HAPTER Modulation and Demodulation MIT（HAPTER调制和解调麻省理工学院）.pdfVIP

MIT 6.02 DRAFT Lecture Notes Last update: April 11, 2012 Comments, questions or bug reports? Please contact {hari, verghese} at CHAPTER 14 Modulation and Demodulation This chapter describes the essential principles behind modulation and demodulation, which we introduced briefly in Chapter 10. Recall that our goal is to transmit data over a commu- nication link, which we achieve by mapping the bit stream we wish to transmit onto analog signals because most communication links, at the lowest layer, are able to transmit ana- log signals, not binary digits. The signals that most simply and directly represent the bit stream are called the baseband signals. We discussed in Chapter 10 why it is generally un- tenable to directly transmit baseband signals over communication links. We reiterate and elaborate on those reasons in Section 14.1, and discuss the motivations for modulation of a baseband signal. In Section 14.2, we describe a basic principle used in many modulation schemes, called the heterodyne principle. This principle is at the heart of amplitude modulation (AM), the scheme we study in detail. Sections 14.3 and 14.4 describe the “inverse” process of demodulation, to recover the original baseband signal from the received version. Fi- nally, Section 14.5 provides a brief overview of more sophisticated modulation schemes. 14.1 Why Modulation? There are two principal motivating reasons for modulation. We described the first in Chap- ter 10: matching the transmission characteristics of the medium, and considerations of power and antenna size, which impact portability. The second is the desire to multiplex, or share, a communication medium among many concurrently active users. 14.1.1 Portability Mobile phones and other wireless devices send information across free space using electro- magnetic waves. To send t