Options

• Definition:
  The options allow you to change the way you view the current simulation. Changes to options take effect immediately, and also carry over to subsequent simulations. There are four different options all of which are independent of each other. To open the options dialog box press the "Options" button in the Setup Panel.
  • The Normalize Path Measures option lets you choose whether to normalize the Path Measures of the states on a "per trellis section" basis.
    
    • If the Channel Model is BSC then the path measures are probabilities. Normalizing these corresponds to scaling the values so that the states' path measures sum to 1. This scaling does not affect the Viterbi algorithm at all. It is useful to normalize the path measures because otherwise the values will get very small and eventually could cause number underflow if the trellis is long enough.
      
    • If the Channel Model is AWGN then the path measures are the negative logarithms of probabilities. In this case, values are added to find new path measures. So, to normalize the states in a trellis section we can subtract out the smallest path measure from all the states in the section. This does not affect the Viterbi algorithm at all. This is useful since the path measures can get pretty large after a while.
      
  • The Show Dead Paths option lets you choose whether to prune out potential paths as they die in states with no surviving outgoing branches. If this option is selected, these paths will not be pruned and the Long Trellis Diagram will show all surviving branches. The trace back will then show how the paths converge amongst this web of surviving branches.
    With this option off you can interactively see how the selection of winning branches in the forward direction affects the convergence of the paths.
    
  • The Show Trace Convergence option when selected makes the algorithm trace back all possible paths from the end of the block. This is good for showing where the paths converge(if at all). It should be noted that the path that is used in case of an ambiguity is the same one that is used when this option is not selected. When the Decode Style is "Tailing" this option has no effect since the only path that ever need be traced back is the one starting at the zero state.
    
  • The Shift Speed option refers to how quickly the trellis diagrams draw the animation of trellis sections and blocks shifting to the right or left. If you have a slow machine you may consider switching the shift speed to "NONE". This will stop the diagrams for animating the shift. If, however, you have a very fast machine, you may want to consider slowing the shift speed down to get a better look at the animation.

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