Isa Mammedov, Azerbaijan Technical University, Baku, Azerbaijan, email@example.com
Ilham Afandiyev, State Management of Radiofrequencies of Ministry of Transport, Communication and High Technology, Baku, Azerbaijan, firstname.lastname@example.org
SYNCHROINFO JOURNAL. Volume 6, Number 3 (2020). P. 24-28.
Is studied the effect of measurement error on the accuracy of selecting the modulation type in the transmitter of the base station. The article discusses changes in the statistical characteristics of the measured signal as it passes through the stage of the feedback loop of the system of the LMDS type. Probabilistic characteristics of pilot-signal are determined at the output of this loop. The aim of the work was to find a compromise between the type of modulation and the power of the BS transmitter to maintain the transmission faithfulness within the given limits in the forward channel of an interactive wireless broadband radio access system of the LMDS. In this system a transition is made from one type of modulation to another in order to maintain the noise immunity of the system within the admissible limits. An adaptive controlling method of modulation type makes it possible to obtain an energy gain in the forward channel of the system.
It is useful to determine the type of modulation by measuring the current value of the signal-to-interference ratio at the base station (BS) receiver. For this purpose a pilot signal is transmitted on the reverse channel of the system. This signal, passing through the turbulent medium, undergoes attenuation, and therefore at the input of the BS receiver we have a random process. Moreover, the distribution law of this process can be different and is determined by the communication channel model. The problem consisted in determining the necessary probabilistic characteristics of the output process by using the system parameters and probabilistic characteristics of the input random process. For this purpose the cumulants of the input random process and the random process at the output of the linear inertial system are determined. An expression for the probability density of the random process at the output of a linear inertial system is obtained.
The graphics of the error probability on the energy parameter at 16-QAM and 64-QAM type of modulation are constructed. The influence of the measurement error on the accuracy of the choice of the modulation type in the transmitter of the BS is studied. The limits of the change in the energy parameter are defined graphically, under which the modulation type changes to ensure a given error probability. It is determined, that the system is more critical to the measurement error at high admissible error probability, i.e. at high error probabilities, small errors in the measurement of the signal-to-noise ratio make it necessary to transition to a more noise-immune modulation mode.
Keywords: broadband wireless access system, base station, amplitude-phase modulation, power control, cumulant, adaptive selection of the modulation type