Development of an Optical Slice for an RF and Optical Software Defined Radio

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 AppSTAR("TM") 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 AppSTAR("TM") platform, the design of the SCPPM waveform, and the development of the optical mezzanine card

Single-Photon Avalanche Diodes in CMOS Technologies for Optical Communications

As optical communications may soon supplement Wi-Fi technologies, a concept known as visible light communications (VLC), low-cost receivers must provide extreme sensitivity to alleviate attenuation factors and overall power usage within communications link budgets. We present circuits with an advantage over conventional optical receivers, in that gain can be applied within the photodiode thus reducing the need for amplification circuits. To achieve this, single-photon avalanche diodes (SPADs) can be implemented in complementary metal-oxide-semiconductor (CMOS) technologies and have already been investigated in several topologies for VLC. The digital nature of SPADs removes the design effort used for low-noise, high-gain but high-bandwidth analogue circuits. We therefore present one of these circuit topologies, along with some common design and performance metrics. SPAD receivers are however not yet mature prompting research to take low-level parameters up to the communications level

A comparison of APD and SPAD based receivers for visible light communications

Visible light communications (VLC) is an alternative method of indoor wireless communications that requires sensitive receivers. Ideally, single photon avalanche detectors (SPADs) could be used to create more sensitive receivers. However, the dead-time, finite output pulse width and photon detection efficiency of existing SPAD arrays limits their sensitivity and bandwidth. In this paper an accurate equation for the impact of dead-time on the sensitivity of a SPAD array is presented. In addition the impact of the width of the output pulses on the on-off keying (OOK) data rate is investigated. Finally, a comparison between receivers containing an APD and a large array of SPADs shows that although the receiver containing the SPAD is more sensitive in the dark the APD-based receiver is more sensitive in normal operating condition. However, the models that predict the performance of both receivers suggest that newer SPAD arrays will enable significant improvements in receiver sensitivity

Time-Gated Photon Counting Receivers for Optical Wireless Communication

Photon counting detectors such as single-photon avalanche diode (SPAD) arrays are commonly considered for reliable optical wireless communication at power limited regimes. However, SPAD-based receivers suffer from significant dead time induced intersymbol interference (ISI) especially when the incident photon rate is relatively high and the dead time is comparable or even larger than the symbol duration, i.e., sub-dead-time regime. In this work, we propose a novel time-gated SPAD receiver to mitigate such ISI effects and improve the communication performance. When operated in the gated mode, the SPAD can be activated and deactivated in well-defined time intervals. We investigate the statistics of the detected photon count for the proposed time-gated SPAD receiver. It is demonstrated that the gate-ON time interval can be optimized to achieve the best bit error rate (BER) performance. Our extensive performance analysis illustrates the superiority of the time-gated SPAD receiver over the traditional free-running receiver in terms of the BER performance and the tolerance to background light

A SPAD-Based Photon Detecting System for Optical Communications

A small array of single photon avalanche detectors (SPADs) has been designed and fabricated in a standard 0.18 μm CMOS process to test a new photon detecting system for optical communications. First numerical results are presented which show that using arrays of SPADs reduces the optical power density required at the receiver. Experimental results then show that the new system preserves the photon counting ability of the SPADs. Finally a simple method is presented which can be used to estimate the size of array needed to achieve a particular target bit error rate at a specific optical power density. Together these results indicate that by replacing the avalanche photodiode in a receiver with the new system it will be possible to count the received photons

Statistical Modeling of Single-Photon Avalanche Diode Receivers for Optical Wireless Communications

In this paper, a comprehensive analytical approach is presented for modeling the counting statistics of active quenching and passive quenching single-photon avalanche diode (SPAD) detectors. It is shown that, unlike ideal photon counting receiver for which the detection process is described by a Poisson arrival process, photon counts in practical SPAD receivers do not follow a Poisson distribution and are highly affected by the dead time caused by the quenching circuit. Using the concepts of renewal theory, the exact expressions for the probability distribution and moments (mean and variance) of photocounts in the presence of dead time are derived for both active quenching and passive quenching SPADs. The derived probability distributions are validated through Monte Carlo simulations and it is demonstrated that the moments match with the existing empirical models for the moments of SPAD photocounts. Furthermore, an optical communication system with on-off keying and binary pulse position modulation is considered and the bit error performance of the system for different dead time values and background count levels is evaluated

SPAD-Based Optical Wireless Communication with Signal Pre-Distortion and Noise Normalization

In recent years, there has been a growing interest in exploring the application of single-photon avalanche diode (SPAD) in optical wireless communication (OWC). As a photon counting detector, SPAD can provide much higher sensitivity compared to the other commonly used photodetectors. However, SPAD-based receivers suffer from significant dead-time-induced non-linear distortion and signal dependent noise. In this work, we propose a novel SPAD-based OWC system in which the non-linear distortion caused by dead time can be successfully eliminated by the pre-distortion of the signal at the transmitter. In addition, another system with joint pre-distortion and noise normalization functionality is proposed. Thanks to the additional noise normalization process, for the transformed signal at the receiver, the originally signal dependent noise becomes signal independent so that the conventional signal detection techniques designed for AWGN channels can be employed to decode the signal. Our numerical results demonstrate the superiority of the proposed SPAD-based systems compared to the existing systems in terms of BER performance and achievable data rate

Receiver design for SPAD-based VLC systems under Poisson-Gaussian mixed noise model

Single-photon avalanche diode (SPAD) is a promising photosensor because of its high sensitivity to optical signals in weak illuminance environment. Recently, it has drawn much attention from researchers in visible light communications (VLC). However, existing literature only deals with the simplified channel model, which only considers the effects of Poisson noise introduced by SPAD, but neglects other noise sources. Specifically, when an analog SPAD detector is applied, there exists Gaussian thermal noise generated by the transimpedance amplifier (TIA) and the digital-to-analog converter (D/A). Therefore, in this paper, we propose an SPAD-based VLC system with pulse-amplitude-modulation (PAM) under Poisson-Gaussian mixed noise model, where Gaussian-distributed thermal noise at the receiver is also investigated. The closed-form conditional likelihood of received signals is derived using the Laplace transform and the saddle-point approximation method, and the corresponding quasi-maximum-likelihood (quasi-ML) detector is proposed. Furthermore, the Poisson-Gaussian-distributed signals are converted to Gaussian variables with the aid of the generalized Anscombe transform (GAT), leading to an equivalent additive white Gaussian noise (AWGN) channel, and a hard-decision-based detector is invoked. Simulation results demonstrate that, the proposed GAT-based detector can reduce the computational complexity with marginal performance loss compared with the proposed quasi-ML detector, and both detectors are capable of accurately demodulating the SPAD-based PAM signals