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Re: [Discuss-gnuradio] Fwd: Introducing noise/ considerable BER

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Fwd: Introducing noise/ considerable BER
Date: Tue, 09 Aug 2011 23:54:02 -0400
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On 08/09/2011 11:12 PM, shantharam balasubramanian wrote:
> Hello Tom, Marcus and Martin,
> Thanks a lot for your replies. One of the objective of our experiment
> is find to the capacity of a path, i.e, we want to transmit a random
> binary sequence between two USRP nodes. We want to find the maximum
> data transfer rate (in bit/sec) with a "small" bit error rate /
> probability of error.
> Tom said, "The way things are in our benchmark code, a single bit
> error means that an entire packet is lost." Based on this statement, I
> have the following questions:
> 1. From Tom's reply, it seems like the benchmark_Rx and benchmark_Tx
> programs already have error correction coding blocks in themselves.
> Therefore, it is hard for us to calculate the bit error rate and find
> the capacity of the path. Is there any program in gnuradio repository
> that does not employ error correction codes?
The benchmark stuff doesn't, as far as I know, have FEC in it.

> 2. In our experiments, we have received packets that has some bit
> error rate (e.g. 15% bit error rate, 20% bit error rate, etc.). If a
> single bit error means that an entire packet is lost, do you know why
> that phenomenon occurs? (i.e. some packets get lost but some other
> packets have small erroneous bits)
> Your feedback will be very appreciated.
> Thanks
I think Tom was referring to single-bit errors in the so-called "access
code" at the beginning of the
  frame.  If there are bit errors there, then the frame is necessarily
discarded, since it's not
  *recognized* as a valid frame.

But apart from that, if the frame sequence is valid, then the packet is
"recognized", and punted up
  through the RX packet callback mechanism, which means there could
still be bit errors in the
  payload section of the packet.  In the "real world" this would still
cause the packet to be discarded
  entirely, because the FCS (CRC-32, usually) at the end of the packet
wouldn't validate, and somewhere
  between the PHY and MAC layer, it would get discarded.

This is all from my having spent 10 minutes looking at the code--I don't
have as deep an understanding
  of it as Tom.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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