On the Capacity of Free-Space Optical Intensity Channels

New upper and lower bounds are presented on the capacity of the free-space optical intensity channel. This channel is characterized by inputs that are nonnegative (representing the transmitted optical intensity) and by outputs that are corrupted by additive white Gaussian noise (because in free space the disturbances arise from many independent sources). Due to battery and safety reasons the inputs are simultaneously constrained in both their average and peak power. For a fixed ratio of the average power to the peak power the difference between the upper and the lower bounds tends to zero as the average power tends to infinity, and the ratio of the upper and lower bounds tends to one as the average power tends to zero. The case where only an average-power constraint is imposed on the input is treated separately. In this case, the difference of the upper and lower bound tends to 0 as the average power tends to infinity, and their ratio tends to a constant as the power tends to zero.Comment: To be presented at ISIT 2008 in Toront

Study of a novel free-space optical communication system with structured light beams

Since the advent of modern telecommunications, the necessity to exploit the same link to put in communication more users or to increase the amount of transmitted information has boosted the exploration of the different degrees of freedom of light, leading to the development of several division multiplexing techniques based on wavelength, time, polarization, phase/amplitude, and, more recently, space. Space division multiplexing (SDM) relies on structuring the intensity or phase distribution of electromagnetic waves over a set of non-interfering spatial configurations to be used as distinct information channels at the same frequency in combination with standard modulation formats, offering a way out to the impelling problem of networks saturation (optical crunch) and a wider alphabet of states for quantum protocols. That requires the choice of a suitable family of orthogonal beams and the design of specific devices, i.e., the multiplexer and the demultiplexer, realizing their superposition at the transmitter stage and separation at the receiver one. \\ In this thesis, a novel and innovative framework will be considered and investigated for SDM, based on the exploitation of a new type of beams characterized by multipole phases. The purpose of this work is the design and simulation of a free-space optical link based on multipole-phase division multiplexing. The generation (multiplexing), transmission, and sorting (demultiplexing) of multipole-phase beams will be analysed with both a theoretical and numerical approach, in order to engineer the design of efficient and compact all-optical devices for the manipulation and control of this new type of structured beams. This new spatial multiplexing technique provides an innovative and revolutionary solution in the scenario of free-space optical communication, from the optical up to the radio regimes, promising to solve the still-opened issues of previous techniques, as those based on orbital angular momentum, and offering an efficient and practical method for high-capacity transmission.Since the advent of modern telecommunications, the necessity to exploit the same link to put in communication more users or to increase the amount of transmitted information has boosted the exploration of the different degrees of freedom of light, leading to the development of several division multiplexing techniques based on wavelength, time, polarization, phase/amplitude, and, more recently, space. Space division multiplexing (SDM) relies on structuring the intensity or phase distribution of electromagnetic waves over a set of non-interfering spatial configurations to be used as distinct information channels at the same frequency in combination with standard modulation formats, offering a way out to the impelling problem of networks saturation (optical crunch) and a wider alphabet of states for quantum protocols. That requires the choice of a suitable family of orthogonal beams and the design of specific devices, i.e., the multiplexer and the demultiplexer, realizing their superposition at the transmitter stage and separation at the receiver one. \\ In this thesis, a novel and innovative framework will be considered and investigated for SDM, based on the exploitation of a new type of beams characterized by multipole phases. The purpose of this work is the design and simulation of a free-space optical link based on multipole-phase division multiplexing. The generation (multiplexing), transmission, and sorting (demultiplexing) of multipole-phase beams will be analysed with both a theoretical and numerical approach, in order to engineer the design of efficient and compact all-optical devices for the manipulation and control of this new type of structured beams. This new spatial multiplexing technique provides an innovative and revolutionary solution in the scenario of free-space optical communication, from the optical up to the radio regimes, promising to solve the still-opened issues of previous techniques, as those based on orbital angular momentum, and offering an efficient and practical method for high-capacity transmission

Analog free-space optical links.

Free-space optics (FSO) communications is a technology that uses modulated infrared optical beams to transmit information line-of-sight through the atmosphere. There has been a substantial increase in the use of FSO technology over the last few years, mainly for "last mile" applications, because FSO links provide the transmission capacity to overcome bandwidth bottlenecks between backbone optical fiber links and metropolitan concentrations of end users. Optical fiber has been traditionally deployed for the transmission of both digital and analog signals. While transmission techniques for analog radio frequency (RF) intensity-modulated signals over optical fibers is well-established, prior to the investigations presented in this dissertation, there is no report of research on the efficiency of FSO for transmission of analog signals in the technical literature. This dissertation research investigated the effectiveness of FSO to transport modulated RF analog signals and compares key performance measures against those of fiber optic links. In addition, a new method to setup temporary IS-95 CDMA microcells or permanent IS-95 CDMA macrocells using FSO was proposed and its viability investigated. Finally, a new transmission technique for transmitting multiple RF signals (channels) over a single FSO link using wavelength division multiplexing (WDM) technology for potential CATV applications was demonstrated

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

Free Space Optical Polarization De-multiplexing and Multiplexing by means of Conical Refraction

Polarization de-multiplexing and multiplexing by means of conical refraction is proposed to increase the channel capacity for free space optical communication applications. The proposed technique is based on the forward-backward optical transform occurring when a light beam propagates consecutively along the optic axes of two identical biaxial crystals with opposite orientations of their conical refraction characteristic vectors. We present experimental proof of usefulness of the conical refraction de-multiplexing and multiplexing technique by increasing in one order of magnitude the channel capacity at optical frequencies in a propagation distance of 4m

Orbital Angular Momentum-based Space Division Multiplexing for High-capacity Underwater Optical Communications

To increase system capacity of underwater optical communications, we employ the spatial domain to simultaneously transmit multiple orthogonal spatial beams, each carrying an independent data channel. In this paper, we multiplex and transmit four green orbital angular momentum (OAM) beams through a single aperture. Moreover, we investigate the degrading effects of scattering/turbidity, water current, and thermal gradient-induced turbulence, and we find that thermal gradients cause the most distortions and turbidity causes the most loss. We show systems results using two different data generation techniques, one at 1064 nm for 10-Gbit/s/beam and one at 520 nm for 1-Gbit/s/beam, we use both techniques since present data-modulation technologies are faster for infrared (IR) than for green. For the higher-rate link, data is modulated in the IR, and OAM imprinting is performed in the green using a specially-designed metasurface phase mask. For the lower rates, a green laser diode is directly modulated. Finally, we show that inter-channel crosstalk induced by thermal gradients can be mitigated using multi-channel equalisation processing.Comment: 26 pages, 5 figure

Capacity Results on Multiple-Input Single-Output Wireless Optical Channels

This paper derives upper and lower bounds on the capacity of the multiple-input single-output free-space optical intensity channel with signal-independent additive Gaussian noise subject to both an average-intensity and a peak-intensity constraint. In the limit where the signal-to-noise ratio (SNR) tends to infinity, the asymptotic capacity is specified, while in the limit where the SNR tends to zero, the exact slope of the capacity is also given.Comment: Submitted to IEEE Transactions on Information Theor