Re: UDR56k-4 Sensitivity & BER

Bryan Hoyer <bhhoyer@...>
 

On the topic of Modulation

The data-sheet lists the modulation types implemented in the first release. These types support existing digital standards for compatibility.

The UDR uses an IQ XCVR and as such is capable of doing other modulation types, limited by the regs and the linearity of the PA. The modem is implemented in software and can be updated in the field.

We will release a full briefing on our software architecture at DCC.

Cheers,
Bryan K7UDR

On Aug 7, 2013, at 9:23 AM, "Michael E. Fox - N6MEF" <n6mef@...> wrote:

 


Around here in order to test anything I have to buy more radios and computers, there's no-one else to talk to.  That said, I'm interested in getting more radios :)

Absolutely!  I’m all for that!

Interesting, thanks for the info.  This was using Connected Mode AX.25 or TCP/IP over Disconnected Mode?

We tried both.  It doesn’t matter.  The errored packet still needs to be retransmitted, regardless of whether it is AX.25 or TCP/IP that makes the decision.




Sure.   This is why i thought quoting the sensitivity for a BER of 10^-3 was a bit odd.  I assume that post-correction residual BER figures for modems with FEC would be used in the sensitivity figures when presented in the specs.

I believe that’s the case.  BER is certainly important and one needs to know where that break point is.  But it’s also important to understand that you can’t design a system to operate at the worst case 0% BER point.  The reason is that, with FEC, BER stays at 0% until the signal is so dirty that error correction can’t help it any more.  At that point, BER jumps up very quickly from 0% to 5% or more with just a dB or two of reduced C/I (Carrier to Interference ratio).  In other words, a 0% BER signal and a total unusable signal can be very, very close in C/I.  So, at the point where 0% BER is lost, you have little to no fade margin and your system will not be reliable.  In practice, the modulation fidelity value is monitored during such things as drive tests because it changes gradually as the signal quality gets worse.  That can help you determine the type of fade margin you need to build into your link budget.  Then, depending on the fade margin you need for your environment, you can then determine how much signal you need to stay away from the danger point.

 

On another point, I note that the UDR56K datasheet says the modulation types are FSK and GMSK.  I also know that the digital radio community is moving to PSK as a more reliable way to transmit higher bandwidths within the same spectrum.  I’m not a modulation engineer, but as I understand it, it’s evidently possible to switch more quickly and accurately between phases than it is to switch between frequencies, making for less bit/symbol errors.  I wonder if PSK, QPSK, etc. emission types are even allowed by our FCC Part 97 rules which govern ham radio.  Hmm… something to check.

 

Michael

N6MEF



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