Digital Communications Chapter 2 Formatting and Baseband Modulation

Digital Communications Chapter 2 Formatting and Baseband Modulation Signal Processing Lab

Block Diagram of a DCS Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Basic Digital Communication Transformations Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Formatting and Transmission of Baseband Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Seven-bit ASCII Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

EBCDIC Character Code Set Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

2. 3 Messages, Characters, and Symbols Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Format Analog Signals q To transform an analog waveform into a form that is compatible with a digital communication, the following steps are taken: Ø Sampling Ø Quantization and encoding Ø Baseband transmission Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Sampling Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Sampling (cont´d) Sampling Theorem q q 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. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Sampling Aliasing (cont´d) Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Undersampling Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Quantization and Encoding q Quantization: Sampled values are quantized to values of a finite set. q Pulse code modulation (PCM): Encoding the quantized PAM signals into a digital word (PCM word or codeword). ü Each quantized sample is digitally encoded into an ℓ bits codeword where ℓ=log 2 L and L in the number of quantization level ü The analog information can be recovered from the quantized samples, but not precisely Ø Ø 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. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Sampling and Quantization Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Quantization Noise q Quantization noise is a round-off or truncation error which is inversely proportional to the number of quantization levels. ü Quantization noise for a uniform (linear) quantizer • Average quantization noise power • Signal peak power • Signal power to average Quantization noise power Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

PCM Sequence Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Statistics of Speech Amplitudes q In speech, weak signal are more frequent than strong ones q 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. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Non-Uniform Quantization Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Non-Uniform Quantization q Non-uniform quantization implies varying step sizes; smaller steps for weak signals as compared to strong ones ü It is done by uniformly quantizing the compressed signal ü At the receiver, an inverse compression characteristic, called expansion is employed to avoid signal distortion Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Companding Characteristics Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

2. 8 Baseband Transmission q To transmit information through physical channels, PCM sequences (code words) are transformed to pulses (waveforms). ü Each waveform carries a symbol from a set of size M. ü Each transmit symbol represents k=log 2 M bits of the PCM words. ü PCM waveforms (line codes) are used for binary symbols (M=2). ü M-ary pulse modulation are used for non-binary symbols (M>2). Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Waveform Representation Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

PCM Waveforms q PCM waveforms category 1. Nonreturn-to-zero (NRZ) 3. Return-to-zero (RZ) 2. Phase encoded 4. Multilevel binary Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Baseband Transmission (cont´d) PCM Waveforms (cont´d) q Criteria for comparing and selecting PCM waveforms ü Spectral characteristics (power spectral density and bandwidth efficiency) ü Bit synchronization capability ü Error detection capability ü Interference and noise immunity ü Implementation cost and complexity Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

Spectral Densities of various PCM Waveforms Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

M-ary Pulse Modulation q M-ary pulse modulations category M-ary pulse-amplitude modulation (PAM) Ø M-ary pulse-position modulation (PPM) Ø M-ary pulse-duration modulation (PDM) Ø ü 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. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

M-ary Pulse-Modulation Waveforms Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.

M-ary Pulse Modulation (cont´d) q For a given data rate, M-ary PAM(M>2) requires less bandwidth than binary PCM q For a given average pulse power, binary PCM is easier to detect than M-ary PAM(M>2) Signal Processing Lab. , http: //signal. korea. ac. kr Dept. of Elec. and Info. Engr. , Korea Univ.