There has been a growing interest in employing single photon avalanche diode (SPAD) array detectors for laser communication systems in deep space communications. The most important factor affecting the performance of SPAD array in communication systems is the dead time. In this paper, we investigate the photocount distribution for a small-scale SPAD array with a fixed short dead time. We employed Gaussian, Poisson, and shifted Poisson to fit the SPAD photocount distribution and compare the goodness of fit for various array sizes. With the SPAD photocount distribution, the capacity of the PPM-SPAD system is investigated. In addition, we proposed a new log-likelihood ratio (LLR) formula using shifted Poisson fitting results. The error performance of PPM-SPAD under a deep space communication scenario is simulated. When the dead time of a SPAD to the counting interval ratio θ = 0.6, the new LLR provides about 2 dB coding gain over the commonly used Poisson LLR. These results give helpful insight for designing practical SPAD-based optical communication systems.
KEYWORDS: Laser communications, Ranging, Telecommunications, Avalanche photodetectors, Laser communication terminals, Performance modeling, Laser systems engineering, Simulations, Monte Carlo methods
Combination of deep space laser communication and ranging is nowadays well-appreciated. For purpose of improving the communication and ranging performance, this paper proposes direct sequence spread spectrum (DSSS) assisted high receiving sensitivity inter-satellite integrated laser communication/ranging link. A thorough analysis for our proposed system is carried out, including optical receiving model, laser communication and ranging model, and theoretical performance analysis. DSSS performance in laser communication and ranging system is first researched by not only simulations but experimental results. Experimental, Monte Carlo, and theoretical results agree with each other. These results show that over inter-satellite link, on one hand, DSSS technique is an effective method of improving the receiving sensitivity of laser communication. On the other hand, DSSS effectively increases the distance and the precision of laser ranging.
In wireless laser communication systems, beam acquiring, pointing, and tracking (APT) technology is the key to establishing and maintaining a laser communication link. In order to solve the problem that the capture cameras used in APT systems usually have a small field of view and are inefficient in scanning the uncertain area and capturing the target spot, which leads to the inability to precisely locate the spot position, an image processing-based omnidirectional laser communication spot image detection technology solution is designed under the condition of using a large field of view of the capture camera, which has the characteristics of strong anti-interference capability and high detection accuracy. Firstly, the block design was simulated on matlab platform, and then the APT system of omnidirectional laser communication was built based on the panoramic camera with large field of view and two-dimensional turntable to verify the scheme. The scheme is implemented in 0.1° steps to sample the laser spot within 180° of the horizontal and tilt directions and obtain the spot coordinates. The spot position information is sent to the two-dimensional turntable through the serial port, and the 0.1° pointing of the turntable to the laser light source is accomplished, which verifies the feasibility and accuracy of the system solution.
We study the frequency offset correction for high-speed laser communication system with Pulse Position Modulation (PPM) and Superconducting Nano-wire Single Photon Detector (SNSPD). In particular, we consider that the dead time is longer than the symbol time. We consider an inter-symbol guard time-based sliding window synchronization algorithm, and evaluate its performance under various window sizes, sampling rates, and frequency offsets. These studies provide a design guideline for building practical communication system.
We present a high-speed, low-complexity, CCSDS-compatible SCPPM decoder architecture. At the algorithmic level, we use a combination of low-complexity Max-Log-MAP and SF-Max-Log-MAP algorithm. For both outer code and inner code SISO decoding module, we halve the delay by using a bidirectional calculation structure. We adopt the parallel-edge pre-computation and propose a symbol splitting method to reduce the complexity of the inner APPM decoder. We implement our proposed SCPPM design for 16-PPM in the FPGA platform. The implemented decoder achieves a throughput rate of more than 800Mbits per second at 250MHz clock. Meanwhile, our proposed SS-APPM inner decoder has an 80% reduction in FPGA logic resource utilization compared to the conventional APPM inner decoder.
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