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VSAT technology and installation >> iDirect Forum: hubs and terminals >> Channel Spacing..PCMA implementation

Message started by wbailey on Apr 14th, 2010 at 12:24am

Title: Channel Spacing..PCMA implementation
Post by wbailey on Apr 14th, 2010 at 12:24am
Hub Owners...

Has anyone gone past 1.2 on an S2 network for their channel spacing? I've been thinking about going to 1.1 but have not heard of anyone even attempting to make this work...

I have a 20megasymbol S2 downstream with 4 x 3mbps carriers under it.

Mike.. Eric.. thoughts?

Title: Re: Channel Spacing..
Post by Forum Admin on Apr 14th, 2010 at 10:54am
Normal carrier spacing is about 1.35 to 1.4 x symbol rate.  If you move closer the interference from the adjacent carrier increases, if you move further apart the interference decreases.  Be careful if you have a high spectral density carrier next to a tiny low power spectral density carrier.

If you are looking on a spectrum analyser at TDMA carriers set Peak Hold ON (for 10 minutes or so if the traffic is light) and have the resolution bandwidth very narrow so you can accurately assess the gaps between the carriers.

If you lease a block of bandwith you can try different spacings between your carriers, provided the edge carriers don't differ from what the satellite operator has agreed with you. Keep them informed however of carrier frequencies and bandwidths so their spectrum monitoring software does not raise alarms.

I don't understand your 20 Msps/s carrier with 4 x 3Mbps carrier under it.  Do you mean "by the side of it" ?

Title: Re: Channel Spacing..
Post by TDMAMike on Apr 15th, 2010 at 11:46am
I think by saying "under it" he is stating the inroute group handling the network returns (iBuilder speak/terminology).  That or this network is using a PCMA canceller.  

I always stick with 1.2 or 1.25.   If you are determined to squeeze a little more our of it, you could always experiment with it and watch for any warnings.   There are warnings on the returns for interference, and naturally the downstream will exhibit SCPC errors. Are you using UPC on your forward?

Title: Re: Channel Spacing..
Post by DeanFM on Apr 20th, 2010 at 6:31am
Hi Mike,

What is the relation between interference on return channel and SCPC error on downstream?because I experience this SCPC error frequently. currently I'm using 1.26 spacing for all carrier.


Title: Re: Channel Spacing..
Post by wbailey on Apr 22nd, 2010 at 11:58pm
PCMA .. So I have a very large downstream with multiple upstreams under (really...) the carrier. My problem is .. I'm running 1.2 right now just fine.. But I haven't heard of anyone pushing the limits, so to say, with anything more aggressive. I take SCPC errors, but I don't believe that's a result of anything carrier spacing wise. I think it's probably more of a remote that is bursting a little hotter than it should (fade condition, for example) and drowning out my downstream for other remotes..

Title: Re: Channel Spacing..
Post by TDMAMike on Apr 25th, 2010 at 5:39pm
I am surprised you are willing to live with those (occasional) SCPC Errors.  How does it affect TCP/connection-based traffic? Or do you use a UDP encapsulation based accelerator? I am assuming the latter..

Title: Re: Channel Spacing..
Post by Eric Johnston on Apr 25th, 2010 at 6:29pm
This is really interesting. I am surprised it works at all.

Does this describe the system correctly ?

Operation is in one beam, where the hub and all remotes can see all of the signals.

There is a PCMA canceller at the hub that knows the 20 Msps uplink signal and can cancel it as it comes back down to the hub, providing a relatively low noise floor for the four wanted return link carriers.

The remote sites receive the outlink carrier, which is presumably of very high power spectral density so that the return link burst interference is not so bad as to cause serious interference.  You see frequent/occasional SCPC errors at remote sites, receiving the outlink carrier.

It is quite possible that the errors occur (at the remotes) if one remote transmit is running 'hot'. Its burst directly degrades the outlink C/N for all remote sites.  Another possibility is that with 4 return link carriers, operating simultaneously, and all being noise like modulated that the four carriers occasionally all add up in voltage and produce a brief power spike of interference. Maybe, since TDMA carriers are only active for some proportion of time, the errors occur whenever 4 remote sites transmit simultaneously.

Maybe you need to carefully examine the powers of the remotes.  Use a spectrum analyser at the hub looking at the cancelled return link spectrum.  Put peak hold ON and wait till the return link carrier outlines build up.  You can then see which return link has the highest power remotes.  Look more closely at that return link, adjust the sweep rate so you see the levels of individual bursts.  Keep clearing the display and sending test loop traffic via the sites till you find the bad one.

Can you reduce the power of all remotes and use uplink power control (at the remotes) to keep the return link total power to a minimum?

The return link TDMA bursts probably start with a very brief period of CW. It may help to change the remote site transmit frequencies so that these bursts do not fall at exact frequencies within the 20 Msps carrier where it is sensitive to interference. Digital carriers normally suffer from interference regardless of the frequency of the CW interferer within the top central section of the carrier spectrum.  However there are some scrambler systems and demodulators that are sensitive to interference at certain spot frequencies.  In your case +/- 10 MHz either side of the centre might be sensitive frequencies, as might +/- 5 MHz.  If you have 4 x 3 Msps return link carriers, I would space them at least 4.2 MHz apart (1.4 x symbol rate). Your 20 Msps carrier probably occupies something like 28 MHz of transponder bandwidth so there should be room to try different spacings. Putting the outer pair as far apart as possible will help.

Do you have any alternative scrambler codes for use on the 20 Msps outlink ?  

I'm most impressed that you have a PCMA system that is working to a significant degree (i.e. with some interference) with just one canceller at the hub and more than 2 sites in total. - unless I misunderstand the network design...

Best regards, Eric.

Title: Re: Channel Spacing..
Post by TDMAMike on Apr 26th, 2010 at 12:07pm
Successfully employed PCMA also fascinates me.   However, I have been somewhat skeptical as to its value added due to the high probability of errors (and wbailey mentions above) and the power requirements.

W, are you using this on Ku and C band networks?  Or is there a specific band that you use the PCMAs?

Title: Re: Channel Spacing..
Post by wbailey on Apr 27th, 2010 at 2:15pm
I'm using PCMA across an entire transponder (36mhz) for 3 downstreams (1S2 and 2 infiniti) on my C Band network. 3 downstreams, and 2 upstreams (5mbps) under each carrier. C band is 13M to 3.6's mostly. On Ku i have a 9m going out to 1.2-1.8m (normally 1.8m) on an S2 network. Its a 25msym network with 4 XLC-M 2D16State 4mbps upstreams centered under that carrier.

Eric is right when he says that we do occassionally see power spikes. A remote an burst and overcome the noise of the larger carrier, in which you actually see the spikes on the carrier. The rule of thumb we've developed is: 3db over nominal operating power for intial and 2db over that for max transmit. After you set those levels, your PCMA network will live forever. I'm cancelling 36 full mhz with them, and have NEVER had a problem. My C Band networks have about 150 customers and Ku has about 120 customers. All online, PCMA'd up and working nicely.

Am I starting to get the impression I have balls of steel?? ;)

Title: Re: Channel Spacing..
Post by TDMAMike on Apr 28th, 2010 at 11:56am
You are one of the few (that I know) who have successfully employed this technology.   I am naturally interested in it due to the BW savings (MHz = $$$) but I have my reservations due to the aperture sizes I have on the remote ends.  

It sounds like you are doing a "typical" commissioning (3dB up, and 2 additional dB for the max), but I would be very curious of the power delta between remote nodes operating in cancelled (PCMA) space segment, and those operating in a non-PCMA enviro (crosspol).  I am naturally curious, because I am concerned with the potential for ASI when using sub-1m's.   I am glad to hear you have been successful.  

Title: Re: Channel Spacing..PCMA implementation
Post by wbailey on May 5th, 2010 at 4:08am
you take a 1db hit on your downstream snr (c/n) and generally the upstream power really isnt affected. Especially since I am using 2D16State 438 4/5 as my forward fec. The only thing that is really tough is getting the 70mhz in the middle leveled out. Other than that, stuff works peachy.

TDMAMike, I suppose since you are really not power limited in your circumstances (downstream or up) it would probably work pretty well for you. I do know that the PCMA equipment has a max power input, but I'm pretty sure it's not going to be less than the max tx of your line cards.

My next magic show, will be - PCMA (So downstream and upstream together) and Spready Spectrum (SS 16) across an entire transponder (I own the adjacent pol's). I did the numbers tonight, and I'm not going to be power limited.. Who wants to donate a few em1d1's to the cause?? ;)

If anyone is interested in glowlink plots of this stuff, let me know :)

Title: Re: Channel Spacing..PCMA implementation
Post by nexlinx on Oct 21st, 2010 at 10:33pm
Are you guys using Viasat or comtech . In my implementation when we insert the PCMA cancellers the forward link SNR drops by around 3 db but the returns just get killed with a 5 dB drop . Our normal return Eb/No without PCMA is around 8-9 but with PCMA around 4 . Is there any way to optimize this ... what could be causing such a massive drop !!

Tks ,


Title: Re: Channel Spacing..PCMA implementation
Post by Eric Johnston on Oct 21st, 2010 at 10:47pm
PCMA works best with two similar carriers, one in each direction, between a pair of similar large earth stations, using two cancellers, one at each site.

I assume you are at the hub teleport.  You refer to "returns" so I guess you are not expecting the remotes achieve full cancellation.  I wish you good luck.
If you have three return carriers, each remote with a canceller, will have uncancelled  interference from the  two other remotes.

At the hub, when you insert the canceller your self reception of your outlink carrier (after cancellation !) should virtually disappear but the nearby return links should remain unchanged.

Is that the case ?  If not, try to improve the cancellation. The sample of your original transmit outlink carrier and your received version of your outlink carrier must match as far as possible.  Transmit and receiver amplitude and group delay filter adjustments ?  Linearity of up/down converters and LNB/HPA ?  

Once the canceller is working well, try putting a remote on the same frequency as the outlink. You should see the return link as almost normal and be able to receive it at the hub.  Note that remote sites receiving the outlink will all see a real degradation due to uncancelled interference into the outlink, so this idea will only work if the aggregate power of all the return carriers (co-frequency with the outlink) minus one is very low compared with the outlink.  I assume each remote has a canceller to deal with its own transmit signal - but this will have less effect the more remotes you have.

You might like to email the above contributors to get their attention; they may not monitor this forum frequently.

Best regards, Eric.  

Title: Re: Channel Spacing..PCMA implementation
Post by ScotlandTDMA on Oct 26th, 2010 at 9:09pm
I would sugest going with the Viasat Unit it works alot better than the comtech equivelent

We are using this on two networks one of a full transponder DVB-S2 downstream with 15 upstream carriers ranging in size QPSK and 2D16 and we have seen no issues with it

also the second PCMA carrier is smaller 8Mhz outbound with three returns

there are the occasional SCPC error and CRC error but nothing to be worried about


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