1. Interference Mitigation by Dynamic
self-Power Control in Femtocell
scenarios in LTE Networks
Presented by:
Yara Ali
Supervised by:
Dr. Mohsen Tantawy

2. Index











Abstract
Introduction
Related work
Contributions
System Description
Model
Analysis
Assumptions (game)
Results ( Simulation )
Conclusion
Future work (research)

3. Abstract

Although the base station performs a smart
scheduling scheme for resource allocation,
the expected result could be affected due to
interference, specially in femtocell scenarios.

We are going to investigate the problem of
interference in femtocell networks in LTE.

4. Introduction

In LTE architecture, the QoS management is
handled by the base station called “ eNodeB “
or eNb

The eNb experiments hard problems due to
geographical position of users. ( i.e. When a
user is close to the eNb it will experience a
very good QoS otherwise the performance
might not be as expected )

5. Introduction

As a solution of this problem, operators started
to propose femtocell (It’s a small base station
installed at home in order to assure a
closeness to the terminal aiming to improve
the signal and consequently the QoS.

Femtocells in the LTE specifications are called
HeNb ( Home eNodeB ). They are installed by
users through a broadband over the internet.

6. Introduction

The main goal of HeNb is:
1.
To provide a private mobile coverage inside
buildings to free radio resources on the outdoor
network
Increase the total capacity of the mobile system
by avoiding several wall penetration losses.
2.

Although femtocells grant a better quality of
signal to users, the scheduling is capable of
interference.

7. Introduction

Kinds of interference in femtocell scenarios:
1.
Macro-Femto
Femto-Femto
2.

8. Introduction
Macro-Femto
Open-access femtocells
Closed-access femtocells
Users are allowed to be
handed over to the
corresponding HeNb
The HeNb only grants
access to a particular set
of authorized users

9. Introduction

Femto-Femto

Femto-Femto interference is caused due to
neighboring issues. ( i.e. Geographical distribution of
buildings does not follow any standard, therefore
HeNbs will be positioned in a random manner. This
causes home cell edge interference between
apartements and officers in small coverage area )

Unlike the macrocell, femtocell can be installed by
users in random manner making it difficult to handle
the interference problem.

10. Related work
1.
Graph-based dynamic frequency:

Aims to decrease the interference by dividing the
available frequency band into sub-bands which are
distributed among femtocells in a way that directly
adjacent cells do not use the same sub-bands

It works well but the cost to pay is the decrease of bit
rate.

Unfortunately, this proposed method might be hard to
be adapted to the LTE needs because the core of LTE
is focused on RT services such as video and VoIP and
this scheme does not satisfy RT needs.

11. Related Work
2.
Mechanism to distributed interference
management and scheduling for downlink over
LTE:

It is based on only one round of exchange of very
few bits of information in each subframe ( one
transmission time interval TTI )

Problem: The basic well known constraints of QoS
such as PLR, throughput, total spectral cell
efficiency are not studied, therefore, the impact of
this method on the performance of QoS is not
clear.

12. Related Work
3. Power Control

Well known method for interference
mitigation ( will be discussed in more details
in the next slides )

13. Contributions

Introducing a method to mitigate interference in
femtocell scenarios by using a dynamic self power
transmission control. This solution differs from
others in:
1.
As interference and throughput are not directly
proportional, our method performs a constant
bargain between interference and throughput
efficiency in order to find an optimum tradeoff
which is represented by the femto transmission
power value.

14. Contributions
2. Our proposed constant bargaining has a
low complexity.
3. Since LTE characteristics are focused on
RT services, We test our proposed method
in a scenario which uses video and VoIP
flows.

15. System Description

The architecture of the 3GPP ( 3rd generation
partnership project ) LTE system consists of
several base stations called “eNb” or
“macrocell”.

Inside of the eNb coverage, it’s possible to install
small base stations called “HeNb” or
“Femtocells”

The macro and micro-base stations transmit with
different power.

16. System Description

To perform the message exchange between macrobase stations the X2 interface is used (X2 is a pointto-point interface), message exchange between
femto base stations is made in the same way.

To make the eNb – HeNb communication the S1
interface (The S1 interface supports a many-tomany relation) is used and the message exchange
must pass by the MME / SGW or by the HeNb GW.
-MME : Mobility Management Entity
-SGW: Serving Gateway

17. System Description

Users report their instantaneous downlink
channel conditions ( e.g. SINR ) to the
serving eNb/HeNb at each TTI (Transmission
time interval)
-“SINR is Signal to Interference plus Noise
Ratio that is calculated as SINR = P / (I + N)
where P is signal power, I is interference
power and N is noise power. ”

18. System Description

At the eNb/HeNb, the packet scheduler
performs a user selection priority
procedure, based on criteria such as:
1.
Channel Conditions
Head of line (HOL) packet delays
Buffers status
Service types
2.
3.
4.

19. System Description

The eNb/HeNb has a complete information about
the channel quality by the use of channel state
information (CSI).

The QoS aspects of the LTE downlink are affected
by a large number of factors such as:
Channel conditions
Resource allocation policies
Available resources
Delay and sensitive/insensitive traffic
interference
1.
2.
3.
4.
5.

20. System Description

In LTE, the resource is allocated to a user in
the downlink system, contains frequency and
time domains, and is called resource block.

The resource allocation is realized at every
TTI, that is exactly every two consecutive
resource blocks.

21. Model

22. Model

23. Model

24. Model

25. Model

26. Model (modulation and coding
scheme)

27. Analysis

There exist close femto neighbors asking for
resources at the same time as could happen in a
metropolis building.

If 2 or more femtocells set their transmission power
values to the highest level in a specific case where
user (belonging to different femtocells) ask for
resources, each femtocell will assign all their
subbands to their users.

This will cause interference and this interference will
avoid an optimal packets transmission performance.

28. Analysis

Considering that if femto owners decide to set the
transmission power level to the lowest allowed
value, logically interference will decrease, but
coverage signal quality and MSC values will
decrease as well

If MSC values are small, TBS (Transport Block Size)
values will also be small, therefore throughput gain
will not get high level which is not an optimal
solution when transmission packets belong to realtime flows.

29. Analysis

Now the key of this game is focused on setting the
transmission power value as a dynamic variable
which changes depending on the scenario changes.
(i.e. each user computes its SINR estimation and
reports its MSC values to the HeNb, each HeNb will
decide the value of transmission power depending
on the user position, in order to maintain the power
value as low as possible but without affecting the
throughput gain)

30. Assumption ( Game )

1.
2.

Players:
Throughput
SINR
Target:
Players compete for transmission power
value at equilibrium point

31. Assumption ( Game )

How it works:
In order to increase
its level, SINR will
propose to set the
power as low as
possible and on the
other hand throughput
will propose to set the
transmission power
value as high as
possible to increase its level
as we can see on the 1st move.

32. Assumption ( Game )

33. Results ( Simulation )

Our Femtocell scenario is set as follows:

The number of femtocells which are neighbors relatively
close start from 1 until 10, increasing in one unit in order
to increase the interference.

In the time domain, radio resources are distributed every
TTI, each one lasting 1 ms.

Each TTI is composed by 2 time slot of 0.5 ms,
corresponding to 14 OFDM symbols in the default
configuration with short cyclic prefix ( i.e. 10 consecutive
TTIs form the LTE frame )

34. Results ( Simulation )

We have tested one scenario on VoIP flows.

Macrocell interference is not taken into
account in this study.

Users are constantly moving at speed of 1
kmph in random directions ( random walk )

35. Results ( Simulation )

36. Results ( Simulation )

37. Results ( Simulation )

38. Results ( Simulation )

39. Conclusion

By simulation we have shown the effect that
interference causes in their performance regarding a
degradation of capacity if the transmission power
value is not set to an optimum level specially in
femtocell scenarios where signal coverage surfaces
are small.

In order to solve this problem we proposed a
method based on controlling the transmission power
value that changes dynamically this value
depending on the interference level.

40. Conclusion

The power level value plays an important role in
performing a good QoS.

REMEMBER, Neglecting control of power level
value could be important cause to perform a
degradation of throughput.

The proposed scheme allows a low complexity
implementation, which is suitable for practical
wireless systems.

41. Future Work ( Research )

Could be focused on finding out a way to
include the macrocell into this scenario,
which can not be neglected in real
transmission systems scenario.

42. Thank You !