通信原理（英文版）全套PPT课件.pptx

1Principles of Communications 2Chapter 1 Introduction1.1 Historical Review of CommunicationOrigin of ancient communicationTwo modes of communicationDevelopment of modern communication 31.2 Message, information signalMessage：speech, letters, figures, images…Information：effective content of message. Different types of messages may contain the same informationSignal：the carrier of message What transmitted in a communication system is signal. 4Measurement of information： # “quantity of message” ? information content # Ex: “Rainfall will be 1 mm tomorrow” – information content small “Rainfall will be 1 m tomorrow” – information content large “The sun will rise in the east tomorrow morning” – information content equals zero # Information content I = I [ P(x) ]，P(x) – Occurrence probability # Definition：I = loga [1/P(x)] = -logaP(x) # Usually, set a = 2, the unit of the information content will be called a bit. # For an equal probability binary symbol: I = log2 [1/P(x)] = log2 [1/(1/2)] = 1 bit 5# For an equal probability M-ary symbol: I = log2 [1/P(x)] = log2 [1/(1/M)] = log2 M bit If M = 2k ，then I = k bit 61.3 Digital Communication 1.3.1 Basic conceptTwo categories of signals ? Analog signal：Its voltage or current can be expressed by a continuous function of time. For example, speech signal. ? Digital signal： Its voltage or current can only take finite number of discrete values. For example, digital computer data signal. 7Analog Signal Digital SignalAnalog signalsDigital signalsts(t)ts(t)s(t)tSymbolts(t) 8Two kinds of communication systems ? Analog communication system Requirement － High fidelity Criterion － Signal to noise ratio Basic issue － parameter estimation ? Digital communication system Requirement － correct decision Criterion － Error probability Basic issur － statistical decision theory 91.3.2 Advantages of Digital CommunicationFinite number of possible values of signalsCorrect decision may be