Heterogeneous Networks(HetNets)

Heterogeneous Network (HetNet): A paradigm shift in
cellular networks
Devendra Singh
Under the Guidance of Mr. Ramesh Babu Battula
MNIT,Jaipur
November 17, 2014
Devendra Singh (2013pcp5144) M.Tech CS 3rd Semester November 17, 2014 1 / 30

Table of Contents
Introduction
Growing demand of data
Growth in Mobile Broadband
What is HetNet ?
Multiple cell sizes
Requirements for Future Networks
HetNets oer Promising Solution
Deployment scenarios
Changes/Challanges from Traditional Networks to HetNets
Conclusion

Introduction
The use of mobile devices has been increasing in an exponential
fashion from past few years. The exponential increase in the use of
interconnected mobile devices will result in the growth of data trac.
This will force the network operators to increase the capacity of the
networks in a dramatic fashion to meet the upcoming demands.
The problem currently being faced by the networks operators is not
the coverage { which is now nearly universal { but the capacity.
Because there are too many users demanding too much data.

Growing demand of data
Figure : Growth of Data Demand in dierent areas between 2011 { 2016.
Exponential rise in demand for data esp. in Asia Paci

c, North
America and Europe.

Growth in Mobile Broadband
Figure : Fixed Broadband vs Mobile Broadband.
Thus rapid adoption of cellular based mobile broadband .

What is HetNet ?
HetNets are typically composed of a variety of formats of base station,
radio access networks, transmission solutions and power levels.
But it also presents problems in terms of ubiquity(omnipresence) and
operation with such a variation of technologies and approaches.
In this way HetNets have several aspects :
Use of multiple radio access technologies
Operation of dierent cell sizes and approaches
Backhaul

What is HetNet ?
Figure : Heterogeneous Network (HetNet): A network that consists of a mix of high power
macro cells and low-power nodes, e.g. Pico, Femto cells..

Multiple cell sizes
Small cells: smaller coverage and capacity.
Small cells consist of:
Pico-cells
Femto-cells
Micro-cells
Mobile operators mainly use small-cells to extend and maximize their
service coverage and/or to increase the network capacity.
Macro cells: larger coverage and capacity.
High power cellular BS(tower)

Multiple cell sizes
Properties of dierent cells :
Figure : comparision of femto, pico, micro and marco cells .

Requirements for Future Networks
Future networks should support explosive mobile data trac growth
driven by :-
Large screen devices
Multimedia applications
More connected users and devices
Future networks should be optimized for mobile broadband trac :-
Eciently support low-mobility trac, mobile video, internet
applications
Future networks should be low power, and environmentally friendly
Future networks should interwork eciently with other radio
technologies :-
Converged multi-access networks and terminals

HetNets oer Promising Solution
Future networks should support explosive mobile data trac growth
HetNets oer linear capacity scaling with number of BS nodes
Future networks should be optimized for mobile broadband trac
HetNets bring serving BS closer to user, eciently supporting low
mobility and high rate trac
Future networks should be low power, and environmentally friendly
HetNets oer opportunity to lower transmission power, saving energy
at BS and battery life at clients
Future networks should interwork eciently with other radio
technologies
HetNets enable seamless integration of un-licensed LAN/PAN
technologies into cellular networks

Deployment scenarios
By using small cells in Hotspots(where user density is more.) we
achieve spectrum reuse via cell splitting.
Additional low power nodes
In dense user locations
Small Coverage area
Localized interference
Figure : Deployment of low power nodes (small cells) throughout a macro-cell layout

Outdoor Macro-Pico Deployment
Picos are deployed on the macro cell edge or hotspot to improve coverage or
throughput.
Picos are open to all UEs(User Equipments)
Picos can be used for both indoor/outdoor purpose
Their coverage area is around in the radius of 200 meters.
Usually they serve around 32 to 100 users
Figure : Outdoor Macro-Pico Deployment

Indoor Macro-Femto Deployment
Femtos are deployed indoor to enhance the throughput
Femto cells are open to speci

c UEs { called CSG (Closed Subscription Group)
A UE close to femto cant connect to femto if it is not in CSG (connects to macro
instead)
Usually serve 4 to 16 users
Coverage in the radius of 10 to 50 meters.
Figure : Indoor Macro-Femto Deployment

HetNet Deployment: Macro+small cells
Heterogeneous Network(HetNet) consists of two levels of BS: Macro-Cellular
BSs(MCBSs) and underlying Small-Cell BSs(SCBSs); these can use the same
technology(e.g. LTE) or dierent technologies(LTE and WiFi).
Figure : HetNet Deployment: Macro + small cells

Changes/Challanges from Traditional Networks to HetNets
HetNets require the changes in following important aspects :
Performance Metrics
Network Topology
Cell Association
Downlink{Uplink Relationship
The Backhaul Bottleneck
Mobility
Interference Management
Mobility and Interference Management are the most challanging
issues in HetNets.
Many of these issues are still under the research work for betterment.

Performance Metrics
Traditional Cellular:
Outage/coverage probability distribution in terms of
SINR(signal{to{interference{plus{noise{ratio) or spectral
eciency(bps/Hz)
HetNet:
Outage/coverage probability distribution in terms of Rate or Area
spectral eciency(bps/Hz/m2)
Recommendation: Stop measuring performance with
BER(bit{error{rate) or SINR distribution, or with spectral eciency.
These metrics are no longer very relevant. Instead, use the rate
distribution (user-perceived, i.e., accounting for load) or area spectral
eciency.

Performance Metrics
Figure : Downlink (left) and uplink (right) Max-SINR association regions for a three-tier
network with macros(red), picos(green), and femtos(black). Note that the BSs are in the same
locations in these two plots, but the regions are very dierent.

Network Topology
BSs spaced out, have distnict coverage areas. Hexagonal grid is an
ubiquitous model foe BS locations.
HetNet:
Nested cells(pico/femto) inside macrocell. BSs are placed
opportunistically and their locations are better modeled as a random
process.
Recommendation: Phase out the grid model for BS locations, which
is neither tractable nor realistic for HetNets. Instead adopt a random
spatial model for the BS locations.

Network Topology
Figure : The macro-pico model used by 3GPP(3rd Generation Partnership Project). Other
3GPP models include randomly located picocells inside the macrocell area.

Cell Association
Usually connect to stongest BS, or perhaps two strongest during soft
handover.
HetNet:
Connect to BS(s) able to provide the highest data rate rather than
signal strength. Use biasing for small BSs.
Recommendation: Initial work shows that load balancing through
cell range extension is very valuable in a HetNet, and that biasing is
nearly optimum compared to a centralized optimization, which is
perhaps surprising.

Cell Association
Figure : Base station associations in a three-tier HetNet using the traditional max SINR
criteria (left) or a revised max sum log rate criteria (right). The right

gure results in more
balanced load and higher achieved data rates, especially for users who were previously on the
macrocell edges.

Downlink{Uplink Relationship
Downlink and Uplink to a given BS have approximately the same SINR.
The best DL BS is usually the best in UL too.
HetNet:
Downlink and Uplink can have very dierent SINRs; should not
necessarily use the same BS in each direction.
Recommendation:The DL and UL need to be considered as two
dierent networks, and will require dierent models for interference,
cell association, and throughput.

The Backhaul Bottleneck
BSs have heavy{duty wired backhaul, are connected into the core
network. BS to MS connection is the bottleneck.
HetNet:
BSs often will not have high speed wired connections. BS to core
network (backhaul) link is often the bottleneck in terms of performance
and cost.
Recommendation: One clever approach is the idea of caching
popular content such as video clips or other common downloads at
the small cells. Such content can be updated periodically at a time of
low{backhaul load. The gain of such innovations on network
performance can be large.

Mobility
Hando to a stronger BS when entering its coverage area, involves
signaling over wired core network.
HetNet:
Handos and dropped calls may be too frequent if use small cells when
highly mobile, overhead is a major concern.
Recommendation: Improved mobility modeling, handover
optimization, and mobility-aware interference management are all
challenging topics for future work.

Interference Management
Employ (fractional) frequency reuse and/or simply tolerate very poor
cell edge rates. All BSs are available for connection, i.e. open access.
HetNet:
Manage closed access interference through resource allocation; users
may be in one cell while communicating with a dierent BS;
interference management is hard due to irregular backhaul and sheer
number of BSs
Recommendation: Ecient interference management in a HetNet
relies on reasonable models for all the previous topics discussed until
now. Interference management is also another challenging topic for
research and future work.

Conclusion
HetNets will play a key role in future of mobile broadband cellular
networks.
Support shift from spectral eciency to network eciency
High capacity at low cost
Flexible deployment and self-management
Low power and emissions(Green)
Enable mass devices to connect to cellular network
Active area of research in enabling technologies: mobility, interference
management, self-organization, topology etc.
Standardization is under way to realize bene

Heterogeneous Networks(HetNets)

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