UNIVERSITY OF SOUTHAMPTON ELEC6214W1
SEMESTER 2 EXAMINATIONS 2016-2017
ADVANCEDWIRELESS COMMUNICATIONS NETWORKS AND SYSTEMS
DURATION: 150 MINS (2.5 Hours)
This paper contains 5 (FIVE) questions.
Answer THREE questions out of FIVE questions.
An outline marking scheme is indicated in brackets to the right of each
question.
Only University approved calculators may be used.
A foreign language dictionary is permitted ONLY IF it is a paper version of
a direct ‘Word to Word’ translation dictionary AND it contains no notes,
additions or annotations.
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1.
a) The power delay profile of a mobile radio channel is given in Figure 1.
The system’s carrier frequency is fc = 3GHz, the transmitted signal
bandwidth is BS = 200 kHz, the symbol period of the system is TS =
5s, and the propagation speed is c = 3 108ms1.
0 10.0
1.0
0.8
0.6
0.4
2 3 4 8 96 7 10 11
P( )τ
0.2
τ ( s)µ5
Figure 1
The root mean square delay spread is defined as =
q
2 ()2,
where and 2 are the first and second moments of the channel’s
power delay profile, respectively, and the 50% coherence bandwidth
that is given by BC = 15 .
i) Calculate the 50% coherence bandwidth of the channel, and de-
cide whether an equaliser is required for this mobile communica-
tion system. [5 marks]
ii) You are making a mobile call at a high-speed train travelling at
a speed of 360 km=hr. Estimate the Doppler spread of the cor-
responding channel, and calculate the normalised Doppler fre-
quency of this fading channel. [5 marks]
b) The received signal power PRx at receiver is related to the transmitted
signal power PTx at transmitter by the following equation
PRx = PTx h r
where h is the small-fading channel gain which is assumed to be a con-
stant for all links, r is the distance from the transmitter to the receiver,
and is the pathloss exponent which is given by = 2.
Question 1 continued on the next page
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The spatial locations of source S, destination D, and potential relay R
are illustrated in Figure 2. For the receiver to successfully detect the
transmitted signal, the received signal power should be no less than
the threshold Pth, i.e., PRx Pth.
θ θ
4 kmkm4
D
R
S
Figure 2
i) Assume that = 60. In order to minimise the total required trans-
mit power, should source S directly transmit to destination D? or
should S transmit to relay R and let R forward the signal toD? and
why? (hint: cos 60 = 12 :) [4 marks]
ii) If = 30, repeat question i) above. In this case, calculate the
ratio of the total required transmit power for S ! D link to the total
required transmit power for S ! R! D link. (hint: cos 30 =
p
3
2 :)
[6 marks]
c) i) With the aid of sketches, briefly explain why orthogonal frequency
division multiplexing (OFDM) is an effective technique for combating
both time-domain fading and frequency selective channels.
[10 marks]
ii) What is the main drawback of OFDM transmission technique? You
should explain this drawback from the characteristics of OFDM trans-
mission signal. [3 marks]
TURN OVER
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2.
a) Draw the block diagram of the time-two carrier recovery scheme for bi-
nary phase shift keying (BPSK) transmission, clearly label the diagram
and explain the operation. [6 marks]
b) With the aid of a clearly labelled block diagram and signal plots, de-
scribe the time-two clock recovery scheme for BPSK transmission.
[6 marks]
c) i) Explain the frequency division duplexing (FDD) and time division
duplexing (TDD). In which of these two duplexing based systems, will
the uplink channel and the downlink channel be reciprocal? Explain
why. [8 marks]
ii) With FDD or TDD, a mobile user requires two resource blocks (two
frequency slots or two time slots) to achieve full duplexing, namely, one
resource block for transmitting and the other for receiving. Name the
the duplexing scheme that is capable of achieving full duplexing with
only single resource block. Explain how this full duplexing scheme
works with the aid of a system block diagram. [7 marks]
d) Explain why it is inappropriate to apply carrier sense multiple access
(CSMA) for wireless ad hoc LANs, and briefly describe the basic idea
of the contention algorithm, multiple access with collision avoidance
(MACA), for this type of wireless LANs. [6 marks]
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3.
a) Explain the three main parameters for designing a symbol constella-
tion, using the square 16-quadrature amplitude modulation (16-QAM),
star 16-QAM and 16-phase shift keying (16-PSK), as illustrations.
[7 marks]
b) Consider the 4-QAM communication system.
i) Derive the average error probability of the 4-QAM scheme over the
additive white Gaussian noise (AWGN) channel, expressed as a
function of the channel’s signal to noise ratio (SNR) Es=N0, where
Es is the average symbol energy and N0=2 is the two-side power
spectral density of the noise. [4 marks]
ii) Based on the result of question i) above, derive the average error
probability of the 4-QAM scheme over the Rayleigh fading chan-
nel, whose Rayleigh fading envelope has the probability density
function
p() =

20
e
2
22
0 ; 0;
where 20 is the second moment of the Rayleigh distribution. (You
may find the following integration formula useful.)
Z 1
0
2Q
p
2x

ex
2
x dx =
1
2

1 p
+ 2
!
:
[3 marks]
iii) Comment on the effect of fading to the achievable system perfor-
mance, and suggest a counter fading measure. [2 marks]
Question 3 continued on the next page
TURN OVER
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c) Classify various multiple-input multiple-output (MIMO) systems based
on multiple-antenna techniques into three types and briefly discuss
their main purposes. [6 marks]
d) In traditional networks with distributed access, such as ad hoc net-
works, there are no central access points.
i) Briefly explain how information can be transmitted from source to
destination in traditional ad hoc networks. [3 marks]
ii) What does the communication strategy you outlined in i) rely on?
[2 marks]
iii) In emerging networks known as delay tolerance networks, can the
communication strategy you outlined in i) still be applied? and
why? [2 marks]
iv) Explain briefly the new communication strategy or paradigm for
delay tolerance networks. [4 marks]
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4.
a) For the binary Bose-Chaudhuri-Hocquenhem code BCH(7; 4; 3), de-
scribed by the generator polynomial g(x) = 1+x+x3, draw the encoder
circuit, construct and draw the state-transition diagram or the state di-
agram, marking explicitly all the states and state-transitions, labelled
by the associated output bit. [7 marks]
b) For this BCH(7; 4; 3) system, the receiver demapper produces a log
likelihood ratio sequence: +0:8;1:2;+0:6;2:2;0:4;1:3;0:9, where
the left most value corresponds to the left-most position of the trellis.
The soft-input hard-output Viterbi algorithm is used for decoding.
Draw the associated trellis diagram for decoding, clearly marking all
the transitions and the associated branch metrics. Find the most likely
transmitted information sequence. [7 marks]
c) With the aid of a diagram, explain the operations of orthogonal space-
time block codes (OSTBCs). Clearly indicate what OSTBCs aim to
achieve and their associated drawbacks. [7 marks]
d) In a multiple-input multiple-output (MIMO) system, the base station
(BS) equipped with M antennas serves K single-antenna mobiles
based on the time division duplexing (TDD) protocol.
i) In uplink reception, what the BS needs in order to perform multi-
user detection (MUD)? Describe how the BS obtains this informa-
tion and give an MUD scheme based on this information.
[6 marks]
ii) In downlink transmission, what the BS needs in order to carry out
multi-user transmission (MUT) precoding? Describe how the BS
obtains this information and provide an MUT precoding scheme
based on this information. [6 marks]
TURN OVER
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5.
a) A multiple-input multiple-output (MIMO) system, consisting of nT trans-
mit antennas and nR receive antennas, communicates over flat fading
channels. The system is described by the following MIMO model
x(k) =H s(k) + n(k) ; (Eq: 5:1)
whereH is the nR nT channel matrix, s(k) = [s1(k) s2(k) snT (k)]T
is the transmitted symbol vector of the nT transmitters with E
hjsm(k)j2i
= 2s for 1 m nT , x(k) = [x1(k) x2(k) xnR(k)]T is the received
signal vector, and n(k) = [n1(k) n2(k) nnR(k)]T is the complex-
valued Gaussian white noise vector associated with the MIMO chan-
nels with E
h
n(k)nH(k)
i
= 22nInR. A bank of the spatial filters
ym(k) = w
H
mx(k); 1 m nT ;
are used to detect the transmitted symbols sm(k) for 1 m nT ,
where wm is the nR-dimensional complex-valued weight vector of the
m-th detector. During training, the m-th error signal for updating the
m-th detector’s weight vector is given by
"m(k) = sm(k) ym(k) :
i) Give the mean square error (MSE) expression, J(wm)=E
hj"m(k)j2i,
for the m-th detector. You should express the MSE in terms of the
MIMO system’s parametersH, 2n and 2s . [6 marks]
ii) What are the necessary and sufficient conditions for a detector
weight vector cwm to be a minimum point of the mean square error?
[4 marks]
iii) From these conditions, determine the minimum mean square error
(MMSE) solution cwm of the m-th detector’s weight vector.
[2 marks]
Question 5 continued on the next page
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b) Consider the MIMO system described by (Eq. 5.1) in a). The transmit-
ted data symbols take the values from theM -ary constellation set
sm(k) 2 S = fs(1); s(2); ; s(M)g
i) Assume that the MIMO channel matrixH is known at the receiver.
Write down the expression of the maximum likelihood (ML) solu-
tion for the optimal detection of the transmitted symbol vector s(k).
[4 marks]
ii) During the training period, the receiver has the training data
X = [x(1) x(2) x(K)]; S = [s(1) s(2) s(K)]
Write down the expression of the least squares (LS) estimate for
the MIMO channel matrix. [4 marks]
c) With the aid of clearly labelled block diagram and well-defined equa-
tions, briefly describe the operations of the coherent space-time shift-
keying transmitter and receiver. [13 marks]
END OF PAPER
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