consumption, and limited working area. We have
designed an anchor node with an antenna array that
uses multi-radio modules and directional antennas to
collect signal strengths when receiving wireless data
from the target node. The angle of the node can be cal-
culated by comparing signal strengths and combining
the distance reckoned by the signal strength. With this
design, the node position can be determined by a signal
anchor node. This method does not require data
exchange or extra ranging process and can meet the
requirements of real-time positioning. Besides static
WSNs, the proposed solution is also suitable for more
complex networks including mobile WSNs,
34,35
wireless
sensor, actuator, and robot networks (WSARNs),
36
or
heterogeneous networked cooperating objects.
37–39
However, the directional antenna transmission
characteristic is complex, which is different from omni-
directional antennas like 3-dBi loaded antennas.
Furthermore, antennas interact with each other and
change antenna array radiation characteristics, so free
space radio channel propagation model does not apply.
In this article, we studied directional antenna trans-
mission characteristics measured and analyzed antenna
array RSSI data, following a single anchor node posi-
tioning method for WSNs. Experimental evaluation of
the algorithm and comparison of the algorithm with the
currently used algorithms indicate a potential improve-
ment in real-time positioning methods and suggest a
new design for positioning systems.
Anchor node design based on antenna
array
Basic rule of antenna array design
For successful antenna design, several principals were
considered. First, directional antennas should be
applied to measure angle or direction, and these
antennas should have wide beamwidths and regular
radiation patterns. Meanwhile, antenna array consists
of same antennas, and all antennas are mounted on the
base board with a strict equal intersection angle.
Directional antenna
Although there are some manufactured directional
antennas available, they could not meet our design
requirements due to big size, irregular radiation pat-
tern, poor consistency, or difficulty in installation. It
was therefore necessary to design and fabricate a novel
antenna. According to our investigation, the printed
circuit board (PCB) resonant antenna in Su’s
40
work
has good performance, which is a dual polarization
antenna with a 147° beamwidth and a front-to-back
ratio of 20 dB. Most importantly, this antenna struc-
ture is simple, and it is printed on a low-cost FR4 sub-
strate. The antenna characteristic is tested and is
consistent with those in the previous report, so we
chose this antenna as our array antenna. A constructed
prototype is shown in Figure 1. Figure 1(a) shows a sin-
gle antenna with a separation between the antenna and
reflection board of 10 mm. The antenna size (including
backboard) is 144 mm 3 55 mm. Figure 1(b) is the
radiation pattern of the antenna at the distance of
1.2 m. Figure 1(c) is the antenna array prototype,
which consists of four directional antennas, and all
antennas are mounted on the base board with an inter-
section angle of 90°. The antenna array size is
300 mm 3 300 mm 3 144 mm.
Although single antenna radiation pattern is regular
as shown in Figure 1(b), the infinite ground plane con-
dition in single antenna board changes when the
antenna is mounted on the base board; however, anten-
nas affect each other in multi-antenna arrays. As a
result, the antenna radiation patterns would change in
Figure 1. Directional antenna array: (a) prototype of the directional antenna, (b) polar diagram of the directional antenna, and
(c) prototype of the directional antenna array.
Fu 3