Wavelength-tunable true time delay for multi-beam radio
beamformer in multi-Gbps satellite communication
Tessema, N.M.; Cao, Z.; van Zantvoort, J.H.C.; Tangdiongga, E.; Smolders, A.B.; Koonen,
A.M.J.
Published in:
ECOC 2016, 42nd European Conference on Optical Communication, 18-22 September 2016, Dusseldorf,
Germany
Published: 01/09/2016
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Citation for published version (APA):
Tessema, N. M., Cao, Z., van Zantvoort, J. H. C., Tangdiongga, E., Smolders, A. B., & Koonen, A. M. J. (2016).
Wavelength-tunable true time delay for multi-beam radio beamformer in multi-Gbps satellite communication. In
ECOC 2016, 42nd European Conference on Optical Communication, 18-22 September 2016, Dusseldorf,
Germany (pp. 1-3). Piscataway: IEEE.
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ECOC 2016
42nd European Conference and Exhibition on Optical Communications  September 18 – 22, 2016  Düsseldorf
Wavelength-tunable True Time Delay for Multi-beam Radio
Beamformer in Multi-Gbps Satellite Communication
N. Tessema, Z. Cao, J.H.C. van Zantvoort, E. Tangdiongga, A.B. Smolders, A.M.J. Koonen
COBRA Research Institute, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven,
The Netherlands, Email: [email protected]
Abstract We present a Si 3 N 4 photonic integrated chip providing wavelength-dependent true time
delay for forming multiple radio beams: a number of multi-Gbps connections from home-to-multiple
satellites can be supported simultaneously.
Introduction
to N satellites. The beamforming in the FPA
enables a higher gain than a traditional reflector
antenna. Within a home network, an optical fibre
link can be used to provide seamless wideband
connectivity to the devices per room.
Radio beamforming is an ultimate solution to
satisfy the increased bandwidth demand of
wireless communication users. Electronic
beamformers using traditional phase shifters
have intrinsic narrowband characteristics. The
realization of photonic integrated true time delay
(TTD) devices is proven to be the ultimate
solution for a wideband beamformer1-3.
Several realizations of an optically-controlled
multi-beam RF beamformers have been
reported using wavelength-dependent TTD
(WD-TTD) based on discrete components.
Integrated Si 3 N 4 optical ring resonator (ORR)
test structures with WD-TTD bandwidth of 1
GHz were realized and characterized for multibeamforming application as given in3. The
design implemented a Vernier approach which
involves cascading two or more ORRs with
different free spectral range (FSRs). This
configuration provides WD-TTD at the expense
of increased device complexity.
With the ever increasing demand for high
capacity links, home-satellite communication is
a key enabler technology for users located in
remote
areas.
Home-to-multiple-satellite
connections are highly beneficial since they
enable users to simultaneously tune into a wide
range of services such as TV, internet, mobile
connections provided by different satellites.
Currently, motorized dish alignment techniques
are used to switch connections between
satellites supporting a single satellite connection
at a time. In order to setup multiple satellite
connections simultaneously, the home user
terminals need to deploy multi-beam radio
beamforming. Fig. 1(a) shows such a homesatellite system with an optically-controlled radio
beamformer. An antenna array mounted in the
focal plane of a dish reflector, known as a focal
plane array (FPA) is controlled by a TTD. We
use N optical wavelengths in the TTD to create
different delays for feeding the antenna arrays.
In this way, N radio beams simultaneously point
In this paper, we present a novel approach in
which only a single ORR is used to generate the
WD-TTD with a bandwidth of 2 GHz. The
measured delays can be used in the realization
of multi-beam RF beamforming. Since only a
single ORR per antenna element is used, the
device complexity is reduced significantly. The
2-3
works given in did not show data transmission
through TTD chip. In this paper, we present
successful transmission of Nyquist-subcarrier
multiplexed data through the TTD chip. The nonorthogonality
in
the
Nyquist-subcarrier
multiplexed signals (as opposed to an OFDM
(b)
(a)
Remote antenna site
Central site
Ȝ1
1
FPA
M
EM
EM
fibre
link
ȜN
PIC : Wavelengthdependent TTD
fibre
antenna
HCC
LO2
~
LON
LO1
De-mux
...
fibre 1
LD
~
~
M :1
PD combiner
fibre M
fibre 2
fibre
Ȝ2
...
EM
dish
2
MUX
FPA
De-mux
Ȉ
Beam-N: șN
Beam-2: ș2
Beam-1: ș1
De-mux
Fig. 1: (a) A home-to-multiple-satellite system (HCC: home communications controller) (b) optically-controlled multi-beam
radio beamformer
ISBN 978-3-8007-4274-5
307
© VDE VERLAG GMBH  Berlin  Offenbach
ECOC 2016
42nd European Conference and Exhibition on Optical Communications  September 18 – 22, 2016  Düsseldorf
signals) is suitable for combating carrier
synchronization offset in the satellite channels4.
In addition, a reduced peak-to-average-power
ratio can be achieved since the number of subcarriers is not very large.
(a)
TLS
PC
power (dBm)
LO @ 20
GHz
PC- offline
processing
×
MZM
PC
RF out
Vector Network
Analyzer
RF in
ORR
delay(ps)
Ȝ1
Ȝ2
400
400
ȜN
200
200
0
1552.7
1552.8
1552.9
wavelength(nm)
N=0.22
N=0.33
N=0.5
N=0.65
N=0.75
1553
Fig. 2: (a) Schematic of the TTD unit (b) Theoretical
group delay of an ORR: wavelength delay tuning
variation in a given wavelength range. The
higher the slope, the narrower the TTD
bandwidth. On the other hand, the higher the
slope, the larger delay tuning range per unit
wavelength. Therefore, the TTD bandwidth and
the resulting delay variation per unit wavelength
can be thermo-optically optimized by varying ࣀ.
Experimental Setup
The experimental setup is given in Fig. 3 (a).
The first part of the experiment involves
measurement of the delay. A vector network
analyser (VNA) is used to sweep an RF signal
from 18 to 20 GHz, while the laser wavelength is
incrementally tuned. The second part of the
experiment involves the transmission of data
signal through the optical chip at 20 GHz. Eight
non-orthogonal subcarriers were modulated by a
QAM-16 signal which is band-limited by a
Nyquist filter. The roll-off factor Į of the Nyquist
filter was optimized per subcarrier. The guard
band between the subcarriers is 30 MHz. The
signal format is given in Fig. 3(b) and has a data
rate of 5.2 Gbps. A fibre-chip-fibre insertion loss
of 11 dB was measured.
Results and discussions
The phase response measurement for TTD
bandwidth of 2 GHz is shown in Fig. 4(a). During
the measurement, heater E voltage was tuned
Nyquist-subcarrier multiplexed signal
(b)
LO @
20 GHz
×
PD EA Tx Ant Rx Ant
T
D
600
0
-20
OA
C
wavelength delay
tuning
uni
600
Digital
Oscilloscope
100 GS/s
-40
-60
-80
1552.6 1552.8 1553
wavelength (nm)
waveguide
heater
OSBF
(b)
Wavelength dependent TTD
The realized TTD is a cascade of an optical side
band filter (OSBF) and an ORR as shown in Fig.
2(a). The OSBF is designed for single side band
filtering of radio frequency (RF) signals of 20
GHz and higher. It is provided with heaters A, B,
and C which allow thermo-optic tuning of the
OSBF response. Heater D of the ORR enables
tuning of the resonance wavelength and heater
E enables tuning the coupling ratio ࣀ. Fig. 2(b)
shows the theoretical plot of the group delay of
the realized ORR under different coupling
conditions. The wavelength delay tuning is done
by placing the wavelengths [Ȝ 1 , Ȝ 2 … Ȝ N ] in the
ORR delay response as shown in Fig. 2(b). For
lower coupling ratios, it is seen that the
wavelength vs delay slope is significantly large,
and a large delay variation happens in a narrow
wavelength range. For higher coupling ratios of
0.5 and higher, the wavelength vs delay slope is
significantly lower, and shows smaller delay
Wave form
Generator
24 GS/s
B
E
Operation Principle of an Opticallycontrolled multi-beam RF beamformer
The diagram in Fig. 1(b) illustrates a multi-beam
radio beamformer which is controlled by a TTD
implemented in a photonic integrated circuit
(PIC). The remote antenna site and the central
site are connected by a short fibre-link of less
than 1000m. In the WD-TTD PIC, N
wavelengths originating from N lasers are
delayed
differently.
N×M
delays
are
simultaneously generated by using M ORRs,
where M is the number of antenna elements of
the FPA. In this way, multiple radio beams can
be steered simultaneously in the desired
directions [ș 1 , ș 2 , …,ș N ]. We realized and
characterized a WD-TTD PIC intended to
operate in such a home-satellite system.
(a)
A
Subcarrier Frequency
(GHz)
Baud rate
(MHz/s)
Roll-off
(Į )
1
2
0.12
0.3
120
170
0.1
0.2
3
4
0.5
0.7
170
170
0.2
0.2
5
0.9
170
0.2
6
7
1.1
1.3
170
170
0.2
0.2
1.5
170
0.2
Antenna system
replaced with electrical
8
waveguide
PC :Offline processing
Fig. 3: (a) Experimental setup (b) Transmitted Nyquist-subcarrier multiplexed signal
ISBN 978-3-8007-4274-5
308
© VDE VERLAG GMBH  Berlin  Offenbach
ECOC 2016
42nd European Conference and Exhibition on Optical Communications  September 18 – 22, 2016  Düsseldorf
(b)
(a)
Ȝ
1
2
3
4
5
6
7
Ȝ(nm)
1552.63
1552.64
1552.65
1552.66
1552.67
1552.68
1552.69
-120
(iii)
12
6
18.5
19
19.5
frequency (GHz)
7
20
Steering angle (degrees)
150
100
14
50
15
0
power in dBm
160
-50
120
1552.62 1552.64 1552.66 1552.68
Wavelength (nm)
1552.7
root mean square error (degrees)
(iv)
2.5
2
14
1.5
15
10
5
0
2
4
6
8
sub carrier number
subcarrier 1
subcarrier 8
(iv)
3
13
1
0.5
frequency in GHz
20
(ii)
140
1.5
15
1552.64 1552.66 1552.68
Wavelength (nm)
-40
-60
0
180
12
13
-30
1
(iii)
0.5
Quadrature
-100
200
12 V
13 V
14 V
15 V
rms evm (%)
5
group delay (ps)
-80
220
Voltage E (V)
phase(degrees)
-60
-140
18
Voltage E (V)
1,2 (ii)
240
3
4
-20
Quadrature
12 V
(i)
(i)
0
-0.5
-0.5 0 0.5
In-Phase
1552.64 1552.66 1552.68
Wavelength (nm)
0.5
0
-0.5
-0.5 0 0.5
In-Phase
Fig. 4: (a) Phase measurement results (b) data transmission results
from 12 to 15 V, while the wavelength was tuned
from 1552.63 nm to 1552.69 nm. The phase
response at 12 V is shown in Fig. 4(a, i), in
which the solid lines represent the measured
phase responses and black dotted lines
represent their best linear fits. The phase slope
of the linear fits were used to obtain the group
delays values. It can be seen from Fig. 4(a, ii)
that by tuning the heater E voltage, thus tuning
coupling
ratio
ࣀ,
results
in
different
delay variation per unit wavelength. With 7
wavelengths and 4 thermo-optic voltage settings
a total of 28 delays were measured which yield
28 steering angles as shown in Fig. 4(a, iii). The
steering angles were calculated for a 2×1
beamformer at 20 GHz. It can be seen that
steering angles from 0° to 158° can be realized.
The measured delays can be employed to steer,
N beams at the same time for a 1-by-M antenna
array; (while N, M satisfy, N×M=28), and when
M ORRs are used. The root mean square error
(rmse) quantifies the deviation of the measured
phase responses from the corresponding best
linear fits and is given in Fig. 4(a, iv). The rmse
values range from 0.8° - 4.6°. The smallest rmse
corresponds to the smallest wavelength vs delay
slope and the largest rmse corresponds to the
largest wavelength vs delay slope. The impact
of these phase errors on the beamformer
performance is expected to be minimal. Fig. 4(b,
i) shows the received Nyquist-subcarrier
multiplexed signal. High-quality signal reception
was achieved with a receive SNR of 20 dB per
subcarrier. The 6 dB roll-off across signal
bandwidth is the result of the impact of various
RF components in the experimental setup. The
ISBN 978-3-8007-4274-5
rms error vector magnitude (evm) per subcarrier
is given in Fig. 4(b, ii). The smallest evm of 8.4
% was measured at subcarrier 1 and the largest
evm of 12.3% was measured at subcarrier 8.
Clear constellation diagrams were obtained in
both cases as shown in Fig. 4(b, iii) and Fig. 4(b,
iv).
Conclusions
We have presented on-chip WD-TTD based on
a single ORR that can potentially be employed
in a multi-beam RF beamformer. Multiple beams
can be used to provide broadband multi-satellite
connections to users. A Nyquist-subcarrier
multiplexed signal format suited for satellite
communication has been employed to achieve
5.2 Gbps transmission capacity.
Acknowledgements
We wish to thank the Dutch STW and the ERC FP7
for the financial support in the context of FREEBEAM
and BROWSE projects, respectively.
References
[1] J. Campany et al., “Microwave Photonics combines two
worlds,” Nat. Photonics, Vol. 12, no. 5, p. 319 (2007).
[2] M. Burla et al., “Multiwavelength-Integrated Optical
Beamformer based on Wavelength Division Multiplexing
for 2D Phased Array Antennas,” J. Light wave Technol.,
Vol. 32, no.20, p. 3509 (2014).
[3] L. Zhuang et al., “Novel low-loss waveguide delay lines
using Vernier ring resonators for on-chip multi-Ȝ
microwave
photonic
signal
processors,”
Laser
&
Photon.Rev., Vol. 7, no.10, p. 994(2013).
[4] J. Dommel et al., "5G in space: PHY-layer design for
satellite communications using non-orthogonal multicarrier transmission," ASMS/SPSC, Livorno (2014).
309
© VDE VERLAG GMBH  Berlin  Offenbach