Showing papers on "Multipath propagation published in 1969"

Effects of multipath transmission on the measured propagation delay of an FM signal

Abstract: Position location techniques based on propagation delay have been proposed previously. A narrow-band version of this technique involves the transmission of RF carrier, modulated by a single audio frequency. At a receiver, the audio phase is a measure of distance provided the propagation delay is less than one quarter cycle. The transmission medium introduces multipath distortion and the received signal consists of a set of signals, each an attenuated and delayed replica of the transmitted signal, having traversed a different path. When FM is employed, the phase of the demodulated composite is a nonlinear function of the parameters of the multipath structure of the channel. In this paper, this functional relationship is derived.

Multipath compensation system

Abstract: A system for compensating for multipath distortion in a synthetic phase isolator predetection combiner. Since echos in the signal can be detected with respect to both magnitude and sense, this information can be used to suppress echos or to control time delay compensation elements to thereby provide a more coherent output signal. Multipath distorted signals are received at a plurality of channels, each of which channels effectively shifts the input signal phase by predetection combination with the sum of the signal outputs of the other channels such that each channel output is substantially independent of the input signal phase.

Indirect atmospheric measurements utilizing rake tropospheric scatter techniques—Part I: The rake tropospheric scatter technique

Abstract: Unusually detailed multipath resolution of the tropospheric-scatter medium has been obtained by incorporating Rake instrumentation into tropospheric transhorizon microwave experiments. Radio-frequency carriers at approximately 900 MHz, each binary phase-shift keyed at 10 megabauds by a pseudorandom sequence of digits and subsequently filtered to a 10-MHz bandwidth, were transmitted during separate experiments in February and in November and December, 1965, over two paths--a 480-km overland path near the eastern coast of the United States and a 614-km over-water path in the Caribbean. At the receiver an identical binary stream was used in a series of 10 cross-correlators (Rake taps) where each stream was delayed 0.1 µs with respect to the stream in the preceding tap. This provided a multipath resolution of 0.1 µs that, depending upon the path length and altitude, corresponded to a separation of the common volume into ellipsoidal shells varying in thickness from about 200 to 550 meters, The principal goal of the experiments was to record the randomly varying in-phase and quadrature components of each tap output and thus investigate the fluctuations in signal phase, as well as in amplitude, caused by fading. The test procedures also permitted multipath structure and Doppler shifts to be directly observed. Scattering functions, which clearly illustrate the distribution of received signal energy as a function of multipath time delay and Doppler frequency shift, were calculated. In contrast to a multipath time-delay spread that was ordinarily less than 1 µs for the overwater path, spreads of over 3 µs were common for the shorter, overland path. Typical fading rates and Doppler spreads were also markedly different--several fades per second over land versus one fade per several seconds over water. Although some of the observed variations can be attributed to differences in antenna size (8.5-meter diameter for over land, 18-meter diameter for over water) and pointing angle, others apparently result from different typical refractive structures (meteorological conditions) within the atmospheric common volume of the respective radio links. Doppler shifts of as much as 15 Hz, which probably resulted from off-great-circle antenna alignment, were observed on the overland but not the overwater path.

Mobile radio diversity reception

Abstract: This paper examines a particular kind of diversity system, under conditions of multipath fading, when there is interference from either random noise or from an unwanted station The transmitter sends a pilot wave along with the modulated signal The receiver's mixer stage heterodynes the signal with the pilot (instead of with a locally generated tone) Doppler phase distortion, which affects the signal and pilot in nearly the same way, cancels out during mixing The diversity system with N antennas adds the outputs from N such mixers This kind of diversity tends to add the N signal outputs in phase, while random noise components as well as certain other interferences add powerwise In the presence of an interfering station, diversity smooths out amplitude fluctuations It thereby reduces the probability that the interference will override the desired station

Time-frequency-phase in-band coded communications system

Abstract: A communication system which uses a diversity technique to overcome problems of signal fading and multipath distortion. Frequency shift keying (FSK) and differential phase shift keying (DPSK) are employed in combination to code a sequence of signals responsive to a word represented by a stream of input binary digits. The sequences of signals to be transmitted are arranged such that identification of the frequency, phase shift and location of one signal in the sequence uniquely defines the input word.

Near Earth Millimeter Wave RADAR and Radiometry

Abstract: The Army is interested in detection systems operating on the earth or at relatively low altitudes . Hopefully, these systems will detect targets, identify them, and guide missiles to the targets. Since grazing angles of 0.1 to 1 degree are involved, multipath signal distortion occurs. The multipath signal causes antenna pattern distortion and inaccurate pointing.

Note on the optimal transversal equalizer for a dual multipath noiseless channel

Abstract: Optimal transversal equalization is considered for a noiseless multipath channel comprising a main signal followed by a larger magnitude ghost. It is shown that for the optimal transversal equalizer, the output residual intersymbol variance decreases with increase of processing delay. For the same processing delay, the optimal transversal equalizer intersymbol variance is always larger than that of the finite optimal Wiener equalizer, especially as the magnitudes of the main signal and ghost approach equality.

Signal Characteristics of a Very-High-Frequency Satellite-to-Aircraft Communications Link

Abstract: The results of a series of very-high-frequency (VHF) aeronautical communications tests conducted with NASA Applications Technology satellites are summarized. The tests were undertaken to collect data which could be compared to the results obtained from a previously derived analytical model. Specifically, the objective of the program was the measurement of the received signal strength and fading characteristics, as received on board a typical commercial jet aircraft to determine the effectiveness of communication and to establish parameters which can be useful to the communication system designer. A discussion of the analytical model, including the antenna characteristics, is followed by a brief description of the test program. The results and conclusions included consideration of the signal amplitude behavior, signal-to-noise ratio, antenna pattern effects, multipath, and system gain and noise. Probability distributions of signal fading and examples of unusual signal reception are included. The probability distributions provide a guide to system designers in establishing communications reliability for specific systems. It was concluded from the tests that the received signal level exceeded the voice intelligibility threshold more than 98 percent of the time using the test system.

Preliminary study of satellite multipath interference characteristics

Abstract: The effects of multipath arising when a signal transmitted from a User satellite is scattered by the earth before reaching a tracking and delay relay satellite (TRDS) in stationary orbit are considered. An upper bound for the probability of the scattered power density is derived, and numerical results are presented for wide ranges of parameters. In particular, the probability is calculated and displayed for vertical and horizontal polarizations, frequencies of 136 MHz and 2. 3 GHz, wind speeds from 1 m/sec to 12 m/sec, and User satellite heights from 6.578 km to 10.378 km.

Multipath Fading in FSK Communication Links - An Experimental al Investigation

Abstract: The results of an experimental investigation of binary error rates in an FSK channel experiencing nonselective fading are presented. For all cases considered, the received frequency uncertainty is large compareded to the bit rate, requiring the use of an envelope detector rather than a matched filter. Both slow and fast fading rates are considered and include the effects of differential Doppler shift between the direct and reflected energy. A simplified mathematical analysis is presented to support the observed results. Both the theoretical examination and the data obtained demonstrate that fast fading and/or differential Doppler generally improve the link error rate performance with respect to the nonfading case.

Radio direction finder signal processing means

Abstract: Apparatus for processing radio direction finding signals and the like including an electronic circuit including means for generating an output signal proportional to the amplitude of a first input signal times a function of the phase difference between two other input signals, and applications of the same. The apparatus also includes means for substantially reducing display azimuth bearing errors such as are caused by multipath reflection interference and the like.

Note on FM transmission of video between moving vehicles over a flat earth

Abstract: A signal-to-interference ratio for the transmission of scanned images and digital data in the dynamic multipath environment was derived in an earlier paper The dynamic case depends upon a Doppler environment-signal parameter ratio, while the static result is independent of the signal structure The dynamic case is asymptotic to, but always worse than, the static one For a Doppler-signal ratio of less than 10 percent the dynamic signal-to-noise ratio is essentially equal to the static one Most real systems today fall in this region This indicates why certain simple multipath analyses in use today have been successful The method of designing a signal to fall below the 10-percent value is indicated Signal-to-noise ratio curves are provided for system design purposes These results permit system analysis and tradeoff studies to be based on earth reflection coefficient distributions and aircraft or satellite operation envelopes