ECE 4710: Lecture #27 1 QPSK & MPSK QPSK and MPSK if baseband m(t) is rectangular pulse then...

ECE 4710: Lecture #27 1

QPSK & MPSK

QPSK and MPSK if baseband m(t) is rectangular pulse then envelope of RF signal

is constant (excluding bit transitions)

QPSK signal constellation

Signal points located on

circle of constant radius = Ac

“00”“11”I

Q)(tg

cAcA

“01”

“10”

cjA

cjA


QAM

Quadrature Amplitude Modulation = QAM Signal constellation NOT restricted to having signal points

on circle of constant radius = Ac

» Envelope of RF signal will vary in amplitude

Quadrature phase information required for symbol detection

Amplitude amplitude information also required for symbol detection

QPSK is special case of QAM when M = 4 QAM is most general form of combined AM and PM

digital modulation method


QAM

General QAM signal

I & Q time domain waveforms

)()()()()( where

)2sin()()2cos()()(tj

cc

etRtyjtxtg

tftytftxts

n

nn

n DnthytyD

nthxtx 11 )( & )(

amplitudes IQ permitted,

rate symbol

shape pulse symbol)( where 1

nn yx

RD

th


QAM Generation

signal digital level- 2 DACsbit 2Two M spsRD

R bits/sec


QAM Signal Constellation

M = 16 Symbol Constellation 16-QAM

» xi & yi are each M =4 multi-level signals

4 bits/symbol Rectangular constellation

» Amplitude modulation!!

QPSK has circular

constellation» Constant envelope

16-QAM used for 2400 bps

V.22 Modem standard

I

Q

xi

yi


FSK & QAM Modem Standards


Other QAM Dial Up Modems

Type Data Rate Modulation

V.32 9,600 32-QAM

V.32bis 14,400 128-QAM

V.34 28,800 960-QAM

V.90 56,000 PCM


V.32 Modem Standard

Option 1: 32 QAM or QPSK

Option 2: 16 QAM or QPSK

Option 1 Data Rate Modulation

Option 2 Data Rate Modulation

Error Coding!!

4 data bits +1 coding bit =5 bits/symbol

4 data bits/symbol


V.33 with 128 QAM

6 data bits +1 coding bit =7 bits/symbol


QAM Modems

Most dial-up modems transmitted at 2400 sps VF spectrum on phone line limited to 300 to 3600 Hz

» Lower frequencies are noisy from 60 Hz AC power supply + harmonics

Signal BW 1 / Ts 2.4 kHz < 3.3 kHz phone line BW

As # of QAM levels M increases the data rate increases for same symbol rate and signal BW

Why not increase M indefinitely to get higher data rates on phone lines?? As constellation diagram becomes more dense, the spacing between signal

points becomes smaller thereby increasing likelihood of noise corrupting data

» Required S/N increases for same BER

» BER increases for same S/N


Channel Capacity

Shannon’s formula Available phone line BW 3300 Hz (e.g. 3600 – 300)

S/N (dB) C (kbps) C (kbps)

10 11 8

20 22 16

30 33 24

40 44 32

50 55 40

NSBC 1log2

Ideal Realistic


Channel Capacity

S/N’s of 30-50 dB required for reasonable data rates over VF phone lines Phone line S/N varies with time on single line and between different

lines and circuits

Average (typical) U.S. phone line can support 24-32 kbps

Modern modems negotiate data rate based upon S/N Modem drops to “fallback” lower data rate if S/N cannot support

maximum data rate offered by modem S/N monitored throughout connection to adjust modem speed as

necessary


Channel Capacity

High density QAM signal constellations are only appropriate for high S/N communication systems Normally this implies it is a wired network

» Twisted pair, T1, T3, fiber optic, etc.

40-60 dB S/N needed for high density QAM» 128 QAM V.32bis» 960 QAM V.34» 1664 QAM V.34+


Channel Capacity

Wireless Communication Channels Attenuation in channel is much larger than wired channel

for same distance S/N ratios are much lower due to wireless channel loss Typical S/N ratios for wireless cellular mobile phones are

8-12 dB Low S/N ratios cannot support high density signal

constellations Typical signal constellations used for wireless mobile

» QPSK, OQPSK, /4 DQPSK all are M = 4!!

ECE 4710: Lecture #27 1 QPSK & MPSK QPSK and MPSK if baseband m(t) is rectangular pulse then...

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