We studied a three-wave coupling process occurring in microwave power transmission (MPT) experiment in the ionospheric plasma by performing computer experiments with one-dimensional electromagnetic PIC (Particle-In-Cell) model. In order to examine the spatial variation of the coupling process, we continuously emitted intense electromagnetic wave from an antenna located at a simulation boundary. In the three-wave coupling, a low-frequency electrostatic wave is excited as the consequence of a nonlinear interaction between the forward propagating pump wave and backscattered one. In the computer experiments, low-frequency electrostatic bursts are discontinuously observed in space. The discontinuity of the electrostatic bursts is accounted for by the local electron heating due to the bursts and associated modification of the wave dispersion relation. In a case where the pump wave propagates along the geomagnetic field Bext, several bursts of Langmuir waves are observed. Since the first burst consumes a part of the pump wave energy, the pump wave is weakened and cannot trigger the three-wave coupling beyond the region where the burst occurs. Since the dispersion relation of the Langmuir wave is variable due to the local electron heating by the burst, the coupling condition eventually becomes unsatisfied and the first interaction becomes weak. Another burst of Langmuir waves is observed at a different region beyond the location of the first burst. In the case of perpendicular propagation, the upper hybrid wave, one of the mode branches of the electron cyclotron harmonic waves, is excited. Since the dispersion relation of the upper hybrid wave is less sensitive to the electron temperature, the coupling condition is not easily violated by the temperature increase. As a result, the three-wave coupling periodically takes place in time and eventually the transmission ratio of the microwaves becomes approximately 20% while almost no attenuation of the pump waves is observed after the first electrostatic burst in the parallel case.