-Photon Avalanche Diode

In this letter, an optical orthogonal frequency division multiplexing (OFDM) system based on a single-photon avalanche diode (SPAD) receiver is presented. SPAD detectors do not require a transimpedance amplifier as they operate in the Geiger mode. This poses challenges on detecting continuous data carrying signals. However, in this letter, we show that OFDM signals can indeed be detected, and the bit error ratio performances of both dc-biased optical OFDM and asymmetrically clipped optical OFDM are investigated. The investigations are carried out in the absence of background light. The results are compared with those of standard optical OFDM with convetional photodiode receivers. The SPAD-based OFDM system shows superior power efficiency over the state-of-the-art counterpart, and it can reach receiver sensitivities that are comparable with existing radio frequency systems.

Diversity gain analysis of free-space optical communication systems

In this paper, we present a diversity gain analysis for free-space optical (FSO) diversity systems operating in log-normal atmospheric turbulence channels. We consider a multi-input multi-output (MIMO) system and two decode-and-forward (DF) relay-assisted systems based on either serial (i.e., multi-hop transmission) or parallel (i.e., cooperative diversity) configurations. We adopt the recently introduced so-called ldquoasymptotical relative diversity orderrdquo (ARDO) in our analysis and derive ARDOs for FSO diversity systems under consideration. Assuming M transmit and N receive apertures in MIMO scheme, we demonstrate that an ARDO of 4sigmachi 2 / ln(1 + (exp(4sigmachi 2)-1)/MN) is achievable where sigmachi 2 is the variance of the fading log-amplitude. For weak turbulence conditions (i.e., sigmachi ~ 0.1 ), ARDO is well approximated as MN. For DF relaying schemes with K relays each of which is equipped with single transmit/receive apertures, we demonstrate that ARDOs of (K +1)11/6 and 211/6 K are achievable, respectively, for serial and parallel relaying configurations.

Signalling over nonlinear fibre-optic channels by utilizing both solitonic and radiative spectra

Using the inverse scattering method known for solving partial differential equations such as nonlinear Schrödinger equation, we analyse a fibre optic communication system which maps data on both discrete and continuous spectra defined by nonlinear Fourier transform. By signalling over both spectra, the degrees of freedom of the channel can be exploited to potentially increase the achievable data-rate in an attempt to approach the capacity of the optical fiber. Considering such a system, our results demonstrate the signal-dependency of the noise mapped to the nonlinear frequency domain. The temporal broadening factor is also studied for different pulse shapes and pulse sequence lengths modulated over the continuous spectrum. Finally, the error rate performance of the communication system signalling over both discrete and continuous spectra is presented.

Efficient optical wireless communication in the presence of signal-dependent noise

This work investigates the problem of efficient signalling for optical wireless communication systems in the presence of signal-dependent noise. We propose a novel scheme based on signalling in the square-root domain (as defined by a square-root transform) where the optical channel originally distorted by signal-dependent noise is transformed into a stationary white Gaussian noise channel. The bit error rate (BER) of the proposed scheme is derived and compared with the BER performance of the conventional signalling in time domain. As an alternative solution, signalling in frequency domain is analyzed in the presence of signal-dependent noise.

Relay-Assisted Quantum-Key Distribution Over Long Atmospheric Channels

In this paper, we investigate a terrestrial relay-assisted scheme for a free-space quantum key distribution (QKD) system based on BB84 protocol. We consider the deployment of passive relays which simply redirect the qubits to the next relay node or to the receiver without performing any measurement or detection process. Based on a near-field analysis, we derive an upper bound on quantum bit error rate (QBER) of the relay-assisted QKD system. Our results demonstrate that the relay-assisted scheme is able to outperform point-to-point direct transmission for long link ranges in which turbulence effects are particularly degrading.

Capacity Analysis of Signaling on the Continuous Spectrum of Nonlinear Optical Fibers

This paper investigates the capacity of signaling on the continuous spectrum (CS) of optical fibers as defined by the nonlinear Fourier transform (NFT). The channel model of such NFT-based optical fiber communication systems is studied based on the behavior of the propagated optical signal in the time domain for an asymptotically long fiber length. The derived channel model is validated using simulation for different scenarios of practical interest. An important characteristic of the channel in the nonlinear spectral domain is the strong dependence of noise to signal. A variance normalizing transform is applied as a tool to obtain an estimate on the capacity of the underlying channel. The results predict a remarkable capacity for signaling on CS, which can be utilized as an extra degree of freedom along with discrete spectrum (i.e., soliton transmission). It is further observed that the channel capacity of signaling on CS saturates at high signal power despite the linearizing capability of the NFT.