Full-duplex wireless-over-fibre transmission incorporating a CWDM ring architecture with remote millimetre-wave LO delivery using a bi-directional SOA

Abstract

We demonstrate the first full-duplex wireless-over-fibre transmission between a central station and a CWDM ring architecture with remote 40 GHz LO delivery using a bi-directional semiconductor optical amplifier.

Full text

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Full-duplex wireless-over-fibre transmission incorporating a
CWDM ring architecture with remote millimetre-wave LO
delivery using a bi-directional SOA

T. Ismail, C. P. Liu, J. E. Mitchell and A. J. Seeds
Department of Electronic and Electrical Engineering
University College London, Torrington Place, London, WC1E 7JE, United Kingdom
E-mail: a.seeds@ee.ucl.ac.uk

X. Qian, A. Wonfor, R. V. Penty and I. H. White
Centre for Photonic Systems, Cambridge University Engineering Department,
William Gates Building, 15 JJ Thomson Avenue, Cambridge, CB3 0FD, United Kingdom

Abstract: We demonstrate the first full-duplex wireless-over-fibre transmission between a central
station and a CWDM ring architecture with remote 40 GHz LO delivery using a bi-directional
semiconductor optical amplifier.
2005 Optical Society of America
OCIS codes: (060.2330) Fibre optics communications: (060.2360) Fibre optics links and subsystems

1. Introduction
Wireless-over-fibre is an attractive technology for future broadband wireless access systems operating in the
millimetre-wave frequency range. In previously reported wireless-over-fibre systems separate uni-directional erbium
doped fibre amplifiers (EDFAs) were used to provide the necessary gain for the downlink and uplink [1,2]. In this
paper, we report the first demonstration of the use of bi-directional semiconductor optical amplifiers (SOAs) to
provide gain for both downlink and uplink optical signals simultaneously, together with the first experimental results
for a coarse wavelength division multiplex (CWDM) ring architecture which allows distribution of a centrally
generated 40 GHz local oscillator (LO) signal and the connection of multiple base stations in a wireless-over-fibre
network.
The use of bi-directional SOAs in place of uni-directional EDFAs, together with the use of CWDM and optical
distribution of the millimetre-wave local oscillator signal allows substantial reduction in overall systems complexity
and cost, as will be necessary for the widespread adoption of millimetre-wave wireless access systems. It also
increases utilisation of the deployed fibre, an economic advantage where leased fibre has to be used.

2. System Overview
The system comprises a 12.8 km standard single mode fibre (SMF) backbone link with a 2.2 km fibre ring for
distribution to the wireless base stations. 3.5 MSymb/s wireless data signals to and from the base stations are
distributed at intermediate frequencies (IFs) in the 2.40 GHz to 2.50 GHz band to overcome fibre dispersion using
directly modulated uncooled distributed feedback (DFB) lasers. The laser emission wavelengths are selected on a 20
nm pitch CWDM grid to route signals to/from the required base station. The LO source at 40 GHz is generated
optically at the central station and is delivered to multiple remote base stations. The delivery of the LO signal is also
tolerant to fibre dispersion since it is generated in single sideband form. The common LO signal is used at each base
station to up-convert down-link IF signals and down-convert received up-link signals.

3. Optically Generated 40 GHz Local Oscillator Source
The 40 GHz LO source is optically generated at the central station by injection locking two slave DFB lasers to two
sidebands of an externally modulated master tunable laser. Fig. 1 illustrates the experimental arrangement of this 40
GHz LO source.
In Fig. 1 the Mach-Zehnder (MZ) modulator externally modulates the output of the CW tunable master laser
source whose centre wavelength is set at 1543.6 nm (λ0) with 8.1 dBm output power. Because the MZ modulator is
biased at 7.3V (Vπ), the unmodulated optical transmission is minimum. When the MZ is driven by a 20 GHz
microwave signal generator, its optical output then contains mainly two modulation sidebands located at ± 20 GHz
offset from the suppressed 1543.6 nm line. The two main sidebands are therefore 40 GHz apart and effectively the
MZ is functioning as a frequency doubler. Slave DFB 1 is injection locked by one sideband and Slave DFB 2 by the
other. The injection locked outputs of the two slave lasers are then combined in a 50%/50% coupler forming a 40
GHz heterodyne beat signal. Previously Braun et al. reported generation of a 64 GHz signal by optical injection

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locking two slave DFB lasers to the –10th and +10th modulation sidebands of a master DFB laser which was
directly and heavily modulated at 3.2 GHz with +23.4 dBm input [3].
The advantages of heterodyning the two injection locked slave DFB lasers instead of simply taking the
frequency doubled output directly from the MZ modulator are the 15 dB increase in the optical output power level
and the further suppression of the tunable laser source centre wavelength at 1543.6 nm. Since the two modulation
sidebands are each offset by 20 GHz from 1543.6 nm, insufficient suppression of this centre wavelength would
result in the generation of an unwanted 20 GHz signal in the detected RF spectrum of the LO source output. Fig. 2
shows the detected RF spectrum of this 40 GHz heterodyne LO source in a full 50 GHz span.

4. Wireless-over-fibre transmission experiment and results
Fig. 3 shows the complete experimental arrangement of the wireless-over-fibre transmission system. Full-duplex
transmission occurs between the Central Station and Base Station 1. Base Station 1 is connected via optical add-drop
multiplexer (OADM) 1 to a 2.2 km single-mode fibre (SMF) ring. The fibre ring in turn is connected to the Central
Station via a bi-directional SOA and a 12.8 km back-bone SMF. The OADMs used are constructed using low cost
CWDM filters and are employed for routing different wavelengths to their intended remote base stations. Measured
optical and detected electrical spectra at various points in the transmission experimental arrangement are also shown
in Fig. 3.
Slave DFB 1
Slave DFB 2
MZ
Modulator
20GHz Microwave
Signal Generator
Tunable Laser
Polarisation
Controller
Polarisation
Controller
Optical
Circulars
50/50
Output
Couplers
50/50
Input
Couplers
Optical
Output 2
Optical
Output 1
Polarisation
Controller
λ0: 1543.65 nm

Fig. 1. An optically generated 40 GHz LO source using heterodyning and
injection locking
-120
-100
-80
-60
-40
-20
0.0010.020.0
Frequency (GHz)
30.040.050.0
Detected Electrical Power (dBm)

Fig. 2. Detected RF spectrum of the LO source output
Bi-directional
SOA
MUX
12.8 km
SMF
2.40 GHz
3.5 MSymb/s
QPSK 223-1 PRBS
(Channel 1)
Vector Signal
Generator
Optically generated
40 GHz LO source
Vector Signal
Analyser
1 nm optical
bandpass filter
tuned to λ2
Optical
circulator
Optical
circulator
λ2
2.2 km
SMF
40 GHz
Vector Signal
Analyser
2.41 GHz
3.5 MSymb/s
QPSK 223-1 PRBS
(Channel 2)
Vector Signal
Generator
Up-converting
mixer
Down-converting
mixer
Amplifiers
40 GHz
± 2.40 GHz
40 GHz
LO
Ch. 1
λ2
Ch. 1
OADM 1
λ1
λ0
λ0
50/50
splitter
λ0
λ0
λ1
Base Station 1
Base Station 2OADM 2
Fibres
Fibre
Ring
Central Station
λ0, λ2
λ2
Laser 1
Laser 2
λ1
λ2
-70
1493.5 1513.5 1533.5 1553.5 1573.5 1593.5
Wavelength (nm)
10
λ1
-60
-50
-40
-30
-20
-10
0
Optical Power (dBm)
Resolution Bandwidth = 0.06 nm
λ1
-70
1522.6
-60
-50
-40
-30
-20
-10
0
1527.61532.61537.61542.6
Optical Power (dBm)
Wavelength (nm)
Resolution Bandwidth = 0.06 nm
λ2
-120
-100
-80
-60
-40
-20
2.3802.3902.400
Frequency (GHz)
2.4102.4202.430
Detected Electrical Power (dBm)
Resolution Bandwidth = 300 kHz
Channel 2
Channel 1
-60
1480.0 1500.0 1520.0 1540.0 1560.0 1580.0
Wavelength (nm)
-50
-40
-30
-20
-10
0
Optical Power (dBm)
Resolution Bandwidth = 0.06 nm
λ2
λ0
-120
-100
-80
-60
-40
-20
2.37502.38752.40002.41252.4250
Detected Electrical Power (dBm)
Frequency (GHz)
Resolution Bandwidth = 300 kHz
Channel 1
-120
-100
-80
-60
-40
-20
37.57537.58837.60037.61237.625
Detected Electrical Power (dBm)
Frequency (GHz)
Resolution Bandwidth = 300 kHz
Channel 1
-70
1493.5 1513.5 1533.5 1553.5 1573.5 1593.5
Wavelength (nm)
-60
-50
-40
-30
-20
-10
0
Optical Power (dBm)
Resolution Bandwidth = 0.06 nm
λ0


Fig. 3. Full-duplex wireless-over-fibre transmission experimental arrangement with electrical and optical spectra at various points

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modulated with 3.5 MSymb/s QPSK data at 2.40 GHz carrier frequency (Channel 1) provided by a Vector Signal
Generator (VSG). λ1 is multiplexed with the output of the optically generated 40 GHz LO source whose two
sidebands are centred around wavelength λ0. Both λ0 and λ1 are then transported to the fibre ring via the 12.8 km
SMF and the bi-directional SOA. At OADM 1, the λ1 signal, carrying the QPSK data, is dropped and delivered to
Base Station 1. Around 50 % of the λ0 power carrying the LO is also dropped and delivered to Base Station 1 by
OADM 1 whilst the remaining 50 % is allowed to proceed to OADM 2 and to be used for further frequency
conversions elsewhere. After photo-detection, the QPSK data is up-converted to 40 GHz ± 2.4 GHz using the
remotely delivered 40 GHz LO source. In order to demodulate the downlink signal and to assess the transmission
performance, the up-converted signal is then down-converted back to 2.4 GHz using an independent 40 GHz LO
before the QPSK data is detected by a Vector Signal Analyzer (VSA).
In the uplink direction, Laser 2 of 1532.7 nm wavelength (λ2) at Base Station 1 is directly modulated with 3.5
MSymb/s QPSK data at 2.41 GHz (Channel 2) provided by another VSG. λ2 now carrying the uplink QPSK signal
is first sent round the fibre ring before going through the SOA and the 12.8 km SMF, and finally arriving at the
Central Station. A 1 nm optical bandpass filter is used to select λ2 which is then photo-detected and subsequently
demodulated by the VSA. As can be seen in Figure 3, the detected electrical spectrum of λ2 contains both Channel 1
and Channel 2 signals even though λ1 is not present after the optical bandpass filtering. The transfer of the Channel
1 signal onto λ2 can be attributed to the cross-gain modulation characteristic of the SOA where both λ1 and λ2 are
simultaneously present. Since the downlink and uplink signals use different subcarrier frequencies (2.40 GHz and
2.41 GHz, respectively), Channel 2 can be easily selected for demodulation by simple electrical filtering.
Fig. 4(a) shows the constellation, electrical spectrum, eye-diagram and signal statistics for the downlink
Channel 1 QPSK signal measured and demodulated by the VSA while Fig. 4(b) shows the corresponding results for
the uplink Channel 2 signal. Successful full-duplex transmission has been achieved as evident from the clear and
well-defined constellations and eye-diagrams as well as low 10.5% and 7.8 % error vector magnitude (EVM) values
for the downlink and uplink directions, respectively.
In the down-link direction, Laser 1, operating at 1512.3 nm wavelength (λ1), at the Central Station is directly
5. Conclusions
A full-duplex 3.5 MSymb/s QPSK wireless data over 12.8 km fibre transmission with a bi-directional SOA
incorporating a 2.2 km CWDM fibre ring architecture has been experimentally demonstrated for the first time.
Successful transmission has been achieved as evident in the clear and well-defined constellations and eye-diagrams
as well as low 10.5% and 7.8 % EVM values for the downlink and uplink directions, respectively. The cost savings
obtained from the use of CWDM and bi-directional SOA technology, together with the improved fibre utilisation
resulting will make such solutions more attractive for future millimetre-wave access systems.

6. References
[1] C. Lim, A. Nirmalathas, D. Novak, R. Waterhouse, G. Yoffe, “Millimetre-wave broad-band fiber-wireless system incorporating baseband
data transmission over fiber and remote LO delivery,” IEEE J. Lightwave Technol, 18, 1355-1363 (2000).
[2] G. H. Smith, D. Novak, “Broadband millimetre-wave fiber-radio network incorporating remote up/down conversion,” IEEE Mtt-s Int Symp
Dig, 3, 1509-1512 (1998).
[3] R.-P. Braun, G. Grosskopf, D. Rohde and F. Schmidt, “Low-Phase-Noise Millimeter-Wave Generation at 64 GHz and Data Transmission
Using Optical Sideband Injection Locking,” IEEE Photon Technol Lett, 10, 728-730, (1998).




Fig. 4. Demodulated 3.5 MSymb/s QPSK data for the downlink and the uplink. The four quadrants show the constellation
diagram, electrical spectrum, eye diagram, and signal statistics.





(a) Downlink (b) Uplink