通信工程文献翻译——在数字通信系统中，需要将输入的数字序列映射为信号波形在信道中传输，此时信源输出数字序列，经过信号映射后成为适于信道传输的数字调制信号。数字序列中每个数字产生的时间间隔称为码元间隔，单位时间内产生的符号数称为符号速率，它反映了数字符号产生的快慢速度。由于数字符号是按码元间隔不断产生的，经过将数字符号一一映射为相应的信号波形后，就形成了数字调制信号。根据映射后信号的频谱特性，可以分成基带信号和频带信号。通常基带信号指信号的频谱为低通型，而频带信号的频谱为带通型的。由消息转换过来的原始信号所固有的频带称为基本频带，简称“基带”。 来自数据终端的原始数据信号，其频谱一般是从零开始的，往往包含丰富的低频成分，甚至直流分量，故称之为数字基带信号。用数字基带信号直接进行传输，即不搬移基带信号的频谱或只经过简单的码型变换就直接进行传输的方式称为数字基带传输。基带信号所包含的频率范围很宽，可以从直流到高频，所以对用于传输基带信号的信道是有限制的。在某些具有低通特性的有限信道，传输距离不太远的情况下，可以进行数字基带信号直接传输。而对大多数信道，数字基带信号必须经过载波调制，把频谱搬移到较高的载频处才能在信道中传输，这种传输方式称为数字频带传输。

数字基带传输系统的基本结构主要由基带信号形成器，信道，接收滤波器，同步提取电路和抽样判决器组成。基带信号形成器的作用是将原始的数字信号变换成适合于信道传输的数字基带信号，这种变换主要是通过码型变换和波形变换来实现的，其目的是与信道匹配，便于传输，减少码间串扰，利于同步提取和抽样判决。信道是信号通过的媒介，其传输特性......

In digital communication systems, digital mapping needs to be input for the signal transmission in the channel, the source output digital sequence, after the signal mapping into digital modulation signal suitable for transmission channel. Each digital sequence of time intervals called a symbol interval, number of symbols are produced per unit time is called the symbol rate, which reflects the digital symbol generating speed. Because the digital symbol by symbol interval continuously produces, through digital symbol one one is mapped to the corresponding waveform, on the formation of the digital modulation signal. According to the spectrum characteristics of signal mapping, can be divided into baseband signal and the frequency band signal. Usually the baseband signal to the frequency spectrum of the signal is low-pass type, and frequency band signal is band-pass type. The original signal is converted by the news of the band called the fundamental frequency band, referred to as "baseband". The original data signal from the data terminal, its spectrum is generally start from scratch, often contain abundant low frequency components, and even the DC component, it is called a digital baseband signal. Direct transmission digital baseband signal, which is not moving frequency baseband signal or only after code conversion simply direct transmission mode is called the digital baseband transmission. The frequency range ......