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Understanding Coherent and Non-Coherent Decoding in Communication Systems

This document explores the concepts of coherent and non-coherent decoding within communication systems, focusing on the modulation processes of AM waves and various decoder types. It examines key techniques such as phase-locked loops (PLL), voltage-controlled oscillators (VCO), and their roles in demodulating signals. The paper also discusses the implications of errors in frequency and phase for performance, along with applications in signal recovery and Clock generation. Basic principles of spread spectrum and the functioning of single-sideband modulation are also covered.

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Understanding Coherent and Non-Coherent Decoding in Communication Systems

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  1. ECE4331, Fall, 2009Communication Systems Zhu Han Department of Electrical and Computer Engineering Class 4 Sep. 4th, 2009

  2. Review + + Coherent Decoding and Amplitude Decoding

  3. Illustrating the amplitude modulation process. (a) Baseband signal m(t). (b) AM wave for | kam(t) | < 1 for all t. (c) AM wave for | kam(t) | > 1 for some t.

  4. Figure 2.4(a) Spectrum of baseband signal. (b) Spectrum of AM wave.

  5. + vc(t) - AM signal R C AM Noncoherent 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.

  6. Coherent detector for demodulating DSB-SC modulated wave.

  7. Carrier Recover Error • DSB: e(t)=2m(t)cos(wct)cos((wc+ w)t+) e(t)=m(t) cos((w)t+) • Phase error: if fixed, attenuation. If not, shortwave radio • Frequency error: catastrophic beating effect • SSB, only frequency changes, f<30Hz. • Donald Duck Effect • Crystal oscillator, atoms oscillator, GPS, … • Pilot: a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes.

  8. Phase-Locked Loop • Can be a whole course. The most important part of receiver. • Definition: a closed-loop feedback control system that generates and outputs a signal in relation to the frequency and phase of an input ("reference") signal • A phase-locked loop circuit responds both to the frequency and phase of the input signals, automatically raising or lowering the frequency of a controlled oscillator until it is matched to the reference in both frequency and phase.

  9. Voltage Controlled Oscillator (VCO) • W(t)=wc+ce0(t), where wc is the free-running frequency • Example

  10. Ideal Model • Model • Si=Acos(wct+1(t)), Sv=Avcos(wct+c(t)) • Sp=0.5AAv[sin(2wct+1+c)+sin(1-c)] • So=0.5AAvsin(1-c)=AAv(1-c) • Capture Range and Lock Range LPF VCO

  11. Carrier Acquisition in DSB-SC • Signal Squaring method • Costas Loop • SSB-SC not working

  12. Costas receiver

  13. PLL Applications • Clock recovery: no pilot • Deskewing: circuit design • Clock generation: Direct Digital Synthesis • Spread spectrum: • Jitter Noise Reduction • Clock distribution

  14. Quadrature-carrier multiplexing system. (a) Transmitter. (b) Receiver.

  15. x + m(t) ~ + Hilbert Transformer X SSB Generator • Phase shift method using Hilbert transformer • Non-causal filter, approximations SSB (Upper sideband) SSB 0

  16. Filtering scheme for the generation of VSB modulated wave.

  17. Figure 2.15(a) Idealized magnitude spectrum of a transmitted TV signal. (b) Magnitude response of VSB shaping filter in the receiver.

  18. Figure 2.16Block diagram of mixer.

  19. Figure 2.17(a) Spectrum of modulated signal s1(t) at the mixer input. (b) Spectrum of the corresponding signal s´(t) at the output of the product modulator in the mixer.

  20. Block diagram of FDM system.

  21. Illustrating the modulation steps in an FDM system

  22. FMA of SSB for Telephone Systems

  23. FMA of SSB for Telephone Systems

  24. FMA of SSB for Telephone Systems

  25. Speed, Wavelength, Frequency • Light speed = Wavelength x Frequency = 3 x 108 m/s = 300,000 km/s

  26. Type of waves

  27. Radio Frequency Bands

  28. GPS Orbits

  29. GPS Position • By knowing how far one is from three satellites one can ideally find their 3D coordinates • To correct for clock errors one needs to receive four satellites • Differential GPS: local FM

  30. Homework 1.6, 1.22, 2.5, 2.8, 2.10, 2.14, 2.15 Due on 9/15

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