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Digital Communications I: Modulation and Coding Course

Spring - 2015 Jeffrey N. Denenberg Lecture 4: BandPass Modulation/Demodulation. Digital Communications I: Modulation and Coding Course. Last time we talked about:. Another source of error due to filtering effect of the system: Inter-symbol interference (ISI)‏ The techniques to reduce ISI

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Digital Communications I: Modulation and Coding Course

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  1. Spring - 2015 Jeffrey N. Denenberg Lecture 4: BandPass Modulation/Demodulation Digital Communications I:Modulation and Coding Course

  2. Lecture 7 Last time we talked about: Another source of error due to filtering effect of the system: Inter-symbol interference (ISI)‏ The techniques to reduce ISI Pulse shaping to achieve zero ISI at the sampling time Equalization to combat the filtering effect of the channel

  3. Lecture 7 Today, we are going to talk about: Some bandpass modulation schemes used in DCS for transmitting information over channel M-PAM, M-PSK, M-FSK, M-QAM How to detect the transmitted information at the receiver Coherent detection Non-coherent detection

  4. Lecture 7 Block diagram of a DCS Source encode Channel encode Pulse modulate Bandpass modulate Format Digital modulation Channel Digital demodulation Source decode Demod. Sample Channel decode Format Detect

  5. Lecture 7 Bandpass modulation Bandpass modulation: The process of converting a data signal to a sinusoidal waveform where its amplitude, phase or frequency, or a combination of them, are varied in accordance with the transmitting data. Bandpass signal: where is the baseband pulse shape with energy . We assume here (otherwise will be stated): is a rectangular pulse shape with unit energy. Gray coding is used for mapping bits to symbols. denotes average symbol energy given by

  6. Lecture 7 Demodulation and detection Demodulation: The receiver signal is converted to baseband, filtered and sampled. Detection: Sampled values are used for detection using a decision rule such as the ML detection rule. Decision circuits (ML detector)‏

  7. Lecture 7 Coherent detection Coherent detection requires carrier phase recovery at the receiver and hence, circuits to perform phase estimation. Sources of carrier-phase mismatch at the receiver: Propagation delay causes carrier-phase offset in the received signal. The oscillators at the receiver which generate the carrier signal, are not usually phased locked to the transmitted carrier.

  8. Lecture 7 Coherent detection .. Circuits such as Phase-Locked-Loop (PLL) are implemented at the receiver for carrier phase estimation ( ). PLL Used by correlators Oscillator 90 deg. I branch Q branch

  9. Lecture 7 Bandpass Modulation Schemes One dimensional waveforms Amplitude Shift Keying (ASK)‏ M-ary Pulse Amplitude Modulation (M-PAM)‏ Two dimensional waveforms M-ary Phase Shift Keying (M-PSK)‏ M-ary Quadrature Amplitude Modulation (M-QAM)‏ Multidimensional waveforms M-ary Frequency Shift Keying (M-FSK)

  10. Lecture 7 One dimensional modulation, demodulation and detection Amplitude Shift Keying (ASK) modulation: On-off keying (M=2): “0” “1” 0

  11. Lecture 7 One dimensional mod.,… M-ary Pulse Amplitude modulation (M-PAM)‏ 4-PAM: “00” “01” “11” “10” 0

  12. Lecture 7 Example of bandpass modulation:Binary PAM

  13. Lecture 7 Coherent detection of M-PAM One dimensional mod.,...–cont’d ML detector (Compare with M-1 thresholds)‏

  14. Lecture 7 Two dimensional modulation, demodulation and detection (M-PSK)‏ M-ary Phase Shift Keying (M-PSK)‏

  15. Lecture 7 Two dimensional mod.,… (MPSK)‏ “0” “1” “011” “010” “001” “000” “110” “00” “01” “111” “100” “101” “11” “10” BPSK (M=2)‏ 8PSK (M=8)‏ QPSK (M=4)‏

  16. Lecture 7 Two dimensional mod.,…(MPSK)‏ Coherent detection of MPSK Compute Choose smallest

  17. Lecture 7 Two dimensional mod.,… (M-QAM)‏ M-ary Quadrature Amplitude Mod. (M-QAM)‏

  18. Lecture 7 Two dimensional mod.,… (M-QAM)‏ 16-QAM “0000” “0001” “0011” “0010” 3 “1000” “1001” “1011” “1010” 1 -3 -1 1 3 -1 “1100” “1101” “1111” “1110” -3 “0100” “0101” “0111” “0110”

  19. Lecture 7 Two dimensional mod.,… (M-QAM)‏ Coherent detection of M-QAM ML detector Parallel-to-serial converter ML detector

  20. Lecture 7 Multi-dimensional modulation, demodulation & detection M-ary Frequency Shift keying (M-FSK)‏

  21. Lecture 7 Multi-dimensional mod.,…(M-FSK)‏ ML detector: Choose the largest element in the observed vector

  22. Lecture 7 Non-coherent detection Non-coherent detection: No need for a reference in phase with the received carrier Less complexity compared to coherent detection at the price of higher error rate.

  23. Lecture 7 Non-coherent detection … Differential coherent detection Differential encoding of the message The symbol phase changes if the current bit is different from the previous bit. 0 1 2 3 4 5 6 7 1 1 0 1 0 1 1 1 1 1 0 0 1 1 1 Symbol index: Data bits: Diff. encoded bits Symbol phase: 0 0 0

  24. Lecture 7 Non-coherent detection … Coherent detection for diff encoded mod. assumes slow variation in carrier-phase mismatch during two symbol intervals. correlates the received signal with basis functions uses the phase difference between the current received vector and previously estimated symbol

  25. Lecture 7 Non-coherent detection … Optimum differentially coherent detector Sub-optimum differentially coherent detector Performance degradation about 3 dB by using sub-optimal detector Decision Delay T Decision Delay T

  26. Lecture 7 Non-coherent detection … Energy detection Non-coherent detection for orthogonal signals (e.g. M-FSK)‏ Carrier-phase offset causes partial correlation between I and Q branches for each candidate signal. The received energy corresponding to each candidate signal is used for detection.

  27. Lecture 7 Non-coherent detection … Non-coherent detection of BFSK Decision stage: + -

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