The authors discuss several designs for self-steering array combiners together with a simple frequency-offset tagging method which permits locking of combiner to one of several cochannel but spatially separate signals. The combiners use the output signal as a reference for extraction of phase information from each individual element-signal, thereby utilizing the full transmitted signal for element phase measurement. Designed for receiving PSK signals, the combiner inherently provides, as part of its design, the CW signal required for coherent demodulation of the combined array output. Laboratory measurements made with a four-branch combiner for two-phase PSK signals demonstrate the salient features of the combiner.

We present block diagram designs for the baseband processing modules required to implement high-speed (600 Mbit/s) burst modems for use in TDMA satellite systems. Our main concern is to obtain designs which can be implemented with a minimum of high-speed (ECL) devices so as to reduce design and hardware costs. A major consideration in the modem operation is to accurately synchronize it to a reference frame-marker broadcast by a master earth station. Since frame mark recovery must be done at the full serial bit rate, the proposed design relies on the periodic nature of the frame-marker occurrences to enable utilization of short unique words and simple highSpeed electronics to provide extremely reliable marker recovery with virtually no false alarms (or missing markers), even on very noisy channels. Estimates of false alarm rates and the mean time to lock are given. Considerations of clock stability indicate that short-term (150 ms) instabilities must be on the order of a part in 107in order to maintain proper operation of the synchronization circuitry; such stability also minimizes guard time requirements between bursts from the several ground stations sharing a given transponder.

We discuss a possible method for interference control in radio systems whose desired signal is analog FM. The method utilizes the fact that the desired signal has a nearly constant envelope; with interference present, the composite envelope becomes noisy. A controlling processor measures this additional envelope noise, and iteratively adjusts the complex combining gain of an auxiliary signal to minimize the interference. The expected residual interference remaining after minimization is proportional to the reciprocal of the square of the rms envelope fluctuation of the desired signal. This result was confirmed in a laboratory simulation of an intersecting radio route case using both large and small index FM. Beginning with signal-to-interference ratios of 30 to 40 dB, the controller increased these numbers to about 60 dB, the expected theoretical limit for a signal with 0.1 percent rms envelope fluctuation.

We describe exploratory work designed to demonstrate the feasibility of directly interfacing terrestrial digital carrier systems to digital satellite systems. Designed to accept up to 30 T1 carrier systems on the terrestrial side, the interface is designed as a common-control, bus-oriented time-division switch using a common time-slot interchanger for simultaneous switching of up- and down-links. The output bit rate is about 50 Mbits/s. The controller is implemented as two Motorola M6800 microcomputer chips and associated memory. The switch can communicate with a variety of terrestrial originating switches, including those using either register-sender or cut-through operation. By proper software, a simultaneous mix of such originating switches can be handled, and either delay-start dial, wink-start or dialtone start outpulse control can be used in either direction of call setup (i.e., to or from a terrestrial switch connected to the satellite earth station).

Using simulated Rayleigh-fading FM signals, the baseband performance of a diversity system that uses a separate pilot is studied. The effect of various RF/audio bandwidth expansion ratios is included. These results show an approach to a constant asymptotic baseband SNR for large expansion ratios, with diversity increasing this asymptote by 5 dB in the case of moderate to large interference and/or RF noise levels (9 to 15 dB below the carrier), to over 10 dB in the case of lower impairment levels (21 to 27 dB below the signal level). Assuming the usual hex-cell radio coverage plan, and a propagation law of the formr^{-n}, n = 3or 4, bandwidth expansion ratios of about three to six seem to yield the most efficient RF spectrum utilization in many practical situations. The number of required channel subsets (cells) is strongly influenced by the propagation law ranging from about 19 forn = 3, to about 7 forn = 4.

Modeling the occurrence times as a Poisson process, the spectrum of a carrier frequency modulated by a random pulse train is studied. In general, the corresponding phase modulation is nonstationary, but the modulated carrier is shown to be wide-sense stationary, so that its RF spectrum can be obtained as the Fourier transform of its correlation function. Closed-form asymptotic results show that RF spectrum is proportional to ω-4if the frequency modulation is discontinuous, and as ω-6if it is continuous, where ω is the radian frequency measured from the carrier frequency. Numerically evaluated spectra are obtained for the particular case of a stationary tone-burst modulation, a form used in a proposed mobile-radio feedback diversity system. These results show that even for modulation indices as small as 0.5, the spectrum of the tone bursts themselves is not a good approximation to the RF spectrum. Finally, the spectral broadening caused by adding tone-burst modulation to a carrier already modulated by a flat-band Gaussian process is evaluated numerically. These results show that the modulation index of the bursts should be kept to unity or less in order to restrict the spectral broadening to 10 percent or less if the rms phase deviation of the Gaussian modulation is less than three radians.

Predictions of the response probability density function for bandlimited white noise excitation of nonlinear systems often employ the so-called Fokker-Planck technique. While these predictions are exact for white noise, there exist no general criteria for assessing their accuracy in physical situations. This paper endeavors to shed some light on the problem and perhaps point the way toward better understanding of the inherent difficulties involved. First, a brief discussion of more typical response functions for a first-order nonlinear system is presented on the basis of a very narrow-band low-pass excitation and on a very wide-band low-pass excitation spectrum. Such limiting cases generally exhibit substantially different behavior. Next, some experimental results for the same system driven by variable-bandwidth low-pass Gaussian noise is presented. These results indicate substantial deviation from the Fokker-Planck predictions when the excitation bandwidth is comparable with an equivalent system bandwidth. Moreover, it was found that the data at high response levels were bounded by the Fokker-Planck predictions on one hand and by a prediction based on a pointwise transformation of an extremely low-pass excitation process on the other. Bandwidths ranging between three and fifteen times the system's equivalent bandwidth are required for validity of the Fokker-Planck approximation depending on the nature of the non-linearity and the response level in question.