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13.75-Gb/s OFDM Signal Generation for 60-GHz RoF Systemwithin 7-GHz License-Free Band via Frequency Sextupling
Po-Tsung Shih(1)
, Chun-Ting Lin(1)
, Han-Sheng Huang(1)
, Wen-Jr Jiang(1)
, Jyehong Chen(1)
,Anthony Ngoma
(2), Michael Sauer
(2), Sien Chi
(1,3)
(1)
Department of Photonics, National Chiao-Tung University, Hsinchu 300, Taiwan.,jinting@ms94.url.com.tw; boris.eo95g@nctu.edu.tw(2)
Corning Incorporated, One Science Center Dr., Corning, NY 14831, USA.(3)
Department of Electrical Engineering, Yuan-Ze University, Chung-Li 320, Taiwan.
AbstractA 60-GHz RoF system with frequency sextupling is experimentally demonstrated. Based on modified
SSB, a 13.75-Gb/s OFDM signal is generated within the 7-GHz license-free band. No penalty is observed after
transmission over 25-km single-mode fiber.
Introduction
Broadband wireless systems have been considered
as a potential solution for future quad play services.
With the release of a 7-GHz wide license-free band,
60-GHz millimeter-wave wireless systems have beenwidely investigated recently. Many standards and
applications at this 60-GHz band have been proposed,
such as IEEE 802.15.3c WPAN, IEEE 802.16 WiMAX,
and WirelessHD. However, the range of 60-GHz
wireless signals is rather limited due to the high path
and atmospheric losses. To extend the signal
coverage, radio-over-fiber (RoF) techniques are a
promising solution for broadband wireless networks.
The generation of 60-GHz RoF signals remains a
great challenge. Electro-absorption-modulation (EAM)
is one of the solutions to generate 60-GHz RoF
signals. Although EAMs have been shown to providehigh radio frequency (RF) modulation bandwidth up to
60 GHz, there is dispersion-induced performance
fading in the generated double sideband (DSB) signal
[1]. Moreover, costly 60-GHz RF devices are required
in EAM systems. To generate 60-GHz RoF signals
with frequency multiplication and to overcome the
dispersion-induced fading issue, double sideband
modulation with carrier suppression (DSB-CS)
scheme using Mach-Zehnder modulators (MZM) was
proposed [2]. However, only on-off-keying (OOK)
modulation can be used in the DSB-CS systems. To
provide very high throughput services within the 7-
GHz license-free band at 60 GHz, high spectral
efficiency modulation formats are necessary.
In this work, a modified single sideband (SSB)
modulation scheme with frequency sextupling using
two dual-parallel MZM is proposed. A modified SSB
signal with frequency doubling is generated using a
dual-parallel MZM. Then, the modified SSB signal is
sent into the proposed colorless optical up-conversion
system with frequency quadrupling. To select the
desired optical sidebands and obtain the modified
SSB signal with frequency sextupling, an optical
interleaver is utilized. Due to the modified SSB
modulation scheme, there is no dispersion-induced
performance fading, and high spectral efficiency13.75-Gb/s orthogonal frequency-division multiplexing
(OFDM) modulation format is experimentally
demonstrated. The receiver performance can be
optimized by adjusting the optical power ratio (OPR)
between the OFDM modulated and the unmodulated
optical sidebands because the optical powers of the
OFDM-modulated and of the unmodulated optical
sidebands can be freely adjusted by controlling the
amplitude of the driving signals. Wavelength-division-
multiplexing (WDM) up-conversion sharing only one
optical up-conversion system can be achieved due to
the colorless up-conversion system. Most importantly,only approx. 10-GHz bandwidth components are
required in the proposed system, resulting in lower
cost compared to higher-frequency components.
Experimental Setup and Results
Figure 1 shows the experimental setup. There are two
stages of the proposed system. The first stage is the
modified SSB generation with frequency doubling
using one dual-parallel MZM [3]. The second stage is
the colorless optical up-conversion with frequency
quadrupling using another dual-parallel MZM [4]. A
distributed-feedback laser serves as the optical
source.
10GHz
O/E60 GHz
BPFScope
55 GHz
Wireless
Remote Node
SMF
V
V
Laser
+90
/2
MZ1-a
MZ1-b
MZ2-a
MZ2-b
10GHz
FrequencyQuadrupling
AWG +90
+90
0V
0V
1 54 7. 5 1 54 7. 6 1 54 7. 7 1 54 7. 8 1 54 7. 9
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Level(dBm)
WaveLength(nm)
1 5 47 . 2 1 5 47 . 4 1 5 47 . 6 1 5 47 . 8 1 5 48 . 0 1 5 48 . 2
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Level(dBm)
WaveLength(nm)
1547.2 1547.4 1547.6 1547.8 1548.0 1548.2
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0
10
Level(dBm)
WaveLength(nm)
20G
Hz
60
GHz
(a)
(b)
(c)
Fig. 1 Experimental setup of the proposed system.
ECOC 2009, 20-24 September, 2009, Vienna, Austria Paper 4.5.4
978-3-8007-3173-2 VDE VERLAG GMBH
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In the first stage, sub-MZM (MZ1-a and MZ1-b) of the
dual-parallel MZM are biased at the null point, while
the main MZM is biased at the quadrature point. Two
signals are generated to drive the first dual-parallel
MZM. One is the OFDM signal which is generated by
an arbitrary waveform generator (AWG) and up-
converted to 10-GHz using an electrical mixer. The
other signal is the 10-GHz LO signal which generates
the unmodulated optical sideband. The up-converted
OFDM signal is generated with 20-GHz DAC
sampling rate, 256 FFT size, 88 subcarriers, and
6.875-GHz total bandwidth. After the division of the
signals using two electrical hybrid couplers, a 90 deg
phase delay is introduced to the upper arm of the
OFDM signal while a -90 deg phase delay isintroduced to the lower arm of the LO signal. After the
respective combination of the upper and lower arm
signals, the combined signals are sent into the MZM.
Then, a modified SSB signal with frequency doubling
as shown in the inset (a) of Fig. 1 is obtained at the
output of the first stage of the proposed system.
In the second stage, another dual-parallel MZM is
employed for the colorless optical up-conversion with
frequency quadrupling. The sub-MZM (MZ2-a and
MZ2-b) are biased at the full point while the main
MZM is biased at the null point. There is a 90 deg
phase difference between the 10-GHz driving signals
of MZ2-a and MZ2-b. After the second dual-parallel
MZM, both the OFDM modulated and unmodulated
optical sidebands are up-converted in frequency by a
factor of four. An optical spectrum as shown in the
inset (b) of Fig. 1 is obtained at the output of the
second stage. Then, an optical interleaver is utilized
to select the desired optical sidebands for the 60-GHz
RoF signals as shown in the inset (c) of Fig. 1. After
transmission over standard single mode fiber, the 60-
GHz modified SSB signal is received and down-
converted to 5-GHz using an electrical mixer. The
waveform of the down-converted signal is captured
with a real time oscilloscope for off-line analysis.
Since the OPR between the OFDM-modulated and
unmodulated optical sidebands can be freely adjusted
by controlling the amplitude of the driving signals, the
60-GHz OFDM signal can be optimized by adjustingthe OPR. Fig. 2 shows the experimental results of the
OPR optimization. The OPR is defined as the ratio of
the unmodulated sideband optical power to the
OFDM-modulated sideband optical power. When the
OPR equals to 6 dB, the receiver has the best
performance. The constellation diagrams for different
OPR values are also shown in the insets of Fig. 2.
Fig. 3 shows the estimated bit error rate (BER) curves
of the 60-GHz 13.75-Gb/s OFDM signal. After
transmission over 25-km single-mode fiber, no
significant receiver power penalty was observed. The
constellation diagrams of the QPSK-OFDM signals
are also shown in the insets of Fig. 3. After
transmission over 25 km of standard single-mode
fiber, no signal degradation was observed.
Conclusion
A 60-GHz RoF signal generation and distribution
system with frequency sextupling is experimentally
demonstrated. Because of the colorless optical up-
conversion system with frequency quadrupling, WDM
up-conversion can be achieved. By using the
modified SSB modulation approach, a high spectral
efficiency was achieved. A total bit-rate of 13.75-Gb/s
using QPSK-OFDM modulation within the 7-GHz
license-free band was experimentally demonstrated.
Due to SSB modulation, no dispersion-induced
performance fading was observed. After transmission
over 25-km single-mode fiber, no significant receiver
power penalty and signal distortion was observed.
References
1 Y. X. Guo et al., Proc. GSMM08, Invited paper
(2008).
2 H. C. Chien et al., Opt. Express. 17, 3036 (2009).
3 C. T. Lin et al., Photon. Technol. Lett, 20, 1106
(2008).
4 P. T. Shih et al., Opt. Express. 17, 1726 (2009).
-2 0 2 4 6 8 10
12
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-log(B
ER)
OPR
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-8
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Q
I
(a)
(b)
(c)
(a) (b)
(c)
Fig. 2 Optimization of optical power ratio (OPR) between the
optical carrier and OFDM sideband.(OPR=unmodulated sideband power/modulated sideband
power)
-13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2
9
8
7
6
5
4
3
BTB
25km
-log(B
ER)
Level(dBm)
-5 -4 -3 -2 -1 0 1 2 3 4 5-5
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Q
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BTB
25km
-5 -4 -3 -2 -1 0 1 2 3 4 5-5
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Q
I
Fig. 3 BER measurement and constellation diagrams ofthe received OFDM signals.
ECOC 2009, 20-24 September, 2009, Vienna, Austria Paper 4.5.4
978-3-8007-3173-2 VDE VERLAG GMBH
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