probability & random process problems
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Probability & Random Process ProblemsTRANSCRIPT
Unit – I (Random Variables)
• Problems on Discrete & Continuous R.Vs
1) A random variable X has the following probability function:
X 0 1 2 3 4 5 6 7
P(X) 0 K 2K 2K 3K K2
2K2
7K2 + K
a) Find K .
b) Evaluate (((( )))) (((( ))))6 , 6P X P X< ≥< ≥< ≥< ≥ .
c) Find (((( )))) (((( )))) (((( ))))2 , 3 , 1 5P X P X P X< > < << > < << > < << > < < .
d) If (((( )))) 12
P X C≥ >≥ >≥ >≥ > , find the minimum value of C .
e) (((( ))))1.5 4.5 / 2P X X< < >< < >< < >< < >
2) The probability function of an infinite discrete distribution is given by
(((( )))) 1, 1,2,3...
2 jP X j j= = = ∞= = = ∞= = = ∞= = = ∞ . Find the mean and variance of the distribution.
Also find (((( ))))X is evenP , (((( ))))5P X ≥≥≥≥ and (((( ))))X is divisible by 3P .
3) Suppose that X is a continuous random variable whose probability density function is
given by (((( ))))24 2 , 0 2
( )0, otherwise
C x x xf x
− < <− < <− < <− < <====
(a) find C (b) find (((( ))))1P X >>>> .
4) A continuous random variable X has the density function
2( ) , 1
Kf x x
x= − ∞ < < ∞= − ∞ < < ∞= − ∞ < < ∞= − ∞ < < ∞
++++. Find the value of K ,the distribution function and
(((( ))))0P X ≥≥≥≥ .
Probability & Random Process
Problems
5) A random variable X has the p.d.f 2 , 0 1
( )0, otherwise
x xf x
< << << << <====
. Find (i) 12
P X >>>>
(ii)
1 32 4
P X < << << << <
(iii)3 1
/4 2
P X X > >> >> >> >
(iv) 3 1
/4 2
P X X < >< >< >< >
.
6) If a random variable X has the p.d.f
1, 2
( ) 40, otherwise
xf x
<<<<====
. Find (a) (((( ))))1P X <<<<
(b) (((( ))))1P X >>>> (c) (((( ))))2 3 5P X + >+ >+ >+ >
7) The amount of time, in hours that a computer functions before breaking down is a
continuous random variable with probability density function given by
100, 0( )0, 0
x
e xf xx
λλλλ−−−− ≥≥≥≥====
<<<<
. What is the probability that (a) a computer will function
between 50 and 150 hrs. before breaking down (b) it will function less than 500 hrs.
8) A random variable X has the probability density function
, 0( )
0, otherwise
xxe xf x
λλλλ −−−− >>>>====
. Find (((( )))) (((( )))), . . , 2 5 , 7c d f P X P Xλλλλ < < ≥< < ≥< < ≥< < ≥ .
9) If the random variable X takes the values 1,2,3 and 4 such that
(((( )))) (((( )))) (((( )))) (((( ))))2 1 3 2 3 5 4P X P X P X P X= = = = = = == = = = = = == = = = = = == = = = = = = . Find the probability
distribution.
10) The distribution function of a random variable X is given by
(((( ))))( ) 1 1 ; 0xF x x e x−−−−= − + ≥= − + ≥= − + ≥= − + ≥ . Find the density function, mean and variance of X.
11) A continuous random variable X has the distribution function
4
0, 1
( ) ( 1) , 1 3
0, 30
x
F x k x x
x
≤≤≤≤= − < ≤= − < ≤= − < ≤= − < ≤ >>>>
. Find k , probability density function ( )f x , (((( ))))2P X <<<< .
12) A test engineer discovered that the cumulative distribution function of the lifetime
of an equipment in years is given by51 , 0( )
0, 0
x
e xF xx
−−−− − ≥− ≥− ≥− ≥==== <<<<
.
i) What is the expected life time of the equipment?
ii) What is the variance of the life time of the equipment?
• Moments and Moment Generating Function
1) Find the moment generating function of R.V X whose probability function
1( ) , 1,2, ...
2xP X x x= = == = == = == = = Hence find its mean and variance.
2) The density function of random variable X is given by ( ) (2 ), 0 2f x Kx x x= − ≤ ≤= − ≤ ≤= − ≤ ≤= − ≤ ≤ .
Find K, mean, variance and rth moment.
3) Let X be a R.V. with p.d.f31
, 0( ) 3
0, Otherwise
x
e xf x
−−−−>>>>====
. Find the following
a) P(X > 3).
b) Moment generating function of X.
c) E(X) and Var(X).
4) Find the MGF of a R.V. X having the density function, 0 2
( ) 20, otherwise
xx
f x ≤ ≤≤ ≤≤ ≤≤ ≤====
. Using
the generating function find the first four moments about the origin.
5) Define Binomial distribution and find the M.G.F, Mean and Variance of the Binomial
distribution.
6) Define Poisson distribution and find the M.G.F, Mean and Variance of the Poisson
distribution.
7) Define Geometric distribution and find the M.G.F, Mean and Variance of the
Geometric distribution.
8) Write the pdf of Uniform distribution and find the M.G.F, Mean and Variance.
9) Define Exponential distribution and find the M.G.F, Mean and Variance of the
Exponential distribution.
10) Define Gamma distribution and find the M.G.F, Mean and Variance of the Gamma
distribution.
11) Define Normal distribution and find the M.G.F, Mean and Variance of the Normal
distribution.
• Problems on distributions
1) The mean of a Binomial distribution is 20 and standard deviation is 4. Determine the
parameters of the distribution.
2) If 10% of the screws produced by an automatic machine are defective, find the
probability that of 20 screws selected at random, there are (i) exactly two defectives
(ii) atmost three defectives (iii) atleast two defectives and (iv) between one and
three defectives (inclusive).
3) In a certain factory furning razar blades there is a small chance of 1/500 for any
blade to be defective. The blades are in packets of 10. Use Poisson distribution to
calculate the approximate number of packets containing (i) no defective (ii) one
defective (iii) two defective blades respectively in a consignment of 10,000 packets.
4) The number of monthly breakdown of a computer is a random variable having a
Poisson distribution with mean equally to 1.8. Find the probability that this
computer will function for a month
a) Without a breakdown
b) With only one breakdown and
c) With atleast one breakdown.
5) Prove that the Poisson distribution is a limiting case of binomial distribution.
6) If the mgf of a random variable X is of the form 8(0.4 0.6)te ++++ , what is the mgf of
3 2X ++++ . Evaluate (((( ))))E X .
7) A discrete R.V. X has moment generating function
51 3
( )4 4
tXM t e
= += += += +
. Find
(((( ))))E X , (((( ))))Var X and (((( ))))2P X ==== .
8) If X is a binomially distributed R.V. with ( ) 2E X ==== and 4
( )3
Var X ==== , find [[[[ ]]]]5P X ==== .
9) If X is a Poisson variate such that [[[[ ]]]] [[[[ ]]]] [[[[ ]]]]2 9 4 90 6P X P X P X= = = + == = = + == = = + == = = + = , find the
mean and variance.
10) The number of personal computer (PC) sold daily at a CompuWorld is uniformly
distributed with a minimum of 2000 PC and a maximum of 5000 PC. Find the
following
(i) The probability that daily sales will fall between 2,500 PC and 3,000 PC. (ii) What is the probability that the CompuWorld will sell at least 4,000 PC’s?
(iii) What is the probability that the CompuWorld will exactly sell 2,500 PC’s?
11) Suppose that a trainee soldier shoots a target in an independent fashion. If the probability that the target is shot on any one shot is 0.8. (i) What is the probability
that the target would be hit on 6th attempt? (ii) What is the probability that it takes
him less than 5 shots? (iii) What is the probability that it takes him an even number
of shots?
12) A die is cast until 6 appears. What is the probability that it must be cast more than 5
times?
13) The length of time (in minutes) that a certain lady speaks on the telephone is found
to be random phenomenon, with a probability function specified by the function.
5 , 0( )0, otherwise
x
Ae xf x
−−−− ≥≥≥≥====
. (i) Find the value of A that makes f(x) a probability
density function. (ii) What is the probability that the number of minutes that she will
talk over the phone is (a) more than 10 minutes (b) less than 5 minutes and (c)
between 5 and 10 minutes.
14) If the number of kilometers that a car can run before its battery wears out is
exponentially distributed with an average value of 10,000 km and if the owner
desires to take a 5000 km trip, what is the probability that he will be able to
complete his trip without having to replace the car battery? Assume that the car has
been used for same time.
15) The mileage which car owners get with a certain kind of radial tyre is a random
variable having an exponential distribution with mean 40,000 km. Find the
probabilities that one of these tyres will last (i) atleast 20,000 km and (ii) atmost
30,000 km.
16) If a continuous random variable X follows uniform distribution in the interval (((( ))))0,2
and a continuous random variable Y follows exponential distribution with
parameter αααα , find αααα such that (((( )))) (((( ))))1 1P X P Y< = << = << = << = < .
17) If X is exponantially distributed with parameter λλλλ , find the value of K there exists
(((( ))))(((( ))))
P X ka
P X k
>>>>====
≤≤≤≤.
18) State and prove memoryless property of Geometric distribution.
19) State and prove memoryless property of Exponential distribution.
20) The time required to repair a machine is exponentially distributed with parameter ½.
What is the probability that the repair times exceeds 2 hours and also find what is
the conditional probability that a repair takes at least 10 hours given that its
duration exceeds 9 hours?
21) The weekly wages of 1000 workmen are normall distributed around a mean of Rs. 70
with a S.D. of Rs. 5. Estimate the number of workers whose weekly wages will be (i)
between Rs. 69 and Rs. 72, (ii) less than Rs. 69 and (iii) more than Rs. 72.
22) In a test on 2000 electric bulbs, it was found that the life of a particular make, was
normally distributed with an average life of 2040 hours and S.D. of 60 hours.
Estimate the number of bulbs lilkely to burn for (i) more than 2150 hours, (ii) less
than 1950 hours and (iii) more than 1920 hours but less than 2160 hours.
• Function of random variable
1) Let X be a continuous random variable with p.d.f, 1 5
( ) 120, otherwise
xx
f x < << << << <====
, find the
probability density function of 2X – 3.
2) If X is a uniformly distributed RV in ,2 2π ππ ππ ππ π−−−−
, find the pdf of tanY X==== .
3) If X has an exponential distribution with parameter 1, find the pdf of Y X==== .
4) If X is uniformly distributed in (((( ))))1,1−−−− , find the pdf of sin2X
Yππππ ====
.
5) If the pdf of X is ( ) , 0xf x e x−−−−= >= >= >= > , find the pdf of 2Y X==== .
6) If X is uniformly distributed in (((( ))))0,1 find the pdf of 1
2 1Y
X====
++++.
Unit – II (Two Dimensional Random Variables)
• Joint distributions – Marginal & Conditional
1) The two dimensional random variable (X,Y) has the joint density function
2( , ) , 0,1,2; 0,1,2
27x y
f x y x y++++= = == = == = == = = . Find the marginal distribution of X and Y
and the conditional distribution of Y given X = x. Also find the conditional
distribution of X given Y = 1.
2) The joint probability mass function of (X,Y) is given by
(((( ))))( , ) 2 3 , 0,1,2; 1,2,3P x y K x y x y= + = == + = == + = == + = = . Find all the marginal and conditional
probability distributions. Also find the probability distribution of X Y++++ and
(((( ))))3P X Y+ >+ >+ >+ > .
3) If the joint pdf of a two dimensional random variable (X,Y) is given by
(6 ) ,0 2, 2 4( , )
0 ,otherwise
K x y x yf x y
− − < < < <− − < < < <− − < < < <− − < < < <====
. Find the following (i) the value of K;
(ii) (((( ))))1, 3P x y< << << << < ; (iii) (((( ))))3P x y+ <+ <+ <+ < ; (iv) (((( ))))1/ 3P x y< << << << <
4) If the joint pdf of a two – dimensional random variable (X,Y) is given by
2 ,0 1, 0 2( , ) 3
0 ,otherwise
xyx x y
f x y + < < < <+ < < < <+ < < < <+ < < < <====
. Find (i) 12
P X >>>>
; (ii) (((( ))))P Y X<<<< ; (iii)
1 1/
2 2P Y X < << << << <
. Check whether the conditional density functions are valid.
5) The joint p.d.f of the random variable (X,Y) is given by
(((( ))))2 2
( , ) , 0 ,x y
f x y Kxye x y− +− +− +− += < < ∞= < < ∞= < < ∞= < < ∞ . Find the value of K and Prove that X and Y
are independent.
6) If the joint distribution function of X and Y is given by
(((( )))) (((( ))))( , ) 1 1 , 0, 0x yF x y e e x y− −− −− −− −= − − > >= − − > >= − − > >= − − > > and "0" otherwise . (i) Are X and Y
independent? (ii) Find (((( ))))1 3, 1 2P X Y< < < << < < << < < << < < < .
• Covariance, Correlation and Regression
1) Define correlation and explain varies type with example.
2) Find the coefficient of correlation between industrial production and export using the following data:
Production (X) 55 56 58 59 60 60 62
Export (Y) 35 38 37 39 44 43 44
3) Let X and Y be discrete random variables with probability function
( , ) , 1,2,3; 1,221
x yf x y x y
++++= = == = == = == = = . Find (i) (((( )))),Cov X Y (ii) Correlation co –
efficient.
4) Two random variables X and Y have the following joint probability density function.
2 , 0 1, 0 1( , )
0, otherwise
x y x yf x y
− − ≤ ≤ ≤ ≤− − ≤ ≤ ≤ ≤− − ≤ ≤ ≤ ≤− − ≤ ≤ ≤ ≤====
. Find (((( ))))Var X , (((( ))))Var Y and the
covariance between X and Y. Also find Correlation between X and Y. ( ( , )X Yρρρρ ).
5) Let X and Y be random variables having joint density function.
(((( ))))2 23, 0 , 1
( , ) 20, otherwise
x y x yf x y
+ ≤ ≤+ ≤ ≤+ ≤ ≤+ ≤ ≤====
. Find the correlation coefficient ( , )X Yρρρρ .
6) The independent variables X and Y have the probability density functions given by
4 , 0 1( )
0, otherwiseX
ax xf x
≤ ≤≤ ≤≤ ≤≤ ≤====
4 , 0 1
( )0, otherwiseY
by yf y
≤ ≤≤ ≤≤ ≤≤ ≤====
. Find the correlation
coefficient between X and Y .
(or)
The independent variables X and Y have the probability density functions given by
4 , 0 1( )
0, otherwiseX
ax xf x
≤ ≤≤ ≤≤ ≤≤ ≤====
4 , 0 1
( )0, otherwiseY
by yf y
≤ ≤≤ ≤≤ ≤≤ ≤====
. Find the correlation
coefficient between X Y++++ and X Y−−−− .
7) Let X,Y and Z be uncorrelated random variables with zero means and standard
deviations 5, 12 and 9 respectively. If U X Y= += += += + and V Y Z= += += += + , find the
correlation coefficient between U and V .
8) If the independent random variables X and Y have the variances 36 and 16
respectively, find the correlation coefficient between X Y++++ and X Y−−−− .
9) From the data, find
(i) The two regression equations. (ii) The coefficient of correlation between the marks in Economics and
Statistics.
(iii) The most likely marks in statistics when a mark in Economics is 30.
Marks in Economics 25 28 35 32 31 36 29 38 34 32
Marks in Statistics 43 46 49 41 36 32 31 30 33 39
10) The two lines of regression are 8x – 10y + 66 = 0, 40x – 18y – 214 = 0. The variance
of X is 9. Find (i) the mean values of X and Y (ii) correlation coefficient between X
and Y (iii) Variance of Y .
11) The joint p.d.f of a two dimensional random variable is given by
1( , ) ( ); 0 1, 0 2
3f x y x y x y= + ≤ ≤ ≤ ≤= + ≤ ≤ ≤ ≤= + ≤ ≤ ≤ ≤= + ≤ ≤ ≤ ≤ . Find the following
(i) The correlation co – efficient.
(ii) The equation of the two lines of regression (iii) The two regression curves for mean
• Transformation of the random variables
1) If X is a uniformly distributed RV in ,2 2π ππ ππ ππ π−−−−
, find the pdf of tanY X==== .
2) Let (X,Y) be a two – dimensional non – negative continuous random variables having
the joint probability density function (((( ))))2 2
4 , 0, 0( , )0, elsewhere
x yxye x yf x y
− +− +− +− + ≥ ≥≥ ≥≥ ≥≥ ≥====
. Find the
density function of 2 2U X Y= += += += + .
3) X and Y be independent exponential R.Vs. with parameter 1. Find the j.p.d.f of
U X Y= += += += + andX
VX Y
====++++
.
(Or) (The above problem may be ask as follows)
The waiting times X and Y of two customers entering a bank at different times are
assumed to be independent random variables with respective probability density
functions. , 0
( )0, otherwise
xe xf x
−−−− ≥≥≥≥====
and , 0
( )0, otherwise
ye yf y
−−−− ≥≥≥≥====
Find the joined p.d.f of the sum of their waiting times, U X Y= += += += + and the fraction of
this time that the first customer spreads waiting, i.e X
VX Y
====++++
. Find the marginal
p.d.f’s of U and V and show that they are independent.
(Or)
If X and Y are independent random variable with pdf , 0xe x−−−− ≥≥≥≥ and , 0ye y−−−− ≥≥≥≥ , find the
density function of X
UX Y
====++++
and V X Y= += += += + . Are they independent?
4) If X and Y are independent exponential random variables each with parameter 1,
find the pdf of U = X – Y.
5) Let X and Y be independent random variables both uniformly distributed on (0,1).
Calculate the probability density of X + Y.
6) Let X and Y are positive independent random variable with the identical probability
density function ( ) , 0xf x e x−−−−= ≥= ≥= ≥= ≥ . Find the joint probability density function of
U X Y= += += += + andX
VY
==== . Are U and V independent?
7) If the joint probability density of X1and X2 is given by (((( ))))1 2
1 21 2
, 0, 0( , )
0, elsewhere
x xe x xf x x
− +− +− +− + > >> >> >> >====
, find the probability of 1
2 2
XY
X X====
++++.
8) If X is any continuous R.V. having the p.d.f2 , 0 1
( )0, otherwise
x xf x
< << << << <====
, andXY e−−−−==== , find
the p.d.f of the R.V. Y.
9) If the joint p.d.f of the R.Vs X and Y is given by 2, 0 1
( , )0, otherwise
x yf x y
< < << < << < << < <====
find the
p.d.f of the R.V. X
UY
==== .
10) Let X be a continuous random variable with p.d.f, 1 5
( ) 120, otherwise
xx
f x < << << << <====
, find the
probability density function of 2X – 3.
• Central Limit Theorem
1) If 1 2, , ... nX X X are Poisson variables with parameter 2λλλλ ==== , use the Central Limit
Theorem to estimate (120 160)nP S< < where 1 2 ...n nS X X X= + + + and
75n = .
2) The resistors 1 2 3 4, , and r r r r are independent random variables and is uniform in
the interval (450 , 550). Using the central limit theorem, find
1 2 3 4(1900 2100)P r r r r≤ + + + ≤ .
3) Let 1 2 100, ,...X X X be independent identically distributed random variables with
2µ = and2 1
4σ = . Find 1 2 100(192 ... 210)P X X X< + + + ≤ .
4) Suppose that orders at a restaurant are iid random variables with mean .8Rsµ =
and standard deviation .2Rsσ = . Estimate (i) the probability that first 100
customers spend a total of more than Rs.840 (ii) 1 2 100(780 ... 820)P X X X< + + + ≤ .
5) The life time of a certain brand of a Tube light may be considered as a random
variable with mean 1200 h and standard deviation 250 h. Find the probability, using
central limit theorem, that the average life time of 60 light exceeds 1250 h.
6) A random sample of size 100 is taken from a population whose mean is 60 and
variance is 400. Using Central limit theorem, with what probability can we assert
that the mean of the sample will not differ from 60µ = by more than 4.
7) A distribution with unknown mean µ has variance equal to 1.5. Use central limit
theorem to determine how large a sample should be taken from the distribution in
order that the probability will be at least 0.95 that the sample mean will be within
0.5 of the population mean.
Unit – III (Classification of Random Processes)
• Verification of SSS and WSS process
1) Define the following: a) Markov process.
b) Independent increment random process.
c) Strict – sense stationary process.
d) Second order stationary process.
2) Classify the random process and give example to each.
3) Let cos( ) sin( )nX A n B nλ λλ λλ λλ λ= += += += + where A and B are uncorrelated random variables
with (((( )))) (((( )))) 0E A E B= == == == = and (((( )))) (((( )))) 1Var A Var B= == == == = . Show that nX is covariance
stationary.
4) A stochastic process is described by ( ) sin cosX t A t B t= += += += + where A and B are
independent random variables with zero means and equal standard deviations show
that the process is stationary of the second order.
5) If ( ) cos sinX t Y t Z tω ωω ωω ωω ω= += += += + , where Y and Z are two independent random variables
with 2 2 2( ) ( ) 0, ( ) ( )E Y E Z E Y E Z σσσσ= = = == = = == = = == = = = and ωωωω is a constants. Prove that
{{{{ }}}}( )X t is a strict sense stationary process of order 2 (WSS).
6) At the receiver of an AM radio, the received signal contains a cosine carrier signal at
the carrier frequency 0ωωωω with a random phase θθθθ that is uniformly distributed over
(((( ))))0,2ππππ . The received carrier signal is (((( ))))0( ) cosX t A tω θω θω θω θ= += += += + . Show that the
process is second order stationary.
7) The process {{{{ }}}}( ) :X t t T∈∈∈∈ whose probability distribution, under certain conditions,
is given by (((( )))) (((( ))))1
1
( ), 1,2...
1( )
, 01
n
n
atn
atP X t nat
nat
−−−−
++++
====
++++= == == == = ==== ++++
. Show that it is not stationary .
• Ergodic Processes, Mean ergodic and Correlation ergodic
1) Consider the process ( ) cos sinX t A t B tω ωω ωω ωω ω= += += += + where A andB are random variables
with ( ) ( ) 0E A E B= == == == = and ( ) 0E AB ==== . Prove that {{{{ }}}}( )X t is mean ergodic.
2) Prove that the random processes (((( ))))( ) cosX t A tω θω θω θω θ= += += += + where A and ωωωω are
constants and θθθθ is uniformly distributed random variable in (((( ))))0,2ππππ is correlation
ergodic.
3) Consider the random process {{{{ }}}}( )X t with (((( ))))2( ) cosX t A A t φφφφ= += += += + , whereφφφφ is a
uniformly distributed random variable in (((( )))),π ππ ππ ππ π−−−− . Prove that {{{{ }}}}( )X t is correlation
ergodic.
Note: The same problem they may ask by putting 10A ==== .
4) Let {{{{ }}}}( )X t be a WSS process with zero mean and auto correlation function
( ) 1XXRT
ττττττττ = −= −= −= − , where T is a constant. Find the mean and variance of the time
average of {{{{ }}}}( )X t over (((( ))))0,T . Is {{{{ }}}}( )X t mean ergodic?
Note: The same problem they may ask by putting 1T ==== .
5) Given that the autocorrelation function for a stationary ergodic process with no
periodic components is 2
4( ) 25
1 6XXR ττττττττ
= += += += +++++
. Find the mean and variance of the
process {{{{ }}}}( )X t .
• Problems on Markov Chain
6) Consider a Markov chain{{{{ }}}}; 1nX n ≥≥≥≥ with state space {{{{ }}}}1,2S ==== and one – step
transition probability matrix0.9 0.1
0.2 0.8P
====
.
i) Is chain irreducible?
ii) Find the mean recurrence time of states ‘1’ and ‘2’.
iii) Find the invariant probabilities.
7) A raining process is considered as two state Markov chain. If it rains, it is considered
to be state 0 and if it does not rain, the chain is in state 1. The transitions probability
of the Markov chain is defined as0.6 0.4
0.2 0.8P
====
. Find the probability that it will
rain for 3 days. Assume the initial probabilities of state 0 and state 1 as 0.4 and 0.6
respectively.
8) A person owning a scooter has the option to switch over to scooter, bike or a car next time with the probability of (0.3, 0.5, 0.2). If the transition probability matrix is
0.4 0.3 0.3
0.2 0.5 0.3
0.25 0.25 0.5
. What are the probabilities vehicles related to his fourth
purchase?
9) Assume that a computer system is in any one of the three states: busy, idle and
under repair respectively denoted by 0, 1, 2. Observing its state at 2 pm each day,
we get the transition probability matrix as
0.6 0.2 0.2
0.1 0.8 0.1
0.6 0 0.4
P
====
. Find out the 3rd
step transition probability matrix. Determine the limiting probabilities.
10) Two boys 1B and 2B and two girls 1G and 2G are throwing a ball from one to the
other. Each boys throws the ball to the other boy with probability 1/2 and to each
girl with probability 1/4. On the other hand each girl throws the ball to each boy
with probability 1/2 and never to the other girl. In the long run, how often does each
receive the ball?
11) A housewife buys 3 kinds of cereals A, B, C. She never buys the same cereal in
successive weeks. If she buys cereal A, the next week she buys cereal B. However if
she buys B or C the next week she is 3 times as likely to buy A as the other cereal.
How often she buys each of the 3 cereals?
12) Three boys A, B, C are throwing a ball each other. A always throws the ball to B and B always throws the ball to C, but C is just as likely to throw the ball to B as to A. Find
the transition matrix and classify the states.
13) The transition probability matrix of a Markov chain {{{{ }}}} 1,2,3...n nX
====having 3 states 1, 2
and 3 is
0.1 0.5 0.4
0.6 0.2 0.2
0.3 0.4 0.3
P
====
and the initial distribution is (((( ))))(0) 0.7,0.2,0.1P ==== . Find
(((( ))))2 3P X ==== and (((( ))))3 2 1 02, 3, 3, 2P X X X X= = = == = = == = = == = = = .
14) The tpm of a Markov chain with three states 0, 1, 2 is
3 / 4 1 / 4 0
1 / 4 1 / 2 1 / 4
0 3 / 4 1 / 4
P
====
and
the initial state distribution of the chain is (((( ))))0 1 / 3, 0,1,2P X i i= = == = == = == = = . Find (i)
(((( ))))2 2P X ==== and (ii) (((( ))))3 2 1 01, 2, 1, 2P X X X X= = = == = = == = = == = = = .
• Poisson process
1) Define Poisson process and obtain its probability distribution. 2) Prove that the Poisson process is Covariance stationary. 3) Show that the sum of two independent Poisson process is a Poisson process.
4) Suppose that customers arrive at a bank according to a Poisson process with a mean
rate of 3 per minute; find the probability that during a time interval of 2 mins.
(i) Exactly 4 customers arrive and
(ii) More than 4 customers arrive.
5) If customers arrive at a counter in accordance with a Poisson process with a mean
rate of 3 per minute, find the probability that the interval between 2 consecutive
arrivals is
(i) more than 1 minute
(ii) between 1 minute and 2 minutes
(iii) 4 minutes or less
6) A radar emits particles at the rate of 5 per minute according to Poisson distribution.
Each particles emitted has probability 0.6. Find the probability that 10 particles are
emitted in a 4 minutes period.
7) Queries presented in a computer data base are following a Poisson process of rate
6λλλλ ==== queries per minute. An experiment consists of monitoring the data base for
m minutes and recording ( )N m the number of queries presented
i) What is the probability that no queries in a one minute interval?
ii) What is the probability that exactly 6 queries arriving in one minute
interval?
iii) What is the probability of less than 3 queries arriving in a half minute
interval?
• Normal (Gaussian) & Random telegraph Process
1) Let {{{{ }}}}( )X t is a Gaussian random process with {{{{ }}}}( ) 10X tµµµµ ==== and
1 2
1 2( , ) 16 t tXXC t t e− −− −− −− −==== . Find the probability that (i) (10) 8X ≤≤≤≤ (ii) (10) (6) 4X X− ≤− ≤− ≤− ≤ .
2) Prove that a random telegraph signal process ( ) ( )Y t X tαααα==== is a wide sense
stationary process when αααα is a random variable which is independent of ( )X t ,
assume values 1−−−− and 1++++ with equal probability and 1 22 ( )1 2( , ) t t
XXR t t e λλλλ− −− −− −− −==== .
Unit – IV (Correlation and Spectral densities)
Section – I
1) Determine the mean and variance of process given that the auto correlation
function (((( )))) 2
425
1 6XXR ττττττττ
= += += += +++++
.
2) A stationary random process has an auto correlation function and is given by
(((( ))))2
2
25 366.25 4XXR
ττττττττττττ
++++====++++
. Find the mean and variance of the process.
3) If {{{{ }}}}( )X t and {{{{ }}}}( )Y t are two random processes then
( ) (0) (0)XY XX YYR R Rττττ ≤≤≤≤ where ( )XXR ττττ and ( )YYR ττττ are their respective auto
correlation function.
4) If {{{{ }}}}( )X t and {{{{ }}}}( )Y t are two random processes then
1( ) (0) (0)
2XY XX YYR R Rττττ ≤ +≤ +≤ +≤ + where ( )XXR ττττ and ( )YYR ττττ are their respective auto
correlation function.
Section – II
5) State and Prove Wiener – Khinchine theorem.
6) The auto correlation of a stationary random process is given by
( ) , 0bXXR ae bττττττττ −−−−= >= >= >= > . Find the spectral density function.
7) The auto correlation of the random binary transmission is given by
1 , ( )
0, XX
for TR T
for T
ττττττττττττ
ττττ
− ≤− ≤− ≤− ≤====
>>>>
. Find the power spectrum.
Note: By putting T = 1, the above problem can be ask1 , 1
( )0, 1XX
forR
for
τ ττ ττ ττ τττττ
ττττ − ≤− ≤− ≤− ≤====
>>>>.
8) Show that the power spectrum of the auto correlation function 1e ατατατατ α τα τα τα τ−−−− −−−− is
(((( ))))3
22 2
4ααααα ωα ωα ωα ω++++
.
9) Find the power spectral density of a WSS process with auto correlation function 2
( ) , 0XXR e αταταταττ ατ ατ ατ α−−−−= >= >= >= > .
10) Find the power spectral density of the random process, if its auto correlation
function is given by ( ) cosXXR e α τα τα τα ττ βττ βττ βττ βτ−−−−==== .
11) Find the power spectral density function whose auto correlation function is given by 2
0( ) cos( )2XX
AR τ ω ττ ω ττ ω ττ ω τ==== .
Section – III
12) If the power spectral density of a WSS process is given by
(((( )))) , ( )
0, XX
ba a
aSa
ω ωω ωω ωω ωωωωω
ωωωω
− ≤− ≤− ≤− ≤==== >>>>
, find the auto correlation function of the process.
13) The power spectral density of a zero mean WSS process {{{{ }}}}( )X t is given by
1, ( )
0, elsewhereXX
aS
ωωωωωωωω
<<<<====
. Find ( )XXR ττττ and show that ( )X t and X taππππ ++++
are
uncorrelated.
14) Find the autocorrelation function of the process {{{{ }}}}( )X t , for which the spectral
density is given by
21 , 1( )
0, 1S
ω ωω ωω ωω ωωωωω
ωωωω
+ ≤+ ≤+ ≤+ ≤==== >>>>
.
15) The cross – power spectrum of real random processes {{{{ }}}}( )X t and {{{{ }}}}( )Y t is given by
, 1( )
0, elsewhereXY
a jbS
ω ωω ωω ωω ωωωωω
+ <+ <+ <+ <====
. Find the cross – correlation function.
Section – IV
16) If ( ) ( ) ( )Y t X t a X t a= + − −= + − −= + − −= + − − ,prove that
( ) 2 ( ) ( 2 ) ( 2 )YY XX XX XXR R R a R t aτ τ ττ τ ττ τ ττ τ τ= − + − −= − + − −= − + − −= − + − − Hence prove that
2( ) 4sin ( ) ( )YY XXS a Sω ω ωω ω ωω ω ωω ω ω==== .
17) {{{{ }}}}( )X t and {{{{ }}}}( )Y t are zero mean and stochastically independent random process
having autocorrelation function ( )XXR e ττττττττ −−−−==== , ( ) cos 2YYR τ πττ πττ πττ πτ==== respectively. Find
(i) the auto correlation function of ( ) ( ) ( )W t X t Y t= += += += + and ( ) ( ) ( )Z t X t Y t= −= −= −= −
(ii) The cross correlation function of ( )W t and ( )Z t .
18) If {{{{ }}}}( )X t and {{{{ }}}}( )Y t are independent with zero means. Find the auto correlation
function of {{{{ }}}}( )Z t where ( ) ( ) ( )Z t a bX t cY t= + += + += + += + + .
19) If (((( ))))( ) 3cosX t tω θω θω θω θ= += += += + and ( ) 2cos2
Y t tππππω θω θω θω θ = + −= + −= + −= + −
are two random processes
where θθθθ is a random variable uniformly distributed in (((( ))))0,2ππππ . Prove that
(((( )))) (((( )))) (((( ))))0 0XX YY XYR R R ττττ≥≥≥≥ .
20) Two random process {{{{ }}}}( )X t and {{{{ }}}}( )Y t are given by (((( ))))( ) cosX t A tω θω θω θω θ= += += += + ;
(((( ))))( ) sinY t A tω θω θω θω θ= += += += + where A and ωωωω are constants and " "θθθθ is a uniform random
variable over 0 to 2ππππ . Find the cross – correlation function.
21) If {{{{ }}}}( )X t is a process with mean ( ) 3tηηηη ==== and auto correlation
(((( )))) 0.2, 9 4XXR t t e ττττττττ −−−−+ = ++ = ++ = ++ = + . Determine the mean, variance of the random variable
(5)Z X==== and (8)W X==== .
Unit – V (Linear systems with Random inputs) 1) Prove that if the input ( )X t is WSS then the output ( )Y t is also WSS.
2) If ( )X t is the input voltage to a circuit and ( )Y t is the output voltage, {{{{ }}}}( )X t is a
stationary random process with 0xµµµµ ==== and2( )XXR e ττττττττ −−−−==== . Find yµµµµ , ( )XXS ωωωω and
( )YYS ωωωω , if the system function is given by1
( )2
Hi
ωωωωωωωω
====++++
.
3) If {{{{ }}}}( )X t is a band limited process such that ( ) 0, XXS ω ω σω ω σω ω σω ω σ= >= >= >= > , prove that
2 22 (0) ( ) (0)XX XX XXR R Rτ σ ττ σ ττ σ ττ σ τ− ≤− ≤− ≤− ≤ .
4) Let {{{{ }}}}( )X t be a random process which is given as input to a system with the system
transfer function 0 0( ) 1, H ω ω ω ωω ω ω ωω ω ω ωω ω ω ω= − < <= − < <= − < <= − < < . If the autocorrelation function of the
input process is 0 . ( )2
N δ τδ τδ τδ τ , find the auto correlation of the output process.
5) If (((( ))))0( ) cos ( )Y t A t N tω θω θω θω θ= + += + += + += + + where A is a constant, θθθθ is a random variable with a
uniform distribution in (((( )))),π ππ ππ ππ π−−−− and {{{{ }}}}( )N t is a band limited Gaussian white noise
with a power spectral density 0( )2NN
NS ωωωω ==== for 0 Bω ω ωω ω ωω ω ωω ω ω− <− <− <− < and
( ) 0NNS ωωωω ==== ,elsewhere. Find the power spectral density of ( )Y t , assuming that
( )N t and θθθθ are independent.
6) Consider a white Gaussian noise of zero mean and power spectral density 0
2N
applied to a low pass RC filter whose transfer function is1
( )1 2
H fi fRCππππ
====++++
. Find
the autocorrelation function of the output random process.
7) A WSS random process ( )X t with auto correlation ( )XXR Ae α τα τα τα τττττ −−−−==== where A and αααα
are real positive constants, is applied to the input of an linear time invariant (LTI)
system with impulse response ( ) ( )bth t e u t−−−−==== where b is a real positive constant.
Find the auto correlation of the output ( )Y t of the system.
8) An linear time invariant (LIT) system has an impulse response ( ) ( )th t e u tββββ−−−−==== . Find
the output auto correlation function ( )YYR ττττ corresponding to an input ( )X t .