Interference Mitigation by Dynamic self-Power Control in Femtocell scenarios in LTE Networks

Presented by:

Yara Ali

Supervised by:

Dr. Mohsen Tantawy

Index

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

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.

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 )

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.

Introduction

The main goal of HeNb is:

1. To provide a private mobile coverage inside buildings to free radio resources on the outdoor network

2. Increase the total capacity of the mobile system by avoiding several wall penetration losses.

Although femtocells grant a better quality of signal to users, the scheduling is capable of interference.

Introduction

Kinds of interference in femtocell scenarios:

1. Macro-Femto

2. Femto-Femto

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

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.

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.

Related Work2. 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.

Related Work

3. Power Control

Well known method for interference mitigation ( will be discussed in more details in the next slides )

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.

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.

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.

System Description

To perform the message exchange between macro-base stations the X2 interface is used (X2 is a point-to-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-to-many 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

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. ”

System Description

At the eNb/HeNb, the packet scheduler performs a user selection priority procedure, based on criteria such as:

1. Channel Conditions

2. Head of line (HOL) packet delays

3. Buffers status

4. Service types

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:

1. Channel conditions2. Resource allocation policies3. Available resources4. Delay and sensitive/insensitive traffic5. interference

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.

Model

Model

Model

Model

Model

Model (modulation and coding scheme)

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.

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 real-time flows.

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)

Assumption ( Game )

Players:

1. Throughput

2. SINR

Target:

Players compete for transmission power value at equilibrium point

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.

Assumption ( Game )

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 )

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 )

Results ( Simulation )

Results ( Simulation )

Results ( Simulation )

Results ( Simulation )

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.

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.

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.

Thank You !