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Nuclear Scintillation Mitigation by Matched Channel Filtering

Description:

TECHNOLOGY AREA(S): Nuclear

OBJECTIVE: The objective of this SBIR topic is to conduct research, development, and demonstration for a new method to mitigate digital communications message errors resulting from communication overnuclear-disturbed RF propagation channels.Such errors can occur on SATCOM links which must pass through magneto-ionic media generated by nuclear weapons detonationin the high atmosphere.Accordingly, the objective of this research is to define a “Matched Channel Filter” (MCF)—matched to the then current scintillated communication channel filter function, and to demonstrate the mitigation offered by the new MCF filter via experiments with GFE HWIL scintillation simulators.

DESCRIPTION: Previous DTRA scintillation research has successfully formulated models of propagating channels disturbed by high-altitude nuclear weapons detonations. Detailed computer simulations to analyze and predict the disturbed channel performance have also been developed.DTRA has also developed Hardware-in-the-Loop (HWIL) fading channel simulators (like the WCS, CoLTS, RNECS, and others). These simulators have been used for developmental and acceptance testing of strategic RF communication systems and components which must operate with transmitters propagating in a scintillation medium.Two important DoD capabilities have been derived from these previous efforts: 1) the ability to harden the design of strategic RF systems against scintillation, and 2) the ability to test the performance of fielded RF systems under simulated wartime (nuclear-disturbed atmosphere conditions).Antenna Channel Impulse Response Function (ACIRF)The principal DTRA modeling tool which is used for analysis and testing of scintillated communication links is the Antenna Channel Impulse Response Function (ACRIF--Ref 2).DTRA’s ACIRF code provides the DoD strategic communication and radar communities with a formal method of representing disturbed trans-ionospheric propagation channels for SATCOM, HF communications, or even two way radar propagation channels.ACIRF also models the filtering effects of the receive antenna. ACIRF output files, called “realizations” when associated with a particular link, are usually pre-computed in accordance with link specifications (frequency, modulation bandwidth, geometry, and either measured or derived channel parameters).ACIRF generates pseudo-random baseband equivalent realizations of RF fading channels, and are considered to be the channels impulse response function.These realizations are stored to hard drive for use in analysis codes or in HWIL test systems.Channel Realization Generator (CReG) Although the ACIRF code has proven to be a valuable tool, it has limitations. It runs off-line to generate one fixed-length realization that must be stored and repeatedly played back to represent a fading channel effects over a long duration communication link testing scenarios.Secondly, any test or analysis application using ACIRF realizations must have a large storage and retrieval capability to accept and playback ACIRF realizations needed during HWIL testing, rather than just accepting a stream of channel realization updated in real test time. Thirdly, ACIRF only generates complex baseband realizations with Rayleigh or Rician amplitude statistics.This is appropriate for the highly-disturbed propagation paths associated with extreme wartime conditions, but not for propagation paths disturbed by distant high altitude explosions, or by natural phenomena such as tropospheric scatter or auroral effects.Consequently, DTRA has recently developed a real-time channel realization generation code (called CReG), and is in process of implementing it in the newest HWIL simulator (now under development—called CoLTS-AD (Configurable Link Test System--All Digital).A full description of the CReG code and its capabilities can be available to bidders on this topic within SBIR contractor rights restrictions (Ref 3), as well as a description of the CoLTS-AD HWIL system (Ref 4) within the same SBIR rights restrictions.Matched Propagation Channel Filter FunctionThe concept of matched channel filter synthesis for compensating the effects of a disturbed magneto-ionic propagation channel has been investigated in the past. Then, Halpin and Urkowitz of GE Aerospace (Ref 5) considered implementing an intentionally pre-distorted wideband chirp radar waveform before transmission through a scintillated propagation channel.The pre-distortion essentially implemented a matched propagation channel filter at the Cobra Dane radar transmitter.Halpin chose to implement transmit waveform pre-distortion so that the propagation channel itself would filter the pre-distorted waveform, allowing linear de-chirp radar return signal processing to proceed at the radar receiver as if the propagation path had been through free space.In this SBIR effort, we wish to find the channel filter of the scintillated channel itself in real time, and then implement the matched channel filter on receive as the method to mitigate errors imposed by the transmit channel scintillation.The primary intent of this research is to define a Matched Channel Filter (MCF) —matched to the then current scintillated communication channel filter function, where Hs(ω) is the MCF function and hs(t, τ) is the scintillated communication channel impulse response (the channel response at time t to an impulse applied at t-τ).It is believed that hs(t, τ), may be found by measuring the then current channel parameters of the nuclear-disturbed channel directly, and therefrom computing the scintillated channel filter function.Then, it is desired to synthesize a “matched filter” function for the newly found fading channel filter.The matched channel filter function may be found by conjugating the determined current channel filter function, but other syntheses may also be pursued.For this application, it remains to be shown that the MCF is the conjugate of the channel filter function.

PHASE I: Phase I investigation will be a modeling study using the DTRA code ACIRF, CReG or other to define a scintillated Channel Impulse Response Function (CIRF), as well as the scintillation channel parameters themselves which are necessary to define that CIRF.Then, the modeling study will continue with the mathematical synthesis to find the Matched Channel Filter (MCF)--matched to the fading channel transfer function, H(w,t).The critical channel parameters needed to synthesize the MCR will also be subject to a sensitivity study in order to bound the needed accuracy of their measurement for satisfactory synthesis of the MCF.A necessary part of the Phase I investigation is to suggest how these channel parameters may be estimated or measured in real time (probably by some channel sounding technique).The Phase I modeling activity will conclude with the (almost) error free recovery of the communication waveform after having been passed through both a scintillated propagation channel (CIRF) and then recovered in a receiver outfitted with the new matched propagation channel filter for that link (MCF).Phase I modeling will affirm the feasibility of the proposed mitigation concept via matched channel filter processing.As noted, Phase I will also investigate and describe possible means of determining the real-time channel parameters for any scintillated communication link. The channel parameters to be found by real time channel sounding techniques (or other) are: frequency selective bandwidth, scintillation delay tau0, scintillation index, own antenna dimensions, and Line-of-sight Total Electron Content (TEC).

PHASE II: Phase II will proceed to incorporate the propagation matched filter defined during Phase I into a stand-alone software emulation for the purpose of demonstrating the mitigating capability of the matched filter function.This demonstration is intended to quantify the mitigation capability of the matched channel filter (probably by measuring Bit Error Rate) for the disturbed digital communication waveform, based on how well the matched channel filter can be defined and implemented.The matched channel filter can be programmed into one of the DTRA HWIL simulator test channels, and exercised when presented with the disturbed channel impulse response communications waveform.Phase II will be completed when a detailed plan for incorporating the new matched channel filter in a scintillation HWIL test set has been prepared and delivered to the sponsor. Candidate HWIL sets include WCS, CoLTs, CoLTs-LC, and the new all-digital Scintillator (CoLTS-AD) now being designed and built for DTRA (Ref 4).

PHASE III: The commercial market for the recently developed software and run-time CReG codes and for the matched channel filter mitigation being developed herein, includes two communities: 1) the academic community where a number of communication channel simulators have been developed for general purpose use (as for example at the Naval Post Graduate School where a CoLTs-LC scintillator has been in university research use),and 2) the commercial marketplace where tools such as the MATLAB Communications Toolbox are available.Market spaces such as commercial SATCOM, cellular phone, GPS, and Wi-Fi should all be interested in the natural multipath-induced fading channel impulse response calculation of the CReG code and of the matched Channel Filter mitigation being developed herein.In the academic marketplace the matched channel filter development would be of specific interest at Virginia Polytechnic Institute and State University where scintillation mitigation research is underway for a 28 MHz wide area communications network (Ref 6).

KEYWORDS: Nuclear technology, Survivable communications,Nuclear Scintillation, Scintillation Mitigation

References:

[1] Bello, P. A., “Characterization of randomly time-variant linear channels”, IEEE Trans. On Comm. Systems, CS 11, Dec. 1963, pp. 360-393. [2] Dana, R. A., ACIRF User’s Guide for the General Model (Version 3.5), DNA-TR-91-162, June 1992, code available from DTRA at: dtra.belvoir.rd.mbx.reachback-software-distribution@mail.mil [3] CReG description document (Sawyer, et.al, Welkin Sciences, (within SBIR-rights restrictions)[4] CoLTS-AD description document (Sawyer, et.al., Welkin Sciences (within SBIR-rights restrictions) [5] Halpin, Urkowitz, and Maron, “Propagation Compensation by Waveform Pre-Distortion”, GE Aerospace, Government Electronic Systems Division, Moorestown, NJ, Proceedings of the IEEE International Radar Conference, 1990. [6] “Channel Impulse Response and Its Relationship to Bit Error Rate at 28 GHz Wide Area Communications Network” by Mary Miniuk, Master Thesis, Department t of Electrical Engineering, Virginia Technical University, 2007.

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