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Published in IET Communications
Received on 17th March 2012
Revised on 18th July 2012
doi: 10.1049/iet-com.2012.0149

ISSN 1751-8628

Transmission of 60 GHz wired/wireless based on

full-duplex radio-over-fibre using dual-sextupling
frequency
K. Yang X.G. Huang J.H. Zhu W.J. Fang
Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectric Science and
Engineering, South China Normal University, Guangzhou 510006, People’s Republic of China
E-mail: huangxg0@163.com

Abstract: A novel full-duplex radio-over-fibre dual-sextupling frequency system with thin-film filter (TFF) was proposed
and simultaneously demonstrated. A light wave generated by a distribution feedback laser diode (DFB-LD) is modulated
by an RF signal to generate optical sidebands. This system used a dual-drive LiNbO3 Mach – Zehnder modulator (DD-
LN-MZM) and an optical TFF to generate dual-sextupling frequency optical millimetre waves. The optical millimetre
waves were carrying downstream signals to transmit through optical fibres to the two-base stations (BSs). Upstream
signals were transmitted in the same optical fibres by using a circulator. The simulation results revealed that the power
penalties for the downstream and upstream signals of both BSs are less than 0.6 dB. One of the sidebands in the BSs
was used as a carrier for uplink connection. The laser diodes can be used instead of uplink sidebands in order to
reduce the cost of the BSs. Also the users can pay less for the expensive laser diodes. The cost of the new system is
largely reduced.

1 Introduction ROF systems be practical for commercial deployment. The

mm-wave bands would be utilised to meet the requirement
Radio-over-fibre (ROF) is more powerful to meet the growing for broadband service and overcome the frequency
demands of large capacity, low-power consumption, wide congestion in the future ROF-based optical-wireless network.
bandwidth and effective cost in access service of In an ROF system the CO is connected to many functionally
telecommunication system. Optical millimetre-wave (mm- simple BSs via optical fibre. Almost all processing including
wave) generation is one of the key technologies in the ROF modulation, demodulation, coding and routing are performed
systems. Recently, several techniques have been proposed to at the CO. The main function of the BS is to realise optical/
implement mm-wave generation [1–7]. The 40–60 GHz wireless conversion and broadcasting by antenna [17, 18].
mm-wave has gained much attention and will probably be Some cost-effective ROF system projects have been
the first choice for ROF wireless access system. 60 GHz proposed and experimentally investigated, such as the
wireless standards, such as IEEE 802.15 wireless personal technique for sharing single light source for both downlink
area network (WPAN), IEEE 802.16 Worldwide and uplink signals. The electrical components and
Interoperability for Microwave Access (Wi-MAX) and equipments such as RF carrier generators, mixers and
wireless high definition video services (wireless HD), have synthesisers at frequencies 60 GHz are very expensive.
been proposed. Nevertheless, the transmission distance of Additionally, as the RF frequency increases, the effect of
60 GHz wireless signal is limited by the high path and dispersion is even more pronounced and the fibre-link
atmospheric losses. To extend the coverage of 60 GHz distance is severely limited [19]. It could be significant for
wireless signal, ROF technique becomes a promising reducing the cost of the ROF system, compared with ROF
solution because of the low transmission loss and unlimited systems based on direct ultrahigh RF oscillator and based on
bandwidth of optical fibres [8–12]. However, 60 GHz optical frequency doubling or sextuple RF oscillator.
vector signal generation still remains a great challenge. In this paper, the frequency of RF is 60 GHz, both the
Although direct modulation is by far the simplest, because of downstream and upstream signals have less than 0.6 dB
the limited modulation bandwidth of the laser, this is not power penalties. The thin-film filter (TFF) with two outputs,
suitable for 60 GHz mm-wave bands [13, 14]. External each of them with a 30 GHz bandwidth is used to generate
intensity modulation is the simplest and the most accurate the mm-wave. The structure of a BS is simplified without
one, and shows great potential application for producing laser, the circulator is used for uploading signals to CO, and
high-frequency mm-wave signals [15, 16]. A low-cost BS or the devices used in BSs are less than before. The frequency
central office (CO) is one of the key techniques to make of local oscillator (LO) signal is largely reduced because of

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the frequency sextupling mm-wave techniques. The system [20], the light-wave can be written as
has a better commercial value.   
E0(t) V1 (t) Vb1
E1 (t) = (a/20) b. exp jp + jp
10 Vp Vp
2 Principle  
V (t) V
The principle of the proposed novel ROF architecture is + (1 − b). exp jp 2 + jp b2 (1)
Vp Vp
shown in Fig. 1. A continue wavelength (CW) light from
distributed feedback laser diode (DFB-LD) with an angular
where V1(t) ¼ VLOcos(vLOt) and V2(t) ¼ VLOcos(vLOt + u)
frequency vc is driven by an external modulator. The light
are the LO clock signals which is, respectively, driving the
can be represented as E0(t) ¼ Acos(vct). Here A is the
two arms of the intensity modulator (IM), here vLO and
electric field strength. After being modulated by the dual-
VLO are the angular frequency and the amplitude voltage of
drive LiNbO3 Mach– Zehnder modulator (DD-LN-MZM)
LO clocks, u is the relative phase of the clocks between the
two arms of the LiNbO3 modulator. Vb1 and Vb2 are,
respectively, the bias voltages of the two arms, which are
used to adjust an optical phase of FDC ¼ 2kp (k ¼ 1, 2, 3,
. . .) to suppress the odd- or even-order optical sidebands
[21]. b is the power splitting ratio of the two arms of the
MZM, a is the insertion loss of the MZM. For a good
MZM, b ¼ 0.5, a ¼ 0. Thus the components of the optical
spectrum depend on the bias voltages and the relative phase
u, one can adjust them to generate optical mm-waves with
different spectra. According to (1), the output light-wave
intensity can be written as
  
E0 pV pV
E1 = cos vc t + LO cos(vLO t) + b1
2 Vp Vp
 
pV pV
+ cos vct + LO cos(vLO t + u) + b2 (2)
Vp Vp

Equation (2) can be expanded by using Bessel format, and

then the electrical field at the output of the MZM becomes
(see (3))
Here n ¼ pVLO/Vp is the phase modulation index. If the
relative phase is set to be u ¼ p and both the bias voltages
of the LiNbO3 modulator are set to be Vb1 ¼ Vb2 ¼ 0, so it
represents cos(2mu) ¼ 1, sin(2mu) ¼ 0, cos[(2m 2 1)u] ¼
21, sin[(2m 2 1)u] ¼ 0
⎧ ⎫
⎪
⎪ E0J 0(n) cos wct ⎪
⎪
⎨ ⎬
− 2E0J2 (n)[ cos(vc t + 2vLO t) + cos(vc t − 2vLO t)]
E1 =
⎪ + 2E0J4 (n)[ cos(vc t + 4vLO t) + cos(vc t − 4vLO t)] ⎪
⎪ ⎪
⎩ ⎭
− 2E0 J6 (n)[ cos(vc t + 6vLO t) + cos(vc t − 6vLO t)]
Fig. 1 Experiment setup of 60 GHz wired/wireless based on (4)
full-duplex radio-over-fibre using dual-sextupling frequency
According to (4), it shows the mm-wave signal has only even
LD, laser diode; MZM, Mach –Zehnder modulator; TFF, thin-film filter;
FBG, fibre Bragg grating; IM, intensity modulator; OC, optical coupler; order, an optical pass-band filter is used to suppress seventh
SMF, single-mode fibre; IGF, inverse Gauss filter; EDFA, erbium-doped and higher-order sidebands, the Bessel function higher than
optical fibre amplifier; LPBF, low-pass Bessel filter; BERT, bit error tester; J6(n) can be ignored, and the left sidebands are the zero,
CO, central office; BS, base station the second, the fourth and the sixth-order sidebands. From

⎧  1
 ⎫
⎪
⎪ ⎪
⎪
⎪
⎪ cos( v ct + f ) J (n) + 2 (−1) m
J 2m(n) cos(2m v t) ⎪
⎪
⎪
⎪
1 0 LO
⎪
⎪
⎪
⎪
m=1
⎪
⎪
⎪
⎪ 1 ⎪
⎪
⎪
⎪ + 2 sin( v ct + f ) (−1) m
J 2m − 1(n) cos[(2m − 1)v t] ⎪
⎪
⎪
⎪ 1 LO ⎪
⎪
⎪
⎪ m=1 ⎪
⎪
⎨
E0 + cos(vct + f2 ) ⎬
E1 =   (3)
2 ⎪
⎪
1 ⎪
⎪
⎪ J0 (n) + 2
⎪ (−1)m J2m (n)[cos(2mvLO t) cos(2mu) − sin(2mvLO t) sin(2mu)] ⎪
⎪
⎪
⎪ ⎪
⎪
⎪
⎪ m=1 ⎪
⎪
⎪
⎪ + 2 sin(vct + f2 ) ⎪
⎪
⎪
⎪ ⎪
⎪
⎪
⎪ 1 ⎪
⎪
⎪
⎪ ⎪
⎩ (−1) J2m−1 (n)[cos[(2m − 1)vLO t] cos[(2m − 1)u] − sin[(2m − 1)vLO t] sin[(2m − 1)u]] ⎪
m
⎭
m=1

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the table for Bessel functions of the first kind, one can obtain down to the BS1. The circulators are used to transmit
that for upstream signals. In the BS1, the 22 sideband reflected by
fibre bragg grating (FBG)1 is modulated by the IM1, which
n = 7.32, J 0(7.32) = 0.28651, J2 (2) = −0.26244, is driven by 2.5 Gb/s PRBS 231 2 1 electrical upstream
signals, after passing the two circulators the signals are
J 4(7.32) = 0.0726926, J 6(7.32) = 0.352091
uploaded to the CO. The optical spectrum and sideband
location is shown in Fig. 2e. One can see that the power of
Thus, the modulation voltage of VLO ¼ 12 V should be 22 sideband is less than 220 dBm. Fig. 2d shows the
chosen to achieve the value of n ¼ pVLO/Vp ¼ 7.32, given optical sidebands after passing FBG1. For the downstream
a half-voltage of Vp ¼ 5 V. According to the above transmission, FBG2 is used to reflect the 26 order sideband,
formulation, one can use a TTF to separate the light-wave and then the 26 order sidebands as a downstream carrier are
sidebands as dual-sextupling optical mm-waves. The modulated by the IM2. The IM2 is driven by 2.5 Gb/s PRBS
expressions of the electrical fields after passing through the 231 2 1 electrical downstream signals. Fig. 2f shows the
optical interleaver from two output ports are, respectively modulated optical signals after reflecting by the FBG2, in
order to make amplitudes of the modulated 26 sidebands
E2 = and the un-modulated 24, 0 sidebands carrier almost
⎧ ⎫ equivalent, we use an optical coupler (OC) of 0.7 to combine
⎨ E0 ⎬
J 0(n)coswct − 2E0J2 (n)cos(vc t + 2vLO t) the modulated 26 sideband and the un-modulated 24, 0
2
⎩ + 2E0J (n)cos(v t + 4v t) − 2E J (n)cos(v t + 6v t) ⎭ sidebands, the combined sidebands and location are showed
4 c LO 0 6 c LO in Fig. 2g.
(5) The inversed Gauss filter (IGF) is used to filter the 24
sideband, which the central frequency is 193.06 THz and
E3 = the bandwidth is 2 GHz. The location and spectrum after
⎧ ⎫ passing the IGF is represented in Fig. 2h. One can see
⎨ E0 ⎬
J 0(n)coswct − 2E0J2 (n)cos(vc t − 2vLO t) that the 26 sideband and the central sideband have a
2
⎩ + 2E0J (n)cos(v t − 4v t) − 2E J (n)cos(v t − 6v t) ⎭ frequency spacing of 60 GHz. The two components of
4 c LO 0 6 c LO optical mm-wave signals are beaten to generate mm-
(6) waves by a PIN receiver with 3 dB bandwidth of 60 GHz,
signal – ASE noise, ASE – ASE noise, thermal noise and
From (5) and (6) the sidebands are separated into 0, 2, 4, 6 and slot noise are all considered in the PIN photodiode. The
26, 22, 24, 0. After being propagated in a long-distance 3R (re-amplification, reshaping, retiming) receiver is used
single-mode fibre (SMF)-28 fibre, in the BS1 the light to recover signals. The reference rate is 2.5 Gbit/s, the
beam with the frequencies of vc and (vct 2 6vLOt) will be decision instant is 0.5 bit, and the threshold is 0.5 au. In
beating to convert into an RF signal with frequency of the BS1 the system can perform well while using a 3R
6vLO . Also the light beam with the frequencies of vc and receiver, the S/N is increased and the Q factor is 7.32
(vct + 6vLOt) will generate 6vLO frequency RF signal in compared with 7.26 without 3R receiver in the
BS2. If vLO is set to be 10 GHz, the carrier frequency of downstream, in the upstream the Q factor is 6.64 with 3R
the optical mm-wave and the frequency of the RF signal in receiver and 6.62 without 3R receiver. In the BS2 the Q
photo-detector are as high as 60 GHz. factor is 6.99 with 3R receiver and 6.93 without 3R
receiver in downstream, in the upstream the Q factor is
3 Simulated experiment and results 5.69 with 3R receiver and 5.67 without 3R receiver. The
system has a good performance of S/N with the 3R
The simulated experiment setup is shown in Fig. 1. At the receiver. After a 3R receiver and low pass Bessel filter
CO, a DFB laser at 193.1 THz is employed as the light (LPBF), the generated downstream data are sent by the
source. The CW light-wave is intensity modulated via a switch connecting 1′ to launch by the antenna for wireless
DD-LN-MZM driven by a 10 GHz RF sinusoidal clock. transmitting or connecting 2′ for wired signals
The power of the light is 6 dBm, and the linewidth transmitting to bit error tester (BERT) for measurement,
is 10 MHz. The output optical spectrum after the DD- then the data are received by the user. For the above
LN-MZM is shown in Fig. 2a. As the picture depicts, the channel, the optical mm-wave signals are beaten to
modulated signals consists of seven sidebands which are the generate mm-waves to be used by different users in BS2.
central carrier, the +2 order sidebands, the +4 order In the CO, the upstream signals that are carried by the 22
sidebands and the +6 order sidebands. However, the other sideband are amplified by Erbium-doped optical fibre
existing small high-order sidebands are ignored. The optical amplifier (EDFA) and then detected by a photo-detector.
TFF is used to separate the sidebands by the one team is 0, Then the signals pass a 3R receiver and LPBF measured
2, 4, 6 order. The power and location is shown in Fig. 2c, by the BERT.
the other team is 26, 24, 22, 0 order, shown in Fig. 2b. The bit error rate (BER) performances for both downlink
From the picture one can see that the power of each and uplink are measured before they are filtered with LPBF
sideband is almost the same. The central sideband is spliced with the bandwidth of 0.75 bit rate Hz, The insets of Fig. 3
by twice using the TFF. The downstream and upstream illustrates the eye diagrams of the downstream and upstream
optical signals in two paths are similar with each other signals from BS1 have been transmitted over 50 km, in
when they are transmitting in the system shown in the Fig. 3a one can see that the power penalties are 0.3 dB at
picture. So we take the optical transmitting signals above BER of 1029, the eye diagrams are still open after 50 km
channel as an example, the below one is in the same transmitting. In Fig. 3c the penalties for uplink data are
representation. The signals with 26, 24, 22, 0 order 0.4 dB at BER of 1029. One can see that for both downlink
sidebands are transmitted through a 50 km SMF-28 with a and uplink data, after transmitting over 50 km the power
dispersion of 17 ps/nm/km and attenuation of 0.2 dB/km penalties are, respectively, less than 0.4 dB at BER of 1029.

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Fig. 2 Results of optical spectrums at different location

a the optical spectrum after the DDLN-MZM
b the spectrum of the sidebands separated by TFF in up road
c the spectrum of the sidebands separated by TFF in down road
d the spectrum after passing FBG1
e the spectrum modulated by 2.5Gbit/s upstream data after reflected by FBG1
f the spectrum modulated by 2.5Gbit/s downstream data after reflected by FBG2
g the spectrum combined by 3-dB OC
h the spectrum of mm-wave

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Fig. 3 Eye diagrams and BER of downlink and uplink

a Eye diagrams and BER curves of downlink data in the up road
b Eye diagrams and BER curves of downlink data in the down road
c Eye diagrams and BER curves of uplink data in the up road
d Eye diagrams and BER curves of uplink data in the down road

Fig. 4 RF signal in frequency-domain

a the 60GHz RF signal in frequency-domain in the up road
b the 60GHz RF signal in frequency-domain in the down road

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Fig. 5 Relationship between laser linewidths and BER

a Relationship between laser linewidths and BER in the up road
b Relationship between laser linewidths and BER in the down road

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