Sample Question Paper for 9210-119
Graduate Diploma in Engineering
Wireless and mobile communications
Duration: three hours
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, ruler
The following data is attached
• Q-function
• Q-function table
General instructions
• This paper consists of nine questions.
• Answer five questions.
• All questions carry equal marks. The maximum marks for each section within a
question are shown.
• Clearly state any assumptions made.
© The City and Guilds of London Institute
1
a
Briefly describe the design considerations that are specific to wireless communication
systems.
(5 marks)
b
State three main applications of fixed wireless access.
(3 marks)
c
One of the main characteristics of cellular systems is frequency reuse. Briefly explain
the concept of frequency reuse, its advantages and disadvantages.
(3 marks)
d
In a communication link, the receiver is situated 2 km away from the transmitter. A
transmitter has a power output of 150 W. The transmitter and receiver antenna gains
are 10 dBi and 8 dBi, respectively. The carrier frequency is 800 MHz. Free space
propagation is assumed.
i
The Friis free space equation gives the received signal strength when the
transmitter and receiver have a clear unobstructed line of sight (LoS) path
between them. State the Friis free space equation, defining each parameter.
(2 marks)
ii
Assuming that the only loss between the transmitter and the receiver is the free
space loss, calculate the received signal power in dBm.
(2 marks)
iii
Find the path loss in dB.
iv
Find the magnitude of the E-field at the receiver antenna.
(2 marks)
v
Assume that the receiver antenna has a purely real impedance of 60 : which is
matched to the receiver. Find the root mean square (rms) voltage applied to the
receiver input.
(2 marks)
(1 mark)
2
2
a
Suppose that a digitally modulated signal is transmitted through a channel with the
following power delay profile:
Ph (W )
0.8G (W ) 0.4G (W 10 6 ) 0.2G (W 3 u 10 6 ) 0.5G (W 5 u 10 6 ) PW ,
where delay W is in seconds and G is the Dirac delta function.
i
Sketch the power delay profile of the channel in dBm.
(3 marks)
ii
Determine the mean delay and the root mean square (rms) delay spread of the
channel.
(3 marks)
iii
Find the maximum symbol rate (Rs symbols/s) such that the signal transmitted
through these environments experiences negligible inter symbol interference
(ISI).
(3 marks)
iv
If the bandwidth of the transmitted signal is 200 kHz, determine whether the
received signal will undergo frequency-flat or frequency-selective fading.
(4 marks)
Justify your answer.
b
In mobile radio environments, the Rayleigh distribution is commonly used to describe
the time-varying nature of the received signal envelope. The instantaneous power
P of the received signal with a Rayleigh-distributed envelope has the following
exponential distribution:
f P ( p)
p ·
§
exp¨ 2 ¸
2V
© 2V ¹
1
2
p t 0,
where V 2 is the variance of both the in-phase and the quadrature phase components.
i
Show that the average received power is given by 2V 2 .
(3 marks)
ii
Assume the average received power is -50 dBm. Find the percentage of time
that the received power is below -80 dBm.
A Q-function table is attached to the end of the paper.
(4 marks)
3
See next page
3
a
The average probability of bit error for coherent binary phase shift keying (BPSK)
in a flat Rayleigh fading channel is
Pe Where b is the average received
(8 marks)
1
b 1 ,
2 1 b Eb
. Here Eb is the average energy per bit at the
N0
receiver, and N o is the power per unit bandwidth of white Gaussian noise.
Assume the data rate is 100 kbit/s and N0 is 0.01 mW/Hz. Determine the additional
(average) power required to achieve a target Pe of 10 3 for coherent BPSK in a
flat Rayleigh fading channel as compared to the additive white Gaussian noise (AWGN)
channel in dBm. Note that coherent BPSK modulation has a bit error probability in
AWGN given by
Pe ( b ) Q(2 b )
where b b
,
Eb
.
N0
In a particular propagation environment, the path loss (in dB) at distance d follows
the model
d PL(d ) PL(d 0 ) 10n log X d0 where PL (d ) is the average large-scale path loss, n 3 is the path loss exponent,
and X represents a normal random variable in dB having standard deviation
5 dB. Assume that the transmitted power is 10 W and the received power
measurement taken at the reference distance d 0 50 m is -10 dBm.
i
Determine the average received power at a distance of 1.5 km in mW.
(3 marks)
ii
Find the average large-scale path loss at a distance of 1.5 km in dB.
(2 marks)
iii Determine the maximum possible distance d max from the transmitter so that the
received signal strength is greater than -100 dBm of 90% of the time.
4
(7 marks)
4
a
Describe, using block diagrams, the transmitter and receiver operations of a typical
frequency hopped spread spectrum (FH-SS) system.
(6 marks)
b
Briefly explain, using a functional block diagram, the operation of the RAKE receiver.
(4 marks)
c
Consider a direct-sequence spread-spectrum (DS-SS) system with K multiple access
users. Assume BPSK modulation is used and each user has a maximal-length pseudo
noise (PN) sequence that is produced from an n-stage linear feedback shift register
(LFSR). All users employ power control such that the received power of each user is
maintained at an average power level of P0 . It can be shown that the average bit
error probability is approximately given by
1
Pe Q
K 1 N0 3N
2 Eb c
where N c is the number of chips per message symbol and N 0 represents the power
per unit bandwidth of white Gaussian thermal noise. Assume an average bit error
probability of less than 10 3 is required to be maintained for each user. Each user has
a baseband data rate of 16 kbps, P0 5 mW and N 0 1 nW/Hz .
i
Determine the minimum length of the PN sequence required to support 15
simultaneous users in this DS-SS system.
(3 marks)
ii
Suppose LFSRs with twice the number of stages as in part i are employed in the
system. Determine the maximum number of simultaneous users that can be
supported by the system.
(3 marks)
iii
Now suppose that the received power level is doubled (i.e., P0 10 mW) and
the PN sequences with the same length as in part i are employed. Determine
the maximum number of simultaneous users that can be supported by the
system. Comment on your answer in comparison to the answer in part ii.
(4 marks)
5
See next page
5
Consider a cellular system with hexagonal cells. All cells have equal radii R. Base stations
are located at the center of each cell. The transmit power of all base stations are equal.
Frequency bands 890-910 MHz and 940-960 MHz have been allocated for the reverse link
and the forward link, respectively. A radio channel of 200 kHz can support eight users
using TDMA.
a
Show that the co-channel reuse ratio is given by
(4 marks)
Q 3N ,
where N i 2 ij j 2 , i and j are non-negative integers.
b
Assume that the effect of the co-channel cells outside the first tier is negligible. All M
co-channel cells in the first tier are equidistant and are at the reuse distance D from
the intended mobile receiver. Consider the worst case scenario where the mobile
receiver of interest is located at one of the cell vertices. Show that when the transmit
power of each base station is equal, the average signal-to-interference ratio (SIR) on
the forward link is
(7 marks)
n
1 D
SIR ,
MR
where n is the path loss exponent. State any other assumptions made.
c
Suppose that for acceptable forward link voice quality it is required to provide a SIR of
20 dB at the mobile receiver. Assume n 4. Determine the cluster size N and the
frequency reuse factor that should be used for maximum capacity.
(5 marks)
d
Determine the number of full-duplex radio channels and user channels assigned to
each cell.
(4 marks)
6
6
a
Consider the uplink of a single-cell CDMA system with K users. Assume that all users
in this cell are power controlled so that the base station receives equal power from all
users and that the system is interference limited. Let Eb denote the average received
bit energy and N 0 denote the power spectral density of the interference within a
sector.
i
(7 marks)
Show that
Eb
W
N0 K
1 Rb
Ns
where W is the total RF bandwidth, Rb is the baseband information bit rate, N s
is the number of sectors at the base station and denotes the voice activity
factor. State any other assumption made.
ii
Assume W 1.25 MHz, Rb 10 kbps, a target
Eb
of 15 dB. Determine the
N0
(2 marks)
maximum number of users that can be supported in a single-cell CDMA system
using an omnidirectional base station antenna and no voice activity detection.
Repeat the problem in part ii, for the case of a single-cell CDMA system using
three-sectors at the base station and voice activity detection with 0.4.
(3 marks)
Discuss and compare four main features of frequency division multiple access
(FDMA) and time division multiple access (TDMA) schemes used in 1G and 2G cellular
mobile systems, respectively.
(8 marks)
iii
b
7
See next page
7
8
9
a
Briefly explain the advantage of the RAKE receiver over a simple single-correlator
receiver.
(3 marks)
b
In GSM systems, when a mobile station (MS) is switched on or moves into a new
location area, before the registration process takes place, the MS has to acquire
frequency and time synchronization. Briefly describe how a MS acquires frequency
and timing synchronization.
(4 marks)
c
Describe the sequence of operations performed in GSM systems when a connection
is established for an incoming call from the public switched telephone network (PSTN)
to an MS. Clearly mention the involvement of different channels and network entities.
(6 marks)
d
i
Describe what is meant by soft handover.
(3 marks)
ii
Explain an advantage and a disadvantage of soft handoff over hard handoff.
(4 marks)
a
Draw a block diagram of the GSM reference system architecture and briefly describe
the network and switching subsystem (NSS).
b
In GSM TDMA systems, the duration of a multiframe is 120 ms. A multiframe consists
of 26 frames. The 13th and 26th frames in a multiframe are not used for traffic but for
control purpose. A frame consists of 8 time slots. By allocating each time slot to a
different user, eight users are accommodated per frame. A normal time slot consists
of six tailing bits, unused guard time of 8.25 bits, 26 training bits and two traffic
bursts. Each traffic burst consists of 57 bits of coded data and a one bit-stealing flag.
(7 marks)
i
Draw the frame structure of the GSM system.
(1 mark)
ii
Find the time duration of a bit.
(2 marks)
iii
Find the raw data rate provided for each user.
(2 marks)
iv
Determine the frame efficiency for each user.
(2 marks)
c
Briefly describe the three main properties of spreading codes which should be taken
into account when a type of spreading code is chosen for a particular CDMA system.
(6 marks)
a
Briefly describe the three main features of CDMA2000 systems which were not
present in IS-95 systems.
(6 marks)
b
State the three key differences between CDMA2000 and WCDMA-UMTS systems.
(6 marks)
c
Describe the role of the Radio Network Controller (RNC) in UMTS networks.
(4 marks)
d
The random access channel (RACH) is the only common channel used in the uplink of
WCDMA-UMTS systems. What is the purpose of the RACH?
(2 marks)
e
Briefly explain the purpose of scrambling codes in the downlink of WCDMA-UMTS
systems.
(2 marks)
8
Data Attachments
9210-119
Q-function
Q( z ) 1
2
xx22 exp
exp
zz 22 dxdx
Q-Function Table
9