Digital Communications I: Modulation and Coding Course Term 3 - 2008 Catharina Logothetis Lecture 7. - презентация

1 Digital Communications I: Modulation and Coding Course Term Catharina Logothetis Lecture 7

2 2 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 73 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 74 Block diagram of a DCS Format Source encode Format Source decode Channel encode Pulse modulate Bandpass modulate Channel decode Demod. Sample Detect Channel Digital modulation Digital demodulation

5 Lecture 75 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 76 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 77 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 78 Coherent detection.. Circuits such as Phase-Locked-Loop (PLL) are implemented at the receiver for carrier phase estimation ( ). PLL Oscillator 90 deg. Used by correlators I branch Q branch

9 Lecture 79 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 710 One dimensional modulation, demodulation and detection Amplitude Shift Keying (ASK) modulation: 0 01 On-off keying (M=2):

11 Lecture 711 One dimensional mod.,… M-ary Pulse Amplitude modulation (M-PAM) 4-PAM:

12 Lecture 712 Example of bandpass modulation: Binary PAM

13 Lecture 713 Coherent detection of M-PAM ML detector (Compare with M-1 thresholds) One dimensional mod.,...–contd

14 Lecture 714 Two dimensional modulation, demodulation and detection (M-PSK) M-ary Phase Shift Keying (M-PSK)

15 Lecture 715 Two dimensional mod.,… (MPSK) QPSK (M=4) BPSK (M=2) 8PSK (M=8)

16 Lecture 716 Two dimensional mod.,…(MPSK) Coherent detection of MPSK ComputeChoose smallest

17 Lecture 717 Two dimensional mod.,… (M-QAM) M-ary Quadrature Amplitude Mod. (M-QAM)

18 Lecture 718 Two dimensional mod.,… (M-QAM) QAM

19 Lecture 719 Two dimensional mod.,… (M-QAM) Coherent detection of M-QAM ML detector Parallel-to-serial converter

20 Lecture 720 Multi-dimensional modulation, demodulation & detection M-ary Frequency Shift keying (M-FSK)

21 Lecture 721 Multi-dimensional mod.,…(M-FSK) ML detector: Choose the largest element in the observed vector

22 Lecture 722 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 723 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 Symbol index: Data bits: Diff. encoded bits Symbol phase:

24 Lecture 724 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 725 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 726 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 727 Non-coherent detection … Non-coherent detection of BFSK Decision stage: + -