Baseband Modulation-Digital Communication Systems-Lecture Slides, Slides of Digital Communication Systems

Download Baseband Modulation-Digital Communication Systems-Lecture Slides and more Slides Digital Communication Systems in PDF only on Docsity!

1

Digital Communication

Systems

Dr. Shurjeel Wyne

Lecture 4

Baseband Modulation

(Line Codes)

2

z Define baseband modulation?

z Symbol and bit duration

z Mapping of bits to symbols

3

Transmission of baseband

signal

Encode

Transmit

Pulse

Sample Quantize modulate

Demodulate/ Detect

Channel

Low-pass Receive filter Decode

Format/Analog-to-Digital Conversion

Format

source

sink

PCM Codeword (Sequence of bits)

Pulses (waveforms)

4

Baseband transmission

z To transmit information through physical

channels, PCM sequences (codewords) are

transformed to pulses (waveforms).

z Each waveform carries a symbol from a symbol-set of size M.

z Each transmit symbol represents a grouping of k bits of the PCM words.

z Binary pulse modulation used for binary symbols (M=2). Waveforms called PCM waveforms or line codes

z M-ary pulse modulation used for non-binary symbols (M>2).

k =log 2 M

7

PCM waveforms

z PCM waveforms are categorized into 4

groups:

ƒ Phase encoded ƒ Multilevel binary

ƒ Nonreturn-to-zero (NRZ) ƒ Return-to-zero (RZ)

1 0 1 1 0

0 T 2T 3T 4T 5T

+V

-V

+V

+V

-V

1 0 1 1 0

0 T 2T 3T 4T 5T

+V

-V

+V

-V

+V

-V

NRZ-L

Unipolar-RZ

Bipolar-RZ

Manchester

Miller

Dicode NRZ

8

PCM Waveform Types - NRZ

z NRZ-L (L for level)

z A binary one is represented by one voltage level and a binary

zero is represented by another voltage level.

z NRZ-M (M for mark) - type of Differential Encoding

z A one (mark) is represented by a change in level and a zero

(space) is represented by no change in level

z NRZ-S (S for space) - type of Differential Encoding

z A one is represented by no change in level and a zero is

represented by a change in level.

Nonreturn-to-zero (NRZ): Pulse lasts an entire bit period

Differential means that waveform value in current bit interval is determined by the waveform value n previous bit interval

9

z Unipolar-RZ

z A one is represented by a half-bit-wide pulse and a zero is

represented by the absence of a pulse.

z Bipolar-RZ

z Ones and zeros are represented by opposite-level pulse that are

one-half bit wide.

z RZ-AMI (AMI for “alternate mark inversion”)

z Consecutive Ones are represented by equal-amplitude

alternating pulses. Zeros are represented by the absence of

pulses.

PCM Waveform Types - RZ

Return to Zero (RZ) - pulse lasts just half of a bit period

RZ-AMI has limited error detection capability: what should Rx do if consecutive pulses received with same polarity?

10

z Bi-Φ-L (bi-phase-level) or Manchester Coding

z Binary one is represented by a half-bit-interval wide pulse,

positioned during first half of the bit interval;

z Binary zero is represented by a half-bit-interval wide pulse,

positioned during second half of the bit interval.

z Bi-Φ-M (bi-phase-mark)

z A transition occurs at the beginning of every bit interval.

z Binary one is represented by a second transition one-half bit

interval later.

z Binary zero is represented by no second transition.

PCM Waveform Types –

Phase Encoded

13

PCM waveform characteristics

z line code are designed to optimize one or

more of the following goals:

z Synchronization capability

z DC component

z Spectral characteristics (power spectral density and bandwidth efficiency)

z Noise immunity

z Error detection capability

z Implementation cost and complexity

14

Synchronization: The receiver’s bit /symbol intervals must correspond exactly to bit/symbol intervals at transmitter in order to correctly interpret the received signals

A self-synchronizing (self-clocking) digital signal includes the timing information in the data being transmitted.

PCM waveform characteristics -

Synchronization Capability

Transmitter

Receiver

Assume synchronization error: Rx clock is faster than Tx clock

15

Unipolar NRZ encoding

ƒSelf-synchronization can be achieved if there are

transitions in the signal that alert the receiver to the

beginning, mid or end of the bit interval.

ƒTransitions can alert the receiver to reset its clock

PCM waveform characteristics -

Synchronization Capability …

Manchester encoding

Good

Bad

16

Some line coding schemes have a DC component, i.e.,

non-negligible power spectral density at zero frequency

DC component in the signal is undesirable

z Signal has to pass through a system that doesn’t allows the passage of DC component, signal becomes distorted and may cause errors.

PCM waveform characteristics -

DC component

Example of a signal with DC component

Example of a signal without DC component

19

PCM waveform characteristics…

Low probability of bit error

z Noise immunity: Some PCM waveforms are more immune to noise than others, e.g., for the same signal-to-noise ratio, NRZ waveforms provide a lower probability of error relative to unipolar RZ

z Error detection: Some PCM waveforms also provide limited error detection capability that contributes to a low probability of error, e.g. NRZ- AMI, a single error will violate AMI rule

20

BASEBAND TRANSMISSION

M-ary pulse modulation

Each transmitted waveform (pulse) carries a symbol from a symbol-set of size M where M>

M-ary pulse modulation categories:

z M-ary pulse-amplitude modulation (PAM)

z M-ary pulse-width modulation (PWM) z M-ary pulse-position modulation (PPM)

21

In M-ary PAM one of the M allowable amplitude levels are assigned to each of the M possible symbol values, each representing bits of PCM sequence

.

z For the same data rate, M-ary PAM ( M>2 ) requires less bandwidth than binary PCM.

z For the same average pulse power, binary PCM will have lower BER than M-ary PAM ( M>2 ).

k =log 2 M

M-ary Pulse Amplitude

Modulation (PAM)

22

Pulse Width Modulation (PWM)

In PWM, the pulse width is varied by an amount that corresponds to

the value of the digital message symbol. The pulse amplitude is held

constant.

Pulse Position Modulation (PPM)

In PPM, the position (or time of occurrence) of each pulse from

some reference time is varied by an amount that corresponds to the

value of the digital message symbol. Both the pulse amplitude and

width are held constant..

M-ary Pulse Width Modulation (PWM)

and Pulse Position Modulation (PPM)

25

z Assume that an analog audio voice-frequency(VF) telephone signal

occupies a band from 300 to 3,400Hz. The signal is to be converted to a

PCM signal for transmission over a digital telephone system.

z The minimum sampling frequency is

z To be able to use a low-cost low-pass anti-aliasing filter, the VF signal is

oversampled with a sampling frequency fs = 8ksamples/s.

z Assume that each sample values is represented by 8 bits; then the bit rate

of the binary PCM signal is

DESIGN OF A PCM SIGNAL FOR TELEPHONE SYSTEMS

(^8) 64 kbit/s

2 x 3.4 KHz = 6.8 ksample/ s

26

Example: Bits per PCM Word and Bits per

Symbol

z Bits per PCM Word

 l = log 2 L

z M-ary Pulse Modulation Waveforms (Bits per Symbol )

 k = log 2 M

z Example:

z The information in an analog waveform, whose maximum

frequency fm=4000Hz, is to be transmitted using a 16-level

PAM system. The quantization distortion must not exceed

±1% of the peak-to-peak analog signal.

(a) What is the minimum number of bits per sample or bits

per PCM word that should be used in this system?

(b) What is the minimum required sampling rate, and what

is the resulting bit rate?

(c) What is the 16-ary PAM symbol Transmission rate?

27