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C Band and 3G/4G/P2P Inteference

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Posts: 1
Nov 17th, 2018 at 1:06pm  
Hi guys. I have few queries about the C Band operations at 6GHz and 4GHz - Up and Down. The queries is as follows and will really appreciate if I can be helped with the following.

1. Is the C Band at 6GHz and 4GHz totally immune to rain fade or there is chances that during rain we can get the CRC8 and CRC32 errors on the iMonitor.

2. We have 3G/4G at 1.8GHz, 1.9GHz and 2.1GHz transmission at about 5 Meters from the Hub Station. Can the CRC8 and CRC32 errors be caused by the 3G/4G at the satellite hub. Our assumption is that because the frequencies are far apart, it would not cause interference. We used the Spectrum Analyzer but could not get any other transmission in the 3GHz to the 7GHz range.

3. Can a P2P Microwave at 18GHz cause the interference on the C Band Tx at 6GHz and Rx at 4GHz. The P2P Microwave is again at about 5 Meters from the Satellite Hub.


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Reply #1 - Nov 19th, 2018 at 12:03pm  
Can you clarify where the CRC errors are being detected. I assume that it is at the hub site TDMA burst receive line card.

I have put many ideas below. Most (or all) of these will be wrong.

If the CRC errors affect all sites.. 

To detect possible hub interference from adjacent terrestrial transmitters etc I suggest using an C band feed, a C band LNB and spectrum analyser. Use a DC block to avoid damage to the spectrum analyser. Hold the feed/LNB and point it around at possible sources.

If the interference is intermittent try to associate it with timing of nearby events. Examples security staff personal radios, vehicle movements, ship with radar passing by, passing aircraft, switching of air-conditioners, neon signs, equipment being powered on/off nearby, daily temperature changes.

Regarding your hub check all LNB cable joints for intermittent connection. Look at the LNB output spectrum over a wide range (e.g 0 Hz to 2 GHz). Use max hold to look for intermittent signals. Some modern analysers have a waterfall display. The noise floor should be clean except for the intended raised floor for the intended bandwidth. The region 10kHz to 5 MHz is worth looking at in case the problem is hub LNB power supply noise.

Nearby transmitters on lower frequencies will radiate harmonics and mixed products, possibly in your intended 4 GHz receive band. Higher frequency signals can get down your C band feed but should be stopped by the transmit reject filter. The highest power potentially interfering signal is your own 6 GHz transmission.

C band LNBs have various input filtering. Older models 3.7-4.2 GHz are useful if the new super-wideband types such as 3.6-4.2 have interference coming in around 3.5/3.6 GHz. Waveguide bandpass filters are available, if really needed, but only if the interference is proven to be NOT actually on your wanted carrier frequency.

If you monitor your rx spectrum in zero span you can record the shape and amplitude of bursts arriving, one after the other. Do any overlap ? Are there lower level interfering bursts coming at the wrong time ? Do any bursts look wrong shape ? Is there a TDMA system operating on the opposite polarisation.

It is worth looking at the opposite polarisation on your satellite and also at adjacent satellites with the same polarisation to see what is happening there. Continuous and intermittent cross-pol interference does happen and is difficult and intermittent to identify if caused by a particular VSAT terminal in another network.

Very heavy rain, particularly at low elevation, will affect C band (attenuation plus increased noise and x-pol interference) but it should be quite clear that the CRC errors are associated with the rain.

If the CRC errors are associated with particular sites..

iDirect remote sites have an uplink power control capability. This means that during rain the remote site modem power output may be automatically be increased to compensate.  This assumes that during clear sky condition that the remote BUC is operating well below its rated output, (e.g. quarter power or - 6dB).  If the hub tells a remote to increase and the increase causes the BUC to saturate then the bursts will be distorted with CRC errors even though the power of the burst is high.

The cable from the modem to the BUC needs to be low DC resistance. BUCs take more DC current during burst transmission and significant voltage drop on the cable may cause the bursts to be mishaped.
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