Digital Communications Chapter 2 Formatting and Baseband Modulation Signal Processing Lab

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Block Diagram of a DCS

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Basic Digital Communication Transformations

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Formatting and Transmission of Baseband Signal

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Seven-bit ASCII

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. EBCDIC Character Code Set

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. 2.3 Messages, Characters, and Symbols

Signal Processing Lab., Dept. 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., Dept. of Elec. and Info. Engr., Korea Univ. Sampling

Signal Processing Lab., Dept. 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

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Sampling Aliasing (cont´d)

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. Undersampling

Signal Processing Lab., Dept. 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=log 2 L 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., Dept. of Elec. and Info. Engr., Korea Univ. Sampling and Quantization

Signal Processing Lab., Dept. 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 Average quantization noise power Signal peak power Signal power to average Quantization noise power

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. PCM Sequence

Signal Processing Lab., Dept. 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., Dept. of Elec. and Info. Engr., Korea Univ. Non-Uniform Quantization

Signal Processing Lab., Dept. 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., Dept. of Elec. and Info. Engr., Korea Univ. Companding Characteristics

Signal Processing Lab., Dept. 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=log 2 M 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., Dept. of Elec. and Info. Engr., Korea Univ. Waveform Representation

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. PCM Waveforms  PCM waveforms category 1. Nonreturn-to-zero (NRZ) 2. Phase encoded 3. Return-to-zero (RZ) 4. Multilevel binary

Signal Processing Lab., Dept. 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., Dept. of Elec. and Info. Engr., Korea Univ. Spectral Densities of various PCM Waveforms

Signal Processing Lab., Dept. 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=log 2 M bits of PCM words

Signal Processing Lab., Dept. of Elec. and Info. Engr., Korea Univ. M-ary Pulse-Modulation Waveforms

Signal Processing Lab., Dept. 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)