Презентация на тему: " Digital Communications I: Modulation and Coding Course Term 3 - 2008 Catharina Logothetis Lecture 7." — Транскрипт:
Digital Communications I: Modulation and Coding Course Term Catharina Logothetis Lecture 7
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
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
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
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
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)
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.
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
Lecture 721 Multi-dimensional mod.,…(M-FSK) ML detector: Choose the largest element in the observed vector
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.
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:
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
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
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.