Asynchronous DS/CDMA systems employing spreading codes generated by chaos maps with Markovity

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In DS-CDMA communications, linear feedback shift register (LFSR) sequences, such as Gold and Kasami codes, are employed as spread spectrum (SS) codes. In mobile cellar systems, propagation delay is randomly spread for uplink channels and synchronization timing error cannot be eliminated. Hence, we have evaluated the chip-asynchronous systems. Our results are summarized as follows 1) Chip-asynchronous version of total square correlation (CA-TSC) was defined as the total value of all users' interference and its lower bound, named chip-asynchronous version of Welch bound, was presented. Mutual interference depends on spreading codes and chip waveforms. They are normally designed independently but joint optimization of them should give better performance. We relaxed the Nyquist condition, which guarantees zero inter symbol interference and is often assumed to be satisfied, to minimize total interference. We showed that the pair of Gaussian pulse and Markov codes give better performance than the conventional pair of raised cosine pulse and LFSR sequences, where both pulses are adjusted to have the same excess-band energies. 2) Based on a posteriori probability calculations, we proposed a new code acquisition algorithm. The proposed method, called "counting method", is very simple and effective; it counts only the number of events that receiver outputs exceed the pre-determined threshold and it realizes faster code acquisition than the serial search. More importantly, this method guarantees the maximum acquisition error rate. 3) Close relation between the associate memory model of neural networks and CDMA system is investigated. Associative memory corresponding to chip-asynchronous CDMA system was proposed, which was shown to memorize Markov patterns better than i.i.d. patterns