Satellite Communications/Satellite Modulation
Thomas E Burke, PhD, PMP wrote at 2014-05-08 16:18:01
In response to your specific question, the reason that the uplinks and downlinks would be transmitted with different polarizations (V and H or RHCP and LHCP) is to minimize interference between the two.
Generally, satellite uplinks and downlinks are allocated different frequencies. For example, US military UHF systems have downlinks around 250 MHz and uplinks around 400 MHz. Using this approach obviates the need for separate polarizations.
Because satellite communication bands are finite, we increasingly see the use of more complex modulation schemes to enable transmission of more information in the allocated frequencies. The simplest is BPSK (binary phase-shift keyed) that can transmit two bits per unit of bandwidth. Next is QPSK (quaternary phase shift keyed) that transmits four bit of information per unit of bandwidth.
The use of cross-polarization techniques (V and H or RHCP and LHCP) in conjunction with advance modulation provides additional benefits.
Please let me know if this addresses your concern or if you need additional information.