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EE 6332, Spring, 2014 Wireless Communication

EE 6332, Spring, 2014 Wireless Communication. Zhu Han Department of Electrical and Computer Engineering Class 12 Feb. 24 nd , 2014. Outline (Chapter 5.2-5.4). Geometric representation of modulation signals Linear modulation BPSK, DPSK; QPSK, offset QPSK, /4 QPSK

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EE 6332, Spring, 2014 Wireless Communication

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  1. EE 6332, Spring, 2014Wireless Communication Zhu Han Department of Electrical and Computer Engineering Class 12 Feb. 24nd, 2014

  2. Outline (Chapter 5.2-5.4) • Geometric representation of modulation signals • Linear modulation • BPSK, DPSK; QPSK, offset QPSK, /4 QPSK • Constant envelope modulation • BFSK, MSK, GMSK • Combined linear and constant envelope modulation • MPSK • QAM • MFSK and OFDM

  3. Analog Modulation: PAM, PWM, PPM, PCM

  4. Geometric Representation of Modulation Signal Digital Modulation involves Choosing a particular signal waveform for transmission for a particular symbol or signal For M possible signals, the set of all signal waveforms are: For binary modulation, each bit is mapped to a signal from a set of signal set S that has two signals We can view the elements of S as points in vector space

  5. Geometric Representation of Modulation Signal Vector space We can represented the elements of S as linear combination of basis signals. The number of basis signals are the dimension of the vector space. Basis signals are orthogonal to each-other. Each basis is normalized to have unit energy:

  6. Example Two signal waveforms to be used for transmission The basis signal Q I Constellation Diagram Dimension = 1

  7. Constellation Diagram Properties of Modulation Scheme can be inferred from Constellation Diagram Bandwidth occupied by the modulation increases as the dimension of the modulated signal increases Bandwidth occupied by the modulation decreases as the signal points per dimension increases (getting more dense) Probability of bit error is proportional to the distance between the closest points in the constellation. Bit error decreases as the distance increases (sparse).

  8. Concept of a constellation diagram

  9. Example of samples of matched filter output for some bandpass modulation schemes

  10. Linear Modulation Techniques Classify digital modulation techniques as: Linear The amplitude of the transmitted signal varies linearly with the modulating digital signal, m(t). They usually do not have constant envelope. More spectral efficient. Poor power efficiency Non-linear

  11. Binary Phase Shift Keying Use alternative sine wave phase to encode bits Phases are separated by 180 degrees. Simple to implement, inefficient use of bandwidth. Very robust, used extensively in satellite communication. Q 0 State 1 State

  12. BPSK Example 1 1 0 1 0 1 Data Carrier Carrier+ p BPSK waveform

  13. BPSK Virtue of pulse shaping

  14. BPSK Coherent demodulator

  15. Differential PSK encoding Differential BPSK 0 = same phase as last signal element 1 = 180º shift from last signal element

  16. DPSK modulation and demodulation 3dB loss EE 542/452 Spring 2008 EE 552/452 Spring 2007

  17. Quadrature Phase Shift Keying Multilevel Modulation Technique: 2 bits per symbol More spectrally efficient, more complex receiver. Two times more bandwidth efficient than BPSK Q 11 State 01 State 00 State 10 State Phase of Carrier: p/4, 2p/4, 5p/4, 7p/4

  18. 4 different waveforms -cos+sin cos+sin 11 01 00 10 cos-sin -cos-sin

  19. QPSK Example

  20. QPSK Virtue of pulse shaping

  21. QPSK modulation

  22. QPSK receiver

  23. Differential Coherent • DBPSK • 3dB loss • QPSK BER, the same as BPSK

  24. Offset QPSK waveforms

  25. Offset OQPSK • QPSK can have 180 degree jump, amplitude fluctuation • By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time. • 90 degree jump

  26. Pi/4 QPSK signaling 135 degree Non-coherent detection

  27. Pi/4 QPSK transmitter

  28. I. Differential detection of pi/4 QPSK

  29. II. IF Differential Detection

  30. III. FM Discriminator detector

  31. Constant Envelope Modulation Amplitude of the carrier is constant, regardless of the variation in the modulating signal Better immunity to fluctuations due to fading. Better random noise immunity Power efficient They occupy larger bandwidth

  32. Frequency Shift Keying (FSK) The frequency of the carrier is changed according to the message state (high (1) or low (0)). One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation) Integral of m(x) is continues. Continues FSK

  33. FSK Bandwidth • Limiting factor: Physical capabilities of the carrier • Not susceptible to noise as much as ASK • Applications • On voice-grade lines, used up to 1200bps • Used for high-frequency (3 to 30 MHz) radio transmission • used at higher frequencies on LANs that use coaxial cable

  34. Multiple Frequency-Shift Keying (MFSK) • More than two frequencies are used • More bandwidth efficient but more susceptible to error • f i= f c+ (2i – 1 – M)f d • f c= the carrier frequency • f d= the difference frequency • M = number of different signal elements = 2 L • L = number of bits per signal element

  35. FSK Coherent Detection

  36. Noncoherent FSK

  37. MSK modulation

  38. MSK reception

  39. Minimum Shift Keying spectra

  40. GMSK spectral shaping

  41. GMSK spectra shaping

  42. Simple GMSK modulation and demodulation EE 552/452 Spring 2007

  43. Digital GMSK demodulator

  44. 8-PSK Signal Constellation

  45. Pulse Shaped M-PSK

  46. QAM – Quadrature Amplitude Modulation • Modulation technique used in the cable/video networking world • Instead of a single signal change representing only 1 bps – multiple bits can be represented buy a single signal change • Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes)

  47. QAM • QAM • As an example of QAM, 12 different phases are combined with two different amplitudes • Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations • With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16) • Combine ASK and PSK such that each signal corresponds to multiple bits • More phases than amplitudes • Minimum bandwidth requirement same as ASK or PSK

  48. 16-QAM Signal Constellation

  49. QAM vs. MFSK

  50. Orthogonal frequency-division multiplexing • Special form of Multi-Carrier Transmission. • Multi-Carrier Modulation. • Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel (using Sub-Carriers)

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