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EE402B – Radio Systems and Personal Communication Networks
Date: XX December 2016 Time Duration: Two hours
Instructions to Candidates
• Answer THREE questions ONLY , All questions carry equal marks
• Students must bring their own calculator which must be from the Casio FX-83, Casio FX-85, HP10s or HP10s+ series.
This must be either an HP10s/HP10s+ or non-programmable from Casio FX- 83 or FX-85 series (No other calculators
are permitted). HP10s/HP10s+ will be provided to students who do not have their own calculator.
QUESTION 1
(a) What is meant by the term ‘directivity’ or ‘directionality’ as applied to an antenna? Calculate the gain of an antenna
with the effective area of 1.8 m2 and operating at a frequency of 3.5 GHz, with respect to an isotropic antenna.
(b) In a line-of-sight microwave communications link of 25 km length, the antennas specified in (a) are used at both the
transmitter and the receiver and the operating frequency is the same as above (3.5 GHz). Calculate
i) the free-space loss for radio transmission across the link, and
ii) the received signal power (in mW) at the input of the receiver when the transmitted power is 20 mW.
(c) Refer again to the link described in (b). If the system noise temperature is 400K and it is necessary to guarantee an
Eb/No= 45 dB for the required BER (bit error rate) level at the receiver, what data rate can be achieved? If the
required signal power at the receiver is increased, discuss how the system could be altered by making technical
adjustments to maintain the performance for the same length of the transmission link.
QUESTION 2
A graph of P(e) plotted against Eb/No is provided for Question 2 in Fig. 1 at the end of the paper.
(a) Compare and contrast the properties of the signal-to-noise ratio S/N and the Eb/No ratio. Show their relationship by
deriving a formula between the two ratios. [7 MARKS]
(b) QPSK (Quadrature Phase Shift Keying) and 8PSK modulation schemes are among a number of modulation schemes
considered in the 3rd
generation of cellular mobile systems. Determine the achievable bandwidth efficiency for each
of these two modulation schemes when they operate at the same modulation rate and use the rolloff factor of r =
0.4. Assume the S/N ratio at the input of the receiver to be 10.9 dB. Find the resulting bit error probability, P(e), for
each of the modulation schemes using the graph of P(e) against Eb/No provided in Fig.1. [12 MARKS]
(c) Discuss the implications of the results obtained in (b) with respect to the performances in efficiency and reliability
of the two modulation schemes. If a wireless communication system were to be designed to meet both the bandwidth
efficiency and bit error probability targets, what technical approaches or resource arrangements could be considered?
QUESTION 3
(a) Compare and contrast between frequency hopping and CDMA technologies. A direct sequence CDMA system has
a processing gain of 18 dB. What is the data rate if the chip rate is expected to be 1.2288 Mchips/s? [7 MARKS]
(b) Consider a CDMA system in which three users A, B and C are assigned Walsh code sequences (-1, -1, 1, 1, -1, -1,
1, 1), (-1, -1, -1, -1, 1, 1, 1, 1) and (-1, 1, -1, 1, -1, 1, 1, 1), respectively. Assume the code for user C to be used at
the receiver. Determine the code sequence at the output of the receiver for the following cases:
i) User A transmits a data bit 1, user B transmits a data bit 1 and user C transmits a data bit 0.
ii) User A transmits a data bit 1 and user C transmits a data bit 0. However, the 7th
chip bit (from the left) of sequence
A and the 5th
chip bit of sequence C are reversed due to error during the transmission. User B does not transmit.
(c) To reduce the bit error rate for transmitting the CDMA sequences used in (b), the (n, k, d) = (14, 8, 3) array code
can be considered for this purpose to encode the original sequences before transmission. If the code is used for error
correction, calculate the probability of block error after decoding at the receiver. Assume a binary symmetric channel
with bit error probability p = 0.02. [5 MARKS]
QUESTION 4
(a) Explain whether or not the virtual circuit approach is the same as circuit switching. Describe how a protocol stack is
implemented by giving an example and showing the implementation process with a diagram. [8 MARKS]
(b) A Globalstar satellite is in orbit 1410 km above an earth station. The downlink operates at a frequency of 5 GHz
with an EIRP (effective isotropic radiated power) of 45 dBW. The receiving antenna at the earth station has a gain
of 35 dBi. The system noise temperature and noise bandwidth are 110 K and 3 MHz, respectively. Calculate
i) the free-space loss, and ii) the carrier-to-noise ratio (C/N). [10 MARKS]
(c) Describe the principle of frequency reuse in the context of a cellular network. A cellular radio system has 1000 RF
channels available. Given the frequency reuse factor N=9, determine
i) the number of radio channels per cell, and ii) the repeat distance of the system if the radius of the cells is 4 km.

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Date: XX December 2017
Question No 1
(a) i) Define the term antenna gain. (2 marks)
ii) Explain the relationship between antenna gain and antenna directivity. (3 marks)
iii)Given a parabolic reflective antenna with a diameter of 2.8 m and the effective area coefficient 0.56, calculate
the gain of the antenna when it operates at frequency of 8 GHz, with respect to an isotropic antenna.
(b) The downlink of a GPS (Global Positioning Systems) satellite operates at 1.38 GHz with an EIRP (effective isotropic
radiated power) of 49 W. The distance between the satellite and a vehicle (the receiver) on earth is 17,800 km. The
receiving antenna gain is 28 dBi, and the system noise temperature and noise bandwidth are 120 K and 2.5 MHz
respectively. Calculate
i) the free-space loss in dB for transmission across the link, (3 marks)
ii) the carrier-to-noise ratio (C/N) in dB. (6 marks) [Subtotal: 9 marks]
(c) i) What is Doppler shift ? (2 marks)
iii)In (b), suppose that the vehicle is moving at a speed of 80 km per
hour, as shown in Figure 1, and that the satellite has a 150° elevation
with respect to the vehicle, determine the Doppler shift observed at
the receiver. iii) Explain the implication of the result obtained.
Question No 3
(a) i) What is meant by spread spectrum as applied to radio
communications? (2 marks)
i) Discuss the main features of two basic spread spectrum techniques: frequency hopping and direct sequencing.
(4 marks) [Subtotal: 6 marks]
(b) In a direct-sequence CDMA system users A and B are assigned code sequences (-1, -1, -1, -1, 1, 1, 1, 1) and (1, -1,
-1, 1, 1, -1, -1, 1), respectively. Assume the code for user B to be used at the receiver. Determine the code sequence
at the output of the receiver for the following cases.
i) User A transmits a data bit 1 and user B transmits a data bit 0; the signal power of user A is two times of that of
user B, i.e., the magnitude of A’s data bit is 2 or –2, as appropriate. (6 marks)
ii) User B transmits a data bit 1, but the 2nd
and 4th
chip bits (from left) of sequence B are reversed due to
transmission error. User A does not transmit. (4 marks) [Subtotal: 10 marks]
(c) Discuss the implications from the results obtained for the two cases in (b) with respect to the performance of this
CDMA system. (3 marks)
The CDMA code sequences used in (b) can be protected through channel coding by employing the (n, k, dmin) = (15,
8, 4) product code. Assume a binary symmetric channel with bit error probability p= 2×10−3
. Calculate the
probabilities of block error at the output of the receiver for
i) transmission of the coded CDMA sequences using the (15, 8, 4) code, (4 marks)
ii) transmission of the CDMA sequences given in (b) without coding. (2 marks)
Question 2
A Graph of P(e) against Eb / N0 for the 16QAM modulation scheme over fading channels.
(a) i) Discuss the function of equalisation in connection with
digital communications. (3 marks)
i)Explain why an adaptive equaliser is essential for the mobile
communication system, and how it works. (4 marks)
(b) The 16QAM (16-level quadrature amplitude modulation)
modulation scheme is adopted in IEEE 802.11 wireless LANs.
i)Determine the bandwidth efficiency that can be achieved by this
modulation scheme given the roll-off factor r = 0.5. (3 marks)
ii) Assume the S/N ratio at the input of the receiver to be 25 dB.
Find the resulting bit error probability, P( )e , using the graph of P( )e
against Eb / N0 shown in Figure 2. (6 marks)
iii) Explain whether or not we can simply conclude that this
modulation scheme is better than 8PSK in terms of the performances on
both bandwidth efficiency and error probability. (4 marks) [Subtotal: 13 marks]
(c) In (b), assume that the effective noise temperature at the receiver is 240 K and the data transmission rate 168 kb/s.
Determine the received signal power. [Subtotal: 5 marks]
1.0E-04
1.0E-03
1.0E-02
1.0E-01
5 10 15 20 25 30
Eb/No (dB)
BiterrorprobabilityP(e)
Figure 1: A scenario for determining the Doppler shift.

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Question No 4
(a) i) In the packet-switched networks shown in Figures 3 & 4, station X sends a message (divided into three packets)
to station Y across the network. (2 marks)
iv) Identify which switching approach is used in each of the networks, and compare and contrast them. (6 marks)
(b) Explain why the hexagon is chosen as the cell geometric pattern in the cellular radio system. (2 marks)
A cellular radio system has 50 cells and an allocated bandwidth of 25 MHz for downlink transmission. The bandwidth
of each channel is 20 kHz. For the given reuse factors 4 and 7, determine
i) the number of channels per cell, (4 marks)
ii) the total number of mobile users that can be served concurrently in the system, respectively. (3 marks)
(c) i) What is meant by protocol stack or architecture with reference to a communication system? (2 marks)
iii)Describe, by giving an example, the implementation process of a protocol stack using a diagram. (6 marks)
2018
Question 1
(a) Do you agree with the statement that the antenna gain is defined as the power enhancement of a transmitter, & why?
i) A parabolic reflective antenna with a diameter of 1.6 m is used for receiving satellite television signal at 10.5
GHz. Find the effective area of the antenna given the coefficient of 0.56. (1 marks).
ii) Determine the gain of the antenna in dBi described in ii) (3 marks)
(b) A synchronous satellite communication system operates at 4.2 GHz for uplink transmission with an EIRP (effective
isotropic radiated power) of 50 dBW. Assuming that the transmitter-receiver link has a distance of 35, 865 km and
the receiving antenna gain of the satellite is 48 dBi, determine:
i) the free-space loss in dB for transmission over this link, (3 marks) ii) the received carrier power at the satellite
in dBm, (2 marks) and
ii) the carrier-to-noise ratio (C/N) in dB given the system noise temperature and noise bandwidth of 90 K and 4
MHz, respectively.
(c) Explain the term small-scale fading. (2 marks)
i) Identify three main influencing factors of this type of fading (3 marks).
ii) Find the far-field distance for an antenna with the maximum dimension of 0.3 m and operating frequency of 3.5
GHz. (3 marks)
Question 2
(a) Discuss the differences between MPSK (multiple phase shift
keying) and MFSK (multiple frequency shift keying)
modulation schemes with regard to their bandwidth
efficiency and reliability performances.
iii)A broadband wireless system offers a service with the
downloading data rate of up to 5 Mbps. What is the
minimum number of modulation levels required if the
modulation rate of a modem used is 250 kbaud?
(b) Mobile communication systems such as 3G/4G employ a
range of Quadrature Amplitude Modulation (QAM)
schemes. Figure 1 shows the reliability performances of 16-
QAM and 64-QAM modulation schemes in terms of bit
error rate (BER) against Eb/No.
i) Discuss the trade-offs between these two modulation schemes in their reliability and efficiency performances.
ii) To achieve the BER target of 10-6
, and transmit data at a rate of 100 kbps, find the received power in watts
required for both modulation schemes, assuming that the noise power density at the receiver is N0 = 10-9
W/Hz.
(c) Briefly describe the main features of the CDMA technology as applied in cellular mobile communication networks.

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i) A direct sequence CDMA system has a processing gain of 20 dB. If the system has a bandwidth of 2 MHz, find
the data rate offered and the bandwidth efficiency of the system. (4 marks)
Question 3
(a) Given a wireless local area network (WLAN) and by applying the layered protocol concept,
ii) describe the functionality of the three essential protocol layers of this network, and (3 marks) ii) explain how
those layers interact with each other. (4 marks)
(b) Define the Shannon channel capacity for the additive white noise Gaussian channel by presenting its formula and
your explanation. (3 marks)
iii)Consider a wireless communication system where the received signal power Pr is given as a function of the
transmitter-receiver distance d: Pr=Pt(d’/d)2
, with the transmitting power Pt = 2 W and reference distance d’ =
10 m. Determine the channel capacity of the system for d = 500 m given the channel bandwidth of 30 kHz and
noise power at the receiver of 3x10-5
W. (3 marks)
iv) Repeat ii) for d = 1 km while other conditions remain unchanged. (3 marks)
v) Discuss the impact of the transmission distance over the achievable channel capacity based on the results
obtained in ii) and iii). (2 marks)
(c) A cellular radio network has 50 cells with a cell radius of 2 km. If the total bandwidth allocated for the network is
40 MHz and the frequency reuse factor is 7, find
i) the number of channels per cell if the bandwidth of each channel is 25 kHz, and (2 marks) ii) the total area
covered by the network. (3 marks)
ii) What is the main challenge for frequency reuse and how can it be addressed? (2 mark)