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Optisches OFDM – Eine neue Technik für die
optischen Datenübertragung
Fred BuchaliAlcatel-Lucent, Bell Labs Germany, Stuttgart, Germany
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Outline
Einleitung
Grundlagen – OFDM
O-OFDM Systeme und deren Eigenschaften
Realisierungsaspekte
Zusammenfassung
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Einleitung
Typisches optisches Übertragungssystem
INPUT
Tx Rx
OUTPUT
Binary data
- Bitraten: 155 Mb/s … 40 Gb/s
- Bei 10 Gb/s mußte die chromatische Dispersion berücksichtigt werden, teilweise auchPMD
- Bei 40 Gb/s muß die chromatische Dispersion nach Anwendung von Kompensationsfasern berücksichtigt werden, sowie PMD in weiteren Anwendungen
- Verzerrungen (CD und PMD) begrenzen Einsetzbarkeit von Systemen höhererSymbolrate
- Spektrale Effizienz erlaubt keine weitere Erhöhung der Bitrate im 50 GHz Raster
- Empfindlichkeit der Systeme muß stetig verbessert werden
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Einleitung
- Bitraten über 40 Gb/s sind nachgefragt, 100 Gb/s ist nächste Stufe
- Derzeit in Diskussion:
- DQPSK bei 50 Gbaud
- Kohärentes QPSK
- Kohärentes O-OFDM
- Kohärente Systeme ermöglichen die Übertragung der gesamten Feldinformation in die elektrische Ebene
- Dabei wird erstmal massiv DSP in physikalischer Ebene eingesetzt, es sind ADCs (und DACs) erforderlich
- PMD und CD können dann vollständig elektrisch kompensiert werden
- System- und Implementierungsaspekte von kohärentem O-OFDM sind wenigerumfassend untersucht
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Grundlagen von optischem OFDM
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Prinzip von OFDM: Vergleich zwischen Einzelträger und Mehrträgerverfahren (1)
- Einzelträgerverfahren:
- Einzelnes Trägerband welches die gesamte Kanalbandbreite belegt
- Serielle Übertragung der Daten
- Störanfällig für Intersymbolinterferenz
time
freq
uenc
y
band
wid
th
Tsc
Transmitted symbol
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Prinzip von OFDM: Vergleich zwischen Einzelträger und Mehrträgerverfahren (2)
- Mehrträgerverfahren:
- Aufteilung der Kanalbandbreite in Unterkanäle
- Serieller Datenstrom wird parallelisiert und auf Unterkanäle aufgeteilt
- beliebiges Modulationsverfahren für jeden Unterkanal
- Symboldauer verlängert sich um Faktor NC gegenüber Einzelträgerverfahren
weniger anfällig für Störung durch ISI
time
freq
uenc
y
band
wid
th
NCxTsc
Transmitted symbol
∆f=B/NC
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Prinzip von OFDM: Frequency Division Multiplexing
- Schutzbänder zwischen den einzelnen Kanälen nötig
- zur Vermeidung von Kanalübersprechen (Inter Channel Interference, ICI)
- zur Rückgewinnung der Kanäle mittels Filtern
Dadurch schlechte Bandbreiteneffizienz
Abhilfe: Orthogonal Frequency Division Multiplexing
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Prinzip von OFDM
- Mehrträgerverfahren:
- OFDM – Unterträgerbänder überlappen
CN
allzero
1 2 k nn -1... ...
f 1 f 2 f k f n
n x f S
frequency
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Prinzip von OFDM
OFDM modulator/multiplexer
OFDM demodulator/demultiplexer
Basic requirement: use of orthogonal basefunctions
Σ
×Cn,0
tje 0ω
×Cn,N-1
tj Ne 1−ω
s(t)
...× Cn,0
tje 0ω−
× Cn,N-1
tj Ne 1−− ω
s(t)
∫ ⋅ST
dt)(
∫ ⋅ST
dt)(Ts
Ts
...
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OFDM: Sender und Empfänger
DAC
Im
Re
DAC
S/P
Zero
s
iFFT
code
r
P/S
ADC
Im
Re
P/S
Zero
s
FFT de
code
r
S/P
ADC
targeted function is relialized by DFT
OFDM signals are in general complex valued
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Prinzip von OFDM: Guard Interval
- Zyklische Erweiterung des OFDM-Signals
- Im Empfangsfenster muß immer volständige Peride jedes Unterträgers vorhanden sein
- Erweiterung reduziert Nettodatenrate
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Prinzip von OFDM: Subcarrier modulation
DAC
Im
Re
DAC
S/P
Zero
s
iFFTco
der
P/S
QPSK is efficient scheme for high sensitivity
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Subcarrier modulation
Investigated schemesQPSK – 2 bit per subcarrier 10 Gb/s
QAM16 – 4 bit per subcarrier 20 Gb/s
QAM64 – 6 bit per subcarrier 30 Gb/s
Increase of number of bit per subcarrier leads to drastic decrease of distance between adjacent states in the constellation
Reduction of SNR -6.9 dB -13.1 dBConv. schemes, e.g. NRZ -3 dB -4.8 dB
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Subcarrier modulation
Comparison of subcarrier modulation schemes
Makes higher level subcarrier modulation sence?
2.41.60.8Spectral efficiency [bit/Hz]
11.04.1Red. In sensitivity [dB] vs. QPSK
4.83.0Comp. to NRZ [dB]
15.87.1Sensitivity below 10Gb/s [dB]
22.715.07.9 Req. OSNR @ 1E-3 [dB]
302010Data rate [Gb/s]
QAM64QAM16QPSK
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O-OFDM transmitter
IQ-MOD
imag
Electr. OFDM signal
e/o MODOptical OFDMsignal
real
imag
Electr. OFDMsignal
IQe/o MOD
opticalOFDMsignal
real
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O-OFDM receivers
real
opticalOFDMsignal
[⋅]2 IQ-DEMElectr.OFDMsignal
imag
opticalOFDMsignal
LO
realopticalOFDMsignal
90° opt. hybrid
Electr.OFDMsignal
imag
[⋅]2
real
IQ-DEMElectr.OFDMsignal
imag
LO
[⋅]2
[⋅]2
Directdetection
Coherent
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O-OFDM spectra
frequency
Electricalbase bandsignal
I and Q component
amplitude
frequency/wavelength
Opticalsignal
λO
Res. opticalcarrier
frequency
Electricalbase bandsignal
I and Q component
amplitude
nahezu rechteckige Spektren
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O-OFDM Systeme und deren Eigenschaften
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Scaling OFDM towards higher bitrates – first estimation
Data rate Subcarrier modulation DAC/ADC rate DAC/ADC bandw. Resol.10G QPSK 10GSa/s < 4 GHz 6 bit
20G QAM16 10GSa/s < 4 GHz 8 bit
40G QAM16 20GSa/s < 8 GHz 8 bit
50G QAM16 25GSa/s < 10 GHz 8 bit
PM 2x50G=100G QAM16 (2x) 25GSa/s < 10 GHz 8 bit
50G QPSK 50GSa/s < 20 GHz 6 bit
PM 2x50G=100G QPSK (2x) 50GSa/s < 20 GHz 6 bit
CO-OFDM transmitter (similar for receiver)
Optical channelData rate Subcarrier modulation Optical bandwidth Spectral efficiency
10G QPSK < 8 GHz 1.4 bit/Hz
20G QAM16 < 8 GHz 2.8 bit/Hz
40G QAM16 < 16 GHz 2.8 bit/Hz
50G QAM16 < 20 GHz 2.8 bit/Hz
PM 2x50G=100G QAM16 (2x) < 20 GHz 5.6 bit/Hz
50G QPSK < 40 GHz 1.4 bit/Hz
PM 2x50G=100G QPSK (2x) < 40 GHz 2.8 bit/Hz
req. OSNR [dB](*)
6.0
13.1
16.1
17.1
20.1
13.0
16.0(*) BER 10-3
Interconnect2x60Gb/s
2x80Gb/s
2x160Gb/s
2x200Gb/s
4x200Gb/s
2x300Gb/s
4x300Gb/s
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+100Gb/s OFDM scenario
multiplexing of OFDM sub bands
spectral shaping of sub bands is suitable for narrow wavelength spacingdemux of sub bands using comb lines aligned to each sub bandhigh capacity scenario without WDM gridhighest spectral efficiencies achievable w/o. WDM grid (e.g. 1.7 b/s/Hz for 9x12.5 Gb/s)
OpticalComb Generator
CyclicDeMux
100GPDM O-OFDM
fN x Δλ
Optical Local Comb Oscillator
4 PDseach
ADCDSPCoherent
mixer
N x 100GN x 100G
RxTx
e.g. 1TbE
e.g. 1TbE
Experimental result 9x12.5Gb/s in 7.5GHz spacing113 Gb/s in 67GHz bandwidth
-35
-25
-15
-5
1552.4 1552.5 1552.6 1552.7 1552.8 1552.9 1553 1553.1 1553.2
Wavelength(nm)P
(dB
)
12.5Gb/s37.5Gb/s113Gb/s
5x80km SMF
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-45
-40
-35
-30
-25
-20
-15
1552.4 1552.6 1552.8 1553 1553.2
Lambda(nm)
P a.
u.
Back-2-Back5Spans
S/P
Map
ping
IFFT
Cyc
lic E
xten
sion
Scal
ing
& Q
uant
izat
ion
LD
A/D
Syn
chro
nisa
tion
FFT
Q-, BER-estimation
Error- count
opticalLO
Data
real
Cha
nnel
co
mpe
nsat
ion
Offline D
ecis
ion
Optical Link 5x80km SMF DCF free
Pilo
ts
Zero
s
P/S
TS I
nser
tion
imag
D/A
D/A
I/Q
90°-
Hybrid B-P
D
B-PD
A/D
Dow
ncon
vers
ion
CP
PF-
Rem
ove
Booster
Preamp
SMF
MZM1
MZM2 CombGenerator(9 lines)
OFDMModulation12.57Gb/s
7.46GHz+
22.38GHz
Setup of 113Gb/s OFDM-System
Sampling rateReq. bandwidthReq. resolutionSubcarrier
modulation
Data rate
Experimental parameter:
•Tx: 10 GS/s (3.5GHz bw) 8bit; RX: 20GS/s (6GHz bw) 10bit interp.
•OFDM: 512-FFT, 342 subcarriers, 4-PSK, 3.2ns CPPF
•Total bitrate 9 x 12.57Gb/s = 113.13Gb/s in 67GHz (1.68bit/s/Hz)
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System performance: Back-2-Back
Increase of datarate x9 => 9.5dB penalty (= 10 log 9)
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
4 6 8 10 12 14 16 18 20 22 24 26 28 30
OSNR/0.1nm [dB]
log(
BER
)
12.5Gb/s B-2-B
113Gb/s- B-2-B
9.5dB
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Measurement of Non-linear-threshold
-5
-4
-3
-2
-1
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
P_launchedlo
g(B
ER)
113Gb/s- 5Spans113Gb/s- (2)+3Spans113Gb/s- (4)+1Spans
-5
-4
-3
-2
-1
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12
P_launched
log(
BER
)
12.4Gb/s- 5Spans12.4Gb/s- (2)+3Spans12.4Gb/s- (4)+1Spans
Transmission over 5 Spans 80km SMF
12.4Gb/s NLT 1Span: 4dBm
113Gb/s NLT 1Span 8.5dBm
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Signalcharakteristik von OFDM-Signalen: Signal to Average Ratio
-Hohe Peakwerte sind problematisch
- Nichtlineare Effekte sind leistungsabhängig
- Signalverfälschung
-Charakterisierung von Peaks bzw. SAR and PAR
- Amplitude des Peaks wird auf mittlere Amplitude des Signals normiert
- i.d.R. Angabe in dB
)()(
log20)( 11
tsts
tSAR ⋅=
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1E-05
1E-04
1E-03
1E-02
1E-01
1E+00
0 2 4 6 8 10 12
20*log(SAR) [dB]
CC
DF
Investigation of signal power distribution
Description of optical signalPAPR: Ratio of max. power to averagebut: ‘density’ of peak values not regarded
CCDF: Complementary Cumulative Distribution Function ⇒ Probability for power (amplitude) exceeds given value
OFDM has statistical distribution of signal amplitudes already at TxStatistics is not influenced by CD
Similar distributions are present in conventional transmission systems w/o. DCF (fiber acts as Fourier transformer)
Multi span systems80km SMF per span
1E-06
1E-05
1E-04
1E-03
1E-02
1E-01
1E+00
-10 -5 0 5 10 15 20
20*log(SAR) [dB]
CC
DF PRBS7
PRBS11PRBS15
FFT 1024
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Transmission on SSMF
Comparison of 10 Gb/s with a 50 Gb/s scenario
Evaluation of peak values from span to span
10G: dispersion is low wrt. bandwidth consumption
•peaks survive over several spans• same peaks meet nonlinear degradation after optical amplification
50G: peak values arise and vanish from span to span
•different peaks meet nonlinear degradation after optical amplification0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 10 12 14 16Number of 80km Spans
Am
plitu
de a
.u.
10G t=149.5 ns50G t=20.82 ns50G t=27.6 ns
In 16 span simulations: NLT rises from 11 to 16 dBm integrated power while increasing the bit rate from 10 to 50 Gb/s
OFDM has high and competitive nonlinear tolerances in high bit rate systems without DCMs
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Power budget - nonlinear threshold
Investigation of power budgetMaximum launch powerMaximum link attenuationPreamp noise figureReceiver sensitivity
Tx Rx
ATT NFPlaunch Sens.
dB58+−−= NFATTPOSNR launch
OSNR at the receiver is given by
To achieve high OSNR highest Plaunch is required, which is limited bynonlinear effects
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Nonlinear thresholds
High nonlinear thresholds support OFDM at highest data rates
5
7
9
11
13
15
17
19
0.1 1 10 100Bitrate [Gb/s]
Non
linea
r thr
esho
ld [d
Bm
] 16 spans 2048 FFT1 span 2048 FFT
5.6 times bitrate
2 times bitrate
2.1 dB NLT improvement
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Realisierungsaspekte
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Realization aspects
Investigation of DSP, ADC and DAC
for 100Gb/s 50 GSa/s components are required
Interface rate beyond 1Tb/s
due to high interface rates integrated converter – DSP chips are first choice
Investigation of DSP complexity
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Setup of 113Gb/s OFDM-System
DSP: Tx
– IFFT
DSP: Rx
– synchronization
- Frequency offset compensation
- FFT
- channel equalization
- polarization demux
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CMOS building blocks
ACSU
Flip-Flop Flip-Flop-IP
Comparator
Sample& HoldCircuit
Comparatoractive Peak.
Investigation of CMOS buildingblocks
CMOS S&H as key element for
ADCs
ACS unit as key element for VE
Testchip realized in 90nm CMOS
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Equalizer building blocks: test of S&H resolution
The corresponding
Marker Delta fdelta=230 MHz Pdelta= –28dB
freqData =19.9 GHz PData = 5 dBmfreqclk = 20 GHz Ampclk = 1.2Vpp
bits
dBSINAD
36.402.6
76.1ENOB
=
−=
Pdelta
Beat frequency test: 4.36 bit resolution
BER method @ 10 GHz: 4.0 bit resolution
90nm CMOS is suitable for low resolution 40 GSa/s ADCs
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Feasibility-check for real time system
No dedicated DSPs for O-OFDM available today
Real time systems should base on FPGAs with subsequent DACs or ADCs
Assuming DAC with 10 GSa/s, 6 bit resolution (faster DACs are available)
FPGA
DAC
DAC
6x10Gb/s12x5Gb/s
6x10Gb/s12x5Gb/s
10GSa/s6 bit res.
10GSa/s6 bit res.
FPGAs (e.g. Xilinx) have high speed interconnects with <10 Gb/s rate,
5 Gb/s rate is highest alternative: 24 interconnects are required for setup
Highest complex FPGAs have 24 high speed interconnects
Today no significant increase for base data rates beyond 10Gb/s possible
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Feasibility-check for real time system
OFDM Tx implementation in FPGAData rate 10 Gb/s, QPSK modulation
Input word length: 6 bit
FFT is highest complexity block in Tx
Radix-8 assumed as an efficient realization
Utilization of FPGA resources for 512-FFTFPGA type for investigations: XCE4VFX140
Flip Flops: 8 %
LUTs: 68%!!
Radix-8 FFT
ConclusionsImplementation of OFDM Tx for 10 Gb/s using FPGA is feasible
Increase of data rate or FFT size is limited by FPGA
• in logic resources
• interconnection resources to ADC and DAC
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Potential hardware implementation: FFT
FFT implementation
Loading of data into a parallel structure50GSa, FFT256: 200 MHz processing in a pipelined FFT, 50 MHz @ FFT1024Very suitable for subrate processing, further parallelization or pipelining possibleStructure is same as proposed for frequency domain CD compensation for coh. QPSK, but for whole data stream required (complexity roughly times 4)
FFT is highest effort block within algorithmButterfly is most importantsub-structure
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Potential hardware implementation: FFT
FFT implementation
Each butterfly consists of 1 complex mul and 2 addNumber of stages 128 @ FFT256, 512 @ FFT1024 Number of butterflys 1024 @ FFT256, 5120 @ FFT1024In sum 4x4.7Mgates = 19 Mgates are required, but distributed to Tx and Rx (half each)
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DSP complexity comparison – 1: FFT
Small relative increase in hardware with FFT increaseOFDM: processing of reduced number of samples
Comp. To QPSK: overlap requires higher number of samples, FFT is more complex
(bases on Carlos inputs)
FFT complexity
0
2000
4000
6000
8000
10000
12000
0 500 1000 1500 2000
FFT size
num
ber o
f but
terfl
ies
0
1
2
3
4
5
6
7
8
9
10
num
ber o
f but
terfl
ies
per i
nput
absoluterelativeCoh. QPSK overlap 256OFDM 20% CPPFCoh. QPSK overlap 380
33% reduced
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Potential hardware implementation: Channel equalization
Frequency
…
Nsc+NPSC
Tim
e
…..
…
…..
Frequency Domain Interpolation
Pilot Subc. Data Subc.
Reference Symbol
Tim
e D
om
ain
Ave
ragin
gn
Application of pilot signals within the OFDM signal
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Potential hardware implementation: LO offset estimation and correction
Coherent RX with LO freq. offset + phase noise
Estimation + Compensation of LO offset
))(2( ttfj offe φπ + kTfj este π2−
T
CPFFT
Equ.
channel est.freq. offset est.
LO offset correction is a simple complex multiplication of time domain samples
Yk
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Potential hardware implementation: Channel equalization
T
receivedNc ,
Na
equalizedNc ,
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Potential hardware implementation: Channel equalization
Mathmatical operations for OFDM are very simple and very few
Adaptation of system is performed by pilot signals and pilot subcarriers, no blind adaptation required
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Comparison QPSK - OFDM
ECOC 2008, We2E4, B. Spinnler, “Adaptive equalizer complexity in coherent optical receivers”
Similar/comparable approaches for QPSK and OFDM Time domain processing is more complexQPSK: full frequency domain processing challenging
monitor channel advantageous for adaptationIn sum: OFDM has half complexity
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Schlußfolgerung
O-OFDM untersucht
Definition von Szenarios für 100 Gb/s Systeme
Diese haben eine gute Empfindlichkeit und hohe nichtlineare Schwellen, sodass das OSNR Budget ein flexibles Systemdesign erlaubt
Erfordert 50 GSa/s DACs und ADCs, Testchips lieferten ermutigendeErgebnisse
Erfordert DSP in Tx und Rx, Gesamtaufwand ist vergleichsweise gering
O-OFDM ist attraktive Technik für optische Systeme bei höherenDatenraten.
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www.alcatel-lucent.com
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Figure 10: Schematic of a 112-Gb/s PDM-OFDM transceiver architecture. PMDC: PMD compensation. EDC: electronic dispersion compensation. PA-CPEC: pilot-assisted common phase error compensation. DAC: digital-to-analog converter. ADC: analog-to-digital converter.
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0 50 100 150 200 250-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
time [a.u.]
ampl
itude
[a.u
.]
PRBS7
Frequency domainQPSK modulated subcarriers (128 subcarriers)
All have same power
Information is coded in phase
Time domainno constant amplitude (envelope)
critical data pattern leading to high signal peak power (PRBS7 and all “0”)
probability of amplitudes depend on FFT length and data pattern (e.g. PRBS length)
0 50 100 150 200 250-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
time [a.u.]
ampl
itude
[a.u
.] all „0“
very high peak values may occur
OFDM signal characteristic
frequency [a.u.]0 50 100 150 200 2500
0.1
0.2
0.3
0.40.5
0.6
0.70.8
0.91
ampl
itude
[a.u
.]