Wireless Networks

Instructor: Fatima NaseemLecture # 03

Computer Engineering Department, University of Engineering and Technology, Taxila

Signal Encoding Techniques

Chapter 6

Digital Signaling

Data source g(t), which may be digital or analog, is encoded as a digital signal, x(t)

Form of x(t) depends on encoding technique that is chosen to optimize the use of transmission medium

Eg. It might be chosen to conserve the bandwidth or minimize errors

Analog Signaling

Carrier signal: a continuous constant frequency signal The frequency of the carrier signal is chosen to be

compatible with transmission medium Data transmitted using carrier signal by modulation

Modulation: process of encoding source data onto a carrier signal

with frequency fc Involve operation on one of the following three

parameters; frequency, amplitude, phase

Analog Signaling

Modulating signal: Input signal m(t) May be analog or digital

Modulated signal: Resulting signal s(t) s(t) bandlimited signal BW on spectrum related to fc, often centered at fc encoding technique is chosen to optimize some

characteristic of transmission

Reasons for Choosing Encoding Techniques

Digital data, digital signal Equipment less complex and less expensive than

digital-to-analog modulation equipment Analog data, digital signal

Permits use of modern digital transmission and switching equipment

Common to digitize voice signals prior to transmission to improve quality

For wireless transmission the resulting digital data must be modulated onto an analog carrier

Reasons for Choosing Encoding Techniques

Digital data, analog signal Some transmission media will only propagate analog

signals E.g., optical fiber and unguided media

Analog data, analog signal Analog data in electrical form can be transmitted easily

and cheaply Done with voice transmission over voice-grade lines Must be modulated on a higher frequency carrier

Signal Encoding Criteria Some important terms:

Digital signal sequence of discrete, discontinuous voltage pulses

Each pulse is a signal element Binary data are transmitted by encoding each

data bit into signal element A digital bit stream can be encoded onto an

analog signal as a sequence of signal elements, with each signal element being a pulse of constant freq, amp, phase

There might be one-to-one, one to many or many to one correspondence

Signal Encoding Criteria

Data signaling rate/data rate: bits per second data is transmitted Duration/length of a bit: amount of time taken by

the transmitter to emit a bit; for data rate R, bit duration is 1/R

Modulation rate: rate at which signal level is changed Expressed in baud/ signal elements per second Depends on nature of encoding technique

Signal Encoding Criteria

Tasks involved in interpreting digital signals at receiver:

Receiver must know timing of each bit i.e. when it starts and when it ends

Receiver must know whether the signal level for each bit position is high or low

For these tasks the sampling of each bit position is done in the middle of interval.

Signal Encoding Criteria

What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth

An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate

Factors Used to CompareEncoding Schemes

Signal spectrum With lack of high-frequency components, less bandwidth

required Transfer function of a channel is worse near band edges

Clocking Ease of determining beginning and end of each bit position

Factors Used to CompareEncoding Schemes

Signal interference and noise immunity Performance in the presence of noise

Cost and complexity The higher the signal rate to achieve a given data rate, the greater

the cost

Basic Encoding Techniques

Modulation involves operation on one of the three characteristics of carrier signal: Amp, Freq, Phase

Digital data to analog signal Amplitude-shift keying (ASK)

Amplitude difference of carrier frequency Frequency-shift keying (FSK)

Frequency difference near carrier frequency Phase-shift keying (PSK)

Phase of carrier signal shifted

Basic Encoding Techniques

Amplitude-Shift Keying

Two binary values represented by two amplitudes of the carrier frequency

One binary digit represented by presence of carrier, at constant amplitude

Other binary digit represented by absence of carrier

where the carrier signal is Acos(2πfct)

ts tfA c2cos

01binary 0binary

Amplitude-Shift Keying

Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber

Binary Frequency-Shift Keying (BFSK)

Two binary digits represented by two different frequencies near the carrier frequency

where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

ts tfA 12cos tfA 22cos

1binary 0binary

Binary Frequency-Shift Keying (BFSK)

Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio

transmission Can be used at higher frequencies on LANs that

use coaxial cable

Multiple Frequency-Shift Keying (MFSK)

More than two frequencies are used More bandwidth efficient but more susceptible to error One signaling element represents more than one bit The transmitted signal for one signal element time is:

f i = f c + (2i – 1 – M)f d

f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L

L = number of bits per signal element

tfAts ii 2cos Mi 1

Multiple Frequency-Shift Keying (MFSK)

To match data rate of input bit stream, each output signal element is held for:

Ts=LT seconds where T is the bit period (data rate = 1/T)

So, one signal element encodes L bits

Multiple Frequency-Shift Keying (MFSK)

Total bandwidth required 2Mfd

Minimum frequency separation required 2fd=1/Ts

Therefore, modulator requires a bandwidth ofWd=2L/LT=M/Ts

Multiple Frequency-Shift Keying (MFSK)

Phase-Shift Keying (PSK)

Two-level PSK (BPSK) Uses two phases to represent binary digits

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos

tfA c2cos1binary 0binary

Anther formulation is if d(t) is a discrete function that takes on values +1 and -1,same results can be achieved

A d(t)d

ts

Phase-Shift Keying (PSK)

Differential PSK (DPSK) Phase shift with reference to previous bit

Binary 0 – signal burst of same phase as previous signal burst

Binary 1 – signal burst of opposite phase to previous signal burst

Phase-Shift Keying (PSK)

Four-level PSK (QPSK) For efficient use of bandwidth each element represents

more than one bit

ts

42cos

tfA c 11

4

32cos

tfA c

4

32cos

tfA c

42cos

tfA c

01

00

10

Phase-Shift Keying (PSK) Multilevel PSK

Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved

D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2L

L = number of bits per signal element

M

R

L

RD

2log

Performance

Bandwidth of modulated signal (BT) ASK, PSK BT=(1+r)R

FSK BT=2DF+(1+r)R

R = bit rate 0 < r < 1; related to how signal is filtered DF = f2-fc=fc-f1

Performance

Bandwidth of modulated signal (BT)

MPSK

MFSK

L = number of bits encoded per signal element M = number of different signal elements

RM

rR

L

rBT

2log

11

R

M

MrBT

2log

1

Quadrature Amplitude Modulation

QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same

carrier frequency

tftdtftdts cc 2sin2cos 21

Quadrature Amplitude Modulation

Any Questions?