A study on asynchronous CDMA systems using analog PN signals produced by code-orthogonalizing filters

码分多址（CDMA）由于其高容量、抗干扰能力强等优点，正被作为未来几代的主要系统而研究。在异步直接序列（DS）/CDMA中，每个多址用户在公共信道上使用单独的伪噪声（PN）序列进行通信。当期望用户序列与其它用户序列之间的互相关引起多址干扰时，系统的误比特率（BER）性能会下降，本文提出了一种新的异步DS/CDMA通信系统。模拟PN序列由称为“码正交化滤波器”的自适应滤波器产生。在基站接收机中，可以自适应地控制COF的抽头系数，以“接近正交化”各种接收的PN序列。模拟PN序列的元素由COF的抽头系数组成，产生的模拟PN序列按顺序分配给每个用户的发射机。首先，在AWGN环境下，选择合适的参数（COF中LMS算法的步长和收敛判决准则，以及模拟PN序列产生过程中的限幅电平），可以获得良好的误码性能;其次，讨论了COF的快速收敛性质。第三，作为实际应用，通过计算机仿真，给出了瑞利衰落环境下系统性能的几个特点。接下来，讨论当模拟PN序列被发送到用户时量化误差的影响。此外，考虑到移动的，对模拟PN序列的产生方法进行了改进。最后，我们评估的影响，考虑多址干扰从邻近的细胞所提出的系统。

Code division multiple access (CDMA) is now being studied as the major system for future generations because of its high capacity, robustness against interference, and so on. ln asynchronous direct sequence (DS)/CDMA, each multiple access user communicates by using an individual pseudo noise (PN) sequence over a common channel. When multiple access interference (MM) arises from cross correlation between the desired user's sequence and other users' sequences, bit error rate (BER) performance will be degraded.This paper proposes a novel asynchronous DS/CDMA communication system using analog PN sequences that have an orthogonal relation for all active users. Analog PN sequences are produced by an adaptive filter called "a code-orthogonalizing filter (COF)'. In a base station receiver, the tap coefficients of the COF can be adaptively controlled "to approach to orthogonalize" various received PN sequences. The elements of the analog PN sequences consist of the tap coefficients of the COF.The analog PN sequence produced is assigned to the transmitter of each user in order. As a result, MAI caused by other users can be reduced considerably.First, good BER properties can be obtained for appropriate parameters (step size and convergence decision criterion of LMS algorithm in COF, and amplitude limiting level in the process of producing the analog PN sequence) in an AWGN environment. Second, fast convergence properties of COF are discussed. Third, as practical applications, several characteristics of the system performance in a Rayleigh fading environment are shown by computer simulations. Next, an influence of quantization error is discussed when the analog PN sequences are transmitted to users. Further, we modify the method of producing analog PN sequences, considering the moving of mobile. Finally, we evaluate the effect considering MAI from the neighboring cell by the proposed system.