Tlcm 311: Analog And Digital Communications Systems Question Paper
Tlcm 311: Analog And Digital Communications Systems
Course:Bachelor Of Science In Telecommunications
Institution: Kabarak University question papers
Exam Year:2011
KABARAK UNIVERSITY
UNIVERSITY EXAMINATIONS
2010/2011 ACADEMIC YEAR
FOR THE DEGREE OF BACHELOR OF SCIENCE IN
TELECOMMUNICATIONS
COURSE CODE: TLCM 311
COURSE TITLE: ANALOG AND DIGITAL COMMUNICATIONS
SYSTEMS
STREAM: Y3S1
DAY: WEDNESDAY
TIME: 2.00 – 5.00 P.M.
DATE: 16/03/2011
INSTRUCTIONS:
Answer ALL questions in PART ONE and any THREE questions in PART TWO.
Where required use;
Boltzman’s constant = 1.38 x 10-23 J/K
Room temperature = 290K
PART ONE
Question One (20 marks)
a. Define;
i. An energy signal (1 mark)
ii. A power signal (1 mark)
iii. A time invariant system (1 mark)
iv. A causal system. (1 mark)
b. A ground station for receiving signals from a satellite has an antenna with a gain of 40dB and
a noise temperature of 60K. The antenna feeds a receiver system whose front end is a
preamplifier. The preamplifier has an effective noise temperature of 125K and a gain of
20dB. The preamplifier is connected to an amplifier by a lossy cable with a noise figure of
3dB. The amplifier has a noise figure of 10dB and a gain of 80dB. The amplifier forms the
back end of the receiver system. The transmission bandwidth is 1MHz. The satellite has an
antenna with a power gain of 6dB and the total path loss from satellite to ground station is
190dB. Calculate;
i. The effective noise temperature of the receiver system. (2 marks)
ii. The average thermal noise power at the receiver system output. (1 mark)
iii. The minimum signal power at the input of the ground station antenna required to
achieve a 60dB signal to noise ratio at the receiver system output. (2 marks)
iv. The minimum satellite transmitter power required to achieve signal to noise ratio at the
receiver output as in (iii) above. (1 mark)
c. Describe two disadvantages of variable length source coding. (2 marks)
d. A Gaussian channel has a bandwidth of 8.5 KHz and a noise power spectral density of 10-15
W/Hz. The signal power at the output of the channel has to be maintained at a level of 0.2
milliwatts. Calculate the capacity (in bits per second) of the channel. (2 marks)
e. A discrete source emits 10 independent symbols. The symbols occur with equal probabilities.
Calculate the entropy of the source. (1 mark)
f. A discrete source emits one of 10 distinct symbols every 0.2 milliseconds. The probabilities
of the symbols are as shown in the table below.
Symbol A B C D E F G H I J
Probability 1/30 4/30 2/30 6/30 3/30 3/30 5/30 2/30 3/30 1/30
Calculate
i. The average information rate of the source (2 marks)
ii. The minimum bandwidth required to transmit the output of the source using binary pulse
amplitude modulation. (1 mark)
g. A transmission channel has a bandwidth of 8000Hz. The channel is Gaussian with a noise
of power spectral density 10-14 W/Hz. Data is transmitted on the channel at a maximum rate
of 100 Kbps. Calculate;
i. The signal to noise ratio (in decibels) at the output of the transmission channel. (1 mark)
ii. The average signal power at the output of the transmission channel (1 mark)
Question Two (10 marks)
a. Describe;
i. What is meant by inter symbol interference
ii. What is meant by aliasing, with respect to sampling (2 marks)
b. In practical sampling describe the negative effects of
i. Flat topped sampling.
ii. Using non ideal reconstruction filters
How can the effects described be compensated for? (2 marks)
c. An analog signal, band limited to 10 KHz, is sampled and then quantized using a uniform
quantizer. An output signal to quantization noise ratio of 40 dB is desired. (Assume that the
signal has a uniform probability density function). Calculate the minimum bandwidth (in
Hz) required if;
i. An M-ARY pulse amplitude modulation system is used to transmit the samples (with M
being equal to the number of quantization levels).
ii. Pulse code modulation / binary pulse amplitude modulation is used to transmit the
digitized signal.
iii. Pulse code modulation / QPSK is used to transmit the digitized signal.
(3 marks)
d. Describe
i. The main advantage of M-ARY pulse amplitude modulation (PAM) compared to binary
PAM schemes
ii. The two main disadvantages of M-ARY PAM schemes as compared to binary PAM
schemes. (2 marks)
e. An analog signal band limited to 5 KHz is sampled and a delta modulation / binary pulse
amplitude system is used to transmit the digitized signal. The bandwidth of the transmission
channel is 30 KHz. Calculate the minimum rate and the maximum rate at which the analog
signal can be sampled. (1 mark)
Question Three (10 marks)
a. Describe why the signal to noise ratio at the output of a coherent double side band (DSB)
demodulator is twice that at the input of the demodulator. (1 mark)
b. Describe what is meant by the threshold effect in amplitude modulation (AM). (1 mark)
c. Describe the main advantage and the two main disadvantages of amplitude modulation (AM).
(2 marks)
d. An AM (amplitude modulation) station is broadcasting with an average transmitted power of
8 kW. The transmission power efficiency is 25 percent. Calculate;
i. The average power in the signal component of the transmitted signal
ii. The average power in the carrier component of the transmitted signal
iii. The average transmitted power if the station was using a single side band (SSB)
modulation scheme with the same average signal and carrier power.
(2 marks)
e. A 10 MHz carrier signal is frequency modulated by a signal such that the peak frequency
deviation of 5 KHz.
i. Calculate the deviation ratio and the approximate bandwidth of the FM signal if the
bandwidth of the modulating signal is 2 KHz.
ii. What will be the transmission bandwidth if double side band modulation was used to
transmit the signal in (i) above.
iii. What is the main advantage of FM as compared to linear continuous wave modulation
techniques (e.g. DSB). (2 marks)
f. A message signal with a bandwidth of 3 KHz is transmitted over a Gaussian transmission
channel with a noise of power spectral density of 10-11 W/Hz. The transmitted power is
8 KW and the power loss in the channel is 40dB
Calculate the signal to noise ratio (in decibels) at the destination if
i. Single side band (SSB) modulation is used.
ii. Frequency modulation (FM) with a deviation ratio of 3 is used. (2 marks)
Question Four (10 marks)
a. With regard to digital modulation schemes;
i. Draw a block diagram of a correlation receiver used in demodulation
ii. Which digital demodulation schemes, between coherent and non coherent schemes, are
suitable for a transmission channel that exhibits fading. Explain your answer.
(3 marks)
b. Give two situations when M-ARY digital modulation signaling schemes are preferred over
binary digital modulation signaling schemes. (1 mark)
c. What is the maximum transmission rate in bits per second for a quadrature phase shift keying
(QPSK) scheme over a medium with a bandwidth of 3000 Hz? Assume that rectangular
pulses are used to generate the QPSK signal. (1 mark)
d. Binary data is transmitted over a transmission channel at a bit rate of 800 Mbps and a
maximum transmit power of 10 KW. The average receive power at the receiver input is
0.01W. The noise power spectral density at input of receiver is 10-12 W/Hz
Calculate;
i. The loss in decibels of the transmission channel. (1 mark)
ii. The probability of error if a coherent amplitude shift keying (ASK) scheme is used for
transmitting the binary data. (2 marks)
iii. The probability of error if a coherent phase shift keying (PSK) scheme is used for
transmitting the binary data. (2 marks)
Question Five (10 marks)
a. Describe a systematic block code. (1 mark)
b. Consider a (7,4) linear code with the following generator matrix.
G =
?
?
?
?
?
?
?
?
?
?
?
?
0 0 0 1 1 1 1
0 0 1 0 1 0 1
0 1 0 0 0 1 1
1 0 0 0 1 1 0
i. Find the parity check matrix for this code.
ii. Use the parity check matrix to locate the error in the vector (1110100)
iii. Find the minimum distance for this code.
(2 marks)
c. A (15, 5) linear cyclic code has the following generator polynomial
g(x) = 1 + x + x2
+ x4
+ x5
+ x8
+ x10
i. Draw a block diagram of an encoder for this code. (2 marks)
ii. Find the code polynomials for the following message vectors in systematic form.
(10100)
(11111) (1 mark)
iii. Calculate the syndrome vectors for the following received vectors.
(001101000010001)
(000011001010101) (1 mark)
d. Explain why convolution codes are not suitable for error detection and retransmission
schemes? (1 mark)
e. A transmission system has a probability of error of Pe = 10-4 and a message bit rate of 0.5
Mbps. Error control coding is used in the system to reduce the probability of error. The error
control coding consists of repeating each bit four times. The decoding algorithm used at the
receiver is given below;
Received Bits Output
0000 0
0001 0
0010 0
0011 Retransmit
0100 0
0101 Retransmit
0110 Retransmit
0111 1
1000 0
1001 Retransmit
1010 Retransmit
1011 1
1100 Retransmit
1101 1
1110 1
1111 1
Calculate the probability of error and the bit error rate for the error control coded system.
(2 marks)
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