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ECE4331, Fall, 2009

ECE4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 3 Sep. 1 st , 2009. Review of last class. Random Process Stationary, Ergodic Mean, correlation, covariance, power spectrum density Gaussian Process Noise Central limit theorem Narrowband Noise

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ECE4331, Fall, 2009

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  1. ECE4331, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 3 Sep. 1st, 2009

  2. Review of last class • Random Process • Stationary, Ergodic • Mean, correlation, covariance, power spectrum density • Gaussian Process • Noise • Central limit theorem • Narrowband Noise • In-phase and Quadrature phase N0/2 power each Gaussian • Envelope (Rayleigh distribution) and phase (uniform) • Ricean distribution • Might not cover everything in the textbook ECE4331

  3. Baseband and Carrier Communication • Baseband: • Describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency • Voice: Telephone 0-3.5KHz; CD 0-22.05KHz http://www.youtube.com/watch?v=YjO_VXHxsRw&mode=related&search= • Video: Analog TV 4.5MHz, TV channel is 0-6MHz. Digital, depending on the size, movement, frames per second, … • Example: wire, coaxial cable, optical fiber, PCM phone • Carrier Communication: • Carrier: a waveform (usually sinusoidal) that is modulated to represent the information to be transmitted. This carrier wave is usually of much higher frequency than the modulating (baseband) signal. • Modulation: is the process of varying a carrier signal in order to use that signal to convey information. ECE4331

  4. Modulation • Modulation • A process that causes a shift in the range of frequencies of a signal. • Gain advantages • Antenna size: half of the antenna size. Thousands of miles for baseband • Better usage of limited bandwidth: less side lopes • Trade bandwidth for SNR: CDMA • Robust to inter-symbol-interference (multipath delay) • Robust to errors and distortions • Types • Analog: AM (DSB, SSB, VSB), FM, Delta modulation • Digital: ASK, FSK, PSK, QAM, … • Pulse modulation: PCM, PDM, … Fiber, phone • Advanced: CDMA (3G), OFDM (WLAN, WMAN), …. ECE4331

  5. Double Sideband • Modulation: time: m(t)cos(wct), freq: 0.5[M(w-wc)+M(w+wc)] • Figure 2.4: LSB, USB, DSB • DSB-SC: suppressed carrier, no carrier frequency • Wc >= bandwidth of the signal to avoid aliasing. • Demodulation: e(t)=m(t)(cos(wct))^2=0.5(m(t)+m(t)cos(2wct)) E(w)=0.5M(w)+0.25(M(w+2wc)+M(w-2wc)) Low pass filter to remove the higher frequency ECE4331

  6. DSB-SC • Example 0 0 Lower sideband (LSB) Upper sideband (USB) 0 ECE4331

  7. Frequency Conversion • Move the signals to other frequency • Multiplying two sinusoids results in two frequencies which are the sum and difference of the frequencies of the sinusoids multiplied. • To change the carrier frequency c of a modulated signal to an intermediate frequency I we use an oscillator to generate a sinusoid of frequency MIX such that BPF@ I EXAMPLE : Let m(t) be as shown. m(t) (t) e1(t) SPECTRA 0 0 0 ECE4331

  8. 0 0 Amplitude Modulation • Why DSB-SC not working: do not know the carrier frequency in receiver. • The last impulse functions indicate that the carrier is not suppressed in this case. For some M() shown, the modulated signal spectrum is as shown. • With this type of AM the demodulation can be performed with/without a local oscillator synchronized with the transmitter. ECE4331

  9. AM Example • m(t) has a minimum value of about -0.4. Adding a dc offset of A=1 results in A+m(t) being always positive. Therefore the positive envelope of is just A+m(t). An envelope detector can be used to retrieve this. A=1 m(t) A+m(t) 0.7 1. 0. -0.4 ECE4331

  10. AM Example (cont.) • The choice of dc offset should be such that A+m(t) should always be positive. Otherwise envelope detector cannot be used, but coherent still ok • For example, the minimum value of m(t) = -0.4 . Therefore A > |min(m(t))| for successful envelope detection. What if A< |m(t) |. • In the previous example let A=0.3. A+m(t) m(t) 0.7 0 0. -0.4 ECE4331

  11. m(t) Modulation Index • Let mp be the absolute negative peak of m(t). • EXAMPLE : Single-tone modulation. Let m(t)=2sin(20t) ECE4331

  12. Sideband and Carrier Power ECE4331

  13. + vc(t) - AM signal R C AM Decoder Coherent detection: Receiver can recover the frequency and phase of the transmitter by PLL. Error of timing causes the performance error floor • Rectifier Detector: synchronous • Envelope Detector: asynchronous Non-coherent receiver has 3dB worst performance than coherent. Cheaper for Non-coherent receiver, Nextel. ECE4331

  14. QAM • AM signal BANDWIDTH : AM signal bandwidth is twice the bandwidth of the modulating signal. A 5kHz signal requires 10kHz bandwidth for AM transmission. If the carrier frequency is 1000 kHz, the AM signal spectrum is in the frequency range of 995kHz to 1005 kHz. • QUADRARTURE AMPLITUDE MODULATION is a scheme that allows two signals to be transmitted over the same frequency range. • Equations • Coherent in frequency and phase. Expensive • TV for analog • Most modems ECE4331

  15. SSB Frequency baseband 0 DSB 0 SSB (Upper sideband) SSB 0 ECE4331

  16. SSB Math How to generate mh(t) ? M() H() Mh() Transfer function of a Hilbert transformer ECE4331

  17. 0 SSB Generator • Selective Filtering using filters with sharp cutoff characteristics. Sharp cutoff filters are difficult to design. The audio signal spectrum has no dc component, therefore , the spectrum of the modulated audio signal has a null around the carrier frequency. This means a less than perfect filter can do a reasonably good job of filtering the DSB to produce SSB signals. • Baseband signal must be bandpass • Filter design challenges • No low frequency components ECE4331

  18. x + m(t) ~ + Hilbert Transformer X SSB Generator • Phase shift method using Hilbert transformer • Non-causal filter, approximations ECE4331

  19. Single Sideband (SSB) • Purpose : to reduce the bandwidth requirement of AM by one-half. This is achieved by transmitting only the upper sideband or the lower sidebband of the DSB AM signal. ECE4331

  20. SSB vs. AM • Since the carrier is not transmitted, there is a reduction by 67%  of the transmitted power (-4.7dBm).   --In AM @100% modulation: 2/3 of the power is comprised of the carrier; with the remaining (1/3) power in both sidebands.  • Because in SSB, only one sideband is transmitted, there is a further reduction by 50% in transmitted power • Finally, because only one sideband is received, the receiver's needed bandwidth is reduced by one half--thus effectively reducing the required power by the transmitter another 50% • (-4.7dBm (+) -3dBm (+) -3dBm = -10.7dBm). • Relative expensive receiver ECE4331

  21. DSB 0 SSB (Upper sideband) 0 VSB Spectrum Vestigial Sideband (VSB) • VSB is a compromise between DSB and SSB. To produce SSB signal from DSB signal ideal filters should be used to split the spectrum in the middle so that the bandwidth of bandpass signal is reduced by one half. In VSB system one sideband and a vestige of other sideband are transmitted together. The resulting signal has a bandwidth > the bandwidth of the modulating (baseband) signal but < the DSB signal bandwidth. ECE4331

  22. m(t) e(t) Hi() LPF Ho() Transmitter Receiver VSB Transceiver ECE4331

  23. Other Facts about VSB • Envelope detection of VSB+C • TV: • DSB, SSB and VSB • DSB bandwidth too high • SSB: baseband has low frequency component, receiver cost • Relax the filter and baseband requirement with modest increase in bandwidth ECE4331

  24. Comparison ECE4331

  25. AM Broadcasting • History • Frequency • Long wave: 153-270kHz • Medium wave: 520-1,710kHz, AM radio • Short wave: 2,300-26,100kHz, long distance, SSB, VOA • Limitation • Susceptibility to atmospheric interference • Lower-fidelity sound, news and talk radio • Better at night, ionosphere. ECE4331

  26. Superheterodyne vs. homodyne • Move all frequencies of different channels to one medium freq. • In AM receivers, that frequency is 455 kHz, • for FM receivers, it is usually 10.7 MHz. • Filter Design Concern • Accommodate more radio stations • Edwin Howard Armstrong ECE4331

  27. Television Digital Display (CRT) Analog Display (TV) Eliminate flicker effects ECE4331

  28. Deflection Signal and Synchronization Deflection signal and synchronization signal 525525 30=8.27M ECE4331

  29. Solar Power and Human Eye ECE4331

  30. RGB, LIQ mL=0.3mr+0.59mg+0.11mb mI=0.6mr+0.28mg-0.32mb mQ=0.21mr-0.52mg+0.31mb ECE4331

  31. Bandwidth VSB and QAM ECE4331

  32. NTSC, PAL, and SECAM • National Television System Committee • Low complexity, higher vertical color resolution • 525 line/60Hz(30frames per second) • Phase Alternative Line: PAL • The phase of the color components is reversed from line to line • Robust to Multipath, phase distortion • 625line/50Hz(25 frames per second), slightly larger bandwidth • SECAM • Requires the receiver to memorize the content of each line • Mono when used for different standards ECE4331

  33. TV standards in the world ECE4331

  34. Analog vs. Digital TV (DT) • OTA Analog transmissions ceased in 2009 after over 50 years. DT offers better picture quality – no “snow”, no geometric distortion, no convergence problems • Each pixel separately generated • Stereo and surround sound (5.1) built-in • MPEG-2 compresses 1 Gb/s video stream • One 6 MHz Channel can carry 20 Mb/sec or several DT (HDTV and SDTV) sub channels and other data (8-VSB and QAM)

  35. Digital TV and HDTV (cont.) • DT Formats – 480i, 576i, 720p, 1080i, 1080p (1080p Blue-Ray, HD-DVD only) (interlaced, progressive scan) • Native HDTV Formats - 720p or 1080i, up or down conversion is required • Aspect Ratios – 4:3, 16:9 – black stripes, missing foreheads or 300-lb bodies • On-Screen Daily Schedules • Old Assignments: VHF (Ch. 2-13) and UHF (Ch. 14-68) • New Assignments: Upper VHF, lower UHF (Ch. 7-53) • Government Will Auction the Vacated Channels for $$ Billions, issued rebate coupons for converter boxes

  36. Transmission and Modulation QAM (Cable) 6 MHz NTSC/ATSC/QAM Tuners 6 MHz Channel + Video Stream 1 Gbit/sec MPEG-2 20 Mbits/sec 8-VSB (OTA) 6 MHz

  37. Plasma, LCD and DLP Displays • Plasma – Generates its own light, vivid colors, best viewing angle, but subject to burn-in, ambient light a problem (glare, reflections) • LCD – Backlit, (Venetian blinds) good pastel colors, poor blacks, low power consumption, low weight, good in lighted areas • DLP – Reflected light (Rear projection), bright colors, relatively expensive, fragile

  38. Plasma Display • Panel Cross Section • Cell arrangement

  39. LCD Display • Polarization

  40. DLP (Projection) Display • How it works

  41. SSB Hilbert • SSB signal can be expressed in terms of m(t) and its Hilbert transform ECE4331

  42. SSB Demodulation Synchronous, SSB-SC demodulation SSB+C, envelop detection ECE4331

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