make_symbols.m

The GSMSP uses the USRP hardware device to receive data from the GSM band. This data is raw and pre

function [ SYMBOLS ] = make_symbols(Lh)
%
% MAKE_SYMBOLS:       
%           This function returns a table containing the mapping 
%           from state numbers to symbols. The table is contained
%           in a matrix, and the layout is:
%
%           -                     -
%           | Symbols for state 1 |
%           | Symbols for state 2 |
%                   :
%                   :
%           | Symbols for state M |
%           -                     -
%
%           Where M is the total number of states, and can be calculated 
%           as: 2^(Lh+1). Lh is the length of the estimated impulse
%           response, as found in the mf-routine. In the symbols for
%           a statenumber the order is as:
%
%           I(n-1) I(n-2) I(n-3) ... I(n-Lh)
%
%           Each of the symbols belong to { 1 , -1 , j , -j }.
%
% SYNTAX:   [SYMBOLS] = make_symbols(Lh)
%
% INPUT:    Lh:      Length of the estimated impulse resonse.
%
% OUTPUT:   SYMBOLS: The table of symbols corresponding the the state-
%                    numbers, as described above.
%
% SUB_FUNC: None
%
% WARNINGS: None
%
% TEST(S):  Compared result against expected values.
%
% AUTOR:    Jan H. Mikkelsen / Arne Norre Ekstr鴐
% EMAIL:    [e-mail] / [e-mail]
%
% $Id: make_symbols.m,v 1.6 1997/09/22 11:38:57 aneks Exp $

% THIS CODE CANNOT HANDLE Lh=1 or Lh>4.
%
if Lh==1,
  error('GSMsim-Error: Lh is constrained to be in the interval [1:4].');
elseif Lh > 4,
  error('GSMsim-Error: Lh is constrained to be in the interval [1:4].')
end

% make initiating symbols
%
SYMBOLS=[ 1; j; -1 ; -j];
% 					
for n=1:Lh-1
  SYMBOLS=[[ SYMBOLS(:,1)*j , SYMBOLS ] ; [ SYMBOLS(:,1)*(-j) , SYMBOLS  ]];
end
%
% NOW WE NEED TO ASSURE THAT THE STATE RELATED TO THE NUMBER ONE
% IS COMPLEX. THIS IS REQUIRED BY THE IMPLEMENTATION OF THE VITERBI 
% ALGORITHM.
%
if isreal(SYMBOLS(1,1)),
  SYMBOLS=flipud(SYMBOLS);
end