Proceedings

Hungarian Academy of Sciences

.iff*,*tl..*,

Thermal Hotel Helia

BUDAPEST, HUNGARY
March 2l-24, 1999
 Optical generation of millimetre waves
for mobile radio systems

T. Bercelio, G. Jir6', T. Marozsikt, A. Flilt*o, S. Mih6ly', E. udvary', A.z6lorry',z.Yatga'

* Tcchnical Univcrsity of Budapcst, Dcpartncnt of Microrravc Tolccornmunietions,
H-l I I I Budapost, Goldmann Gy6rgr-t€,r3, Hungary, c-mail : bc,rcoli@nov.mht.bmo.hu

o Tolocornmunisatioms rnnsv6fi ms Co.

H-1142 Budapost, Ungwrir u.64-66, Huagary

Abstract
A new approach is prese,lrted for the optical ge,neration of millimetre waves with the inte'ntion
to feed the base stations of mobile radio syste,ms. In this approach an optically hansmitted
subharmonic reference signal is used to stabilise a voltage contolled harmonic oscillator
providing the millimefre wave signal. This method offers a simpler and less expe,nsive
solution. It also avoids chrornafic dispersion problems.

Introduction
There are two main approaches to generate millimete waves by opical methods. In the first
approach two lasers are used with off-set frequency stabilisation. Their freque,ncy differe,nce
is kept constant utilising a millimete wave signal as a reference. For the stabilisation one of
the lasers is tuned by a phase locked loop. In the other approach a single laser operating in
two modes is used. The frequency difference between the two laser modes is kept constant by
injection locking techniques .utilising a millimetre wave signal [-4]. The' generated
millimetre wave signal has to be stable and of low noise.
For the optical generation of a stable, low noise signal based on the first approach distributed
feedback (DFB) lasers are used which have a low relative intensity noise @IN) and can be
tuned to accomplish the off:set frequency stabilisation. For the second approach a two mode
laser with a low RIN is needed along with a high mode purity and stability. Beside these
requirements the millimetre wave signal used as a reference in the phase locked loop or for
injection locking has to be stable and of low noise as well. Furthennore at the reception side a
high-speed photo-diode is to be applied. Therefore, tlese methods are rather expe,nsive. l

New optical generation method

The aim of this paper is to present a new method for optical generation.of millimetre wave
signals [5]. It offers a simpler and less expensive solution. The basic principle of this new
method is the following: the millimeie wave signal is generated by a voltage contolled
oscillator (VCO) which is stabilised by a phase locked loop utilising an optically transmitted
subharmonic reference si gnal.
In the new approach a single mode laser is intensity modulated by the subharmonic reference
signal. At the reception side the reference signal is detected by an inexpensive cable TV type
photo diode. The detected signal is used to stabilise the VCO frequency by a phase locked
loop. The block diagram of this system is shown in Fig.1.
Beside the reference signal subcarriers are used'for optical tansmission of information
channels. At the reception side the subcarriers are up-converted utilising the ge,lrerated
millimetre wave as a local oscillator (LO) signal for the up-converter.

37s
The new method has the following advantages:
o a simple laser is applied because there is no need for a tuned laser or double mode
laser,
o there is no need for the off-set frequency stabilisation of two lasers or two modes,
o an inexpensive photo diode is utilised,
o there is no proble,rn with the chromatic dispersion of the fiber because the reference
signal and the subcariers are at low microwave frequencies.
Howwetr, there is a noticeable disadvantage,
namely the more complex receiver. This
problern can be overcome applying the integrated oircuit technology.
The main task is to ensure the low noise property of the millimetre wave signal. Comparing
the well known methods and the new method it is obvious that the electronic system part
producrng the millimetre wave signal provides the same stability and noise performance when
it is applied either in the transmitter or in the receiver. Therefore it is very relevant to use a
quarfu,crystal oscillator with a low noise as the basic reference.
For mobile radio systelns sweral information channels have to be transmitted over the fiber.
That can easily be done by direct bias current modulation of the laser in the new approach.

Measurement results
The systern has been constructed according to the block diagrams of Figs. 2 and 3. In the
receiver a voltage contolled harmonic oscillator (VCHO) serves for generating the millimefie
wave [6, 7]. T[e VCHO has the firndarnental oscillator frequency at 8.44 GHz and its 3'd
harmonic provides the output signal. At the same time the firndame'lrtal frequency of
oscillation is coupled ou! divided by 8 and compared to the reference signal in the phase
detector at 1.055 GHz.
The developed system has been measured in details. Fig.4 presents the phase noise of the
generated millimetre wave signal together with the phase noise of the reference signal as a
firnction of the frequency. Considering the multiplication ntrmber of 24 which means 27 dB
noise increase in principle, there is no noticeable degradation in the noise level.
In Fig.5 the phase jitter dishibution of the millimete wave signal is depicted. The average
phase jitter is 2.25". As the reference signal has a phase jitter of nv0.09o and the multiplication
nnmber of the frequency is24,the noise contribution of the system is negligible.
In Fig.6 the constellation diagram of the 4QAM transmission is presented. Fig. 7 shows the
calculated probability of a give,n angle dwiation at different phase jitter values. For a l6QAM
signal man. 18 degree is allowed, thus this system with its 2.25 degree jitter can reach a BER
less than 104 [S].

Conclusion
A new approach has been presented for the optical generation of millimetre waves with the
intention to feed the base stations of mobil.e radio systems. In this approach an optically
transmitted subharmonic reference signal has been used to stabilise a voltage controlled
harrnonic oscillator providing the millimetre wave signal. This method offers p simpler and
less expensive solution. It also avoids chromatic dispersion problems.

Acknowledgment
The work was financed by the "FRANIS" program of the European Union and Hung artan
National Scientific Research Fund 'OTKA' No. T0 17295, ,F02 4113, T0 14300, T019839 and
T0l 9857 .

376
References
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[5] T.Berceli, "A nev/ approach for optical millimeter wave generation utilizing locking
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l724,Denver, USd June 1997
[6] V.Bir6: "Nonlinear Oscillations in Feedback Systems", ISBN-963-05-3425-9 Akad6miai
Kiad6, Budapest, Hungary, 1985.
[7] A Z6lomy, V. Biro, T. Berceli, G. Jdr6, A. Hilt "Design Of Harmonic Oscillators For
Millimeter Wave Signal Ge,neration In Optical Systems", Conferdnce Proceeding of the 286
EuMC, pp. 75-80, Amsterdam, The Netherlands, October 1998.
[8] T.Marozs6k, T. Berceli, G. J6r6, A.Z6lomy, A. Hilt, S. Mih61y, E. Udvary, Z.Ynga:
'A New Optical Distribution Approach For Millimeter Wave Radio", Technical Digest
MWP'98, pp.63-66, Princeton, New Jersey, USA, October 1998,

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Figure. I The block diagram of the system

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Fig.tre.z The block diagram of the transmitter

377
 Figrrre.3 The block diagfam of the receiver

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Figore 4.Phase noise of the generated MVIW signal

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Figrre 5. Phase distribution of the generated Mlvtw signal

ot Slngle slde angle deviatlon
probabillty 0 10 20
1

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0.01
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Figore 6. 4QAM datatransmission Figur";fr

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378
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