S.Y. Zyablikov, V.G. Alybin, Y. N. Antonov-Antipov, M.B. Zylber, A.Y. Siziakova, A.A. Trofileev
In this paper we consider the problem of optimizing parameters of 4-channels satellite repeater using the criterion of minimum bit error probability (or bit error rate – BER). This is one of important problems for satellite repeaters – minimize BER while a high-speed digital data stream is beeng transferred over data downwlink within limited frequency band (4% relative to carrier ~20 GHz). Terrestial station is equipped with Comtech CDM-700 modem. The satellite moves on inclined HEO and has an antenna with wide coverage zone.
The comparative analysis of TWT amplifier and transistor amplifier transmitter communication line characteristics is carried out.
Initial data for optimization of satellite repeater transmitter with the TWT amplifier were:
Frequency bandwith of the transmitter ~800 MHz;
Electrical parameters of on board and terrestrial station antennas, antenna-feeder system, low noise amplifier on terrestrial station;
The signal of the transmitter with frequency division;
Model of nonlinear effects in the TWT amplifier;
The required bit error probability for signal decoding – 10–8.
A preliminary calculation based on the modem Comtech CDM-700 invariable parameters of satellite repeater transmitter are defined:
Nrmber of frequency channels – 4;
Optimal placement of carrier frequencies;
Channel structure – Nyquist;
Modulation types – QPSK and 8PSK.
Varied parameters of the model were as follows:
Signal to Noise ratio (Eb/N0) in the receiving channel of terrestrial station;
Propagation route, which takes into account the attenuation of the signal in free space, selective signal loss in oxygen and water vapor, rain attenuation, the loss in antenna feeder tract, as well as increased on-board transmitter and a ground receiving antennas;
Gain pre-amplifier, whose value sets the signal level at the transmitter output.
The optimization process was based on the computer simulation program System View.
Computer model of communication line includes the following main functional blocks:
functional block forming the signals such as QPSK and 8PSK with frequency division multiplexing;
pre-amplifier with adjustable gain value, with determiner to set the operating mode of the power amplifier;
TWT power amplifier or 4-transistor amplifier;
unit that simulates the propagation environment on the path (channel), the transmission coefficient which takes into account the gains of transmitting and receiving antennas, the losses in the antenna feeder system of the transmitter, space attenuation, the additional loss in the rain, etc.;
This paper describes the models and characteristics of propagation environment and 8PSK or QPSK demodulator receiver, allowing to carry out statistical calculations of the characteristics that determine the quality of communication line.
The dependences of the probability of symbol error for different values of attenuation on line communication channels are shown from which it follows that 8PSK signals have an advantage over QPSK signals in the transmission rate per channel drugs, but are inferior on SER.
A conclusion was made about the possibility of building communication line, which uses both types of signal the first (8PSK) appropriate to apply under the favorable propagation conditions, while the second (QPSK) – under conditions of high attenuation on the path.
The results can be widely used in the calculation of the parameters for TWT amplifier or transistor amplifier transmitters for communication, navigation, digital TV, especially in modes of direct relaying.