ECE 4371, Fall, 2014 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering.

ECE 4371, Fall, 2014

Introduction to Telecommunication Engineering/Telecommunication Laboratory

Zhu Han

Department of Electrical and Computer Engineering

Class 13

Oct. 8th, 2014

OutlineOutline Partial Response

Carrier systems – ASK, OOK, MASK

– FSK, MFSK

– BPSK, DBPSK, MPSK

– MQAM, MQPR

– OQPSK,

– Continuous phase modulation (CPM): MSK, GMSK

Partial Response SignalsPartial Response Signals

Previous classes: Sy(w)=|P(w)|^2 Sx(w)– Control signal generation methods to reduce Sx(w)

– Raise Cosine function for better |P(w)|^2

This class: improve the bandwidth efficiency– Widen the pulse, the smaller the bandwidth.

– But there is ISI. For binary case with two symbols, there is only few possible interference patterns.

– By adding ISI in a controlled manner, it is possible to achieve a signaling rate equal to the Nyquist rate (2W symbols/sec) in a channel of bandwidth W Hertz.

ExampleExample Duobinary Pulse

– p(nTb)=1, n=0,1

– p(nTb)=1, otherwise

Interpretation of received signal– 2: 11

– -2: 00

– 0: 01 or 10 depends on the previous transmission

Duobinary signalingDuobinary signaling

Duobinary signaling (class I partial response)

Duobinary signal and Nyguist CriteriaDuobinary signal and Nyguist Criteria

Nyguist second criteria: but twice the bandwidth

Differential CodingDifferential Coding The response of a pulse is spread over more than one signaling

interval.

The response is partial in any signaling interval.

Detection :– Major drawback : error propagation.

To avoid error propagation, need deferential coding (precoding).

Modified duobinary signalingModified duobinary signaling

Modified duobinary signaling– In duobinary signaling, H(f) is nonzero at the origin.

– We can correct this deficiency by using the class IV partial response.

Modified duobinary signalingModified duobinary signaling

Spectrum

Modified duobinary signalingModified duobinary signaling

Time Sequency: interpretation of receiving 2, 0, and -2?

Pulse GenerationPulse Generation Generalized form of

correlative-level

coding

(partial response signaling)

TradeoffsTradeoffs Binary data transmission over a physical baseband channel can

be accomplished at a rate close to the Nyquist rate, using realizable filters with gradual cutoff characteristics.

Different spectral shapes can be produced, appropriate for the application at hand.

However, these desirable characteristics are achieved at a price :– A large SNR is required to yield the same average probability of

symbol error in the presence of noise.

ExerciseExercise What is the differential Duobinary output

What is the modified Duobinary output and decoded signal? Interpretation of receiving 2, 0, and -2?

What is the modified differential Duobinary output

Other types of partial response signalsOther types of partial response signals

Type r0 r1 r2 r3 r4 p(t) P(W) Levels

ideal 1 2

I 1 1 3

II 1 2 1 5

III 2 1 -1 6

IV 1 0 -1 3

V -1 0 2 0 -1 5

paper

ASK, OOK, MASKASK, OOK, MASK The amplitude (or height) of the sine wave varies to transmit the

ones and zeros

One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)

Binary amplitude shift keying, Bandwidth

d ≥ 0 related to the condition of the line

B = (1+d) x S = (1+d) x N x 1/r

Implementation of binary ASK

OOK and MASKOOK and MASK OOK (On-OFF Key)

– 0 silence.

– Sensor networks: battery life, simple implementation

MASK: multiple amplitude levels

Pro, Con and ApplicationsPro, Con and Applications Pro

– Simple implementation

Con– Major disadvantage is that telephone lines are very susceptible to

variations in transmission quality that can affect amplitude

– Susceptible to sudden gain changes

– Inefficient modulation technique for data

Applications– On voice-grade lines, used up to 1200 bps

– Used to transmit digital data over optical fiber

– Morse code

– Laser transmitters

Example

We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 1?

Solution– The middle of the bandwidth is located at 250 kHz. This

means that our carrier frequency can be at fc = 250 kHz. We can use the formula for bandwidth to find the bit rate (with d = 1 and r = 1).

Frequency Shift KeyingFrequency Shift Keying

One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)

Represent each logical value with another frequency (like FM)

ts tfA 12cos tfA 22cos

1binary 0binary

FSK BandwidthFSK Bandwidth Limiting factor: Physical capabilities of the carrier Not susceptible to noise as much as ASK

Applications– On voice-grade lines, used up to 1200bps– Used for high-frequency (3 to 30 MHz) radio transmission– used at higher frequencies on LANs that use coaxial cable

Example

We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 1?

Solution– This problem is similar to Example 5.3, but we are modulating

by using FSK. The midpoint of the band is at 250 kHz. We choose 2Δf to be 50 kHz; this means

Multiple Frequency-Shift Keying (MFSK)Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used

More bandwidth efficient but more susceptible to error

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

Phase Shift KeyingPhase Shift Keying

One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)

ts tfA c2cos tfA c2cos

1binary 0binary

DBPSK, QPSKDBPSK, QPSK Differential BPSK

– 0 = same phase as last signal element

– 1 = 180º shift from last signal element

Four Level: QPSK

ts

42cos

tfA c 11

4

32cos

tfA c

4

32cos

tfA c

42cos

tfA c

01

00

10

QPSK ExampleQPSK Example

BandwidthBandwidth Min. BW requirement: same as ASK!

Self clocking (most cases)

Concept of a constellation diagram

MPSKMPSK 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

QAM – Quadrature Amplitude ModulationQAM – Quadrature Amplitude Modulation

Modulation technique used in the cable/video networking world

Instead of a single signal change representing only 1 bps – multiple bits can be represented buy a single signal change

Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes)

QAMQAM QAM

– As an example of QAM, 12 different phases are combined with two different amplitudes

– Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations

– With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16)

– Combine ASK and PSK such that each signal corresponds to multiple bits

– More phases than amplitudes– Minimum bandwidth requirement

same as ASK or PSK

QAM and QPRQAM and QPR QAM is a combination of ASK and PSK

– Two different signals sent simultaneously on the same carrier frequency

– M=4, 16, 32, 64, 128, 256

Quadrature Partial Response (QPR)– 3 levels (+1, 0, -1), so 9QPR, 49QPR

tftdtftdts cc 2sin2cos 21

Offset quadrature phase-shift keying (OQPSK)Offset quadrature phase-shift keying (OQPSK)

QPSK can have 180 degree jump, amplitude fluctuation

By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time.

ECE 4371 Fall 2008

Continuous phase modulation (CPM)Continuous phase modulation (CPM)

CPM the carrier phase is modulated in a continuous manner

constant-envelope waveform

yields excellent power efficiency

high implementation complexity required for an optimal receiver

minimum shift keying (MSK)– Similarly to OQPSK, MSK is encoded with bits alternating between

quarternary components, with the Q component delayed by half a bit period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a constant-modulus signal, which reduces problems caused by non-linear distortion.

Gaussian minimum shift keying Gaussian minimum shift keying

GMSK is similar to MSK except it incorporates a premodulation Gaussian LPF

Achieves smooth phase transitions between signal states which can significantly reduce bandwidth requirements

There are no well-defined phase transitions to detect for bit synchronization at the receiving end.

With smoother phase transitions, there is an increased chance in intersymbol interference which increases the complexity of the receiver.

Used extensively in 2nd generation digital cellular and cordless telephone apps. such as GSM