Aerospace and Electronic Systems Magazine October 2017 - 47

Changey et al.

WIFI TRANSMISSION SYSTEM AND ANTENNA
In addition to the transmission of high-level commands by the 2.4GHz ZigBee protocol, a second bidirectional radio link between
the GLMAV and the ground station is implemented. It allows the
setting of the image pickup device and the transmission and retrieving of real-time images to the ground station. The choice of
this second link is a WiFi connection (also in the 2.4-GHz band)
for several reasons:
C

existence of the WiFi module onboard the Gumstix system;

easy deployment using a laptop at the ground station;

C
C

quick replacement of defective parts due to the use of a standardized connection and low-cost materials available on the
shelf;
reduced size of the antenna (a half wavelength λ/2 = 60 mm
at 2.4 GHz); and
relatively high rate that is necessary for the image transmission.
choice has also some drawbacks:
limited range due to the allowed power limited to 20 dBm in
the 2.4-GHz industrial, scientific, and medical (ISM) band;

2.4-GHz ISM band "saturated" in urban zones;

to increase the range by adding a power amplifier; and

speed rate dependent on the received signal level (decreasing with distance) and on other operating systems transmitting in the same band.

The performances offered by the WiFi system are attractive
and sufficient to demonstrate the technological brick. The increase
of the communication distance can be resolved by the development of a dedicated transmitter or receiver operating in another
frequency band for which a higher transmission power is allowed.
It is the same for the availability of channels in the selected frequency band.

Tests were carried out on the ISL proving ground to check the
transmission range. On the hillock (estimated height of 12 m), the
Gumstix of the GLMAV was connected to interchangeable antennas (including those sized for GLMAV). A very simple receiving
station consisting of a directional antenna and a laptop with a WiFi
card was installed at a distance of 500 m. The images taken by
the GLMAV camera were transmitted in real time to the receiving
station by the WiFi link. Compressed JPEG images having a resolution 640 pixels × 480 pixels (video graphics array [VGA] quality), with a compression ratio of 14:1, were received with a rate of
10 frames per second. The latency time was estimated to 100 ms;
43 ms for the acquisition, 53 ms for the compression and the rest
for the transmission. These tests conducted in an undisturbed environment demonstrate the feasibility of transmitting images at the
distance of 500 m by WiFi connection. The operational use of the
drone requires certainly a more secure means of communication in
a less saturated band than the 2.4-GHz ISM one.
A ZigBee connection must be established simultaneously with
the WiFi connection to transmit the high-level orders of the vehicle
control. Therefore, close to the WiFi module, an Xbee module was
connected to a dipole antenna dimensioned for the GLMAV. These
ZigBee and WiFi standards communicate in the 2.4-GHz ISM band.
It is, therefore, imperative to ensure that there is no interaction between the two systems and that the independent and simultaneous operation of these two links is effective for everyone. The channels used
by both WiFi and ZigBee were selected in the lower part of the band
for WiFi and in the upper part for ZigBee (Figure 14). The immunity,
with respect to each other and their mutual good operation, were validated by other tests on the hillock on the ISL proving ground.
The antennas used for the GLMAV need to have an omnidirectional radiation in the horizontal plane to ensure communication,
regardless of the orientation of the drone. They must also have a
relatively open lobe to have sufficient antenna gain when the drone
flies between ground level and 100-m altitude, no matter what
inclination has the GLMAV has. These criteria and the necessary
bandwidths (2.4 GHz-2.48 GHz) led to the choice of wire antennas
(dipole type) integrated into the top of the GLMAV. Placing them
in an envelope involved transparency to electromagnetic radiation.
Their placement in the lower part was compromised by the pres-

Figure 14.

ZigBee and Wifi coexistence

OCTOBER 2017

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine October 2017