% Cassioli Indoor UWB Channel Model
% Jasurbek Khodjaev - MCL -2006
function G_i = uwb_channel(d)
% d - Transmitter- Receiver seperation in meters
time_t = time_t * 1e9;
sample_t = round(time_t / 0.05);
delay_t = zeros(1,sample_t);
%================= Initialization ===================
N_bins = 100; % NUmber of Bins

epsulon = normrnd(16.1, 1.27);  % Decay constant
epsulon = 10^(epsulon/10);
r = normrnd(-4,3); % Power ratio
r = 10^(r/10);
delta_tau = 100;  % Bin width

for ii = 1:N_bins
    tau(ii) = (ii-1) * delta_tau + time_t;
end

%----------------------------------------------------
%=============== Path-Loss & Shadowing ==============

% Path-Loss is expressed in dB, d0 = 1 m is the 
% reference distance
if d <= 11
    path_loss = 20.4 * log10(d/1); 
else
    path_loss = -56 + 74 * log10(d/1);
end


%================ Exponential decay =================
F = 1/(1 - exp( - delta_tau / epsulon));

% Total Average Normilized Energy: G_nor_tot
% Shadowing
% path_loss = 10^(path_loss/10);
G_avr_tot = normrnd(-path_loss, 4.3);
G_avr_tot = 10^(G_avr_tot/10);
G_avr(1) = (G_avr_tot/(1 + r * F));
% G_avr(1) = (G_nor_tot/(1+r*F))*r*exp(-(tau(k)-tau(2))/epsulon)
% G_avr(1) = 10^(G_avr(1)/10);

for k =2:N_bins
    tt = tau(k) - tau(2);
    part2(k-1) = r * exp(-tt/epsulon);
    G_avr(k) = G_avr(1) * part2(k-1);
end

% G_avr = G_avr/norm(G_avr);
% G_avr = (abs(G_avr)./10);%.* (binornd(1,0.5, 1, length(G_avr)) * 2-1);


for k = 1:N_bins
    mu(k) = 3.5 - tau(k)/73;
    sigma(k) = 1.84 - tau(k)/160;
    m(k) = sqrt(sigma(k)) * randn + mu(k);
    if m(k) < 0.5
    	m(k) = 0;
    end
    G_i(1,k) = gamrnd(m(k),G_avr(k));
end

% G_i = [xx G_i(1:end-length(xx))];
G_i = [delay_t G_i(1:end-length(delay_t))];
% G_i = G_i .* (binornd(1,0.5, 1, length(G_i)) .* 2 - 1);
%====================== END of FUNCTION ============================