Free space optical (FSO) communications technology has potential applications in the military sector to provide a secure, high speed communication channel, and in the civilian sector as a "last mile" carrier solution. It was proposed by other researchers that a multi-rate communication system that utilizes Meyer wavelets would achieve the greatest bandwidth and highest reliability possible for an FSO system. In order to generate Meyer wavelets from femtosecond laser pulses, filtering must be performed optically to produce the desired pulse shape. One of the simplest ways to produce an arbitrary pulse shape from a laser pulse is with a tunable liquid-crystal spatial light modulator (LC-SLM) in a zero-dispersion pulse compression system. The simplest approach to determine the correct mask pattern for an LC-SLM is to utilize adaptive, global optimization methods. Since it takes several milliseconds to adjust the attributes of each pixel of an LC-SLM and there are typically over one-thousand pixels, it is important to determine the fastest algorithm for determining the optimum mask pattern. Several global optimization methods, including simulated annealing, exhaustive search, and random search, a hybrid of the other two algorithms, were characterized. It was found that exhaustive search can be used to form waveforms with negligible inaccuracies at rates of about 5 times faster than simulated annealing and about 3 times faster than random search, but that simulated annealing provides the highest accuracy. However, the difference in accuracy between all of these algorithms is less than 10-5.
Free Space Optical Communications through cloud-obscured channels suffer from severe degradations due to
multiscattering, which induces delay spread in the received signal. The delay spread of FSO channel can vary
considerably according to channel conditions, varying from nanoseconds to microseconds for clear and cloudy
channel, respectively. In this paper, we present the use of return-to-zero (RZ) modulation in conjunction with
multirate modulation schemes as viable means for minimizing the dispersive effects of the channels. We show that
multirate communications provide diversity against channel degradation and fluctuation, while RZ modulation is
more robust to dispersive channels than non return-to-zero (NRZ) modulation, thus providing an overall
improvement in system performance, reliability and availability.
Free space optical (FSO) communication is a promising candidate for emerging broadband applications,
considering that RF spectrum is already congested, rendering accommodation of additional RF broadband channels difficult
and costly. Communications via RF signals are reliable but cannot support emerging high data rate services unless they use a
large portion of the costly radio spectrum. FSO communications offer enormous data rates but operate much more at the
mercy of the atmospheric environment, such as scintillation and multi-scattering through fog and clouds. Since RF paths are
relatively immune to these phenomena, combining the attributes of a higher data rate but bursty link (FSO) with the
attributes of a lower data rate but reliable link (RF) could yield attributes better than either one alone, enabling a high
availability link at high data rates. This transmission configuration is typically called a hybrid RF/FSO wireless system. The
focus of this paper will be on applying well-known equalization techniques to FSO, to further enhance availability of RF/FSO
Wireless Free Space Optics (FSO) is one of the most promising candidates for future broadband communications, offering transmission rates far beyond possible by RF technology. However, free space wireless optical channel presents far more challenging conditions for signals than typical RF channels, making system availability and throughput a critical issue. Multirate fractal modulation has been considered to address adverse channel conditions in FSO systems.
In this paper, channel coding for such a multirate communications scenario is discussed. Specifically, we have used Fountain Codes, a new class of erasure correction codes, in concatenation with an inner Convolutional code. We argue that for a parallel multirate system Fountain coding is a flexible method for receiving data from multiple streams.
Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. For this architecture, we developed a channel model integrating both the RF and FSO channels. Based on the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link is used by itself, clouds hamper availability, due to introduction of attenuation and temporal dispersion. On the contrary, RF signals are relatively immune to the cloud effects, thus improve the availability in a hybrid of RF and FSO links, significantly.
Wireless Free Space Optics (FSO) is one of the most promising candidates for future broadband communications, offering transmission rates far beyond possible by RF technology. However, free space wireless optical channel presents far more challenging conditions for signals than typical RF channels, making system availability and throughput a critical issue. A novel design for an FSO system based on integration of ultra-short pulse lasers and advanced signal processing techniques is presented. Simulations indicate that the novel design promises considerably improved availability and throughput compared to traditional FSO systems.
For free space optics to support multi user communications, multi-access schemes need to be incorporated.
Conventional asynchronous optical code division multiple access (OCDMA) using optical orthogonal codes (OOCs)
cannot deliver the required performance due to code interference, which sets a lower bound on the achievable bit error
rate. Thus we introduce the use of complementary Walsh Hadamard codes as a promising solution to support
synchronous OCDMA, capable of achieving required performance due to inherent interference cancellation, in addition
to supporting a higher number of users while maintaining short code lengths, and alleviating some of the stringent
requirements on the receiver hardware.