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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

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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

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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

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ER)

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(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)

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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