The combination of a transmitter and receiver in the satellite is known as transponder. The basic function of a satellite transponder is amplification and frequency translation.
An Up link signal at 6 Ghz is received by the satellite antenna and applied to a low noise amplifier. The amplified signal is then «down converted» frequency changed from 6Ghz to 4Ghz by means of mixing with a 2Ghz signal from the local oscillator. This 4Ghx signal is amplified by the power amplifier and connected to the common antenna through a device called as «deplexer»
Deplexer will connect the incoming signal to the LNA and output of PA to the antenna but will isolate the LNA and PA from each other. The reason for frequency translation is that a transponder cannot transmit and receive on the same frequency.