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60 GHz radio-over-fiber technologies
for broadband wireless services
[Invited]
Andreas Stöhr,1,*Akram Akrout,2Rüdiger Buß,1Benoit Charbonnier,3
Frederic van Dijk,2Alain Enard,2Sascha Fedderwitz,1Dieter Jäger,1
Mathieu Huchard,3Frédéric Lecoche,3Javier Marti,4Rakesh Sambaraju,4
Andreas Steffan,5Andreas Umbach,5and Mario Weiß1
1Universität Duisburg-Essen, Optoelektronik, Lotharstrasse 55,
47057 Duisburg, Germany
2Alcatel-Thales III-V Labs, Route de Nozay, 91460 Marcoussis, France
3France Telecom Research and Development, 2 Avenue Pierre Marzin,
22300 Lannion, France
4Universidad Politécnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain
5u2t Photonics AG, Berlin, Reuchlinstrasse 10-11, 10553 Berlin, Germany
*
Corresponding author: andreas.stoehr@uni-due.de
Received December 2, 2008; revised March 17, 2009;
accepted March 18, 2009; published April 23, 2009 共Doc. ID 104786兲
Some of the work carried out within the European integrated project Inte-
grated Photonic mm-Wave Functions for Broadband Connectivity (IPHOBAC)
on the development of photonic components and radio-over-fiber technologies
for broadband wireless communication is reviewed. In detail, 60 GHz outdoor
radio systems for ⬎10 Gbits/ s and 60 GHz indoor wireless systems offering
⬎1 Gbit/ s wireless transmission speeds are reported. The wireless transmis-
sion of uncompressed high-definition TV signals using the 60 GHz band is also
demonstrated. © 2009 Optical Society of America
OCIS codes: 060.0060, 060.5625.
1. Introduction
The past few years have witnessed the emergence of several new bandwidth-hungry
multimedia applications such as high-definition TV (HDTV), which is a driving force
behind recent endeavors developing very-high-speed wireless communication systems.
Although conventional wireless local-area network (WLAN) systems (IEEE 802.11a,b,
and g) offer data rates theoretically of up to 54 Mbits/s, more modern alternatives
such as ultrawideband (UWB) and multiple input, multiple output (MIMO) systems
are able to extend the wireless data speed up to several hundred megabits per second,
targeting 1 Gbit/ s per user in the near future. However, even this speed is not suffi-
cient for live broadcasting of HDTV signals since just a single uncompressed HDTV
(1080i) stream already requires a data rate of about 1.5 Gbits/s. A solution to this
bottleneck is seen in the development of wireless systems operating at much higher
carrier frequencies in the millimeter-wave (mm-wave) range where more bandwidth is
available. Especially around 60 GHz, a bandwidth of about 7 GHz is allocated for
wireless communications, 57–64 GHz in North America and South Korea and
59–66 GHz in Japan, and the European Union is currently in the process of creating
similar allocations. Consequently, broadband wireless systems operating at around
60 GHz are currently being studied worldwide, e.g., in the IEEE 802.15.3c group
focusing on short-range (path length up to 10 m) mm-wave indoor wireless systems
for the provision of more than 1 Gbit/ s. Even the introduction of 10 Gbits/ s Ethernet
共10 GbE兲wireless standards is expected, supporting the convergence of wired and
wireless systems in the access and thus ensuring a suitable mobile telephony network
backhauling function in the near future. Further applications are seen in storage area
networks (SANs).
Especially for long-range fixed wireless access (FWA) applications, other mm-wave
bands such as the E-band 共60–90 GHz兲or the F-band 共90– 140 GHz兲are considered
because those bands offer lower atmospheric gaseous losses. Nevertheless, the poten-
tial of 60 GHz for medium-range broadband wireless transmission has not been fully
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 471
1536-5379/09/050471-17/$15.00 © 2009 Optical Society of America
exploited yet, but the rising interest in 60 GHz technology has already led to higher
component availability and lower component cost. Also, future 60 GHz radio-
frequency complementary metal-oxide semiconductor (RF-CMOS) technology implies
further cost reductions. Altogether, this justifies scenarios using the 60 GHz band for
medium-range outdoor wireless point-to-point (p2p) transmission offering speeds
⬎10 Gbits/ s as well as for ⬎1 Gbit / s indoor short-reach radio systems.
With the objective of developing microwave photonic components and integration
technologies for broadband millimeter-wave wireless systems, 11 European partners
have initiated a joint European project entitled Integrated Photonic mm-Wave Func-
tions for Broadband Connectivity (IPHOBAC) [1]. This paper will report on the
progress in broadband radio communication systems based on the utilization of
advanced micorwave photonic components and radio-over-fiber (RoF) technologies
achieved in the IPHOBAC project. One of the targeted applications is the provision of
very-high-speed point-to-point radio links operating in the 60 GHz frequency band for
future mobile network backhauling or high-speed WLAN bridging [2–4]. A second
application is the provision of more than 1 Gbit / s short-reach radio communication
systems [5–7] such as the one studied in the IEEE802.15.3c working group [8],
capable, e.g., of broadcasting uncompressed HDTV signals. In Section 2of this paper,
we will at first report on key photonic components such as 60 GHz mode-locked-laser
diodes (MLLDs) [9–16] and 100 GHz photodetectors [17–19] that were developed in
the IPHOBAC project especially for broadband wireless applications. In Section 3,we
report on 60 GHz short- to medium-range fixed wireless outdoor systems using optical
on–off-keying (OOK) modulation and we demonstrate an ultrabroadband radio system
offering data speeds of up to 12.5 Gbits/ s [2,4]. Based on the experimental achieve-
ments and further theoretical calculations we predict maximum path lengths up to
the kilometer range for 10 Gbits/ s wireless transmission at 60 GHz [20]. We further-
more discuss advanced photonic vector modulation techniques [21,22] required for
high spectral efficiency and present experimental demonstration of 10 Gbits/s 60 GHz
carriers modulated by 4-quadrature amplitude modulation/quadrature phase-shift
keying (4-QAM/QPSK). In Section 4, we report on the provision of a photonic-assisted
radio transmission system for indoor applications distributing and delivering through-
out a building a UWB 60 GHz radio signal carrying 3 Gbits/ s [5,6]. Finally, we report
on a compact photonic 60 GHz RoF wireless link demonstrator for transmitting
uncompressed high-definition video/audio (HD V/A) signals.
2. Millimeter-Wave Photonic Technologies
Within the IPHOBAC project a number of different types of lasers, photodetectors,
modulators, and transceivers are being developed for applications not only in broad-
band wireless communications but also in instrumentation and radar/sensor applica-
tions [1]. In the following subsections, key components required for broadband wire-
less systems developed in the IPHOBAC project will be presented. In detail, 60 GHz
band 1.55
m mode-locked Fabry–Perot (FP) lasers utilizing a quantum-dash (QD)
active material and the development of ultrabroadband 共⬎100 GHz兲1.55
m wave-
guide photodetectors are reported.
2.A. 60 GHz Band Mode-Locked Quantum-Dash Fabry–Perot Lasers
Mode-locked laser sources are very attractive solutions for various applications such
as pulse generation, clock extraction from digital data, and optical microwave signal
generation and processing [8]. Previously published work showed how FP QD mode-
locked lasers could be used to achieve low-phase-noise oscillators at 39.8 GHz [10]. To
achieve devices adapted for 60 GHz wireless transmission, mode-locked lasers oscillat-
ing within this frequency range have been specifically fabricated for direct generation
of the millimeter-wave tone without any external reference oscillator. The linewidth of
the tone is expected to be sufficiently narrow to be used as a carrier for wireless trans-
missions. In addition, the ability to perform direct modulation of the laser can be used
to modulate the transmitted data on the carrier using the same device, avoiding the
need for an additional external modulator.
The studied semiconductor lasers are made of a buried ridge structure and contain
an active layer based on QDs on an InP substrate. The vertical structure is described
in a previously published work [10]. Both facets are cleaved, forming a 774
m long
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 472
FP cavity. The QD FP laser was mounted on an AlN carrier with a ground-signal-
ground (GSG) coplanar guide for biasing and direct modulation.
Passive-phase-modulation-type mode-locking has been obtained with these devices
without the use of any specific saturable absorber. Figure 1shows an example of the
beating spectrum observed at a DC bias current of 370 mA, with the resolution band-
width of the electrical spectrum analyzer (ESA) set to 3 kHz. One can observe a self-
pulsation frequency close to 54.8 GHz, corresponding to the inverse of the round-trip
time of the optical wave in the cavity. A nearly Lorentzian line shape is obtained,
exhibiting a −3 dB linewidth narrower than 18 kHz. A Lorentzian fit is also shown in
the figure. The extremely narrow linewidth for QD lasers is believed to be a conse-
quence of the reduced spontaneous emission rate coupled with the lasing mode and
sufficient four-wave mixing in these QD structures [11].
The electrical power that is obtained from a mode-locked laser source depends on
the beating between the optical modes during the photodetection process. The signal
obtained after photodetection will be composed of the sum of all the beating signals
corresponding to the frequency of interest in phase and amplitude. The highest
millimeter-wave power will be obtained if the relative phase difference between the
adjacent modes is the same. Thus, the RF signal will be even more sensitive to the
phase dispersion because the number of optical modes is large. There are two main
contributions to the relative phase: the dispersion in the laser itself and the disper-
sion associated with the optical fiber used for the transport of the optical signal. Opti-
mizing the transmission efficiency will require these two contributions to be mini-
mized. Studies of the phase dispersion of mode-locked lasers have shown that with
semiconductor lasers the dispersion has a strong linear part [12–14]. This dispersion
was shown to be compensated using standard single-mode fiber, enabling subpicosec-
ond pulses to be achieved [13–16]. Improving the electrical power available at the fun-
damental frequency of a mode-locked laser requires the same conditions as those
needed to reduce the width of a pulse. Minimizing the dispersion between the differ-
ent beat notes will also increase the amplitude at the corresponding frequency. We
have measured the electrical power after photodetection from the mode-locked laser.
The laser was biased at 370 mA. At this bias level the coupled optical power was
11 mW. Detection was made using the commercial XPDV2020R 50 GHz photodiode
(PD) from u2t. The electrical power at 58.4 GHz was measured using an Agilent
E4448A ESA coupled to a V-band 共50– 75 GHz兲H-P 11974V preselected harmonic
mixer. The measurements were performed after propagation through different lengths
of standard single-mode fiber. Figure 2presents the results of these measurements
(diamonds) with a correction used to remove the contribution of the coupling losses
associated with the numerous sections of fiber that had to be used. This was done by
a normalization using the values of the measured DC photocurrents. Without correc-
tion, the electrical power was −19.7 dBm when measuring directly at the output of the
laser and was improved to −6.8 dBm for a fiber length of 50 m and −15 dBm for a
fiber length of 2370 m.
The theoretical variation can be calculated by assuming a linear dispersion from
both the laser itself and the fiber. Experimentally, a dispersion of −1.2 ps/nm was
found for the laser itself. The calculated results are shown in Fig. 2. In the calcula-
Fig. 1. Self-pulsation electrical spectrum at 370 mA.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 473
tion, the relative power of the different modes of the optical spectrum needed for the
model was taken from the spectrum of Fig. 1, and a dispersion of the fiber of
17.8 ps/ nm / km is used. The agreement between the measured and the calculated
data is good, showing that in the laser linear dispersion is dominant. Thus, this part
of the dispersion can be compensated using the appropriate length of standard single-
mode fiber.
From these measurements it can be observed that an improvement of the electrical
power of more than 15 dB compared with the laser alone can be obtained just by using
65 m of standard single-mode optical fiber. It is important to note that the optimum
fiber length ranges are narrow. The length of the optical fiber must be precisely
adjusted in order to achieve efficient power generation.
The shapes of the pulses have been measured for the different fiber lengths using
an autocorrelator. Figure 3presents one of the traces obtained after propagation
through 65 m of single-mode fiber. The measured pulse had a FWHM of 722 fs, result-
ing in 480.5 fs after deconvolution, assuming a sech2shape.
2.B. 100 GHz Broadband and High-Output-Power Photodiodes
Advanced broadband photodiodes are key components for several mm-wave applica-
tions in broadband wireless, radar/sensing, and instrumentation. A key challenge in
that regard is the development of ultrawideband photodiodes capable of high-output-
power millimeter-wave signal generation. In IPHOBAC, various types of high-output-
power PDs are developed. This includes evanescent coupled pin waveguide PDs [17],
waveguide-coupled uni-traveling-carrier (UTC) PDs [18], as well as partially doped
partially nonabsorbent traveling-wave (TW) PDs [19]. Also, for enabling system-level
demonstrations such as optical mm-wave generation, we have developed a small form
factor fiber-optic package with a hermetic housing and a coaxial RF output connector.
Fig. 2. Corrected photodetected electrical power as a function of fiber length.
Fig. 3. Autocorrelator trace after propagation through 65 m of single-mode fiber, sech2fit.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 474
The general concept of the developed housing is based on a PD package with a
V-connector for operation up to 50 GHz. Based on this concept, a package with a w1
connector 共1mm兲has been developed and is shown in the photo of Fig. 4. Here, a criti-
cal point is the RF connection from the photodetector chip to the outer coaxial connec-
tor, which must be very broadband, highly efficient, low loss, and without resonances.
A specially designed grounded coplanar wave (CPW) substrate acts as the connecting
part between the photodetector chip and the coaxial connector. The measured fre-
quency response of the packaged devices is shown in Fig. 4(right). As can be seen
from this figure the packaged devices exhibit a 3 dB cutoff frequency at around
100 GHz. The total frequency roll-off from DC to 110 GHz is approximately 5 dB.
3. 60 GHz RoF System for up to 12.5 Gbits / s Wireless Access
3.A. 60 GHz RoF System Setup
In this section, we report on the development of a RoF system offering data rates
⬎10 Gbit0/s that would potentially allow transmission of 10 Gbit Ethernet signals for
future mobile network backhauling or high-speed WLAN bridging. Figure 5shows the
system configuration of the developed radio-over-fiber link. It consists of a 60 GHz
optical carrier generation unit followed by a broadband optical data modulation, a
wireless RoF transmitter making use of the broadband photodiode, as well as a wire-
less 60 GHz receiver.
For generating the optical 60 GHz carrier signal, light from a 1.55
m external-
cavity laser is modulated by a single-drive Mach–Zehnder modulator (MZM-1). The
bias of MZM-1 is set to the minimum transmission point for generating an optical
double-sideband signal with a suppressed carrier (DSB-SC). Hence, the frequency of
the driving local oscillator (LO) is half of the required wireless RF frequency, i.e.,
fLO/2 =30 GHz. The generated optical mm-wave signal is then coupled to a second
Mach–Zehnder modulator (MZM-2), biased to the quadrature point and modulated by
non-return-to-zero OOK (NRZ-OOK) data. For our experiments, we used a pseudoran-
dom binary sequence with a word length of 231 − 1 and data rates of up to 12.5 Gbits / s.
An optical bandpass filter is used to remove amplified spontaneous emission (ASE)
noise from the erbium-doped fiber amplifier (EDFA) and an optical attenuator is used
to control the optical power. After fiber-optic transmission the signal is detected using
a broadband photodiode described in Section 2, amplified up to approximately
+11 dBm and transmitted using a standard horn antenna with 20 dBi gain. The wire-
less 60 GHz signal is detected in the wireless receiver unit using an identical 20 dBi
horn antenna and it is further amplified by a low-noise amplifier (LNA). Finally, the
received 60 GHz signal is downconverted directly to baseband using a low-loss
custom-design balanced mixer and an amplifier for performing bit error rate (BER)
measurements.
3.B. Broadband Short-Range Indoor Experiments
At first, we investigated the performance of the constructed RoF system described
above within a laboratory indoor environment allowing a maximum wireless path
length of approximately 11 m. For the indoor measurements, no amplifier was used in
the wireless RoF transmitter, but the 60 GHz signal was transmitted directly after
optical/electrical (o/e) conversion. At first, BER measurements were carried out for a
wireless path length of 2.5 m at various data rates of 5, 7.5, 10, and 12.5 Gbits / s. As
Fig. 4. Packaged photodiode (left) using a package with a w1 coaxial output connector and fre-
quency response (right).
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 475
can be seen from Fig. 6, no error floor was observed even for BERs of 10−11.ForaBER
of 10−9 (231 − 1, NRZ) the measured receiver sensitivities at 5, 7.5, 10, and 12.5 Gbits / s
were −51.8, −50.1, −47.6, and −45.4 dBm, respectively. Thus the receiver sensitivity is
reduced by approximately 2 dB when the data rate is increased by 2.5 Gbits/s. We
also verified the maximum wireless path length the system could accommodate with-
out using any RF amplification in the wireless transmitter unit for a fixed safe optical
input power to the PD of +10 dBm. Under these conditions, the maximum wireless
path length achievable for a 10 Gbits/ s signal were approximately 8.5 and 5 m for
BERs of 10−4 and 10−9, respectively.
3.C. Broadband Medium-Range Outdoor Experiments
For studying the system’s performance for medium-range broadband mm-wave wire-
less transmission, outdoor experiments were carried out on the university’s campus.
Fig. 5. Setup of the photonic 60 GHz wireless RoF link for broadband wireless trans-
mission up to 12.5 Gbits/s. The system consists of a optical mm-wave generation and
modulation unit, a 60 GHz wireless transmitter, as well as a 60 GHz wireless receiver.
Fig. 6. Measured bit error rates with respect to the power of the received 60 GHz band
wireless signal. Data shown represents the measured BER for 5, 7.5, 10, and
12.5 Gbits/ s after wireless transmission over 2.5 m.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 476
Figure 7shows a photo of the setup with the constructed 60 GHz RoF wireless trans-
mitter in the front and the 60 GHz wireless receiver located at a distance of up to
40 m. The measurements were carried out under fair weather conditions, as can be
seen from Fig. 7. The system setup was surrounded by buildings, limiting the maxi-
mum wireless path length to 40 m, and the height over ground was only about
120 cm. No precautions were taken to avoid multipath propagation, e.g., due to
ground reflections. The outdoor experiments were performed at data rates of 5, 7.5,
10.3125 (gross data rate required for 64/66 coded 10 Gbit Ethernet), and 12.5 Gbits/ s
(gross data rate required for 8/ 10 coded 10 Gbit Ethernet).
Figure 8shows the BER characteristics after 20 and 40 m wireless transmission.
From the results, a sensitivity of −46 dBm for error-free 共BER⬍10−9兲transmission of
10.3125 Gbits/ s is observed. The sensitivity for 10.3125 Gbits/ s operation after 40 m
wireless path length is slightly better than for 20 m, which is attributed to reflections
from buildings. As can be seen from Fig. 8, the maximum data rate of 12.5 Gbits / s
was achieved for 20 m wireless transmission. However, error rates were limited to
approximately 5⫻10−7 in this case. This can be traced back solely to the power link
budget limitation because the indoor experiments reported above have clearly
revealed that there is no error floor even for 12.5 Gbits/ s. Thus we expect that error-
free transmission of 12.5 Gbits/ s is achieved when using either a slightly larger RF
gain or antennas with a higher directivity. We furthermore generally expect an
improved receiver sensitivity due to reduced multipath propagation by placing the
wireless transmitter and receiver at a higher position over ground, e.g., on the roofs of
two buildings.
3.D. Wireless Range Extension to the Kilometer Range
Based on the above-mentioned experiments, we further investigated the potential for
wireless range extension to the kilometer range by using high-gain antennas such as
50 dBi Cassegrain antennas [23]. Although higher mm-wave frequencies in the E- and
F-bands offer lower gaseous attenuation, the investigated 60 GHz system is expected
to allow wireless distances up to the kilometer range, even when considering heavy
rain fall. Figure 9shows the most significant contributions to the total wireless path
loss for a 1 km long air transmission within the V-band. Although gaseous losses peak
at around 60 GHz due to oxygen absorption, the free-space path loss (FSPL) is clearly
the dominating contribution to the total loss within the V-band with a loss figure of
Fig. 7. Photo of the broadband outdoor fixed wireless access experiments showing the 60 GHz
wireless transmitter and receiver. The experiments were carried out 120 cm above ground and
were surrounded by buildings. Maximum wireless line-of-sight distance was environmentally lim-
ited to 40 m.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 477
more than 100 dB. Also, rain attenuation can be neglected against the FSPL. Consid-
ering further that the FSPL rises with the square of the carrier frequency, this result
shows that for medium path lengths up to the kilometer range the 60 GHz band is
still favorable as compared with wireless systems operating at higher carrier frequen-
cies such as 77, 120, and 300 GHz. This is true although oxygen absorption peaks at
60 GHz.
The results presented in Fig. 9are theoretically determined based on the model
from van Vleck, Liege, and Blake for gaseous attenuation as well as from Olson and
Rogers for rain attenuation [24]. Rain amount data from a middle-European country
was taken from [25].
To study the capability of the developed system for wireless path extension up to
the kilometer range we have calculated the power received by the wireless receiver as
a function of wireless length. For this study, the 60 GHz free-space propagation loss
and a maximum gaseous attenuation of 15.5 dB/ km were considered. Also, to study
the link availability with respect to weather conditions, different rain attenuation fig-
ures based on sample rain data from a middle European country were considered. In
detail, the rain attenuation figures used for a link availability of 99%, 99.99%, and
99.999% are 1.3, 10.1, and 32.5 dB/ km, respectively [24,25]. Figure 10 shows the
received power versus wireless path length in case 50 dBi gain antennas are used.
The corresponding receiver sensitivities for achieving a BER of 10−9 for data rates of
10.3125 Gbits/ s (gross data rate required for 10 Gbit Ethernet transmission using
64/66 coding) and 12.5 Gbits/ s (gross data rate required for 10 Gbit Ethernet trans-
mission using 8/ 10 coding) are also indicated by the dashed lines.
Fig. 8. Measured bit error rates as a function of received power of the 60 GHz band
wireless signal. BERs are shown for 5, 7.5, 10.3125, and 12.5 Gbits/ s for wireless path
lengths of 20 and 40 m.
40 50 60 70 8
0
10−1
100
101
102
103
10
4
Frequency (GHz)
Path loss
(
dB
)
L1: FSPL (dB)
L2: Gaseous att. (dB)
L3: Rain att. 25mm/h (dB)
L1+L2+L3 (dB) after 1km
Fig. 9. V-band FSPL, gaseous loss, and rain attenuation 共25 mm/ h兲as well as total
path loss after 1 km wireless transmission over air.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 478
As can be seen from Fig. 10, the maximum wireless distances for a 10 Gbits/ s sig-
nal 共BER=10−9兲ensuring link availabilities of 99.999%, 99.99%, and 99% are 600,
1100, and 1500 m, respectively.
3.E. Fiber-Optic Range
In general, optical mm-wave double-sideband (DSB) transmission over fiber leads to
severe effects of chromatic dispersion assuming a dispersion coefficient of D
=17 ps / nm / km at 1550 nm for standard single-mode fiber. The power fading due to
phase shift between the upper and lower optical sidebands may result in destructive
interference of the beating products (each sideband with the optical carrier) during o/e
conversion, depending on the fiber length, thus significantly reducing the received
power [26]. Considering an eye diagram, this would cause a vertical closure of the eye.
In our system we have an optical carrier suppression of 26 dB; i.e., we are using an
optical carrier-suppressed double-sideband signal, and thus the power penalty due to
power fading is severely reduced.
A second effect is the phase shift of the data within the upper and lower optical
sideband during o/e conversion corresponding to a horizontal eye closure [27]. Here,
the data rate (i.e., the bit period) determines how much phase shift is tolerable. Fig-
ure 11 shows the relationship between the data rate and the maximum allowable fiber
length for causing a dispersion-induced power penalty (DIPP) due to data phase shift
of 3 dB [28].
As can be seen from Fig. 11, a fiber length of approximately 3 km causes a 3 dB
DIPP for a 10 Gbits/ s signal.
3.F. Photonic Vector Modulation
In this subsection, the technique of photonic vector modulation for generating spec-
trally efficient modulation formats is described. Using photonic vector modulation,
millimeter-wave wireless links with various advanced modulation formats such as
Fig. 10. Received power as a function of wireless path length when using highly direc-
tive antennas with an antenna gain of 50 dBi. Dashed lines represent the 60 GHz wire-
less receiver sensitivities for 10.3125 and 12.5 Gbits/ s.
Fig. 11. Fiber length causing a 3 dB power penalty due to phase shift of the data signal
versus data rate for DSB-SC modulation.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 479
QPSK, 16-QAM can be generated, and up to 10 Gbits/ s 16-QAM-modulated
millimeter-wave carrier generation is demonstrated [21]. In the photonic vector modu-
lator, two DFB lasers at 1554.14 and 1558.17 nm wavelengths, with a modulation
bandwidth of 4 GHz are directly modulated by two (Iand Q) 5 Gbits/ s 27−1 PRBS
data streams, respectively. Figure 12 shows the schematic of the experimental setup.
The optical carriers with the Iand Qdata modulated in a NRZ-OOK format are
individually modulated by an electrical carrier of fLO/2=30 GHz using two 45 GHz
Mach–Zehnder modulators biased at the minimum transmission point. The bias of the
MZM is chosen such that an optical carrier suppression (OCS) modulation is gener-
ated, and harmonics separated at 60 GHz are generated. The Q-arm optical signal is
now delayed by fLO/4, which corresponds to a 90° phase shift between the Iand Q
electrical carriers. The two optical signals are combined using a 3 dB coupler and pho-
todetected using a 100 GHz photodetector with a responsivity of 0.5 A / W. The input
power to the photodetector was measured as −14 dBm.
The photodetector output is a 10 Gbits/ s 4-QAM-modulated 60 GHz carrier. Based
on the photodetector input optical power and the responsivity, the output RF power
was calculated as −48 dBm. The 10 Gbits/ s 4-QAM signal was amplified using a LNA
with a gain of 16 dB. The RF signal was later filtered using a bandpass filter with a
bandwidth of 10 GHz. Another high-power amplifier (HPA) with a gain of 27 dB was
used to amplify the signal further. To emulate the effect of wireless transmission,
50 dB attenuation was added, which can be translated into a distance of 40 m if
antennas with a gain of 20 dBi are used. The RF signal was later amplified with
another LNA and HPA before downconversion using a broadband mixer. For demodu-
lation of the QPSK signals, the RF signals were mixed with a copy of the 60 GHz local
oscillator. An electrical phase shifter was used in the LO to tune the phase, and thus
the Iand Qbaseband components were demodulated, one at a time. The eye diagrams
of the demodulated Iand Qdata are shown in Fig. 13.
The baseband data were directly fed into a bit error rate tester (BERT) to measure
the BER of the demodulated signals. BERs of 4⫻10−8 and 8.7⫻10−8 were measured
for Iand Qdata, respectively. The BER shows a good quality of the 10 Gbits / s
4-QAM-modulated 60 GHz carriers. The quality can be further improved by increas-
ing the optical power input to the photodetector, and by using high-gain antennas,
which will also improve the achievable transmission distance.
4. 60 GHz RoF System for Indoor ⬎1 Gbit/ s Transmission
In this section, we report on the use of the mode-locked Fabry–Perot laser (ML-FPL)
diode described in Section 2to perform first the frequency upconversion of a high-
speed UWB radio signal from a carrier frequency of 4.5 to 60 GHz and second the fre-
quency downconversion from 60 to 4.5 GHz. The upconversion relies on directly modu-
lating the ML-FPL with the intermediate frequency (IF) radio signal while the
downconversion uses an external modulator. The distribution of the optical radio sig-
nal over 50 m of optical fiber is also demonstrated.
To realize a wireless network capable of delivering high-speed data 共⬎1 Gbit/ s兲
with housewide coverage, it is necessary to deploy several radio access points (RAPs)
Fig. 12. Schematic of the 10 Gbits/ s QPSK 60 GHz link using a photonic vector
modulator.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 480
around the home, linked together by a wired backbone network to convey the data
between the different high-speed picocells. In such a scenario, the support of an opti-
cal backbone combined with radio-over-fiber techniques has been demonstrated to be
very efficient and advantageous to distribute the radio signal to the different RAPs
while centralizing the main RF and baseband functions at a single location [6]. In this
architecture, three main functions are needed: frequency upconversion, frequency
downconversion, and signal distribution over up to 100 m. In this section, we will at
first describe the radio signal standard that we are using for our experiments. Then,
we describe a system using advanced photonic components capable of realizing the
three functions cited above.
4.A. IEEE802.15.3c Radio Signal
In the experiments below we use a radio signal taken from the IEEE 802.15.3c pre-
standard [23]. We chose the orthogonal frequency-division multiplexing (OFDM) vari-
ant among the different available modulation formats because it is the most suscep-
tible to nonlinearity. The signal is generated with a sampling rate of 2.595 GHz and
the 336 data subcarriers are QPSK modulated leading to a data rate of 3.03 Gbits/ s.
The 16 pilot subcarriers are used for signal equalization at the receive side. The crite-
rion for the signal to be received successfully is to measure an error vector magnitude
(EVM) [29] less than 23%. This value corresponds to an error rate lower than 10−6.
4.B. Frequency Upconversion and Distribution
The experimental setup is shown in Fig. 14. The radio signal under test is created on
a PC using Matlab following the specification of the IEEE 802.15.3c group [23]. The
signal is generated by a 10 GS/ s dual-output arbitrary waveform generator (AWG),
and both outputs (representing both Iand Qcomponents) are sent to a RF mixer to
generate the radio signal on a 4.5 GHz carrier. At this point, the spectrum of the sig-
nal extends from 3.5 to 5.4 GHz and the available RF power is −15 dBm. In Figs.
14(bគTX) and 14(bគRX), this signal is subsequently amplified to approximately
+15 dBm and is used to modulate the bias current of the ML-FPL (average bias cur-
rent set to 260 mA). The optical output power out of the ML-FPL is +6 dBm. The laser
pulses with a repetition rate of 54.8 GHz. Its modulation produces a mixing between
the pulsating frequency and the IF carrier, leading to an optical frequency upconver-
sion of the original signal to 59.8 GHz. To simulate the distribution of the radio signal
within the home, 50 m of standard single-mode fiber (SMF) is used to link the laser to
Fig. 13. In-phase and quadrature demodulated eye diagrams.
Fig. 14. Schematic of the upconversion and distribution experiment: (a) OFDM signal
generation using an arbitrary waveform generator and upconversion to 4.5 GHz. (bគTX)
Photonic upconversion based on direct modulation of ML-FPL and optical transmission.
(bគRX) Optical reception and amplification. (c) Electronic upconversion to be used as a
benchmark. (d) Downconversion and digitization of the output IF signal using a real-
time oscilloscope.
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 481
a commercial 70 GHz photodetector that is followed by a low-noise amplifier (LNA
−G=18 dB from 55 to 65 GHz), a bandpass filter 共58.5 to 64 GHz) and a high-power
amplifier 共HPA−G= 31 dB from 59 to 63.5 GHz). The available RF power at this point
is +4 dBm. To take a reference measurement, an all-electronic upconverter is used for
comparison. The signal out of Fig. 14(a) is amplified to approximately 0 dBm and sent
into a commercial mixer fed with a +10 dBm, 54.5 GHz local oscillator. The output RF
signal is filtered, and a power of −13 dBm is obtained. In Fig. 14(d), to measure the
quality of the received 60 GHz radio signal out of Figs. 14(b) or 14(c), it is first attenu-
ated to the optimal power level (around −22 dBm); then it is downconverted using a
conventional electrical mixer fed with a 54.5 GHz LO, and finally, it is captured using
a 40 GS/ s real-time scope. OFDM demodulation and EVM evaluation are performed
offline using Matlab. Each capture records a total of 44 OFDM symbols over 10
s,
representing 296,000 bits of data.
Results are presented in Fig. 15. The spectrum of the received OFDM signal and
the associated constellation diagram obtained after demodulation are shown. The
mean EVM is 10.5% for a signal-to-noise ratio (SNR) of 23 dB. For reference, the
results using an all-electronic upconversion (as depicted in Fig. 15, right) yield an
EVM of 9% with a SNR of 25.2 dB. The results are marginally better, but it has to be
underlined that the photonic upconversion provides as well the ability to transport
and distribute the 60 GHz radio signal over several tens of meters. These results vali-
date the use of the ML-FPL for radio signal upconversion and transport.
4.C. Frequency Downconversion and Distribution
The experimental setup is depicted in Fig. 16. The OFDM signal under test is created
on a PC using Matlab and generated by a 10 GS/ s AWG with its bandwidth centered
at 1.6 GHz. It is then upconverted using a RF mixer in the IF band 4.8– 7.0 GHz [Fig.
16(a)]. The IF signal is electrically upconverted to 60 GHz [Fig. 16共abis兲] then sent
through the photonic downconverter [Figs. 16(bគTX) and 16(bគRX)] before analysis
[Fig. 16(c)]. The analysis of Fig. 16(c) uses a 50 GS/ s real-time scope to capture the
output IF signal. OFDM demodulation and EVM evaluation are then performed
offline using Matlab. The operating principle of the photonic downconverter is the
same as in the upconversion scheme. The incoming mm-wave OFDM signal modulates
Fig. 15. Spectra (down) and constellations (up)—data and pilots—obtained after pho-
tonic upconversion (left) or electrical upconversion (right).
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the MLL output through a 60 GHz Mach–Zehnder modulator (MZM, RF power of
+7 dBm). The MZM is connected to a 10 GHz PD with SMF. The original 60 GHz
OFDM signal is thus downconverted to approximately 5 GHz (the radio carrier fre-
quency minus the MLL pulse frequency). The RF power at PD output is typically as
low as −50 dBm.
Spectra of the received OFDM signal and associated constellation diagram obtained
after demodulation are shown in Fig. 17. The mean EVM is 15.2% for a SNR of
20.9 dB. The obtained experimental EVM validates the use of the ML-FPL for radio
signal frequency downconversion and transport.
5. 60 GHz RoF System for Uncompressed HDTV Transmission
Another key application of the developed broadband RoF technology is seen in
enabling the wireless transmission of uncompressed high-density video/audio (HD
Fig. 16. Schematic of the downconversion and distribution experiment: (a) OFDM sig-
nal generation using an arbitrary waveform generator and upconversion to 6 GHz. 共abis兲
Electrical upconversion to 60 GHz. (bគTX) Photonic downconversion based on external
modulation of ML-FPL and optical transmission. (bគRX) Optical reception and amplifi-
cation. (c) Digitization of the output IF signal using a real-time oscilloscope.
Fig. 17. Spectra (down) and constellations (up)—data and pilots—obtained after pho-
tonic upconversion (left) or electrical upconversion (right).
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 483
V/A) signals. Transmitting uncompressed HD V/A signals using a 60 GHz wireless
system would not only preserve the best video and audio quality but also avoid encod-
ing latencies. Even more important would be the fact that such systems would offer
reduced complexity and lower cost by utilizing the license-free 60 GHz band and nei-
ther a video encoder nor a decoder would be required. On top of this, a 60 GHz wire-
less HD V/A link would further be free from interference with other existing wireless
standards at 2.4 GHz, 5 GHz, or UWB. These advantages clearly justify the develop-
ment of a 60 GHz RoF wireless link technology for HD V/A transmission.
For the wireless delivery of a high-quality uncompressed 1080p HD V/A signal, a
maximum data rate of approximately 3 Gbits/ s is required. Because this data rate is
significantly lower than the maximum data rate of 12.5 Gbits/s offered by the 60 GHz
RoF system described in Section 3, we have developed a more compact and signifi-
cantly less costly 60 GHz RoF wireless link system that is especially suited for HD
V/A wireless transmission over several meters.
Fig. 18. Setup of the constructed 60 GHz RoF wireless link for transmitting high-density video/
audio signals. The optical mm-wave generation and modulation unit just consists of a 60 GHz
band mode-locked laser diode. For simplicity and cost reasons, the constructed 60 GHz wireless
receiver utilizes an envelope detection concept.
Fig. 19. Photo showing the developed units of the 60 GHz wireless HD V/A link. The optical mm-
wave generation and modulation unit (top right-hand module) is put on top of the 60 GHz wire-
less RoF transmitter (bottom right-hand module). The 60 GHz wireless receiver can be seen on
the left-hand side of the photo.
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The system configuration of the 60 GHz RoF HD V/A wireless link system is shown
in Fig. 18. As can be seen, the optical mm-wave generation unit of the system only
consists of a single 60 GHz MLLD as described in Section 2, which can be either
directly or externally modulated with the serialized HD V/A signal. This way, the
necessity for the two MZMs required for the 12.5 Gbits/ s system (see Fig. 5)was
avoided for the sake of significantly reduced cost and complexity. It should be pointed
out here that, of course, only the optical mode-locking frequency of the MLLD is in the
mm-wave range; the laser itself only requires a low-cost fiber-optic package. In the
wireless transmitter unit of the 60 GHz RoF HD V/A system, we employed the devel-
oped photodetector described in Section 2. Due to the relaxed system requirements as
compared with the 12.5 Gbits/ s system, the wireless receiver of the HD V/A system
can accomplish a much higher noise figure, and thus we made use of a simpler enve-
lope detection approach. This way, not only were costly phase-locked loops or self-
heterodyning mixers avoided, but this receiver approach also allows the use of a
MLLD with a mode-locking frequency slightly different from 60 GHz. This is possible
because the 60 GHz band offers sufficient bandwidth (about 7 GHz), and thus the
mode-locking frequency of the MLLD can vary slightly within a range of 1– 2 GHz.
Since the mode-locking frequency of the MLLD depends on the length of the laser chip
(see Section 2), a less stringent requirement significantly increases yield and reduces
the cost of the MLLD. The actual MLLD employed in the system has a mode-locking
frequency of 58.8 GHz.
Figure 19 shows a photograph of the developed wireless system. On the right-hand
side of the photo, the optical mm-wave generation and modulation unit that was
placed on the 60 GHz wireless transmitter unit can be seen. Since optical fiber
enables the low-loss transport of the 60 GHz signal over several kilometers, the opti-
cal unit can be placed far away from the transmitter, e.g., close to a recording HD
camera. The size of the constructed optical generation/modulation unit is approxi-
mately 10⫻20⫻8cm
3. The wireless receiver can be seen on the left-hand side of the
photo. It should be noted that the wireless path length between the transmitter and
receiver was 10 m, but with respect to the results reported in Section 3, we expect a
wireless path extension up to the kilometer range.
Using the developed 60 GHz RoF HD V/A wireless link system, the transmission of
high-definition video/audio signals was successfully demonstrated. Figure 20 shows
the developed compact wireless HD TV system displaying a 1080i HD V/A movie. This
system was showcased at the ICT 2008 event.
6. Conclusions
Advanced photonic components and radio-over-fiber techniques for broadband wire-
less communications have been studied in this paper. Mode-locked FP lasers utilizing
a quantum-dash active material and advanced broadband photodiodes were developed
for 60 GHz signal generation. Utilizing those components and RoF techniques we
developed a photonic wireless link and demonstrated wireless transmission at data
Fig. 20. This photo shows the 60 GHz receiver unit and the HD TV screen displays the trans-
mitted and downconverted HD V/A signal (1080i).
Vol. 8, No. 5 / May 2009 / JOURNAL OF OPTICAL NETWORKING 485
rates of 10.3125 Gbits/ s over 40 m of air with the potential of wireless path extension
into the kilometer range. Also, we reported on a photonic vector modulation approach
for generating a 10 Gbits/ s 4-QAM/QPSK-modulated 60 GHz carrier required for
future spectrally efficient wireless systems. Furthermore, we developed a home-area
wireless 60 GHz UWB system capable of carrying data up to 3 Gbits/ s, and finally, we
demonstrated a compact photonic wireless 60 GHz RoF link for transmitting uncom-
pressed HD video/audio signals at speeds exceeding 3 Gbits/ s.
Acknowledgments
This work was supported by the European Commission and carried out within in the
framework of the European integrated project IPHOBAC under grant 35317.
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