| In this research, the design concept and analysis of different architectures of telescope array-based receivers for an inter-planetary optical communications link between Earth and Mars are presented. Pulse-position modulation (PPM) is used at the transmitter end and photon-counting detectors, along with the direct-detection technique, are employed at each telescope element in the array. First, models for the received signal photons and background noise photons are developed to simulate an optical communications channel between Earth and Mars. A method for optimization of various important system parameters such as detector sizes (i.e., receiver field-of-view), PPM slot-width Ts, and PPM order M, is presented to maximize the communications system performance. Then, the performance of different array architectures is evaluated through analytical techniques and Monte-Carlo simulations for a broad range of deep-space operational scenarios, such as Earth-Mars conjunction, Earth-Mars opposition, and different background and turbulence conditions. It is shown that the performance of array-based receivers consisting of up to 100, 1 m telescopes is almost equivalent to a single large telescope with 10 m aperture diameter. It is also revealed that compared to current radio frequency (RF) technology, telescope array-based optical receivers can provide several orders of magnitude greater data rates from Mars.However, array architectures for deep-space optical communications have several unique challenges. Due to very narrow optical beams, the requirement of spatial tracking of the transmitter line-of-sight at the receiver telescopes (to minimize power losses caused by the tracking errors) is very stern. In addition, detected signals at individual telescope elements in an array need to be synchronized with the receiver clock and with each other before data decoding. Compared to a monolithic large telescope, individual telescope elements in an array receive and detect much less optical power. This phenomenon renders the tracking and synchronization tasks at individual telescopes quite difficult. In the next step, the design of tracking and synchronization subsystems for the array receiver is discussed. The performance of different array architectures, after incorporation of these subsystems, is evaluated for a deep-space optical communications link between Earth and Mars operating in the presence of random tracking and synchronization errors. It is shown that even in the worst-case channel conditions, the designed subsystems successfully perform the tracking and synchronization functions the impact of synchronization and tracking errors is almost negligible for an array consisting of 100, 1 m telescopes. The tracking and synchronization analysis further solidifies the theoretical foundations and feasibility investigation of telescope arrays for deep-space optical communications.Atmospheric turbulence and diffused background light from the sky during daytime are the major limiting factors in a deep-space optical communications link. This part of the research is focused on developing techniques to mitigate these deleterious effects. Adaptive optics (AO) technology is commonly employed in astronomy to mitigate the turbulence effects. First, laser guide star (LGS)-based AO systems are designed and incorporated in array receivers, and their performance is analyzed for a communications link between Earth and Mars in extreme turbulence and background conditions. It is shown that the incorporation of LGS-based AO systems results in a substantial improvement in the performance of array receivers. Next, a novel space-time adaptive processor (STAP) is developed for post-detection processing and mitigation of background noise effects. The STAP processor can be thought of an electronic counterpart of an active AO system and is very easy and cost-effective to implement. The performance analysis shows that the incorporation of the STAP processor results in several orders of magnitude performance improvement in strong background conditions. The experimental investigation of the use of adaptive optics (AO) subsystems for turbulence and background noise compensation is also carried out.In the last part of the thesis, short-range, terrestrial, free-space optical (FSO) communications links are analyzed. It is believed that FSO systems can solve the last mile connectivity problem faced by the current commercial telecom market. An efficient general-purpose simulation tool is developed that can model and predict the parameters of interest of a laser beam propagating through a turbulent channel in FSO systems. (Abstract shortened by UMI.)... |
