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Resource Allocation Strategy using Optimal Power Control for Mitigating
Two-Tier Interference in Heterogeneous Networks
Shovon pal, Shifath Shams, Atiqur Rahman
North South University
Electrical Engineering and Computer Science DepartmentBangladesh
Toha Ardi Nugraha
IT Convergence Department, Kumoh National Institute of Technology,
South Korea
111
Presented by
Shovon Pal, Shifath Shams
North South University
IEEE WCNC WORKSHOP 2014 1

Table Of Contents
• Introduction
• Why next generation networks
• HETNET
• Interference Scenarios
• Solution schemes for interference problems
• System model designing
• Simulations and results
• Conclusion
• Further Contribution

Introduction

Evolution of Wireless devices

Traffic Growth of Cellular network
Transition to LTE will increase
capacity trough more spectrum,
better scheduling, and MIMO, but
these gains are not sufficient.
Energy consumption of networks
will become an increasing
problem. “More of
the same” will not be good
enough.

The Exponential Market Growth
No other industry has ever seen a growth as
fast as that seen in the mobile
telecommunications sector.
This has also made the mobile phone the
single most widespread information and
communication technology (ICT) to date.
Within a span of five years, the proportion of
mobile subscriptions originating from the
developing world exploded to 64%

Capacity gains in wireless network
Wireless Network Capacity
Gains 1950-2000
15x by using more spectrum (3 GHz vs 150 Mhz)
5x from better voice coding
5x from better MAC and modulation methods
2700x from smaller cells
Total gain 1 million fold
Source: William Webb, Ofcom.

Why next generation network
High data rate and improved quality-of-services to
subscribers
Eliminate dead holes in existing network footprint
Mobility Optimization
Extended battery life of mobile phones
Mitigate spectrum underutilization problem
Taking DATA network to the very next level to get connected 24/7

Heterogeneous Networks

Heterogeneous Networks
 Heterogeneous networks (Hetnets) consisting of Macro-cells
and Small-cells (e.g. Femto-,Metro-, Pico-cell) are capable of
extending cellular coverage and increasing capacity
 The Macro-eNBs are used to offer coverage over a wide
area, while the low power Small-cells are typically deployed
in hot-spots and indoor are to offload traffic from the Macro
layer

Multi-tire Cellular Network
Improvement of cell coverage, network capacity, and
betterquality-of-service (QoS) provisioning are some of the
major challenges for next generation cellular networks.
Universal frequency reuse and make transmitters and
receivers closer
Hierarchical layering of cells, an efficient solution to improve
cell coverage and network capacity
Adopted in the evolving Long Term Evolution (LTE)/LTE-
Advanced (LTE-A) systems 3GPP Release-8 (LTE), 3GPP
Release 10 onwards (LTE-Advanced)

LTE-A Network
LTE-Advanced systems are designed to support high-speed packet-switched
services in 4G cellular wireless networks.
The cells or radio base stations in LTE-A can be classified as: i) macrocell base
station (referred as MeNB), and ii) small cells (e.g., microcells, picocells,
femtocells).
“Small cell” is an umbrella term for low-power radio access nodes that operate
in both licensed and unlicensed spectrum and have a range of 10 meter to
several hundred meters.
Small cells will improve the cell coverage and area spectral-efficiency (capacity
per unit area).

LTE-A Network

Small Cells - A Necessary Topology Evolution
for Future Data Growth
Moving to hierarchical cell
structures with small cells can:
 Significantly increase the
capacity in the same bandwidth
 Significantly reduce the energy
consumption of networks

Interference Scenarios

Macro-Femto Interference
This type of interference commonly
occurs when a Macro Base Station is
very near to a Femtocell region. The
Femtocell operates at 20mW, whereas a
Macrocell operates way over the kW
range. Considering they both relate to
the same network,

Femto-Femto Interference
When two femtocells are close to each other,
it tends to effect the Femtocell’s cell edge
user the most. Although the users are in their
respective region, the neighboring
Femtocell’s transmission power interferes
with the cell edge user.

Macrocell User Interference
When a macrocell user (MU) is in a
Femtocell region, the downlink signal of
MU will interfere with any FUE near the
MU.

Solution schemes for interference problems
Femto-Aware Spectrum Arrangement Scheme
Fractional Frequency Reuse (FFR)
Strict Fractional Frequency Reuse (Strict FFR)
Soft Frequency Reuse (SFR)
Poisson Point Process (PPP)

Soft Fractional Frequency Reuse (SFFR)
SFFR (Soft Fractional Frequency Reuse) is one effective solution of inter-cell interference control.
SFFR can control the interference in cell edges to enhance the frequency reuse factor and
performance in the cell edges.
System Layout Model

Scenario SFFR
Sub-Carrier
Identification User Femto Femto
Center A 50 % 40% 60%
Sector 1 B (50/3 )% (60/3) % (40/3)% Random
Sector 2 C (50/3 )% (60/3) % (40/3)% Random
Sector 3 D (50/3 )% (60/3) % (40/3)% Random
Cell Edge
Cell Center
Total System
BW
Frequency
Cell1Power
Frequency
Cell1Power
Cell Edge
Cell Center
Total System
BW
Frequency
Cell1Power
Frequency
Cell1Power
Cell Edge
Cell Center
Total System
BW
Frequency
Cell1Power
Frequency
Cell1Power
Bandwidth : 10 MHz = 600 sub-carrier
Ex : 50%, cell center 300 subcarrier,
cell edge @100 sub-carrier

Point Process
A stochastic point process is a type of
random process for which any one
realization consists of a set of isolated
points either in time or geographical space,
or in even more general spaces.

Usefulness of PPP
PPP provides tractable results that help understanding the relationship
among the performance metrics and the design parameters.
PPP can model random network with randomized channel access.
Provides tight bound for networks with planned deployment and networks
with coordinated spectrum access.
Most of the available literature assume that the nodes are distributed
according to a PPP.
Results obtained using PPP are accurate (within 1-2dB) with those
obtained for legacy cellular networks as well as multi-tier cellular
networks.

Definition of PPP

Cellular networks and the PPP

SPPP on small cell deployment

System model designing
Considering two-tier Macro/Femto or
with combination C of |C| Macro-cells and |C| -1
underlying channels in the Femto-cells which are
denoted as Cc. The active mobile users are in the
areas have been denoted through j of |j| sets.
If the set of users |S| of S are denoted users which
are served by the base stations then 1
...o C
J S S 
  

SINR Modeling(Stochastic geometry model)
,1 ,S
( ,..., )c
c c c
P P P
1 10( , ,..., )CP P P P 
• Power allocation vector by
• Two-tier vector
So SINR from the User form is
Range of the base station, the users communicates with the
with its corresponding base stations at the transmission power
𝛾 𝑐,𝑗 =
𝐺 𝑐,𝑗 𝑃𝑐,𝑗
𝐺 𝑐,𝑗
𝑗′≠𝑗
𝑗′∈𝑆 𝑐
𝑃𝑐,𝑗 +
𝑐′≠𝐶
𝑐′∈𝐶
𝐺 𝑐′,𝑗( 𝑘∈𝑆 𝑐′
𝑃 𝑐′,𝑘) + 𝑛0
𝑃𝑐

SINR Modeling(Stochastic geometry model)
Transmitting power vector
, 2 ,
1
( ( )) log (1 ( ))j c j c jU P K P
T
  

Cell segmentation
1. Small coverage is divided into two
sectors.( Cell center, Cell edge)
2. Determines the location of each user
periodically.
3. Adaptive “Smart” small cell chooses
best scheme for the user.

SON PRB(Physical resource Block)
Initialization
Neighboring Cell Info
Neighboring Cell Info
Update
QoS Guarantee PRB Allocation

Cell Edge optimization on SFFR
o In order to improve the performance in
cell-edge, Soft Fractional Frequency Reuse
(SFFR) scheme is introduced, which is
based on Soft Frequency Reuse (SFR).
o Users in each cell are divided into two
major groups according to their geometry
factors.

Adaptive Power Algorithm
,varcP
Step 1
At the center of the Small-cells, there is no specified
interference reduction technique.
Assuming center cell signal is much stronger
Maximum two-tier utility performance is achieved if all of the resources of all the base
stations are given to their respective cell users, and transmit at their maximum power
For a cell wϵC, at any given , power allocation vector such that
Which improves unity base power allocation and keeping h>1
,var
Min Max
C c CP P P c C   

Adaptive Power Algorithm
The SINR gets a proper improvement and the SINR turns in to
, ,
,
, , , ,k 0
, , , ,
, , , ,k 0 , , , ,k 0
( )
( )
. .
. ( ) . . ( ) .
c
c
c c
c c
c j c j
c j j S c C
c j c j c j cj j c C k S
c j c j c j c j
j S c C j S c C
c j c j c j c c j c j c j cj j c C k S j j c C k S
G P
P
G P G P n
hG P hG P
hG P G P n hG P h G P hn


 
  
    
      
           

  
  
 
   
  
     

Step-2
𝑇𝐹𝑅
, ( ) 2 2
ˆ ˆ
( )
y y
SFR e T FR FR
Z Z
Pg r g r
F P T T T
PI PI
 
 
   
 
  
 
Cell Center:
No interference and strong signal strengths so
The The coverage probability of an FFR cell center
user whose initial SINR is<
=
=
( , , , )
1 ( , , , )
c
c FR
T
p
p T

 

 

 
2
2(1 2 ( , , , )) ( )
0
, , ,1 ( )
FR FR
T
v T T T v
P
c TR
e
dv
p T

    
 
 

   


Where

Step 2(Cont)

Middle Outer region
This midsector of small covers an area
10~20m away from the small BS. Here we use
the orthogonal frequency-division multiple
access (OFDMA) technology which is
intensely considered by the 3GPP LTE
[Resource allocation with Interference
Avoidance – Yu-Shan Liang].
Assigning physical resource
block
User classification
Increasing PRB efficiency
In OFDMA-based cellular system, the whole
spectrum is split into orthogonal sub-channels.
Achievable capacity (bps/Hz/cell),
Where Ṝ is the average delivered rate in the
past, measured over a fixed window of
observation.

Sector 3: Cell Edge (20~25m)
In order to improve the performance in cell-edge, the Soft fractional frequency reuse
(SFFR) scheme is introduced, which is based on soft frequency reuse but with a much
greater performance. Specially, users in each cell are divided into two major groups
according to their geometry factors. In cell-edge group, users are interference-limited
due to the neighboring cells, whereas in cell-centre group users are mainly noise-
limited.
Considering fo as the center
frequency and f1, f2, & f3 as non-
crossing frequencies the fractional
reuse factor Fr is,

Simulation and Results

Simulation Parameter

SFFR approach: Hexagonal grid
simulations

Cell Edge improvement

Coverage probability
J. Andrews, F. Baccelli, and R. Ganti, “A Tractable Approach to Coverage and Rate in Cellular Networks,” IEEE
Transactions on Communications, vol. 59, no. 11, pp. 3122–3134 November 2011.

Improved Coverage probability

SINR Comparison

Result analysis
Assumptions
1. We consider Hetnet with 3 Macro-cell as Tier-1 and 6 Femto-cell in Tier-2. The cell
edge zone is 0.6 % of the coverage.
2. The path loss is being measured with different 3 different values which are 7dB,
10dB and 13dB for the three regions which have been discussed earlier in the
paper.
3. TFR which is mentioned as FFR Threshold has been kept 3dB for this 2 tier HetNet
deployment.
Results
1. Noticeable increase at the cell-edge Macro-cell, (SFFR technique)
2. Overall throughput increases by almost 15 %
3. At β= 4, SFFR provides the best coverage probability with the minimum SINR about -10dB
compared to FFR.
4. Final results show that almost 16% development of the SINR distribution for the cell edge.

Conclusion
 Here we proposed a Adaptive power algorithm with SFFR approach
 This strategy is analyzed in a multi-cell systems with coexistence of Femto-cell
on the Macro-cells layers
 network. With a better coverage probability we can conclude with the
enhancement in the SINR of the cell edge users. The final result shows
usalmost 16% development the SINR distribution for the cell edge.

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