Interference mitigation and User Association algorithms for resource optimization in future DenseNets

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Abstract

Traffic demands in mobile networks are expected to grow substantially in the next years, both in terms of total volumes and in bit-rate required by individual users. It is generally agreed that the only possible solution to overcome the current limitations is to deploy very dense and heterogeneous wireless networks, which we call DenseNets. In this work we show a method to face the Access Selection issue in a scenario with Macro-cells and femto-cells, taking into account two imminent problems of DenseNets: the backhaul constraints and the control overhead, such as handover events. Then we propose an algorithm, called Demand-Supply algorithm, to approximate the formu-lated mathematical model. Furthermore, the increase of network node density, due to the several subscriber access points and to a quick increase of mobile device, has a negative consequence: the inter-cell interference. Interference is becoming the most se-rious obstacle towards spectral efficiency. Therefore, considering that radio resources are limited and expensive, new techniques are required for efficient radio resource al-location in the next generation of cellular networks. We formulate two scheduling algorithms, which determine the nodes can be activated to simultaneously transmit, without causing excessive interference to the other nodes. The proposed algorithms are based on time scheduling and they are in line with the ABS (Almost Blank Subframe) technique, recently standardized at the 3GPP. The first algorithm is the improvement of BASICS algorithm, BASICS+_{, and it is formulated both in uplink and downlink}

version; the second algorithm is based on a heuristic approach and it is called min-distance scheduling algorithm. By means of simulations, we test the eﬀectiveness of algorithms realized. Ultimately, we propose a functional scheme architecture of a DenseNet central controller, which includes both functionalities studied: Access Selec-tion and Scheduling techniques.

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Acknowledgments

My study path ends with this Master Degree in Telecommunication Engineering Fac-ulty of Pisa University. Several professors of the Information Engineering Department of Pisa have contributed to my professional education, and in particular I thank my supervisor and Professor Filippo Giannetti for his professionalism in all courses and two degree in which he took me. My traineeship was carried in Intecs s.p.a. firm, which I thank for giving me this opportunity. I thank all the people of Telecommuni-cation Group of Intecs, and in particular, the software developer Claudio Bottai and my two company tutors Claudio Cicconetti and Arianna Morelli for the fundamental help they provided me. I thank my special English teacher, Joanne Spataro, for the review of this document and for giving me strong tools to interface with the World.

Particular thanks go to my parents Pietro and Alba, without which I could not get here. The psychological and economical support were fundamental to live far away from home. The pieces of advices, the recommendations and the support coming from my brother, Marco and his new wife, Celeste, have been educative for my personal growth. Furthermore, in more than five years the strength to go ahead comes from little things, such as: funning with my dog Willy, going out and relying with Ivan, Gabriele, Daniele, Sara, Emiliano and many other friends, going to the gym, watching movies, listening to music and simply having the passion to do whatever.

Pisa, April 2014

S.M.

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Introduction

Mobile Broadband has developed considerably in the last years. Mobile network de-vices have today become the main access medium to most important Internet serde-vices, such as Google, YouTube, Facebook, Twitter. More than half of the Internet connec-tions are wireless. The wireless flexibility has brought to overtake the threshold of mobile devices 9 billion and forecasts tell us about 50 billion in 2020.

The technological evolution path has been defined by standardization institutions, before with ETSI (European Telecommunications Standards Institute) and then with 3GPP (Third Generation Partnership Project). International standards have deter-mined the success of the telecommunication market, because the interoperability, roaming and large scale economies. After a lasting specification of GSM(Global Sys-tem for Mobile Communications) and GPRS(General Packet Radio Service )/EDGE( Enhanced Data rates for GSM Evolution) ETSI systems, a fast 3GPP technology evo-lution has occured. Evoevo-lutionary steps have led us from a few tens of Kbps GPRS transmission rate to more than 100 Mbps with LTE(LONG TERM EVOLUTION), in about 10 years. The principal 3GPP technologies started with UMTS(Universal Mobile Telecommunications System) in 2000, to continue with HSDPA(High Speed Downlink Packet Access) and IMS(IP Multimedia Subsystem) in 2002, HSUPA(High-Speed Uplink Packet Access) in 2004, LTE or EPS(Evolved Packet System) in 2008 and LTE-Advance in 2010. In common language, network and mobile services have crossed a lot of system generations: 2G, 2.5G, 3G, 3.5G, 4G. At the same time, with the smartphone and tablet advent, mobile devices have transformed by simple terminal for SMS and voice to super-computer with multi-tasking and multi-tech. Multimedia Broadcast Multicast Services (MBMS) have already existed since 3GPP Release 6. It has than continuously been enhanced in the following versions of the specification, focusing mainly on good coverage over the entire cell and on power consumption. To tackle these aspects, in 2004, 3GPP has started the standardization of a new LTE ultrabroadband technology, called EPS. From Release 8, 3GPP has talked about eM-BMS (Evolved MeM-BMS).

1.1 The incoming Generation

In accordance with the GSA(Global mobile Suppliers Association), LTE is the mobile technology with the fastest growth at world level. The most active continents in LTE launch have been North America and Asia in 2009. Europe follows with Sweden and Germany while Italy has seen LTE technology at the end of 2012. But, what is LTE?

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In [1] the ecosystem concept is used to describe a complex and interlaced system such as LTE. The principal elements that interwork are: access network, mobile devices, service platform and applications. Just a few months ago, the mobile market was based principally on voice and SMS. Nowadays, and ever more, subscribers demands is changing quickly. Innovative services, based on Internet, are going to be increasingly widespread, such as Cloud services, Mobile App, Rich Call, Web Browsing, Social Network and Multidevice Access needs to ultrabroadband technology and high rate. User terminals are becoming real drivers and controlled devices, toward IoT (Internet of Things). They have entered in a lot of contexts: entertainment and multimedia, health sector, security and logistics. Technology innovation is not only mobile ultra-speed, but even and mainly high level QoE (Quality of Experience).

LTE technology is based on a new access technique and on new network nodes implementation, with 20MHz bandwidth and actual 3 category terminals which can reach 100 Mbps in downlink and 50 Mbps in uplink upper rate. With the completion of Release 8 and the grant of a larger spectrum by the ITU, the workgroup 3GPP has started to look for enhancing existing technology for improving the use of available spectrum and obtain higher user rate.With EPS, in Release 8, some new basilar con-cepts and nodes were introduced: radio access network E-UTRAN (Evolved UMTS Terrestrial Radio Access Network), Core Network EPC (Evolved Packet Core) which works only with packet, voice service both on circuit CSFB (Circuit Switched Fall-Backand) and on packet VoLTE (Voice over LTE) with IMS architecture. The two principal actors that communicate on air interface LTE-Uu are User Equipment (UE) and evolved NodeB-eNB(see Fig. 1.1). The UE is any device used directly by an end-use to communicate. eNB is the hardware element of E-UTRA that connects UE with the mobile phone network. ENB uses X2 protocol on X2-AP interface to communicate with other eNBs, S1-AP protocol on S1-MME interface to communicate with the Mobility Management Entity(MME) for control plane traﬃc and GTP-U pro-tocol on S1-U interface to communicate with Serving-Gateway(S-GW) for user plane traﬃc(not shown in Fig. 1.1).

1.1.1 Small Cells

Small Cells concept was already known in GSM, but only with 3G they came out and it is envisaged that about 5 million small cells will ship annually by 2017([2]). Small cells are low-powered radio access nodes that operate in licensed and unlicensed spectrum which have a range of 10 m to 1 Km. These elements seem to be vital to manage LTE-Advanced spectrum more eﬃciently compared to using just macro-cells, which have a coverage range of a few tens of kilometres. Small cells encompass femto-cells, pico-cells and micro-pico-cells. The coverage area dimension is wider in order of presentation. Pico-cells and micro-cells can have a self-organising and self-management capabilities. Femto-cells, instead, usually do not have this function. Small-cell networks can also be realized by means of distributed radio technology consisting of centralized baseband units. A kind of this central management is proposed by CROWD, as it will be seen in the next chapter. A Home eNB-HeNB is an LTE residential femto-cell. Wi-Fi is a small cell but does not operate in licenced spectrum and therefore cannot be managed as eﬀectively as the small cells utilising licensed spectrum. About 10 million residential femto-cells have been deployed, representing 56% of all base stations globally, as of February 2013. As base stations are intended macro-cells, micro-cells, pico-cells, femto-cells and WiFi access point. In total, almost 11 million small femto-cells encompassing public, enterprise and residential have been deployed by 47 operators worldwide.

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Figure 1.1: EUTRAN architecture as part of LTE network (http://en.wikipedia.org/wiki/E-UTRA).

1.1.2 HetNets

Smalls cell are an integral part of future LTE networks. In 3G networks, small cells are used for offload traffic in a complementary technique to deliver cellular traffic. In 4G networks, the principal of heterogeneous network - HetNet is introduced where the mobile network is constructed with layers of small and large cells. An example of heterogeneous network with three whatever kind of wireless technologies and five different frequency ranges(Fig. 1.2,from [3]). Future mobile network will require col-laboration among different technology to provide high capacity and enough UE access flexibility, independently by its position. To do this it will be necessary to dispose and manage networks able to support different kind of standard technologies, and which works in various frequency ranges and on various cellular layer. To manage these net-works it serves a join management of radio resources, called Common Radio Resource Management (CRRM) is needed. There are several benefits for a HetNet respect to a traditional homogeneous network, such as greater reliability, higher spectral efficiency and better coverage. On the contrary, there are several problems still to be solved in HetNet such as:

• determining the theoretical capacity; • interoperability of technology;

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Figure 1.2: An example of HetNet([3])

• handover; • mobility;

• quality of service(QoS) and quality of experience(QoE); • interference between RATs.

In a future scenery, workgroup 3GPP would like to meet the requirements of mobile network operators for the evolution of LTE, named LTE-Advanced and specified as LTE Release 10 and beyond.

1.2 LTE-A: Requirements and Key Challenges

3GPP created a set of LTE-Advanced requirements in [4], following ITU requests about IMT-Advanced systems. In particular, the key features that should have the upcoming radio interface technologies are given below ([5]):

• a high number of worldwide functions which permit great flexibility to support a wide range of services and applications in a cost eﬃcient manner;

• compatibility of services within IMT and with fixed networks; • capability of interworking with other radio access systems; • high quality mobile services;

• UE suitable for worldwide use;

• user-friendly applications, services and equipment; • worldwide roaming capability;

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• enhanced instantaneous peak data rates to support advanced services and appli-cations (100 Mbps for high and 1 Gbps for low mobility).

Actually these requirements were those required for 4G. Therefore LTE Release 9 is still pre-4G, because it does not meet the fundamental requirements to be identified as new generation. From Release 10 we have 4G. LTE-A includes a lot of technology intended mainly to improve mobile radio network performances, in terms of throughput and latency. Release 10 principally works on access network, and it allows to obtain peak data rate until 600 Mbps with 20 MHz bandwidth. Last LTE specification, Release 11, was completed in June 2013(Fig. 1.3, [6]). In radio field, some optimization about Release 10 technologies, with the future prospective to reach up to 1 Gbps with UE 8 Category has been introduced.

Figure 1.3: Main LTE-A technology components([6]) Principal elements proposed to tackle all new generation issues are:

• Carrier Aggregation: it lets to interlock bandwidth, also with diﬀerent spectral width, belonging to diﬀerent ranges. In this way it is possible to obtain an ”equivalent continuous bandwidth”. The goal is to increase UE data rate. • MIMO(Multiple Input Multiple output) system: up to 8 transmit and receive

antennas are employed and Beamforming technique is used to increase spectral eﬃciency.

• AAS (Active Antenna System): to avoid cable and connector losses, a remote module is directly mounted on the antenna to generate RF signal. Signal is drawn by a digital main module through a fiber. With Digital Beam Forming it is possible to obtain cell-splitting, increasing system capacity.

• CoMP(Coordinated Multi Point): signal is transmitted and received by diﬀerent points which operate in coordinated way. Expected techniques are Coordinated Scheduling, Coordinated Beamforming, Dynamic Point Selection and Joint Pro-cessing. All these solutions aim to reduce interference, to increase UE throughput and to provide a better cell-edge coverage.

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• Relay Node: it is employed a third node, other than Source and Destination, to expand the cell coverage. Relay node can be a simple repeater signal or even a regenerator signal.

• SON (Self Organizing Networks): it concerns Self-Configuration to arrange net-work nodes, Self-Optimization to accommodate parameters and Self-Healing to recognize and repair breakdowns. These functionalities are managed by dis-tributed or centralized algorithms.

• eICIC(Enhanced Inter-Cell Interference Coordination): it lets the cells coexis-tence, coordinating micro,pico and femto nodes with macro node through X2 interface. The aim is to reduce interference produced by nodes towards UE attached to other nodes. Two techniques are introduced, ABS (Almost Blank Subframe) and CRE (Cell Range Expansion). We will talk widely about them in next Chapter.

Figure 1.4: LTE technology components dependences from Release 8 to 11([5])

All these techniques and others(shown in Fig. 1.4, [5]) are currently being studied by various organizations. The European Union has created programmes in order to support and encourage research in this area(Fig. 1.5). In the Future Networks context, the RAS is a cluster activity comprising a portfolio of more than 20 research projects participating in the 7th Framework Program (FP7) and investigating Radio Access and Spectrum aspect1.

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Figure 1.5: Evolution of 3GPP standards ([3])

1.3 CROWD project

CROWD(Connectivity management for eneRgy Optimised Wireless Dense networks) is one of the FP7 project co-funded by the European Commission under the ICT theme(Call 8)2_{. The CROWD consortium consists of a mixture of competences and}

profiles, including Intecs S.p.A. (Italy, co-ordinator), Alcatel-Lucent Bell Labs France (France), France Telecom S.A.(France), Universidad Carlos III de Madrid (Spain), Universidad Paderborn (Germany), Fundación IMDEA Networks (Spain), Signalion GMBH (Germany)(Fig. 1.6).

Figure 1.6: CROWD project partners slogan

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1.3.1 Problem Statement

Nowadays the necessity to have Internet everywhere and all the time is increasing. Number of wireless users and high-quality services rapidly grow up and thus the offered load doubles every year. With this trend, it is not sufficient provide high-capacity in-frastructure. One immediate solution to cope this growing traffic demand entails using more access point, by increasing their density and using different wireless technology, thus employing HetNets. CROWD worries about very dense and heterogeneous net-works, which CROWD calls DenseNets([7]). Also network operators have encouraged to undertake these studies because the immediate solution to face the growing traffic demand in certain area has been to install further Wi-Fi hotspots and to build micro-, pico- and femto-cells in public areas to inject capacity where the data traffic demand is particularly high. CROWD argue that increasing the number of access points does not remove capacity bottleneck issue. Furthermore, a large number of deployed access points also influences the energy cost. In particular, all today’s base stations, i.e. LTE eNBs and Wi-Fi access points, running at zero-load consume almost as much energy as when running at full capacity. As a result, whom an example of DenseNets can lead to wireless chaos and energy waste is shown in Fig. 1.7([8]). Therefore, the conclusion is that wireless networks will become denser and more heterogeneous, but the pace at which new deployments will be realised depends on how sustainable the densification process, in terms of both energy consumption and base stations/backhaul installation costs, which in turn depends on the following challenges:

1. reducing the interference at the network and increasing the real bandwidth that can be obtained from the network;

2. achieving traﬃc proportional energy consumption, at Radio Access Network (RAN) and interconnecting backhaul;

3. reducing the complexity of base stations;

4. enhancing backhaul with less hardware via its dynamic and automatic reconfig-uration;

5. designing fast mobility solutions to alleviate load problems in core networks, which is next bottleneck candidate once the RAN and backhaul become more eﬃcient.

1.3.2 CROWD approach

While PHY(PHysical laYer) approaches have been widely investigated to deal with very dense networks, they take a restricted PHY perspective; they do not consider that higher-layer mechanisms are required to fully optimise global per-flow perfor-mance by orchestrating mechanisms at diﬀerent layers and subsystems. Furthermore PHY-based optimisations do not scale with network density and cannot be easily extended to the case of heterogeneous wireless technologies. In fact, the complexity required to optimise multiple nodes in real time becomes prohibitively high when nodes use heterogeneous PHYs. CROWD aims at developing a novel networking framework that can satisfy future traﬃc demands in DenseNets, which assume the overlapping between LTE cells and WiFi hotspots. The other key component of the CROWD framework is the wireless backhaul, in such a way to have flexible deployment and fast reconfiguration backhaul resources. In a nutshell, the CROWD project aims at

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Figure 1.7: An oncoming scenario for dense and heterogeneous networks([8])

building high-capacity energy and resource-efficient wireless dense networks. To do so, the project will devise novel mechanisms for connectivity management, energy efficient operation, scheduling and random access MAC enhancements, and dynamic backhaul optimisation. These mechanisms will be mutually integrated with each other and span across cell boundaries, technology boundaries, and access/backhaul network boundaries, jointly optimising the performance metrics of these subsystems. The need to introduce reconfigurability in the wireless world has been identified as a key requirement to efficiently deploy and maintain converged networks, where reconfigu-ration involves the ability to change, e.g., the radio technology or MAC parameters. Existing solutions like SDR or SON do not provide suitable control and optimisation mechanisms for DenseNets. In fact, in DenseNets, SDR or SON localised taking of control decisions could result in wireless chaos([9]). The preferred solution is the dis-tributed algorithm, according to which some functions today implemented in the base stations are progressively removed and substituted by an open interface. Control deci-sions which were once taken inside the base station are now delegated to an external controller. Since the interface is open, third parties can provide network operators with optimised multi-vendor algorithms. The emerging SDN paradigm is a natural candidate to match the distributed control approach. CROWD adopts the following definition of SDN: ”SDN is a design approach for the architecture and protocols of a networked system such that its elements have an open interface that allows dedicated logical elements, called controllers, to influence their behaviour over time”. Most of the existing OpenFlow controllers directly expose the low-level OpenFlow Application Pro-gramming Interfaces (APIs) to the higher layer control applications, which makes the network programming a cumbersome and delicate task. These issues have triggered the design and development of high-level languages that can facilitate programming of OpenFlow switches, which is also a major topic within the Internet Research Task Force (IRTF) Software Defined Network Research Group (SDNRG).

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1.3.3 Architecture

In this section a high-level overview of the CROWD proposed network architecture is described. To deepen about it, refer to [10]. It encompasses LTE (macro/pico/femto) and WiFi cells, but the work will mainly focus on LTE side. CROWD proposes a dynamic, two-tier SDN controller hierarchy with two types of controllers:

• a CLC(CROWD Local Controller) which can take fast, short time scale decisions on a limited but fine grain scope;

• a CRC(CROWD Regional Controller) which can take slower, long time scale decisions with a broader but more coarse grain scope.

These two types of controllers are the central parts of overall architecture, which is structured into two logical tiers called district and region. A district consists of base stations, i.e., LTE eNB and WiFi AP(Access Point), as well as interconnecting backhaul links that are assumed without loss of generality to be reconfigurable via some protocol, e.g. OpenFlow (OF). Operation within a district is optimised by appli-cations connected to the so-called CLC via a set of APIs, called North-Bound (NB) interface in SDN terminology. Two types of APIs are foreseen:

• Technology-Specific (TS), which expose fine-grained details as acquired from the base stations (e.g., sub-frame utilisation in LTE) and other methods which are only valid for the specific communication protocol;

• Technology-Agnostic (TA), which expose abstract and aggregated data (e.g., average node utilisation) and other generic modifiers which may be valid for a wide range of technologies and capabilities (e.g., switch oﬀ a node).

A special use of the TA API is to connect a CLC to its higher-level controller, i.e., the CRC, which operates inside a region. The region is defined as a logical area including several districts. A simplified architecture is presented in Fig. 1.83. It shows principally the control plan of a couple CLC-CRC. In addition to NB, there are also South-Bound (SB) interfaces. CLC and CRC interact with physical network elements, e.g. base stations, switches and router, via the respective southbound in-terface. Each southbound is a combination of technology and possibly vendor-specific protocols which will be used to implement the CLC control functions. For instance also an SDN protocol, such as Open-Flow(OF), can be included in the mix if there are compatible devices in the network.

In the case of LTE the eNBs have a split connection: the control path and the data path. The first one goes entirely through the CLC and uses the 3GPP S1-MME (Mobility Management Entity), not shown in Fig. 1.8, and X2 interfaces. The sec-ond one is directed to the Distributed Mobility Management (DMM) Gateway (GW), which is a novel element proposed in CROWD. A CLC application can interject the communication between the eNB and the MME, i.e. on S1-MME interface. Note also that CROWD proposes the use of the X2 interface for collecting fast and de-tailed measurements from the LTE eNBs, since it already supports a wide range of data, even though the standard assumes that information is exchanged between peer eNBs. The proposed architecture enables an incremental penetration of the technology into existing deployments. In fact, CLC and CRC applications can be considered as

3_{X2’, i.e. an extension of X2 protocol, is just the first proposal of communication protocol. The}

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second-order optimisation in an operational network. One specific design choice is to have remote access for API, and thus to have the controller and application software decoupled.

1.3.4 MAC-layer enhancements and Backhaul Management

To optimise the performance of distributed and coordinated MAC technologies, such as WiFi,and LTE respectively, in extremely dense scenarios, algorithms will be run within CLC applications. In fact the dynamics of the MAC protocol happen at very short time scales, which can be as short as 1ms. The following CLC-internal services are foreseen:

• a database to store/cache the configuration and measurements;

• a collector which periodically acquires field measurements from the physical network elements;

• a broker which takes a final;

• a discovery service which is used for self-configuration at start up and it is queried by the CRC.

Figure 1.9 shows the initial set of control applications proposed by CROWD, and the relative control functions.

CRC operates on large-scale view, concerning backhaul and long-term configura-tion. Since CRC and regional applications are not fundamentally important for this out work, we will only mention the CRC control functions and applications (Fig. 1.10).

1.3.5 Connectivity Management

One of the principal objective of CROWD is to ensure the best connectivity of the user in a scenario characterised by the extreme number of overlapping cells. Each cell corresponds to an access point, which can be used by the terminal as potential points of attachment. In such scenario, the terminal must face the difficult problem of choosing the best candidate point of attachment, among several hundred of WiFi APs and LTE HeNBs. In addition, this decision must be taken in a timely manner and must consider information coming from both the network and the terminal. In order to cope with the amount of backhaul and core bandwidth required to serve the next generation user’s requirements, the network needs to cooperate with the terminal in such a way that the point of attachment selected is not only the best one based on user requirements, but also the most suitable from the network perspective. This can be achieved by the sharing, aggregation and usage of network information to influence the candidate point of attachment selection performed by the terminal. In a scenario composed of a multitude of little-area cells, in which the user handoffs very frequently, an approach requiring the handling of IP mobility for each change of point of attachment would be extremely expensive in terms of signalling and will not scale as required. Hence, the CROWD project envisions a two layered approach to mobility. In this model, the mobility within the boundaries of a technology is performed through layer-2 specific solutions while the roaming to a different district, will be handled through the use of DMM.

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Figure 1.9: MAC-layer CLC Applications and Functions(from CROWD documenta-tion)

Figure 1.10: CRC functions and relative applications(from CROWD documentation)

Definitively CROWD wants to tackle three kind of challenges: • Access Selection;

• Enhanced traﬃc management in dense wireless networks; • Session continuity and distributed anchoring.

Connectivity Management functionality is divided in 3 diﬀerent locations:

District includes at least one DMM-GW which controls the mobility of the users at

L2 (within a technology, LTE or WLAN) and L3. Following the SDN approach of CROWD, the L2 mobility is handled by interfacing with the CLC functionality;

User Terminal is composed of Connection Manager and IP Mobility Terminal

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Core Network in order to support better scalability, it gather information about

location and load of districts, technologies available or backhaul resources.

1.3.6 Use Cases

CROWD project proposes to carry out 15 tasks as part of activity within work projects, shown in Fig. 1.11. These use cases will let the CROWD controllers to have function-ality described above.

Two of the tasks presented in table, UC1 and UC11. UC11, concerning Connec-tivity Management, is the common issue of ConnecConnec-tivity Management and it will be addressed in Chapter 3. On the contrary, UC1 is faced by CROWD in a specific and custom manner. For this reason, thi work presents how the function relative to eICIC application is tackled in next subsection.

In conclusion, the CROWD approach fully endorses the SDN principles of control and data path separation and dynamic reconfigurability of the network elements. In this section the major challenges ahead for three important aspects have been identi-fied: MAC layer reconfiguration, dynamic backhaul reconfiguration, and connectivity management. Future activities within the project will lead to the detailed definition of open interfaces for local and regional controllers, and to the design of prototype con-trollers and optimisation applications. Further details about the project can be found in the website4_{. To receive news you can follow the twitter channel FP7CROWD is}

suggested.

1.3.7 eICIC application

In order to exploit the cooperation between different LTE eNBs and HeNBs operating within the same area, an eICIC application is proposed in CROWD. The application requires continuously live statistics about channel condition experienced by the users by means of a monitoring control function. Moreover, a network discovery function will provide the eICIC application with timely information about topology changes in the network. Furthermore, the eICIC application instructs any single eNB/HeNB about the configuration setting to be adopted in order to avoid inter-cell interference, by means of a function: base station scheduling in time domain. In fact, the proposed use case entails the definition of an eICIC scheme relying on the ABS function offered by LTE. The typical situation follows: many UEs that move in the local area; the eNB communicates with neighbour eNBs by X2 interface and all the eNBs send info to the CLC about the load of their cell, which is used for load balancing. Because of UEs moving if one eNB has too many UEs to manage, it could not be able to guarantee QoS no longer. The CLC will then inform all eNBs to balance their traffic load and switch off eNBs that have no traffic. With an SDN approach each node has a specific cell agent in charge of performing the interface between the local hardware and the remote controller. SDN has been designed to configure in a more dynamical and open way the switches, by separating the data flow with its real-time policies from a central remote controller that performs the control on data flow. The very same approach can be applied to radio nodes and other network nodes found in an LTE network. An eNB is basically a switch that commutes packets. The essential difference with a switch lies in the fact that the medium is shared between several nodes, hence inducing possible collisions, congestion and packet loss. In the eICIC approach, there is still have the local agent on the eNB that runs on top of the hardware.

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Figure 1.11: Use Cases: network and terminal characteristics(from CROWD docu-mentation)

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Diﬀerently from existing approaches, in CROWD the problem of cell inter-ference mitigation from the innovative perspective of scheduling base stations rather than users is tackled. In particular, with the project proposal, base stations down-load activities are coordinated on a sub-frame basis in order to mitigate the inter-ference caused to neighboring cells. Moreover, the mechanism designed by CROWD is scalable since it does not touch the user scheduling performed by each single base station. The scheduling decisions consist in gating base station downlink transmis-sions in each downlink sub-frame, adopting a regular base station scheduling pattern. Every base station, after receiving the scheduling pattern, must remain silent dur-ing the subframes for which the base station was not scheduled. This behaviour can be implemented by means of a recently proposed LTE feature, defined in the 3GPP specifications and explained in third chapter, called ABS (Almost Blank Subframe). However, 3GPP specifications do not define the mechanism that decides when ABS is activated. Therefore, CROWD proposal would work as an ABS policer running on a base station district rather than on a single base station.

1.4 Thesis Goal

This work, within the CROWD project, focuses on LTE system, leaving out WiFi technology and the coexistence of the two. So, this work will treat about eNBs, the unique BSs placed in the scenario, and about UE, the user terminal. Definitively DenseNets will be called all and only dense networks, without any mix of technology. Precisely, this study will treat about district issues, handled to CLC. Therefore the work will take part of functionality multitude belonging to CLC applications. The work will focus on application layer and hence it will not treat about interfacing issues between block systems. Actually, as said before, applications could run in a separate physical machine and thus have a CLC remote control. Definitively, it could be assumed to communicate with network and other system block, e.g. gathering information coming from nodes, without worrying about communication protocols. The simplified architecture to be followed it is shown in Fig. 1.12.

About the contribution of this work to the CROWD project, it will focus on MAC-layer enhancement and Connectivity Management aspect. In particular, the applica-tions treated will be LTE eICIC and LTE access selection, extracted by Figure 1.9 in Figure 1.13.

As regard to LTE eICIC the attention will be concenntrated on the following functionality:

• Scheduling Policy Control: establishing scheduling pattern for each district eNB. However command to control eNB status will not be treated.

• ABS control: choosing, on time domain, which eNB to switch on and switch oﬀ in adaptive mode in such a way to improve network performance.

As regard to Connectivity Management this study will deal with the Access Se-lection part, which corresponds also to one MAC-layer enhancement: LTE access selection. The task will be to find a good User Association policy for all network UE, with regard to backhaul constraints. This study will focus on capacity improvement, without considering the Energy Eﬃciency issue. An observation is due: in following Chapters, backhaul is considered both RAN and intermediate link between backbone and RAN. RAN is intended as the small subnetworks at the edge of the entire hierar-chical network, and it may be realized with cabled or wireless links.

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Figure 1.12: CROWD architecture simplified for subsequent work

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In terms of project tasks, among the overall Use Cases will be extracted those of interest to this study (Fig. 1.14). EICIC functionality are related to UC1, meanwhile Access Selection is related to UC11. One consideration is due about backhaul for both cases: the backhaul reconfigurability will not be treated, so it will be simpler to consider cabled resources and not wireless ones.

Figure 1.14: Use Cases of interest

1.5 Thesis overview

The remainder of this document is structured as follows. In Chapter 2 we will illustrate the state of art of the new generation mobile system, LTE-A. The discussion will cover the PHY-layer and MAC-layer aspect, focusing on interference and wireless resources issues. Then legacy methods for User Association and new interference mitigation techniques proposed by 3GPP are shown. In Chapter 3 we will discuss the reference scenario which has been followed to tackle the work. Subsequently the methodologies to face the main issues are presented. The methodologies are used to build specific functions in diﬀerent black boxes. After a little study on backhaul constraints we will build a possible controller functional scheme, which uses functional blocks. Chapter 4 is introduced with the presentation of simulation program used to test the functions. Then the contribution of this study to the program is described followed by the main DenseNet issue to be addressed. The work then presents how the methods are implemented in the simulator and finally, the results obtained are shown. The First part of results shows that the legacy user association policy is just overcome by one version of the algorithm implemented. Second part explains why only one interference mitigation technique presented the expected results, the heuristic approach, and goodness margin is quantified. Third, some considerations and possible future simulator improvements are discussed. Finally, concluding remarks are argued in Chapter 5, where details for future works are suggested.

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Chapter 2

LTE-A: state of art

In the first Chapter the cellular network development has been argued. In particular, the requirements demanded to new generation, LTE-A, have been discussed. To tackle issues presented in CROWD, how LTE-A is going to work in future networks needs to be studied For this reason the focus of this work has been on aspects fundamentals, i.e. PHY(physical) and MAC(media access control) layers, among all system features. Indeed the controller functions, discussed in 1.3, concern:

• some PHY layer concepts: - Modulation;

- Radio Interface;

- Electromagnetic spectrum frequency allocation and analog bandwidth; - Specification of signal strength, like minimum SINR.

• some of the MAC functionalities:

- Frame delimiting and frame composition;

- Access control of the physical transmission medium according to a multiple access protocol.

In first section, some basic information about LTE-A technology has been shown starting from lower layer, i.e. transmission media, and going toward upper layer, i.e. MAC layer. Then some specific features about PHY-layer have been presented, e.g. measurements and reports channels. The second section is concerned with legacy user association method. In the third section, instead, interference issues in dense networks and the new proposed techniques to fight them are treated.

2.1 LTE legacy: an overview

All the optimization techniques and methodologies presented in the first chapter are studied to improve actual situation, based on a system structure. This structure is founded on a solid PHY layer, which joins all technology employed in LTE.

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2.1.1 Modulation

LTE devices use several modulation techniques to modulate information signal. These modulation techniques are the following: QPSK, 16QAM and 64QAM. All these mod-ulation techniques are supported in uplink and downlink direction, apart 64QAM, which is optional for uplink direction. A modulation technique is selected based on the measured signal to interference plus noise ratio (SINR) and specific SINR thresh-olds. Subscribers with lower SINR values use a more robust modulation scheme (e.g. QPSK) with lower throughput, meanwhile subscribers with higher SINR values can use less robust modulation schemes(e.g. 64QAM) and require higher throughput. The procedure to choose modulation scheme will be shown later.

2.1.2 Frequency

An essential feature about radio transmission frequencies concerns the coverage area compared to frequency range used. In particular, a BS with central frequency higher is able to cover a smaller cell area than a BS operating in frequency range lower. LTE uses two kinds of transmission mode: FDD(Frequency Division Duplexing) and TDD(Time Division Duplexing). In FDD mode, UL(UpLink) and DL(DownLink) work on diﬀerent range bandwidth. On the contrary, in TDD, the bandwidth belonging to two links is the same but two directions alternate themselves on time domain. LTE work frequency, in Europe, on which bandwidths are centered in:

• 800MHz, 900 MHz in FDD mode, with large coverage and good indoor penetra-tion;

• 1800MHz in FDD mode, with medium performances about coverage and indoor penetration;

• 2600MHz in both mode, with least performances about coverage and indoor penetration, so it is used principally for hot spot outdoor.

In cell design, each frequency range is usually related to a set of different and overlap coverage layers. Each layer corresponds to a different kind of cell (macro,micro,pico,femto). The overlapping of these cells and their layers takes a greater management complexity but higher flexibility. On each central frequency, cell spectrum can be 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz wide, according to the standard. Moreover, Car-rier Aggregation functionality lets to have a join between different range bandwidths. In this way the resulting bandwidth is equal to the sum of separate bandwidth.

2.1.3 Basic Transmission Scheme based on OFDM

Channel access control mechanisms makes it possible for several stations connected to the same physical medium to share it, using a physical layer multiplex scheme. Channel access control mechanisms, or multiple access protocol, in LTE, is OFDMA (Orthogonal Frequency Division Multiple Access) in DL and SC-OFDMA (Single Car-rier OFDMA) in UL. Bandwidth is split in channels, which are separated by a guard band. Spectrum Information flows are contained in channels, which are defined or-thogonal on frequency domain. The whole eNB bandwidth is centred in a carrier frequency among them listed in previous section, while subcarriers are centred in sub-bandwidth(SB), related to channels. Scheduler manages SBs, assigning them to UEs for a multiple of TTI(Transmission Time Interval), equal to 1msec. The orthogonality

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among UEs is due to the lack of overlapping in time and in frequency domain, as shown in Fig. 2.1([1]). Downlink and Uplink transmissions are organized into radio

Figure 2.1: Scheduling OFDMA([1])

frames with 10ms duration([11]). Two radio frame structures are supported:

• Type 1, applicable to FDD (Fig. 2.2). Each 10ms radio frame is divided into ten equally sized subframes. Each subframe consists of two equally sized slots. For FDD, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmissions in each 10ms interval. Uplink and downlink transmissions are separated in the frequency domain.

• Type 2, applicable to TDD(Fig. 2.3). Each 10ms radio frame consists of two half-frames of 5ms each. Each half-frame consists of eight slots of length 0.5ms and three special fields: DwPTS, GP and UpPTS. Uplink and downlink trans-missions are separated in the time domain.

Figure 2.2: Type 1 frame structure

The transmitted signal in each slot is described by one or several resource grids of

NDL

RBNscRB subcarriers and NsymbDL OFDM symbols. N min,DL

RB = 6 and N

max,DL

RB = 110

are the smallest and largest downlink bandwidths. See Fig. 2.4 for downlink resource grid ([12]). For simplicity, only DL case is shown, but UL is similar. The number of OFDM symbols in a slot , for a traditional configuration, is 7. The OFDM sub-carrier spacing is deltaf = 15 kHz. 12 consecutive sub-carriers during one slot correspond to

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Figure 2.3: Type 2 frame structure

one downlink resource block (RB). RB is the reference entity for various procedure, such as describing the mapping of certain PHY channels to resource element(RE). The resource element is the smallest time-frequency unit for transmission, composed by a subcarrier and an information symbol.

2.1.4 Physical Channels and Physical Signal

A physical channel corresponds to a set of resource elements carrying information orig-inating from higher layers and is the interface. The following uplink physical channels are defined: Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Chan-nel (PUCCH), Physical Random Access ChanChan-nel (PRACH). The following downlink physical channels are defined: Physical Downlink Shared Channel (PDSCH), Physical Broadcast Channel (PBCH),Physical Multicast Channel (PMCH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH), Physical Hybrid ARQ Indicator Channel (PHICH), Enhanced Physical Downlink Con-trol Channel (EPDCCH). An example of PHY channel follows. The scheduler is in charge of generating specific structures called Downlink Control Information (DCI), which are then transmitted by the PHY of the eNB to the connected UEs on the PDCCH, in order to inform them of the resource allocation on a per subframe basis. In doing this, the scheduler has to fill some specific fields of the DCI structure with all the information, such as: the Modulation and Coding Scheme (MCS) to be used and the allocation bitmap which identifies which RBs will contain the data transmitted by the eNB to each user.

An uplink physical signal is used by the physical layer but does not carry infor-mation originating from higher layers. In uplink only the Reference signal is defined, meanwhile Synchronization signal is defined in downlink. Six types of downlink refer-ence signals are defined: Cell-specific Referrefer-ence Signal (CRS), MBSFN referrefer-ence sig-nal, UE-specific Reference Signal (DM-RS) associated with PDSCH, DeModulation Reference Signal (DM-RS) associated with EPDCCH, Positioning Reference Signal (PRS), Channel State Information-Reference Signal (CSI-RS) by release 10. Focusing

on CSI-RS, the resource occupied by CSI reference resource for a serving cell is: • in the frequency domain by the group of downlink physical resource blocks

cor-responding to a certain band;

• in the time domain, the CSI reference resource is defined by a single downlink subframe.

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To perform channel estimation, reference symbols (reference signals) are embedded in the Physical resource block. Channel state reports need a known reference signal as input to the CSI (Control State Information) computation. The CRS signals can be used for this purpose. CSI-RS also enable the UE to estimate the CSI for multiple cells rather than just its serving cell. Each CSI process is associated with a CSI-RS resource and a CSI-interference measurement (CSI-IM) resource.

2.1.5 Measurements and CQI

The UE and the eNB are required to make physical layer measurements of the radio characteristics. The measurement definitions are specified in [13]. Measurements are reported to the higher layers and are used for a variety of purposes including inter-RAT handover, access selection and interference mitigation. Both the eNodeB and the UE measure signal quality using the Reference Signals. The Reference Signals carry a known (pseudo-noise) bit pattern at a boosted power level. The eNodeB always controls and selects the modulation and coding scheme for both the downlink and uplink. The principal PHY layer measurements on downlink reference signals(RS) are the Reference signal receive power (RSRP) and the Reference signal receive quality (RSRQ). The UE uses PUCCH control information, with CSI (Channel State Infor-mation), to relay an estimate of the channel properties to the base station. Channel status reports include:

• Channel Quality Indication(CQI) represents the recommended modulation scheme and coding rate that should be used for the downlink transmission;

• Rank Indication(RI) provides information about the rank of the channel, which is used to determine the optimal number of layers that should be used for the downlink transmission (only used for spatial multiplexed systems);

• Precoder Matrix Indication(PMI) provides information about which precoding matrix to use (only used in closed loop spatial multiplexing systems).

The UE determines CQI to be reported based on the measurements of the downlink reference symbols. The UE shall derive by the observed SINR a CQI value to report in uplink. There are 15 diﬀerent values ranging from 1 to 15. The 3GPP specification does not state how CQI should be generated, but the requirement is that it must satisfy following condition: a single PDSCH transport block with a combination of modulation scheme and the transport block size corresponding to the CQI index, and occupying a group of downlink physical resource blocks termed the CSI reference resource, could be received with a transport block error probability not exceeding 0.1 ([14]). If there is not any CQI value comprised between 1 and 15 that satisfy the condition, CQI index 0 is assigned. CQI is reported to eNB associated with a delay respect the downlink reference resource because of measurement and CSI processing. Delay is the smallest value greater than or equal to 4 and 5 subframes for TDD and FDD respectively, so that it corresponds to a valid downlink subframe. On contrary, if there is not a valid downlink subframe for the CQI reference resource, CQI reporting is omitted in uplink subframe. Two reporting modes are supported for the DL channel measurements: an aperiodic reporting on PUSCH and a periodic reporting on PUCCH. The aperiodic reporting is triggered by setting the CQI request bit, configured by higher-layer signaling. A UE is not expected to receive more than one aperiodic CSI report request for a given subframe. Under periodic CQI reporting, on the contrary, UE is pre-configures via higher-layer signaling to transmit CQI reports

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Figure 2.5: 4-bit CQI table

periodically. It has to be determined the periodicity and offset, in the subframe scale, for CQI/PMI reporting. The reference is [14],section 7.2, for details. LTE supports wideband and sub-band CQI reporting. A wideband CQI value is a single 4-bit integer that represents an effectiveSINR as observed by the UE over the entire channel bandwidth(Fig. 2.5). To have the variation in the SINR across the channel due to frequency selective nature of the channel each UE needs to report the CQI with a fine frequency granularity, which is possible with sub-band CQI reporting. A sub-band CQI report consists of a vector of CQI values where each CQI value is representative of the SINR observed by the UE over a sub-band. A sub-band is a collection of n adjacent Physical Resource Blocks (PRBs), where the value of n can be 2, 3, 4, 6, or 8. In general, the wideband feedback should be configured periodically to provide basic information about the downlink channel information to the eNB. In addition to this, the narrowband feedback can be configured as needed to support frequency-selective scheduling. The choice of periodic versus aperiodic narrowband reporting depends mostly on the DL data traffic characteristics and overhead considerations. Naturally, the reporting mode should match the traffic pattern. For instance, if the mean time between DL data traffic is long, the aperiodic reporting can be used to supplement wideband feedback to reduce uplink overhead. On the other hand, for traffic with periodic transmission such as video conferencing, the periodic reporting should be used. Within each reporting mode, different types of reports can be configured ([15]). In DCI the way the UEs have take measurements is specified and report the CQI, i.e. if it is triggered the periodic or the aperiodic mode, if it is triggered wideband or the sub-band CQI reporting. The CQI reported values are used by the eNB for DL scheduling and link adaptation, which are important features of LTE. How often and when is the UE feedbacks CQI controlled by the eNB. As we said before, eNB decides the modulation and the code rate for both uplink and downlink. This functionality has been shifted on the central controller, which gathers information from all the network nodes and takes decisions consequently. Moreover, the controller also uses CQIs to manage connectivity and other functions, such as the interference mitigation technique. We will see their use in the section 2.4.

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2.2 Access Selection

As we have seen, in 1.3.1 subsection, one of the principal issues to tackle in future dense networks, is the connectivity management. Access selection is the functional-ity that investigates how the terminals, assisted by the network, are able to select the best candidate BS for handover or first access. In a LTE-A HetNets perspective, the terminal needs to perform target network selection among several point of access (PoA)of different technologies. The terminal gathers information located in the net-work through any of the protocols defined for this purpose (e.g. ANDSF, IEEE 802.21) and it performs a scanning of the neighbour APs for each technology the terminal sup-ports. Ultimately, the terminal selects the best PoA which optimises a target. As we already said in 1.4 section, the work task aims to find a good access selection mecha-nism for only one technology, LTE. Typically, in a cellular network it is assumed that UEs connect to the BS which offer the best SINR, thus the strongest one. However, it commonly happens that some BSs are not fully loaded. In such a case, an intelligent cell association policy would be the one in which UEs are assigned to BSs accord-ing to the best user-perceived rate, also called offered rate, rather than based on the strongest signal. This causes that the cell association should not be based just on signal strength or SINR. In fact, load is often more important. Indeed there is a need to use the rate distribution on UE, i.e. the offered rate. The necessary information to take decisions in this direction, the network should provide the terminal with the load at the access network and the backhaul information. Any other information is consid-ered local and can be obtained by the terminal. The local information obtained by the terminal should not go directly to the network, since it will overload the network. The information received should instead be filtered and processed previously to the handover decision. A network operator, which has the complete view of its network, can convey this information and influence the handover decision. In a DenseNet sce-nario, the handover operation could be a frequent event for each UE. It is easy to think that, in a scenario with few meters radius cells, a UE could make a lot of handover operations with just little movements. Another issue related to the access selection is the user scheduling, which is the way in which the resources are distributed among UEs by a central scheduler. Even if Proportional Fairness(PF) opportunistic sched-uler is implemented in 3G systems after a long and concentrated study on this topic, there are other schedulers proposed in the literature, which promise better throughput performance than PF, at the expenses of either fairness or increased complexity.

The access selection mechanism, or User Association method, can even consider diﬀerent criteria to perform the selection, such as wireless network status, energy eﬃ-ciency, backhaul capacity, handover events number, UE fairness, etc. . . . In conclusion, a user association policy is based on one or more criteria and it is determined fixing an objective function. In 3.2 section we show a new method.

2.3 Interference in DenseNets and Mitigation

Tech-niques

As we have already said in the first Chapter, one of the principal questions to face in future dense networks will be interference. A simple and general case is shown in Fig. 2.6([16]). Future dense networks (DenseNets), in general HetNets, are networks comprised of diﬀerent types of wireless access points with diﬀerent capabilities, con-straints and operating functionalities. In particular, macro-eNB(MeNB), pico-eNB,

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Figure 2.6: Interference concept([16])

relay nodes, remote radio heads (RRHs), and femto/Home-eNB(HeNB) are examples of the available types of access points in LTE release 10 (LTE-Advanced) ([17]). Re-spect to the actual situation, which includes only MeNB and few HeNBs, the main challenge is the great overlapping among cells. The interference that occurs between diﬀerent and analog kind of cells can limit performance increase, which is gained due to the spatial spectrum reuse. Indeed, most severe interference occurs when the small cells and macro-cells are deploying on the same carrier frequency. This co-channel deployment is normal in the future LTE network, where we can imagine a dense het-erogenous network with many self-configuring HeNBs. As a matter of fact, HeNBs are always deployed by the consumer in an ad-hoc planning, and operators do not control either the number or the location of these cells. The unplanned deployment of HeNBs generates a strong interference with MeNB and others HeNB subscribers([18]). In order to limit interferences from HeNBs to macro eNBs the transmitter output power of the HeNB is limited. According to [19], the Home BS maximum output power is 20dBm, meanwhile -20dBm is the minimum value. In a DenseNet, a UE may connect to a closer HeNB even though the receiver power from a further MeNB could be higher. This can lead to a net gain in throughput if resource allocation is done carefully to ensure that the loss in rate due to higher interference does not dominate the gain in rate due to higher spatial reuse ([20]). To note that small-cells, i.e. femto-/pico-/micro-cells, typically serve far fewer users than the macro-cells. Hence, the traﬃc

in femto-cells is less aggregated, and so the load can vary at a much faster timescale. A common situation includes a HeNB that serves only one UE, then the queue of packets at the base-station awaiting transmission can transition between empty and non-empty on the order of a few ms. Hence, to use resources more eﬃciently, resource coordination across cells has to occur much more dynamically than the macro-only case. At the same time in DenseNets coexistence of macro-cells and small-cells has to be managed. Interference can be handled by handover operation, managing UE association between macro-cells and open-access small cells [18]. With Release 9, a new hybrid cell concept is introduced in addition to the already known closed and open access control mechanisms of Release 8: a closed access allows only access for subscribers users in a CSG (Closed Subscriber Group), in contrast to the open access, where all users are allowed to access the HeNB with its oﬀered services. A hybrid cell provides open access to all users, but subscribed users can be handled with priority in contrast to unsubscribed users. Release 9 introduces some additional parameter

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con-trollable by operator, such as CSG identifier(ID) and HeNB name. Also a proximity reports defined, thus the UE is able to send in the uplink a proximity indication to the source eNB([17]). Access control will be done by the MME based on the CSG ID and the membership status of the subscriber. The UE is able to identify that it is nearby a HeNB, and can inform the network components to prepare the necessary actions for a possible handover to the HeNB. For a handover decision to a HeNB, the macro eNB has to acquire some UE measurements from the target HeNB. For these tasks interface X2 among eNB from Release 11, also belonging to diﬀerent types, is used. In the following only open-access small-cells and macro-cells with unique opera-tor will be used. In a scenario with MeNBs, its UEs associated(MUEs),HeNBs and its UEs associated(HUEs), several kinds of interference may occur([18]). The interference cases, valid both TDD and FDD system, are the following:

• Downlink, Macro to femto: when UE is connected to HeNB but is far from it and/or closer to MeNB the interference is high, considering also that transmit power of MeNB is much higher than HeNB one. It would be useful protect data and control channels monitoring macro-cell transmission.

• Downlink, femto to Macro: if a UE is associated to MeNB and it is in a femto-cell coverage, it is under interference condition. Closer UE to macro-cell edge, more critical the condition is. Potential interference mitigation approaches could be: limit the downlink power of HeNB and/or time-domain coordination.

• Uplink, Macro to femto: when UE is associated to HeNB and a MUE is situated indoor the interference that occurs at HeNB may be severe. This problem is espe-cially true as femto-cell is located close to the macro-cell edge, where interfering MUE transmits at high power. Potential interference mitigation approaches may be an uplink power control by HeNB to HUE.

• Uplink, femto to Macro: MeNB suﬀers interference from all HUE transmissions, and this impact becomes severe when there are many femto-cells in the coverage area of MeNB. Moreover the intensity of interference is higher when HUE is located outdoor, in fact it transmits with a strong power and the path loss toward MeNB is low. In this scenario a potential approach for mitigate interference are similar to the previous case, i.e. uplink power control.

• Downlink, Femto to Femto: remaining in a building, in a near future we suppose there are many HeNB placed one or more per apartment, that are one or more per floor. So a HUE will suﬀers a strong interference depending by the number and arrangement of apartments. If HeNBs exchange their information with a controller or with each other, they could use some scheme of interference mitigation. In this way the idea may be to coordinate HeNBs throw a centralized or distributed approach. However, the common idea is to find an orthogonal set of patterns to be used by neighbouring HeNBs.

• Uplink, Femto to Femto: transmission from one HUE suﬀer interference by trans-mission from another HUE connected to another HeNB. A potential solution to interference mitigation for this scenario is the uplink power control.

In all these kind of cases almost one idea or technique to mitigate interference is pro-posed. Some of these and other ideas are introduced in new Releases as matter of study. The attention in focused on the interference reduction techniques in uplink and downlink 3GPP released two technical reports on LTE FDD ([21]) and LTE TDD

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([22]). In order to reduce the unwanted interferences, one has to identify the interfer-ences before taking any actions. In uplink, the HeNB could use its own receiver to monitor the interferences and measure the received interference power. On the other hand, UEs measure downlink SINR(Signal to Interference Noise Ratio) from eNBs and report CQI to controller, which gather all information from network and hence extract interference levels. When specific functionality are active, like an interference mitigation technique, measurement executed by nodes can follow diﬀerent strategies from traditional ones (see 2.4.6 subsection). Interference is one of the main issue that immediately 3GPP wanted to face, and thus one of the first LTE technology compo-nent, included in Release 8, was ICIC(Inter-Cell Interference Coordination). Going ahead improvements and new techniques are added, so to get eICIC(enhanced ICIC) in Release 10 and feICIC(further eICIC) in Release 11. The focus is on eICIC for a scenery with macro-cell and femto-cells as shown in Fig. 2.7([16]). A MUE is ex-posed to dominant interference in the downlink from the femto-cell, and the HUE is exposed to strong uplink interference from the MUE. There are three diﬀerent

cate-Figure 2.7: Interference in partial view of scenario with Macro-cell and femto-cell, [16] gories of eICIC solutions that are proposed to mitigate the interference, as presented in Fig. 2.8([16]):

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

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Figure 2.8: Categorization of eICIC Techniques([16])

2.3.1 Frequency-domain Technique

In frequency-domain eICIC solutions, control and physical channels 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. To note that Rel-8/9 UE access and operate on the reduced bandwidth without change, and 10 UE access the reduced bandwidth as a Rel-8/9 UE, but may be scheduled over the entire bandwidth. This means that in Rel-10 the orthogonality in frequency domain is for control channels only, but data channels may be scheduled over the entire bandwidth available for the serving cell.

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2.3.2 Power Control Technique

Interference cases between MeNB and HeNB in HetNets presented in 2.3.1 subsection argue the main issue about reduction of HeNBs radiated power to improve the per-formance of victim MUEs. However, the consequence of this action brings to reduce total throughput of the femtocell users. Diﬀerent proposals are discussed and they will be presented as follows:

• Backhaul Signalling of Power Reduction: power reduction based on backhaul signalling is recommended between eNBs due to the existence of the backhaul X2 interface. Actually this communication interface is only between MeNBs, meanwhile in a prediction of a future system, e.g. system based on CROWD, all eNBs have an X2 interface. This interface lets indeed to have a minimum signaling delay, which is not guaranteed by a broadband connection(like DSL) of actual consumers HeNBs. If X2 interface would be available among all cell layers, e.g. between MeNB and HeNB, a new technique proposed could be im-plementable: CRE (Cell Range Expansion). The basic idea of CRE concerns the introduction of a balancing factor which brings to favor UE connection toward HeNB even if the best server of signal is MeNB. This situation can occur when MeNB is overloaded and surrounding HeNBs have no UEs associated.

• Power Setting based on HeNB Measurements: the main concept of this approach is to control the radiated power of the HeNB to limit its coverage to the premises boundary of the owner of that HeNB. Diﬀerent proposals are discussed in 3GPP for this purpose where all of them depend on the measurements performed locally by the HeNB. The transmit power of the HeNB is adjusted based on: the mea-surements of received power from the strongest co-channel MeNB in ”Strongest MeNB at HeNB” proposal, the measurements of pathloss between HeNB and MUE in ”HeNB to MUE Pathloss Measurement”, minimum received SINR at HUE in ”Objective SINR of HUE”, minimum received SINR at MUE in ”Objec-tive SINR of MUE”, channel condition and PBCH decoding ability at HUE in PBCH Measurement([16]).

2.3.3 Time-domain Technique

In time-domain eICIC techniques interference is avoided or at least mitigated through the coordinated usage of the time-domain resources such as the subframes or the OFDM symbols. Techniques based on subframes and symbols cab be splitted in two categories. Focusing on subframe techniques category, which corresponds to the prin-cipal study matter in interference mitigation techniques, it is fundamental to have synchronization between MeNB and HeNB, in terms of control and data channels. In order to do this, backhaul signalling is used to align subframe boundaries. When the subframes of the MeNB and HeNBs are aligned, their control and data channels overlap with each other. The basic idea with time-domain eICIC in order to coordi-nate inter-cell interference and protect control channels is that an aggressor node uses protected subframes for victim nodes by reducing its transmission activity in certain subframes. Whereby, transmissions of the victim users are scheduled in time-domain resources where the interference from other nodes is mitigated. The concept relies on accurate time and phase synchronization on subframe resolution between all eNBs within the same geographical area, i.e. a district in CROWD. This approach involves periodically muting the transmissions of entire subframes from nodes that cause

Interference mitigation and User Association algorithms for resource optimization in future DenseNets

Abstract

Acknowledgments

Contents

Chapter 1

Introduction

1.1

The incoming Generation

1.1.1

Small Cells

1.1.2

HetNets

1.2

LTE-A: Requirements and Key Challenges

1.3

CROWD project

1.3.1

Problem Statement

1.3.2

CROWD approach

1.3.3

Architecture

1.3.4

MAC-layer enhancements and Backhaul Management

1.3.5

Connectivity Management

1.3.6

Use Cases

1.3.7

eICIC application

1.4

Thesis Goal

1.5

Thesis overview

Chapter 2

LTE-A: state of art

2.1

LTE legacy: an overview

2.1.1

Modulation

2.1.2

Frequency

2.1.3

Basic Transmission Scheme based on OFDM

2.1.4

Physical Channels and Physical Signal

2.1.5

Measurements and CQI

2.2

Access Selection

2.3

Interference in DenseNets and Mitigation

Tech-niques

2.3.1

Frequency-domain Technique

2.3.2

Power Control Technique

2.3.3

Time-domain Technique