PTP 820-C, PTP 850-C XPI Alignment

Summary:
XPIC (Cross Polarization Interference Cancellation) is a feature used on Carrier-Class Microwave Link installations to increase capacity and spectral efficiency of a link.
A Microwave Link using XPIC technology capabilities effectively doubles the potential capacity of a Microwave Path.
XPIC allows the assignment of the same frequency to both the vertical & horizontal Polarization on a Path. Where available frequencies are limited then it is possible to assign the same frequency twice on the same path using both Polarizations.

Cause:
When XPIC is enable, the XPI value must be maintained between 25 to 30 dBm.

Solution:
This article explains the steps for XPIC alignment.
You can refer to below community link for XPIC Configuration before alignment:

Below picture explains the radio and OMT port details:

Picture1

The first step to verify the XPI alignment is to check this via water level procedure.
Refer below image:

Picture2

  • If both ends of the link are level, the XPI values should be in the preferred range of 25 to 30 dB.
  • If the XPI values are still not in the preferred range then we can follow the below procedure for additional XPI alignment.

Procedure to verify your XPI parameters:

  1. Align the antennas for the first carrier. While you are aligning these antennas, mute the second carrier.

  2. Adjust the antenna alignment until you achieve the maximum RSL for the first-carrier link (the "RSLThis RSL should be no more than +/-2 dB from the expected level.
    Record this as RSL wanted.

  3. Measure the RSL of the second carrier. Record this as RSL unwanted.

  4. Determine the XPI, using either of the following two methods:
    a) To calculate the XPI, subtract the RSL unwanted from the RSL wanted.
    b) Read the XPI from the Modem XPI field of the Radio Parameters page in the Web EMS.

  5. The XPI should be between 25dB and 30dB. If it is not, you should adjust the OMT assembly on the back of the antenna at one side of the link until you achieve the highest XPI, which should be no less than 25dB. Adjust the OMT very slowly in a right-left direction. OMT adjustment requires very fine movements and it may take several minutes to achieve the best possible XPI.

  6. Unmute all the carriers and check the RSL levels of all the carriers on both sides of the link. The RSL of the horizontal carrier of the local unit should match the RSL of the vertical carrier of the remote unit, within ±2dB.The RSL of the vertical carrier of the local unit should match the RSL of the horizontal carrier of the remote unit, within ±2dB.

  7. Check the XPI levels of both carriers on both sides of the link by checking the Modem XPI field of the Radio Parameters page in the Web EMS. See Viewing the Radio Status and Settings. All four carriers should have approximately the same XPI value. Do not adjust the XPI at the remote side of the link, as this may cause the XPI at the local side of the link to deteriorate.

Note:
In some cases, the XPI might not exceed the required 25dB minimum due to adverse atmospheric conditions. If you believe this to be the case, you can leave the configuration at the lower values, but be sure to monitor the XPI to make sure it subsequently exceeds 25dB. A normal XPI level in clear sky conditions is between 25 and 30dB.

Once above points are checked and still there is any scope of XPI Alignment then follow below procedure:

Step1:
When aligning a cross-polar link (a link that has two signals: one vertically polarised (V) and the other horizontally polarised (H)), each link end’s polarities must be aligned:

– V to V
– H to H

Step 2:

a) Slightly twisting one link end’s OMT.
b) Adjust the OMT assembly on the back of the antenna at only one side of the link to maximize the XPI.
c) Adjust the OMT by rotating the OMT and its attached ODU on the back of the antenna.
d) Loosen the four bolts that connect the OMT to the antenna.
e) Move the OMT in small increments. After each increment, refresh the Web EMS screen and review the XPI reading
f) Instead of slightly twisting one link end’s OMT, ensure that both OMTs are level!

Note: Antennas (typically larger than 6ft) may have separate feedhorns
– one for V and one for H
– These separate feed horns may require individual twist alignment
– Follow the antenna manufacturer’s instructions to ensure that the link end polarities are aligned.

1 Like