Scattering regimes for underwater optical wireless communications using Monte Carlo simulation

Optical wireless communications has shown tremendous potential for underwater applications as it can provide higher bandwidth and better security compared to acoustic technologies. In this paper,  an investigation on scattering regimes for underwater links using Monte Carlo simulation has been presented.While the focus of this paper is on diffuse links, the simulation results of collimated links is also provided for comparison purpose. Three types of water namely clear, coastal and turbid water are being used in the simulation. It is shown that the effect of scattering on the path loss cannot be accurately modeled by the existing channel model; ie. Beers-Lambert (BL) law.  It has been shown that  the distance at which the unscattered light drops to zero can be used to estimate the transition point for the scattering regimes in case of diffuse links. The transition point for diffuse links in coastal water and turbid water can be estimated to be around 22 m and 4 m respectively. Further analysis on the scattering order probability at different scattering regimes illustrates how scattering is affected by beam size, water turbidity and distance. From the frequency response plot, it is estimated that the bandwidth of several order of GHz can be achieved when the links are operating in the minimal scattering region and will reduce to several hundreds of MHz when the link is operating in multiple scattering region

Optical wireless channel characterisation in guided structures (vehicle applications)

The field of automotive electronics is growing exponentially in terms of devices related to safety, driver assistance and a variety of other nodes connected to infotainment systems which become standards with every automobile. Networking protocols connect these systems to provide assistance to drivers. However, the demand of high-bandwidth to serve applications lead to the necessity of a more flexible communication network within the vehicle This thesis proposes using optical wireless links in intra-vehicle applications where different parts of the vehicle can form signal transferring media. A vehicle chassis can be represented as an optical wireless waveguide, where Line-of-sight (LOS) or a diffuse optical wireless link exists. In order to predict the validity of the idea, optical wireless channel characteristics should be determined. This thesis describes the design of a modified Monte Carlo simulation tool for modelling a waveguide optical wireless channel. The simulator has the ability to determine the channel characteristics, and it can also be used more generally for indoor systems. The simulator studies the effect of purely diffuse and purely reflective materials as well as mixed diffuse-specular materials. The program was validated by comparing it with other indoor simulation studies and with a laboratory experiment for straight waveguide. Emphasis was placed on understanding the requirements of LOS and diffusing optical wireless communication links. Results are presented for straight and bent waveguides received power, path loss and bandwidth for a series of receiver areas and fields-of-view. Studying the coupling effect between two waveguides which are different in dimensions has been assessed for series of transmitter directions. All studies have been undertaken for two types of materials (scattered and reflective) and two kinds of transmitters (omnidirectional and directional). The study shows that the waveguides are plausible candidates to convey signals with high bandwidth, but a high power transmitter or an array of Light Emitting Diodes (LEDs) is needed due to power limitations

Orientation Effects For LOS And NLOS OWC Characterisation Within Small Structures

This paper reports an experimental investigation of the orientation effect towards optical wireless channel parameters within small structures.The experiment has characterised the orientation effect of the line-of-sight (LOS)and non-line-of-sight (NLOS) infrared transmission when using different material and geometrical properties on significant infrared channel parameters.Two measurement setups were used,(1) the straight guiding structures,and (2) bending guiding structures,with three different bending angles (30o,45o and 60o bend).In each of the measurement,the receiver/transmitter is rotated in the steps of 15˚ each time.The results revealed that the characteristic of the channel depends on the physical geometries,the orientation of the transmitter/receiver and also depend on reflection coefficient of the materials.The results are valid for both LOS and NLOS transmission

Free space optical in vehicular networks using rectangular guiding models

This letter introduces Free-Space Optical (FSO) Communication links in vehicular applications using potential guiding structures around a vehicle. An optical wireless communication system simulation is described which delivers received power, bandwidth, root mean square delay spread channel impulse response for purely diffuse and diffuse-specular materials with omnidirectional and directed transmitters. In the former case, a bandwidth of 225 MHz with a power deviation of 25% results at the exit. For the latter, a 75GHz bandwidth is available at best but with a power deviation of over 99% making receiver positioning critical. The impulse response is calculated using a Modified Monte Carlo algorithm taking into account up to 15 reflections. The effect of the pipe bend angle on the path loss is also presented and the simulation is supported with experimental work

NSC98657

Nonlinearity in the response of the optical source responsible in the electrical-to-optical conversion process limits the dynamic range performance in transmitting analog and multilevel digital signals over fiber optic link channels. Fiber-based wireless access schemes provide a unique possibility for digital linearization. We describe a technique for predistorting the dynamic response of the optical source using a Radial Basis Functional Neural Network (RBFNN). The input and output data from the link provide the samples used to train the RBFNN. A simple method employing the Least Squares (LS) algorithm is used to provide an optimization to the predistorter NN. Simulation results demonstrate the simplicity of this method compared to other NN architectures and linearization techniques. This result is used in this paper to demonstrate the feasibility of the technique in providing an adaptive predistortion solution for wireless application