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Digital modulation basics(nnm)

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Digital modulation basics

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Digital modulation basics(nnm)

  1. 1. Digital Modulation Basics N N Maurya
  2. 2. Why Digital Modulation ?  More information capacity  Compatibility with digital data services  Higher data security  Better quality communications  Digital modulation schemes have greater capacity to convey large amounts of information than analog modulation schemes
  3. 3. Constraints ?  Available bandwidth  Permissible power  Inherent noise level of the system
  4. 4. Trading off  There is a fundamental tradeoff in communication systems  This tradeoff exists whether communication is over air or wire, analog or digital  Spectrally efficient transmission techniques require more and more complex hardware
  5. 5. Trends in the Industry
  6. 6. Block Diagram of Digital Communication System
  7. 7. Classification of Communication System  Most communication systems can be classified into one of three different categories: – Bandwidth efficient Ability of system to accommodate data within a prescribed bandwidth – Power efficient Reliable sending of data with minimal power requirements – Cost efficient System needs to be affordable in the context of its use
  8. 8. Types of Digital Modulation System  COHERENT  NON- COHERENT  Coherent (synchronous) detection: process receives signal with a local carrier of same frequency and phase  Non coherent (envelope) detection: requires no reference wave
  9. 9. TYPES OF DIGITAL MODULATION SYSTEM……  Coherent detection  Receiver uses the carrier phase to detect signal  Cross correlate with replica signals at receiver  Match within threshold to make decision  Non coherent detection  Does not exploit phase reference information  Less complex receiver, but worse performance
  10. 10. Hierarchy of digital modulation technique
  11. 11. Digital modulation techniques  Amplitude shift keying (ASK)  Frequency shift keying (FSK)  Phase shift keying (PSK)  Quadrature phase shift keying (QPSK)  Quadrature amplitude modulation (QAM)
  12. 12. Metrics for Digital Modulation  Power Efficiency  Power efficiency is a measure of how much signal power should be increased to achieve a particular BER for a given modulation scheme  Ability of a modulation technique to preserve the fidelity of the digital message at low power levels  Designer can increase noise immunity by increasing signal power  Signal energy per bit / noise power spectral density: Eb / N0
  13. 13. Metrics for Digital Modulation…  Bandwidth Efficiency  Ability to accommodate data within a limited bandwidth  Tradeoff between data rate and pulse width  Data rate per hertz: R/B bps per Hz  Shannon Limit: Channel capacity / bandwidth  C = B log2(1 + S/N) OR  C/B = log2(1 + S/N)
  14. 14. Considerations in Choice of Modulation Scheme  High spectral efficiency  High power efficiency  Robust to multipath effects  Low cost and ease of implementation  Low carrier-to-cochannel interference ratio  Low out-of-band radiation  Constant or near constant envelope  Constant: only phase is modulated  Non-constant: phase and amplitude modulated
  15. 15. Amplitude Shift Keying Digital information 1 0 1 1 0 0 1 0 1 0 Carrier wave ASK modulated signal Carrier present Carrier absent Amplitude varying- frequency constant
  16. 16. ASK Generation Lower Side band Upper Side band Band width=2 X Modulating freq. 00 1)2cos( )( tfA ts cc
  17. 17. 1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1 t t tf tASK tf tASK tA cc sin n nn awhere T nTt rectatf 0 1 tAtft ccASK sin 0 sin tA t cc ASK (logic 1) (logic 0) ASK Generation…
  18. 18. ASK Detection
  19. 19. Necessity to shape the pulse A pulse contains infinite number of harmonics and hence bandwidth
  20. 20. Frequency shift keying Digital information 1 0 1 1 0 0 1 Carrier 1 (frequency #1) FSK modulated signal Carrier 2 (frequency #2) Frequency varying- amplitude constant
  21. 21. FSK Generation 0)2cos( 1)2cos( )( 2 1 btfA btfA ts c c
  22. 22. FSK Detection
  23. 23. Minimum Shift Keying  When the frequency of the separation becomes lowest it is known as minimum shift keying (MSK)
  24. 24. Phase Shift Keying
  25. 25. PSK Generation 1800 shift 0)2cos( 1)2cos( )( btfA btfA ts cc cc
  26. 26. PSK Generation… n nn awhere T nTt rectatf 0 1 tAtft ccPSK cos tf tPSK tA cc cos 1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1 t t tf tPSK ~ ~ tf02cos tf02cos
  27. 27. Signal Vector Representation Phase S 0 degrees I Q I-Q Plane s(t) = Ac(t) cos (2 fct + θ(t)) fixed!!! t = 0 t = t θ = 90 θ = 0
  28. 28. S1 S2 I Q Magnitude Change S1 S2 I Q Phase Change S1 S2 I Q Magnitude & Phase Changes I-Q Diagrams or Constellations Signal Changes: Representation in the I-Q plane
  29. 29. Constellation
  30. 30. QPSK  The only way to achieve high data rates with a narrowband channel is to increase the number of bits/symbol  The most reliable way to do this is with a combination of amplitude and phase modulation called quadrature amplitude modulation (QAM)  Quadrature Phase Shift Keying is effectively two independent BPSK  QPSK systems (I and Q), exhibits the same performance but twice the bandwidth efficiency of that of BPSK.  Large envelope variations occur during phase transitions, thus requiring linear amplification.
  31. 31. QPSK constellation Basic QPSK constellation QPSK constellation Shifted by 450
  32. 32. QPSK…… Carrier phase is changed by 450 ,1350, 2250, 3150 00 10 11 10 01
  33. 33. QPSK Generation
  34. 34. QPSK Detection
  35. 35. Types of QPSK  Conventional QPSK has transitions through zero (i.e. 180 phase transition). Highly linear amplifier required.  In Offset QPSK, the transitions on the I and Q channels are staggered.  Phase transitions are therefore limited to 90degrees.  In /4-QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations.  All QPSK schemes require linear power amplifiers.
  36. 36. Offset QPSK
  37. 37. /4 QPSK
  38. 38. Multi-level (M-ary) Phase and Amplitude Modulation  Amplitude and phase shift keying can be combined to transmit several bits per symbol  These modulation schemes are often referred to as linear, as they require linear amplification  Amplitude modulation on both quadrature carriers  2^n discrete levels, n = 2 same as QPSK  16-QAM has the largest distance between points, but requires very linear amplification. 16-PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise  M-ary schemes are more bandwidth efficient, but more susceptible to noise.
  39. 39. 16-QAM
  40. 40. 16-QAM Generation
  41. 41. 16-QAM Detection
  42. 42. 16QAM with different impairments AWGN Loss of Sync Interference Phase Noise
  43. 43. Application
  44. 44. Bandwidth efficiency limits
  45. 45. Thank You!
  46. 46. Actual example  Here is a 16-level constellation which is reconstructed in the presence of noise -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Eb/No=5 dB
  47. 47. Defining decision regions  An easy detection method, is to compute “decision regions” offline. Here are a few examples decide s1 decide s2 s1s2 measurement decide s1 decide s2 decide s3 decide s4 s1s2 s3 s4 decide s1 s1
  48. 48. On-off keying (OOK)  Simplest/oldest form of modulation  Morse code (1837) – developed for telegraphy
  49. 49. Eye diagrams

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