/usr/share/octave/packages/communications-1.1.1/prbs_sequence.m is in octave-communications-common 1.1.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 | ## 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
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