Cross-Pol Discrimination
How XPD Sets the Frequency-Reuse Ceiling
Cross-pol discrimination is the single number that decides whether two signals can share the same frequency band by riding on orthogonal polarizations, vertical and horizontal in a linear system or right-hand and left-hand in a circular one. When the transmitter radiates a perfectly pure polarization, an ideal receiver would see zero energy in the orthogonal port. Real antennas, feeds, and ortho-mode transducers always leak a little, and the propagation path adds more, so a fraction of the wanted carrier appears in the cross-polar channel as interference. XPD measures that leakage as a power ratio, and it directly becomes the carrier-to-interference (C/I) floor for the reused channel.
Because XPD is a power ratio, it adds and combines like any other dB budget. The end-to-end link XPD is the worst-case combination of the transmit antenna XPD, the receive antenna XPD, and the depolarization contributed by the atmosphere. A 40 dB transmit antenna paired with a 35 dB receive antenna already cannot exceed about 33 dB on a clear day, and a Ku-band or Ka-band rain event can subtract another 10 to 15 dB. Designers therefore specify clear-sky XPD with substantial headroom so that the rain-faded value still clears the modulation's required C/I margin at the worst-case availability.
Where physical XPD is insufficient, terrestrial microwave radios add cross-polar interference cancellation (XPIC). An XPIC engine samples the orthogonal channel, estimates the leakage transfer function, and subtracts a scaled copy from the wanted channel, recovering 15 to 20 dB of effective XPD. This is what lets 256-QAM and higher constellations run reliably on co-channel dual-polarized links that would otherwise be limited by a 35 dB antenna.
Governing Relationships
XPD = 10 log10(Pco / Pcross) dB
XPD from linear polarization tilt error φ:
XPD ≈ −20 log10(tan φ) dB
XPD from circular axial ratio AR (linear, ≥1):
XPD = 20 log10[(AR + 1) / (AR − 1)] dB
Rain depolarization (ITU-R P.618 form):
XPDrain ≈ U − V × log10(Ap) dB, V ≈ 20 to 23
Where Pco = co-polar power, Pcross = cross-polar power, φ = polarization misalignment angle, AR = voltage axial ratio of a circularly polarized wave, Ap = co-polar rain attenuation in dB, and U, V are frequency- and elevation-dependent coefficients. Example: a 1° tilt error gives XPD ≈ 35 dB; AR = 1.06 (0.5 dB) gives XPD ≈ 30 dB.
XPD Targets by System and Modulation
| System / Use Case | Modulation | Required C/I | Clear-Sky XPD Target | Notes |
|---|---|---|---|---|
| Satellite earth station (Ku) | QPSK / 8PSK | 12 to 18 dB | 30 dB min over contour | Per ITU-R S.731 / operator spec |
| Satellite earth station (Ka) | 16APSK | 18 to 22 dB | 33 to 35 dB | Rain depolarization dominant |
| Terrestrial PtP microwave | 64-QAM | 25 to 28 dB | 33 to 35 dB | XPIC optional |
| Terrestrial PtP microwave | 256-QAM | 31 to 34 dB | ≥ 35 dB + XPIC | XPIC adds 15 to 20 dB |
| Reflector boresight (good feed) | n/a | n/a | 40 to 45 dB | Degrades off-axis and in rain |
| Linear feed, 1° tilt error | n/a | n/a | ~35 dB | −20 log(tanφ) limit |
Frequently Asked Questions
What is the difference between XPD and cross-polarization isolation (XPI)?
Both are dB power ratios but measured differently. XPD transmits one polarization and compares the co-polar received power to the cross-polar power from that same signal. XPI transmits both polarizations at once and compares the wanted carrier to the interference leaking from the orthogonal channel. On a clear, aligned path they agree within about 1 dB, but rain depolarization can degrade XPI faster since both channels are perturbed simultaneously. Antenna specs and frequency-reuse budgets are normally written in XPD because a single-feed measurement can verify it.
How much XPD do I need for dual-polarization frequency reuse?
Work backward from the modulation's C/I need. QPSK wants C/I above about 15 dB, met comfortably by 27 to 30 dB XPD. 64-QAM needs roughly 25 to 28 dB C/I, so target 33 to 35 dB clear-sky XPD so the rain-faded value still clears threshold. Satellite operators commonly mandate 30 dB minimum over the coverage contour, while 256-QAM terrestrial radios with XPIC start from 35 dB and recover another 15 to 20 dB electronically. Always size for the worst-case availability rain rate.
Why does rain reduce cross-pol discrimination?
Falling raindrops flatten into oblate shapes with a canting angle, so the horizontal and vertical dimensions differ. Each polarization then sees different attenuation and phase shift through the rain volume, and this differential rotates wanted energy into the orthogonal port. The effect grows with rain rate and path length and is worst above 10 GHz. ITU-R P.618 models XPD degrading as roughly 20 to 23 times the log of co-polar attenuation, so a 6 dB Ku-band fade can pull a 35 dB clear-sky XPD down toward 23 dB.