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EE 6331, Spring, 2009 Advanced Telecommunication

EE 6331, Spring, 2009 Advanced Telecommunication. Zhu Han Department of Electrical and Computer Engineering Class 10 Feb. 19 th , 2009. Outline. Review Rayleigh and Ricean Distributions Project 1 Modulation Basics Analog modulation. Rayleigh Distributions.

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EE 6331, Spring, 2009 Advanced Telecommunication

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  1. EE 6331, Spring, 2009Advanced Telecommunication Zhu Han Department of Electrical and Computer Engineering Class 10 Feb. 19th, 2009

  2. ECE6331 Spring 2009 Outline • Review • Rayleigh and Ricean Distributions • Project 1 • Modulation • Basics • Analog modulation

  3. ECE6331 Spring 2009 Rayleigh Distributions Describes the received signal envelope distribution for channels, where all the components are non-LOS: i.e. there is no line-of–sight (LOS) component.

  4. ECE6331 Spring 2009 Ricean Distributions Describes the received signal envelope distribution for channels where one of the multipath components is LOS component. i.e. there is one LOS component.

  5. ECE6331 Spring 2009 Rayleigh Fading

  6. ECE6331 Spring 2009 Rayleigh Fading Distribution The Rayleigh distribution is commonly used to describe the statistical time varying nature of the received envelope of a flat fading signal, or the envelope of an individual multipath component. The envelope of the sum of two quadrature Gaussian noise signals obeys a Rayleigh distribution.  is the rms value of the received voltage before envelope detection, and 2 is the time-average power of the received signal before envelope detection.

  7. ECE6331 Spring 2009 Rayleigh PDF 0.6065/s mean = 1.2533s median = 1.177s variance = 0.4292s2 5s s 2s 3s 4s

  8. ECE6331 Spring 2009 Ricean Fading Distribution When there is a dominant stationary signal component present, the small-scale fading envelope distribution is Ricean. The effect of a dominant signal arriving with many weaker multipath signals gives rise to the Ricean distribution. The Ricean distribution degenerates to a Rayleigh distribution when the dominant component fades away. The Ricean distribution is often described in terms of a parameter K which is defined as the ratio between the deterministic signal power and the variance of the multipath. K is known as the Ricean factor As A0, K  - dB, Ricean distribution degenerates to Rayleigh distribution.

  9. ECE6331 Spring 2009 PDF Probability density function of Ricean distributions: K=-∞dB (Rayleigh) and K=6dB. For K>>1, the Ricean pdf is approximately Gaussian about the mean.

  10. ECE6331 Spring 2009 Small-scale fading mechanism Assume signals arrive from all angles in the horizontal plane 0<α<360 Signal amplitudes are equal, independent of α Assume further that there is no multipath delay: (flat fading assumption) Doppler shifts

  11. ECE6331 Spring 2009 Small-scale fading: effect of Doppler in a multipath environment fm, the largest Doppler shift

  12. ECE6331 Spring 2009 Carrier Doppler spectrum Spectrum Empirical investigations show results that deviate from this model Power Model Power goes to infinity at fc+/-fm

  13. ECE6331 Spring 2009 Fading Model: Gilbert-Elliot Model Fade Period Signal Amplitude Threshold Time t Bad (Fade) Good (Non-fade)

  14. ECE6331 Spring 2009 Simulating 2-ray multipath a1 and a2 are independent Rayleigh fading 1 and 2 are uniformly distributed over [0,2)

  15. ECE6331 Spring 2009 Simulating multipath with Doppler-induced Rayleigh fading

  16. ECE6331 Spring 2009 Review

  17. ECE6331 Spring 2009 Project 1 Due 3/3. Electrical version 3 Tasks Investigate the large scale fading such as free space, reflection, and diffraction. Test log-scale propagation loss and shadowing model Investigate small scale fading with multipath and Doppler shifting using Clarke model. Write a report!!! Exam 3/5 50% why why why question 50% likes homework Cover until last class (class 9)

  18. ECE6331 Spring 2009 Project 1 ht=2m, hr=1.5, Freq.=2.4GHz, hf=2.5m, d1=0.5m, d=1-10m 10dB diffraction loss Light diffraction hf df

  19. ECE6331 Spring 2009 Project 1 Two ray model path loss

  20. ECE6331 Spring 2009 Project 1 Two ray model phase

  21. ECE6331 Spring 2009 Project 1 Diffraction Model Path Loss

  22. ECE6331 Spring 2009 Project 1 Diffraction Phase

  23. ECE6331 Spring 2009 Project 1 Combined path loss

  24. ECE6331 Spring 2009 Project 1

  25. ECE6331 Spring 2009 What is modulation Modulation is the process of encoding information from a message source in a manner suitable for transmission It involves translating a baseband messagesignal to a bandpass signal at frequencies that are very high compared to the baseband frequency. Baseband signal is called modulating signal Bandpass signal is called modulated signal

  26. ECE6331 Spring 2009 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 • 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. • Example on the board.

  27. ECE6331 Spring 2009 Modulation Techniques Modulation can be done by varying the Amplitude Phase, or Frequency of a high frequency carrier in accordance with the amplitude of the message signal. Demodulation is the inverse operation: extracting the baseband message from the carrier so that it may be processed at the receiver.

  28. ECE6331 Spring 2009 Analog/Digital Modulation Analog Modulation The input is continues signal Used in first generation mobile radio systems such as AMPS in USA. Digital Modulation The input is time sequence of symbols or pulses. Are used in current and future mobile radio systems

  29. ECE6331 Spring 2009 Goal of Modulation Techniques Modulation is difficult task given the hostile mobile radio channels Small-scale fading and multipath conditions. The goal of a modulation scheme is: Transport the message signal through the radio channel with best possible quality Occupy least amount of radio (RF) spectrum.

  30. ECE6331 Spring 2009 Frequency versus Amplitude Modulation Amplitude Modulation (AM) Changes the amplitude of the carrier signal according to the amplitude of the message signal All info is carried in the amplitude of the carrier There is a linear relationship between the received signal quality and received signal power. AM systems usually occupy less bandwidth then FM systems. AM carrier signal has time-varying envelope.

  31. ECE6331 Spring 2009 Amplitude Modulation • The amplitude of high-carrier signal is varied according to the instantaneous amplitude of the modulating message signal m(t). AM Modulator m(t) sAM(t)

  32. ECE6331 Spring 2009 Modulation Index of AM Signal For a sinusoidal message signal Index is defined as: SAM(t) can also be expressed as: g(t) is called the complex envelope of AM signal.

  33. ECE6331 Spring 2009 AM Modulation/Demodulation Source Sink Wireless Channel Modulator Demodulator Baseband Signal with frequency fm (Modulating Signal) Bandpass Signal with frequency fc (Modulated Signal) Original Signal with frequency fm fc >> fm

  34. ECE6331 Spring 2009 AM Modulation - Example 1/fmesg 1/fc

  35. ECE6331 Spring 2009 AM Spectrum

  36. ECE6331 Spring 2009 + vc(t) - AM signal R C AM Decoder • Rectifier Detector: synchronous • Envelope Detector: asynchronous

  37. ECE6331 Spring 2009 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.

  38. ECE6331 Spring 2009 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

  39. ECE6331 Spring 2009 Angle Modulation Angle of the carrier is varied according to the amplitude of the modulating baseband signal. Two classes of angle modulation techniques: Frequency Modulation Instantaneous frequency of the carrier signal is varied linearly with message signal m(t) Phase Modulation The phaseq(t) of the carrier signal is varied linearly with the message signal m(t).

  40. ECE6331 Spring 2009 Angle Modulation FREQUENCY MODULATION kf is the frequency deviation constant (kHz/V) If modulation signal is a sinusoid of amplitude Am, frequency fm: PHASE MODULATION kq is the phase deviation constant

  41. ECE6331 Spring 2009 FM Example 4 0 - + - - + 1.5 0.5 1 2 -4 Message signal FM Signal Carrier Signal

  42. ECE6331 Spring 2009 FM Index W: the maximum bandwidth of the modulating signal Df: peak frequency deviation of the transmitter. Am: peak value of the modulating signal Example: Given m(t) = 4cos(2p4x103t) as the message signal and a frequency deviation constant gain (kf) of 10kHz/V; Compute the peak frequency deviation and modulation index! Answer: fm=4kHz Df = 10kHz/V * 4V = 40kHz. bf = 40kHz / 4kHz = 10

  43. ECE6331 Spring 2009 Spectra and Bandwidth of FM Signals An FM Signal has 98% of the total transmitted power in a RF bandwidth BT Carson’s Rule Upper bound Lower bound Example: Analog AMPS FM system uses modulation index of Bf = 3 and fm = 4kHz. Using Carson’s Rule: AMPS has 32kHz upper bound and 24kHz lower bound on required channnel bandwidth.

  44. ECE6331 Spring 2009 FM/PM Example (Frequency)

  45. EE 542/452 Spring 2008 FM Demodulator Convert from the frequency of the carrier signal to the amplitude of the message signal FM Detection Techniques Slope Detection Zero-crossing detection Phase-locked discrimination Quadrature detection

  46. EE 542/452 Spring 2008 Slope Detector Vin(t) Vout(t) V2(t) V1(t) Limiter Differentiator Envelope Detector Proportional to the priginal Message Signal

  47. ECE6331 Spring 2009 Comparison of analog modulation systems

  48. ECE6331 Spring 2009 Frequency versus Amplitude Modulation Frequency Modulation (FM) Most popular analog modulation technique Amplitude of the carrier signal is kept constant (constant envelope signal), the frequency of carrier is changed according to the amplitude of the modulating message signal; Hence info is carried in the phase or frequency of the carrier. Has better noise immunity: atmospheric or impulse noise cause rapid fluctuations in the amplitude of the received signal Performs better in multipath environment Small-scale fading cause amplitude fluctuations as we have seen earlier. Can trade bandwidth occupancy for improved noise performance. Increasing the bandwith occupied increases the SNR ratio. The relationship between received power and quality is non-linear. Rapid increase in quality for an increase in received power. Resistant to co-channel interference (capture effect).

  49. ECE6331 Spring 2009 Satellite FM: XM vs. Sirus

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