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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.
 2N QAM processes N bits in a single channel, so it has N times
spectral efficiency compared with OOK.
直交位相(Q) (Q)
Quadrature-phase
0000
0100
1100
0001
0101
1101
Quadrature-phase (Q)
1000
r
θ
1001
In-phase (I)
同位相(I)
0011
0111
1111
1011
0010
0110
1110
1010
Constellation map for 16 (=24) QAM
0
With OOK
1
In-phase (I)
Various modulation formats for microwaves and
their spectral efficiencies [1]
ASK type
PSK type
MSK type
FSK type
•Multi-level FSK
・16 QAM
・64 QAM
Correlation
Coded
PSK
Satellite
communication
・256 QAM
Coded modulation
Fixed amplitude
Adoption of
coding technique
•Duobinary
FSK
(-1.6 dB)
Quadrature
modulation
type
•Quadrature
modulation
•Associated
quadrature
modulation
C/W (bit/s/Hz)
Large
Small
Amplitude change
Shannon limit
64
M-QAM
256
1024
16
4
Mobile communication
Fixed wireless
communication
Eb/N0 (dB)
Increase in spectral efficiency
Increase in power efficiency
Modulation schemes and their application fields
C: Channel capacity (bit/s), W: Bandwidth (Hz)
Eb/N0: Energy to noise power density ratio per bit
Eb/N0 at BER = 10-4
synchronous detection
is
shown
assuming
Spectral efficiency of various
modulation schemes
[1] Y. Saito, “Modulation and demodulation in digital wireless communication,” IEICE (in Japanese)
Advantages of QAM optical transmission
Microwave transmission
Drawbacks of QAM wireless or
metallic cable transmission:
Obstacle
Transmitted
point
Fading noise caused by
obstacles
Free space
Received
point
Narrow bandwidth transmission
Metallic cable
Optical fiber transmission
Integrated global
network
Regional IP backbone
network
User access
network
Advantages of QAM optical
transmission:
100 Gb/s~1 Tb/s
per wavelength
10 Gb/s~40 Gb/s
per wavelength
10 Mb/s~1 Gb/s
No fading noise in optical
fibers
Broad bandwidth transmission
Configuration for QAM coherent transmission
IF signal
fIF=fs- fL
Optical fiber
Coherent
light source
fs
PD
QAM
modulator
fL
Local oscillator
(LO)
Optical phaselocked loop
(OPLL) circuit
Key components of QAM coherent transmission:
- Coherent light source: C2H2 frequency-stabilized laser
- QAM modulator: Single sideband (SSB) modulator
- OPLL circuit: Tunable tracking laser as an LO
- Demodulator: Digital demodulator using a software (DSP)
Demodulator
A C2H2 frequency-stabilized fiber laser[1]
0
1.5 GHz
反射率 [dB]
[dB]
Reflection
-5
1.48 mm LD
-10
PZT
-15
-20
WDM
VPZT
EDF
-25
-30
Cavity Length ~ 4 m
(FSR= 49.0 MHz)
PM- FBG[2]
-35
-40
1538.7
1538.72
1538.74
1538.76
1538.78
MLP
Wavelength
波長 [nm][nm]
Circulator
80/20 Coupler
Phase Sensitive
Detection Circuit
Feedback Low Pass
Circuit
Filter
Electrical
Amp
DBM
Electrical
Amp
PD
Coupler
1.54 mm Optical Output
(No Frequency Modulation)
Single-frequency Fiber Ring Laser
13C H
2 2
Cell
LN Frequency
Modulator
Laser Frequency
Stabilization Unit
• Frequency stability: 2x10-11
• Line width: 4 kHz
[1] K. Kasai et al., IEICE ELEX., vol. 3, 487 (2006).
[2] A. Suzuki et al., IEICE ELEX., vol. 3, 469 (2006).
QAM modulator[1]
Electrical magnitude of optical signal
I data RFA: F1(t)+DCA
time
MZA
MZC
Optical input
MZB
Q data RFB: F2(t)+DCB
p/2
Optical output
Electrical magnitude of optical signal
DCC
p
2
time
MZ: Mach-Zehnder interferometer
Configuration of QAM modulator
I data
Q data
DCC
[1] S. Shimotsu et al., IEEE Photon. Technol. Lett., vol. 13, 364 (2001).
OPLL circuit with a tunable fiber laser as an LO[1]
Resolution: 10 Hz
20
RF spectrum analyzer
fs
DBM
Loop filter1
(Fast operation: 1 MHz)
PD
fsyn
Synthesizer
fL
Loop filter2
(Slow operation: 10 kHz)
Power [dB]
IF signal: fIF=fs-fL
0
500 Hz
Less than
10 Hz
-20
-40
-60
-80
-100
-1
to PZT
-0.5
0
0.5
1
Frequency [kHz]
IF signal spectrum
LO
to LN phase modulator
-40
Phase error:
0.3 deg.
SSB phase noise
[dBc/Hz]
Tunable fiber laser
- Linewidth: 4 kHz
- Bandwidth of frequency
response：1.5 GHz
-60
-80
-100
-120
-140
10 Hz
[1] K. Kasai et al., IEICE ELEX., vol. 4, 77 (2007).
Frequency offset
1 MHz
SSB phase noise spectrum
Configuration of digital demodulator
DSP (Software Processing)
I (t )  I (t ) cos2wIF t  2f0   Q(t ) sin2wIF t  2f0 
QAM Signal
LPF
S(t) = I(t)cos(wIFt+f0)
-Q(t)sin(wIFt+f0)
A/D
I(t)
2cos(wIFt+f)
Clock
signal
Save to file
Decode
p/2
0, 1, 0, 0, • • •
-2sin(wIFt+f)
LPF Q(t)
Q(t ) - Q(t ) cos2wIF t  2f0   I (t ) sin2wIF t  2f0 
Accumulation of
QAM Data Signal
Bit Error Rate
Measurement
Digital Demodulation Circuit
Our system operates in an off-line condition by using softwares.
Polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s)
coherent optical transmission system[1]
Q
Q
Arbitrary Waveform
Generator
QAM( ) Delay Line
⊥
QAM
Modulator
2.5 GHz
Optical Frequency
QAM(//)
QAM
Modulator
C2H2 FrequencyStabilized Fiber
Laser
QAM data Pilot
signal
Intensity
I
I
Arbitrary Waveform
Generator
Optical Filter (~ 5nm)
PBS Att
EDFA
PC
OFS
Pilot
DSF
75 km
(MUX)
(fOFS =2.5 GHz)
2 GHz FBG IF Signal
fIF=fsyn+fOFS=4 GHz
PD
Att
PBS
(
or
(DEMUX)
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
A/D
DSF
75 km
Digital Signal
Processor
)
DBM
PD
(fsyn= 1.5 GHz)
Synthesizer
Feedback
Circuit
LO
[1] M. Nakazawa, et al., OFC2007, PDP26 (2007).
Pilot(⊥)
(//) LO((//))
QAM data
signal ((//))
2.5GHz 1.5GHz
Intensity
Intensity
Electrical spectrum of IF signal
4 GHz
4 GHz
Optical Frequency
Optical Frequency
0
0
Demodulation bandwidth
2 GHz
Demodulation bandwidth
2 GHz
-20
Power [dB]
-20
Power [dB]
(//)
Pilot(⊥)
(//) LO (//)
QAM data
signal (//)
(//)
2.5GHz 1.5GHz
-40
-60
-40
-60
-80
-80
-100
-100
1
2
3
4
5
Frequency [GHz]
(a) Orthogonal polarization
6
1
2
3
4
5
Frequency [GHz]
(b) Parallel polarization
6
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
1.2
1.2
1
1
Amplitude
H(f)
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0
0
-1.5 -1 -0.5
0
0.5
1
1.5
-0.2
Normalized frequency
Transfer function
-4
-2
0
2
4
Symbol period
Impulse response
Bandwidth narrowing
f
Data signal spectrum
f
[1] H. Nyquist, AIEEE Trans, 47 (1928).
Pilot(⊥)
(//) LO((//))
QAM data
signal ((//))
2.5GHz 1.5GHz
QAM data
signal ((//))
2.5GHz 1.5GHz
4 GHz
4 GHz
Optical Frequency
Optical Frequency
0
0
Demodulation bandwidth
2 GHz
Demodulation bandwidth
1.5 GHz
-20
Power [dB]
-20
Power [dB]
Pilot(⊥)
(//) LO((//))
Intensity
Intensity
Electrical spectrum of IF data signal
-40
-60
-40
-60
-80
-80
-100
-100
1
2
3
4
5
Frequency [GHz]
(a) Without Nyquist filter
6
1
2
3
4
5
Frequency [GHz]
(b) With Nyquist filter
Roll off factor: 0.35
6
Experimental result for polarization-multiplexed 1 Gsymbol/s, 64
QAM (12 Gbit/s) transmission over 150 km[1]
Q
Q
Q
(b) 150 km transmission
for orthogonal data
(Received power: -26 dBm)
(c) 150 km transmission
for parallel data
(Received power: -26 dBm)
Constellation
diagram
Eye pattern
(I)
Eye pattern
(Q)
(a) Back-to-back
(Received power: -29 dBm)
[1] K. Kasai et al., OECC2007, PDP, PD1-1 (2007).
Bit error rate (BER) characteristics
Bit Error Rate
Orthogonal polarization (Back-to-back)
Orthogonal polarization (150 km transmission)
Parallel polarization (Back-to-back)
Parallel polarization (150 km transmission)
10
-3
10
-4
3 dB
10
-5
-38
-36
-34
-32
-30
-28
Received Power [dBm]
-26
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.

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