Tests solely designed to test performance of spacecraft communication and ranging equipment under conditions of severe fading are usually impossible to perform, and when possible they are of too short duration to allow system modification to improve performance.
This paper describes a fading channel simulator which is readily constructed and flexible in operation. By use of this simulator communication and ranging equipment can be tested and optimized for operation during periods of severe fading.
The fading channel simulator described in this paper gives fading rates varying from about 2 to 30 cps.
Frequency and time scaling of the problem can be used if the fading rates of the channel to be simulated are outside this range 

Manders, Arnfinn M.

English

Embry-Riddle Aeronautical University

The Space Congress® Proceedings 1966 (3rd) The Challenge of Space Mar 7th, 8:00 AM A Multipath Simulator for use in Evaluation of Spacecraft Ranging and Communication Systems Arnfinn M. Manders Associate Professor, University of Florida Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Manders, Arnfinn M., "A Multipath Simulator for use in Evaluation of Spacecraft Ranging and Communication Systems" (1966). The Space Congress® Proceedings. 1. https://commons.erau.edu/space-congress-proceedings/proceedings-1966-3rd/session-1/1 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact commons@erau.edu. A MULTIPATH SIMULATOR FOR USE IN EVALUATION OF SPACECRAFT RANGING AND COMMUNICATION SYSTEMSArnfinn Moe -MandersAssociate ProfessorUniversity of FloridaGENESYS, Port Canaveral, FloridaSummaryTests solely designed to test performance of spacecraft communication and ranging equipment under conditions of severe fad- ing are usually impossible to perform, and when possible they are of too short duration to allow system modification to improve performance.This paper describes a fading channel simu- lator which is readily constructed and flex- ible in operation. By use of this simulator communication and ranging equipment can be tested and optimized for operation during periods of severe fading.The fading channel simulator described inthis paper gives fading rates varying fromabout 2 to 30 cps.Frequency and time scaling of the problem can be used if the fading rates of the channel to be simulated are outside this range .IntroductionCommunication and ranging are two of the most vital support operations in connect- ion with spacecraft systems. In spite of this, equipment designed to measure the characteristics of the RF channel can only rarely be placed on board a space- craft because of the great competition for the available payload by all interested parties. Channel measurements can thus usually only be obtained by analysis of incidental data from the operating communi- cation and ranging links. Fading and multipath conditions occur most frequently during powered flight and when the space- craft is close to the horizon. The dura- tion of this condition is therefore usually very short, typically a minute or less. This time does not allow the system de- signer much time during which he can modify the system to improve its performance during these critical periods.A convenient approach that allows the system designer to try different approaches in order to optimize the system performance in the laboratory is to use a multipath simulator.This paper describes a multipath simulator that has been constructed in the Communication Laboratory of the University of Florida, GENESYS, at Port Canaveral.Concept and SystemThe multipath channel simulator described in this paper consists of a 5 x 3 x 2 ft. plywood tank partially filled with water. See Fig. 1. Instead of electromagnetic waves used in a radio link, accoustic waves are used for transmission.In place of receiving and transmitting antennas, two ultrasonic transducers are used at the ends of the channel. The beamwidth of these transducers is quite narrow. This in connection with sound absorbent material placed along the walls of the tank results in freedom from undesirable reflections and standing wave patterns .The nominal carrier frequency used is 200 kHz. The velocity of a 200 kHz ultrasonic signal in fresh water at room temperature is about 1500 m/sec. resulting in a wave- length of 7.5 mm. This short wavelength results in strong reflections from relat- ively small objects. A wide range of dif- ferent multipath conditions can therefore be simulated in this manner. Of particular interest are two types.One is the case where all received signal is due to reflections from random scatter- ers . This is accomplished by lining the walls of the tank with sound absorbing material and reflecting the transmitted signal off air bubbles introduced through small orifices placed at the bottom of the tank. See Fig. 2.The other channel type is where there is a strong steady signal component in addition to considerable multipath caused by random scattering. This second type of channel can be simulated by placing a solid reflecting object near the location where the air bubbles are introduced into the tank. See Fig. 3.The communication system currently used in connection with the multipath simulator is shown in Fig. 4. The transmitter has a crystal controlled carrier frequency of 200 kHz. This carrier is SSB modulated with a 4 kHz subcarrier. The resulting SSB signal is fed to the transmitting transducer where it is radiated into the water. The receiving transducer picks up the signal reflected from the air bubbles and other reflecting objects placed in intersection of the two transducer patterns. This signal is amplified and69demodulated in the reciever. The receiver performs both envelope and product demodu- lation. The output from the envelope demodu- lation provides information about the fading of the envelope of the received signal. The output from the product demodulator is used to extract information about the phase of the fading signal. Reference signals for the product demodulator and for the sub- carrier phase comparison are obtained dir- ectly from the transmitter by means of cables. This results in one great additional advantage accrued by use of the multipath simulator rather than "the real thing". Since the reference signals used at the receiver can be obtained directly from the transmitter they do not have to be generated locally at the receiver. This eliminates a great source of uncertainty present in all actual system experiments, namely if the effects observed are due to the behavior of the channel or if they are due to inaccurate time, frequency and phase references used at the receiver.Applicat ionsThe multipath simulator described in this paper is well suited to be used in experimental work whose goal is to improve the performance of existing spacecraft communication and ranging systems under multipath conditions. It is also well suited for experimental verifications of theoretical results that may be used in future spacecraft systems.As an example of the former category is some work currently under way at University of Florida, GENESYS. The goal of this work is improvement of the stability of a phase locked loop (PLL) under conditions of severe multipath. It is a common experience that PLL's tend to loose lock under this condition. Most PLL's currently in use are optimized for use in an additive Gaussian noise environment. By use of the multipath simulator experimental optimization of the PLL under mul- tipath conditions can be achived; theoretical results can thus be verified and modified as required. In the second category falls various other more sophisticated methods of estimating the phase of the transmitted carrier.The rate of deep fades obtainable with this multipath simulator is typically 5 to 10 fades per sec. Frequently the fading rate of the channel to simulated is outside this range. If this is the case, frequency and time scaling can be used to bring the problem within the range of the simulator.This scaling is accomplished in the follow- ing manner. Let the ratio between the simu- lator and actual fading rates be K. In order to bring the problem within the range of the simulator all subcarrier frequencies must be multiplied by K. The value of all capacitors and inductors used in the signal circuits must be divided by K. The resulting system will yield similar performance when used on the simulator as the original circuit willgive on the actual channel. The only dif- ference is that the time scale must be stretched by _. Nearly any type of multi-Kpath channels can thus be simulated by suit- able reflectors, air bubbles and use of time and frequency scaling.TestsInvestigation of various simulated channels have been carried out during the time since in Fig. 4. The two most interesting channel conditions are the one where the transmitted signal, is reflected by a large number of incoherent scatters and the case where there is a strong -specular component besides the incoherent scatter. These two types of channels are representative ofconditions encountered during poweredflight.When all the received signal is reflected by incoherent scattering one experiences, as one would expect, the most serious fading conditions. Fig. 5 shows a typical spectrum of the amplitude of the received signal. We see that the amplitude spectrum has a width of about 30 Hz at the - 25 DB lavel. At the - 6 DB level it has a width of about 10 Hz. From Fig.6 we see that this agrees well with the time records of the amplitude variations which show about 10 deep and 30 minor fades per sec.Statistical analysis of the envelope and phase of the received signal has been carried out. This analysis shows that when the received signal is due to a large number of random scatters the probability density of the received carrier amplitude is approximately Rayleigh while the phase is uniformly distributed over the range 1 £. When the phase variations are measured over the range ± 4ir the resulting measured probability distrivution is approximately triangular as seen from Fig. 8.When the received signal has a strong specular component beside the random reflections the power spectrum shows a steady signal at the carrier frequency. The part of the spectrum due to incoherent scattering is essentially unchanged. The probability of amplitude of the received carrier follows approximately the Rice distribution as can be seen from Fig. 9.The probability density function for 'the phase shows the most interesting difference between the two simulated channels. It is now triangular but with a range * TT rather than ± 4ir. This result can ^ be seen from Fig. 10.Conclus ionsA multipath simulator has been describedthat is readily constructed and flexiblein operation. The multipath characteristicsof the channel can be varied over a widerange of conditions allowing simulation ofnearly all desired types of multipath channels,Since the transmitter and receiver are located70close together, reference and synchronization signals required at the receiver can, if de- sired, be obtained by cables directly from the transmitter. Difficulties with the local reference signals can thus be avoided, allowing a more exact investigation of the effects of multipath condition on the transmitted signal and on the operation of the receiver.AcknowledgementThe author wishes to express his appreciation to Dean T.L. Martin and Dr. J.L. Hummer, System Director of GENESYS, for providing the facilites used in this project, and further appreciation to Dr. J.V. Wait for designing and operating the hybrid statistical analyzis and to W.R. Nyren for building the equipment used in this experiment .71IX \\\\\\\\\\\\\\MSound absorbent material l\\\\\\\\\\\ V1TankFig. 1 Multipath channel simulatorl\ \\\\\\\\\\\\\\\lS3 \\mm\^y Air bubbles^ O\\1 \\X\\\\\\\\ VIT?i%. 2 Multipath channel where the received signal is due to a large number of incoherent scatterers.72S]\\•••* * •"••.* S p e c u 1 a rX^^\ reflectorl\\\\\\\\babbleso[Ty|Fig. 3 Multipath channel with a strong specular signal component in addition to the randomly scattered "> i gnal .200 kH zAmpI IEnvelope Demod1 ' \Prod Demuct od1 ''''<'/ /\*,';-;-— s v Y/ //////i VA — -4—1 ^ s H^^ J JTransI^ ,200 kH^Os cPhaseDemod^ SubcarrierGenW-iEnvelope Subcarrier PhaseFig. A. Block diagram of the GENESYS roultipath channel simulatorsy s t em *73-10-20-.-23---30 -10 0 10 30 HzFig. 5 Spectrum of the envelope of the fading carrier. Incoherently scattered signa.Fig. 6 Amplitude record of the fading carrier. Incoherent scatter* 20 mm/sec.74-2 -1Fig. 7 Probability density of the envelope of the fading signal. Incoherent scattering.P(e)-Air 0Fig. 8 Probability density of the phase of the fading carrier. Incoherent Scattering.75P (v)0Fig. 9 Probability density of the envelope of the fading carrier. Soecular ^n<* incoherent reflections.• ifI0Fig* 10 Probability density of the phase of the fading carrier* Specular and incoherent, reflections.76

A Multipath Simulator for use in Evaluation of Spacecraft Ranging and Communication Systems

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A Multipath Simulator for use in Evaluation of Spacecraft Ranging and Communication Systems

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