Quadrature Amplitude Modulation (QAM) format

workablejeansMobile - Wireless

Nov 21, 2013 (3 years and 10 months ago)

64 views

Quadrature Amplitude Modulation (QAM) format

Features of QAM format:


Two carriers with the same frequency are amplitude
-
modulated
independently.


The phase of the two carriers is 90 deg. shifted each other.


2
N

QAM processes N

bits in a single channel
, so it
has
N

times

spectral
efficiency
compared with OO
K
.

Constellation map for 16 (=2
4
) QAM

0000

0100

1000

1100

0101

1101

1001

0001

1111

0011

0111

1011

0110

0010

1110

1010

r
θ
同位相
(I)
直交位相
(Q)
In
-
phase (I)

Quadrature
-
phase (Q)

0

1

With OOK

In
-
phase (I)

Quadrature
-
phase (Q)

Modulation schemes and their application fields

E
b
/N
0
(dB)

C/W (bit/s/Hz)

M
-
QAM

4

16

64

256

1024

(
-
1.6 dB)

C
:

Channel

capacity

(bit/s),

W
:

Bandwidth

(Hz)

E
b
/N
0
:

Energy

to

noise

power

density

ratio

per

bit



E
b
/N
0

at

BER

=

10
-
4

is

shown

assuming

synchronous

detection

[1] Y. Saito, “Modulation and demodulation in digital wireless communication,” IEICE (in Japanese)

Various modulation formats for microwaves and

their spectral efficiencies
[1]

Shannon limit

Increase in power efficiency

Increase in spectral efficiency


16 QAM


64 QAM


256 QAM

Amplitude change

Fixed amplitude

Large

Small

Satellite

communication

Adoption of

coding technique

Fixed wireless
communication

Mobile communication

ASK type

PSK type

MSK type

FSK type

Coded

Correlation

PSK


Quadrature


modulation


Associated


quadrature


modulation


Multi
-
level FSK


Duobinary


FSK

Coded modulation

Spectral efficiency of various

modulation schemes

Quadrature
modulation
type

Advantages of QAM optical transmission

Received
point

Transmitted
point

Obstacle

Free space

Metallic cable

Microwave transmission

Drawbacks of QAM wireless or
metallic cable transmission:

Fading noise caused by
obstacles

Narrow bandwidth transmission

Optical fiber transmission

Regional IP backbone

network

Integrated global

network

10 Mb/s~1 Gb/s

10 Gb/s~40 Gb/s

per wavelength

User access

network

100 Gb/s~1 Tb/s

per wavelength

No fading noise in optical
fibers

Advantages of QAM optical
transmission:

Broad bandwidth transmission


Key components of QAM coherent transmission:

-

Coherent light source: C
2
H
2
frequency
-
stabilized laser

-

QAM modulator: Single sideband (SSB) modulator

-

OPLL circuit: Tunable tracking laser as an LO

-

Demodulator: Digital demodulator using a software (DSP)

IF signal

Coherent

light source

QAM
modulator

Local oscillator

(LO)

Demodulator

Optical fiber

f
s

f
IF
=f
s
-

f
L

f
L

PD

Configuration for QAM coherent transmission

Optical phase
-

locked loop

(OPLL) circuit

[1] K. Kasai et al., IEICE ELEX., vol. 3, 487 (2006).

[2] A. Suzuki et al., IEICE ELEX., vol. 3, 469 (2006).

-40
-35
-30
-25
-20
-15
-10
-5
0
1538.7
1538.72
1538.74
1538.76
1538.78
反射率 [dB]
波長
[nm]
1.5 GHz

Reflection [dB]

Wavelength [nm]

DBM

Coupler

1.54
m
m Optical Output

(No Frequency Modulation)

WDM

V
PZT

80/20 Coupler

PZT

EDF

1.48
m
m LD

Circulator

PM
-

FBG
[2]


MLP

Cavity Length ~ 4 m

(FSR= 49.0 MHz)

Feedback
Circuit

LN Frequency
Modulator

13
C
2
H
2

Cell

PD

Phase Sensitive

Detection Circuit

Low Pass
Filter

Electrical
Amp

Electrical
Amp

Single
-
frequency Fiber Ring Laser

Laser Frequency
Stabilization Unit

A C
2
H
2
frequency
-
stabilized fiber laser
[1]



Frequency stability: 2x10
-
11



Line width: 4 kHz

Optical input

I data

MZ
A

Q data

MZ
B

MZ
C

Optical output

RF
A
:
F
1
(t)+DC
A

RF
B
:
F
2
(t)+DC
B

p
/2

time

time

MZ: Mach
-
Zehnder interferometer


Configuration of QAM modulator

I data

Q data

Electrical magnitude of optical signal

Electrical magnitude of optical signal

2
p
QAM modulator
[1]

[1] S. Shimotsu et al., IEEE Photon. Technol. Lett., vol. 13, 364 (2001).

DC
C

DC
C

-100
-80
-60
-40
-20
0
20
-1
-0.5
0
0.5
1
Power [dB]
Frequency [kHz]
OPLL circuit with a tunable fiber laser as an LO
[1]

SSB phase noise
[dBc/Hz]

SSB phase noise spectrum

Frequency offset

10 Hz

1 MHz

-
40

-
60

-
80

-
100

-
120

-
140

Phase error:
0.3 deg.

IF signal spectrum

Resolution: 10 Hz

Tunable fiber laser


-

Linewidth: 4 kHz


-

Bandwidth of frequency


response

1.5 GHz

PD

Synthesizer

f
syn


LO

DBM

RF spectrum analyzer

f
L

to LN phase modulator

to PZT

f
s

IF signal: f
IF
=f
s
-
f
L

Loop filter1

(Fast operation: 1 MHz)

Loop filter2

(Slow operation: 10 kHz)

[1] K. Kasai et al., IEICE ELEX., vol. 4, 77 (2007).

Less than
10 Hz

500 Hz


Our system operates in an off
-
line condition by using softwares.

2cos(
w
IF
t
+
f
)

0, 1, 0, 0,
• • •

p
/2

LPF

LPF

I
(
t
)

Q
(
t
)

S
(
t
) =
I
(
t
)cos(
w
IF
t
+
f
0
)


-
Q
(
t
)sin(
w
IF
t
+
f
0
)

QAM Signal

-
2sin(
w
IF
t
+
f
)

DSP

Decode

Save to file

(Software Processing)

Clock
signal

A/D

Accumulation of

QAM Data Signal

Digital Demodulation Circuit





0
0
2
2
sin
)
(
2
2
cos
)
(
)
(
f

w

f

w

t
t
Q
t
t
I
t
I
IF
IF




0
0
2
2
sin
)
(
2
2
cos
)
(
-
)
(
f

w

f

w
t
t
I
t
t
Q
t
Q
IF
IF
Bit Error Rate
Measurement


Configuration of digital demodulator

QAM

Modulator

Polarization
-
multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s)
coherent optical transmission system
[1]

PC

QAM(//)

QAM( )

Pilot

LO



(MUX)

(DEMUX)

Optical Filter (~ 5nm)

DSF

75 km

DSF

75 km

Att


A/D

Digital Signal

Processor

IF Signal

f
IF
=f
syn
+f
OFS
=4 GHz

PD

PD

Synthesizer

(f
syn
= 1.5 GHz)

DBM

2 GHz FBG

(f
OFS
=2.5 GHz)

Att

EDFA

EDFA: Erbium
-
doped Fiber Amplifier

PC: Polarization Controller

OFS: Optical Frequency Shifter

PBS: Polarization Beam Splitter

DSF: Dispersion
-
shifted Fiber

FBG: Fiber Bragg Grating

PD: Photo
-
detector

DBM: Double Balanced Mixer

QAM

Modulator

PBS

PBS

Q

I

C
2
H
2

Frequency
-
Stabilized Fiber
Laser

I

Q

Arbitrary Waveform

Generator

Delay Line

( or )

OFS

Feedback

Circuit

Arbitrary Waveform

Generator

Optical Frequency

Pilot

QAM data
signal

Intensity

2.5 GHz

[1] M. Nakazawa, et al., OFC2007, PDP26 (2007).

LO (//)
Pilot(

)
QAM data
signal (//)
4 GHz
2.5GHz
Intensity
1.5GHz
Optical Frequency
Electrical spectrum of IF signal

-100
-80
-60
-40
-20
0
1
2
3
4
5
6
Power [dB]
Frequency [GHz]
-100
-80
-60
-40
-20
0
1
2
3
4
5
6
Power [dB]
Frequency [GHz]
Demodulation bandwidth

2 GHz

2 GHz

(a) Orthogonal polarization

(b) Parallel polarization

(//)

( )

( )

LO (//)
Pilot(

)
QAM data
signal (//)
4 GHz
2.5GHz
Intensity
1.5GHz
Optical Frequency
(//)

(//)

(//)

Demodulation bandwidth

Experimental result for polarization
-
multiplexed 1 Gsymbol/s, 64
QAM (12 Gbit/s) transmission over 150 km

Constellation
diagram

Eye pattern


(I)

Eye pattern
(Q)

(a) Back
-
to
-
back

(Received power:
-
29 dBm)

(b) 150 km transmission



for orthogonal data

(Received power:
-
26 dBm)

(c) 150 km transmission




for parallel data

(Received power:
-
26 dBm)

Improvement of spectral efficiency by using
a Nyquist filter
[1]

Nyquist filter:


Bandwidth reduction of data signal without intersymbol interference

[1] H. Nyquist, AIEEE Trans,
47

(1928).

Data signal spectrum

Bandwidth narrowing

f

f

Impulse response

-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-4
-2
0
2
4
Amplitude
Symbol period
0
0.2
0.4
0.6
0.8
1
1.2
-1.5
-1
-0.5
0
0.5
1
1.5
H(f)
Normalized frequency
Transfer function

-100
-80
-60
-40
-20
0
1
2
3
4
5
6
Power [dB]
Frequency [GHz]
LO (//)
Pilot(

)
QAM data
signal (//)
4 GHz
2.5GHz
Intensity
1.5GHz
Optical Frequency
-100
-80
-60
-40
-20
0
1
2
3
4
5
6
Power [dB]
Frequency [GHz]
Demodulation bandwidth

2 GHz

1.5 GHz

(a) Without Nyquist filter

(b) With Nyquist filter


Roll off factor: 0.35

(//)

( )

( )

LO (//)
Pilot(

)
QAM data
signal (//)
4 GHz
2.5GHz
Intensity
1.5GHz
Optical Frequency
( )

(//)

Demodulation bandwidth

( )

Electrical spectrum of IF data signal

Constellation
diagram

Eye pattern


(I)

Eye pattern
(Q)

(a) Back
-
to
-
back

(Received power:
-
29 dBm)

(b) 150 km transmission



for orthogonal data

(Received power:
-
26 dBm)

(c) 150 km transmission




for parallel data

(Received power:
-
26 dBm)

Q

Q

Q

Experimental result for polarization
-
multiplexed 1 Gsymbol/s, 64
QAM (12 Gbit/s) transmission over 150 km
[1]

[1] K. Kasai et al., OECC2007, PDP, PD1
-
1 (2007).

Orthogonal polarization (Back
-
to
-
back)

Orthogonal polarization (150 km transmission)

Parallel polarization (Back
-
to
-
back)

Parallel polarization (150 km transmission)

10
-5
10
-4
10
-3
-38
-36
-34
-32
-30
-28
-26
Bit Error Rate
Received Power [dBm]
3 dB

Bit error rate (BER) characteristics

Conclusion

Two

emerging

optical

transmission

technologies

were

described
.



(
1
)

Ultrahigh
-
speed

OTDM

transmission



160

Gbit/s
-
1
,
000

km

transmission

was

successfully

achieved

by

combing

DPSK

and

time
-
domain

OFT
.


OFT

has

crucial

potential

especially

for

high

bit

rate,

thus

it

is

expected

to

play

an

important

role

for

OTDM

transmission

at

320

Gbit/s

and

even

faster
.


(
2
)

Coherent

QAM

transmission



We

have

successfully

transmitted

a

polarization
-
multiplexed

1

Gsymbol/s,

64

QAM

(
12

Gbit/s)

coherent

optical

signal

over

150

km

within

an

optical

bandwidth

of

1
.
5

GHz

using

a

Nyquist

filter
.



Thus,

a

spectral

efficiency

of

8

bit/s/Hz

has

been

achieved

in

a

single
-
channel
.