通信工程文献翻译——基带信号传输中的接受波形已经是脉冲形式，为什么还需要解调器来恢复脉冲波形呢？原因是到达的基带波形不是理想脉冲（每个码元只占据自己的码元间隔）。由于发送端滤波器和信道的原因，接受脉冲序列存在着码间串扰（ISI）从而产生拖尾信号，这些信号不利于采样检测。解调器（接收滤波器）的目的是消除码间干扰，恢复具有大信噪比（SNR）的基带信号。本章讲述的均衡技术就能够实现这个目的。虽然不是所有的通信信道都需要均衡，但由于它是一种补偿信道干扰的综合信号处理技术，因此成为许多系统非常重要的组成部分。

1 、信号与噪声

1.1 通信系统中差错性能的劣化

检测器的目的，是以尽可能少的差错从已经产生失真的接收波形中恢复原信号流。引起差错性能下降的主要原因有两个。第一个原因是3.3节将介绍发送端、信道和接收端的滤波的影响，非理想的系统传递函数会引起码元“拖尾”而产生码间串扰（ISI）。

另一个原因是电子噪声以及其他各种噪声源的干扰，如宇宙大气噪声、开关瞬态噪声、互调噪声和来自其他噪声源的干扰信号（这些将在第5章讨论）。采用适当的预防措施可以有效的减少甚至消除许多接收器中的噪声和干扰。但是，有一种噪声是无法消除的，就是导体中电子热运动产生的热噪声，该噪声是一种加性噪声，存在于放大器和电路中。运用量子力学已获知了热噪声的统计特性。

1.2 数字通信系统中的信噪比参数

学过模拟通信的读者都很熟悉一个指标，即信号平均功率和噪声平均功率的比值，简称为信噪比（或称为SNR）。在数字通信系统中，通常用信噪比的归一化形式作为性能指标。为每比特能量，等于信号能量与每比特持续时间的乘积; 是噪声功率谱密度，等于噪声功率与带宽之比；又因为每比特持续时间与比特速率互为倒数，可用代替，因此有下列表达式成立：......

In the case of baseband signaling, the received waveforms are already in a pulse-like form. One might ask , why then ,is a demodulator needed to recover the pulse waveforms? The answer is that the arriving baseband pulses are not in the form of ideal pulse shapes, each one occupying its own symbol interval. The filtering to suffer from inter-symbol interference (ISI) and thus appear as an amorphous “smeared” signal, not quite ready for sampling and detection .The goal of the demodulator (receiving filter) is to receiver a baseband pulse with the best possible signal-to-noise ration (SNR),free of any ISI. Equalization, covered in this chapter, is a technique used to help accomplish this goal. The equalization process is not required for every type of communication channel. However, since equalization embodies a sophisticated set of signal-processing techniques, making it possible to compensate for channel-induced interference, it is an important area for many systems. 

1 SIGNALS AND NOISE

1.1 Error-performance Degradation in Communication Systems

The purpose of the detector is based on as few errors as possible to restore the original signal stream from the received waveform distortion. There are two main reasons for the error performance degradation. The first reason is that 3.3 will introduce a sender channel and receiver filter, non-ideal system transfer function will cause the symbol "tail" arising from inter-symbol interference (ISI).

Another reason is that the electronic noise and other noise sources, such as atmospheric noise of the universe, the switching transient noise, inter-modulation noise and interference signals from other noise sources (these will be discussed in Chapter 5). Appropriate preventive measures can effectively reduce or even eliminate the number of receiver noise and interference. However, there is a noise can not be eliminated, is the electronic thermal motion of the conductor thermal noise, the noise is an additive noise, exist in the amplifier circuit. The use of quantum mechanics has been informed of the......