Technical English 09课件.ppt

Technical English;Unit Nine;概述;time slot;使许多用户同时使用同一个固定的无线电频带 ;FDMA, TDMA and CDMA are the three major methods of sharing the available bandwidth to multiple users in wireless system. There are many extensions, and hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA systems. However, an understanding of the three major methods is required for understanding of any extensions to these methods. ;In Frequency Division Multiple Access (FDMA), the available bandwidth is subdivided into a number of narrower bands. Each user is allocated a unique frequency band in which to transmit and receive. During a call, no other user can use the same frequency band. Each user is allocated a forward link channel (from the base station to the mobile phone) and a reverse channel (back to the base station), each being a single way link. ;每个信道只支持一个用户 ;Unit 9;待发送的输入数据在前一帧期间被缓存，在分配给该信道的时隙中以较高速率爆发式发送出去。 ;TDMA不能直接传送模拟信号因为它需要使用缓冲 ;TDMA is normally used in conjunction with FDMA to subdivide the total available bandwidth into several channels. This is done to reduce the number of users per channel allowing a lower data rate to be used. This helps reduce the effect of delay spread on the transmission. ;将基于FDMA的各信道进一步用TDMA划分，从而多个用户可以在同一信道上发送信号 ;窄带的消息（典型的是数字话音）被乘以一个宽带的伪随机噪声（PN码）信号 ;发射的信号通过与发送者用的同样PN码相关而恢复出来。 ;当时研究人员受到激励以寻求安全和能够在干扰中正常工作的通信方式。 ;For many years, spread spectrum technology was considered solely for military applications. However, with rapid developments in LSI and VLSI designs, commercial systems are starting to be used. ;系统处理增益是指扩频系统通过扩频和反扩频的性质所表现出来的增益或信噪比的提高。 ;其中BWRF是数据扩展以后的发射带宽，BWinfo是所发送信息数据的带宽 ;在本例中处理增益为125因为扩频带宽是数据带宽的125倍。 ;它包括要求的信号加上背景噪声，以及其他CDMA用户或无线电信号源的干扰。 ;也可以通过与信号相乘得到，只要数据和PN序列都用1和?1表示而不是1和0。 ;短的PN码（典型长度10～128码片）可用于调制每一个数据比特。 ;所以它们有益于增加安全性，因为更难以解码，;CDMA系统中从基站到移动电话的前向链路可以使用称为Walsh码的特殊正交码来将同一信道的多用户分开。 ;It is generated from the basis that Walsh(1) = W1 = 0 and that: where Wn is the Walsh matrix of dimension n. For example: ;这是因为任何两行之间都有一半的比特相同，另一半不同。 ;Un