transmission of rf and microwave signals by optical fiber
TRANSCRIPT
Transmission of RF and Microwave Signals by Optical Fiber
Jean-Pierre VILCOT, Christophe LETHIEN Institut d'Electronique, de Microélectronique et de Nanotechnologie, UMR CNRS 8520
Transmanche Centre for Telecommunications Research Université des Sciences et Technologies de Lille
Avenue Poincaré, BP 69 59652 Villeneuve d'Ascq cedex FRANCE
Abstract
We will present the different indoor applications and relative device requirements for the transmission of RF or microwave signals by optical means. Systems are based on the picocell distribution scheme which uses the remote antenna concept consisting in a base station which optically feeds several remote antenna units. Two kinds of systems can be distinguished. The first one deals with RF signals in the 1 GHz to 5 GHz frequency band and is mainly dedicated to the optical transport of mobile communication bands (GSM, DECT, DCS, UMTS, Hiperlan,…). The second one is more advanced and is based on an optically generated micro- or even millimeter wave signal (40 GHz and 60 GHz frequency bands); the achievable data bitrate could be then as high as several hundred of Mbits/s. 1. Introduction
Radio over Fiber (RoF) systems use optical carriers to distribute micro- or millimeter-wave signals. Such systems are used on applications, such radars, on several kilometers spans. An interesting use of these systems in telecommunications is to distribute wireless telecom signals all over an indoor or shadowed area. In that case, multiple access points are needed on a coverage zone that is typically less than 1km diameter wide. The concept is then declined as a picocellular system in which each elementary cell covers a maximum of some hundreds of meters. But most of time these elementary cells will cover a unique space or room (figure 1). The coverage of a building can then be done using a base station which receives/emits signals either by hertzian or fiber optics systems and distributes them to a multitude of remote antenna units which are optically fed. These antenna units deliver the downlink signals and receive the uplink ones in their picocell area.
Two kinds of systems can be distinguished. The first one deals with RF signals in the 1 GHz to 5 GHz frequency band and is mainly dedicated to the optical transport of mobile communication bands (GSM, UMTS, Hiperlan,…). In that case, the RoF system acts
as a relay for these signals. The second one is more advanced and is based on an optically generated micro- or even millimeter wave signal (40 GHz and 60 GHz frequency bands); the achievable data bitrate could be then as high as several hundred of Mbits/s and up to the Gbits/s. These systems are dedicated to the transfer of huge amount of data within the same building such as hospitals or banks. They can act as stand-alone internal systems without any external connections.
Figure 1. Schematic of radio over fiber picocellullar
indoor system. We will now describe these systems depending on the frequency range they use: (i) RF over fiber systems up to 5 GHz and, (ii) millimeter-wave systems for 40 or 60 GHz range systems. 2. RF over fiber systems 2.1. General considerations
For indoor or shadow area coverage of mobile communications systems, picocellular systems are fine alternatives since they allow distributing signals where global distribution systems are ineffective. Their
Remote Antenna
Unit
Base station
Picocell
fiber
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coverage range is some tens to hundred meters. A typical system uses a base station which optically fed remote antenna units with the RF signal which is modulated on the optical carrier. Since the coverage is rather small, electromagnetic pollution can be greatly reduced since emitted RF power can be 30 dB lower than for classical, i.e. 20 mW in spite of 20W.
- RF over fiber. This is the simplest way of implementation as well as the lower cost for the remote antenna units. The RF signal directly modulates the emitter and is directly re-emitted at the remote antenna unit (figure 4). But this scheme requires enhanced performance for the optical emitters and receivers since they have to transmit the carrier frequency of the mobile telecom signals, i.e 900 MHz for GSM band, 1800 MHz for DCS, 2 GHz for UMTS,….
This optical carrier is carried on optical fiber up to the remote antenna units. Three types of optical fiber can be distinguished, each of them corresponding more or less to a specific wavelength of the optical carrier: (i) single mode fiber (SMF) for mainly 1300 nm, (ii) multimode fiber (MMF) for mainly 850 nm, and (iii) polymeric optical fiber for visible or even infrared wavelengths. This does not prevent the existence of "cross-mixed" systems such as the use of multimode fiber at 1300nm.
Base station
OERF
Base station
EO
Figure 4. RF over fiber transmission scheme Whatever the optical carrier and wavelength
are, the transmission of signals can also been done using mainly three main schemes:
In the following, we will consider the last case,
i.e. RF over fiber since it is potentially THE low cost solution owing to the fact that very few electronics are required within the remote antenna units. This solution will be even more effective if transmission can be done on MMF that constitutes the main part (more than 90%) of the pre-installed fibers in buildings for gigabit ethernet applications. The modal characteristic of this kind of fibre (bandwidth of 50/125 MMF: 500 MHz.km) restricts the system bandwidth: in fact, there is a lot of propagated modes in the fibre which interact. Therefore, we demonstrated that it is possible to overcome this limitation even transmitting complex modulation schemes (32 QAM) on a RF sub-carrier (2 GHz) trough 1 km of MMF at 1.3µm [1]. On figure 5, the constellation as well as the eye diagrams of such a signal is represented before and after the optical transmission. As it can be observed, no real degradation appears even considering the bandwidth of the optical fibre is 500 MHz.km.
- data over fiber. The received mobile telecom signal is received and the data are extracted (A/D). These data are use to modulate the optical emitter (OE). At the remote antenna unit, once detected (EO) they modulate again (D/A) a RF carrier (LO) that is locally generated (figure 2). This scheme adds complexity and cost in each remote antenna due to the embedded required electronics. Anyway, it is the less stringent on optical link requirements since only the data stream (few tens of Mbits/s) is carried on it. MMF is a cost effective solution here but obviously SMF can be used.
˜ SYNCMMF
˜
Base stationRemote antenna unit
OEEOIFRF RF
A/D D/A
LO LO˜ SYNCMMF
˜
Base stationRemote antenna unit
OEEOIFRF RF
A/D D/A
LO LO
Figure 2. Data over fiber transmission scheme
- IF over fiber. The received mobile telecom signal is down-converted to a lower frequency and the resulting IF signal is sent through the fiber. At the remote antenna unit, this IF signal is mixed with a local oscillator one to re-generate the RF signal (figure 3). This scheme adds lower complexity at remote antenna units that the previous one since only RF circuitry is needed. Same comment as above concerning the type of fiber that can be used.
Base station
OE
˜
Remote antenna unit
RF
LO˜
Base station
EOIFRF
LO
Base station
OE
˜
Remote antenna unit
RF
LO˜
Base station
EOIFRF
LO
Figure 5. Constellation (top) and eye (bottom) diagrams of a 32 QAM signal on 2 GHz RF carrier before (left) and after (right) its transmission on a
MMF based RoF system [1].
Figure 3. IF over fiber transmission scheme
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2.2. Required devices It is composed of a MQW structure which realizes the electro-absorption modulator at λup and the detector at λdown. The uplink wavelength λup (continuous wave) is generated within the base station and distributed to each remote antenna unit avoiding the need of optical emitter in these.
Data or IF over fiber systems do not require the
development of new devices, current commercially available ones fulfill their needs. As we saw, RF over fiber is particularly interesting since it can lead to cost effective remote antenna units. Nevertheless, since the carrier frequency of wireless systems can reach 5 GHz, some developments are still needed. The direct modulation of laser diodes is commonly used. This kind of modulation can be made either on VCSELs or edge emitting devices since commercially available components allow such high modulation rates. Optical detectors with such bandwidths are also available.
In order to get a transmission type device, GaAs substrate shall be removed since it is absorbing at 850 nm. Once device fabricated, its substrate is removed and its active part is transferred onto a silica substrate (figure 7) [4]. Some interesting developments could be to move to higher wavelength where GaAs substrate is no more absorbing but GaAs material line materials are optically active, i.e. 980 nm.
Taking into account the bi-directional aspect of
the RoF system, the MMF compatibility and the overall system cost, it is interesting to evaluate breakthrough devices that can relax the need of optical emitters in the remote antenna units. The Electro-Absorption Modulator (EAM) can be used as a detector for the downlink and a modulator for the uplink assuming alone all the functionalities that are required at the remote antenna unit [2] (Figure 6)
Polymer optical fiber (POF) could be potentially used for data over fiber transmission schemes. But, up to now, no real development is made on that. Main advantage of POF is its simplicity of use.
2.3. System performance
Right now, all existing systems are working at
infrared wavelengths either on SMF (RF over fiber) or MMF (data or IF over fiber) systems. Some commercial links are listed at the end of the references chapter.
Remote antennaunit
downlink
uplink
Base station
EAM
EO
OE
Remote antennaunit
downlink
uplink
Base station
EAM
EO
OE
3. Millimeter-wave over fiber systems 3.1. General considerations
Picocellular systems are also studied using higher frequencies such as 40 or 60 GHz. The main problem is here to generate the millimeter wave within the remote antenna unit since either direct or external modulation can not be used (due to modulation bandwidth limitation and chromatic dispersion in the fiber) to optically launch the microwave carrier signal into the system. All these systems have been studied at 1550 nm and with SMF, up to now. Several schemes of carrier generation have been developed that can be roughly separated in two families.
Figure 6. Optimized RoF scheme using an EAM
device at the remote antenna unit.
Several devices have already been reported for infrared wavelength and SMF [3]. In order to benefit of potential lower costs of 850 nm wavelength and MMF systems, such a device is currently under investigation.
The first one is mainly using "electrical"
solutions: a "low" frequency (i.e. 7 GHz) is transmitted via the fiber using, for example, direct modulation of a laser diode; once detected, this carrier is electrically multiplied (i.e. x8) to create the millimeter wave (56 GHz) carrier, the baseband data are generally sent on a different fiber and mixed at the remote antenna unit [6] (Figure 8). Three wavelengths are used transporting respectively, the downlink data (λ0), the downlink millimeter-wave subharmonic (λ1) and the uplink data (λ2). No specific optoelectronic component is required and systems can be built with already commercially available devices, main developments are reported on the electronic part. In that case, remote antenna units are not specifically low cost since they require a lot of
SiO2 s ubs t rat e
Ac t ive MQW s t ruc t ure
Figure7. 850 nm transmission type electro-absorption modulateur transferred on SiO2
substrate.
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electronics but dedicated integration rules can lower the cost.
Det.
Det.
data DFB
l.o. DFB
λ0 fi fi
fi’
fi’
fdown
fup
flo
Laser
x
x
λ1 x nDet.λ1
λ0
fi
fi’
fdownflo
x
x
Mod. IQ
Base station
TransducerM
obile
~Modem IQ
λ 2λ 2
To demodulation
~
fup
fsub
U/C
D/C
U/C
D/C
Down link
Up link
Figure 8. Millimeter-wave over fiber using electrical frequency multiplication within the remote antenna
unit. The other principle which is widely used for
creating millimetre-wave at the remote antenna unit is based on optical heterodyning. Two optical carriers (one of these two is modulated with data) mix within the photodetector and generate a frequency which is directly the difference of their own respective frequency. High detected power as well as high frequencies can then be generated. The other advantage of this solution is that it reduces greatly the chromatic dispersion effect due to the optical path within the fibre which acts lowering the phase noise of received signals (important for OFDM or QPSK type phase modulated data).
"master"Laser(fopt1)
Sub-harmonicOscillator (fmm/n)
"slave"Laser(fopt2)
Mod.
data
fopt1
fmm/n
fopt1 fopt2 fopt1 fopt2
polar.att.
"master"Laser(fopt1)
Sub-harmonicOscillator (fmm/n)
"slave"Laser(fopt2)
Mod.
data
fopt1
fmm/n
fopt1 fopt2 fopt1 fopt2
polar.att.
Figure 9. OIL technique principle. The synchronization signal coming from the "master"
laser is launched in the "slave" laser using an optical circulator.
The main drawback of that solution is that the
two optical carriers shall be phase correlated in order to lower the phase noise of generated millimetre-wave signal. This leads to add complex structures of
synchronization such as Optical Phase Locked Loops (OPLL) [7] or Optical Injection Locking (OIL) [8] to reduce this effect.
The OIL technique (figure 9) is may be the simplest way to achieve optical phase lock of two lasers: a "master" laser is modulated with a subharmonic of requested millimetre-wave carrier (fmm/n), a part of its output power his launched in a "slave" laser which frequency corresponds to one of the harmonic sidebands which are generated by the "master" laser; this locks the "slave" laser emission wavelength to the "master" one. Main disadvantage of that technique is a low detuning range. A technique combining the two above mentioned ones, the Optical Injection Phase Locked Loop (OIPLL) [9] technique allows to gather the advantages of those two but results in a very complex system to handle.
So, several solutions including master-slave laser arrangements, optical phase loops,… have been reported. Nevertheless, no real compact and versatile solutions have been reported today.
3.2. Required devices
On the contrary of RF over fiber systems, the
development of specific optoelectronic components is required here. As examples, the best results on master-slave laser arrangement have been obtained using specifically fabricated laser devices or complex arrangement of more or less on-the-shelf components.
Except for the first solution where compactness and performances can be achieved by integrating electronics, the heterodyne generation scheme needs still some device developments.
Photodetection requires high bandwidth and if
possible high power detector capabilities. This allows generating high power millimeter-wave signals limiting even avoiding the need of electrical amplification. Such detectors have been studied and fabricated; they are declined as traveling-wave (TW-PD) [10] and uni-traveling carrier (UTC-PD) [11] photodetectors.
light
absorption region
carrier collecting
layer
diffusion barrier
layer
P
I N+
P+P
I
e-
h+
space-charge region
Figure 10. Schematic band diagram of UTC-PD
UTC-PD's seem the most promising and cost
effective devices for RoF systems. A UTC-PD (figure 10), contrary to a classical PIN photodiode absorbs light
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in the P-doped region. As a consequence, photo-generated holes response is governed by the dielectric relaxation time which is very short. So, only photogenerated electrons travel into the undoped layer leading to a quick response time and, by the way, to the name of that device. Cut-off frequencies greater than 100 GHz and millimeter-wave power greater than 0 dBm are commonly achieved. Some devices are commercially available since soon.
However laboratory experiments have showed very interesting results such the transmission of 140 MBits/s ASK and 68 Mbits/s DPSK signals modulated on a 36GHz carrier error free and without any amplification on a 65 kms span using an OIPLL generation scheme [9, 15, 16] 4. Conclusion
RF over fiber systems are currently available under different forms. Anyway the cost effective solution which consists in transmitting directly the RF modulated signals on a MMF network still needs some developments in order to cover all the mobile telecommunications standards from GSM 900MHz to Hiperlan/2 around 5GHz.
The emission of a dual-mode optical spectrum requires specific devices. Even considering the OIL technique which is represented on figure 8, it can be seen that the "slave" laser is not so classical since an optical input is needed for the laser device. This means that it is ever a standard (one output) telecom device without any optical isolator or a specific device (one input, one output) with adequate facet coating.
Millimetre-wave over fiber systems can provide large data bitrates. The carrier frequency requires huge bandwidth detectors and possibly large output power in order to minimize the remote antenna electronics. Such kind of devices are not or few commercially available but their development is enough mature to envisage production. The critical point is concerning the millimeter-wave optical generation. Research demonstration has showed the feasibility of pure millimeter-wave carrier generation. Nevertheless, these systems need the development of either new devices or integration solution in order to be affordable and reliable in commercial systems.
Focusing on semiconductor based solutions since they are the best candidates to provide compact systems, specific semiconductor laser sources can also be investigated. As examples, dual mode DFB devices have been made either in in-line [12] or Y [13] configuration. More recently BiVCSEL structures have also shown some potentialities [14]. As an example of OIL technique, the electrical spectrum of detected signal at the photoreceiver output (figure 11) is shown when the system is unlocked (free-running) and locked. The behavior of the locking system is obvious.
17.8 18.0 19.018.2 18.618.4 18.8Frequency (GHz)
-20
-40
-60
-80
-100
dBc/
Hz
Injection locked
Free running
However, the "all electrical" solution using the transmission of a subharmonic of the millimeter-wave carrier frequency has shown reliable results. Few developments in millimeter-wave circuits can quickly lead to commercial systems; 5. Acknowledgements
Parts of this work are supported under
"ROSETTE" British-French INTERREG III and "NEFERTITI" Vth PCRD Network of Excellence European projects.
Figure 11: Photogenerated spectrum from a dual
source laser using OIL technique [13]. Authors want also to thank D. Wake from Microwave Photonics for fruitful discussions.
Some research is made on polymer fiber which is investigated as transport medium at 1300nm [5] but right now, research is beginning using POF or GIPOF.
6. References
[1] D. Wake, S. Dupont, C. Lethien, J-P. Vilcot and D. Decoster, "Radiofrequency transmission of 32-QAM signals over multimode fibre for distributed antenna system applications", Electronics Letters, vol. 37(17), p 1087-1089 (2001).
3.3. System performance Millimeter-wave over fiber systems have been widely studied. Nevertheless, no commercial product is today available mainly for two reasons: (i) affordable, reliable and compact components are not currently available mainly concerning the dual-mode optical source that has to be included in the base station, (ii) no significant demand on high bitrates has encouraged the development of this missing block.
[2] D. Wake, D. Moodie, and F. Henkel, "The electroabsorption modulator as a combined photodetector / modulator for analogue optical systems", Proceedings of Electron Devices for Microwave and Optoelectronic Applications Workshop, King’s College London, (1997).
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[3] Chin-Pang-Liu, A.J. Seeds, J.S. Chadha,-.; P.N. Stavrinou, G. Parry, M. Whitehead, A.B. Krysa and J.S. Roberts, "Design, fabrication and characterization of normal-incidence 1.56- mu m multiple-quantum-well asymmetric Fabry-Perot modulators for passive picocells", IEICE-Transactions-on-Electronics, E86-C(7), pp. 1281-1289 (2003). [4] C. Lethien, J-P. Vilcot and D. Decoster, "850 nm Transmission Type ElectroAbsorption Modulator on SiO2 substrate", Proceedings of SOTAPOCS XL, The Electrochemical Society, San Antonio, (2004). [5] T.I. Monroy, H.P.A. vd-Boom, A.M.J. Koonen, G.D. Khoe, Y. Watanabe, Y. Koike, T. Ishigure, "Data transmission over polymer optical fibers", Optical-Fiber-Technology:-Materials,-Devices-and-Systems, vol. 9(3), pp. 159-171 (2003). [6] S. Dupont, C. Loyez, J-P. Vilcot, N. Haese and D. Decoster, "Optical millimeter-wave system using sub-carrier local oscillator generation", Proceedings of European Microwave Conference, Vol. 30 , n° Vol.3, pp. 197-200 (2000) [7] L.N. Langley, M. D. Elkin, C. Edge, M.J. Wale, X. Gliese, X. Huang and A.J. Seeds, "Packaged semiconductor laser optical phase-locked loop for photonic generation, processing and transmission of microwave signals", IEEE Trans. On Micr. Th. And Techn., MTT 47(7), pp. 1257-1264 (1999) [8] Y.J. Wen, H.F. Liu, D. Novak and Y. Ogawa, "Millimeter-wave signal generation from a monolithic semiconductor laser via subharmonic optical injection", IEEE Photonics Techn. Lett., PTL 12(8), pp. 1058-1060 (2000) [9] L.A. Johansson and A.J. Seeds, "36 GHz 140 MBits/s radio over fiber transmission using an optical injection phaselock loop source", IEEE Photonics Techn. Lett., PTL 13(8), pp. 893-895 (2001) [10] A. Stöhr, R. Heinzelmann, A.Malcoci and D. Jäger, "Optical heterodyne millimetre-wave generation using 1.55µm travelling-wave photodetectors", IEEE Trans. On Micr. Th. And Techn., MTT 49(10), pp. 1926-1933 (2001)
[11] A. Hirata, M. Harada and T. Nagatsuma, "120GHz wireless link using photonic techniques for generation, modulation and emission of millimetre-wave signals", IEEE J. of Lightwave Techn., JLT 21(10), pp. 2145-2153 (2003) [12] D. Wake, C.R. Lima and P.A. Davies, "Optical generation of millimetre-wave signals for fibre-radio systems using a dual mode DFB semiconductor laser", IEEE Trans. On Micr. Th. And Techn., MTT 43(9), pp. 2270-2276 (1995) [13] C. Laperle, M. Svilans, M. Poirier and M. Tetu, "Frequency multiplication of microwave signals by sideband optical injection locking using a monolithic dual-wavelength DFB laser device", IEEE Trans. On Micr. Th. And Techn., MTT 47(7), pp. 1219-1224 (1999) [14] M. Brunner, K. Gulden, R. Hovel, M. Moser, J.F. Carlin and M. Ilegems, "Continuous wave dual wavelength laser emission from BiVCSEL device", Proc. of CLEO'00 Conference, p. 3 (2000) [15] L.A. Johansson, C.P. Liu, and A.J. Seeds, "A 65-km unamplified transmission of 36 GHz radio over fiber signals using an optical injection phase lock loop", IEEE Photonics Techn. Lett., PTL 14(11), pp. 1596-1598 (2002) [16] L.A. Johansson and A.J. Seeds, "Generation and transmission of millimetre-wave data-modulated optical signals using an opticall injection phase lock loop", IEEE J. of Lightwave Techn., JLT 21(2), pp. 511-520 (2003) Links to commercial existing systems: - data over fiber (MMF or SMF) Digivance™: http://www.adc.com/Library/Literature/1646.pdf - IF over fiber (MMF) LGCell: http://www.lgcwireless.com/products/lgcell.html - RF over fiber (SMF) BriteCell®: http://www.andrew.com/products/inbuilding/ FiberDAS™: http://www.avitec.se/files/pdf/FiberDAS/
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