KEYWORDS: Field programmable gate arrays, Sensors, Analog to digital converters, Tunable filters, Receivers, Photon counting, Picosecond phenomena, Electronic filtering, Clocks, Signal detection
We present a Field-Programmable Gate Array (FPGA) implementation of a single photon-counting receive modem for a pulse position modulated signal. The modem is compliant with the Consultative Committee for Space Data Systems (CCSDS) High Photon Efficiency (HPE) Optical Communications Coding and Synchronization standard and is capable of a maximum data rate of 267 Mbps. The system is designed on a commercial off-the-shelf FPGA platform and utilizes superconducting nanowire single photon counting detectors, Analog to Digital Converters (ADCs) to sample the detectors, and two FPGAs. Symbol timing recovery, photon counting, convolutional deinterleaving, and codeword synchronization are performed in the first FPGA. The second FPGA performs iterative decoding on each codeword of the Serially Concatenated Pulse Position Modulated (SCPPM) signal. A digital filter is included to compensate for timing jitter of the detector, and the decoder throughput can be adjusted through reconfigurable parallelization. The decoder also implements a resource-efficient, algorithmic polynomial interleaver and deinterleaver. Both FPGAs can be reconfigured to switch between Pulse Position Modulation (PPM)-16 and PPM-32 with code rates 1/3, 1/2, and 2/3. In this paper, we describe the receiver architecture and FPGA implementation of the timing recovery loop and SCPPM decoder, FPGA utilization for the different modes, and receive modem characterization test results.
The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) has developed a photon-counting optical ground receiver for pulse-position modulated signals. The real-time receiver system includes a fiber interconnect, superconducting nanowire single-photon detectors (SNSPDs), and a real-time field programmable gate array (FPGA) based receiver. The fiber interconnect and SNSPDs are implemented with two different configurations. In the first, a 7-channel few-mode fiber photonic lantern couples the light from the telescope to 7 single-pixel few-mode fiber coupled SNSPDs. In the second configuration, a few-mode fiber couples light to a 16-pixel monolithic SNSPD array. The real-time FPGA-based receiver performs combining of up to 16 SNSPD channels, symbol timing recovery, demodulation, and decoding. The system is scalable with data rates ranging from 20 Mbps to 267 Mbps. It is compliant with the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization Standard. This standard will be used in NASA deep space and other low photon flux missions, such as in the Orion Artemis-2 Optical Communications System (O2O) demonstration, planned for the first crewed flight of Orion. This paper describes the scalable real-time optical receiver system and presents characterization test results.
KEYWORDS: Receivers, Optical communications, Photon counting, Monte Carlo methods, Signal detection, Photodetectors, Optical testing, Field programmable gate arrays
The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) is developing a low cost, scalable, photon-counting receiver prototype for space-to-ground optical communications links. The receiver is being tested in a test bed that emulates photon-starved space-to-ground optical communication links. The receiver uses an array of single-pixel fiber-coupled superconducting nanowire single-photon detectors. The receiver is designed to receive the high photon efficiency serially concatenated pulse position modulation (SCPPM) waveform specified in the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization Blue Book Standard. The optical receiver consists of an array of single-pixel superconducting nanowire detectors, analog phase shifters for channel alignment, digitizers for each detector channel, and digital processing of the received signal. An overview of the test bed and arrayed receiver system is given. Simulation and system characterization results are presented. The data rate increase of using a four-channel arrayed detector system over using one single pixel nanowire detector is characterized. Results indicate that a single-pixel detector is capable of receiving data at a rate of 40 Mbps and a four-channel arrayed detector system is capable of receiving data at a rate of 130 Mbps.
KEYWORDS: Signal detection, Signal to noise ratio, Signal processing, Sensors, Neural networks, Photodetectors, Photon counting, Interference (communication), Single photon, Receivers
Optical communication links, operating in low photon flux conditions, rely on an array of single photon counting detectors to receive the signal. Due to the reset time of these detectors, many separate detectors must be used to receive a continuous signal. Photon counting, channel combining, channel alignment, and digitization of the detected signal can be complex and expensive due to the parallel hardware required for each channel. This issue is compounded as the system scales to greater numbers of detectors due to the amount of hardware required and alignment requirements between each channel. The purpose of this research is to examine a photon counting and channel combining method which allows for photon detection channels to be summed into a single signal before digitization, eliminating the need for parallelized hardware. This reduction in parallel hardware has the potential to reduce the cost and complexity of the system. In this paper, a single layer fully connected neural network architecture is explored as a possible solution for the photon counting of summed photon detection channels. The signal to noise ratio of the combined signal was lower than that of the individual channels and was inversely proportional to the root of the number of channels being summed. The neural network signal processing implementation produced signal gain when the symbol phase remains constant. This is most likely due to the network exploiting the modulation structure of the signal and possibly offsets the losses incurred during analog summation.
The purpose of this study is to investigate a new approach to the modulation of an optical signal which requires high extinction ratio (ER). A deep space, optical, pulse position modulated (PPM) link, may require an extinction ratio approaching 33 dB1 which is not easily achieved through the use of a single optical modulator. In a system where the slot clock is equal to the slot width, it is often not possible to meet ER requirements due to Inter Slot Interference (ISI). Furthermore, the high frequency ER of state of the art optical amplitude modulators is not large enough to allow for implementation losses. By using a second optical modulator in series with the first, it is possible to address both issues. A phase delay placed between each modulator allows for precision control of the pulse width, reducing ISI. While the attenuation in the off slots combines linearly, increasing attenuation in the off slots. Using this approach a series of 1 ns pulses was measured at a series of phase delays to approximate PPM pulses. These measurements were used to extrapolate the ER of a PPM signal at various PPM orders. An ER above 33 dB was observed for all PPM orders of 16 and above. At PPM 256 an ER of 48.2 dB was achieved.
The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) has developed a laboratory transmitter and receiver prototype of a space-to-ground optical communications link. The system is meant to emulate future deep space optical communication links, such as the first crewed flight of Orion, in which the transmitted laser is modulated using pulse position modulation and the receiver is capable of detecting single photons. The transmitter prototype consists of a software defined radio, a high extinction ratio electro-optic modulator system, and a 1550 nm laser. The receiver is a scalable concept and utilizes a single-pixel array of fiber coupled superconducting nanowire single photon detectors. The transmit and receive waveforms follow the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization Standard. A software model of the optical transmitter and receiver has also been implemented to predict performance of the optical test bed. This paper describes the transmitter and receiver prototypes as well as the system test configuration. System level tests results are presented and shown to align with predictions from software simulations. The validated software model can be used to in the future to reduce the design cycle of optical communications systems.
KEYWORDS: Field programmable gate arrays, Radio optics, Clocks, Telecommunications, Modulation, Signal processing, Optical communications, Integrated optics, Digital signal processing
A key component in the Integrated Radio and Optical Communications project at the National Aeronautics and Space Administration’s (NASA) Glenn Research Center (GRC) is the radio frequency (RF) and optical software defined radio (SDR). A NASA RF SDR might consist of a general purpose processor to run the Space Telecommunications Radio System (STRS) Architecture for radio command and control, a reconfigurable signal processing device such as a field programmable gate array (FPGA) which houses the waveform, and a digital to analog converter for (DAC) transmitting data. Prior to development, SDR architecture trades on how to combine the RF and optical elements were studied. A modular architecture with physically separate RF and optical hardware slices was chosen and the optical slice of an SDR was designed and developed. The Harris AppSTARTM platform, which consists of an FPGA processing platform with a mezzanine card targeted for RF communications, was used as the base platform in prototyping the optical slice. A serially concatenated pulse position modulation (SCPPM) optical waveform was developed. The waveform follows the standard described in the Consultative Committee for Space Data Systems (CCSDS) Optical Communions Coding and Synchronization Red Book. A custom optical mezzanine printed circuit board card was developed at NASA GRC for optical transmission. The optical mezzanine card replaces the DAC, which is used in the transmission of RF signals. This paper describes RF and optical SDR architecture trades, the Harris AppSTARTM platform, the design of the SCPPM waveform, and the development of the optical mezzanine card.
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