It is shown that, when the input signal-to-interference ratio (SIR) is small, a biased nonlinearity that has a dead zone below some threshold value can provide a large enhancement in output SIR and signal-to-interference-plus-intermodulation ratio (SIIMR) if the threshold is set equal or close to the amplitude of the strong interference. In the absence of noise, the output SIR and SIIMIR are virtually independent of the input SIR, however low the latter may be. It Is shown that, for a dead-zone limiter, the output SIIMR in this case is -4.93 dB regardless of the input SIR. Under strong interference, any noise present in the input reduces the SIR and SIIMR improvement, but a biased nonlinearity can still provide an output SIR and SIIMR superior to the input SIR, i.e. an output SIIMR 6.35 dB below the input signal-to-noise ratio (SNR) instead of 6.02 dB below the input SIR as is the case with hard limiting.>

Expressions for the cutoff rate of memoryless channels and certain channels with memory are derived assuming decoding with approximate channel statistics. For channels with memory, two different decoding techniques are examined: conventional decoders in conjunction with ideal interleaving/deinterleaving, and maximum likelihood decoders that take advantage of the channel memory. As a practical case of interest, the cutoff rate for the band-limited nonlinear satellite channel is evaluated where the modulation is assumed to be M-ary phase shift keying (MPSK). The channel nonlinearity is introduced by a limiter in cascade with a traveling wave tube amplifier (TWTA) at the satellite repeater while the channel memory is created by channel filters in the transmission path.

The Military Satellite Communications Systems Office (MSO) of the Defense Communications Agency has recently prepared an architectural framework for the evolution of military satellite communications systems through the end of this century. The realization of the goals of the architecture should provide significantly enhanced military utility to three functional classes of users. The strategic forces would be directed and managed through a two-way, highly survivable, jam-resistant Strategic Satellite System (SSS). The Tactical Satellite System (TACSATCOM II) offers a jam-resistant, low probability of detection, communications system for employment of ground mobile forces as well as tactical Naval and Air Forces. The future Defense Satellite Communications System (DSCS III U/IV) will provide the high data-rate users with increased jam resistance to perform their missions. These satellite systems will use higher frequency bands (EHF), on-board processing, and improved antenna techniques. To implement the EHF technology in a coherent fashion, the MSO has also prepared a Technology Development Program Plan which identifies and plans for the needed R&D efforts.

As technology advances, satellite communications traffic is required to support both civil and military, current, and projected needs. These needs can sometimes be better satisfied if the communication system operates in the EHF band as opposed to the SHF or lower frequency bands. This paper addresses those factors that are of considerable interest to the system designer. In particular, the candidate frequency bands are placed in evidence; then the effects of propagation losses, orbit parameters, terminal antenna gain limitations, EIRP, and low probability of intercept are considered.

It is shown that many signal-detection problems involving binary phase and frequency modulation can be solved by considering the problem as the convolution of two independent phase distributions. When the two distributions are similar and each represents the probability density function of the phase of a sinusoid in noise, the probability of error is described in terms of modified Bessel functions by the expression obtained recently by Jain and Blachman. The probability of error can also be expressed in terms of Rice'sIefunction. By appropriate definition of the signal-to-noise ratios, this general expression can be used to determine the error probability in a number of cases of practical interest, such as detection of a hard-limited phase-shift keyed (PSK) signal, PSK detection with noisy reference, DPSK detection, frequency-shift keying (FSK) detection, binary FM with discriminator detection, and binary pulse-position modulation (PPM).

This paper considers the problem of the detection of a binary phase-modulated carrier which has been transmitted along with noise through a hard-limiting repeater, corrupted by additional noise, and demodulated by cross correlation and sampling at the receiver. Three equivalent expressions are obtained for the error probability of the receiver output. Two of these expressions take the form of an infinite series involving either confluent hypergeometric functions or modified Bessel functions. A third form allows representation of the error probability in terms of Rice'sIefunction. In the special case where the signal-to-noise ratios\rho_1^2and\rho_2^2at the limiter and receiver inputs are equal, the error probability is simply \begin{equation} P_e = \frac{1}{2} \exp (-p_1^2). \end{equation} Asymptotic expressions for the error probability for large and small signal-to-noise ratios are also derived. The error probability is found to be smaller than that of a linear system for all practical values of limiter and receiver input signal-to-noise ratios. The optimum repeater non-linearity is investigated and is shown to be a limiter at such signal-to-noise ratios.

The effect of ideal bandpass limiting on a signal lying in narrowband Gaussian noise is analyzed. General analytic expressions for the limiter output components are derived using an integral representation for the limiter characteristic. This method allows retention of the phases of all the signals and the intermodulation products at the limiter output, which are destroyed in the characteristic-function method generally used in limiter studies. Expressions for the desired signals and intermodulation product amplitudes are obtained for the case when the limiter input consists of three angle-modulated sinusoids and noise. The analysis is extended tonmodulated sinusoids plus noise, and approximate expressions for the signal, intermodulation product, and noise terms are derived. Numerical results are presented for the signal suppression and the limiter output signal amplitudes for the case of three input signals, two of equal amplitude.