1 / 105

MITP 413: Wireless Technologies Week 5

MITP 413: Wireless Technologies Week 5. Michael L. Honig Department of ECE Northwestern University. April 2005. Binary Frequency-Shift Keying (FSK). Bits: 1 0 1 1 0. Amplitude Shift Keying (4-Level ASK). Bits: 00 01 10 11. Baseband signal.

dennya
Download Presentation

MITP 413: Wireless Technologies Week 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. MITP 413: Wireless TechnologiesWeek 5 Michael L. Honig Department of ECE Northwestern University April 2005

  2. Binary Frequency-Shift Keying (FSK) Bits: 10110

  3. Amplitude Shift Keying (4-Level ASK) Bits: 0001 10 11 Baseband signal symbol duration

  4. Binary Phase Shift Keying (BPSK) Bits: 101 10 Baseband signal

  5. Quadrature Phase Shift Keying (QPSK) Bits: 0001 10 11

  6. Baseband  RF Conversion Passband (RF) signal Baseband signal sin 2fct time X T fc is the carrier frequency Power Power signal bandwidth is roughly 1/T frequency frequency fc 0  0

  7. Selection Criteria How do we decide on which modulation technique to use? • Performance: probability of error Pe. • Probability that a 0 (1) is transmitted and the receiver decodes as a 1 (0). • Complexity: how difficult is it for the receiver to recover the bits (demodulate)? • FSK was used in early voiceband modems because it is simple to implement. • Bandwidth or spectral efficiency: bandwidth (B) needed to accommodate data rate R bps, i.e., R/B measured in bits per second per Hz. • Power efficiency: energy needed per bit to achieve a satisfactory Pe. • Performance in the presence of fading, multipath, and interference.

  8. Probability of Error 4-ASK BPSK 7 dB (factor of 5) Signal-to-Noise Ratio (dB)

  9. How to Increase Bandwidth Efficiency? • Increase number of signal levels. • Use more bandwidth efficient modulation scheme (e.g., PSK). • Apply coding techniques: protect against errors by adding redundant bits. • Note that reducing T increases the symbol rate, but also increases the signal bandwidth. There is a fundamental tradeoff between power efficiency and bandwidth efficiency.

  10. Shannon Capacity noise Information Source Encoder Channel Decoder bits input x(t) output y(t) Estimated bits Channel capacity: C = B log(1+S/N) bits/second B= Bandwidth, S= Signal Power, N= Noise Power No fading

  11. Colored Balls 2

  12. Binary Phase Shift Keying (BPSK) Bits: 101 10 Baseband signal

  13. Minimum Bandwidth (Nyquist) Pulse Shape This pulse has the minimum bandwidth for a given symbol rate. Given bandwidth B, the maximum symbol rate without intersymbol interference (ISI) is B, the “Nyquist rate”.

  14. Pulse Shape and Bandwidth Rectangular pulse Power amplitude Bandwidth B > 2/T 2/T T frequency 0 time Nyquist pulse Power bandwidth B = 1/T frequency time T

  15. Shifted Nyquist Pulses Bits: 1 1 0 0 1 0

  16. Baseband Waveform(Nyquist Signaling) . . . Bits: 1 1 0 0 1 0 . . .

  17. Baseband  RF Conversion Passband (RF) signal Baseband signal sin 2fct X T time fc is the carrier frequency Power Power signal bandwidth is roughly 1/T frequency frequency fc 0  0

  18. Passband Signal with Different Carrier Frequencies

  19. Pulse Width vs. Bandwidth T time Perfect synchronization T time Offset causes severe ISI!

  20. Raised Cosine Pulses Minimum BW frequency time

  21. Raised Cosine Pulses Minimum BW 50% excess BW 100% excess BW frequency time Excess bandwidth= (Total bandwidth – Nyquist bandwidth)/Nyquist bandwidth

  22. Circle Vocoder

  23. Blocks Transition

  24. Blocks Stretched 8000 Bits Per Second

  25. Additional Bits Additional Bits Error Correction Control Information • Channel • Ground Station • Power • Identity • Billing

  26. 6000 Hz Available 6000 Hz Available Bandwidth Nyquist’s Theorem: Can transmit 4000 symbols per second through a 6000 Hz channel

  27. Colored Balls 2 Nyquist’s Theorem: Can transmit 4000 symbols per second through a 6000 Hz channel

  28. BPSK 4000 < 8000 • BPSK: 1 Bit Per Symbol • 4000 Bits Per Second (bps) • 4000 bps < 8000 bps (Vocoder rate) • BPSK

  29. QPSK 8000 BPS • QPSK: 2 Bits Per Symbol • 2 X 4000 = 8000 Bits Per Second (bps) • 8000 bps = Vocoder rate • Need more bits for error correction and control! • QPSK

  30. 8PSK 12000 BPS • 8PSK: 3 Bits Per Symbol • 3 X 4000 = 12,000 Bits Per Second (bps) • 8000 bps + 4000 bps • Vocoder rate + Error Correction and Control • 8PSK

  31. 16 PSK 16000 BPS • 16 phases: 4 Bits Per Symbol • 4 x 4000 = 16,000 bps • More than enough for vocoderrate + overhead

  32. MOTION BLUR

  33. QPSK w/Bit Labels 90˚ 0˚ 180˚ 270˚

  34. 8PSK 90˚ 45˚ 135˚ 0˚ 180˚ 225˚ 270˚ 315˚

  35. QPSK Signal Constellation amplitude = 1 x 1 x x x

  36. “Rotated” QPSK Signal Constellation

  37. “Rotated” QPSK Signal Constellation amplitude = 1 x x 1 x x

  38. In-Phase/Quadrature Components x (a,b)  a sin 2fct + b cos 2fct 1 x x b is the “in-phase” signal component a is the “quadrature” signal component x

  39. In-Phase/Quadrature Components x x 1 x x

  40. Example Constellations BPSK QPSK x x x x 16-QAM quadrature 8-PSK x x x x x x x x x x x x in-phase x x x x

  41. Example Constellations quadrature BPSK QPSK in-phase x x x x x x 16-QAM quadrature 8-PSK x x x x x x x x x x x x x x x x x x x in-phase x x x x x

  42. Quadrature Modulation in-phase signal even bits Baseband Signal X Split: Even/Odd transmitted (RF) signal source bits + Baseband Signal X odd bits quadrature signal

  43. Modulation for Fading Channels • Problems: 1. Amplitude variations (shadowing, distance, multipath) 2. Phase variations 3. Frequency variations (Doppler) • Solution to 1: • Avoid amplitude modulation • Power control

  44. Modulation for Fading Channels • Problems: 1. Amplitude variations (shadowing, distance, multipath) 2. Phase variations 3. Frequency variations (Doppler) • Solution to 1: • Avoid amplitude modulation • Power control • Solution to 2 & 3: • Avoid phase modulation (use FSK) • “Noncoherent” demodulation: does not use phase reference Differential coding/decoding • “Coherent” demodulation:Estimate phase shifts caused by channel. • Increase data rate/Doppler shift ratio

  45. Binary Frequency-Shift Keying (FSK) Bits: 10110

  46. Minimum Shift Keying (MSK) Bits: 10110 • Frequencies differ by ½ cycle • Used in GSM

  47. Binary Differential Modulation (i+1)st bit = 0: 0o phase shift waveform for ith symbol (i+1)st bit = 1: 180o phase shift

  48. Binary Differential Modulation (i+1)st bit = 0: 0o phase shift waveform for ith symbol (i+1)st bit = 1: 180o phase shift Drawback: a detection error for the ith bit propagates to the (i+1)st bit.

  49. Example: DQPSK bits: 00 x 10 x x 01 x x x 11 x x constellation for ith symbol constellation for (i+1)st symbol Used in IS-136

  50. Coherent Phase Modulation • More complicated than noncoherent (e.g., differential) modulation. • Receiver requires a pilot signal. • Transmit known symbols, measure phase. • Pilot symbols are overhead (not information bits). Receiver estimates phase offset

More Related