1 / 13

EE354 : Communications System I

EE354 : Communications System I. Lecture 15,16: Linear Modulations Aliazam Abbasfar. Outline. Amplitude Modulation DSB/AM/SSB/VSB. DSB modulation. Double sideband modulation x o (t) = A c x(t) cos ( w c t ) Lowpass signal x I (t) = x(t), x Q (t) = 0 A(t), Q (t)= ?

evonne
Télécharger la présentation

EE354 : Communications System I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. EE354 : Communications System I Lecture 15,16: Linear Modulations Aliazam Abbasfar

  2. Outline • Amplitude Modulation • DSB/AM/SSB/VSB

  3. DSB modulation • Double sideband modulation • xo(t) = Ac x(t) cos(wct) • Lowpass signal • xI(t) = x(t), xQ(t) = 0 • A(t), Q(t)= ? • Xo(t) = ½ Ac [ X(f-fc) + X(f+fc) ] • Symmetric spectrum around fc • Bandwidth : 2W • Upper and lower sidebands • Transmitted power • GXo(t) = ¼ Ac2 [ GX(f-fc) + GX(f+fc) ] • PXo = ½ Ac2 PX = PcPX • Pc = Unmodulated carrier power

  4. DSB demodulation • Coherent demodulation • y(t) = 2A cos(wct) xo(t) = A Ac x(t) + A Ac x(t) cos(2wct) • Filter out x(t) cos(2wct) by a LPF • z(t) = A Ac x(t) = K x(t) • Coherent demodulation • Phase and frequency of the carrier is known • Phase offset • z(t) = K cos(f) x(t) • Lower gain • Frequency offset (Df) • z(t) = K cos(2pDf t) x(t) • Distortion

  5. AM modulation • Amplitude modulation • xo(t) = Ac (1+mx(t)) cos(wct) • m : modulation index • (1+mx(t)) > 0 (m<= 1 if |x(t)|<1) • Signal DC value = 0 • Lowpass signal • xI(t) = 1+mx(t), xQ(t) = 0 • A(t) = 1+mx(t), Q(t)= 0 • Xo(t) = ½Ac [d(f-fc) + d (f+fc) ] + ½m Ac [ X(f-fc) + X(f+fc) ] • Symmetric spectrum around fc • Additional tone at fc • Transmitted power • PXo = ½ Ac2 (1+m2) PX = Pc(1+m2 PX) • Efficiency: hAM = m2PX / (1+m2 PX) <= 50%

  6. AM demodulation • Envelope detection • Very simple circuits • Using non-linear circuits • Half-wave/full-wave rectifier • Good for radio broadcast • Expensive TX (only 1) • Cheap RXs (many)

  7. SSB modulation • Single sideband modulation • Send only one of the sidebands • LSSB or USSB • Filter out other sideband • Signal usually has a DC hole • Xo(f) = X(f-fc)u(f-fc) ; f>0 • Bandwidth : W • Spectrally efficient • Not symmetric • Transmitted power • PXo = ½ PDSB = ½ PcPX • Good for FDM • Low bandwidth • Low power

  8. SSB modulation - 2 • Lowpass signal • xI(t) = ½ x(t), xQ(t) =  ½ x(t) • IQ modulator • Weaver modulator

  9. SSB demodulation • Coherent demodulation • y(t) = 2A cos(wct) xo(t) = ½ A Ac x(t) [1+cos(2wct)] - ½ A Ac x(t) sin(2wct) • Filter out high freq. terms by a LPF • z(t) = ½ A Ac x(t) = K x(t) • Phase offset • z(t) = K cos(f) x(t) + K sin(f) x(t) • Lower gain + distortion • Frequency offset (Df) • z(t) = K cos(2pDf t) x(t) + K sin(2pDf t) x(t) • Distortion • IQ demodulator • Multiply with both cos(wct) and sin(wct) • Complex demodulator • No distortion

  10. VSB modulation • Vestigial sideband modulation • Send one of the sidebands and and part of other • Filter out part of other sideband • Keeps signal DC components • Xo(f) = X(f-fc) H(f) ; f>0 • Bandwidth > W • Not symmetric • Transmitted power • PSSB < PXo < PDSB • Used in video broadcast • Low bandwidth • Keeps low frequencies • Low power

  11. VSB modulation • H(f) has odd symmetry around fc • H(f) = (1 +j HVSB )/2 • HVSB is a realizable filter • Lowpass signal • xI(t)= ½ x(t), xQ(t) =  ½ x(t)*hVSB(t) • IQ modulator HVSB

  12. VSB demodulation • Coherent demodulation • y(t) = ½ A Ac x(t) [1+cos(2wct)] - ½ A Ac x’(t) sin(2wct) • Filter out high frequency terms by a LPF • z(t) = ½ A Ac x(t) = K x(t) • Phase offset • z(t) = K cos(f) x(t) + K sin(f) x’(t) • Lower gain + distortion • Frequency offset (Df) • z(t) = K cos(2pDf t) x(t) + K sin(2pDf t) x(t) • Distortion • Use IQ demodulator

  13. Reading • Carlson Ch. 4.2, 4.3 and 4.4 • Proakis 2.5, 3.1, 3.2

More Related