Microwave_Tower_AndrewIn the last “Back to Basics” blog post, Derren Oliver wrote about the ability of an antenna to focus the radio waves in the main beam, hence maximizing the signal level and reducing the interference to and from other links. Here, I’ll be explaining the microwave antenna’s ability to maintain radiated or received polarization purity between horizontally and vertically polarized signals. This is called cross-polar discrimination, or XPD.

In transmit mode, XPD is the proportion of signal that is transmitted in the orthogonal polarization to that which is required. In receive mode, it is the antenna’s ability to maintain the incident signal’s polarization purity.

For example, if a perfectly vertically polarized signal (containing no horizontal component) were incident upon a single polarized receive antenna, electrical and mechanical imperfections will introduce a small amount of ellipticity to the polarization of the signal. The signal can be thought of as having both vertical and horizontal components. The ratio of the consequential horizontal to vertical components is the XPD.

Here at CommScope we use the following formal definition of XPD: “Cross-Polarization Discrimination, in dB, is the difference between the peak of the co-polarized main beam, and the maximum cross-polarized signal over an angle twice the 3dB beamwidth of the co-polarized main beam.”

The angular region is included in the definition to account for any antenna movement caused by wind induced twist or sway.

We monitor these potential contributors to XPD in the factory by our Interport Isolation (IPI) measurement process. IPI has a relationship to XPD, and although it is not the only contributing factor, you can say that XPD will never be greater than IPI. As we have mentioned in previous blogs, to really understand and measure XPD you require a full range test, which CommScope performs in the design phase of all its antennas.

XPD is an important characteristic, particularly in dual-polarized systems, where cross-talk between polarizations can prevent the system’s quality objectives from being achieved. Radios can use cross-polar interference cancellers (XPICs) to isolate polarizations and compensate for any link or propagation induced coupling. However, good polarization purity from the antenna is important to allow the XPICs maximum flexibility to compensate for these dynamic variations.

Obviously, the subject of cross-polar discrimination is quite detailed. It is an important characteristic to understand when comparing microwave antennas. XPD can degrade network performance, meaning slower speeds, wasted spectrum and unhappy customers. We’ve got two more topics to introduce in the next blog post—return loss and voltage standing wave ratio (VSWR).

Any questions about XPD?

About the Author

Jim Syme

Jim Syme is product line manager for CommScope’s Microwave Systems division, responsible for business development in the Middle East, Africa and Asia-Pacific regions with additional global management responsibility for several major OEM customers. Jim began his 27-year career with Andrew Corporation in the microwave antenna design engineering group and is a regular participant at conferences on all matters related to microwave antenna systems. Prior to joining Andrew Corporation, Jim was a design engineer for the United Kingdom Atomic Energy Authority.

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Comments

12 comments for "Back to Basics in Microwave Systems: Cross-Polar Discrimination"
OLATAYO OLALERE Monday, November 21, 2016 8:38 AM

Need to know what is the maximum and minimum value expected for a good link with XPD

Thanks

Patrick Baffoe-korang Wednesday, January 03, 2018 8:58 AM

Can the XPD value of a given antenna change after it has been installed in the field?

Jim Syme Monday, January 08, 2018 4:32 PM

Hi Patrick

There is no reason for the XPD to degrade over time in an ideal world - the most likely scenario would be installation not being carried out as per the supplied bulletins and the antenna misaligning over time.

For example, if the antenna has been subjected to a SURVIVAL wind load occurrence then the antenna will almost certainly need realigned.

Environmental conditions can also degrade the performance (especially rain) but this should be cyclic as once back to clear, dry conditions the antenna should return to meet the RF spec.

The installation and interference from other new links can also impact the XPD.

I hope this answers your questions but, if not, please don't hesitate to contact us.

Best regards
Jim

Shyam Monday, January 15, 2018 3:36 PM

Pls any one tell me minimum and maximum xpd value, coupler details

Jim Syme Wednesday, January 17, 2018 2:58 PM

Hi Shyam

The minimum XPD of the antenna is what is specified in the antenna data sheets on www.commscope.com and this is measured under clear, dry conditions and assumes that the antenna has been installed and aligned correctly.

In an ideal world, the cross pol would be nulled in the azimuth and elevation planes but very small imperfections, that can't be avoided in antenna production, will start to fill these nulls (e.g. slight ellipticity in the circular waveguide, small misalignments during the assembly of feed components etc.).

Poor antenna installation (antennas not correctly aligned) can also degrade XPD as cross pol will increase in regions away from the antenna boresight.

Hopefully this answers your questions however please feel free to contact me if you have any further questions.

Best regards
Jim

ANUP JAIN Friday, April 27, 2018 7:48 PM

Hi,

Here are some question regarding XPI against observations.

Observation

• XPI value is varying out of range in the day time and it is coming to Normal range in night time
• During these time , there is no variation in RSL level. But XPI varies out of range.
• Currently we are facing this issue in some MW Links.

Clarification / support required

• What may be the root cause of above mentioned issue
• Is it temperature sensitivity issue of any vendor radio? Because XPIC value is coming to normal range during night time.
• If the XPI value is coming to normal range, hope we should not take any action of OMT alignment / Re installation / cleaning ports etc.

Thanks & Regards
Anup Jain

Jim Syme Tuesday, May 01, 2018 9:35 AM

Hi Anup

The antenna should be resilient to temperature fluctuations and the fact that the RSL is constant would lead me to believe the antenna is OK.

I think the radio is a more likely candidate for the changes and I'd suggest you contact them to discuss this.

Best regards
Jim

Shafiq .I .S.Marie Sunday, July 01, 2018 11:05 AM

Some times we do XPD test with a values more than 30 db , but some times we have 30db in one terminal between H&V and 34 db
Is there any thing called balance in the reading I Nmean it should read the same values or the reading should be more than 30 db and thats all

Ahmed Soboh Thursday, September 20, 2018 7:36 AM

Hi Jim

I have some cases for 4+4 XPIC_SD configuration where I'm facing link XPD value fluctuation. how to judge if the XPD is healthy. also what about the ATPC feature enabling effect with SD links on XPD value?

Jim Syme Thursday, September 27, 2018 8:12 AM

Hi Ahmed.

I assume you are talking about a Space Diversity set up here.

Fluctuation in XPD value on a space diversity link is almost inevitable as different Tx/Rx Antennas become more/less dominant in the link. Also, the fade/multipath effect is different from each polarisation so this will also cause fluctuation in XPD level not necessarily in line with co-polar fluctuation.

We don’t think the ATPC (Auto Tx Power Control) should cause XPD variation.

Hopefully this answers your questions.

Hanumant Wednesday, November 07, 2018 10:00 AM

Hi,

Does this XPD value varies as modulation increases?

We have set 2048QAM utmost peak modulation in Double XPIC hop. where consistenly getting Hop utilization ~100% .
Raised the TT to vendor TAC , they have suggested us to crosscheck for installation of MW links.

Thanks in advance
Hanumant

Jim Syme Wednesday, November 07, 2018 3:55 PM

Hi Hanumant

XPD of the antenna is fixed and independent of the modulation rate and should not change.

We agree with the feedback to do a cross check.

Best Regards
Jim

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