Shaul Laufer

Chaotic signals for multiple access communications

Implementation of chaotic signals and of chaotic sequences in Code Division Multiple Access (CDMA) communication systems is considered. The performance of such CDMA direct sequence spread spectrum systems is analyzed based on the cross correlation between the chaotic signals (sequences). The cross correlation is found to be between the upper bounds obtained for m-sequences and for Gold sequences. Receiver architectures which exploits the self synchronization feature of chaotic systems are proposed, for a single user receiver and for a two users receiver. In a two users network, with negligible background noise, the performance of the self synchronized two users receiver is superior to the performance of the conventional single user receiver.

Feedforward multiple access satellite communications

Satellite communications modeled by a feedforward random multiple-access system are considered. The transmission times of the users are governed by protocol signals. The authors consider the model of a slot-synchronized collision channel without feedback, proposed by J.L. Massey and P. Mathys (1985), augmented by interleavers that are essential for enabling the use of practical short codes. They assess the throughput of the channel on the basis of a short-coding error parameter. The set of parameters of all users characterizes the rate region achievable by employing short block codes with nonideal interleaving. The short-coding error parameter corresponds to a bound on the probability of error which is tighter than the bound expressed with the aid of the conventional cut-off rate. The analysis takes into account practical communication aspects, such as the effect of interleaving span and noise in noninterfered slots. >

Convolutionally coded frequency-hopping communications with nonideal interleaving

The interleaving span of coded frequency-hopped (FH) systems is often constrained to be smaller than the decoder memory length, i.e. nonideal interleaving is performed. An upper bound on the performance of a Viterbi decoder of a convolutional code with nonideal interleaving is presented. A soft decision diversity combining technique is introduced, and the performance of combined convolutional and diversity coding subject to worst-case partial band noise jamming is investigated. Optimization of the FH system performance subject to constraints of allowed delay and synthesizer settling time provides the best combination of interleaving span and hopping rate. The FH system considered employs M-ary frequency-shift key (MFSK) modulation and noncoherent demodulation with 2-b soft decision based on Viterbis ratio-threshold technique. >

Short coding for the collision channel without feedback

Multiaccess communications over a slot-synchronized collision channel without feedback is studied. Collision immunity is gained by employment of short codes, followed by periodic non-ideal interleaving. The authors examine the achievable rate region and throughput in terms of both the capacity and the short-coding error parameter (SCEP), In case the channel utilization factor does not exceed 1, the throughputs are shown to be lower bounded by positive numbers, which are also the limit values for infinitely many users. The optimal value of the channel utilization factor is found to be less than 1. The throughput of a network that employs a family of RS codes of fixed length is found to be in accordance with the SCEP-based throughput.<<ETX>>

A short-coding error parameter for multi-user communications

Multiaccess communications, governed by protocol signals, over a slot-synchronized collision channel without feedback, is considered. More specifically, the authors consider the channel model proposed by Massey and Mathys (see IEEE Trans. Inf. Theory, vol.IT-31, no.2, p.192-204, 1985), augmented by interleavers that are essential for enabling the usage of practical short codes. They assess the performance on the basis of a (so called) short-coding error parameter. The set of the parameters of all users characterizes the rate region achievable by using short block codes with nonideal interleaving. The short-coding error parameter corresponds to a bound on the probability of error which is tighter than the bound obtained by using the conventional cutoff rate. The analysis brings into account practical communication aspects, such as the effect of interleaving span and background noise in noninterfered slots.<<ETX>>

On the Coding Channel Capacity and Cutoff Rate of Frequency-hopping communications with non-ideal interleaving

Frequency-hopping communications over channels with partial band noise jamming, both with and without side information, is considered. The capacity and the cutoff rate are calculated both for the case of ideal interleaving and the case of non-ideal interleaving. It is shown that both the capacity and the cutoff rate decrease with an increase of the interleaving span. When side information is not available, the capacity in the case of non-ideally interleaved channel approaches monotonically the capacity of the ideally interleaved channel as the interleaving span tends to infinity, whereas the cutoff rate behaves in an opposite manner.