Interference Analysis of Femtocells

INTERFERENCE
ANALYSIS OF
FEMTOCELLS

Chapter 1:
Cellular Communications

1.1 History

O Guglielmo Marconi demonstrated wireless
abilities of radio in 1897.
O Since then, this topic has been researched
by many scientists and engineers.
O Communication was done through
Telegraph and Postal service before wireless
communications was introduced in the
market.

O Cellular communications came into existence with
the introduction of ‘Cellular Concept’ by Bell
Laboratories in 1960’s
O This concept revolutionized the communication
industry and people were eager to try this new
technology.
O One example of this was seen when there were
3700 people on the waiting list of Bell Mobile
Phone Service, which had a capacity of 12
channels and 543 customers in 1976.

1.2 Cellular Generations
O We will give a brief
introduction of
cellular generations
in the section. A
figure showing the
timeline of cellular
generations is shown
below.

0-G
O Very first mobile communication systems.
O Also called as ‘Pre-Cellular systems’ or
‘Mobile Radio Telephony’.
O Mostly worked on AM, FM modulation
schemes.
O Poor capacity, low efficiency, fewer
customers.

1-G

O Called as First Generation systems.
O Born after introduction of cellular concept in
1970’s.
O Still poor capacity, low efficiency and analog
systems.
O AMPS, TACS, NMT were 1-G.

2-G
O First popular cellular mobile communication
systems.
O Called as Second-Generation systems.
O Electronics field was experiencing revolution
during 1-G period, hence 2-G was the First
Digital Cellular Communication Systems.
O Due to digitalization; efficiency increased, higher
performance, higher capacity, roaming abilities
and increased battery life.
O GSM, D-AMPS, PDC, IS-95 were 2-G.

3-G
O Called as Third-Generation Systems.
O 2.5G(GPRS), 2.75G(EDGE) had data capabilities
and formed the basis of 3G systems.
O Better modulation techniques and improvement
in electronics helped in gaining higher speeds,
better interference, video abilities and lower
power requirements.
O W-CDMA and cdma2000 were two main
sections of 3G systems.
O HSPA, EVDO were 3G.

4-G
O Next generation of cellular communications
expected in a couple of years.
O Some of the advanced features in
communication industry like MIMO, AMC,
UWB and Software Radio are expected to be
part of 4G.
O Broadband speeds, better coverage, high
capacity, low interference are some aspects
of 4G.
O WiMAX, LTE are 4G.

1.3 Cellular Architecture
O As stated earlier, Bell Labs came up with an
innovative solution of ‘Cellular Concept’
to address the ever-green capacity question.
O The concept said that “the entire geography
was divided in to smaller ‘cells’ in which
each cell uses a different frequency in order
to increase capacity”, an example of the
same is shown in the next slide.

O We can see that one
large cell is divided
into 7 smaller cells.
O Each cell with a
different number
uses different band
of frequencies.
O Each smaller cell is
placed at a minimum
re-use distance from
its interfering cell.
O This pattern is a 7-
cell Reuse pattern

O Because of introduction of cellular concept,
capacity increased as shown below.

1.3.1 Problems
O With invention of every new technology
comes its problems.
O Problem with cellular arrangement was that
1. Lower Re-use factor: Larger cells, High
interference due to close interfering cells.
2. Higher Re-use factor: Smaller cells, Lower
capacity, Complex hand-over abilities.

1.3.2 Solutions
1. Introduce new
channels until
saturation
2. Cell splitting
3. Cell sectoring
4. Cellular hierarchy

Cellular Hierarchy
O Soon the other three solutions came to
saturation and engineers had to come up
with a new solution.
O Engineers brought in cellular hierarchy in to
the existing cells to reduce load on cellular
base station and increase coverage.
O This meant reducing the size of the cell into
still smaller cells creating a cellular pattern
into the present pattern.

Thus creating the need for femtocells

1.4 Interference perspective
O There are basically two types of interference
based on frequency spectrum that occurs:
O Cross-technology interference: Interference
happens when two technology work on same
frequency. Example: Bluetooth and Wi-Fi work on
same 2.4GHz ISM band
O Cross-hierarchy interference : Interference
happens when two hierarchies of the same
technology work on same frequency. Example:
Macrocells and Femtocells share spectrum
allocated to the technology.

Focus of the project

O The next type of interference occurs on the
basis of channels used.
O Co-Channel Interference : This type of
interference occurs when the interferer is on
the same channel as the one being interfered.
O Adjacent Channel Interference: This type of
interference occurs when the information on
the adjacent channel seeps into passband of
the channel being transmitted, due to which
performance of the main channel is degraded.


Interference pattern in
cellular communications - 1

2.1 Definition
As given in the Femto forum;
“ Femtocells are low-power wireless access points that
operate in licensed spectrum to connect standard
mobile devices to a mobile operator’s network using
residential DSL or cable broadband connections ”
According to the definition we can deduce that they
are (similar to Wi-Fi) hotspots which will connect the
subscriber to the cellular network using Broadband
connection at subscriber's home/office.

2.2 Need for femtocells
Coverage
•90% data services and 2/3 calls are expected
to be indoors.
If we try to address these figures using conventional
macrocell arrangement, we will surely get BLIND
SPOTS and DROPPED CALLS.
Femtocells can prove to be a rational solution to this
problem.
Femtocells can provide “high coverage” to customers
indoor and can address the issue above.

Capacity

Currently, there is a lot of load on macrocells because
it provides services to both indoor and outdoor
customers.

Femtocells will reduce load on Macrocells by taking
up all the indoor subscribers.
As a result, capacity of both hierarchies, Femtocells
and Macrocells will increase.

Backward compatibility

People are reluctant to switch to new technology
because of the cost factor in changing to a new mobile
device.

Femtocells will be working on WiMAX, LTE
technologies; as a result they will be backward
compatible to all the devices working on the above
technologies.
A user can use any device like laptop, palmtop, smart
phones when he/she is in femtocell coverage area.

Data rates

As we can see from the figure below that data usage in
mobile devices is going to increase almost three times in
the coming years.
This would mean providing high data rates to users.

Femtocells will be working on WiMAX, LTE which
would mean that users will automatically be serviced
with high data rates thanks to the underlying
technologies.

2.3 Facts/Predictions about
Femtocells
The operator members of Femto Forum currently
serve 1.7 billion subscribers, across Wi-MAX, CDMA
and UMTS technologies and represent a third of total
mobile customers globally.
It is projected that femtocell deployments will reach
around 50 million by the end of
2014 as shown in the figure

Revenues generated by femtocells would be
comparable to cellular networks by the end of 2013
and the value is estimated to be around $2.4bn.

60% of the surveyed consumers are interested in
trying out femtocells.

Figures indicated above show that the world will
experience mass deployment of femtocells in the
coming years, hence it becomes a priority topic for
researchers and engineers to successfully get it into
the market.

2.5 FAP (Femtocell Access Point)

2.6 WiMAX
“Worldwide inter-operability for microwave access”
4-G technology
Fixed Wireless Broadband and Mobile Wireless
Broadband
IEEE 802.16 is the group responsible for
standardization of WiMAX
Several standards/revisions have come up for
WiMAX, as given in the next slide.

2.6.1 WiMAX features
WiMAX supports high data rates addressing several
customers at the same time ( considered as rival to Wi-Fi
802.11n)
WiMAX suffices MIMO specifications.
WiMAX has IP capability, thus considered as forerunner
for femtocell deployment.
From an operator’s perspective, Wi-MAX helps to achieve
high spectral efficiency with reduced cost per bit and
flexible radio planning.
Wi-MAX has an aptitude to satisfy all technical challenges
that have been put forward for broadband wireless
systems.

According to various references,

 WiMAX can always provide 100% coverage to indoor users
which is an essential requirement for femtocells.

 Areal capacity(throughput per unit area) gains of 300 can be
achieved in WiMAX femtocell scenarios.

 Various other features such as time synchronisation, mobility
management, list of neighbouring macrocells, interference
management and more can be offered by WiMAX femtocells.

2.7 LTE
“Long Term Evolution”
4G Technology
Basically a project under 3GPP, a team to determine
technicalities of UMTS technology.
Comprises of two basic components:
 Evolved-UMTS Terrestrial Radio Access – that consists of air
interface specifications along with UE.
 Evolved-UMTS Terrestrial Radio Access Network – that
consists of RNC and base station

We can see that LTE possess similar features as
WiMAX except a couple of them.

According to various references,

 LTE can deliver high data rates even under extreme
interference conditions in a femtocell implementation
scenario.

2.9 OFDM
One common aspect seen in both WiMAX and LTE is
the use of OFDM as a physical layer technology.
“Orthogonal Frequency Division Multiplexing”.
A digital multi-carrier transmission technology that
uses DFT for efficient modulation and demodulation
of the baseband signal.

We can see that because of orthogonality feature of
subcarriers, a lot of bandwidth is saved.

2.9.1 OFDM – Cyclic prefix
Prime reason for OFDM to be resistant to multi-path effects and
ISI.

It converts linear convolution to circular convolution which
makes it easier to recover signal at receiver.
OFDM has become popular because of the orthogonality
amongst the subcarriers and this feature is given by Cyclic
prefix.

Multi-path propagation

delayed duplicates

Misalignment of OFDM sinusoids

Loosing its Orthogonality
SOLUTION :

Cyclic
Prefix

2.9.2 OFDM advantages
Resistance to narrowband interference or frequency
selective fading.
With an OFDM system we omit the inclusion of an
equalizer in the system, because we can easily
implement it using FFT algorithms, which makes it
easier for FAP’s to be designed by manufacturers.
OFDM systems allow adaptive modulation and
coding schemes, which is useful when users have
different SNR values on their subcarrier.

2.10 Femtocell advantages
 High coverage
 Extremely low power operation (10mW – 100mW)
 High UE battery life
 ‘Green’ Technology
 Safe to be deployed in homes with children around
 Low electric bills
 High capacity
 Cheap installation (can be done by users)
 Femtocells work on IP, hence ‘IPSec’ protocol can be used as an extra
security feature
 Backward compatibility
 Operators can implement femtocells in their currently allocated
frequency spectrum of macrocells
 Work is under progress to bring in more applications in femtocell
technology.

2.11 Femtocell disadvantages

Since femtocells and macrocells work in the same
frequency band, they are bound to experience
INTERFERENCE from each other.

IP section of femtocells is handled by ISP, air interface
section of femtocells is handled by Cellular operators.
Thus TROUBLESHOOTING can be a hassle

Chapter 3:
Femtocells and Macrocells
The game of Hide and Seek

As highlighted in the previous slide, the
major disadvantage of femtocells is its co-
existence with macrocells.
We will thus evaluate the performance, in
terms of SINR, of femtocell and macrocell
in the same environment.
This chapter will simulate all possible
cases of CCI between femtocells and
macrocells.

Pre-Requisites
Downlink: Transmission in the direction
from the base station to cellular mobile
user.
Uplink: Transmission in the direction from
the cellular mobile user to base station.

Path-Loss:
◦ Signal decays as it passes through free space.

Macrocell Environments:
• There are many models prescribed for path-
loss in macrocell environments.
• Okumura-Hata model is the most widely used
model, which is also used it in our simulations.

where a(hmu) is given by equations,

We fix certain parameters for our
simulation as shown in the MATLAB
function below

Femtocell Environments:
Above mentioned path-loss model cannot
be implemented into femtocell
environments, thus we introduce new
model for the same.

In this case,
• q = 1 ( number of walls )
• W = 5dB ( wall loss = 5 dB )
• Ignore Window loss here ( Low = 0 )

SINR:
◦ SINR = Signal power
(Interfering power + Noise power)

Link Budgeting:
◦ Calculation of received power considering all
losses and gains in the system

Noise:
◦ A common degradation that all signals are
subjected to.
◦ We cannot completely make a signal noise-
free but we can remove noise from the signal.
◦ Most common type of noise considered is the
AWGN, which has also been considered in
the simulations.

From here on, we will follow a defined
pattern to explain every situation.

3.1 Femtocell downlink with
Macrocell

Description:
◦ Interfered: Femtocell User
◦ Interferer: Macrocell User
◦ State: Downlink
◦ Femtocell User receives stronger Macrocell
signal that affects the performance of the
femtocell user

Design:

◦ PT_macroBS: Macrocell Base station power = 10W (40dBm)
◦ PT_femtoBS: Femtocell Base station power = 0.1W (20dBm)
◦ Gain_macroBS: Macrocell Base station antenna gain = 15dBi
◦ Gain_femtoBS: Femtocell Base station antenna gain = 2dBi
◦ Gain_femtoMS: Femtocell User mobile antenna gain = 0dBi
◦ Bandwidth = 10MHz
◦ Thermal noise amplitude = -174+10*log10(Bandwidth)
◦ Channel : Rayleigh channel over 100 iterations

Architecture:
◦ Macrocell Radius = 1.5km
◦ Femtocell Radius = 250m

Results:

PERFORMANCE: POOR

3.2 Femtocell uplink with Macrocell

Description:
◦ Interfered: Femtocell User
◦ Interferer: Macrocell User
◦ State: Uplink
◦ Femtocell User receives stronger Macrocell
femtocell user

Design:

◦ PT_femtoMS: Femtocell User mobile power = 0.5W (27dBm)

3.3 Macrocell downlink with
Femtocell

Description:
◦ Interfered: Macrocell User
◦ Interferer: Femtocell User
◦ State: Downlink
◦ Macrocell User receives Femtocell signal that
affects the performance of the macrocell user

Design:

◦ PT_macroBS: Macrocell Base station power = 10W (40dBm)
◦ PT_femtoBS: Femtocell Base station power = 0.1 (20dBm)

Results:

PERFORMANCE: EXCELLENT

3.4 Macrocell uplink with Femtocell

Description:
◦ Interfered: Macrocell User
◦ Interferer: Femtocell User
◦ State: Uplink
◦ Macrocell User receives Femtocell signal that
affects the performance of the macrocell user

Design:

◦ PT_macroMS: Macrocell User mobile power = 0.5W
(27dBm)

3.5 Femtocell downlink with
Femtocell

Description:
◦ Interfered: Femtocell User 1
◦ Interferer: Femtocell User 2
◦ State: Downlink
◦ Femtocell User 1 receives Femtocell User 2
Femtocell user 1

Design:

◦ PT_femto1BS: Femtocell Base station 1 power = 0.1 (20dBm)
◦ PT_femto2BS: Femtocell Base station 2 power = 0.1 (20dBm)
◦ Gain_femto1BS: Femtocell Base station 1 antenna gain = 2dBi
◦ Gain_femto1MS: Femtocell User 1 mobile antenna gain =
0dBi

Architecture:
◦ Femtocell Radius = 250m

Results:

PERFORMANCE: GOOD

3.6 Femtocell uplink with Femtocell

Description:
◦ Interfered: Femtocell User 2
◦ Interferer: Femtocell User 1
◦ State: Uplink
◦ Femtocell User 2 receives Femtocell User 1
Femtocell user 2

Design:

◦ PT_femto2MS: Femtocell User mobile 2 power = 0.5
(27dBm)
◦ Gain_femto2MS: Femtocell User 2 mobile antenna gain =
0dBi

3.7 Motivation
 Scenarios that have been simulated are quasi-practical and
will not occur in actual world, but they will help us form the
basis of the main project and the motivation to undertake
this research work.
 We see from the above simulations that the performance
analysis of first and second scenarios are the worst amongst
the rest of them and thus it forms our cue to the main
research work.

 Metaphorically, in the game of ‘hide and seek’ between
femtocells and macrocells; it is usually the femtocell user that
loses against other players in the game.
 Thus, increase the femtocell user’s chances of winning in this
game, we have to simulate and analyse the user’s gameplay to
give him a fair chance to play and win the game.

Chapter 4:
A Femtocell
Downlink Scenario

4.1 Description
 Femtocell user experiences interference from other
femtocells and macrocells in downlink state.

 Scenario takes inspiration from Section 3.1, where a
femtocell user was interfered with a macrocell user.

 We have made the simulation more practical by introducing
variables instead of fixed values.

 The parameters will have the same definition by will be more
precise in order to emulate the outside world conditions.

 Channel considerations:

 In the previous simulations, we have considered Rayleigh
channel.

 In this case, we will consider Rayleigh fading channel.

 Rayleigh fading channel will help us to consider multi-path situations.
 The number of multi-path reflections are called as ‘taps’.
 This could be due to reflection, refraction and scattering.
 In our case, we consider a 10-tap Rayleigh fading channel.

4.2 Design
 Parameters:

 noFperM: number of Femtocells per Macrocell (10-40)
 PTx_macro: Macrocell Base station power = 20W (43dBm)
 PTx_femto: Femtocell Base station power = 0.1W (20dBm)
 GTx_macro: Macrocell Base station antenna gain = 15dBi
 GTx_femto: Femtocell Base station antenna gain = 2dBi
 GRx_user: Femtocell User mobile antenna gain = 0dBi

 We consider femtocell user in a random femtocell in a
random macrocell for every channel simulation.

 We will then take the average of these simulations to get a
precise figure of SINR value for that particular case.

 Considering the user in a random position will help us get the
range of operation of user will all the possible interference.

 FFT-1024 subcarrier arrangement.

 Noise considerations, keeping SNR levels at 20dB

4.3 Architecture

 Macrocell Radius = 866m
 Femtocell Radius = 50m

Case 1 : 10 femtocells / macrocell

4.6 Extensions
 Once we have the SINR we can find
 Shannon limit data rate of the system
 Best Modulation and coding scheme of the system

 M can only take values in powers of 2. Hence we can find the
best available MCS and data rate of the system using the
table below.

 We can add the below extension to our main program in
order to find the data rate of the system as shown below:

Chapter 5:
A Femtocell
Uplink Scenario

 Femtocell user experiences interference
from other femtocell users and macrocell
users in uplink state.
 This situation takes inspiration from section
3.2, where a femtocell user was interfered
by a macrocell user.
 More practical approach
 We consider only those interferers who share the
same subcarrier index as the user in
consideration.
 This scenario will randomly allocate subcarriers
to users and then check for interferers.

 Random allocation of spectrum:
 We will randomly allocate subcarriers to all the
users in that femtocell/macrocell
 Spectrum will be shared among 720 subcarriers
upon the number of users present in the
femtocell/macrocell
 A MATLAB function was defined to do the same

 Power-control for Macrocell users
 In order to avoid blinding the femtocell completely due to macrocell
user uplink power, we have introduced power-control for macrocell
users.
 The function will take into consideration the distance of the macrocell
user from Macrocell base station and allocate appropriate uplink power
to the user.

 Step 1: We first define the necessary parameters
required for simulation

 nooffemtocells: Number of femtocells in the macrocell (50-200)
 PTx: Femtocell User mobile power = 0.2 (23dBm)
 GTx: Femtocell User mobile antenna gain = 0dBi
 GRx_macro: Macrocell Base station antenna gain = 15dBi
 GRx_femto: Femtocell Base station antenna gain = 2dBi

 Step 2: Architecture

 Macrocell Radius = 1km
 Femtocell Radius = 50m

1. Generate a macrocell of radius 1km in order to
accommodate femtocells and macrocell users.
2. Generate ‘nooffemtocells’ number of femtocells
randomly.
3. Generate macrocell users; make sure they are
placed out of the femtocells to distinguish them
from femtocell users. Get final count of
macrocell users.
4. Place 1 to 4 random number of users in random
femtocells. Get final count of femtocell users in
each femtocells and overall.
5. Place the user under consideration in a random
femtocell. Get the position of the user in terms
of total femtocell users.

Case 1: 50 femtocells/macrocell

 Step 3: Path-loss is calculated as the
previous example

 Step 4: Channel considerations with a 10-tap
Rayleigh fading channel

 Step 5: Allocating subcarrier to femtocell
users
 Afterallocating thee subcarriers we will store
the user subcarrier index for further processing

 Step 6: Finding femtocell interferers
 This step will check for any femtocell interferers
comparing the subcarrier index of the user
under consideration
 Once we have found the interferers, we will find
the exact position of the interferers to find
their stats like path-loss, distance from BS,
channel conditions and others
 A unique list of interferers is produced after
checking for duplicates

 Step 7: Allocating subcarrier indices to
Macrocell users and finding interferers

 Step 8: Assigning transmit power conditions
on interfering macrocell users

 Step 9: SINR calculation
 After calculating all necessary requirements for
SINR calculation, we find SINR
 We will then average the SINR over channel
iterations

 Step 10: Plotting the results

 We will modify the above program by
comparing the performance of the femtocell
user with regards to number of macrocell
users.
 We will consider a random number of
macrocell users first.
 After eliminating the macrocell users in the
femtocells, we will get the final count of
macrocell users.
 We will fix the number of femtocells to 50
with random number of femtocell users (1-4)
in each femtocell.

n this research work, we have simulated the effect of
other users on a femtocell user in both uplink and
downlink situation

imulations in chapter 3 were only for understanding
and no conclusions will be derived from them

e will be deducing reasonable conclusions for chapter
4 and chapter 5 since they were the focus of our
research work

lthough we have got the predicted results, it is not the
same every time because the user maybe subjected to

6.1 Conclusions from chapter 4
he performance evaluation of a femtocell user who is communicating
with its base station and being interfered by the other femtocells and
macrocells

e have increased the number of femtocells in each case from 10 to 40

he effective SINR value of the user in first case is around 43dBm
which reduces to 20dBm for the last case which is a dip of around 50%
in performance

the user is expected to achieve SINR levels in the range of 20dBm to
40dBm in a femtocell downlink scenario and the effect of number of
femtocells over SINR is highly dependent on surrounding location of the

ecommendations

• Well-defined macrocell range considerations:
This would be helpful so that macrocell power will not overpower the
femtocell power

• Proper distribution of femtocells across the geography:
Femtocells can be implemented in areas where macrocells cannot service its
customers, like blind spots.

• Implementing appropriate methods of channel
estimation be applied in femtocells in order to enhance
the user performance in downlink scenario.

6.2 Conclusions from chapter 5
he interference impact of number of femtocells and number of
macrocell users over a femtocell user in uplink state, who is randomly
located in any of the femtocell in the geography

e have analysed the uplink case in a way that there will be
interference only if the other user is on the same subcarrier index as
the user under microscope

he user achieves a SINR level of 15dBm with the least number of
femtocells in the macrocell and the level deteriorates to -10dBm with
more number of femtocells

the user is able to achieve SINR in the range of 20dBm to -20dBm in the
uplink scenario and the performance degradation is more severe due to
presence of macrocell user interferers rather than the femtocell user

ecommendations

• Proper power-control for macrocell users
This should be done so that power levels of femtocells and macrocells can
be easily differentiated

• Common subcarrier allotment system for the geography
This will eliminate any chances of interference based on subcarriers.

• Appropriate channel estimation techniques

6.3 Factor responsible for varied behavior in
cellular systems
1. User position: We do not expect a user to be at a single place all the time and hence user
position plays a pivotal role in interference analysis.

2. Path Loss: The most important factor that determines path loss is the distance between
the user and base station. Other factors include:
• number of walls between user and femtocell base station
• height of macrocell base station
• height of mobile user from the ground
• operating frequency.

3. Allocation of bandwidth and subcarriers to user: Interference happens when there is
overlap between same subcarrier of two users. Thus, allocation of subcarrier indices and
hence the bandwidth to femtocell users and macrocell users will play an important role in
interference analysis.

4. Channel conditions: This is the most fragile of all the conditions because of its high level
of randomness.

5. Noise levels: Noise levels affect the signals upto a certain level after which they can be
neglected.

6.4 Future Work

o investigate effects of interference over a user in a constant position
in all the cases
• This analysis would also have helped to understand the effect of user position on the
performance, which was one of the factors that affect the performance of the user in both
the transmission scenarios.

FDMA based system simulation
• The analysis performed takes into consideration an OFDM system with 10MHz bandwidth.
Implementing OFDMA system will help to simulate the system on a link-layer basis

Interference Analysis of Femtocells

Interference Analysis of Femtocells

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