Download - Digital Communications
Digital CommunicationsDigital CommunicationsChapter 2
Formatting and Baseband ModulationSignal Processing Lab
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Block Diagram of a DCS
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Basic Digital Communication Transformations
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Formatting and Transmissionof Baseband Signal
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Seven-bit ASCII
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
EBCDIC Character Code Set
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
2.3 Messages, Characters, and Symbols
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Format Analog Signals To transform an analog waveform into a form that is compatible with a digital communication, the following steps are taken:
Sampling
Quantization and encoding
Baseband transmission
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Sampling
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Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Sampling (cont´d)Sampling Theorem
Sampling theorem : A bandlimited signal with no spectra components beyond can be uniquely determined by values sampled at uniform intervals of
The sampling rate, is called Nyquist rate
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Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Sampling Aliasing (cont´d)
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Undersampling
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Quantization and Encoding Quantization: Sampled values are quantized to values of a finite set.
Pulse code modulation (PCM): Encoding the quantized PAM signals into a digital word (PCM word or codeword).
Each quantized sample is digitally encoded into an ℓ bits codeword where ℓ=log2L and L in the number of quantization level
The analog information can be recovered from the quantized samples, but not precisely
the fidelity increases as the number of quantization level increases.
This causes bandwidth expansion for real-time application
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Sampling and Quantization
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Quantization Noise Quantization noise is a round-off or truncation error which is inversely proportional to the number of quantization levels.
Quantization noise for a uniform (linear) quantizer
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• Average quantization noise power
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• Signal power to average
Quantization noise power
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Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
PCM Sequence
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Statistics of Speech Amplitudes In speech, weak signal are more frequent than strong ones
Adjusting the step size of the quantizer by taking into account the speech statistics improves the SNR for the input range
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Non-Uniform Quantization
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Non-Uniform Quantization Non-uniform quantization implies varying step sizes; smaller steps for weak signals as compared to strong ones
It is done by uniformly quantizing the compressed signal
At the receiver, an inverse compression characteristic, called
expansion is employed to avoid signal distortion
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Companding Characteristics
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
2.8 Baseband Transmission To transmit information through physical channels, PCM sequences (code words) are transformed to pulses (waveforms).
Each waveform carries a symbol from a set of size M.
Each transmit symbol represents k=log2M bits of the PCM words.
PCM waveforms (line codes) are used for binary symbols (M=2).
M-ary pulse modulation are used for non-binary symbols (M>2).
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Waveform Representation
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
PCM Waveforms PCM waveforms category
1. Nonreturn-to-zero (NRZ) 2. Phase encoded3. Return-to-zero (RZ) 4. Multilevel binary
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Baseband Transmission (cont´d)PCM Waveforms (cont´d)
Criteria for comparing and selecting PCM waveforms
Spectral characteristics (power spectral density and bandwidth efficiency)
Bit synchronization capability
Error detection capability
Interference and noise immunity
Implementation cost and complexity
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
Spectral Densities of various PCM Waveforms
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
M-ary Pulse Modulation
M-ary pulse modulations category
M-ary pulse-amplitude modulation (PAM) M-ary pulse-position modulation (PPM) M-ary pulse-duration modulation (PDM)
M-ary PAM is a multi-level signaling where each symbol takes one of the M allowable amplitude levels, each representing k=log2M bits of PCM words
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
M-ary Pulse-Modulation Waveforms
Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.
M-ary Pulse Modulation (cont´d)
For a given data rate, M-ary PAM(M>2) requires less bandwidth than binary PCM
For a given average pulse power, binary PCM is easier to detect than M-ary PAM(M>2)