Publications
Far-echo cancellation in the presence of frequency offset (full duplex modem)
June 1, 1989
Journal Article
Published in:
IEEE Trans. Commun., Vol. 37, No. 6, June 1989, pp. 635-644.
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Summary
In this paper, we present a design for a full-duplex echo-cancelling data modem based on a combined adaptive reference algorithm and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of the far-end signal based on receiver decisions. This technique provides a new approach for full-duplex far-echo cancellation in which the far echo can be cancelled in spite of carrier frequency offset. To estimate the frequency offset, the system uses a separate receiver structure for the far echo which provides equalization of the far-echo channel and tracks the frequency offset in the far echo. The feasibility of the echo-cancelling algorithms is demonstrated by computer simulation with realistic channel distortions and with 4800 bits/s data transmission at which rate frequency offset in the far echo becomes important.
Summary
In this paper, we present a design for a full-duplex echo-cancelling data modem based on a combined adaptive reference algorithm and adaptive channel equalizer. The adaptive reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of...
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Phase coherence in speech reconstruction for enhancement and coding applications
May 26, 1989
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, Vol. 1, Speech Processing 1, 23-26 May 1989, pp. 207-209.
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Summary
It has been shown that an analysis-synthesis system based on a sinusoidal representation leads to synthetic speech that is essentially perceptually indistinguishable from the original. A change in speech quality has been observed, however, when the phase relation of the sine waves is altered. This occurs in practice when sine waves are processed for speech enhancement (e.g., time-scale modification and reducing peak-to-RMS ratio) and for speech coding. This paper describes a zero-phase sinusoidal analysis-synthesis system which generates natural-sounding speech without the requirement of vocal tract phase. The method provides a basis for improving sound quality by providing different levels of phase coherence in speech reconstruction for time-scale modification, for a baseline system for coding, and for reducing the peak-to-RMS ration by dispersion.
Summary
It has been shown that an analysis-synthesis system based on a sinusoidal representation leads to synthetic speech that is essentially perceptually indistinguishable from the original. A change in speech quality has been observed, however, when the phase relation of the sine waves is altered. This occurs in practice when sine...
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Speech-state-adaptive simulation of co-channel talker interference suppression
May 23, 1989
Conference Paper
Published in:
Proc. IEEE Int. Conf. on Acoustics, Speech and Signal Processing, ICASSP, 23-26 May 1989, pp. 361-364.
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Summary
A co-channel talker interference suppression system processes an input waveform containing the sum of two simultaneous speech signals, referred to as the target and the jammer, to produce a waveform estimate of the target speech signal alone. This paper describes the evaluation of a simulated suppression system performing ideal suppression of a jammer signal given the voicing states (voiced, unvoiced, silent) of the target and jammer speech as a function of time and given the isolated target and jammer speech waveforms. By applying suppression to select regions of jammer speech as a function of the voicing states of the target and jammer, and by measuring the intelligibility of the resulting jammer suppressed co-channel speech, it is possible to identify those regions of co-channel speech on which interference suppression most improves intelligibility. Such results can help focus algorithm development efforts.
Summary
A co-channel talker interference suppression system processes an input waveform containing the sum of two simultaneous speech signals, referred to as the target and the jammer, to produce a waveform estimate of the target speech signal alone. This paper describes the evaluation of a simulated suppression system performing ideal suppression...
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ASR-9 weather channel test report
May 3, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-165
Summary
The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. This report presents results of an assessment of the ASR-9 weather channel performance. Two issues addressed are: (1) whether the ASR-9 weather channel performs according to FAA specifications, and (2) whether the ASR-9 weather channel adequately represents weather reflectivity for ATC purposes. These measurement results are intended to support the FAA in developing the operational use of ASR-9 weather information. Comparisons between data from an ASR-9 in Huntsville, Alabama, recorded during design qualification and testing, and data from two other "reference" radars were used as the basis for the assessment. Several storm cases were analyzed, comprised of stratiform rain, isolated convective storms, squall lines, and cold fronts containing multiple simultaneous convective storms. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the "reference" radars, the ASR-9 weather channel seems to perform according to FAA specifications. Although the ASR-9 products give a reasonable representation of the extent and severity of potentially hazardous weather in Huntsville, the results suggest that the static storm model used to determine beamfill corrections for the ASR-9 should be optimized for the particular climatic region in which an ASR-9 will be operated.
Summary
The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. This report presents...
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ASR-9 weather channel test report, executive summary
May 3, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-168
Summary
The ASR-9, the next generation Airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. Comparisons between data from an ASR-9 in Huntsville, Alabama, recorded during design qualification and testing, and data from two other "reference" radars, were used as the basis for assessment of ASR-9 weather channel performance. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the "reference" radars, the ASR-9 weather channel seems to perform according to FAA specifications.
Summary
The ASR-9, the next generation Airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. Comparisons between data...
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Storm models for end-to-end TDWR signal processing simulation tests
May 1, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-155
Summary
End-to-end qualification testing of teh Terminal Doppler Weather Radar (TDWR) contractor signal processing system will be accomplished by a signal processing simulation test. Government furnished storm models will be used to provide inputs to the signal processor. The corresponding hazardous weather product results will be compared to hte results determined by the detection algorithm developers. This report examines the role of the end-to-end tests in the context of overall TDWR qualification testing and concludes that the signal waveform/velocity ambiguity resolution should be the principal focus of the signal processing simulation testing. Salient characteristics of the initial pair of storm models (a high reflectivity microburst observed in Huntsville, AL, and a series of low-to-moderate reflectivity microburst storms observed in Denver, CO) are described as well as desirable characteristics of additional storm models to be provided later.
Summary
End-to-end qualification testing of teh Terminal Doppler Weather Radar (TDWR) contractor signal processing system will be accomplished by a signal processing simulation test. Government furnished storm models will be used to provide inputs to the signal processor. The corresponding hazardous weather product results will be compared to hte results determined...
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Notes and correspondence - Correcting wind speed measurements for site obstructions
April 1, 1989
Journal Article
Published in:
J. Atmos. Oceanic Technol., Vol. 6. No. 2, April 1989, pp. 343-352.
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Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem for scientific and operational wind shear measurement system This paper considers an improved version of the technique used by Fujita and Wakimoto for compensating the obstruction effects by the use of mathematical models relating the unobstructed wind speed to the measured wind speed and the observed obstructions at each site. Over eight million wind speed measurements gathered over 197 days (15 February–31 August) were used. The effects of obstructions at a particular site were evidenced by a strong negative correlation between the observed wind speed transmission factors and the obstruction angles as measured from panoramic photographs taken of the horizon around each station. The functional relationship between them was modeled as a decaying exponential plus a constant, and an iterative least squares regression technique was used on data from all of the stations at once in deriving the three parameters of the equation. It was found that the first 8° of obstruction have the greatest blockage effects, and that even a 2° or 3° high isolated clump of trees can have a pronounced effect on the measured wind speeds from that direction. The possibility that the transmission factors are scale dependent and time dependent is explored.
Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem...
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The FLOWS automatic weather station network
April 1, 1989
Journal Article
Published in:
J. Atmos. Oceanic Technol., Vol. 6, No. 2, April 1989, pp. 307-326.
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This report describes in detail the FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) automatic weather station network which is being used in the Terminal Doppler Weather Radar program to assess the radar detectability of wind shear and to help gain an understanding of microburst forcing mechanisms. The weather stations are descended from the PROBE stations originally operated by the Bureau of Reclamation. The current instrumentation has been modified slightly but is largely the same as that originally used as is the hardware structure, but the data collection platforms are entirely new. Each station in the 30-station network transmits 1 min averages of temperature, relative humidity, barometric pressure, wind speed, wind direction and precipitation amounts, as well as peak wind speed, on a single GOES satellite channel. Performance results from the first 3 yr (1984-86) of mesonet operations are presented. During June and July 1986 the FLOWS network was collocated with the NCAR PAM-II network near Huntsville, Alabama to measure surface data on microbursts as part of the Cooperative Huntsville Meteorological Experiment (COHMEX). A preliminary assessment of the overall performance of the two networks suggests that they performed with comparable accuracy for those meteorological characteristics most important to the detection of microbursts. While differences and discrepancies were noted, none would preclude treating PAM-II and FLOWS data together as if they were generated by a single network.
Summary
This report describes in detail the FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) automatic weather station network which is being used in the Terminal Doppler Weather Radar program to assess the radar detectability of wind shear and to help gain an understanding of microburst forcing mechanisms. The weather stations are descended...
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Analysis of microburst observability with Doppler radar through comparison of radar and surface wind sensor data
March 27, 1989
Conference Paper
Published in:
24th Conf. on Radar Meteorology, 27-31 March 1989, pp. 171-174.
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Summary
As part of the FAA Terminal Weather Doppler Weather Radar (TDWR) measurement program in Huntsville, AL and Denver, CO during 1986 and 1987, respectively, the ability of a single Doppler weather radar to observe microburst outflow signatures (i.e., show identifiable radial velocity patterns) was assessed by comparing radar-observed microbursts with those identified by joint use of both radar and data from a mesoscale network (mesonet) of surface meteorological stations (Clark, 1988; DiStefano, 1988). Observability by radar must be considered together with pattern recognition algorithm performance for observable microbursts (Campbell et al., 1988) in order to fully assess the potential effectiveness of an automated microburst detection system which relies on data from a single Doppler radar. The comparison of radar and surface sensor data presented here investigates the possibility that some outflows may not be observable by radar due to: (1) low SNR (signal-to-noise ratio), (2) very shallow outflows for which the radar beam is scanning too high above the surface, (3) blockage of the beam, and/or (4) asymmetry in the surface outflow causing the radar to significantly underestimate the magnitude of the surface wind shear (Eilts and Doviak, 1987; GAO, 1987). Also addressed is the possibility that microbursts are not observed by the mesonet surface sensors because the spacing between stations is too great, or because the microburst outflow does not reach the surface due to a dense layer of cold air at the surface.
Summary
As part of the FAA Terminal Weather Doppler Weather Radar (TDWR) measurement program in Huntsville, AL and Denver, CO during 1986 and 1987, respectively, the ability of a single Doppler weather radar to observe microburst outflow signatures (i.e., show identifiable radial velocity patterns) was assessed by comparing radar-observed microbursts with...
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Range obscuration mitigation by adaptive PRF selection for the TDWR System
March 27, 1989
Conference Paper
Published in:
24th Conf. on Radar Meteorology, 27-31 March 1989, pp. 175-178.
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Summary
The Federal Aviation Administration has recently awarded a contract for the procurement of 47 Terminal Doppler Weather Radar (TDWR) systems to be sited near high traffic airports. These systems will collect and process Doppler radar data that will be used by fully automated algorithms to identify hazardous meteorological wind shear events in real time (eg., microbursts and gust fronts.) This information will the be conveyed to aircraft pilots in order that potentially hazardous takeoffs or landings be averted. In a pulsed Doppler weather radar, one of the most serious causes of data quality degradation is due to range aliased echoes from distant storms [3]. This range contamination can occur in the immediate vicinity of a meteorological hazard, possibly obscure the event, and thus decrease the probability of detecting it. In other instances, range contaminated data can present a radar signature similar to that of a wind shear hazard, and perhaps cause an algorithm to issue a false alarm. In order for the TDWR system to achieve a high probability of detecting meteorological hazards, while maintaining a low probability of false alarms, an effective means of dealing with range contamination is required. An adaptive procedure by which to select the radar's pulse repetition frequency (Pm) has been developed as a primary means by which to minimize range contamination within the operationally significant coverage area of a TDWR system. This procedure will be developed within this paper and a quantitative assessment as to the anticipated effectiveness of this technique in the TDWR system will be provided.
Summary
The Federal Aviation Administration has recently awarded a contract for the procurement of 47 Terminal Doppler Weather Radar (TDWR) systems to be sited near high traffic airports. These systems will collect and process Doppler radar data that will be used by fully automated algorithms to identify hazardous meteorological wind shear...
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