Direct-sequence spread-spectrum (DS-SS) transmission are difficult to intercept. The theoretical analysis of intercept performance against such systems has always been a difficult task if the noise samples are correlated. Thus, many results are purely experimental and lack a generality which can only be achieved from theoretical considerations. The target system described is a DS-SS communications system operating environment which consists of additive white Gaussian noise and continuous wave interference. The interceptor uses a radiometer in conjunction with interference excision techniques to detect the presence of the target signal. It is shown that much of the interceptor's effectiveness is restored by the interference excision.>

The transmitted data rate is a function of the channel state and is varied in such a way as to keep the probability of bit error approximately constant. The performance measure of the system is the probability of successful message completion in a given time span T/sub D/. Optimum operational signal-to-noise ratios are found as well as practical limits on the maximum and minimum transmission data rates. It is shown that the adaptive data rate feature provides a significant improvement in system performance as compared to a system transmitting at a fixed data rate. The performance improvement which can be obtained by the use of forward error correction coding is also analyzed. The codes considered are Reed-Solomon codes with rates of 1/3, 1/2, and 2/3. A much simpler expression for the probability of successful completion of a message is derived and used in the optimization search.>

An analysis is presented of a BPSK-PN (binary phase shift-keyed with pseudonoise) demodulator when present narrowband interference is attenuated using transform domain filtering. The signal of interest is a wideband direct-sequence spread-spectrum signal (DS-SS). The interference is a narrowband signal which may be of high power and which is either intentionally or unintentionally collocated in frequency and interfering with the signal of interest. The effect of windowing the input signal by a rectangular or Hamming windows as well as the use of an overlap and save scheme is presented. It is shown that the Hamming window far outperforms the rectangular window, and that the interference is for all purposes completely eliminated when the Hamming window is used in conjunction with the overlap and save scheme. The criterion used to evaluate the effectiveness of the different interference rejection techniques is the bit error rate (BER) as measured at the output of the demodulator.>

In this paper, we briefly describe the physical characteristics of a meteor-burst (MB) communication channel, and then present the results of an analysis showing the expected throughput and the probability of completing a given message under two different scenarios. In the first scenario, the transmitter is presumed to have knowledge (obtained by appropriate probe signals) of the entire duration of a given MB. In the second, only the starting time of each MB is known. Numerical results based on the analysis are provided, and it is shown what types of delays must be tolerated in order to achieve a given degree of reliability of having the transmitted message correctly received at its destination.

In this paper, the results of comparing the performance of various forward error correction techniques and several modulation formats when used over a nonselective Rayleigh fading channel in the presence of a pulse-burst jammer are presented. Both binary and nonbinary codes are considered, as well as concatenated codes consisting of either a block or a convolutional inner code and a Reed-Solomon outer code. Finally, the use of side information to allow the decoding of both erasures and errors is also analyzed.

This paper describes a rapid acquisition technique which can be used in direct sequence spread-spectrum systems. It is sequential in nature and is based on a double threshold which defines when a decision can be made. The values of the thresholds change at every examination instant. The procedure makes use of a bound to the partial crosscorrelation function of PN sequences which ensures that only the sequence length needs to be given as far as the PN sequence particulars are concerned. A decision is made only if the probability of error associated with a decision at that point is equal to or better than a prespecified value. Finally, it is shown that most of the computational difficulties associated with the bound as given by a recursive algorithm can be eliminated when using a low-order polynomial approximation to the bound. However, either by using the bound or its polynomial approximation, a significant improvement in the acquisition time is obtained.

The Chernoff bound is used to upper bound the average probability of error of a direct sequence spread spectrum communication system under the conditions of multiple-tone interference. A special case is analyzed using a Gaussian approximation to the interference, and this latter result is compared to that predicted by the Chernoff bound. The limitations of the Gaussian approximation are then carefully explained.

This paper presents the results of a study in data compression of adaptive delta modulated video signals. The Song mode ADM is first investigated at a sampling rate of 16 Mbits/s and shown not to produce enough redundancy to warrant entropy encoding. We then present a modified adaptive delta modulator algorithm that operates at a sampling rate of 16 Mbits/s and does produce sufficient redundancy to yield a 40-50 percent data compression by using a simple code on 4 bit data blocks. Other techniques such as field interpolation and direct substitution are shown to increase the possible data compression further without noticeable degradation in the two input images used in this investigation. The effects of channel errors in the transmission of packet video over a computer network are considered. A leaky integrator is used to reduce the effects of channel errors in the data bits. We show that the effects of channel errors can be reduced by field interpolating those packets that can be shown to contain errors.