("g"")

\..... ../:
............................

MULTIPLE ANTENNA TECHNIQUES

9.1 MIMO SYSTEM

9.1.1 Introduction

~ Multiple Input Multiple Output (MIMO) is an antenna technology for

wireless communications in which multiple antennas are used for single user
at both the source (transmitter) and the destination (receiver) to exploit the
spatial domain for spatial multiplexing and/or spatial diversity to increase the
channel capacity and data rate.

User data CHANNEL User data

stream stream

~R
Fig. 9.1. MIMO antenna configuration

~ The antennas at each end of the communications circuit are combined to

I
minimize errors and optimize the data speed.
9.2 Wireless Communication

~ Generally, this type of multiplexing is used to increase the channel capacity

by transmitting simultaneously an independent data streams using the same
time slot and frequency band from each transmitting antenna and
differentiating multiple data streams at the receiver end using the channel
information about each propagation path.
J;, MIMO system offer much larger channel capacity over traditional SISO
system. Recently, many transmit beam forming algorithms have been
developed to exploit the high capacity in the MIMO systems.

9.1.2 Shannon's Channel Capacity Theorem

~ Shannon's channel capacity law defines, "the maximum rate at which error
free data can be transmitted over a given bandwidth in the presence of noise".

C = Wlog 2 (1+ SNR) ... (1)

where
C- Channel capacitytbps),
W- Bandwidth (Hz), and
SNR- Signal to Noise Ratio (unit less)

9.1.3 Basic Structures of MIMO System

There is a number of different MlMO configurations or formats that can be

mainly used in OFDM. They are

(i) Single- Input and Single -Output (SISO):

TX

Fig.9.2.SISO System
Multiple Antenna Techniques 9.3

"£ It uses single antennas both at the transmitter and receiver sides. There is no
diversity and no additional processing required and channel capacity of the
system expressed as,

C=Wlog 2(1+SNR) ... (2)

(ii) Single- Input and Multiple -Output (SIMO):

"£ Transmitter has a single antenna and the receiver has N multiple antennas,
which is also known as receiver diversity. The receiver system that receives
signals from a number of independent sources to combat the effect of fading.
"£ It has been used for many years with short wave listening/receiving stations to
combat the effects of ionosphere fading & interference.

TX
»)~ RX

Fig. 9.3. SIMO System

"£ It is easy to implement but an additional processing is required in the receiver.

The channel capacity of SIMa system expressed as,

C = Wlog 2 (1+ N R XSNR) ... (3)

where, NR - Number of receiving antenna.

(iii) Multiple- Input and Single -Output (MISO):

TX RX

Fig.9.4. MISO System

"£ The same data is transmitted from more than one antenna (M) and received by
single a~tenna. MISa is also termed as transmitter diversity. The receiver is
9.4 Wireless Communication

able to receive the optimum signal which is used to receive exact required
data. The channel capacity of MISO system expressed as,

." (4)

where, NT - Number of transmitting antenna.

(iv) Multiple- Input and Multiple -Output (MIMO):

~ It uses multiple antennas both for transmission and reception. Multiple

transmitting and receiving antennas will achieve antenna diversity without
reducing spectral efficiency.
~ It can utilize coding on the channel to separate the data from the different
paths. They can be used to provide improvements in both the channel
robustness as well as channel throughput to the radio link by improving the
Signal to Noise Ratio (SNR) or by increasing the link data capacity.

)>» 'V

TX :J>$ r" RX

Fig.9oS. MIMO System

~ The channel capacity of MIMO system can be expressed as,

.. , (5)

9.1.4 Advantages and Applications of MIMO

* Advantages:
(i) Higher channel capacity,
(ii) Better spectral efficiency,
(iii) Increased coverage,
(iv) Improved user position estimation,
(v) Lower power consumption,
Multiple Antenna Techniques 9.5
(vi) Minimize the errors,
(vii) Faster speeds, and
(viii) Higher data rate.
The main disadvantage is that it is complex.

* Applications
(i) MIMO is currently being used within the telecommunications and networking
industries, that is, cellular, WMAN, WWAN, and so forth.
(ii) MIMO is used largely in cellular towers.
(iii)· 'It is used in modern wireless standards, including in 3GPP LTE, and mobile
WiMAX systems.
(iv) MIMO - OFDM is considered a key technology in emerging high-data rate
systems such as 4G, IEEE 802.16 and IEEE 802.11n.

9.1.5 MIMO Implementation Techniques

J;, In MIMO, two techniques are used to transmit data using MIMO across the
given channel.

(i) Spatial diversity (or) simply diversity, and

(ii) Spatial multiplexing.

J;, When MIMO is implemented using diversity techniques, it provides diversity

gain in order to improve the reliability.
J;, When MIMO is implemented using spatial-multiplexing techniques, it
provides degrees of freedom or multiplexing gain in order to improve the
data rate of the system.

9.2 SPATIAL MULTIPLEXING(SM or SMX)

J, Spatial multiplexing is a very powerful technique for increasing channel

capacity at higher signal-to-noise ratio (SNR) and it is used to maximize the
data rate.
~~ It is a transmission technique in MIMO wireless communication to transmit
an original high-data stream is multiplexed into several parallel streams,
each of which is sent from one transmit antenna element, Therefore, the space
dimension is reused, or multiplexed, more than one time.
~ The channel "mixes up" these data streams, so that each of the receive
antenna elements sees a combination of them. An appropriate signal
processing at the receiver can separate the data streams,

Fig.9.6. Spatial multiplexing

~ A basic condition here is that, the number of receive antenna element should
be atleast as large as the number of transmit data streams.
~ If the transmitter is equipped with NT antennas and the receiver has NR
antennas, the maximum spatial multiplexing order (the number of streams) is,
n, ::::min(NpN R)

~ If a linear receiver is used, then Ns streams can be transmitted in parallel,

ideally leading to an Ns increases the spectral efficiency.
J, The practical multiplexing gain can be limited by the spatial correlation,
which means that some of the parallel streams may have very weak channel
gains,

* Difference Between Diversity and Spatial Multiplexing

~ Following Fig.9.7. illustrates the difference between diversity and spatial
multiplexing, In the transmit diversity technique, the same information is sent
Multiple Antenna Techniques 9.7
across different independent spatial channels by placing them on three
different transmit antennas. Here, the diversity gain is 3 (assuming 3xl MISO
configuration) and multiplexing gain is O.
Diversity gain = NT X N R
Transmit antennas Transmit antennas

101 1

101 o
101-~----' 101---';~----'

101 1
(a) (b)
Fig.9.7. (a) MIMO with diversity (b) MIMO with spatial multiplexing

~ In the spatial multiplexing technique, each bit of the data stream (independent
information) is multiplexed on the three different spatial channels thereby
increasing the data rate. Here, the diversity gain is 0 and the multiplexing gain
is 3 (assuming 3x3 MIMO configuration).
Multiplexing gain = N s =min(NpNR )

9.3 MIMO SYSTEM MODEL

~ Fig.9.8, shows the system model of MIMO. At the TX, the data stream enters
an encoder, whose outputs are forwarded to NTtransmit antennas.
~ From this antenna, the signal is sent through the wireless propagation channel,
which is assumed to be quasi-static andfrequency flat.
~ Quasi-static means that the coherence time ofthe channel is so long that is <fa
large number" ofbits can be transmitted within this time.
 Wireless Communication

Transmitter Channel Receiver

RX

Fig. 9.8. Block diagram ofa MIMO system model.

1& MIMO wireless systems utilize a matrix mathematical approach to analyze the
system. Consider the data streams ti, t2, t3 that can be transmitted from
antennas 1,2 and 3.

~YISil
III
"'::.. » y, y-~
~.III
h
I
n
r1
User data
stream
y 82 • ~ Yz Y User data
stream

CHANNEL
Y~T Marrlx H

~
S )'"
Transmitted vector Received vector
Fig.9.9. Representation of MIMO system model

Jv The channel transfer function h2l represents that the data traveling from the
first transmit antenna to the second receiving antenna. The received signals
can then be expressed as,
rr = h11 tl + hzr tz +h31t3 (1a)
rz = h12 h + hzz 12 +h3zb (1b)
f3 = h13 t] + h23 tz +h3313 (1e)
Multiple Antenna Techniques 9.9

where
r1- Signal received at antenna at 1, and
rz- Signal received at antenna at 2 and so forth.
~ In matrix format the above expression can be represented as,
[R] = [H] [T] ... (2)
~ The received signal vector Y can be expressed using the transmit signal
vector S as,
Y=HS +n=X+n ... (3)
where, 'n' is the additive white Gaussian noise
~ The Transfer function (channel matrix) H vector which is denoted by the
NT x N R matrix of the channel and it is given as:

H=

where, hij is a complex Gaussian random variable that models-

fading gain between the i th transmit and fh receive antenna.

9.4 PRE-CODING

9.4.1 Introduction

~ The main difficulty in MIMO channels is the separation of the data streams
which are sent in parallel. To decrease the multi-user interference and
increase the data rate in MIMO system, we are using the pre-coding
technique.
~ It is a preprocessing technique that performs transmits diversity and it is
similar to equalization, but the main difference is that we are in need to
optimize the precoder with a decoder. The Channel equalization aims to
 Wireless Communication

minimize channel errors, but the precoder aims to minimize the error in the
receiver output.
J. The precoding is a technique which exploits transmit diversity by weighting
information stream, reo the transmitter sends the coded information to the
receiver in order to get the prior knowledge of the channel. The receiver is a
simple detector, such as matched filter, and does not have to know the channel
side information. This technique will reduce the corrupted effect of the
communication channel.

l{ ~!L~
". I~~~~

..
~
,. Linear
1.......•.•...
')

~
,
........
.. •
Detection
&
output
.-"'0...
,.
precoderk· ........
' ..~ decoding
__..,,_ _., NT ~ NR
..~ _- . - - - - .

Y CSIT I
Fig.9.10. Block diagram ofMIMO pre-coding

CSIT- Channel State Information at Transmitter

J. Pre-coding or pre-equalization of the transmitted signals for MIMO systems is
the type of processing at the transmitter requires the Channel State
Information (CST).
J. In order to obtain CSI at the transmitter, the channel should be fixed (non-
mobile) or approximately constant over a reasonably large time period.
J, If CSI is available at the transmitter end then the transmitted symbols either
for a single-user or for multiple users can be partially separated by means of
pre-equalization at the transmitter.

9.4.2 Vertical Bell Labs Layered Space-Time Architecture (V-BLAST)

J, V-BLAST is a transmitter-receiver architecture which is mainly used to

implement multiplexing in MIMO.
Multiple Antenna Techniques 9.11

J;, This architecture can then achieve atmost a diversity order of NR, since each
coded symbol is transmitted from one antenna and received by NR antennas.
J;, By coding across the sub-channels, BLAST can average over the randomness
of the individual sub-channels and get better outage performance.

Coder
Modulator Stream 1 ~I""-----JI'
~
Data Coder
source Modulator Stream2 . /

Coder _~
Modulator Stream M

Fig.9.11. BLAST structure

9.4.3 Working Principle

J;, For example, you are sending information's' and it will pass through the
channel, 'h' and Gaussian noise, 'n'. Then, the received signal at the receiver
front-end will be,
r=sh +n
J;, The receiver will have to know the information about 'h' and 'n'. It will
suppress the effect of 'n' by increasing SNR. It needs information about the
channel 'h' and this will increase the complexity.
J;, The receiver mobile units have to be simple for many reasons like cost, size of
mobile unit. So, the transmitter, base station, will do the hard work and
predicts the channel.
J;, Let us call the predicted channel hest and for a system with precoder, the

information will be coded: (_!-J.

he.,·,
The received signal will be r=(~)s
h
+ n.
es'

J;, If your prediction is perfect, hes, = hand r ;::: s+ n and it turns out to be the
detection problem in Gaussian channels which is simple.
9.12 Wireless Communication

9.4.4 Types of Pre-Codlnq

1., Precoding can be realized without requiring channel state CSI at the
transmitter, while such information is essential to handle the inter-user
interference in multi-user systems.
1., In multi-user MIMO, a multi-antenna transmitter communicates
simultaneously with multiple receivers (each having one or multiple
antennas). This is known as Space-Division Multiple Access (SDMA). From
an implementation perspective, precoding algorithms for SDMA systems can
be sub-divided into,

(i) Linear precoding, and

(ii) Nonlinear precoding.

1., The capacity achieving algorithms are nonlinear, but linear precoding
approaches usually achieve a reasonable performance with much lower
complexity .
1., Linear precoding strategies include Maximum Ratio Transmission (MRT),
Zero-Forcing (ZF) precoding, and Transmit Wiener precoding,
1., The Nonlinear precoding is designed based on the concept of Dirty Paper
Coding (DPC), which shows that any known interference at the transmitter
can usually be subtracted without the penalty of radio resources if the optimal
precoding scheme can be applied on the transmit signal.
/l)~
§
E
<S
~
~

Compexity
Fig. 9.12. Linear Vs nonlinear precoding.
Multiple Antenna Techniques 9.13

9.5 BEAM FORMING

9.5.1 Introduction

~ Beam forming techniques can be used in any antenna system, particularly in

MIMO systems in order to create a certain required antenna directive pattern
to give the required performance under the given conditions.
~ Beamforming is the combination of radio signals from a set of small non-
directional antennas in order to simulate a large directional antenna.
Aligning the transmit signal in the direction of the transmit antenna array
pattern is called transmit beamforming.
~ It takes advantages of an interference to change the directionality of the
antenna. Beam former controls the phase and relative amplitude of the signals
atTX.
. ..
·
r--.,.· :.
·
·: MS

BS

MS

Interferece User
Fig.9.13. Beam forming in MIMO

~ Smart antennas are normally used because it can be controlled automatically

according to the required performance and the prevailing conditions. It is
divided into two groups namely,
9.14

(i) Phased Array Systems (PAS): Phased array systems are switched and have a
number of pre-defined patterns ~ the required one being switched according to
the direction required.

(ii) Adaptive Array Systems (AAS): This type of antenna uses an infinite number
of patterns and can be adjusted based on the requirements in real time.

Antenna
Array

hlt:~ferin~,

Conventional
V'
Switched
LJSer .~
Iii
~ V
Beamformlng Antenna Adaptive
Array Array Antenna Array
Fig. 9.14. Types of beam forming
- - ~ ~

1& MIMO beam forming using phased array systems requires the overall system
to determine the direction of arrival of the incoming signal and then switch in
the most appropriate beam. This is something of a compromise because the
fixed beam is unlikely to exactly match the required direction.
1& Adaptive array systems are able to direct the beam in the exact direction
needed, and also move the beam in real time, This is a particular advantage in
mobile telecommunications. However the cost is the considerable extra
complexity required.

9.5.2 Working Principle

1& In MIMO, beam forming sends the same symboi over each transmit antenna
with different scale factor.
J, At the receiver, all received signals are coherently combined using different
scale factor.
Multiple Antenna Techniques 9.15

;,'£, This produces a transmit/receiver diversity system, whose SNR can be

maximized by optimizing the scale factors (MRC).
;,'£, Beam forming leads to a much higher SNR than on the individual channels in
the parallel channel decomposition.
;,'£, For beamforrning CSI at the receiver is typically assumed since it is required
for coherent combining. A max 'p' is the SNR when H is known at the
receiver.
;,'£, When the channel is not known at the transmitter, the transmit antenna
weights are all equal, so the received SNR equals y = IIHu*lI, where the
normalized transmit vector 'u' is chosen to maximize y.

'* Advantages:

(i) Increase SINR, and

(ii) Support higher user densities.
9.16 Wireless Communication

9.6 POSSIBLE TWO MARKS QUESTIONS AND ANSWERS

1. What is MIMO systems?

Multiple Input Multiple Output (MIMO) is an antenna technology for wireless

communications in which multiple antennas are used for single user at both the
source (transmitter) and the destination (receiver) to exploit the spatial domain
for spatial multiplexing and/or spatial diversity to increase the channel capacity
and data rate.

2. State the Shannon's channel capacity theorem.

Shannon's channel capacity law defines, "the maximum rate at which error free
data can be transmitted over a given bandwidth in the presence of noise".
c = W log, (1 + SNR)
where
C- Channel capacity (bps),
W- Bandwidth (Hz), and
SNR- Signal to Noise Ratio (unit less)

3. Mention the advantages ofMIMO.

Advantages of MIMO are,

(i) Higher channel capacity,
(ii) Better spectral efficiency,
(iii) Increased coverage,
(iv) Improved user position estimation,
(v) Lower power consumption,
(vi) Minimize the errors,
(vii) Faster speeds, and
(viii) Higher data rate.
The main disadvantage is that it is complex.
Multiple Antenna Techniques 9.17
4. List the applications ofMIMO.

The applications of MIMO are,

(i) MIMO is currently being used within the telecommunications and
networking industries, that is, cellular, WMAN, WWAN, and so forth.
(ii) MIMO is used largely in cellular towers.
(iii) It is used in modern wireless standards, including in 3GPP LTE, and
, mobile WiMAX systems.
(iv) MIMO - OFDM is considered a key technology in emerging high-data
rate systems such as 4G, IEEE 802.16 and IEEE 802.11n.

S. What is meant by spatial multiplexing?

Spatial multiplexing is a very powerful technique for increasing channel capacity

at higher signal-to-noise ratio (SNR) and it is used to maximize the data rate.
It is a transmission technique in MIMO wireless communication to transmit an
original high-data stream is multiplexed into several parallel streams, each of
which is sent from one transmit antenna element. Therefore, the space dimension
is reused, or multiplexed, more than one time.
The channel "mixes up" these data streams, so that each of the receive antenna
elements sees a combination of them. An appropriate signal processing at the
receiver can separate the data streams.

6. Give the simple MIMO system model equation.

The simple system model is given as,

Y=HS +n=X+n
where,
Y- Received signal,
H- Transfer function of the channel,
X- Original information signal,
S- Transmitted signal, and
n- Additive white Gaussian noise.
9.18 WirelessCommunicatioll

7, Write a note on precoding.

Pre-coding or pre-equalization of the transmitted signals for MIMO systems is

the type of processing at the transmitter requires the Channel State Information
(r'~T\
\ '-"V.l.j_

It is a technique which exploits transmit diversity by weighting information

stream, i.e. the transmitter sends the coded information to the receiver in order to
get the prior knowledge of the channel. The receiver is a simple detector, such as
matched filter, and does not have to know the channel side information. This
technique will reduce the corrupted effect of the communication channel.

8. Differentiate between the types ofprecoding.

Precoding can be sub-divided into,

(i) Linear precoding, and
(ii) Nonlinear preceding.
The capacity achieving algorithms are nonlinear, but linear precoding approaches
usually achieve a reasonable performance with much lower complexity. Linear
precoding strategies include Maximum Ratio Transmission (MRT), Zero-
Forcing (ZF) precoding, and Transmit Wiener preceding.
The nonlinear precoding is designed based on the concept of Dirty Paper
Coding (DPe), which shows that any known interference at the transmitter can
usually be subtracted without the penalty of radio resources if the optimal
precoding scheme can be applied on the transmit signal.

9. What is beam forming?

Beam forming techniques can be used in any antenna system, particularly in

MIMO systerps in order to create a certain required antenna directive pattern to
give the required performance under the given conditions.
Multiple Antenna Techniques 9.19

10. Mention the types ofbeam forming in MIMO system.

Smart antennas are normally used in beam forming because it can be controlled
automatically according to the required performance and the prevailing
conditions. It is divided into two groups namely,
(i) Phased Array Systems (PAS): Phased array systems are switched and
have a number of pre-defined patterns - the required one being switched
according to the direction required.
(ii) Adaptive Array Systems (AAS): This type of antenna uses an infinite
number of patterns and can be adjusted based on the requirements in real
time.

11. List the advantages ofbeam forming.

The main advantages of beam forming are,

(i) Increase SINR, and
(ii) Support higher user densities.
9.20 Wireless Communication

9.7 POSSIBLE SIXTEEN MARKS QUESTIONS

1. Explain MIMO system in detail with a neat diagram.

2. With neat sketches, explain about spatial multiplexing.
3. Define MIMO system. Explain system model in detail,
4. Why precoding is necessary? Also explain the working principle of MIMO
system.
5. Explain beam forming technique in detail.

=*=*=*=*=*=
 (';0'·')
..................................~~::~

MULTIPLE ANTENNA - II

10.1 TRANSMITTER OR TRANSMIT DIVERSITY

10.1.1 Introduction

~ In transmitter diversity, multiple antenna elements are required at the

transmitter and only one antenna element at the receiver end provides better
performance. The transmit power is divided among these antennas and it is
shown as

Fig.l0.1. Transmitter diversity

~ Transmitter diversity can achieve diversity in the downlink, even if the mobile
has only a single antenna.
~-------- Wireless Communication

;x;, This method uses multiple base station antennas and make their signals
separated by assigning them different CDMA spreading codes (or different
delay of the same code) or by using space-time codes.
fio.
'1"
Transmit
d(l) antenna 1
Data
symbols cl(O)

Transmit
t-'" antenna 2

Fig.l0.2. Data transmission in transmitter diversity

1 MIMO increases the robustness of data transmission especially in fading

scenarios via transmit diversity. Each transmit antenna transmits the same
stream of data. This might increases the signal to noise ratio at the receiver
end.

10.1.2 Types of Transmit Diversity

The two main different types considered under transmit diversity are:
(i) Transmitter diversity with the channel-state information (closed loop transmit
diversity), and
(ii) Transmitter diversity without the channel -state information (open loop
transmit diversity).

(1) Closed Loop Transmit Diversity:

~ In the first case, the transmitter knows perfectly about the channel. This
information is obtained by means of feedback from the receiver.
Multiple Antenna- II 10.3

Transmit
delivery Receiver
encoder

Feedback channel Cht, h2 , ... h~

Fig. 10.3. Closed loop transmit diversity

~ If the selected antennas maximizes the Signal to Interference Noise Ratio

(SINR), then the transmit encoder receives feedback channel state information
from the receiver. This attempts to fine encoding matrix which also maximizes
SNR at the receiver.

ITransmitter I I Receiver I

Space time • Space time

t-----:j~
sequence encoder dencoder Estimated bit
sequence

Fig.l0A.STB encoder in transmitter diversity

~ Space-Time Block Coder (STBC) has emerged as an efficient means of

achieving near optimal transmitter diversity gain. Assume that two transmit
antennas and one receive antenna are using in the transmit diversity.
Transmitter 1 Transmitter 2
Timet Xl X2

Timet+ T -X2* XI*

10.4 Wireless Communication

Where, xi and X2 are the modulated symbols. Antenna 1

transmit with (xp-x;) and antenna 2 transmit with (x2'X~) . Then, the
STBC transmission matrix is expressed as,

'i An optimum transmission scheme linearly weights the signals sent from the
various antenna elements with the complex conjugates of the channel transfer
functions from the transmit antenna elements to the single receive antenna.
.This approach is known as Maximum Ratio Transmission (MRT). Here, the
choice of antenna weights will maximize the received SNR.

(2) Open Loop Transmit Diversity:

~ The transmission of signals from the different antenna elements has to be done
in such a way that it will allow the receiver to distinguish different transmitted
signal components. One such method is called as delay diversity. Here, we
transmit data streams with the delay of one symbol duration from each of the
transmit antennas.

10.2 RECEIVER DIVERSITY

1& In receiver diversity, one transmitting antenna and many receiving antennas
are used. Here, the desired message is transmitted by using single
transmitting antenna and received by multiple antennas. NR different antennas
appropriately separated are deployed at the receiver to combine the
uncorrelated fading signals, It is also called as space diversity.
1& In space diversity, several receiving antennas are spaced sufficiently far apart
(spatial separation should be sufficently large to reduce the correlation
between diversity branches, e.g. > lOA).
~ The space diversity has been widely used because it can be implemented
simply and economically.
Multiple Antenna- II 10.5

Transmitter Receiver

Fig.lO.5. Receiver diversity

* Classification of Space Diversity:

Space diversity reception method can be classified into four categories

namely,

(i) Selection diversity,

(ii) Feedback diversity,
(iii) Maximal ratio combining, and
(iv) Equal gain diversity.

(1) Selection (or) Switching Diversity:

~ Selection diversity is more suitable for mobile radio applications because of its
simple implementation.
~ In this scheme, the receiver monitors the SNR value of each diversity channel
and chooses the one with the maximum SNR value for signal detection.
~ The receiver with 'M' demodulators are used to provide 'M' diversity
branches whose gains are adjusted to provide the same average SNR for each
branch.
~ The receiver branch having the highest instantaneo~s SNR is connected to the
demodulator. The antenna signals themselves could be sampled and the best
one sent to a single demodulator.
W.6 Wireless Communication

SNR Select :rvIAX

Monitor SNR= Ytnax

Transmitter Receiver

Channel M I----+-....

Fig. iO.6.Block diagram for selection diversity

~ Advantage: Selection diversity is easy to implement because all that needed is

a side monitoring station and an antenna switch at the receiver.
~ Disadvantage: It is not an optimal diversity technique because it does not use
all of the possible branches simultaneously.

(2) Feedback or Scanning or Threshold Diversity:

~ Scanning diversity is very similar to selection diversity except that instead of

always using the best of 'n' signals, the 'n' signals are scanned in a fixed
sequence until one is found to be above a predetermined threshold.
'1/ '1/
AntennasT T

" Control Present

----~------------ ~~~ threshold

Fig.iO.7. Block diagram of scanning diversity

~ It is then received until it falls below threshold and the scanning process is
again initiated.
Multiple Antenna- II 10.7

* Merits:
(i) It is very simple to implement.
(ii) Only one receiver is required.

(3) Maximal Ratio Combining (MRC):

~ In this method, the signals from all of the 'M' branches are weighted
according to their individual signal voltage to noise power ratios and then
summed up.
~ Here, the individual signals must be co - phased before being summed which
generally requires an individual receiver and the phasing circuit for each an
antenna element.
~ Maximal ratio combining produces an output SNR which is equal to the sum
of the individual SNRs.

Cophase
and sum Detector

III

Fig.Itl.S. Maximal ratio combiner

* Advantages:
(i) It produces an output with an acceptable SNR even when none of the
individual signals are themselves acceptable.
(ii) It gives the best statistical reduction of fading of any known linear diversity
combines.
10.8 Wireless Communication

(4) Equal Gain Diversity (EGD)

~ It is similar to maximal - ratio combining except that, there will be an

omission of the weighting circuits.

Channel 1 I--.......:...~

Chanriel 2 I----~
Transmitter Receiver

Fig.l0.9. A simple block diagram ofEGD

~ In this method, the branch weights are all set to unity but the signals from
each branch are co-phased to provide an equal gain combining diversity.
~ This allows the receiver to exploit the signals that are simultaneously received
on each branch.
~ The performance improvement obtained by an EGC is slightly inferior to that
of a MRC, since interference and noise corrupted signals may be combined
with high - quality (noise and interference free) signals. EGC is superior to
selection diversity.

10.3 CHANNEL-STATE INFORMATION (CSI)

10.3.1 Introduction

J;, in wireless communication, CSI simply represents the properties of a

communication link between the transmitter and receiver.
1& The CSI describes how a signal propagates from the transmitter to the
receiver and represents the combined effect of, for example, scattering,
fading, and power decay with distance.
Multiple Antenna- II 10.9

J, The CSI makes it possible to adapt transmissions to current channel

conditions, which is crucial for achieving reliable communication with high
data rates in multi -antenna systems.
J, The CSI at the transmitter is vital in MIMO systems in order to increase the
transmission rate, to enhance coverage, to improve spectral efficiency and to
reduce receiver complexity.
J, The CSI is usually estimated at the receiving end and then quantized and fed
back to the transmitter side. The transmitter and receiver can have different
CSI.

CSIT CSIR

Transmitter Receiver

CSI

CSIT - CSI at Transmitter

CSIR - CSI at Receiver
Fig.IO.IO. CSlfrom receiver to transmitter

10.3.2 Types of CSI

There are basically two levels of CSI, namely,

(i) Instantaneous CSI, and
(ii) Statistical CSI.

(i) Instantaneous CSI:

~ Instantaneous CSI is also known as short-term CSI. Instantaneous CSI means

that the current conditions of the channel are known, which can be viewed as
knowing the impulse response of a digital filter.
10.10 Wireless Communication

~ This gives an opportunity to adapt the transmitted signal to the impulse

response and thereby optimize the received signal for spatial multiplexing or
to achieve a low bit error rates.

(ii) Statistical CSI:

Statistical CSI is also known as long-term CSI. It means that a statistical

characterization of the channel is known. This description can include the type
of fading distribution, the average channel gain, the line-of-sight component,
and the spatial correlation.

10.3.3 MIMO-CSI Transmission

The algorithms used for MIMO transmission can be categorized based on

the amount of CSI that they require as follows:

(i) Full CSI at the TX and full CSI at the RX:

In this ideal case, both the TX and the RX have full and perfect knowledge of
the channel, which results in the highest possible capacity. But this case is
very difficult.

(li) Average CSI at the TX and full CSI at the RX:

In this case, the RX has full information about the instantaneous channel state,
but rx knows only the average CSI.

(iii) No CSI at the TX l:md full CSI at the RX:

Without any feedback or calibration, this case can be achieved most easily.
The TX simply does not use any CSI, while the RX learns the instantaneous
channel state from a training sequence or using the blind estimation.

(iv) Noisy CSI:

When we assume "full CSI" at the RX, this implies that the RX has learned
the channel state perfectly. Any received training sequence will gets affected
Multiple Antenna- II 10.11

by an additive noise as well as quantization noise, then RX processes the

signal based on the observed channel Hobs, while the signal pass through the
actual channel H acl•
H act =H obs + L1
L1- Changes due to noise.

(v) No CSI at the TX and no CSI at the RX:

The channel capacity is high when neither the TX nor the RX have CSI.

10.4 CAPACITY OF MIMO

10.4.1 Shannon Capacity Theorem

1& Channel capacity is the maximum mutual information of a channel. Its

significance comes from Shannon's coding theorem and converse, which
shows that the capacity is the maximum error-free data rate a channel can
support.
:L Capacity is a channel characteristic - not dependent on transmission or
reception techniques or limitation.
1& In AWGN,

C = Blog z (1 + rlHlz)
Where,
B - Signal bandwidth,

r =!-
N
- Received Signal-to-Noise Power ratio, and
H - Normalized transfer function from the transmitter to the receiver.

10.4.2Capacity in Fading (or) Frequency Selective Channels

~ Capacity for time-invariant frequency-selective fading channels is hard, which

results in a "water- filling" of power over frequency. For time-varying lSI
channels, the capacity will be unknown.
10.12 Wireless Communication

1& Approximately it is obtained by dividing up the bandwidth subbands of width

equal to that of the coherence Bandwidth. We assume independent fading in
each subband.
J;, The capacity in each subband will be obtained by means of flat-fading
analysis. The power gets optimized over both the frequencies and time.
1& Then the capacity of channel H is given by the sum of the capacities of the sub
channels and it is expressed as,

c == IlOg2[1 + P~ U;]
k=1 Un
... (I)

where,
7 'T· .
U~ - Noise variance,
Pi - Power allocated to the k th antenna,.
RH - No of parallel channels.
u; - Signal variance.
J; If we assume that L P, = P is independent of the number of antennas. Then
the equivalent capacity expression for equation(l) is expressed as,

C=lOg,ldetrIN.
L \
+; HR~Hnll
. JJ
... (2)

where,
I N R is the N R X N R identity matrix,

r is the mean SNR per receiver branch,

Rss is the correlation matrix of the transmit data.
det is represents determinant.
J; The distribution of power among the different antennas depends on the
amount of CSI at the transmitter and assumes that the receiver has perfect
CSI. Then the capacity increases linearly with min(NT'NR,N s)' where Ns is
the number of parallel data stream
Multiple Antenna- II 10.13

(1) No CSI at the TX and full CSI at RX:

~ When the RX knows the channel perfectly, but no CSI is available at the TX,

it is optimum to assign equal transmit power to all TX antennas, Pk =~ ,

NT
and uses an uncorrelated data streams. Then, the capacity is expressed as,

'" (3)

~ The capacity of a MIMO system increases linearly with min(NT' N R)'

irrespective of whether the channel is known at the TX or not.

* Special Cases:

In some special cases, consider that NT = N R =N :

(i) All transfer functions are identical. This case occurs when all antenna
elements are spaced very closely together, and all waves are corning from
similar directions and the rank of the channel matrix is unity. Then, the
capacity is expressed as,

CM /MO = log2(1 + N r) ... (4) .

Here, the SNR is increased by a factor of N compared with the single antenna
case, due to the beamforming gain at the RX. The number of antennas leads
to a logarithmic increase m capacity.
(ii) All transfer functions are different. This case can occur when the antenna
elements are spaced far apart and arranged in a special way. Then the capacity
is expressed as,

C M /MO = NIOg 2(1+ r) '" (5)

Thus, the capacity increases linearly with the number of antenna elements.
10.14 Wireless Communication

(iii) For parallel transmission channels, the capacity also increases linearly with
the number of antenna elements. However, the SNR per channel decreases
with N, then the total capacity is expressed as,

eM",o ~ NIOg{1( + ~-J .. , (6)

(2) Full CSI at TX and full CSI at the RX~ Water Filling Method:

~ When both the RX and TX knows the channel perfectly, then it is not
necessary to distribute power uniformly between the different transmit
antennas but assign it based on the channel state.
c;£ Optimally allocating the power to several parallel channels is difficult because
each has different SNR. This issue can be overcome by using water filling
method.
no power
.
allocated

Fig.lO.n. Principle behind waterfilling.

~ Imagine a number of connected vessels. We may consider that at the bottom

of each vessel is a block of concrete with a height that equal to the noise
power of the corresponding sub channel.
~ You have volume S of water which is used to till a vessel with variable bottom
height. The water will find its level f.1. The amount of poured water is
proportional to the total transmit power that is available.
c;£ Then the height of water in each part of vessel will be equal to the power to be
allocated at the corresponding sub channel.
Multiple Antenna-It 10.15

~ Some parts of vessel will not be covered by water. These parts correspond to
sub channels with so strong noise, that it is better not to use them at all.
~ Obviously, subchannel 1, which has the highest SNR, has the most water in it
and some subchannels that have a poor SNR (like channel m), do not get any
power assigned to them at all.
~ Essentially, water filling makes sure that energy is not wasted on subchannels
that have poor SNR, that is, not wasting power on subcarriers in OFDM that
are in a deep fade.
~ With water filling, power is allocated preferably to subchannels that have a
good SNR, with the transmitter requires making use of the large capacity on
good sub channels and also the transmitter has to adapt the data rate according
to the SNR that is available.
~ Consequently, the coding rate as well as the constellation size of the
modulation method has to be adjusted. The power allocation P n of the nth sub
channel is expressed as,

... (7)

where,
all is the gain of the nth sub channel,

0',7 is noise variance,

P n is the nth sub channel, and
e is the threshold which is determined by the constraint of the total
transmitted power Pas,

... (8)

Thus by using water filling method, the total capacity can be computed as:
N
ClI'tlterflll = ~)Og2 (1 + a; pj0';) ... (9)
11=1
10.16 Wireless Communication

10.4.3 Capacity in Flat-Fading (or) Non Fading Channels

J;, Depends on what is known about the channel (CSI). There are three cases
namely:

(i) Fading statistics known,

(ii) Fade value known at the receiver- No CSI at TX and perfect CSI at the RX.
(iii) Fade value known at the transmitter and receiver- Perfect CSI at the TX and
RX.

J;, When only the fading statistics are known, the capacity will be difficult to
compute. Only known results are there for Finite State Markov channels,
Rayleigh fading channels, and block fading. The two types of capacity
possible in flat-fading MIMO systems are:
(i) Ergodic (Shannon) capacity, and
(ii) Outage capacity.

iIi Ergodic Capacity:

It is the expected value of the capacity, taken over all realizations of the
channel. This quality assumes an infinitely long code that extends over all the
different channel realizations.

* Outage Capacity:
This is the minimum transmission rate that is achieved over a certain fraction
of time.

(1) No CSI at TX and perfect CSI at the RX

Capacity given by C = r Blog 2 (1+ r )p(r )dr bps ... (10)

where, p(r) is the distribution of the fading SNR r

'i By using Jensen's inequality this capacity always less than that of an AWGN
channel. This is the "Average" capacity formula, but transmission rate is
fixed.
Multiple Antenna- II 10.17

(2) Capacity with Fading Known at the Transmitter and Receiver (Full CSI
at TX and full CSI at the RX):

~ For fixed transmit power, the same capacity will be available when only the
receiver knows fading. By Jensen's inequality, fading reduces capacity w.r.t.
AWGN for the fixed transmit power.
~ Transmit power as well as transmission rate can be adapted. If the transmit
power S(y) varies with r with respect to an average power constraint'S',
then the under variable rate and power of the channel capacity will be ,

c s!
= BIOg z(l+ p~)r)p(r)dr ... (11)

where, p(r) is power adaptation.

10.18
_._--_._----~------~--~--
Wireless Communication

10.5 POSSIBLE TWO MARKS QUESTIONS AND ANSWERS

(1) Define transmitter diversity,

In transmitter diversity, multiple antenna elements are required at the

transmitter and one antenna element at the receiver end and provide better
performance. The transmit power is divided among these antennas.

(2) Mention the types oftransmit diversity,

The two main types considered under transmit diversity are:

(i) Transmitter diversity with the channel-state information (closed loop
transmit diversity), and
(ii) Transmitter diversity without the channel -state information (open
loop transmit diversity).

(3) Expand lVlRT,

An optimum transmission scheme linearly weights the signals sent from the
various antenna elements with the complex conjugates of the channel transfer
functions from the transmit antenna elements to the single receive antenna.
This approach is known as Maximum Ratio Transmission (MRT). Here, the
choice of antenna weights will maximize the received SNR.

(4) Define receiver diversity.

In receiver diversity, one transmitting antenna and many receiving antennas are
used, Here, the desired message is transmitted by using single transmitting
antenna and received by multiple antennas. NR different antennas appropriately
separated are deployed at the receiver to combine the uncorrelated fading
signals. It is also called space diversity.
Multiple Antenna- II 10.19

(5) Name the different types of space diversity.

Space diversity reception method can be classified into four categories

(i) Selection diversity,
(ii) Feedback diversity,
(iii) Maximal ratio combining, and
(iv) Equal gain diversity.

(6) What is meant by CSl?

In wireless communication, Channel State Information (CSI) simply represents

the properties of a communication link between the transmitter and receiver.
The CSI describes how a signal propagates from the transmitter to the receiver
and represents the combined effect of, for example, scattering, fading, and
power decay with distance.

(7) What is ergodic capacity and outage capacity ofa flat fading channel?

The two types of capacity possible in flat-fading MIMO systems are:

(i) Ergodic (Shannon) capacity, and
(ii) Outage capacity.
Ergodic capacity is the expected value of the capacity, taken over all
realizations of the channel. This quality assumes an infinitely long code that
extends over all the different channel realizations.
Outage Capacity is the minimum transmission rate that is achieved over a
certain fraction of time.
10.20 Wireless Communication

10.6 POSSIBLE SIXTEEN MARKS QUESTIONS

1. Describe the transmit diversity in MIMO system.

2. Explain receiver diversity technique in detail.
3. Discuss in detail about spatial diversity and its types with neat sketches.
4. What is known as channel state information? Explain in detail.
3. Calculate the capacity of a MIMO system in flat fading and non-fading channels.

=*=*=*=*=*=
 B.EIB.Tech. DEGREE EXAMINATION, NOVIDEC 2015
Fifth Semester
Information Technology

EC6801- WIRELESS COMMUNICATION

(Regulation 2013)
(Common to B.E. (Part - Time) Seventh Semester Regulation 2005)

Time: Three hours Maximum: 100 marks

Answer ALL questions.

Part A - (10 x 2 = 20 marks)

1. Find the far-field distance for an antenna with maximum dimension 012m and
operating frequency of 1GHz.

Solution:
Given:
Operating frequency, f = 1 GHz
Large dimension of antenna, D =2m

~
8
300x10 6
Operating wavelength, A= = 3x10 6 = - - - -
f 1000xl0 1000xl0 6

A=O.3 m

2D 2 2(2Y 8
Far - field distance, dr = - - = - - = -
A 0.3 0.3

I df= 26.67 ill I

Q-2 Wireless Communication

2. Define Coherence time and Coherence bandwidth.

(i) Coherence Time (Tc):

Coherence time (Te) is usually defined as, "the required time interval to
obtain an amplitude correlation of 0.9 or less between two received signals in
multipath propagation".
It is the time duration over which the two received signals have a strong
potential for an amplitude correlation and It is inversely proportional to the
maximum Doppler frequency as

T =_1
c
fm

where, fm - Maximum Doppler frequency.

(ii) Coherence Bandwidth:

The coherence bandwidth is a measure of the maximum frequency

difference (bandwidth) for which the received signals strongly correlated in
amplitude. This bandwidth is inversely proportional to the rms value of time delay
spread (J,,) as,
1
Bex-
c
(J"

3. Define co-channel reuse ratio(Q).

The minimum ratio of D / R that is required to provide a tolerable level of

co - channel interference is called the co - channel reuse ratio and it is given by,
r> D
!.! = -
R
where,
D - Distance between two co - channel cells, and
R - Cell radius.
Solved Anna University Question Papers Q-3

4. Define the Grade ofservice.

The Grade of Service (GaS) is a measure of the ability of a user to access a

trunked system during the busiest hour.

5. Find the 3~dB bandwidth for a Gaussian low pass filter used to produce
0.25GMSK with a channel data rate of Rb = 270 kbps. What is the 90% power
bandwidth in the RF channel?

Solution:
1 6
---3 =3.7xlO =3.7flS '" (1)
270 x 10
Where, BT = 0.25 '" (2)
By substituting equation(l) in equation(2),

B = 0.25 0.25 =67.567 kHz

T 3.7 X 10- 6
Thus the 3-dB bandwidth is 67.567 kHz. To determine the 90% power
bandwidth, 0.57 Rs is the desired value.
RF BW = 0.57 Ri, = 0.57x270x10 3 = 153.9 kHz

6. Mention the difference between FDMA and OFDM.

One of the basic differences between OFDM and FDMA is that spectrum
overlapping is allowed in OFDM which is not possible in the case of FDMA and
hence makes efficient use of the available bandwidth.
Although frequency division multiplexing (FDM) implies multiple data
streams, orthogonal FDM (OFDM) carries only one data stream which is broken
up into multiple signals, Hundreds or Thousands of carriers, known as "sub-
carriers", are used for a single data channel.
 l~Tireless Communication

7,.. What are the factors used in adaptive algorithms?

The performance of an adaptive algorithm is determined by the following factors.
(i) Rate of convergence,
(ii) Misadjustment,
(iii) Computational complexity, and
(iv) Numerical properties.

8. Draw the structure ofa linear transversal equalizer.

delay elements

y{t) + "b(t) ~T'---r---'

clock ~ ! !- i
Taps

Basic linear transversal equalizer structure

9. What is antenna diversity?

Spatial diversity, also known as space diversity or antenna diversity, is any

one of several wireless diversity schemes that uses two or more antennas to
improve the quality and reliability of a wireless link. Often used especially in urban
and indoor environments mainly in base station design.

10. What is meant by CSI?

In wireless communication, Channel State Information (CSI) simply

represents the properties of a communication link between the transmitter and
receiver.
The CSI describes how a signal propagates from the transmitter to the
receiver and represents the combined effect of, for example, scattering, fading, and
power decay with distance.
Solved Anna University Question Papers Q-5

PARK-B - (5* 16 = 80 marks)

11 (a) (i) Explain the advantages and disadvantages ofthe two-ray ground reflection
model in the analysis ofpath loss. (4)
Advantages:
(i) It is used for modeling the land mobile radio. It is a useful propagation
model that is based on geometric optics, and considers both the direct path
and a ground reflected propagation path between the transmitter and
receiver.
(ii) This model is reasonably accurate for predicting the large - scale signal
strength over distances of several kilometersfor mobile radio systems that
use tall towers (height which exceed SOm), as well as for line - of - sight
microcell channels in urban environments.
Disadvantages:
(i) The power decays as the fourth power of distance.

P, (d) = ~ with K being a constant.

(ii) Path loss is independent of frequency(wavelength)

(iii) Received power is also proportional to h,2 and h; meaning, if height of

any of the antennas is increased, received power increases.
(iv) In this model it does not include important factors such as terrain profile
and buildings.

(ii) In the following cases, tell whether the two-ray model could be applied, and
justify why or why not.
Case (i) : ht = 35 m, h- = 3 m, d= 250 m
Case (ii) : h, =30 m, h- =1.5 m, d= 450 m (6)
Generally, when d> lO(ht + h-), we can say that d > > h, + h-, and this
may apply the two ray model.
 (i) h. = 35 m, h, = 3 m, d= 250 m
l Oth. + h-) = 10(35+3) = 380 m
d < IOth. + h-) = 250 m < 380 m
Hence, the two ray model could not be applied.

(ii) h t = 30 m, h. = 1.5 m, d= 450 m

IOth. + h-) = 10(30+1.5) = 315 m
d > 1O(ht + h-) = 450 m > 315 m
Hence, the two ray model could be applied.

L\ = d" -d'= 2h t hr
(iii) Prove that in the two-ray ground reflected model, d (6)
L\ = d"-d'

For d»ht+h ( ht: h -)2 «1, (h dh)2 «1

r
• r I r

Using Taylor series approximation, we have

r fl t r
r , o'
= dll+il :fl j J- dll+il :fl j J
c
r
"\2l .f •
flr
•
r
"\2l

. 1 r(h, + h ')2 ( ht -h. ') 21

= d' 2 ll d
r

) -l d ') J
= _1 [(h ,2 + h; + 2h/tr ) - (ht2 + h; - 2hth r ) ]
2d

L\ = d" -d'= 2ht hr

d
(OR)
Solved Anna University Question Papers Q-7

(b) Derive the Impulse response model ofa multipath channel. (16)

The small- scale variations of a mobile radio signal can be directly related
to the impulse response of the mobile radio channel.
The impulse response model is a wideband channel characterization and
contains all information necessary to simulate or analyze any type of radio
transmission through the channel.
The filtering nature of the channel is caused by the summation of
amplitudes and delays of the multiple arriving waves at any instant of time.
The impulse response model is a useful characterization of the channel, it
may be used to predict and compare the performance of many different mobile
communication systems. and transmission bandwidths for a particular mobile
channel condition.
Mobile radio channel may be modeled as a linear filter with a time
varying impulse response.

Transmit
base station =
d vt
movement

Fig. 1. Fundamental radio propagation and system concepts

In the Figure.I. the receiver moves along the ground at some constant
velocity v. Here time variation is due to receiver motion in space.
For a fixed position d, the channel between the transmitter and the receiver
can be modulated as a linear time invariant system.
 Wireless Communication

Due to the different multipath waves which have propagation delays

which vary over different spatial locations of the receiver, the impulse response
of the linear time invariant channel should be a function of the position of the
receiver.
The channel impulse response can be expressed as h(d, t) and x(t)
represent the transmitted signal, then the received signal y (d, t) at position d
. y(d,t)= ~(t)®h(d,t)
cc

:= f x(r)h(d,t-r)dr ... (1)

where,

1: represents the channel multipath delay for a fixed value of t.

For a causal system, h (d, t) = 0 for t < 0, then equation (1) becomes
t

y(d,t)= fX('C)h(d,t-'C)d'C ... (2)

-00

d Spatia! position

Fig.2. The mobile radio channel as a function oftime and space

The receiver moves along the ground at a constant velocity 11, and the
position of the receiver can be expressed as,
Distance =Velocity x Time
d=vt ... (3)
By substitute equation (3) in equation (2), we get
Solved Anna University Question Papers Q-9
t

Y(vt,t) = Jx(t)h(vt,t-'t)d't ... (4)

Yrvt, .t) is just a function of t, and v is a constant, then equation (4)

becomes
t

Y(t) = Jx('t)h(vt;t-'t)d't
-OQ

= x(t) ® h(vt, t)

= x(t) ®h(d, t] '" (5)

The mobile radio channel can be modeled as a linear time varying channel,
where the channel changes with time and distance.

12. (a) (i) A cellular service provider decides to use a digital TDMA scheme which
can tolerate a signal to- interference ratio of 15 dB in the worst case. Find
the optimal value ofN for
(1) Omni directional antennas (3) .
(2) 120° sectoring (3)
(3) 60° sectoring (3)
(4) Should sectoring be used? If so, which case (60° or 120°) should be
used? (Assume a path loss exponent of n =4 and consider trunking
~~~ rn
Solution
(1) Omni directional antennas:
Given:
Path loss exponent (n) = 4

Tolerable signal to interference ratio, ~ =15 dB = 31.623

I
Assuming 6 interferers from the first tire of co-channel cells,
Q-10 Wireless Communication

.~_ (%)" _(EN)"

I io io

N =4.592
Since we have to choose higher possible value to satisfy the S/I requirement,
N=7. If we calculate sir from N=7, we get 18.66 dB which is better than the
requirement.

(2) 120° Sectoring:

Let us consider first that N=4, (i=2,j=0) .If we see the layout of the cells with
N=4, it seems as below.

It is clear from the diagram that with 120° sectoring and N=4, there are 2
interferers in the first tier of co-channel cells. Taking io=2 in the expression

s (J3N)" _ (-J3X4t = 72 = 18.57 dB

I io 2
With 120° sectoring, the sir obtained is better than required. So N=4 can be
used.
Solved Anna University Question Papers Q-11

(3) 60° Sectoring:

We have to again check for N=3

Let us consider that N=3, (i= 1,j=1) .If we see the layout of the cells with N=3, it
seems as below.

\~

Taking io=2 in the expression

s = (J3Nf = (~y = 40.5 = 16.074dB

I io 2
Since it is lower than the required value, N=3 can be used.

(4) To satisfy the SII requirement, optimum values of N are

N = 7 for omnidirectional antenna

N = 4 for 120° sectoring
N = 3 for 60° sectoring

(ii) If a signal- to - interference ratio of15dB is requiredfor satisfactory forward

channel performance of a cellular system, what is the frequency reuse factor
and cluster size that should be used for maximum capacity of the path loss
exponent is
(a)n =4(b)n =3?
Assume that there are six co-channel cells in the first tier, and all ofthem are
at the same distance from the mobile. Use suitable approximation?
Q-12 Wireless Communication

Solution:
(a) n = 4 .
First let us consider a seven cell reuse pattern (i.e.) cluster size N := 7
n-

Co-channel reuse ratio (Q) == u == .J3N == ~3X7 =.fij

R

IQ ~ ~ ~ 4.5 831
Signal- to - noise interference ratio is given by,

-S = (D/R)fi
-'--'---C.._
I 10

No. of co-channel interfering cell iO =6

-i = (i)X(4.583)4 = (0.167) x (441.16)
I~ ~ 7367 1
~(dB) = 1010gJO 73.67
I

~(dB):= 18.67dB
I

Since this is greater than the minimum requires SII, N := 7 can be used.

(b) n = 3:

First, let us consider a seven cell reuse pattern. (i.e.) cluster size N := 7

IQ ~m ~ J:JN ~ 4.5 831

No. of co-channel interfering cell iO:= 6

~ = (D/R)fi
I r,
:= (!6)I x (4.58~
Solved Anna University Question Papers Q-13

= (0.167) X (96.26)

I~ =16.07 1
~(dB) = 1010glO (16.07)
I

~(dB) = 12.06dB
I

Since this is less than the minimum required SII, we need to use a larger N.
Next consider N = 12
Q= (~) = -J3N = ~3X12
IQ = (D/R)=61
~ = (D~R)n = (~) X(6)3
I 10 6

= (0.167) x (216)

1~=361
S
- (an) = 10 10glO 36 = 15.56dB
I
Since this is greater than the minimum required SII, N = 12is used.

(b) Explain about co-channel interference and system capacity with neat diagrams.
(16)
Refer: chapter 4: page no 4.15- 4.20

13. (a) Explain in detail about Gaussian Minimum Shift Keying(GMSK)

transmission and reception with necessary block diagrams. (16)
Refer: chapter 5: page rio 5.32 - 5. 36
(OR)
Q-14 Wireless Communication

(b) Derive the expression for MSK signal as special type of continuous phase
FSK signal. (16)
Refer chapter 5: page 1105.24 - 5.30

14. (a) Discuss in detail about the frequency diversity with neat sketches. (16)
Refer chapter 5: page 1108.6. - 8. 9.
(OR)
(b) Derive the mean square errorfor a Generic Adaptive Equalizer. (16)
Refer chapter 7: page no 7.2- 7.4 & 7.19 - 7.22

15. (a) Determine the capacity offrequency selectivefading channel and explain
the concept ofwaterfilling/ waterpouring. (16)
Refer chapter 10: page no 10.15-10.16
(OR)
(b) What is known as channel state information? Explain in detail. (16)
Refer chapter 10 : page no 10.9 -10.11
Solved Anna University Question Papers Q-15

B.EIB.Tech. DEGREE EXAMINATION, MAY/JUNE 2016

Fifth Semester
Information Technology

EC6801 - WIRELESS COMMUNICATION

(Regulation 2013)

Time: Three hours Maximum: 100 marks

Answer ALL questions.

Part A - (10 x 2 = 20 marks)

t. Calculate the Brewster angle for a wave impinging on ground having a

permittivity of q. =5.
Solution:

sm e,
• II
= gr-
-2-
1
e; -1

~ ~;,__\ ~ H; ~ H~ 0.41

(J; = sin " (0.41)= 24.2 0

Thus Brewster angle for Cr =5 is equal to 24.2 0

2. Define coherence bandwidth.

The coherence bandwidth is a measure of the maximum frequency

difference (bandwidth) for which the received signals strongly correlated in
. amplitude. This bandwidth is inversely proportional to the rms value of time delay
spread (o-) as,
1
Bc oc--
O't
 Wireless Communication

3. What is soft hand offin mobile communication?

The mobile communicates with two or more cells at the same time and find
which one is a strongest signal base station then it automatically transfers the call
to that base station is called soft handoff.

4. What is multiple access techniques?

Multiple access is a signal transmission situation in which two or more users

may wish to communicate simultaneously with each other using the same
propagation channel.
The sharing of spectrum is required to achieve a high capacity by
simultaneously allocating the available bandwidths to multiple users.

5. Why is MSK referred to as fast FSK?

Minimum Shift Keying(MSK) is a special type of binary Continuous Phase

Frequency Shift Keying (CPFSK) .where in the peak frequency deviation is equal
I
to 1/4 the bit rate and modulation index h = "2' This leads to the minimum

frequency spacing that makes the two FSK signals orthogonal to each other.
MSK is sometimes referred to as fast FSK, as the frequency spacing used is
only half as much as that used in conventional non-coherent FSK.

6. What is windowing?

In communication, window function is a mathematical function that is zero-

valued outside of some chosen interval and is the process of taking a small subset
of a larger dataset, for processing and analysis.

7. Define adaptive equalization.

In a mobile cellular environment, the characteristics of the wireless

dispersive fading channel change randomly with time. In order for an equalizer to
effectively combat lSI, the equalizer coefficients should change according to the
channel status so as to track the time varying characteristics of the mobile channel.
Solved Anna University Question Papers Q-17
Such an equalizer is called an adaptive equalizer as it adapts to the channel
variations.

8. What is the benefit ofRAKE receiver?

RAKE receiver can be used to reduce the multi path interference in CDMA
system by combining direct and reflected signals in order to improve the signal -
to - noise ratio at the receiver.

9. What is MIMO systems?

Multiple Input Multiple Output (MIMO) is an antenna technology for

wireless communications in which multiple antennas are used for single user at
both the source (transmitter) and the destination (receiver) to exploit the spatial
domain for spatial multiplexing and/or spatial diversity to increase the channel
capacity and data rate.

lO. What is transmit diversity ?

In transmitter diversity, multiple antenna elements are required at the

transmitter and one antenna element at the receiver end and provide better
performance. The transmit power is divided among these antennas.

PART-B (5* 16 = 80)

11. (a). Infree space propagation describe how the signals are affected by reflection,
diffraction and scattering. (16)
The three basic propagation mechanisms which impact propagation in a mobile
communication system are
(i) Reflection,
(ii) Diffraction, and
(iii) Scattering.
(1) Reflection
If an object is large compared to the wavelength of the signal, e.g.: huge
buildings, mountains or the surface of the wavelength, the signal is reflected.
Q-18 Wireless Communication

Reflection

Fig.Llleflection of wave
The reflected signal is not as strong as the original, as objects can absorb some
of the signal's power. When a radio wave propagating in one medium impinges upon
another medium having electrical properties, the wave is partially reflected and
partially transmitted.
If the plane wave is incident on a perfect dielectric, part of the energy is
transmitted into the second medium and part of the energy is reflected back into the
first medium, and there is no loss of energy in absorption.
If the second medium is a perfect conductor, then ail incident energy is reflected
back into the first medium without loss of energy.
The reflection coefficient is a function of the material properties, and generally
depends on the wave polarization, angle of incidence, and the frequency of the
propagating wave.
(2) Diffraction

Diffraction

Fig. 2.Diffraction of waves

Solved Anna University Question Papers Q-19

Diffraction occurs when the radio path between the transmitter and receiver is
obstructed by a surface that has sharp irregularities (edges) and propagates in different
directions.
At high frequencies, diffraction like reflection-depends on the geometry of the
object, as well as the amplitude, phase and polarization of the incident wave at the point
of diffraction.
The received field strength decreases rapidly as a receiver moves deeper into the
obstructed (shadowed) region, the diffraction field still exist and often have sufficient
strength to produce a useful signal.
Huygen's principle states that all points on a wave front can be considered as
point sources, for the production of secondary wavelets, and that these wavelets
combine to produce a new wavefront in the direction of propagation.
Diffraction is caused by the propagation of secondary wavelets into a shadowed
region.
Thefield strength of a diffracted wave in the shadowed region is the vector sum
of the electric field components of all the secondary wavelets in the space around the
obstacle.
(3) Scattering
If the size of an obstacle is in the order of the wavelength or less, then waves can
be scattered. An incoming signal is scattered into several weaker outgoing signal.

Fig.3. Scattering
Scattered waves are produced by rough surfaces, small objects, or by other
irregularities in the channel.
Q-20 Wireless Communication
~.~~---------

In practice, foliage, street signs, trees and lamp posts induce scattering in a
mobile pcmmunication system, thereby providing additional radio energy at a
receiver.
Fiat surfaces that have much larger dimension than a wavelength may be
modeled as reflected surfaces.
(OR)

(b) Explain in detail the various parameters involved in mobile multipath

channels. (16)
Refer chapter 2: Page no- 2.3 - 2.5

12. (a) Summarise the features of various multiple access techniques used in
Wireless mobile communication. State the advantages and disadvantages
of each technique. (16)
Refer chapter 3: Page no- 3.4 - 3.21

(OR)

(b) Explain in detail how to improve coverageand channel capacity in cellular

systems. (16)
Refer chapter 4: Page no- 4.23 - 4.27
13. (a) Explain in detail Offset QPSK and xl4 DQPSK linear digital modulation
techniques employed in wirelesscommunication. (16)
Offset QPSK: Refer chapter 5: Page no- 5.15 - 5.17
1[/4 DQPSK: Refer chapter 5: Page no- 5.17 - 5.20.
(OR)

(b) Explain in detail Gaussian Minimum Shift Keying(GMSK) transmission

and reception with neat diagrams. (16)
Refer chapter 5: Page 110- 5. 32 - 5. 36
Solved Anna University Question Papers Q-21

14. (a) Explain in detail about linear and non linear equalizer. (16)
Linear equalizer: Refer chapter 7: Page no- 7.5 & 7.7-7.11
Non linear equalizer: Refer chapter 7: Page no- 7.11 - 7.16
(OR)
(b) Write short notes on: (16)
(i) Space Diversity
Refer chapter 8: Page no- 8.3 - 8.4
(ii) Frequency Diversity
Refer chapter 8: Page no- 8.6 - 8.9
(iii) Polarization Diversity
Refer chapter 8: Page no- 8.10 - 8.13.
(iv) Time Diversity.
The desired message is transmitted repeatedly over several time periods.
The time separation between adjacent transmissions should be larger than the
channel coherence time such that the channel fading experienced by each
transmission is independent of the channel fading experienced by all of the
other transmission.
One modern implementation of the time diversity involves the use of the
RAKE receiver for spread spectrum CDMA, where the multipath channel
provides redundancy in the transmitted message.

15. (a) (i) Explain in detail how inherent delay in a multiuser system is overcome by
beam forming. (8)
Refer: Chapter 9: Page nos: 9.13-9.16
(ii) Explain in detail about spatial multiplexing ofa MIMO system. (8)
Refer: Chapter 9: Page nos: 9.6-9.7
(OR)
(b) Explain with relevant diagrams the layered space time structure with
respect to MIMO systems. (16)
Refer: Chapter 9: Page nos: 9.11 -9.12
Q-22 Wireless Communication

H.E/ B.Tech. DEGREE EXAMINATION, NOVIDEC 2016

Fifth Semester
Information Technology

EC6801 - WIRELESS COMMUNICATION

(Regulation 2013)

Time: Three hours Maximum: 100 marks

Answer ALL questions.

Part A - (10 x 2::: 20 marks)

1. Give the equation for average large scale-path loss between the transmitter and
receiver as a function ofdistance.
The path loss in two - ray model is expressed as,
PL (dB) = 4010gd -(lOlogG( +1010gG r +2010gh( +2010ghJ

where, d - Distance between transmitter and receiver,

G. - Gain of the transmitting antenna,
G r - Gain of the receiving antenna,
h. - Height of the transmitting antenna, and
h- - Height of the receiving antenna.

2. What is frequency selective fading?

If the channel possesses a constant - gain and linear phase response over a
bandwidth that is, smaller than the bandwidth of transmitted signal, then the
channel creates frequency selective fading on the received signal.
A signal undergoes frequency selective fading if
BW of the signal> BW of the channel.
(B, > Be) and
Symbol period < Delay period
rr, <crT)
· Solved Anna University Question Papers Q-23

3. State advantages of CDMA over FDMA.

Sl.No. FDMA CDMA

Channel bandwidth is subdivided The sharing of both bandwidth
1.
into a number of sub - channels. and time takes place.
2. Narrow band system. Wide band system.
Commonly used for the voice and Commonly used for digital voice
3
data transmission. signals and multimedia services.
4. Code word is not required. Code words are required.
5. Hard handoff Soft handoff

4. Define the Grade ofservice.

The Grade of Service (GoS) is a measure of the ability of a user to access a

trunked system during the busiest hour.

5. Give the function of Gaussian filter in GMSK.

The Gaussian filter that is used before the modulator (transmitter) to reduce
the transmitted bandwidth of the signal. GMSK uses less bandwidth than
conventional FSK.

6. hat is Cyclic Prefix(CP)?

The cyclic prefix (CP) or Guard Interval is a periodic extension of the last
part of an OFDM symbol that is added to the front of the symbol in the transmitter
and is removed at the receiver before the demodulation.
It converts a frequency -selective channel into a number of parallel flat-
fading channels. The use of cyclic prefix (CP) can give guarantee for orthogonality
between the sub-carriers, even when they travel through the multi-path channels.

7. What is linear equalizers and non linear equalizers?

The major classification of an equalization techniques are linear and
nonlinear equalization.
 Wireless Communication

If the output d(t) is not used in the feedback path to adapt the equalizer. Then
this type of equalizers are called linear equalizer.
If the output d(t) is fed back to change the subsequent outputs of the
equalizer. Then this type of equalizers are called nonlinear equalizers.

8. What is Macro diversity?

Macro diversity is a kind of space diversity scheme using several receiver

antennas and/or transmitter antennas for transferring the same signal. Here, the
distance between the transmitters is much longer than the wavelength, that is, the
correlation distances for large scale fading are on the order of tens or hundreds of
meters, so that temporal diversity or spatial diversity cannot be used. Thus, to
reduce large scale fading, macro diversity is used.

9. How does spatial multiplexing work?

Spatial multiplexing is a very powerful technique for increasing channel

capacity at higher signal-to-noise ratio (SNR) and it is used to maximize the data
rate.
It is a transmission technique in MIMO wireless communication to transmit
an original high-data stream is multiplexed into several parallel streams, each of
which is sent from one transmit antenna element. Therefore, the space dimension
is reused, or multiplexed, more than one time.
The channel "mixes up" these data streams, so that each of the receive
antenna elements sees a combination of them. An appropriate signal processing at
the receiver can separate the data streams.

The two types of capacity possible in flat-fading MIMO systems are:

(i) Ergodic (Shannon) capacity, and
(ii) Outage capacity.
Solved Anna University Question Papers Q-25

Ergodic capacity is the expected value of the capacity, taken over all
realizations of the channel. This quality assumes an infinitely long code that
extends over all the different channel realizations.
Outage Capacity is the minimum transmission rate that is achieved over a
certain fraction of time.

PART-B (5* 16 = 80 marks)

11. (a) Explain the time variant two-path model ofa wirelesspropagation channel.
(16)
Refer - Chapter 1- Pages 1.7-1.14
(OR)
(b) (i) Explain fading effects due to multipatb time delay spread and fading
effects due to Doppler spread. (10)
Refer - Chapter 2- Pages 2.15- 2.18
(ii) What are thefactors influencing small scalefading? (6)
Refer - Chapter 2- Pages 2.3- 2.5

12. (a) Explain about co-channel interference and adjacent channel interference.
Describe the techniques to avoid interference. (16)
Refer - Chapter 4- Pages 4. 15- 4.20
(OR)
(b) (i) Explain in detail howfrequency is efficiently allocated in an cellular radio
systems. (6)
Refer - Chapter 4- Pages 4. 3- 4.6
(ii) Explain in detail a handoffscenario at cell boundary. (10),
Refer - Chapter 4- Pages 4. 10 - 4.12
13. (a) What is MSK? Also derive the expression ofMSK signal as a special type of
FSK signal and also explain its operations. (16)
Refer - Chapter 5- Pages 5. 24 - 5.30

(OR)
Q-26 Wireless Communication

(b) Draw the basic arrangement of Orthogonal Frequency Division

Multiplexing transceivers and discuss its overall operation. (16)
Refer - Chapter 6- Pages 6. 2 - 6.7

14. (a) Explain in detail the various factors to determine the algorithm for adaptive
equalizer. Also derive the Least Mean Square Algorithm for adaptive
equalizer. (16)
Refer - Chapter 7- Pages 7.16- 7.17 & 7.19-7.21

(OR)
(b) With relevant diagrams explain RAKE receiver. Also discuss how time
diversity is achieved in a CDMA technique using RAKE receiver. (16)
Refer - Chapter 8- Pages 8. 21 - 8.23

15. (a) (i) With a neat diagram explain the system modelfor multiple input multiple
output systems. (8)
Refer - Chapter 9 - Pages 9. 7 - 9.9
(ii) Discuss in detail the classifications ofalgorithms for MIMO based system.
(8)
Refer - Chapter 10 - PagesfO.ll

(OR)

(b) Calculate the capacity of a MIMO system flat fading and non fading
channels.·· (16)
Flatfading: Refer .../Chapter 10 - Pages 10.12-10.17.
Non fading: Refer - Chapter 10 - Pages 10.17-10.18.