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HX50 Rate Codes and Allowed Inroutes

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sun-tracker
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May 26th, 2012 at 7:08am  
Hello,

I had made a previous post in the pointing/aligning forum, and have successfully set up a dual 2000 system under the BW-provided draytek router. Performance has improved!

I also finally got the satellite meter working, and came up with some new questions.

1) Why does the sat meter say "100%" on Quality, but the HX50 never still hovers around a rate code 1/2 or 2/3? How can I get to 4/5 and have it stay there?

2) I saw this post: http://www.satsig.net/cgi-bin/yabb/YaBB.pl?board=direcway;action=display;num=130... and noticed that my rate codes are only "256 1/2", "256 2/3", and "256 4/5". When I go to the allowed inroutes page of the HX50, it shows green under the 256 column, and the 512/1024/2048 columns are purple (purple means "range metric zero"). Under the Dual 2000 system from BW, each HX50 is supposed to offer 1024 kb/s up and 1024 kb/s down (for a combined total of 2048/2048 ). Am I getting the wrong rate codes? BW said it was fine..

Thanks!
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Eric Johnston
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Reply #1 - May 26th, 2012 at 8:53am  
If you have a receive signal level or quality that reaches the maximum of the measurment scale you should note that this makes peaking up difficult as there is then a region around the beam peak where you will get the same maximum reading however you point the antenna.

In such circumstances mispoint the antenna 4 times, up/down/left/right down to some accurately equal degraded value and then set the adjusters mechanically to the exact physical centre in elevation and azimuth.

This is the way get the transmit beam, which is narrower then the receive beam, accurately pointed at the satellite.  You might then find that the system adopts a lower FEC rate in clear sky conditions for burst transmissions to the hub.

Regarding "in routes", i.e. burst transmissions from the remotes, the 256 in these expressions: "256 1/2", "256 2/3", and "256 4/5". refers to the QPSK symbol rate.  So for 256k symbol rate, the transmission data rate is 512 kbit/s.  The information data rate will be 1/2, 2/3 or 4/5 of this.  However the burst length changes for different FEC rates so that the information per burst is the same, whatever the FEC rate.
If the FEC rate drops during heavy rain you will notice no difference when clicking the mouse (sends one burst), but if you sent a very large file, involving a long series of bursts, then it will take longer to send.

The HX modem has the capability to send at 128, 256, 512, 1024 and 2048 symbol rate. Which of these are possible will depend on the hub configuration, link budget and dish size and power rating of the remote BUC.

Best regards, Eric.
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sun-tracker
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Reply #2 - May 26th, 2012 at 9:11am  
Sounds tricky.

What about the rate codes themselves? Is it normal that only 256 1/2, 256 2/3, and 256 4/5 are the only ones available? Thanks
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Eric Johnston
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Reply #3 - May 26th, 2012 at 9:57am  
The HX50 remote terminal modem has only the following transmit FEC possibilities:
Transmit encoding: Rate 1/2, 2/3, 4/5 TurboCode, Rate 1/2 Convolutional

Read more:
HX System overview
HX data sheet

Best regards, Eric
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Reply #4 - May 26th, 2012 at 12:16pm  
Quote:
Sounds tricky.

What about the rate codes themselves? Is it normal that only 256 1/2, 256 2/3, and 256 4/5 are the only ones available? Thanks
Tricky, perhaps. But once the antenna pointing angles are optimized, it becomes an automatic attribute of the modem. Just so there's no confusion, we're discussing a function of your transmit to the hub. Other than feedback, the receive functions of the modem don't come into play here. Modulation type is variable also; 8PSK, QPSK, BPSK, etc. Also - as Eric stated - higher rates are available - but none higher than those associated with your subscription plan are normally listed.

But all that is unrelated to the actual Forward Error Correction (FEC), which is achieved by transmission redundancy. That is, a duplicate bit or bits is sent along for the ride with every packet. Under normal conditions, 4/5 rate should be achievable. When in fair weather 4/5 mode, 1  bit of every 4 is repeated.  If all 5 arrive at the hub undamaged, the redundant bit is simply discarded. If one of the 4 real bits is damaged, there's a 25% chance that the extra one that came along for ride will be a duplicate. If it is, the damaged bit is replaced on the spot. If not, the hub will request a retransmission and tally a stream error. So whereas the symbol rate is 256k, the working data rate is actually 4/5 of that - or 205k.

As conditions deteriorate, the modem will drop to a lower rate; 2/3 for example. In this case, 2 redundant bits are sent along with every 3 data bits. Symbol rate remains 256k, but redundancy drops the actual data rate to 171k. 1/2 rate is full redundancy; every bit is sent twice. Symbol rate again remains 256k, but data rate is reduced to 128k. This is why things slow down as weather conditions deteriorate. Then when 1/2 rate starts racking up too many errors, the symbol rate itself will fall back. Not sure why yours only displays 256k options, mine also lists two 128k and one 64k fallback combos. I know the HX50 isn't spec'd for 64k ops, but I'm surprised you didn't list the 128k rates.

Since you hover between 2/3 and 1/2, it means too many transmission errors are being tallied to support 4/5 rate. The usual reason for transmission errors is atmospheric attenuation (precipitation). But if that's the best you see in fair weather, there's almost certainly a hardware issue at your end. Usually it's an inaccurate POL angle, but can also be attributed to incorrect focal angle or length. A weak transmitter can do it, a fault in the cable path can do it, a bad router can do it, and fringe satellite access can do it.

First thing I'd do is determine your location in relation to the satellite footprint. It may be a simple matter of being moved to a more opportune satellite. If that's discounted, plug one PC straight into the HX50. That eliminates the router. See if that changes anything. If not, go to the dish and optimize the POL angle. I understand that in the case of the HX50, this must be done while in contact with the hub. If those two methods don't work, troubleshoot the cable path. Simplest way to initiate that is to swap RX and TX cables to see if the problem follows. Mark and remove both cables from the back of the modem. Go to the dish and disconnect both cables. Put the TX on the LNB, and the RX on the transmitter. Then go back inside and put the TX on Sat IN and the RX on Sat Out. If that shows improvement, you've narrowed your problem down at least to a TX cable problem. If no change, suspect a weak or failing transmitter.

//greg//
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sun-tracker
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Reply #5 - May 26th, 2012 at 1:45pm  
Very interesting! I was wondering what the "1/2", "2/3", and "4/5" numbers actually meant. Makes sense.

Eric, I see that you said that the 256k symbol rate equates to a transmission rate of 512 kbits/s... so if the Dual 2000 system offers peak capability of 2048 kbits/s up and down, and there are 2 HX50 modems, why do I not have a higher symbol rate of 512?

When I first installed my second new dish, it was at 256 4/5 for several days straight, and now it is at a steady 2/3. Nothing with the system has changed (positioning), and weather is fine.

Does the "S" on the satellite meter refer to signal quality (aka signal-to-noise ratio?)
Slightly tweaking the elevation/azimuth/rotation of the dish and checking the HX50 over and over seems very tedius, especially since it seems to take several minutes for the HX50 to decide to go up or down with its rate code. Hmmmm.
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Admin1
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Reply #6 - May 26th, 2012 at 5:21pm  
Quote:
Does the "S" on the satellite meter refer to signal quality (aka signal-to-noise ratio?)

I don't know what the S means.

Simple 'crystal set' style meters and all complex 'identifier' meters have an initial, broadband noise power measurement mode. This will give an indication on any satellite, and the sun, and will allow you to peak up.

The HX has a function like that above, with a maximum reading of 29.

'Identifier' type meters and the HX can be programmed to tune to a particular L band frequency and symbol rate carrier. This is a simplified satellite TV receiver. When the receiver demodulator locks on to the wanted carrier it is possible to make bit error rate assessment and to display C/N, S/N, Eb/No or whatever.

In the case of the HX the scale reading jumps up into the range 30 - 100, typically about 91.

In any case of an 'identifier' type meter remember that the programmer does not know what LNB you are using, what is its local oscillator frequency, what polarisation you are set to or what is the effect of a 22kHz tone or 13/19 volts.  All these are assumptions that may be wrong.  Also note that there may be similar carriers on other satellites - so identification may sometimes be in error.

Identifier meters are excellent if you are installing several dishes per day to the same satellite - you gain confidence in the meter programming and know exactly what LNB and polarisation to set.

Notes about symbol rate etc.

The required transponder bandwidth for a carrier is about 1.4 x the symbol rate.

With BPSK modulation there is 1 transmission rate bit per symbol.
With QPSK or OQPSK modulation there are 2 transmission rate bits per symbol.
With 8PSK or 8QAM modulation there are 3 transmission rate bits per symbol.
With 16QAM modulation there are 4 transmission rate bits per symbol.

Adding forward error correction bits to the information rate means that the transmission rate is higher than the information rate. FEC ratios vary. Examples are 5/16, 21/44, 1/2, 3/4, 7/8, 0.95
FEC rates like 1/2 are very effective when the signal is weak, such as when the antenna is at beam edge or during rain, but more bandwidth is needed for the higher transmission rate.
Methods of FEC have names like Viterbi, Reed-Soloman, Turbo and Low-density parity-check (LDPC), which refer to the way the information rate bit stream is converted to the higher transmission rate bitstream.

Read more: http://en.wikipedia.org/wiki/Forward_error_correction

If you double the transmit information rate you need approx twice the transmit BUC power or a dish with twice the area.

Best regards, Eric.
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« Last Edit: May 27th, 2012 at 9:15am by Admin1 »  
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