Reliable Communication over Optical Fading Channels

In free space optical communication links,atmospheric turbulence causes random fluctuations in the refractive index of air at optical wavelengths, which in turn cause random fluctuations in the intensity and phase of a propagating optical signal. These intensity fluctuations, termed ``fading,'' can lead to an increase in link error probability, thereby degrading communication performance. Two techniques are suggested to combat the detrimental effects of fading, viz., (a) estimation of channel fade and use of these estimates at the transmitter or receiver; and (b) use of multiple transmitter and receiver elements. In this thesis, we consider several key issues concerning reliable transmission over multiple input multiple output (MIMO) optical fading channels. These include the formulation of a block fading channel model that takes into account the slowly varying nature of optical fade; the determination of channel capacity, viz., the maximum achievable rate of reliable communication, when the receiver has perfect fade information while the transmitter is provided with varying degrees of fade information; characterization of good transmitter power control strategies that achieve capacity; and the capacity in the low and high signal-to-noise ratio (SNR) regimes. We consider a shot-noise limited, intensity modulated direct detection optical fading channel model in which the transmitted signals are subject to peak and average power constraints. The fading occurs in blocks of duration $T_{c}$ (seconds) during each of which the channel fade (or channel state) remains constant, and changes across successive such intervals in an independent and identically distributed (i.i.d.) manner. A single-letter characterization of the capacity of this channel is obtained when the receiver is provided with perfect channel state information (CSI) while the transmitter CSI can be imperfect. A two-level signaling scheme (``ON-OFF keying'') with arbitrarily fast intertransition times through each of the transmit apertures is shown to achieve channel capacity. Several interesting properties of the optimum transmission strategies for the transmit apertures are discussed. For the special case of a single input single output (SISO) optical fading channel, the behavior of channel capacity in the high and low signal-to-noise ratio (SNR) regimes is explicitly characterized, and the effects of transmitter CSI on capacity are studied

Cellular Underwater Wireless Optical CDMA Network: Potentials and Challenges

Underwater wireless optical communications is an emerging solution to the expanding demand for broadband links in oceans and seas. In this paper, a cellular underwater wireless optical code division multiple-access (UW-OCDMA) network is proposed to provide broadband links for commercial and military applications. The optical orthogonal codes (OOC) are employed as signature codes of underwater mobile users. Fundamental key aspects of the network such as its backhaul architecture, its potential applications and its design challenges are presented. In particular, the proposed network is used as infrastructure of centralized, decentralized and relay-assisted underwater sensor networks for high-speed real-time monitoring. Furthermore, a promising underwater localization and positioning scheme based on this cellular network is presented. Finally, probable design challenges such as cell edge coverage, blockage avoidance, power control and increasing the network capacity are addressed.Comment: 11 pages, 10 figure

MIMO Underwater Visible Light Communications: Comprehensive Channel Study, Performance Analysis, and Multiple-Symbol Detection

In this paper, we analytically study the bit error rate (BER) performance of underwater visible light communication (UVLC) systems with binary pulse position modulation (BPPM). We simulate the channel fading-free impulse response (FFIR) based on Monte Carlo numerical method to take into account the absorption and scattering effects. Additionally, to characterize turbulence effects, we multiply the aforementioned FFIR by a fading coefficient which for weak oceanic turbulence can be modeled as a lognormal random variable (RV). Moreover, to mitigate turbulence effects, we employ multiple transmitters and/or receivers, i.e., spatial diversity technique over UVLC links. Closed-form expressions for the system BER are provided, when equal gain combiner (EGC) is employed at the receiver side, thanks to Gauss-Hermite quadrature formula and approximation to the sum of lognormal RVs. We further apply saddle-point approximation, an accurate photon-counting-based method, to evaluate the system BER in the presence of shot noise. Both laser-based collimated and light emitting diode (LED)-based diffusive links are investigated. Since multiple-scattering effect of UVLC channels on the propagating photons causes considerable inter-symbol interference (ISI), especially for diffusive channels, we also obtain the optimum multiple-symbol detection (MSD) algorithm to significantly alleviate ISI effects and improve the system performance. Our numerical analysis indicates good matches between the analytical and photon-counting results implying the negligibility of signal-dependent shot noise, and also between analytical results and numerical simulations confirming the accuracy of our derived closed-form expressions for the system BER. Besides, our results show that spatial diversity significantly mitigates fading impairments while MSD considerably alleviates ISI deteriorations

Adaptive Subcarrier PSK Intensity Modulation in Free Space Optical Systems

We propose an adaptive transmission technique for free space optical (FSO) systems, operating in atmospheric turbulence and employing subcarrier phase shift keying (S-PSK) intensity modulation. Exploiting the constant envelope characteristics of S-PSK, the proposed technique offers efficient utilization of the FSO channel capacity by adapting the modulation order of S-PSK, according to the instantaneous state of turbulence induced fading and a pre-defined bit error rate (BER) requirement. Novel expressions for the spectral efficiency and average BER of the proposed adaptive FSO system are presented and performance investigations under various turbulence conditions and target BER requirements are carried out. Numerical results indicate that significant spectral efficiency gains are offered without increasing the transmitted average optical power or sacrificing BER requirements, in moderate-to-strong turbulence conditions. Furthermore, the proposed variable rate transmission technique is applied to multiple input multiple output (MIMO) FSO systems, providing additional improvement in the achieved spectral efficiency as the number of the transmit and/or receive apertures increases.Comment: Submitted To IEEE Transactions On Communication

Gaussian Entanglement Distribution via Satellite

In this work we analyse three quantum communication schemes for the generation of Gaussian entanglement between two ground stations. Communication occurs via a satellite over two independent atmospheric fading channels dominated by turbulence-induced beam wander. In our first scheme the engineering complexity remains largely on the ground transceivers, with the satellite acting simply as a reflector. Although the channel state information of the two atmospheric channels remains unknown in this scheme, the Gaussian entanglement generation between the ground stations can still be determined. On the ground, distillation and Gaussification procedures can be applied, leading to a refined Gaussian entanglement generation rate between the ground stations. We compare the rates produced by this first scheme with two competing schemes in which quantum complexity is added to the satellite, thereby illustrating the trade-off between space-based engineering complexity and the rate of ground-station entanglement generation.Comment: Closer to published version (to appear in Phys. Rev. A) 13 pages, 6 figure