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Quadrature Amplitude Modulation (QAM) formatQuadrature 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.
Constellation map for 16 (=24) QAM
0000 0100 10001100
0101 1101 10010001
11110011 0111 1011
01100010 1110 1010
rθ 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
Eb/N0 (dB)
C/W
(b
it/s
/Hz)
M-QAM
4
1664
2561024
(-1.6 dB)
C: Channel capacity (bit/s), W: Bandwidth (Hz)Eb/N0: Energy to noise power density ratio per bit Eb/N0 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]
Various modulation formats for microwaves and their spectral efficiencies [1]
Shannon limit
Increase in power efficiencyIncrease in spectral efficiency
・ 16 QAM
・ 64 QAM
・ 256 QAM
Amplitude change Fixed amplitudeLarge Small
Satellitecommunication
Adoption of coding technique
Fixed wireless communication
Mobile communication
ASK type PSK type MSK type FSK type
Coded CorrelationPSK
•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 transmissionAdvantages 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 backbonenetwork
Integrated globalnetwork
10 Mb/s~1 Gb/s
10 Gb/s~40 Gb/sper wavelength
User access network
100 Gb/s~1 Tb/sper wavelength
No fading noise in optical fibers
Advantages of QAM optical transmission:
Broad bandwidth transmission
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)
IF signal
Coherent light source
QAM modulator
Local oscillator(LO)
Demodulator
Optical fiberfs
fIF=fs- fL
fL
PD
Configuration for QAM coherent transmissionConfiguration 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
[dB]
反射
率
波長 [nm]
1.5 GHz
Ref
lect
ion
[d
B]
Wavelength [nm]
DBM
Coupler
1.54 m Optical Output (No Frequency Modulation)
WDM
VPZT
80/20 Coupler
PZT
EDF
1.48 m LD
Circulator
PM- FBG[2]
MLP
Cavity Length ~ 4 m(FSR= 49.0 MHz)
Feedback Circuit
LN Frequency Modulator
13C2H2 Cell
PD Phase Sensitive Detection Circuit
Low Pass Filter
Electrical Amp
Electrical Amp
Single-frequency Fiber Ring Laser Laser Frequency Stabilization Unit
A C2H2 frequency-stabilized fiber laser[1]A C2H2 frequency-stabilized fiber laser[1]
• Frequency stability: 2x10-11
• Line width: 4 kHz
Optical input
I data
MZA
Q data
MZB
MZC Optical output
RFA: 1(t)+DCA
RFB: 2(t)+DCB
/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
QAM modulator[1]QAM modulator[1]
[1] S. Shimotsu et al., IEEE Photon. Technol. Lett., vol. 13, 364 (2001).
DCC
DCC
-100
-80
-60
-40
-20
0
20
-1 -0.5 0 0.5 1
Po
we
r [d
B]
Frequency [kHz]
OPLL circuit with a tunable fiber laser as an LO[1]OPLL circuit with a tunable fiber laser as an LO[1]
SS
B p
has
e n
ois
e [d
Bc/
Hz]
SSB phase noise spectrumFrequency offset10 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
fsyn
LO
DBM
RF spectrum analyzer
fL
to LN phase modulator
to PZT
fs
IF signal: fIF=fs-fL 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(IFt+)
0, 1, 0, 0, • • •/2
LPF
LPF
I(t)
Q(t)
S(t) = I(t)cos(IFt+0)
-Q(t)sin(IFt+0)
QAM Signal
-2sin(IFt+)
DSP
DecodeSave to file
(Software Processing)
Clock signalA/D
Accumulation of QAM Data Signal Digital Demodulation Circuit
00 22sin)(22cos)()( ttQttItI IFIF
00 22sin)(22cos)(-)( ttIttQtQ IFIF Bit Error Rate Measurement
Configuration of digital demodulator Configuration of digital demodulator
QAMModulator
Polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) coherent optical transmission system[1]
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 SignalProcessor
IF SignalfIF=fsyn+fOFS=4 GHz
PD
PD
Synthesizer
(fsyn= 1.5 GHz)
DBM
2 GHz FBG
(fOFS =2.5 GHz)
Att
EDFA
EDFA: Erbium-doped Fiber AmplifierPC: Polarization ControllerOFS: Optical Frequency ShifterPBS: Polarization Beam SplitterDSF: Dispersion-shifted FiberFBG: Fiber Bragg GratingPD: Photo-detectorDBM: Double Balanced Mixer
QAMModulator
PBS
PBS
Q
I
C2H2 Frequency-Stabilized Fiber Laser
I
Q
Arbitrary Waveform Generator
Delay Line
( or )
OFS
Feedback Circuit
Arbitrary Waveform Generator
Optical Frequency
PilotQAM data signal
Inte
nsi
ty
2.5 GHz
[1] M. Nakazawa, et al., OFC2007, PDP26 (2007).
LO (//)Pilot(⊥)QAM data signal (//)
4 GHz
2.5GHz
Inte
nsit
y
1.5GHz
Optical Frequency
Electrical spectrum of IF signal Electrical spectrum of IF signal
-100
-80
-60
-40
-20
0
1 2 3 4 5 6
Po
wer
[dB
]
Frequency [GHz]
-100
-80
-60
-40
-20
0
1 2 3 4 5 6
Po
wer
[dB
]
Frequency [GHz]
Demodulation bandwidth2 GHz 2 GHz
(a) Orthogonal polarization (b) Parallel polarization
(//)
( )
( ) LO (//)Pilot(⊥)QAM data signal (//)
4 GHz
2.5GHz
Inte
nsit
y
1.5GHz
Optical Frequency
(//)
(//) (//)
Demodulation bandwidth
Experimental result for polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) transmission over 150 km
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 transmissionfor 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]
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
Am
plitu
de
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
Po
wer
[dB
]
Frequency [GHz]
LO (//)Pilot(⊥)QAM data signal (//)
4 GHz
2.5GHz
Inte
nsit
y
1.5GHz
Optical Frequency
-100
-80
-60
-40
-20
0
1 2 3 4 5 6
Po
wer
[dB
]
Frequency [GHz]
Demodulation bandwidth2 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
Inte
nsit
y
1.5GHz
Optical Frequency
( )
(//)
Demodulation bandwidth
( )
Electrical spectrum of IF data signal 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 transmissionfor 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]
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
Err
or
Ra
te
Received Power [dBm]
3 dB
Bit error rate (BER) characteristicsBit error rate (BER) characteristics
ConclusionConclusion
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.