1、1Communication Theory2Lecture 8 New Digital Bandpass Modulations Communication Theory38.1 Quadrature Amplitude Modulation (QAM)ExpressionLet Xk = AkcoskYk = -AksinkLecture 8 New digital bandpass modulations4Vector DiagramIf k=/4 or -/4, Ak=+A or -A, QAM signal is QPSK signal.Lecture 8 New digital ba

2、ndpass modulations516QAMAkLecture 8 New digital bandpass modulations6 64QAM 256QAMLecture 8 New digital bandpass modulations78.2 Minimum Shift Keying (MSK)Orthogonal 2FSK with constant envelop, continuous phase, minimum bandwidthLecture 8 New digital bandpass modulations88.2.1 Minimum frequency inte

3、rval for orthogonal 2FSK2FSK signal To be orthogonalLecture 8 New digital bandpass modulations9If 1+0 1, the first and third terms are approximately 0If 1 and 0 are arbitrary, Minimum frequency interval is when m = 1.Lecture 8 New digital bandpass modulations10For coherent detection, we require the

4、initial phase are not arbitrary. Thus, we can let 1 - 0 = 0. To satisfyThus, for coherent detection, minimum frequency interval is 1 / 2Ts.Lecture 8 New digital bandpass modulations11 8.2.2 MSK signalThe kth symbol of MSK signal is expressed aswhere s carrier angular frequency ak = 1 Ts symbol durat

5、ion k the initial phase of the kth symbolLecture 8 New digital bandpass modulations12when ak = +1, f1 is fs + 1/(4Ts); when ak = -1, f0 is fs - 1/(4Ts). Lecture 8 New digital bandpass modulations13The number of periods in a MSK symbol duration Lecture 8 New digital bandpass modulations14The third te

6、rm: sin(2k) = 0The first term: Lecture 8 New digital bandpass modulations15where T1 = 1 / f1 and T0 = 1 / f0When N =1, m = 3, for “1” and “ 0”, a symbol duration includes 2 and 1.5 periods, respectively. Lecture 8 New digital bandpass modulations16Lecture 8 New digital bandpass modulations17 Continu

7、ous phase of MSK signalLecture 8 New digital bandpass modulations18For coherent detection, if k-1=0, can be rewritten as Lecture 8 New digital bandpass modulations19ak =1,1,1,1,1,1,1,1,1,1,1,1,1 k(t)Ts3Ts5Ts9Ts7Ts11Ts0Lecture 8 New digital bandpass modulations20第8章 新型數(shù)字帶通調(diào)制技術(shù)Ts3Ts5Ts9Ts7Ts11Ts0k(t)2

8、18.2.4 PSD of MSK signalLecture 8 New digital bandpass modulations228.3 Orthogonal Frequency Division Multiplexing (OFDM)8.3.1 Introduction Single carrierMulti-carrierLecture 8 New digital bandpass modulations23fttBBTsNTsSingle carrierMulti-carrierf|C(f)|C(f)|ffc(t)tLecture 8 New digital bandpass mo

9、dulations248.3.2 OFDM signalExpression Let a OFDM signal has N subcarriers. For kth subcarrier,Lecture 8 New digital bandpass modulations25Condition for orthogonal Lecture 8 New digital bandpass modulations26Minimum subcarrier interval isfmin = 1/Ts Lecture 8 New digital bandpass modulations27Frequency spectrum of OFDM signal For the kth subcarrier fk, the waveform and the frequency spectrum are as followsffkfk+1/TsTstfk2/Tsfk1/TsfkffLec