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

在DS-CDMA通信中，线性反馈移位寄存器（LFSR）序列，如Gold和Kasami码，被用作扩频（SS）码。在移动的蜂窝系统中，上行链路信道的传播延迟是随机扩展的，并且同步定时误差不能被消除。因此，我们评估了芯片异步系统。1）将码片异步总平方相关（CA-TSC）定义为所有用户干扰的总和，并给出其下界，即码片异步Welch界。相互干扰取决于扩频码和码片波形。它们通常是独立设计的，但它们的联合优化应该会提供更好的性能。我们放宽了奈奎斯特条件，该条件保证零符号间干扰，并且通常假设满足该条件，以最小化总干扰。我们表明，高斯脉冲和马尔可夫码的对比传统的对升余弦脉冲和LFSR序列，其中两个脉冲被调整为具有相同的过剩带能量提供更好的性能。2)基于后验概率计算，提出了一种新的码捕获算法。所提出的方法称为“计数法”，简单有效，只对接收机输出超过预定阈值的事件数进行计数，实现了比串行搜索更快的码捕获。更重要的是，这种方法保证了最大的捕获错误率。3)研究了神经网络的联想记忆模型与CDMA系统的密切关系。针对码片异步CDMA系统提出了一种联想记忆算法，该算法比独立同分布算法具有更好的马尔可夫模式记忆能力。模式

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