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Free-Space Optical communication technology is a potential solution to the last mile or broadband access problem. Conventional free-space optical (FSO) communication is over a single link between two nodes. We explore multi-channel FSO communication system using compact (a maximum of a Sq.Ft) 2-dimensional antennas with multiple communication links between them to achieve very high aggregate bandwidths (100's of Gbps). But, close packaging of optical channels on the arrays causes inter-channel interference, reducing per channel capacity. We model the error due to inter-channel interference for such arrays and estimate the channel capacity. We address the multi-channel interference issue by both array design and by employing optical orthogonal codes (OOCs) for free-space optical communications and show that we can achieve multi Gbps bandwidths using such arrays. Possible applications for such multi-channel FSO systems can be in multi-hop broadband access networks or mesh networks and in back haul, connecting wireless base stations.

Directional communication (for example, using directional antennas or free-space optics) has the potential to dramatically increase capacity reuse in multi-hop wireless mesh and ad hoc networks. What is less appreciated is that directional communications can also simplify network layer functions. In this paper, we show how directionality (combined with ranging) can be used to construct a scalable, fully distributed localization system and assign "relative" coordinates to each node (without the need for GPS) that can be used by highly scalable geographic routing systems. We show that range and orientation-based localization schemes (ROL) scale better than range-only or orientation-only localization schemes (e.g., triangulation) for such applications. We present simulations studying scalability, effect of errors (both range and directionality) and compare performance with triangulation methods. Finally, we discuss how such an ROL scheme can be realized with optical wireless nodes, based upon our ongoing efforts to build ad hoc and meshed networks using both FSO and RF technologies.

In this paper we analyze the error performance of free-space optical (FSO) communication over multiple hops. We first develop an error model for a single hop based on visibility, atmospheric attenuation, and geometric spread of the light beam. We model atmospheric visibility by Gaussian distributions with mean and variance values to reflect clear and adverse weather conditions. Based on this, we find the end-to-end bit error distribution of the FSO link for single hop and multi-hop scenarios. We present simulation results for decoded relaying, where each hop decodes the signal before retransmitting. We demonstrate that multi-hop FSO communication achieves a significant reduction in the mean bit error rate and also reduces the variance of the bit error rate. We argue that by lowering mean error and error variance, multi-hop operation facilitates an efficient system design and improves the reliability of the FSO link by application of specific coding schemes (such as forward error correction techniques).

In this paper we examine the feasibility of using 2-dimensional multiple element array antennas for free-space optical communications. Spatial diversity due to multiple antennas on 2-d arrays can increase aggregate link bandwidth. On the other hand, simultaneous transmissions between the elements on the arrays can cause inter-channel interference, reducing the effective bandwidth. We model this inter-channel interference as noise and find the probability of error due to such noise. Based on this error model, we then derive channel capacity estimations. We present design guidelines based on the link range, number of optical transceivers (elements) that can be packed on a given array, and the achievable aggregate bandwidth.

Existence of line of sight (LOS) and alignment between the communicating antennas are one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new optical antenna design that employs spherical antennas covered with transmitter and receiver modules for maintaining optical links even when antennas are in relative motion. In this paper, for proof-of-concept, we design and test an auto-configurable circuit integrated with light sources and detectors placed on spherical surfaces. We also perform simulation-based analysis of these multi-element FSO modules that can enable mobility and high bandwidth in wireless, particularly indoor, networks. Broader impact of our work is to make FSO communication technology widely applicable in mobile, ad-hoc, and multi-hop wireless networks.

A general approach for synthesizing inter-process communication devices by adapting labeled transition systems is proposed. An approach is also proposed to generate the finite state machine representing the protocol converter. It is assumed that the data path of the protocol converter is already given. The approach is illustrated by generating the communication process between a four phase master and a two phase slave. >

Organizations today are faced with an ever-increasing need to become more efficient in their methods for exchange of information with consumers, collaborators, and partners. Unfortunately existing mechanisms for such information exchange provide nowhere near the levels of both security and automation required to satisfy this need. Effective information sharing and dissemination can take place only if the data holders have assurance that access constraints on the information they own or manage will be respected and that, while releasing information, disclosure of sensitive information is not a risk. The Secure Access Wrapper (SAW) project is a collaborative effort between SRI International and Stanford University to develop techniques that provide substantially more automation and assurance than has previously been available for secure, selective information sharing. The SAW project is sponsored by DARPA ITO under the Wrappers and Composition focus area of the Information Survivability program.