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.