Bruce Moision

Communication Limits Due to Photon Detector Jitter

Detector jitter, the random delay from the time a photon is incident on a single-photon-counting detector (SPD) to the time an electrical pulse is produced in response to that photon, is characterized for a number of SPDs. The jitter is modeled as a weighted sum of Gaussians. The performance in detector jitter is measured by determining the capacity of a communications channel utilizing a given detector. It is observed that the loss, measured as the ratio of the signal power required to achieve a specified capacity in the presence of jitter to that in the absence of jitter, goes as the square of the normalized jitter standard deviation (the standard deviation of the jitter in slotwidths). The loss is small when the normalized jitter is less than one, and becomes significant beyond that point. This loss must be taken into account when evaluating detectors for very high throughput channels.

Maximum likelihood time-of-arrival estimation of optical pulses via photon-counting photodetectors

Many optical imaging, ranging, and communications systems rely on the estimation of the arrival time of an optical pulse. In systems utilizing photon-counting photodetectors, which are finding increased use, the detected process is well modeled as a Poisson point process. In this paper, we analyze the performance of maximum likelihood (ML) estimators of the arrival time of an optical pulse, based on observations of a Poisson process. We develop an analytic model for the mean-square error of the ML estimator, illustrating two phenomena that cause deviations from the Cramér-Rao bound at low signal photon flux. The model accurately predicts the ML performance over all regimes considered. We also derive an approximation to the threshold at which the ML estimator essentially fails to provide better than a random guess of the pulse arrival time.

On approaching the ultimate limits of photon-efficient and bandwidth-efficient optical communication

It is well known that ideal free-space optical communication at the quantum limit can have unbounded photon information efficiency (PIE), measured in bits per photon. High PIE comes at a price of low dimensional information efficiency (DIE), measured in bits per spatio-temporal-polarization mode. If only temporal modes are used, then DIE translates directly to bandwidth efficiency. In this paper, the DIE vs. PIE tradeoffs for known modulations and receiver structures are compared to the ultimate quantum limit, and analytic approximations are found in the limit of high PIE. This analysis shows that known structures fall short of the maximum attainable DIE by a factor that increases linearly with PIE for high PIE.

Multipulse PPM on memoryless channels

We examine several properties of n-pulse pulse position modulation (PPM) [H. Sugiyama et al., 1989] on memoryless channels. We derive the maximum likelihood decision rule and an exact expression for the symbol error rate for nges1, generalizing previous results [R.M. Gagliardi et al., 1976, M. Simon et al., 2003]. A capacity comparison indicates that muItipulse PPM does not produce appreciable gains over conventional PPM except at high average power

An interleaver implementation for the serially concatenated pulse-position modulation decoder

We describe novel interleaver and deinterleaver architectures that support bandwidth efficient memory access for decoders of turbo-like codes that are used in conjunction with high order modulations. The presentation focuses on a decoder for serially concatenated pulse-position modulation (SCPPM), which is a forward-error-correction code designed by NASA to support laser communications from Mars at mega-bits-per-second (Mbps) rates. For 64-ary PPM, the new architectures effectively triple the fan-in of the interleaver and fan-out of the deinterleaver, enabling parallelization that doubles the overall throughput. The techniques described here can be readily modified for other PPM orders

Low-complexity serially-concatenated coding for the deep space optical channel

This paper discusses the iterative decoding of a serial concatenation of a short constraint length convolutional code and coded PPM through a bit interleaver. To reduce the complexity of iterative decoding, we propose to compute and store only a subset of the channel likelihoods. We show that this can be done while suffering a negligible loss in performance. The complexity of implementing the forward-backward algorithm is also reduced when partial likelihoods are retained and when optical links are used to support deep space communication at high data rates.

Optical communications performance of hybrid 34-meter microwave antennas

There is considerable interest in determining whether suitably modified versions of existing 34-meter antennas at NASAs Goldstone Communications Complex, originally designed for X-band (nominally 8 GHz) and Ka-band (32 GHz) operation, could also be used to receive near-infrared optical signals1. The robust backup structure of these antennas, together with extremely large collecting apertures and milli-degree pointing capabilities suggest that dual RF/Optical communications may indeed be possible, at optical data-rates approaching 1 gigabit per second (GBPS) from typical Mars distances2. Several design concepts have emerged as possible candidates, requiring modifications ranging from polishing and coating of the existing aluminum panels of the main reflector, to significant redesign involving replacement of the panels with optical reflectors. Optical receiver parameters such as collecting area, field-of-view (FOV), and immunity to reflected sunlight differ markedly for each design concept, hence will likely lead to different levels of performance in terms of data-throughput at a given BER, and in terms of the ability to point close to the sun. The communications performance of two candidate design concepts operating under realistic daytime conditions is evaluated, with particular emphasis on spatial and temporal acquisition algorithms and receiver optimization to achieve the best possible communication performance at high data rates.

Palomar Receive Terminal (PRT) for the Mars Laser Communication Demonstration (MLCD) Project

Significant technological advances were made toward utilizing the Hale telescope for receiving the faint laser communication signals transmitted from an optical transceiver on a spacecraft orbiting Mars. The so-called Palomar receive terminal design, which would have supported nominal downlink data rates of 1-30 Mbps, is described. Testing to validate technologies for near-Sun (3deg from edge of solar disc) daytime operations is also discussed. Finally, a laboratory end-to-end link utilizing a 64-ary pulse-position modulated photon-counting receiver and decoder that achieved predicted near-capacity (within 1.4 dB) performance is described.

An optical communications link design tool for long-term mission planning for deep-space missions

We describe work on the development of an optical link budget tool for an intensity-modulated direct-detected photon-counting channel utilizing pulse-position-modulation (PPM). We provide new material in several areas. First, we provide an approximation to the channel capacity, which is not known in closed form, to enable efficient search algorithms over the trade space. The expression also illustrates clearly the trade-offs between signal and noise power, and modulation parameters. We provide an approximation to the losses due to log-normal fading, which may be used to model scintillation. We provide approximations for the loss due to photo-detector blocking and jitter. Lastly, we describe a methodology to choose an optimum detector sub-array in the presence of dark noise, blocking, and an arbitrary point spread function.

SAT05-4: Implementation of a Coded Modulation for Deep Space Optical Communications

We present an efficient implementation of a coded modulation for the deep space optical channel. NASA designed this so called serially concatenated pulse position modulation (SCPPM) code to provide an optical link that can operate within one dB signal energy of the Shannon capacity during a nominal mission condition from Mars. Here, we describe some of the challenges in realizing the SCPPM decoder on a field-programmable gate array (FPGA). Through various architectural optimizations, we achieve a 6 Mbps decoder on a single FPGA. Moreover, we demonstrate that it is possible to communicate reliably on an efficient bits-per-photon count in an end-to-end SCPPM coded system.