tx_fre_to_time.asv

OFDM的在瑞利信道下的与单载波信号的识别算法

function time_sig = tx_fre_to_time(fre_sym,N_SPF,pilot_sym,pilot_num)

% Convert frequency symbols into time signals by IFFT
% The IFFT points is 64, oversampling is adopted, and 4 pilot symbols are inserted

% Ralted paramters are as following:
% Number of data subcarriers: Nsd = 48
% Number of pilot subcarriers: Nsp = 4
% Number of total subcarriers: Nst = 52
% Subcarrier frequency spacing: fs = 0.3125MHz ( = 20MHz/64)
% Total bandwidth: ft = fs * Nst = 52 * 0.3125MHz = 16.25MHz
% IFFT/FFT period: T_FFT = 1/fs =1/0.3125M = 3.2us
% Guard interval: T_GI = T_FFT/4 = 3.2us/4 = 0.8us
% OFDM symbol period: T_sym = T_FFT + T_GI = 3.2+0.8 = 4us
% IFFT/FFT sampling point: 64, over sampling
% Sampling rate: T_FFT/64 = 0.05us

Nst = 52;                                     % Number of total subcarriers
Nsd = 48;                                     % Number of data subcarriers
OFDM_sym = reshape(fre_sym, Nsd, N_SPF);           % Reshape the modulated symbols into OFDM symbols
OFDM_sym = OFDM_sym.';
time_sig = zeros(64, N_SPF);

for m = 1: N_SPF 
    temp_sym = OFDM_sym(m,:);% Modulate symbols to be allocated to the mth OFDM symbol of current frame  
    temp_pilot = pilot_sym(pilot_num,:);      % Pilot symbols to be allocated to the mth OFDM symbol of current frame
    
    % Allocate the subcarrier frequency according to IEEE802.11a stand. P23,subclause (17.3.5.9)
    sym_minus_fre = [zeros(1,6),temp_sym(1:5),temp_pilot(1),temp_sym(6:18),temp_pilot(2),temp_sym(19:24)];  % Allocate symbol to the minus frequency band 
    sym_plus_fre = [0,temp_sym(25:30),temp_pilot(3),temp_sym(31:43),temp_pilot(4),temp_sym(44:48),zeros(1,5)]; % Allocate symbol to the plus frequency band 
    sym_IFFT = [sym_plus_fre,sym_minus_fre];  % Total IFFT symbols                          
    temp = ifft(sym_IFFT)*sqrt(64);
    temp = temp.';
    time_sig(:,m) = temp;  % IFFT, convert frequency symbols to time signals
    pilot_num = mod(pilot_num,128) + 1;                % Pilot symbol index + 1 
end

cyc_sig = time_sig(49:64,:);                  % Cyclic prefix interval: T_GI = T_FFT/4 
signal_total = [cyc_sig;time_sig];            % Add cyclic prefix to the time signal
time_sig=signal_total(:).';                   % Parallel to Serial