

Phil Karn karn at philkarn.net
Wed Aug 17 19:13:33 PDT 2011

On 8/17/11 4:24 PM, Alan Cresswell wrote:

> That's interesting.  I have collected all my passes on the TS2000 with the
> AGC on and set to the longest setting.  This is mainly because I often
> record the signal level every 0.5 seconds during a pass which requires the
> AGC to be on and the longest setting irons out any short fades.

The main thing is that the gain remain constant, or nearly so, during
each fade so that the random series of 0's and 1's produced during the
fade on thermal noise is mostly seen as 'weak' 0's and 'weak' 1's.
Increasing the gain during a fade makes those random bits seem stronger
and more certain when they're still just random bits. That can make it
harder for the Viterbi decoder to correct them as errors.

A Viterbi decoder works much like a network routing algorithm that looks
for the cheapest path to a destination. It finds the best path through a
'trellis', a pattern of links corresponding to all possible state
transitions in the convolutional encoder that produced the transmitted
signal. Out of every possible path the decoder finds the one that most
closely matches the received sequence and declares it as the one that
was most likely sent. It can still be wrong, and when it does it usually
emits a burst of several dozen errors until the decoder gets back on the
right path. In BPSK-1000, this error burst causes the HDLC decoder to
abort the current frame or discard it with a CRC error.

The Viterbi decoder tallies up the 'cost' of each link in a path to find
its total 'path cost'. If a particular link assumes that a '0' was sent,
then receiving a strong '0' is the best possible match so that results
in the lowest possible cost for that link. A weak '0' gives a greater
cost, a weak '1' an even greater cost, and a strong '1' gives the
highest possible cost.

Even if one link in a path has a high cost, the complete path will still
be chosen as the winner if all the other paths are worse. When this
happens, typically all the other links in the path will closely match
the received sequence.

So you really want to avoid classifying bits as 'strong' when you know
the signal is gone and the bits can't possibly be right. That means not
boosting the gain (and the noise level) during a fade. The noise level
should be kept constant.


More information about the AMSAT-BB mailing list