4G Heterogeneous Networks (HetNets)

Πρόγραμμα Μεταπτυχιακών Σπουδών: «ΕΠΙΣΤΗΜΗ ΚΑΙ ΤΕΧΝΟΛΟΓΙΑ ΥΠΟΛΟΓΙΣΤΩΝ

(ΕΤΥ)»

ΤΜΗΜΑ ΜΗΧΑΝΙΚΩΝ ΗΛΕΚΤΡΟΝΙΚΩΝ ΥΠΟΛΟΓΙΣΤΩΝ ΚΑΙ ΠΛΗΡΟΦΟΡΙΚΗΣ

ΠΟΛΥΤΕΧΝΙΚΗ ΣΧΟΛΗ, ΠΑΝΕΠΙΣΤΗΜΙΟ ΠΑΤΡΩΝ

Presentation Outline• Introduction

• Benefits and research challenges of 4G HetNets

• Traditional cellular networks vs. 4G Hetnets

• 2-tier HetNet (typical system model and analysis)

• Area Spectral Efficiency (ASE) and Area Green Efficiency (AGE)

• Comparison of various cell-edge related technologies

• Femtocells

Evolution of Mobile Broadband

Progression of cellular peak data rates

Beyond 4G challenges

Mobile Data Growth (1/2)

Mobile Data Growth (2/2)

The Zetabyte era (1021 Bytes )

Reasons for traffic increase

Mobile User Side • Number of subscribers due to

Smart phones and devices (18% increase in sale of smart phones in 2013 and even greater in 2014-15)

• Video and Ultra High Definition (UHD) movies

• E-facilities; online gaming, e-Gov services, highly video content exchange and peer-to-peer networking

• Mobile broadband

Corporate Side • Increase deployment of mobile broadband • Flat rate pricing and other package incentives policy • Launch of high speed protocols such as HSDPA, VoIP and Small-cells • Triple Play (broadband + IP Phone + IP TV) • New corporate services e.g. Machine-Type communication

Moving to 5G

The Roadmap to 1000x capacity

Addition of macro BSs? Probably not!

Small Cell solution is better!

3GPP Hetnets (LTE-A)

Benefits of HetNetsBringing the network close to the subscribers is one of the striking solution to: • Offload the macrocells and reduce the coverage holes. • Enhance the link quality by reducing the distance between TX and RX. • Allow more efficient spectral reuse due to large number of small-cell deployments. • Reduce expenditures due to no or limited upfront planning and lease costs. • Reduced power consumption and thus achieves target rates with low power. • Increase coverage especially for sparsely populated and rural areas.

Challenges of HetNetsThe implementation of HetNets implies its own challenges: • Co-tier and Cross tier interference: unplanned/ad hoc deployments, lack of

coordination among different tiers (e.g. macro and micro tiers).

• Subscription and Connectivity Issues: some cells may operate on restricted mode and thus non-subscribers cannot connect and hand over to the nearest cells (access policy conflicts).

• Handovers and traffic offloading: provide a seamless uniform service when users move in or out of the coverage area at the expense of significant overhead.

• Self-Organization: each cell constantly monitors the network status and optimizes its settings to improve coverage and reduce interference.

• Convergence and Integration: users desired to connect to multi heterogeneous RATs in multi access and multi operator environment based on “Always Best Connected (ABC)” rule (maintain and optimized quality).

Small Cell Deployment

What is a small cell?

• Small-cell is a low power, low cost user/operator deployed base station or access point to complement macrocell setup.

• “Small-cells” concept includes femtocells (up to 100m), picocells (up to 200m), microcells (up to 500m), etc.

• Small-cell can be deployed open access and closed access/restricted. • In home use can support up 4-6 mobile users and for business use can support up to

16-20 users, (dependent on small-cell coverage). • Small-cells connect with the main networks using digital subscribers line (DSL) or

cable broadband (wired or wireless front haul options).

Key features: • Consumer side: coverage; capacity; superior indoor quality of service and improve

battery life. • Operator side: Reduce CAPEX and OPEX of mobile networks.

Range expansion via small cellsRange Expansion is essentially a way to extend the reach of cells through small-cell

deployments so that they cover more and more users in their vicinity and their edge particularly with improved link quality. It is critical for the following important reasons:

1) More users who can be better served by small-cells (than the macro) are being

connected to them;

2) More users can be offloaded from macro network (to small-cells) freeing up resources for the users on the macro.

3) More users can be transmitted with much less uplink power and thereby reducing co-channel interferences.

• And all these are done without any additional spectrum or infrastructure.

Serving the network edges

• LTE Advanced (LTE-A) should target the cell-edge user throughput to be as high as possible, given a reasonable system complexity

• An intelligent interference management control is required to handle cell-edge interferers

• A more homogeneous distribution of the user experience over the coverage

area is highly desirable and therefore a special focus should be put on improving the cell-edge performance

Traditional cellular vs. 4G Hetnets (1/7)Traditional Cellular:• Outage/coverage probability distribution in terms of SINR (signal-to-interference

plus noise ratio) or spectral efficiency (bps/Hz)

4G HetNet:• Outage/coverage probability distribution in terms of Rate or Area spectral

efficiency(bps/Hz/m2)

Change (performance metrics): Stop measuring performance with BER (bit error rate) or SINR distribution, or with

spectral efficiency. These metrics are no longer very relevant. Instead, use the rate distribution (user-perceived, i.e., accounting for load), area spectral efficiency, area green efficiency, etc.

Traditional cellular vs. 4G Hetnets (2/7)Traditional Cellular:• BSs spaced out, have distinct coverage areas. Hexagonal grid is an ubiquitous

model foe BS locations.

4G HetNet:• Nested cells (pico/femto) inside macrocell. BSs are placed opportunistically and

their locations are better modeled as a random process.

Change (network topology): • 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.

Traditional cellular vs. 4G Hetnets (3/7)Traditional Cellular:• Usually connect to strongest BS, or perhaps two strongest during soft handover.

4G HetNet:• Connect to BS(s) able to provide the highest data rate rather than signal strength.

Use biasing for small BSs (i.e. predetermined and modeled conditions).

Change (cell association): • 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.

Traditional cellular vs. 4G Hetnets (4/7)Traditional Cellular:• Downlink and Uplink to a given BS have approximately the same SINR. The best

DL BS is usually the best in UL too.

4G HetNet:• Downlink and Uplink can have very dierent SINRs; should not necessarily use the

same BS in each direction.

Change (downlink-uplink relationship): • The DL and UL need to be considered as two different networks, and will require

different models for interference, cell association, and throughput.

Traditional cellular vs. 4G Hetnets (5/7)Traditional Cellular:• BSs have heav-duty wired backhaul, are connected into the core network. BS to MS

connection is the bottleneck.

4G 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.

Change (backhaul bottleneck): • 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.

Traditional cellular vs. 4G Hetnets (6/7)Traditional Cellular:• Handoff to a stronger BS when entering its coverage area, involves signaling over

wired core network.

4G HetNet:• Handoffs and dropped calls may be too frequent if use small cells when highly

mobile, communication overhead is a major concern.

Change (mobility): • Improved mobility modeling, handover optimization, and mobility-aware

interference management are all challenging topics for future work.

Traditional cellular vs. 4G Hetnets (7/7)Traditional Cellular:• Employ (fractional) frequency reuse and/or simply tolerate very poor cell edge

rates. All BSs are available for connection, i.e. open access.

4G HetNet:• Manage closed access interference through resource allocation; users may be in one

cell while communicating with a different BS; interference management is hard due to irregular backhaul and sheer number of BSs.

Change (interference management): • Efficient 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.

2-Tier HetNet (typical system model)Macrocell network Small cell network

Macrocell-Only network (Monet)

Uniformly Distributed Small Cell (UDC)

Is UDC setup an efficient choice? (1/2)

Is UDC setup an efficient choice? (2/2)

Cell-on-Edge model

Energy efficiency problem and targets

Research towards green edges

• Energy efficiency has been recently marked as one of the alarming bottleneck in the telecommunication growth paradigm mainly due to two major reasons:

1.Slowly progressing battery technology 2.Dramatically varying global climate

What is a “green edge”?

Energy consumption at the downlink

Energy consumption at the uplink

Power Control (PC) techniques

Open Loop PC Closed Loop PC

Uplink power adaptation based on PC

Mobile users are considered to be able to estimate/ compensate their path loss while adjusting their transmit power accordingly.

Uplink receiver at the BS estimates the SINR of the received signal and compares it with the target SINR value. If the received SINR is below the target SINR, a TPC command is transmitted to the mobile user to request an increase in transmit power. Otherwise, the TPC command will request a decrease in transmit power.

Area Spectral Efficiency (ASE) of HetNets

The Area Spectral Efficiency (ASE) of the 2-tier HetNet is defined as the sum of the maximum achievable rates (of all UEs) per unit bandwidth per unit area [bit rate/Hz/macrocell area] jointly supported by the small-cell and macrocell Base Stations.

ASE vs. macrocell radiusIt is clear that the ASE of the COE model has been significantly improved when Small-cells are active in the macrocell compared to the MoNet and UDC models. This is due to the fact that COE deployment restricts only the cell-edge mobile users to communicate with the Small-cells which enhances the overall network ASE compared to UDC and MoNet configurations.

Relative ASE gain vs. number of small cells

Due to reduced link distance between the edge mobile users and their respective small-cell BSs, higher relative ASE gain is observed even with only few active Small-cells in COE configuration. This gain, however, tends to reduce with the further increase in the number of small-cells as the level of interference increases with the population of Small-cells.

Interference problem in 4G Hetnets

4G Hetnets’ interference mitigation

It can be seen that the received interference power at macrocell BS has been significantly reduced due to the presence of Small-cells arranged on the edge of the macrocell. Moreover, it is also noted that the interference received at a Small-cell BS from N-1 small-cells is almost same compared to the interference experienced from two adjacent Small-cells.

ICIC scenarios in 4G Hetnets

Standardization for Hetnet eICIC• The ICIC methods specified in 3GPP Rel. 8 and Rel. 9 do not specifically consider

HetNet settings and may not be effective for dominant HetNet interference scenarios (see previous figure)

• In order to address such dominant interference scenarios, enhanced Inter-Cell Interference Coordination (eICIC) techniques were developed for Rel. 10, which can be grouped under three major categories according to:

• – Time-domain techniques. – Frequency-domain techniques. – Power control techniques.

Time domain techniques for eICIC• Almost Blank Subframes (ABSFs) at femtocells • As shown in next figure, in the ABSFs, no control or data signals, but

only reference signals are transmitted. • When there are MUEs in the vicinity of a femtocell, they can be

scheduled within the subframes overlapping with the ABSFs of the femtocell, which significantly mitigates cross-tier interference.

• Similar eICIC approach using ABSFs can also be used to mitigate interference problems in picocells (and relays) that implement range-expansion.

• When no interference coordination is used for range-expanded picocell users, they observe large DL interference from the macrocell. The interference problem can be mitigated through using ABSFs at the macrocell, and scheduling range-expanded picocell users within the subframes that are overlapping with the ABSFs of the macrocell.

ABSFs for time-domain eICIC

Frequency domain techniques (eICIC) • In frequency-domain eICIC solutions, control channels and physical signals (i.e.,

synchronization signals and reference signals) of different cells are scheduled in reduced bandwidths in order to have totally orthogonal transmission of these signals at different cells. While frequency-domain orthogonalization may be achieved in a static manner, it may also be implemented dynamically through victim UE detection.

• For instance, victim MUEs can be determined by the macro eNBs by utilizing the

measurement reports of the MUEs, and their identity may be signaled by the macro eNB to the home eNB(s) through the backhaul. Alternatively, victim MUEs may also be sensed by the home eNBs.

Power control techniques (eICIC) • Apply different power control techniques at femtocells. • While reducing the radiated power at a femtocell also reduces the total throughput

of femtocell users, it may significantly improve the performance of victim MUEs.

Area Green Efficiency (AGE) of HetNets

AGE vs. macrocell radius

It is illustrated clearly that the COE configuration outperforms the UDC configuration since the deployment of small-cells at the macrocell edge mandates a reduction in the number of edge mobile users transmitting with the maximum power. The AGE improvement is due to the fact that the number of energy efficient users increase in both UDC and COE deployments with the increase in macrocell radius.

Other cell-edge related technologies

Distributed Antennas on Edge (DOE)

Relay-on-Edge (ROE)

D2D deployment at the edge (DCOE)

ROE vs. DOE vs. DCOE vs. FOE

Small cell deployment: Comparison

Femtocells (some basics)• A Femtocell is a low-power access point based on mobile technology which

provides wireless voice and broadband services.

• The Femtocell connects to the mobile operator’s network via a standard broadband connection, including ADSL, cable or fibre.

• Data to and from the femtocell is carried over the Internet.

• Femtocells use standard wireless protocols over the air to communicate with standard mobile devices, including mobile phones and a wide range of other mobile-enabled devices.

• Standard protocols include GSM, WCDMA, LTE, Mobile WiMAX, CDMA and other current and future protocols standardised by 3GPP and 3GPP2

• The use of such protocols allows femtocells to provide services to several billion existing mobile devices worldwide.

Femtocell architecture

Types of Femtocell – Class 1• Similar transmit power and deployment view to Wi-Fi access points,

typically 20 dBm (100 mW) of radiated power or less.– For residential or enterprise application. – They typically support 4–8 users. – Installed by the end-user.

• Class 1 femtocells may be stand-alone devices or integrated with other technology (residential gateways).

• Access will often be closed, restricted to a specified group of users, but may also be open.

Types of Femtocell – Class 2• Higher power typically up to 24 dBm (250 mW) of radiated power,

– Class 2 femtocells may be installed in small-office, in large enterprise buildings or in a corporate campus.

– Support longer range or more users (8–16)– May be installed by the end-user or the operator.

• Will typically support additional functionality such as handover between femtocells, integration with PBX and local call routing.

• Access may be closed or open.

Types of Femtocell – Class 3• Still higher power (+24 dBm) and can be deployed:

– Indoors (e.g. in public buildings) for localised capacity, Outdoors in built-up areas to deliver distributed capacity or in rural areas for specific coverage needs

– longer range or more users (e.g. 16 or greater).– Installed by the operator.

• Class 3 femtocells are used to solve specific coverage, capacity or service issues.

• Typically they are open access.

Comparison table

Deployment scenarios for open access femtocells

Scope of Femto Forum

Femtocell characteristics (1/3)• Generates coverage and capacity.

– Provide reliable coverage throughout the home. This allows users to rely on their mobiles as a prime means of making and receiving calls.

– Femtocells also create extra network capacity, serving a greater number of users.

• Permits low prices. – The large volumes envisaged for femtocells will allow substantial

economies of scale.– Some femtocell operators include special tariffs for calls made or

received on the femtocell

Femtocell characteristics (2/3)• Operate in licensed spectrum.

– By operating in licensed spectrum femtocells provide assured quality of service to customers over the air, free from harmful interference.

• Fully managed by licensed operators. – Femtocells only operate within parameters set by the licensed

operator. The operator is always able to create or deny service to individual femtocells or users.

Femtocell characteristics (3/3)• Self-organising and self-managing.

– Class 1 and Class 2 Femtocells can be installed by the end customer. They set themselves up to operate with high performance, with no need for intervention by the customer or operator.

• Over Internet-grade backhaul. – Femtocells backhaul their data over Internet-grade broadband

connections, including DSL and cable, using standard Internet protocols.

Femtocell vs. WiFi (1/2)• Wi-Fi Access Points always operate in unlicensed spectrum.

– Interfering devices can legally appear close to any given user. Most Wi-Fi devices operate in the 2.4 GHz frequency band, where only three non-overlapping channels are available.

• Femtocells, operate in licensed spectrum. – The operator is in control of every transmitting device and can

manage interference.

Femtocell vs. WiFi (2/2)• Transmit Power Control in Wi-Fi networks

– Wi-Fi access points and client devices all transmit at a power of around 100 mW, which does not change even when far less power is required, increasing the incidence of interference and draining batteries.

• Transmit Power Control in cellular technology– Both the mobile devices and the femtocells continually adjust their

transmit power to the minimum necessary, reducing interference, increasing capacity and increasing battery life.

Challenge 1: Spectrum Allocation • How will a Femtocell adapt to its

surrounding environment and allocate spectrum in the presence of Intra-cell and Inter-cell Interference?

• Due to the absence of coordination between the macrocell and femtocells, and between femtocells, decentralized spectrum allocation is an open research problem.

• Can fractional frequency reuse be used for overlapping macrocell and femtocells?

• Should interference between macrocell and femtocell users be minimized through bandwidth splitting?

• Which is the best scheme in various configurations?

F F

F

F1 F2

F3

F=F1+F2+F3

F1, F2, and F3 are different sets of sub-channels

Challenge 2: IP backhaul bottleneck • How Will Backhaul Provide Acceptable QoS?

• IP backhaul should provide sufficient capacity to avoid creating a traffic bottleneck.• Trials reveal that when users employed Wi-Fi, femtocells experience difficulty

transferring data and even low-bandwidth services like voice.• This is a very important challenge to meet as improved voice coverage is the main

driver for femtocells.• The Femtocell and Wi-Fi architectures should be extended in order to incorporate a

unified bandwidth controlling mechanism responsible for the dynamic allocation of the backhaul bandwidth between the Femtocell, the Wi-Fi and the local wire line network.

• A unified Call Admission Control could also improve the QoS by admitting a new service request either to the Femtocell or to the Wi-Fi network depending on the available respective capacity.

Challenge 3: Open or Closed Access? • Should Femtocells Provide Open or Closed Access?

• A closed access femtocell has a fixed set of subscribed home users that, for privacy and security, are licensed to use the femtocell.

• Open access femtocells, on the other hand, provide service to macrocell users if they pass nearby. Radio interference is managed by allowing strong macrocell interferers to communicate with nearby femtocells.

• Although open access reduces the macrocell load, the higher numbers of users communicating with each femtocell will strain the backhaul to provide sufficient capacity and raise privacy concerns for home users.

• Ethical and legal dilemmas arise on whether a femtocell should service macrocell users for making emergency calls if they are located within its radio range.

• Operators are looking at hybrid models where some of the femto’s resources are reserved for registered family members while others are open for roamers.

Challenge 4: Mobility Management • How Will Handoff be Performed?

• Current cellular systems broadcast a neighbour list of potential handover cells to be used by a mobile attached to the current cell.

• In general, handoff from a femtocell to the macrocell network is significantly easier (as there is only one macro BS) than handoffs from the macrocell to the femtocell.

• • Such a handoff protocol does not scale to the large numbers of femtocells that

“neighbor” (actually underlay) the macrocell.

• The underlying network equipment is not designed to rapidly change the lists as femtocells come and go.

• In open access, channel fluctuations may cause a passing macrocell user to perform multiple handovers.

Challenge 5: Femtocell location• Can Subscribers Carry Their Femtocells for Use Outside the Home Area? • Unlike Wi-Fi networks that operate in unlicensed spectrum in which radio interference

is not actively managed, femtocell networks will operate in licensed spectrum. • Femtocell mobility can cause problems when a subscriber with operator A carries their

femtocell to another location where the only service provider is rival operator B.

• Should the femtocell be allowed to transmit on operator B’s spectrum? How will Femtocells Provide Location Tracking for Emergency calls?

• Femtocell location may be obtained by either: – using GPS inside femtocells (added cost with possibly poor indoor

coverage), – gathering information from the macrocell providing the femtocell falls

within the macrocell radio range,– or even inferring the location from the mobile position (estimated by the

macro network) at handoff to the femtocell.

Towards 5G

Small cells in 5G era

• Objective is NOT just the link spectral efficiency (as in 2G, 3G, 4G) –Area Spectral Efficiency and Energy Efficiency

• 4G is suitable for low density cells, NOT high density cells (small-cells)

– Minimum network management control overheads – Sub-millisecond Air-Interface latency – Flexible for carrier aggregation across highly fragmented spectrum including

license-exempt band – Highly energy efficient (at least 10 times) – Support fast and reliable spectrum sensing for spectrum sharing – Simple Wi-Fi-like MAC – Support of device to device communications – Scalable for Machine type communications

Managing overheads for small cells

Before After

Split Data/Control plane SDN technology in 5G?

Splitting the data and control functionality – scalable deployment solution for small-cells in 5G cellular networks

Open issues• Cooperative wireless communications/Relays

• Mobile cloud computing (MCC)

• M2M/IoT

• Self-organized networks (SON)

• Context aware mobile and wireless networking (CAMoWiN)

• Software-defined networking (SDN) • Network function virtualization (NFV)