Meera Srinivasan

Adaptive detector arrays for optical communications receivers

An optimal adaptive array receiver for use in groundbased optical communications is investigated. Kolmogorov phase screen simulations are used to generate realistic focal-plane distributions of the received optical fields in the presence of turbulence. The array detection concept reduces interference from background radiation by effectively assigning higher confidence levels at each instant of time to those detector elements that contain significant signal energy and suppressing those that do not. A simpler suboptimum structure that replaces the continuous weighting of the optimal receiver by a hard decision over each detector element is also described. It is shown that, for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single-detector photon counting receivers when observing turbulent optical fields in high background environments.

Optical array receiver for communication through atmospheric turbulence

An optical array receiver concept is developed and analyzed. It is shown that for ground-based reception, the number of array elements can be increased without incurring performance degradation, provided the array telescope diameters exceed the coherence-length of the atmosphere. Maximum likelihood detection of turbulence-degraded signal fields is developed for the case of pulse-position modulated signals observed in the presence of background radiation. Performance of optical array receivers is compared to single-aperture receivers with diameters ranging from 4 to 8 m, both in the presence of turbulence and in a turbulence-free environment such as space. It is shown that in the absence of atmospheric turbulence, single-aperture receivers outperform receiver arrays when significant background radiation is present. However, it is also demonstrated that for ground-based reception of deep-space signals, the number of array elements can be as great as several thousand without incurring any performance degradation relative to a large single-aperture receiver.

Optical PPM synchronization for photon counting receivers

The performance of two signal synchronization schemes for photon counting receivers used in optical PPM communication systems operating in low-flux conditions, such as proposed deep space optical communication links, are analyzed. The pilot symbol insertion and inter-symbol guard time synchronization schemespsila performance are compared under conditions of equal throughput and equal peak or average power at representative operating points, including frequency offset due to transmit and receiver oscillator instability.

LLCD operations using the Optical Communications Telescope Laboratory (OCTL)

The Optical Communications Telescope Laboratory (OCTL) located on Table Mountain near Wrightwood, CA served as an alternate ground terminal to the Lunar Laser Communications Demonstration (LLCD), the first free-space laser communication demonstration from lunar distances. The Lunar Lasercom OCTL Terminal (LLOT) Project utilized the existing 1m diameter OCTL telescope by retrofitting: (i) a multi-beam 1568 nm laser beacon transmitter; (ii) a tungsten silicide (WSi) superconducting nanowire single photon detector (SNSPD) receiver for 1550 nm downlink; (iii) a telescope control system with the functionality required for laser communication operations; and (iv) a secure network connection to the Lunar Lasercom Operations Center (LLOC) located at the Lincoln Laboratory, Massachusetts Institute of Technology (LL-MIT). The laser beacon transmitted from Table Mountain was acquired by the Lunar Lasercom Space Terminal (LLST) on-board the Lunar Atmospheric Dust Environment Explorer (LADEE) spacecraft and a 1550 nm downlink at 39 and 78 Mb/s was returned to LLOT. Link operations were coordinated by LLOC. During October and November of 2013, twenty successful links were accomplished under diverse conditions. In this paper, a brief system level description of LLOT along with the concept of operations and selected results are presented.

PN Code Tracking Using Noncommensurate Sampling

The use of a noninteger symbol to sample time ratio, noncommensurate sampling, in pseudonoise (PN) code-tracking loops is investigated. A noncoherent PN code-tracking loop using noncommensurate sampling and single-bit quantization is presented. An analysis method is developed to characterize the system and determine the steady-state error variance of the timing estimate. The performance of an equivalent analog code-tracking loop is derived and compared with the digital loop to investigate the degradation incurred through sampling and quantization

Analysis of sampling and quantization effects on the performance of PN code tracking loops

Pseudonoise (PN) code tracking loops in direct-sequence spread-spectrum systems are often implemented using digital hardware. Performance degradation due to quantization and sampling effects is not adequately characterized by the traditional analog system feedback loop analysis. A low-complexity digital PN code tracking loop with one-bit non-commensurate sampling is analyzed, and the steady-state delay error variance is derived. The results are compared with that of an equivalent analog loop.

Cooperative modulation techniques for long haul relay in sensor networks

Cooperative modulation techniques can reduce the energy requirements for the long haul transmission of data from localized sensor networks. This saving is vital in extending the battery life of power limited sensor nodes communicating over a time-limited channel. Several transmission methods, including a novel scheme that conveys information via node selection, are analyzed. The energy cost in local communications needed to support the cooperative long haul link is determined and used to make comparisons between the techniques.

A receiver for the Lunar Laser Communication Demonstration using the optical communications telescope laboratory

We discuss the design and implementation of a receiver for the Lunar Laser Communication Demonstration based on a 12-pixel array of tungsten silicide superconducting nanowire single photon detectors. The receiver was used to close a software communication link from lunar orbit at 39 and 79 Mbps.

Laboratory characterization of silicon avalanche photodiodes (APD) for pulse position modulation (PPM) detection

Two commercially available large area silicon avalanche photodiodes (APD) were characterized in the laboratory. The response of the APDs to a sequence of 8-bit pulse position modulated (256-PPM) laser pulses, with and without additive background noise, was recorded and stored for post analysis. Empirical probability density functions (pdfs) were constructed from the signal and noise slot data and compared to pdfs predicted by an analytical model based on Webb+Gaussian statistics. The pulse sequence was used to generate bit-error rate (BER) versus signal photons per pulse plots, albeit with large error bars due to the limited number of signal pulses stored. These BER measurements were also compared with analytical results obtained by using the Gaussian and Webb+Gaussian models for APD channel statistics. While the measurements qualitatively reflect features predicted by theory, significant quantitative deviations were displayed between the measurements and theory. The source of these discrepancies is not currently well understood, but it is surmised that inaccurate knowledge of detector parameters such as gain and noise equivalent temperature models may explain the discrepancies.

Optical array receiver for deep-space communications

An optical array receiver concept is developed and analyzed. It is shown that for ground-based reception, the number of array elements can be increased without any performance degradation, as long as the array telescope diameters exceed the coherence-length of the atmosphere. Maximum likelihood detection of turbulence-degraded signal fields is developed for the case of pulse-position modulated (PPM) signals observed in the presence of background radiation. Performance of optical array receivers is compared to single-aperture receivers with diameters ranging from 4 to 8 meters, both in the presence of turbulence and in a turbulence-free environment such as space. It is shown that in the absence of atmospheric turbulence, single-aperture receivers outperform receiver arrays when significant background radiation is present. However, it is also shown that for ground-based reception of deep-space signals, the number of array elements can be as great as several thousand without incurring any performance degradation relative to a large single-aperture receiver.