Modulation of the polarization state provides a means for information transfer in an optical communication channel.
Polarization shift keying (POLSK) has been proposed as a candidate for modulation in optical fiber communications and
shown to have 3dB better sensitivity than an on-off keying (OOK). Unfortunately, the utility of POLSK in standard
fibers is reduced because of birefringence of the fibers. The polarization state changes unpredictably and becomes
difficult to track at the receiver. On the other hand, POLSK does not suffer from these problems in free space optical
(FSO) communication systems. In theory, the atmospheric turbulence channel causes minimal depolarization, and
negligible crosstalk occurs between orthogonal polarization states along the transmission path. The polarization state of a
propagating optical wave is well preserved over link lengths up to several kilometers, which makes polarization state
reliably detectable at the receiver. In this paper, the experimental performance of a POLSK modulation scheme used in
our 1km FSO communication system test-bed is described and its theoretical analysis is also presented.
Atmospheric turbulence is caused by inhomogeneities in the temperature and pressure of the atmosphere, resulting in random variations of the refractive index. A laser beam propagating through such turbulences experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical (FSO) communication systems. In our time delayed diversity (TDD) technique, we transmit twice and take advantage of the fact that propagation along an atmospheric path is statistically uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. Communications performance is improved because the joint probability of error is less than the probability of error from individual channels. In this paper, we describe the theoretical and experimental analyses of FSO systems implementing this novel scheme in various performance scenarios. Theoretical models and performance of TDD systems are derived and characterized. The experimental performance results obtained under weak turbulence conditions are shown to be in good agreement with the theory. Related system design and implementation issues, such as: atmospheric turbulence statistics, laser beam depolarization, and diversity receiver architecture are also discussed.
Hybrid free space optical/radio frequency (FSO/RF) networks promise broadband connectivity, high availability and quality of service (QoS), together with the capability of autonomous reconfigurability to deal with changing atmospheric and traffic conditions in dynamic environments. Nodes with n-connectedness (multiple transceivers) offer great flexibility in constructing new network topologies. Moreover, topologies using hybrid links are more effective in changing atmospheric conditions than those, using either communication modality alone. While FSO links can be expected to be available >99% of the time on links up to 1km in length, high performance RF provides backup connectivity in heavily obscured conditions. We have designed and implemented gimbal-mounted, hybrid FSO/RF nodes with combined apertures for joint pointing, acquisition, and tracking (PAT) operation. These nodes incorporate directional RF antennas for PAT network setup and management, and FSO links for very high data rate transmission. We describe these hybrid nodes and their performance, our hybrid network simulations, and our re-configurable network testbed for high data rate video transmission. Our simulations include realistic modeling of obscuration, traffic management, and topology control to deal with link non-availability and optimization of network performance. Hybrid, directional networks are scalable and provide low probability of intercept/detection (LPI/LPD) operation, especially in FSO mode.
Atmospheric turbulence causes fluctuations in both the intensity and phase of the received signal in an optical wireless communication link. These fluctuations, often referred to as scintillation noise, lead to signal fading, which increase bit errors in digital communication links using intensity modulation and direct detection. The performance of an optical link can be improved by the use of a time delayed diversity technique, which takes advantage of the fact that the atmospheric path from transmitter to receiver is statistically independent for time intervals beyond the correlation time of the intensity fluctuations. We have designed and constructed a prototype optical wireless system using this scheme. Bit-error-rate measurements have been used to characterize the link performance for different delay periods under conditions of controlled simulated turbulence. It has been determined that link performance improves significantly, especially in strong turbulence. In addition, we have implemented orthogonal polarization modulation, which works especially well in optical wireless systems. In contrast to fiber optic communications, the polarization state of a laser beam is well preserved on a free space optical path.
Free space, dynamic, optical wireless communications will require topology control for optimization of network performance. Such networks may need to be configured for bi- or multiple-connectedness, reliability and quality-of-service. Topology control involves the introduction of new links and/or nodes into the network to achieve such performance objectives through autonomous reconfiguration as well as precise pointing, acquisition, tracking, and steering of laser beams. Reconfiguration may be required because of link degradation resulting from obscuration or node loss. As a result, the optical transceivers may need to be re-directed to new or existing nodes within the network and tracked on moving nodes. The redirection of transceivers may require operation over a whole sphere, so that small-angle beam steering techniques cannot be applied. In this context, we are studying the performance of optical wireless links using lightweight, bi-static transceivers mounted on high-performance stepping motor driven stages. These motors provide an angular resolution of 0.00072 degree at up to 80,000 steps per second. This paper focuses on the performance characteristics of these agile transceivers for pointing, acquisition, and tracking (PAT), including the influence of acceleration/deceleration time, motor angular speed, and angular re-adjustment, on latency and packet loss in small free space optical (FSO) wireless test networks.
The worldwide demand for broadband communications is being met in many places through the use of installed single-mode fiber networks. However, there is still a significant 'first-mile' problem, which seriously limits the availability of broadband Internet access. Free-space optical wireless communication has emerged as a technique of choice for bridging gaps in the existing high data rate communication networks, and as a backbone for rapidly deployable mobile wireless communication infrastructure. Because free space laser communication links can be easily and rapidly redirected, optical wireless networks can be autonomously reconfigured in a multiple-connected topology to provide improved network performance. In this paper we describe research designed to improve the performance of such networks. Using topology control algorithms, we have demonstrated that multiply-connected, rapidly reconfigurable optical wireless networks can provide robust performance, and a high quality of service at high data rates (up to and beyond 1 Gbps). These systems are also very cost-effective. We have designed and tested on the University of Maryland campus a prototype four-node optical wireless network, where each node could be connected to the others via steerable optical wireless links. The design and performance of this network and the topology control is discussed.