XPost: comp.dsp, sci.crypt, sci.electronics.design   
   From: spope33@speedymail.org   
      
   Eric Jacobsen   wrote:   
      
   >On 12/20/2009 10:32 AM, Steve Pope wrote:   
      
   >> On Dec 20, 2:36 am, Eric Jacobsen  wrote:   
      
   >>> The point was really that even from an advanced FEC standpoint an input   
   >>> BER of 1 in 10 isn't practical to work with for the described   
   >>> application.   Yielding half the bandwidth to FEC overhead is actually   
   >>> practical, and using R = 1/2 coding over satellite channels is quite   
   >>> common.   Using something like an R = 1/6 capacity-approaching code to   
   >>> be able to handle such low input error rates is, I think, not practical.   
      
   >> Last I checked, such a channel is within the operating range of a rate   
   >> 1/3 binary convolutional code...   
      
   >BICO capacity for R = 1/2 is at about 0.177dB Eb/No, and for R = 1/3   
   >it's about -0.357dB Eb/No.   
      
   Agree   
      
   >Not a lot of difference there from a   
   >capacity perspective.   
      
   Correct; as one gets into lower code rates, going to even lower rates   
   tends to give you no additional normalized coding gain (i.e. the   
   additional coding performs no better than a repetition code).   
      
   >For uncoded, i.e., raw BER of 1e-1 happens at about -1dB Eb/No   
      
   Actually it's about -1.7 dB, but let's say it's -1 for the   
   sake of argument.   
      
   (or about   
   -4dB (equivalent raw Eb/No) at R = 1/2, and -5.77dB (equivalent raw   
   Eb/No) for R = 1/3.   
      
   I'm not sure what this sentence means.   
      
   A rate 1/2 coded system operating at an Eb/No of +2 dB has the   
   same raw BER as an uncoded system operating at an Eb/No of -1 dB.   
      
   A rate 1/3 coded system operating at an Eb/No of +3.77 dB has   
   the same raw BER as an uncoded system operating at -1 dB.   
      
   (Unless I'm confused, which has happened before...)   
      
   >So I'm skeptical.  In a practical system, especially a practical   
   >satellite system, I think it'd be very difficult to operate with an   
   >input BER of 1e-1.   
      
   I'm not too skeptical.  I would posit that GSM phones in their   
   basic 2G mode operate under conditions this bad, and 802.11 systems   
   at 1 mbps might also.   
      
   Steve   
      
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