Lecture 1 signals

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  1. Lecture 1 Communication basics. (2 hours) 01/09/2012 1 2. Contents Further readings: Behrouz, Chapters 3, 4, 5, 7 (section 7.1), 10 ã Signals – Amplitude –…
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  • 1. Lecture 1 Communication basics. (2 hours) 01/09/2012 1
  • 2. Contents Further readings: Behrouz, Chapters 3, 4, 5, 7 (section 7.1), 10 • Signals – Amplitude – Frequency – Phase • Distortion sources – Attenuation – Bandwidth limitation – Delay – Noise – Fading – Shadowing 01/09/2012 2
  • 3. Signals - Definition • Signals - Electromagnetic Waves that carry information. • Information is encoded by the shape of the signals. • Analog signals vary smoothly over time. • Discreet signals maintain constant level for some time then change to another constant level. Digital signals have only 2 levels. 01/09/2012 3
  • 4. Time Amplitude Time Amplitude Analog signal Discreet signal Amplitude Time Digital signal 01/09/2012 4
  • 5. Signals, Representation (1) • Time domain: Amplitude (A) (height of the signal, representing the power) against time. – Periodic Signals: Repeated pattern over a time distance called the period T. The rate of the repetition is called the frequency (f = 1/T). The distance between the two points of repetition is called the wavelength ( = c/f) (c is the speed of the signal). The phase () is the shift of the signal in time from the origin. – Periodic signals can be expressed as series of sin and cos, S(t) = A sin (2ft + ) + … – Aperiodic signals: do not repeat. 01/09/2012 5
  • 6. Periodic signal 01/09/2012 6
  • 7. Concept of phase shifts relative to time zero 01/09/2012 7
  • 8. Signals, Representation (2) • Frequency Domain: Amplitude against the frequencies. The frequency is the number of changes that the signal make every second. The signal is made up of many frequencies, which are multiple of the Fundamental frequency. • Spectrum of a signal: The range of all frequencies that the signal contains. The bandwidth represents the range of the frequencies where most the signal strength is located. 01/09/2012 8
  • 9. Frequency domain Amplitude Fundamental Bandwidth Spectrum Example: S(t) = 15sin(10t) + 4 sin(20 t) + 2 sin (30t) + 0.5sin(40t) 01/09/2012 9
  • 10. Distortion Sources (1) • Attenuation: the strength of the signal decreases as it propagates throughout a transmission medium. – Repeater and Equalizers are normally used. – Loss (dB) = 10 log10 (Pout/Pin) (guided media) • Limited Bandwidth: the bandwidth of the medium filters some of the frequency components of the signal. – Nyquist theorem: To recover the signal the rate of transmission C should be twice the bandwidth W. • C = 2 Wlog2M • M is the levels of the signal (binary M =2) 01/09/2012 10
  • 11. Distortion Sources (2) • Delay distortion: Some frequency components of a signal arrive at different time, so the signal is made up of various delayed frequencies. • Noise: in the absence of transmission there are random perturbations on the line. – Shannon theorem: in a noisy channel the maximum data rate C = W log2(1+S/N) bps, with the signal to noise ratio = 10log10 (S/N)dB. – Sources of Noise: Cross talk, Impulse noise, inter-modulation, thermal (white) noise (N=KTW thermal noise in watts present in a channel of bandwidth W at temperature T kevin , K=1,3808x10-23J/K) 01/09/2012 11
  • 12. Other Impairments • Atmospheric absorption – water vapor and oxygen contribute to attenuation • Shadowing – Obstacles on the way of the transmission absorb the strength from the signal. • Fading – Reception of multiple version of the same signal. – Reflected signals may arrive relatively simultaneously ( local scattering around the receiver.), they could create combined or destructive effects, deep fading (explain…) – Reflected signals may arrive relatively with long intervals, dues to multiple paths. 01/09/2012 12
  • 13. Shadowing (1) (2) • Buildings between the transmitter and receiver may cause temporary degradation to the signal strength. (1) • Temporary line-of sight transmission path would results in abnormally high received power.(2) 01/09/2012 13
  • 14. Multipath Propagation – Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal – Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave – Scattering – occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal or less 01/09/2012 14
  • 15. Quiz • What is the difference between frequency domain and time domain representations? • Sketch a periodic and aperiodic signals. • What happens in the attenuation? • What does -2dB mean? Give an example. • What is the difference between shadowing and fading? 01/09/2012 15
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