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Bandpass Modulation and Coherent & Non-coherent Detection

Learn about various bandpass modulation schemes used in digital communication systems, including M-PAM, M-PSK, M-FSK, and M-QAM. Understand coherent and non-coherent detection methods for accurately recovering transmitted information.

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Bandpass Modulation and Coherent & Non-coherent Detection

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  1. EE 3220: Digital Communication Lec-7: Bandpass modulation and Coherent and non-coherent detection Dr. Hassan Yousif Ahmed Department of Electrical Engineering College of Engineering at Wadi Aldwasser Slman bin Abdulaziz University Dr Hassan Yousif

  2. 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 Dr Hassan Yousif

  3. 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 Dr Hassan Yousif

  4. 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 Dr Hassan Yousif

  5. 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)‏ Dr Hassan Yousif

  6. 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. Dr Hassan Yousif

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

  8. 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) Dr Hassan Yousif

  9. On-off keying (M=2): “0” “1” 0 One dimensional modulation, demodulation and detection • Amplitude Shift Keying (ASK) modulation: Dr Hassan Yousif

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

  11. Example of bandpass modulation:Binary PAM Dr Hassan Yousif

  12. One dimensional mod.,...–cont’d • Coherent detection of M-PAM ML detector (Compare with M-1 thresholds)‏ Dr Hassan Yousif

  13. Two dimensional modulation, demodulation and detection (M-PSK)‏ • M-ary Phase Shift Keying (M-PSK)‏ Dr Hassan Yousif

  14. “0” “1” “011” “010” “001” “000” “110” “00” “01” “111” “100” “101” “11” “10” Two dimensional mod.,… (MPSK)‏ BPSK (M=2)‏ 8PSK (M=8)‏ QPSK (M=4)‏ Dr Hassan Yousif

  15. Compute Choose smallest Two dimensional mod.,…(MPSK)‏ • Coherent detection of MPSK Dr Hassan Yousif

  16. Two dimensional mod.,… (M-QAM)‏ • M-ary Quadrature Amplitude Mod. (M-QAM)‏ Dr Hassan Yousif

  17. 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” Two dimensional mod.,… (M-QAM)‏ Dr Hassan Yousif

  18. Two dimensional mod.,… (M-QAM)‏ • Coherent detection of M-QAM ML detector Parallel-to-serial converter ML detector Dr Hassan Yousif

  19. Multi-dimensional modulation, demodulation & detection • M-ary Frequency Shift keying (M-FSK)‏ Dr Hassan Yousif

  20. Multi-dimensional mod.,…(M-FSK)‏ ML detector: Choose the largest element in the observed vector Dr Hassan Yousif

  21. 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. Dr Hassan Yousif

  22. 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 Dr Hassan Yousif

  23. 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 Dr Hassan Yousif

  24. 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 Dr Hassan Yousif

  25. 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. Dr Hassan Yousif

  26. Non-coherent detection … • Non-coherent detection of BFSK Decision stage: + - Dr Hassan Yousif

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