Regarding uplink to downlink frequency
I am selecting the uplink and downlink frequency for a satellite system. As a consideration, I have taken A, B and C as the three packets of consecutive packets being sent down. If one considers dual modular redundancy( there are better n-modular redundancy types), shouldn't the uplink frequency be selected such that the uplink frequency is more than 2 times the downlink. Is this a correct consideration?
GS
Selecting frequencies for a satellite up- and down-link requires other consideration than you did.
In earlier systems, the satellite transponder was the "bent-pipe" type, so different frequency bands were needed for reasonable filters, to prevent a feedback. Received-signal level at a satellite is ~ -100 dBm while the transmitted power is +40 dBm or more.
Newer satellite systems include digital signal processing and actively pointed beams, so uplink and downlink frequencies can be closer to each other. Propagation effects like rain attenuation and depolarization must be considered, as well as interference protection of your and other satellite systems.
Using 2:1 frequency ratio is technically possible but not reasonable due to the propagation problems. I would suggest you to read some new satellite-communications handbooks to get some insight.
No. If Tx freq is half of RX freq, the TX 2nd Harmonics is RX freq, How can you suppress the TX 2nd harmonics? That could be more expensive and more complex.