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Service interrupted in a RTN950 Xpic link due to cross polarization discrimination problem in HSX10-64-D3M Andrew antennas

Publication Date:  2012-07-25 Views:  70 Downloads:  0
Issue Description
In a RTN950 XPIC link with V100R001C02SPC300(5.76.01.34) Software Version with HSX10-64-D3M Andrew antennas aligned and working without problem with vertical and horizontal polarization on, if in one site both ODUs of one polarity are powered off, services got interrupted.


Alarm Information
MW_LIM, SRV led in yellow




Handling Process
To solve the discrimination problem between both polarities installation personnel had to made the following procedure:

1. In the near end site turn off both ODUs of the same polarity (in this case horizontal)

2. In the far end climb tower to the antenna, connect with a tester in the main horizontal RSSI interface and start rotating feeder until the receive voltage goes to 0 V.

3. After feeder was rotated to guarantee the antenna discrimination, levels received by LCT were verified. With feeder movement, differences between received levels of horizontal and vertical polarities were more than 40 dB.

4. Finally test 3 was repeated (Turn off both ODUs of the same polarity only in one site and check behavior in remote site) and no services were affected and link ID never loose


Root Cause
During the switching tests if both ODUs of the same polarity were switched off, services passing through the other polarity (switched on) were affected no matter which polarization were switched off or switched on.

In order to found problem several things were done:

1. First check physical connections. IF cables between main/stand by IFX configured boards and ODU and check cross polarization cables connections between main/stand by vertical IFX and main/stand by horizontal IFX:

- Everything was correctively connected according the configured Xpic groups

2. Then Turn off completely one side and to check external interferences in remote site:

- This test was made in both sites and no received levels were reported during the test. Always -90 dB

3. Turn off both ODUs of the same polarity only in one site and check behavior in remote site:

- In this test as soon as the ODUs of one polarity were switched off in one site, in remote all the ODUs (main/stand by- vertical/horizontal)  reported high signal levels and a MW_LIM alarm appeared indicating that the received link was lost . The difference between received levels of the active polarity and the non active was of 20 dB and normally in an Xpic link with HSX10-64-D3M Andrew antennas the difference should be of 40 dB (as shown in the attached document
Suggestions
In Xpic links always discrimination polarization test must be done to ensure that vertical and horizontal signals are not interfering each other. During test the idea is to move feeder just in one site. In this test difference between received levels must be at least of 40 dB or more if a High Performance, Super High XPD Parabolic Shielded Antenna is used.




END