Performance evaluation of optical beamforming-based
wideband antenna array
(Invited Paper)
Xingwei Ye (叶星炜), Bowen Zhang (张博文), Yamei Zhang (张亚梅),
Dan Zhu (朱 丹), and Shilong Pan (潘时龙)*
Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University
of Aeronautics and Astronautics, Nanjing 210016, China
*Corresponding author: pans@ieee.org
Received September 19, 2016; accepted December 2, 2016; posted online December 21, 2016
A wideband-generalized pattern multiplication approach to evaluate the performance of an optical beamform-
ing-based wideband antenna array is proposed and experimentally demonstrated, which enables the far-field
measurement of a large wideband array with a small anechoic chamber. Because the optimum reception of
a wideband microwave signal is highly related to the time-domain distortions of the beamforming system, a
correlation-receiver-based radiation pattern is applied to take the fidelity of the wideband signals into account.
A four-element optical beamforming system is built to verify the feasibility of the proposed method. The results
achieved by the proposed method agree well with the conventional direct measurements.
OCIS codes: 280.5110, 060.5625.
doi: 10.3788/COL201715.010013.
Optical beamforming, which is one of the most promising
solutions to the beam-squint problem in wideband array
antenna systems, such as high-resolution radars, electronic
warfare systems, and the upcoming 5G mobile communica-
tion systems, has been a topic of interest in the last few
decades
[1–7]
. Thanks to the intrinsic advantage of large in-
stantaneous bandwidth brought by optical technologies,
broadband and large-range true time delay (TTD) can
be easily implemented in the optical domain
[8–12]
. Thus,
complex waveforms with larger instantaneous bandwidth
can be applied, which enables distinctive features such as
better resolution for ranging
[13]
, larger suppression of gra-
ting lobes
[14,15]
, and higher speed in wireless communica-
tions
[16,17]
. For the optical beamforming-based wideband
antenna array, it is of great importance to know what
the output signal at an arbitrary radiation direction is for
a given wideband input signal. However, the conventional
way to characterize the antenna array is implemented by
measuring the radiation patterns at a specific frequency or
several discrete frequencies in an anechoic chamber
[18,19]
,
which cannot obtain the complete information of the an-
tenna system at a wide bandwidth. To solve this problem,
an integrated antenna pattern (IAP) is defined
[20]
, which is
the integration of the transmitted power at a radiation di-
rection over the whole signal duration. The IAP maps the
two-dimensional angle-time function to a one-dimensional
angle function, which can be used to calculate the radio fre-
quency (RF) energy at certain radiation directions for a
given feeding waveform. However, information about the
time-domain distortion of the wideband signal in the
antenna and the optical beamforming network (OBFN)
cannot be obtained. Because the optimum reception of
a wideband microwave signal is highly related to the
time-domain distortions
[21]
, the wideband antenna system
cannot be characterized by simply measuring the IAP.
Another challenge in evaluating the optical beamform-
ing-based wideband antenna array is that the size of the
required anechoic chamber is nearly proportional to the
square of the scale of the antenna array. According to
the antenna theory, for an antenna with a maximum over-
all dimension of D and a feeding wavelength of λ, the inner
boundary of the far-field region is 2D
2
∕λ
[22]
.IfN antennas
are arranged in a one-dimensional array, and the spacing
between two adjacent antennas is d, the inner boundary
should be larger than 2½ðN − 1Þd
2
∕λ, indicating that a
big anechoic chamber should be used, which would dra-
matically increase the cost in the optical beamforming re-
search. Although the near-field or compact-range antenna
measurement could be resorted, which requires only a small
chamber, some special instruments, such as the planar
scanner for near-field probing and the reflector for com-
pact-range implemen tation, could also bring about an un-
affordable cost. Thi s might be a reason for the fact that
radiation patterns in most works on optical beamforming
were obtained through numerical simulation
[23,24]
rather
than actual field measurement. Actually, the classical an-
tenna theory has provided a pattern multiplication ap-
proach to enable field measurement of a large array with
a small anechoic chamber
[22]
. In the pattern multiplication
approach, the total far-field is equal to the field of a single
element multiplied by the array factor, which is defined as
the summation of the complex feeding amplitudes for all
antenna elements. But, this approach can only be applied
to achieving the radiation pattern at a single frequency.
In this Letter, a correlation-receiver-based radiation
pattern measured by a generalized pattern multiplication
COL 15(1), 010013(2017) CHINESE OPTICS LETTERS January 10, 2017
1671-7694/2016/010013(5) 010013-1 © 2016 Chinese Optics Letters