A.M.Grekhov, D.V.Bezsmertna
National Aviation University, Kyiv, Ukraine
INVESTIGATION OF MODULATION SCHEME AND TRANSMITTER
NONLINEARITY IMPACT ON ADS-B MESSAGES TRANSMISSION VIA
SATELLITE LINK
Air traffic is predicted to increase steadily over the coming years, which influence
on the increase of flights and flights through the Poles and regions with reduced radar
coverage the current ATM network will not be able to handle all the traffic with the
requirements associated to ATC services.
There is a need to ensure that standards of safety and efficiency should be
maintained, or even enhanced. Thus, there arises a necessity of conversion from the
expensive ground-based systems to the cheap satellite-based systems that provide global
coverage. Satellite systems can improve the overall communication system’s availability
and is able to cover large regions of the earth, thus, provide a cost effective solution to
the coverage of both high and low density areas such as oceanic regions where reliable
terrestrial coverage is nonexistent.
As a solution of increasing performance requirements, there are used the latest
Iridium satellite constellation and developed program of Automatic Dependent
Surveillance Broadcast (ADS-B). Iridium Satellite System is LEO satellite
communications services that provide voice and data communication anytime and
everywhere on the Earth surface for a wide range of users. The benefits of this system
are: global coverage, low propagation delays and low power requirements, which
allowing direct interface with the system and service in the wake of a natural disasters.
ADS-B is a kind of surveillance that provide communication between aircrafts and
between aircraft and the ground through data transmission link of broadcast type. This
technology provides an accurate position reports, reduce separation minima and make
flying significantly safer. Iridium satellites with ADS-B receivers will provide global,
real-time aircraft tracking over oceanic airspace, polar routes, remote and mountainous
regions where conventional methods are either impossible or impractical to employ.
The goal of this work is to construct the original models of satellite communication
channel "Equipped ADS-B aircraft-to-Iridium satellite-to-Ground Station" and to
investigate the dependencies of model parameters for ensuring efficient, accurate and
high rate data transfer.
Satellite communication link model consists of a source of information (Bernoulli
Binary block), “Transmitter” (Modulator bank, OFDM Transmitter, Digital PreDistortion and Nonlinear Amplifier, Transmitter Antenna Gain), transmission channel
(Free Space Path Loss, Phase/Frequency Offset), “Satellite Transponder” (Receiver and
Transmitter Dish Antenna Gain, Phase/Frequency offset, Amplifier), “Ground Earth
Station (GES) Receiver” (Receiver Dish Antenna Gain, OFDM receiver, Gain and Phase
Compensator, Extract Data carrier, Demodulator bank), “SNR estimation block”, “Rate
ID block”, “Error Rate Calculation block”.
“Bernoulli Binary Generator” block generates random binary numbers using a Bernoulli
distribution. The transmitter sent data over the channel, but before sending signal has to
be adapted to the channel conditions using a specific adaptive modulation (BPSK,
QPSK, 16QAM, 64QAM) and coding scheme. Data is transmitted using the OFDM
transmission technique that is why signal needs to be conformed into an OFDM symbol
by performing the corresponding operations, which include a frequency-time
transformation and the addition of a guard period. Then, the signal is sent over the
transmission channel where losses in signal strength arise. Transponder changes the
frequency, the polarization and amplification of received signal and retransmitted back
to the ground station. Receiver basically performs the reverse operation as the
transmitter. Bit error rate is used to quantify a channel carrying data by counting the rate
of errors in a data string.
Figure “Aircraft-to-Satellite-to-Ground Earth Station” channel model
In the work was analyzed the probability of error that depends on the normalized signal
to noise ratio that is used to compare the level of a desired signal to the level of
background noise. The model uses an adaptive-rate control scheme that allow to
estimate SNR by varying the data rate according to the channel conditions. There were
also investigated which model parameters influence on the efficient, accurate and high
rate data transfer.
There were researched the dependencies of a SNR on free path losses for different
modulation modes, transponder noise temperatures, channel bandwidths, number of
OFDM symbols and values of cyclic prefix.
Te results of investigation show that the most efficient, accurate high data transmission
can be provided by an optimal combination of model parameters. The 64QAM provides
highest data transfer rate but sensitive to noises and BPSK and QPSK are more robust to
the interferences. The reduction of transponder noise temperature, channel bandwidth
and cyclic prefix leads to more efficient data transfer. The greater the number of OFDM
and higher the average input power more accurate signal is received.