function y = uwb_pulse(x, tau_m)
% The received UWB pulse w_rec (t) as defined in
%   Moe Z. Win and Robert A. Scholtz
%   "Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum
%   Impulse Radio for Wireless Multiple-Access Communications,"
%   IEEE Trans. Commun. vol 48, No. 4, April 2000, pp. 679--691
% where tau_m controls the width of the uwb pulse.  
% eg. tau_m = 0.2877 in the above paper;
T_arg = (x / tau_m).^2;
y = (1 - 4*pi* T_arg ) .* exp(-2*pi* T_arg);