digital communications chapter 2 formatting and baseband modulation signal processing lab

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


Basic Digital Communication Transformations


Formatting and Transmissionof Baseband Signal


Seven-bit ASCII


EBCDIC Character Code Set


2.3 Messages, Characters, and Symbols


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


Sampling

)(*)()()()()( fXfXfXtxtxtx

domainFrequencydomainTime

ss


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

ms fT

2

1

mf

ms

s fT

f 21


Sampling Aliasing (cont´d)


Undersampling


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


Sampling and Quantization


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

12

22 q

• Average quantization noise power

4

222 qL

Vp • Signal peak power

• Signal power to average

Quantization noise power

22

22

312

4L

q

qL

N

S

q


PCM Sequence


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


Non-Uniform Quantization


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


Companding Characteristics


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).


Waveform Representation


PCM Waveforms PCM waveforms category

1. Nonreturn-to-zero (NRZ) 2. Phase encoded3. Return-to-zero (RZ) 4. Multilevel binary


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


Spectral Densities of various PCM Waveforms


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


M-ary Pulse-Modulation Waveforms


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)

digital communications chapter 2 formatting and baseband modulation signal processing lab

Documents

pulse code modulation

number of quantization

bandlimited signal

digital word pcm word

compressed signal

uniform intervals

analog waveform

pcm sequences code words