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We modified the code in wl_example_siso_ofdm_txrx.m as given below
1) transmitter side
% Insert the data and pilot values; other subcarriers will remain at 0
ifft_in_mat(SC_IND_DATA, :) = tx_syms_mat;
ifft_in_mat(SC_IND_PILOTS, :) = pilots_mat;
totalSubcarriers = 256 ;% M.
numSymbols = N_SC;% N.
Q = totalSubcarriers/numSymbols;
subcarrierMapping = 'LFDMA';% Subcarrier mapping scheme.
X=fft(ifft_in_mat, N_SC, 1);
Y = zeros(totalSubcarriers,N_OFDM_SYMS);
%%%%%%%%%%%%%%%% Mapping%%%%%%%%%%%%%%%%.
if subcarrierMapping == 'IFDMA'
Y(1:Q:totalSubcarriers,:) = X;
elseif subcarrierMapping == 'LFDMA'
Y(1:numSymbols,:) = X;
end
M=totalSubcarriers;
%Perform the IFFT
tx_payload_mat = ifft(Y, M, 1);
2) at receiver side
% Take the FFT
syms = fft(payload_mat_noCP, M, 1);
% Equalize (zero-forcing, just divide by complex chan estimates)
syms_f_mat = syms(1:N_SC,:) ./ repmat(rx_H_est.', 1, N_OFDM_SYMS);
if subcarrierMapping == 'IFDMA'
Demap = syms_f_mat(1:Q:M,:);
elseif subcarrierMapping == 'LFDMA'
Demap = syms_f_mat(1:N_SC,:);
end
syms_eq_mat = ifft(Demap);
The problem is that rx_H_est value is 0 at indices 1, 28 to 38. In case of OFDM as there is no IFFT block after FDE there is no problem but in SC-FDMA as we need to perform IFFT after FDE the infinity values in syms_f_mat, Demap cause NaN values in syms_eq_mat. Can anybody help me with this problem?
Last edited by shriramtej (2018-Aug-23 02:01:03)
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The problem is that rx_H_est value is 0 at indices 1, 28 to 38
The WARPLab OFDM example code uses a preamble similar to 802.11a. This preamble only defines non-zero training symbols in subcarriers which carry data/pilot symbols. The DC subcarrier (MATLAB index 1) and the higher-frequency subcarriers (indexes 28:38) are all zero in the training symbols and payload. If you modify the payload to use these subcarriers you must also modify the training symbols to have non-zero values so your Rx code can estimate the corresponding channel coefficients.
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If you modify the payload to use these subcarriers you must also modify the training symbols to have non-zero values so your Rx code can estimate the corresponding channel coefficients.
Thank you for the reply. I did modified lts_f values so that it has all non zero values like
lts_f = [1 1 -1 -1 1 1 -1 1 -1 1 -1 -1 -1 -1 -1 1 1 -1 -1 1 -1 1 -1 1 1 1 (1) 1 1 -1 -1 1 1 -1 1 -1 1 -1 (1) 1 -1 -1 1 1 -1 1 -1 1 1 1 1 1 1 -1 -1 1 1 -1 1 -1 1 1 1 1];
but the results are very bad. Is there any condition for lts_f to satisfy while modifying it?
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The DC (zero frequency) term should be 0. Does your signal actually occupy all 64 subcarriers? This would be unusual- the higher subcarriers are typically reserved to allow better spectral mask compliance.
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Actually we need more subcarriers than 64. As shown in the image if N=64 then M would be greater than 64 say 256. As the code is for 64 subcarriers lets assume N=16 and M=64. So in this case we need 64 subcarriers
https://en.wikipedia.org/wiki/File:Scfdma.jpg
Last edited by shriramtej (2018-Aug-28 04:48:35)
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I'm not familiar with SC-FMDA. In general you must provide training symbols with the same frequency content as the data symbols. In OFDM this means using a training symbol (LTS in 802.11) with known values in the same subcarriers as the data/pilot symbols. You will need to build something similar for your custom waveform.
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