On Optical Wireless Communications Research Articles

Impact of Device Orientation on Error Performance of LiFi Systems

Most studies on optical wireless communications (OWC) have neglected the effect of random orientation in their performance analysis due to the lack of a proper model for the random orientation. Our recent empirical-based research illustrates that the random orientation follows a Laplace distribution for the static user equipment (UE). In this paper, we analyze the device orientation and assess its importance on system performance. The reliability of the OWC channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this paper, the effect of receiver orientation, including both polar and azimuth angles on the LOS channel gain are analyzed. The probability of establishing the LOS link is investigated and the probability density function (PDF) of signal-to-noise ratio (SNR) for a randomly oriented device is derived. By means of the PDF of SNR, the bit-error ratio (BER) of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in additive white Gaussian noise (AWGN) channels is evaluated. A closed-form approximation for the BER of UE with random orientation is presented which shows a good match with Monte-Carlo simulation results. Furthermore, the impact of UE's random motion on the BER performance has been assessed. Finally, the effect of random orientation on the average signal-to-interference-plus-noise ratio (SINR) in a multiple access points (AP) scenario is investigated.

Ultimate channel capacity of free-space optical communications [Invited

Feature Issue on Optical Wireless Communications (OWC) The ultimate classical information capacity of multiple-spatial-mode, wideband optical communications in vacuum between soft-aperture transmit and receive pupils is considered. The ultimate capacity is shown to be achieved by coherent-state encoding and joint measurements over entire code words. This capacity is compared with the capacities realized with the same encoding and homodyne or heterodyne detection, which are single-channel-use measurements. Realistic background spectral-radiance values are used to obtain tight bounds on the capacity of single-spatial-mode, narrowband 1.55 μm wavelength free-space communications in the presence of background light.

Shannon capacities and error-correction codes for optical atmospheric turbulent channels

Feature Issue on Optical Wireless Communications (OWC) The propagation of an on-off keying modulated optical signal through an optical atmospheric turbulent channel is considered. The intensity fluctuations of the signal observed at the receiver are modeled using a gamma-gamma distribution. The capacity of this channel is determined for a wide range of turbulence conditions. For a zero inner scale, the capacity decreases monotonically as the turbulence strengthens. For non-zero inner scale, the capacity is not monotonic with turbulence strength. Two error-correction schemes, based on low-density parity-check (LDPC) codes, are investigated as a means to improve the bit-error rate (BER) performance of the system. Very large coding gains--ranging from 5.5 to 14 dB, depending on the turbulence conditions--are obtained by these LDPC codes compared with Reed-Solomon error-correction codes of similar rates and lengths.

Trends in satellite communications and the role of optical free-space communications [Invited

Feature Issue on Optical Wireless Communications (OWC) The communication needs of Earth observation satellites is steadily increasing. Within a few years, the data rate of such satellites will exceed 1 Gbit/s, the angular resolution of sensors will be less than 1 μrad, and the memory size of onboard data recorders will be beyond 1 Tbyte. Compared with radio frequency links, optical communications in space offers various advantages, such as smaller and lighter equipment, higher data rates, limited risk of interference with other communications systems, and the effective use of frequency resources. This paper describes and compares the major features of radio and optical frequency communications systems in space and predicts the needs of future satellite communications.

Channel modeling for optical wireless communication through dense fog

Feature Issue on Optical Wireless Communications (OWC) The modified vector radiative transfer equation is solved in the frequency domain to study the frequency response of the propagation channel. In the time domain, the impulse response is obtained by taking the inverse Fourier transform of the frequency response. The time-domain analysis investigates the effects of the receiver's field of view on reducing the bit error rate when there is intersymbol interference caused by the temporal spread of the received signal.

Aperture averaging for optimizing receiver design and system performance on free-space optical communication links

Feature Issue on Optical Wireless Communications (OWC) We have developed a flexible, empirical approach for optimizing the design of free-space optical communication links by using multiframe image analysis of received intensity scintillation patterns. This is a versatile way to perform aperture-averaging analysis. A high-performance digital camera with a frame-grabbing computer interface is used to capture received intensity distributions of a He-Ne laser beam propagating in weak and intermediate turbulence conditions. The aperture-averaging results demonstrate the expected reduction in intensity fluctuations due to increasing the receiver aperture diameter for various strengths of turbulence. Aperture averaging improves the bit error rate.

Self-organizing broadband hybrid wireless networks

Feature Issue on Optical Wireless Communications (OWC) We describe what we believe to be a new paradigm for autonomous configuration and self-organization of directional broadband wireless networks. These networks use combined free-space optics (FSO) and directional radio-frequency (RF) wireless communication modalities, along with agile transceivers that can intelligently direct their narrow beams. Autonomous hardware and software control of network topologies is used to set up links or reconfigure them in response to performance degradation. We call our new paradigm for dynamic network operation and self-organization "topology control." We describe research in the design, development, and evaluation of novel, fully optical wireless pointing, acquisition, and tracking (PAT) techniques. We also describe the modeling and simulation of hybrid FSO-RF networks in dynamic environments, which include changing traffic and atmospheric conditions, and node mobility.

Optical wireless communications within fourth-generation wireless systems [Invited

Feature Issue on Optical Wireless Communications (OWC) It is commonly agreed that the next generation of wireless communication systems, usually referred to as fourth-generation (4G) systems, will not be based on a single access technique but will encompass a number of different complementary access technologies. We outline some of the radio frequency (RF) standards that will be used and assess where optical wireless (OW) communications may play a role. Further, we outline some future research challenges and opportunities.

Satellite-based quantum communication terminal employing state-of-the-art technology

Feature Issue on Optical Wireless Communications (OWC) We investigate the design and the accommodation of a quantum communication transceiver in an existing classical optical communication terminal on board a satellite. Operation from a low earth orbit (LEO) platform (e.g., the International Space Station) would allow transmission of single photons and pairs of entangled photons to ground stations and hence permit quantum communication applications such as quantum cryptography on a global scale. Integration of a source generating entangled photon pairs and single-photon detection into existing optical terminal designs is feasible. Even more, major subunits of the classical terminals such as those for pointing, acquisition, and tracking as well as those providing the required electronic, thermal, and structural backbone can be adapted so as to meet the quantum communication terminal needs.

Availability-differentiated service provisioning in free-space optical access networks

Feature Issue on Optical Wireless Communications (OWC)Weather conditions have a significant impact on availability of free-space optical (FSO) access networks. Under fair weather conditions, such networks can use long-range FSO links that span over 1 km. Under severe weather conditions, e.g., dense fog, only the short-range FSO links in the range of 200–400m remain operational. We propose an availability-differentiated networking scheme that supports two classes of service availability in an FSO network consisting of both long-range and short-range FSO links. Under severe weather conditions, the network topology is reconfigured to use only the short-range FSO links. Owing to the reduced network capacity in the reconfigured network, lower-availability services (LAS) will be preempted by higher-availability services (HAS). LAS and HAS have different revenue values. Given a set of hybrid LAS and HAS requests, we maximize the total revenue of LAS + HAS by selectively provisioning a portion of the service requests in the FSO access network. We formulate the problem into a common terminal multicommodity problem and develop an optimal algorithm to solve this problem in polynomial time.

Optical wireless links with enhanced linearity and selectivity [Invited

Feature Issue on Optical Wireless Communications (OWC) Optical wireless is an attractive medium as an alternative to optical fiber communications, and also to RF, because of its high bandwidth and relative ease of use, especially when it comes to deployment in new physical situations. We describe an optical wireless link approach that offers a performance that gives analog transmission with significantly reduced distortion levels and enhanced reception sensitivity by combining a novel hybrid detector-amplifier technique. Reduction of distortion by 40 dB and improvement in sensitivity of 20-30 dB is possible, using the techniques described.

Entangled and non-line-of-sight (NLOS) free-space photon quantum communication [Invited

Feature Issue on Optical Wireless Communications (OWC) We present new quantum communication (QC) schemes suitable for free-space (wireless) QC implementation. In particular, we present several entangled QC schemes and propose non-line-of-sight (NLOS) free-space photon QC. It is shown that in the presence of atmospheric scattering media, UV photons can be used for NLOS QC. Non-Poisson quantum fluctuations in an invariance regime can propagate farther than coherent laser speckle. In such situations the non-Poissonian statistics survive over long distances and should be taggable with a polarization signature. Quantum noise observables scale markedly differently with scattering parameters compared to classical noise observables. Variation of the polarization should allow Yuen-Kumar QC using non-Poissonian statistics of the beam as an authentication of the signal. The NLOS communication in the UV would be expected to be possible to at least a range of 1 km and falls under the category of deliberately short-range QC. Applications of importance for this method include military stealth and optical communication in the presence of obstacles such as a forest or urban environment.

Optimizing IrDA throughput by including processing time with physical layer consideration

Feature Issue on Optical Wireless Communications (OWC) We derive new equations linking the Infrared Data Association (IrDA) data link layer (IrLAP) throughput to the physical layer by providing the bit error rate in relation to link distance and various physical layer parameters. Furthermore, we optimize the link by simultaneously adapting the frame and window size. The optimum equations include the effect of processor speed. The mathematical analysis is validated by comparison with exact numerical methods. Analytical results presented in the paper explore the effect of frame processing time on improving the performance of IrDA links and identify the improvement in throughput performance by utilizing the derived optimum values.

Arbitrary waveform synthesis for communication applications

Feature Issue on Optical Wireless Communications (OWC) For optical wireless laser transmissions we take advantage of a specific comblike structure of the spectrum of mode-locked laser ultrashort pulse trains and present a pulse-shaping technique for arbitrary waveform synthesis. The shaped pulses are free of space-time coupling effects and high-order spatial modes. As examples, we provide simulation results for the synthesis of waveforms that are of particular interest for optical communications, namely, square-root-raised cosines and Meyer wavelets.