Link Engineering

Cross-Polarization Interference

Q: What XPD value is required for 256-QAM on a co-channel dual-polarized link?

A 256-QAM signal needs a carrier-to-interference ratio of roughly 30 to 32 dB at the demodulator for a 1e-6 BER. Because the unwanted cross-polar energy acts as the dominant interferer on a co-channel dual-polarized (CCDP) link, the residual XPD after any cancellation must stay above that CIR plus an implementation margin of 3 to 5 dB. In practice that means about 35 dB of effective XPD. Antennas typically guarantee only 30 dB of boresight XPD and rain depolarization can erode it to 15 dB or worse, so an XPIC canceller providing 20 to 25 dB of additional cancellation is mandatory for 256-QAM and higher.

Q: How does rain affect cross-polarization interference at 18 GHz versus 6 GHz?

Falling raindrops are oblate and tilt with the wind, so they present different attenuation and phase shift to horizontal and vertical polarizations. This differential depolarizes the link and raises the cross-polar level. The effect scales strongly with frequency: at 6 GHz the rain-induced XPD degradation is modest, but at 18 GHz a 50 mm/h rain rate can drop XPD by 15 to 20 dB from the clear-sky value. The ITU-R P.618 model relates cross-polar discrimination to co-polar attenuation by XPD = U - V times log10(CPA), where U is near 30 to 35 dB and V is near 20, so a few dB of co-polar rain fade translates into a large XPD collapse on high-frequency links.

/kraws pol-uh-rih-ZAY-shun in-ter-FEER-uns/ (XPI)

Unwanted signal energy that couples from the orthogonal polarization into the wanted channel of a dual-polarized link. When horizontal and vertical streams reuse the same channel (co-channel dual-polarized operation), imperfect antenna isolation, feeder leakage, and rain depolarization let a fraction of each stream bleed into the other. The leakage level is governed by the system cross-polar discrimination; clear-sky XPD is typically 30 dB at antenna boresight, but at 18 to 38 GHz heavy rain can collapse it below 15 dB. Because the interferer is essentially the full-power co-channel carrier on the other polarization, even a few dB of XPD loss can break a high-order QAM link, which is why an XPIC adaptive canceller is standard on modern microwave backhaul radios.

Category: Link Engineering

Clear-sky XPD: ~30 dB boresight

XPIC cancellation: 20 to 25 dB

Why Orthogonal-Polarization Leakage Limits Spectral Efficiency

Polarization reuse doubles the data rate of a microwave link by sending two independent streams, one horizontally polarized and one vertically polarized, in the same RF channel. The arrangement only works if the two streams stay isolated. In a perfect system the receiving antenna would reject the orthogonal polarization completely, but real antennas, waveguide feeds, and the propagation medium all introduce coupling. The portion of the orthogonal-channel power that ends up in the wanted receiver is the cross-polarization interference, and it sets a hard ceiling on the modulation order the link can carry.

The dominant source of leakage shifts with frequency and weather. Under clear-sky conditions the antenna and orthomode transducer define the floor: a well-aligned parabolic antenna with a quality OMT delivers about 30 dB of boresight XPD, falling off the main beam to 25 dB or less. Once rain enters the path, differential attenuation and differential phase between the two polarizations become the controlling mechanism. Oblate, canted raindrops attenuate and phase-shift horizontal and vertical components unequally, rotating energy from one channel into the other. At 6 GHz the effect is minor, but from 18 GHz upward a 30 to 50 mm/h cell can drop effective XPD by 15 to 20 dB during the worst minutes of a storm.

Cross-polarization interference matters because the interferer is not thermal noise; it is a full-strength co-channel carrier. On a balanced co-channel dual-polarized (CCDP) link with equal transmit powers, the carrier-to-interference ratio the demodulator sees is numerically equal to the residual XPD. A 256-QAM waveform needs roughly 30 to 32 dB of CIR, so 30 dB of raw antenna XPD leaves no margin and a single rain cell will force the link to drop to a lower modulation. The fix is an XPIC (cross-polarization interference cancellation) equalizer that estimates the leakage from the co-located orthogonal receiver and subtracts it, recovering 20 to 25 dB of effective discrimination.

Quantifying XPI, XPD, and the CIR Budget

Cross-polar discrimination (definition):
XPD = 10·log₁₀(P_co / P_cross) dB

Carrier-to-interference ratio on a balanced CCDP link:
CIR ≈ XPD_eff = XPD_antenna ⊕ XPIC_gain

Rain depolarization (ITU-R P.618 form):
XPD_rain ≈ U − V × log₁₀(CPA) (U ≈ 30–35, V ≈ 20)

Residual interference power after cancellation:
P_res = P_cross × 10^{(−XPIC_gain/10)}

Where P_co = co-polar received power, P_cross = cross-polar leakage power, CPA = co-polar rain attenuation in dB, U and V are empirical fit constants, and ⊕ denotes power-domain combination of discrimination terms. Example: 30 dB antenna XPD with 22 dB XPIC gain yields roughly 35 dB effective discrimination after subtracting residual error, enough for 256-QAM.

XPD Required by Modulation Order

Modulation	Required CIR (1e-6 BER)	Min. effective XPD	Clear-sky margin (30 dB ant.)	XPIC needed?
QPSK	~13 dB	~16 dB	14 dB	Optional
16-QAM	~20 dB	~24 dB	6 dB	Recommended
64-QAM	~26 dB	~30 dB	0 dB	Required
256-QAM	~31 dB	~35 dB	−5 dB	Required
1024-QAM	~37 dB	~41 dB	−11 dB	Required + high-XPD antenna

Common Questions

Frequently Asked Questions

What XPD value is required for 256-QAM on a co-channel dual-polarized link?

A 256-QAM signal needs a CIR of roughly 30 to 32 dB at the demodulator for 1e-6 BER. Since the cross-polar carrier is the dominant interferer on a CCDP link, residual XPD must stay above that CIR plus 3 to 5 dB of implementation margin, about 35 dB effective. Antennas guarantee only ~30 dB boresight XPD and rain can erode it below 15 dB, so an XPIC canceller adding 20 to 25 dB is mandatory.

How does rain affect cross-polarization interference at 18 GHz versus 6 GHz?

Oblate, wind-canted raindrops attenuate and phase-shift horizontal and vertical polarizations differently, depolarizing the link. The effect scales with frequency: at 6 GHz the XPD hit is small, but at 18 GHz a 50 mm/h rate can drop XPD by 15 to 20 dB. ITU-R P.618 ties it to co-polar attenuation as XPD ≈ U − V·log₁₀(CPA), so a few dB of rain fade collapses XPD on high-frequency links.

What is the difference between XPI and XPD?

XPD (cross-polar discrimination) is an antenna or channel property: co-polar power over cross-polar power when only the co-polar signal is transmitted. Cross-polarization interference (XPI) is the system result: the actual wanted-to-interfering ratio during dual-polarized operation. On a balanced link they match numerically, but XPI also captures asymmetric power, feeder leakage, and residual XPIC error that XPD alone ignores.

Related Terms

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Link Engineering

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