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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