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function cap = CapacityTableLookup( EsNo_AWGN, capacity_AWGN, slope, gamma )
% CapacityTableLookup returns the instantaneous capacity for a particular
% channel realization by using a precalculated AWGN capacity table.
% Since the table does not generally contain capacity values for the exact
% SNR, linear interpolation is used to estimate the value.
%
% The calling syntax is:
% cap = CapacityTableLoookup( EsNo_AWGN, capacity_AWGN, slope, gamma )
%
% cap = The instantaneous capacity, a column vector with same number of
% rows as gamma.
%
% EsNo_AWGN = The SNR values stored in the table
% capacity_AWGN = The AWGN capacity values stored in the table.
% slope = The slope of the AWGN capacity curve.
% gamma = The instantaneous SNRs for the channel realization.
% Each row correpsonds to a distinct codeword,
% And the columns correspond to the blocks of that codeword.
%
% Copyright (C) 2005, Tarik Ghanim and Matthew C. Valenti
%
% Last updated on Dec. 23, 2005
%
% Function CapacityTableLookup is part of the Iterative Solutions Coded Modulation
% Library (ISCML).
%
% The Iterative Solutions Coded Modulation Library is free software;
% you can redistribute it and/or modify it under the terms of
% the GNU Lesser General Public License as published by the
% Free Software Foundation; either version 2.1 of the License,
% or (at your option) any later version.
%
% This library 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
% Lesser General Public License for more details.
%
% You should have received a copy of the GNU Lesser General Public
% License along with this library; if not, write to the Free Software
% Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
% L is the number of blocks per codeword,
% while trial_size is the number of codewords per trial
[trial_size, L] = size( gamma );
max_SNR = max( EsNo_AWGN );
min_SNR = min( EsNo_AWGN );
max_cap = max( capacity_AWGN );
cap = zeros(size(gamma,1),1);
for codeword=1:trial_size
for block=1:L
if ( gamma(codeword,block) >= max_SNR )
% SNR above smallest value in table; use largest capacity
inst_cap(block) = max_cap;
elseif ( gamma(codeword,block) > min_SNR )
% Need to interpolate
index_below = max( find( EsNo_AWGN < gamma(codeword,block) ) );
correction = slope(index_below)*(gamma(codeword,block)-EsNo_AWGN(index_below));
inst_cap(block) = capacity_AWGN(index_below) + correction;
else
% SNR below smallest value in table; assume capacity is zero
inst_cap(block) = 0;
end
% add instantaneous capacities
cap(codeword) = sum( inst_cap );
end
end
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