Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / communications-1.1.1 / prbs_sequence.m

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+## Copyright (C) 2006 Muthiah Annamalai <muthiah.annamalai@uta.edu>
+##
+## This program is free software; you can redistribute it and/or modify it under
+## the terms of the GNU General Public License as published by the Free Software
+## Foundation; either version 3 of the License, or (at your option) any later
+## version.
+##
+## This program is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+## details.
+##
+## You should have received a copy of the GNU General Public License along with
+## this program; if not, see <http://www.gnu.org/licenses/>.
+
+## Implement book keeping for a Pseudo-Random Binary Sequence ( PRBS )
+## also called as a Linear Feedback Shift Register.
+## 
+## For the given PRBS in a intial state, compute the PRBS sequence length.
+## Length is period of output when the PRBS state is same as 
+## the start state of PRBS.
+## 
+## Example: If you had a PRBS shift register like the diagram
+## below with 4 registers we use representation by polynomial
+## of [ 1 2 3 4], and feedback connections between [ 1 3 4 ].
+## The output PRBS sequence is taken from the position 4.
+## 
+##  +---+    +----+   +---+   +---+
+##  | D |----| D  |---| D |---| D |
+##  +---+    +----+   +---+   +---+
+##    |                 |       |
+##    \                 /      /
+##    [+]---------------+------+
+##   1   +    0.D   + 1.D^2 + 1.D^3
+##
+## The code to implement this PRBS will be 
+## prbs=prbs_generator([1 3 4],{[1 3 4]},[1 0 1 1]);
+## x = prbs_sequence(prbs) #gives 15
+## 
+##
+## See Also: This function is to be used along with functions 
+## prbs_generator.
+
+function [itrs,seq]=prbs_sequence(prbs)
+  if nargin != 1
+    print_usage;
+  endif
+  nstate=zeros(1,prbs.reglen);
+  itrs=0; seq = [];
+  inits = prbs.sregs;
+  
+  ## For each iteration, shift the output bit. Then compute the xor pattern of connections. 
+  ## Finally apply feedback the stuff. Insert the computed pattern.
+  while( true )
+    itrs = itrs + 1;
+    
+    ## compute the feedback.
+    for itr2=1:prbs.conlen
+      val=0;
+      L=length(prbs.connections{itr2});
+      for itr3=2:L
+        val=bitxor(val,prbs.sregs(prbs.connections{itr2}(itr3)));
+      endfor
+      nstate(prbs.connections{itr2}(1))=val;
+    endfor
+    
+    ## rotate the output discarding the last output.
+    seq = [seq, prbs.sregs(end)];
+    prbs.sregs=[0 prbs.sregs(1:prbs.reglen-1)];
+
+    ## insert the feedback.
+    for itr2=1:prbs.conlen
+      prbs.sregs(itr2)=nstate(itr2);
+      nstate(itr2)=0; # reset.
+    endfor
+    
+    if(isequal(prbs.sregs,inits))
+      break
+    endif
+  endwhile
+end