Cross-polarization discrimination (XPD) in SATCOM refers to what?

• A. The isolation between orthogonal polarizations; high XPD reduces leakage and enables polarization diversity
• B. Difference in path lengths between two polarizations
• C. The ratio of transmit to receive polarization gains for cross-polarized links
• D. Cross-talk between adjacent channels

Answer: (A)

Cross-polarization discrimination measures how well the two orthogonal polarizations are kept separate in a SATCOM link. When a signal is transmitted in one polarization, ideally the receiver picks up almost all of it in that same polarization and very little in the orthogonal one. XPD quantifies that separation by comparing the power that leaks into the cross-polarization against the power that stays in the desired polarization. It’s usually expressed in decibels; the higher the XPD, the less leakage there is.

This separation is crucial for polarization diversity and polarization-multiplexed systems because any leakage reduces the effectiveness of using two polarizations for increasing capacity or reliability. Imperfections in antennas, feeds, and reflectors, plus propagation effects like reflections and misalignment, can degrade XPD and cause cross-polar leakage.

The other options describe different concepts: path length differences between polarizations, a ratio of gains for cross-polarized links, or cross-talk between adjacent channels, none of which define XPD.