Mobile ground-based terminals observe significant Doppler on the forward channel when communicating through low Earth orbit (LEO) satellites. This paper deals with the analytic derivation of the Doppler shift measured by a user on the surface of Earth on a signal transmitted by a circular orbit LEO satellite. Two simplifications are performed to obtain the analytical expression of the Doppler shift as a function of time. First, during the visibility duration of the satellite at a terminal, the trajectory of the satellite with respect to the Earth is approximated by a great circle arc. Second, the angular velocity of the satellite with respect to the user is assumed to be constant. Numerical results validate the approximations. Another result of our analysis is an expression for the visibility window duration of a satellite at a terminal as a function of the maximum elevation angle. An algorithm for estimating the parameters of the Doppler curve based on a couple of Doppler and Doppler-rate measurements is also presented.

This paper presents a concept under development, that of dynamically coloring the spectrum of a pseudorandom spreading code in order to aid slow rate data communication in a very poor channel. The method seems to hold promise for some difficult industrial communications problems, especially for wireless random access communications in time varying noisy channels that are difficult to effectively model and are resistant to conventional approaches.

The performance of a countermeasure technique in the presence of the optimal follower multitone jammer is evaluated for frequency-hopped spread spectrum (FHSS) communications. It is shown that, with a certain probability, the optimal jammer will have dual tones in a frequency channel.

A method for optical signaling is investigated that allows multiple transmitters to simultaneously use the same noncoherent receiving aperture. The signaling is based on the properties of laser speckle interference patterns. Capacity for an interference limited system is explored for a correlation receiver based on random coding. A synchronization scheme is presented that allows a practical implementation of the proposed optical multiple access system.>

A countermeasure technique to a follower tone-jammer is evaluated for frequency-hopped spread spectrum communications. This technique utilizes randomized decisions by the transmitter and the receiver to lure the jammer into helping the communicator's performance part of the time. It is shown that the proposed technique reduces the bit error rate of the system.>

We present three unconventional approaches to keying variable management. The first approach is based on using a public key cryptosystem (PKC) that is breakable in short, but on average less, time than it takes to set up an ultrawide bandwidth modem that is then used to transport a keying variable for a classical cryptosystem. The second concept proposes using the characteristics of an urban UHF radio channel, determined by mutual sounding, as the cryptovariable. The third concept encourages research into ill-conditioned problems as potentially fruitful ground for PKCs not based on finite field arithmetic.>

A countermeasure to a partial-band follower jammer is proposed for frequency-hopped spread-spectrum communications. This technique randomizes the transmission technique used by the transmitter (and receiver). Either the information is carried by M tones which are transmitted in a frequency slot, or by M frequency slots which contain signal energy. As a counter-countermeasures, the jammer randomizes between jamming the same frequency slot being used by the communicator, or jamming a subset of the slots not being used by the communicator. The performance for randomized strategies for the communicator and jammer is investigated. It is shown that the proposed technique enhances the system's performance.>

The authors present a method for transporting data through very noisy channels such as those encountered in electronic warfare. This method is based on two very important concepts. One of these is multiplexing via Boolean functions and the other is code combining. This technique spreads synchronization information throughout the message. The multiplexed synchronization technique is costly. The amount of traffic that must be sent per message is often quite large. The technique is robust, however, and may be of value for countering severe random fades and the severe and nonstationary interference processes encountered in electronic warfare.>

It has been shown that an L/sub p/ normed solution to linear prediction equations may be advantageous in certain time-series modeling applications. Specifically, in the context of sinusoidal frequency estimation from data contaminated by impulsive noise, choosing p=1 may result in less biased frequency estimates. Computing L/sub p/ normed solutions requires iterative methods that are computationally expensive. By plotting prediction error filter root migration in the complex z-plane as a function of iteration step one is able to construct a suboptimal weighted least-squares solution. This solution, though not an L/sub p/ normed solution, is still robust against impulsive noise and is not as computationally expensive as iterating to a true L/sub p/ normed solution.>

Further properties of the autocorrelation coefficients of the Thue-Morse sequence are developed which are then used to develop a novel, robust synchronization method. Its behavior is simulated over a highly degraded binary symmetric channel and uses for the synchronization method are proposed. The synchronization method has the potential to maintain synchronization at bit error rates up to 10%.>

The Thue-Morse sequence (TMS) has been proposed as a vehicle for effecting comma-free synchronization. The purpose of this correspondence is twofold. First, we present and examine an expression which describes the TMS power spectral density. Second, we consider the autocorrelation of the TMS sequence and cast it as a random process.

The Thue-Morse sequence (TMS), the "parity check" sequence formed on a normal binary counter, has been proposed as a vehicle for comma-free synchronizatin of binary counters. This note reports further work on the TMS as a vehicle for commafree synchronization of binary counters. Specifically, we present a new synchronization algorithm and a way to implement the algorithm via PSK encoding/DPSK decoding, and we examine and comment on the TMS/BPSK spectrum.

The generation of high-speed maximal length "M"-sequences is at the heart of many diverse techniques, notably code-division multiple access communications systems, or spectrum spreading, and precise ranging disciplines. This paper develops a rich family of M-sequences whose upper speed bound is set by the fastest shift register stage available in the technology under consideration and not by logic gate delays.

The structure of the "parity check" sequence formed on a binary counter has been well studied and is known as a Thue-Morse sequence [1]-[4]. It is shown that this sequence can be used in a simple synchronization scheme that can be engineered with a minimum of hard-ware.