Q.1: Consider the transmission of a sinusoid x(t) = cos(2f0t) through a channel a ected
by multipath and Doppler. Let there be two paths, and assume the sinusoid is being sent
from a moving transmitter so that a Doppler frequency shift occurs. Let the received signal
be
r(t) = 0cos(2(f0 ?? v)(t ?? L0=c)) + 1cos(2(f0 ?? v)(t ?? L1=c))
where 0 i 1 are attenuations, Li are the distances from the transmitter to the receiver
that the signal travels in the ith path i = 1,2, c = 3 108 m/sec, and the frequency shift
v is caused by the Doppler e ect.
(a) Let f0 = 2 KHz, v = 50 Hz, 0 =1, 1 = 0.9 and L0 = 10,000 meters. What would
be L1 if the two sinusoids have a phase di erence of =2 ?
(b) Is the received signal r(t), with the parameters given above but L1 = 10,000, periodic?
If so, what would be its period and how much does it di er from the period of the
original sinusoid? If x(t) is the input and r(t) the output of the transmission channel,
considered a system, is it linear and time invariant? Explain.
(c) Sample the signals x(t) and r(t) using a sampling frequency Fs = 10 KHz. Plot the
sampled sent x(nTs) and received r(nTs) signals for n = 0 to 2000.
(d) Consider the situation where f0 = 2 KHz, but the parameters of the paths are random,
trying to simulate real situations where these parameters are unpredictable, although
somewhat related. Let
r(t) = 0cos(2(f0 ?? v)(t ?? L0=c)) + 1cos(2(f0 ?? v)(t ?? L1=c))
where v = 50 HZ, L0 = 1,000, L1 = 10,000, 0 = 1 - , 1 = 0/10 and is a
random number between 0 and 1 with equal probability of being any of these values
1
(this can be realized by using the rand MATLAB function). Generate the received
signal for 10 di erent events, use Fs = 10,000 Hz as the sampling rate, and plot them
together to observe the e ects of the multipath and Doppler.