基于系统参数的深空光通信下行链路损失预算.pdf

IPN Progress Report 42-154August 15, 2003Deep-Space Optical Communications Downlink Budget from Mars: System ParametersA. Biswas1and S. Piazzolla2This article describes the elements of a design control table for an optical commu- nications link from a spacecraft in Mars orbit to a ground-based receiving station onEarth. A fixed average laser transmitter power of 5 W transmitted through a 30-cm-diameter near-diffraction-limited telescope is assumed, along with a 10-m-diameter ground receiving antenna. Pulse-position modulation of the laser with direct de- tection also is assumed. An end-to-end systems analysis is presented to provide the expected signal and background-noise photons as a function of the Earth–Mars range. The signal and noise photons received are treated using an ideal Poisson channel model in order to predict data rates when Mars is close to conjunction. The data rates range from 5 to 40 Mb/s with a large part of the uncertainty owing its origin to variability of the atmosphere. The article also concludes that further work is required in order to narrow the rather wide range of preliminary data rates presented.I. IntroductionIn this article, an optical communications link from a spacecraft in Mars orbit is studied with the objective of establishing preliminary bounds on the data rates achievable. A pulse-position modulation (PPM) sequence of laser pulses is transmitted to Earth, where direct detection and a ground-based receiving terminal are assumed. The transmitter diameter is 30 cm, with an average laser power of 5 Wat a wavelength of 1064 nm. An effective ground-based collection aperture diameter of 10 m with an ideal photon-counting receiver is assumed.Optical links from Mars are dominated by huge free-space propagation losses. These losses must beovercome so that sufficient laser signal photons can be reliably detected at the ground receiver whileusing fixed average laser power on the spacecraft. The constant average laser power, (Pavg)trans, of thetransmitter, with a fixed pulse width, allows varying pulse energy, Ep, and pulse-repetition frequency (PRF) while satisfying the relation(Pavg)trans= Ep× PRF1Communications Systems and Research Section.2University of Southern California, Los Angeles, California.The research described in this publication was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.1So, at the longest ranges, pulses with sufficient energy to overcome the losses are transmitted to achieve data rates commensurate with the PRF that can be supported. As range decreases, progressively in-creasing data rates can be realized. Thus, channel capacity (bits/second) is traded for channel efficiency (bits/photon) as the communications range increases.PPM lends itself elegantly to this direct-detection scheme. The emitted laser pulse is positioned in one of M time slots, each Tsseconds wide, in order to encode log2M bits of data per PPM symbol. The product of slot width, Ts, and M defines the symbol duration. In the analysis to be presented, fixed slotwidths of 2 ns are considered. In future articles, variable slot widths will be analyzed. The effect of an upper bound on the PPM order also is considered.The communications link equation determines the relation between mean received signal power, (Pavg)recd, and transmitted power, (Pavg)trans:(Pavg)recd= (Pavg)trans× GT× ηT× LTP× Ls× ηatm× ηR× GR× Lother(1)whereGT,R= the transmitter and the ground-receiver gainLTP= the loss allocated for imperfect pointing of the narrow laser beamLS= the free-space lossLother= a miscellaneous loss term explained belowη(T,atm,R)= the transmitter, atmospheric, and receiver efficiencies at the laser wavelengthFor this preliminary report, an ideal photon-counting receiver is considered. Future articles will address deviations expected from this model when realistic device constraints are accounted for. The average received signal power is converted to detected signal photons, ns, after scaling with the quantum efficiencyof the detector. Even though narrow bandpass optical filters are used to reject out-of-band background-noise photons, an in-band fraction contributed by sunlight scattered and reflected by the sky and Mars will be incident upon the detector. The number of background-noise photons is directly proportionalto the detector field of view or solid angle used. Furthermore, in-band noise photons from outside thedetector field of view, referred to as stray light, also may contribute to background noise. The aggregate ofbackground-noise photons scaled by the quantum efficiency results in nbdetected noise photons. Treating the ns+ nbdetected photons by a Poisson [1,2] process allows a determination of the channel capacity. A companion article [3] details the derivation of realizable data rates, given a combination of nsand nb. Synchronization and quantization losses of 1 dB together with a link 。