通信原理#40;英文版#41;6.ppt

Chapter 6 Elementary Digital Modulation System 6.1 Introduction Sinusoidal carrier: or where A －amplitude (V); f0 － frequency (Hz); ?0 = 2? f0 － angular frequency (rad/s); ? － initial phase (rad). Basic modulation systems: ASK FSK PSK Vector representation and vector diagram 6.2 2ASK 6.2.1 Basic principle Expression: where ?0 ＝ 2?f0 --- is the angular frequency of carrier Modulation methods Multiplier method: envelope may be nonrectangular Switching method: envelope is rectangular Demodulation methods Envelope detection method － carrier phase information is not used. Coherent demodulation method － carrier phase information is used. 6.2.2 Power Spectral Density Assume the general expression of the 2ASK random signal sequence is where an － binary unipolar random amplitude g(t) － symbol waveform T － duration of the symbol then the power spectral density of the 2ASK signal can be calculated as 式中， Ps(f) － power spectral density of s(t) PA(f) － power spectral density of A(t) ∴If PA(f) has been found, then Ps(f) can be calculated by substituting PA(f) into the above equation. Find PA(f): from eq.(5.5-29): where fc ＝ 1/T G1(f) － frequency spectrum of g1(t) G2(f) － frequency spectrum of g2(t) ∵Now, g1(t) = 0, ∴The above equation becomes where G(f) = G2(f) Now, the symbol waveform of the baseband signal is a rectangular pulse, so it is known from Fig. 2.2.3 and eq. (2.2-13), G(nfc) = 0 for all integers of n ? 0. Therefore the above equation becomes Substituting the eq. of PA(f) into the eq. of Ps(f), obtain: Find Ps(f): from the above equation: When P = ?, the above equation becomes where Therefore, we have Finally, we obtain Curves of PA(f) and Ps(f) 6.2.3 Symbol Error Probability Assume the received signal and the noise voltage after bandpass filtering equal: where ∵n(t) is a narrow band Gaussian process, we have Substituting the above