Analysis, desing and optimization of partial frequency reuse-aided OFDMA-based heterogeneous cellular networks

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DOCTORAL THESIS 2018

Doctoral Programme of Information and Communications Technology

ANALYSIS, DESIGN AND OPTIMIZATION OF PARTIAL FREQUENCY REUSE-AIDED

OFDMA-BASED HETEROGENEOUS CELLULAR NETWORKS

Jan García Morales

Director/Advisor: Guillem Femenias Nadal

Director: Felip Riera Palou

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Guillem Femenias Nadal, professor del Departament de Matemàtiques i Informàtica de la Universitat de les Illes Balears

FA CONSTAR:

que la present memòria "Analysis, design and optimization of partial frequency reuse- aided OFDMA-based heterogeneous cellular networks" presentada per Jan García Morales per optar al grau de Doctor en Tecnologies de la Informació i les

Comunicacions, ha estat realitzada sota la seva direcció i cumpleix els requisists per ser considerada coma tesi doctoral.

Firma i data

Felipe Riera Palou, professor del Departament de Matemàtiques i Informàtica de la Universitat de les Illes Balears

FA CONSTAR:

que la present memòria "Analysis, design and optimization of partial frequency reuse- aided OFDMA-based heterogeneous cellular networks" presentada per Jan García Morales per optar al grau de Doctor en Tecnologies de la Informació i les

Comunicacions, ha estat realitzada sota la seva direcció i cumpleix els requisists per ser considerada coma tesi doctoral.

Firma i data

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Quiero dedicar esta tesis a Mariset, mi mujer, que me ha brindado sin más su cariño, ternura y amor, por compartir su vida conmigo, en los buenos y también en los malos momentos, y por su inmensa dedicación para conmigo.

A toda mi familia, a Dios, y en especial a mi hijo Gabriel.

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A CKNOWLEDGEMENTS

Traveling back in time to the past, I must honestly admit that I never thought to reach this moment in my life. This journey has led me to climb to the top of my academic career.

The trip to this day has been laborious and complicated, but rewarding. It has been a way of effort, always giving my best. It has also been a path of struggle to improve myself, and to learn from my mistakes every day. However, I must point out that I have never been alone. I have always been guided by those who were willing and dedicated to help me.

For all of them are these acknowledgements and my gratitude!

Therefore and firstly, I would like to render thanks to my thesis directors Profs. Guillem Femenias Nadal and Felip Riera Palou, because without their invaluable help, without their guidance and motivation, and especially, without their unconditional support at important moments in my life, I could not have come this far. Thank you for your trust and friendship!

Thanks in addition to all my colleagues of the mobile communications research group and friends of the telematics group, with whom I have shared countless hours of work and very good moments. I would like to extend my gratitude to the University of the Balearic Islands and to all the staff for making me feel one more of them.

My gratitude is also extended to Prof. John Thompson and his team for hosting me during my research stay at the University of Edinburgh. For the kindness and availability to me, as well as for the professional support which has given me the opportunity to learn and complete an important part of this research.

I am especially grateful to my friend Prof. Samuel Montejo Sánchez whose experi- ence and education have been one of my sources of motivation.

Finally, but not least, I would like to thank my wife Mariset, my brother Shael, my father Juan and in particular my mother Rosa Maria, for being always by my side when I have needed her and because she has supported me unconditionally in every moment of my life. They have made me the man that I am, they have encouraged me to be independent and to grow as a person. To all my family for trust me, support me, encourage and help me, and in general to all those people who have contributed their bit in my growth and training. To all of them, thank you!

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Funding

This work has been supported in part by the Agencia Estatal de Investigación and Fondo Europeo de Desarrollo Regional (AEI/FEDER, UE) under projects AM3DIO (TEC2011- 25446) and ELISA (subproject TEC2014-59255-C3-2-R), Ministerio de Economía y Competitividad (MINECO), Spain, and the Conselleria d’Educació, Cultura i Universitats (Govern de les Illes Balears) under grant FPI/1538/2013 (co-financed by the European Social Fund). The research leading to these results has also received funding from "la Caixa" Banking Foundation.

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L IST OF MANUSCRIPTS

As a compendium of articles, the main body of this thesis consist of the following manuscripts:

• (Chapter 2) J. García-Morales, G. Femenias, and F. Riera-Palou, "Performance Analysis and Optimisation of FFR-Aided OFDMA Networks using Channel-Aware Scheduling," Mobile Networks and Applications. Springer, 2017, vol. 22, no. 6, pp.

1068-1082.

DOI: 10.1007/s11036-016-0730-8.

Journal Citation Reports (JCR 2016) Impact Factor: 3.259, Q1 (Category - Telecom- munications - Journals).

Part of this work was presented in the paper "On the Analysis of Channel-Aware Schedulers in OFDMA-Based Networks using FFR", at the 11th IEEE International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob), October 2015.

• (Chapter 3) J. García-Morales, G. Femenias, and F. Riera-Palou, "Statistical Analy- sis and Optimization of FFR/SFR-aided OFDMA-based Multi-cellular Networks,"

in Statistical Signal Processing Workshop (SSP). IEEE, 2016, pp. 1-5.

DOI: 10.1109/SSP.2016.7551802.

Scimago Journal Rank (SJR 2016) index: 0.246, Q1 (Subject Area - Computer Science - Conferences and Proceedings).

• (Chapter 4) J. García-Morales, G. Femenias, and F. Riera-Palou, "On the Design of OFDMA-based FFR-aided Irregular Cellular Networks with Shadowing,"accepted inIEEE Access. IEEE, 2018.

DOI: 10.1109/ACCESS.2018.2804927.

Part of this work was presented in the paper "FFR-aided OFDMA-based Networks under Spatially Correlated Shadowing", at the 10th IEEE International Workshop on Selected Topics in Wireless and Mobile Computing (STWiMob), October 2017.

• (Chapter 5) J. García-Morales, G. Femenias, and F. Riera-Palou, "Statistical Analy- sis and Optimization of a 5th-Percentile User Rate Constrained Design for FFR/SFR- aided OFDMA-based Cellular Networks,"accepted inIEEE Transactions on Vehic- ular Technology. IEEE, 2017.

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DOI: 10.1109/TVT.2017.2782943.

Part of this work was presented in the paper "Characterizing and Optimizing the Throughput of FFR/SFR-aided OFDMA Networks", at the 27th IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), September 2016.

• (Chapter 6) J. García-Morales, G. Femenias, F. Riera-Palou, and J.S. Thompson,

"Multi-layer FFR-aided OFDMA-based Networks using Channel-Aware Sched- ulers,"accepted inIEEE Access. IEEE, 2017.

DOI: 10.1109/ACCESS.2017.2788049.

• (Chapter 7) J. García-Morales, G. Femenias, and F. Riera-Palou, "Analysis and Optimization of FFR-aided OFDMA-based Heterogeneous Cellular Networks,"

IEEE Access. IEEE, 2016, vol. 4, pp. 5111-5127.

DOI: 10.1109/ACCESS.2016.2599026.

Part of this work was presented in the paper "Analytical Performance Evaluation of OFDMA-based Heterogeneous Cellular Networks Using FFR", at the 81st IEEE Vehicular Technology Conference (VTC-Spring), May 2015.

• (Chapter 8) J. García-Morales, G. Femenias, and F. Riera-Palou, "Channel-aware Scheduling in FFR-aided OFDMA-based Heterogeneous Cellular Networks," in International Conference on Computer and Information Technology (CIT). IEEE, 2015, pp. 44-51.

DOI: 10.1109/CIT/IUCC/DASC/PICOM.2015.10.

Scimago Journal Rank (SJR 2016) index: 0.130, Q3 (Subject Area - Computer Science - Conferences and Proceedings).

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C ONTENTS

Acknowledgements v

Funding . . . vi

List of manuscripts vii Contents ix Abstract xv Resumen xvii Resum xix Acronyms xxi 1 Introduction 1 1.1 Context and motivation . . . 1

1.2 Challenges . . . 4

1.3 Objectives . . . 5

1.4 Contributions . . . 6

References 11 2 Performance Analysis and Optimisation of FFR-Aided OFDMA Networks using Channel-Aware Scheduling 15 2.1 Abstract . . . 17

2.2 Introduction . . . 17

2.3 System model . . . 19

2.3.1 Network topology model . . . 19

2.3.2 Channel model . . . 21

2.3.3 Rate allocation . . . 22

2.4 Throughput analysis . . . 25

2.4.1 PF scheduling . . . 26

2.4.2 MSINR scheduling . . . 28

2.4.3 RR scheduling . . . 28

2.5 Optimal Designs . . . 29

2.5.1 Fixed-spectrum-allocation-factor Design . . . 30

2.5.2 Area-proportional Design . . . 30

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2.5.3 QoS-constrained Design . . . 30

2.6 Numerical results . . . 31

2.6.1 FxD-based FFR design . . . 31

2.6.2 ApD-based FFR design . . . 33

2.6.3 QoScD-based FFR design . . . 34

2.7 Conclusion . . . 37

References 39 3 Statistical Analysis and Optimization of FFR/SFR-aided OFDMA-based Multi-cellular Networks 43 3.1 Abstract . . . 45

3.4 Statistical throughput analysis . . . 48

3.5 Optimal Quality-constrained Design . . . 49

References 53 4 On the Design of OFDMA-based FFR-aided Irregular Cellular Networks with Shadowing 55 4.1 Abstract . . . 57

4.2.1 Background work . . . 58

4.2.2 Contributions of the paper . . . 59

4.2.3 Paper organization . . . 60

4.3 Multicellular network model . . . 60

4.3.1 Network layout . . . 60

4.3.2 Cell coverage area and FFR-related cell regions . . . 61

4.3.4 Cell association and FFR-based user classification . . . 63

4.4 Statistical characterization of the SINR . . . 64

4.5 Average spectral efficiency analysis . . . 65

4.5.1 MSINR scheduling . . . 66

4.6 Optimal designs . . . 67

4.6.1 Fixed spectrum allocation factor design . . . 67

4.6.2 QoS constrained design . . . 67

4.7 Performance evaluation . . . 68

4.7.1 FxD-based results . . . 71

4.7.2 QoScD-based results . . . 73

References 77

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CONTENTS

5 Statistical Analysis and Optimization of a 5th-Percentile User Rate Con- strained Design for FFR/SFR-aided OFDMA-based Cellular Networks 81

5.1 Abstract . . . 83

5.2.1 Background work . . . 84

5.3 FFR/SFR system model . . . 86

5.3.1 Network model . . . 86

5.4 Optimal R5pD-based design . . . 89

5.5 Characterization of the throughput . . . 90

5.5.1 Statistical characterization of the users throughput . . . 90

5.5.2 Cell throughput analysis . . . 92

5.6.1 Background designs . . . 93

5.6.2 Proposed R5pD-based design and Comparison . . . 94

5.6.3 R5pD-based design guidelines . . . 102

5.8 APPENDIX Statistical distribution of the SINR . . . 103

References 105 6 Multi-layer FFR-aided OFDMA-based Networks using Channel-Aware Sched- ulers 109 6.1 Abstract . . . 111

6.2.1 Motivation . . . 111

6.2.2 Related work . . . 112

6.3 Cellular network model . . . 114

6.3.1 Two-layer FFR network layout . . . 114

6.3.2 Four-layer FFR network layout . . . 115

6.3.3 Statistical characterization of the SINR . . . 116

6.4 Throughput analysis . . . 118

6.4.1 PF scheduling . . . 119

6.4.3 WorstMSs’ throughput: PF and RR . . . 121

6.5 Multi-layer Designs and Optimization . . . 122

6.5.2 Free FFR-based Design . . . 123

6.6 Performance evaluation . . . 125

6.6.1 ApD for a fixed value ofM . . . 126

6.6.2 FrD for a fixed value ofM . . . 128

6.6.3 Effects of the number of users per cell . . . 129

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References 131 7 Analysis and Optimization of FFR-aided OFDMA-based Heterogeneous

Cellular Networks 135

7.1 Abstract . . . 137

7.2.1 Motivation . . . 137

7.2.2 Related work . . . 138

7.4 Assumptions and preliminaries . . . 142

7.4.1 Assumptions . . . 142

7.4.2 Signal-to-interference-plus-noise ratio . . . 143

7.4.3 Statistical model of the femtocell distribution . . . 144

7.5 Macrocell throughput analysis . . . 145

7.5.1 Average macrocell throughput . . . 145

7.5.2 Macrocell MSINR scheduling . . . 146

7.5.3 Macrocell RR scheduling . . . 148

7.6 Femtocell throughput analysis . . . 148

7.6.1 Overall femtocell throughput . . . 148

7.6.2 Femtocell MSINR scheduling . . . 149

7.6.3 Femtocell RR scheduling . . . 149

7.7 Optimal Designs . . . 150

7.7.1 Fixed-spectrum-allocation-factor Design . . . 150

7.7.3 QoS-constrained Design . . . 151

7.8.1 FxD-based macrocell FFR . . . 152

7.8.2 ApD-based macrocell FFR . . . 155

7.8.3 QoScD-based macrocell FFR . . . 157

7.8.4 Femtocell network . . . 160

7.10 APPENDIX A Proof of Theorem 7.4.1 . . . 163

7.11 APPENDIX B Proof of Theorem 7.4.2 . . . 164

References 167 8 Channel-aware Scheduling in FFR-aided OFDMA-based Heterogeneous Cellular Networks 171 8.1 Abstract . . . 173

8.3.1 A two-tier network topology model . . . 175

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CONTENTS

8.3.3 Statistical model of the femtocell distribution . . . 177

8.4 Macrocell throughput analysis . . . 178

8.4.1 Overall macrocell throughput . . . 178

8.4.2 Macrocell PF scheduling . . . 178

8.4.3 Macrocell MSINR scheduling . . . 181

8.4.4 Macrocell RR scheduling . . . 181

8.5 Femtocell throughput analysis . . . 181

8.5.1 Overall femtocell throughput . . . 181

8.5.2 Femtocell PF scheduling . . . 182

8.5.3 Femtocell MSINR scheduling . . . 182

8.5.4 Femtocell RR scheduling . . . 183

8.6.1 Macrocell performance . . . 183

8.6.2 Femtocell performance . . . 185

References 189 9 General Discussion and Conclusion 191 9.1 Discussion . . . 191

9.3 Future works . . . 195

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A BSTRACT

Fourth Generation (4G) mobile networks are currently undergoing massive deployment in many parts of the world under the umbrella of the Third Generation Partnership Project (3GPP), Long Term Evolution (LTE) and LTE-Advanced (LTE-A) standards. The latest incarnations versions of these systems aim at downlink peak data rates of 100 Mbit/s and 1 Gb/s for high and low mobility users, respectively. Despite the huge leap forward in capacity with respect to previous second and third generation mobile standards (2G GSM and 3G WCDMA, respectively), demand in mobile data traffic is expected to rise well beyond the capabilities of 4G networks. Orthogonal Frequency Division Multiple Access (OFDMA) has played a crucial role towards the success of 4G cellular systems and an increasing number of actors in the fifth generation (5G) arena strongly advocate for its continuation owing to its high capabilities in terms of spectral efficiency and flexibility.

OFDMA-based networks avoid intra-cell interference. However, due to the common use of universal frequency reuse plans, Inter-Cell Interference (ICI) arises, which critically affects the users located in the cell-edge areas.

In this context, Intercell Interference Coordination (ICIC) strategies are deemed to play a key role in 5G multi-cellular networks based on OFDMA. Two of the most representative ICIC techniques are Fractional and Soft Frequency Reuse (FFR and SFR, respectively), which have already been adopted by emerging cellular deployments as an efficient way to improve the throughput performance perceived by cell-edge users. For this reason, the first part of this thesis presents an analytical framework allowing the performance evaluation of these frequency reuse plans in realistic multi-cellular OFDMA networks, where regular and irregular deployments have been taken into consideration. To this end, a novel statistical characterization of the Signal-to-Interference-Plus-Noise Ratio (SINR) in terms of a Cumulative Distribution Function (CDF) as well as tractable mathematical expressions of the average cell spectral efficiency have been derived for different scheduling policies.

Using this physical layer abstractions, the thesis then focuses on the optimization of FFR/SFR-related designs in order to provide high spectral efficiencies over the whole coverage area and to improve the system capacity while maintaining high Quality-of- Service (QoS) and fairness among users. This optimal designs hallow a tradeoff between throughput performance and fairness by suitably dimensioning the FFR/SFR-defined cell- center and cell-edge areas, the frequency allocation to each region and the corresponding transmit power.

Although no final 5G proposal has yet emerged, it is envisaged that in order to tackle the forecasted traffic demands a radical change in the mobile network architecture is likely to be required. Towards this end, many voices within the research community suggest that the classical cellular architecture based on the deployment of a regular grid of high-power tower-mounted base stations covering extensive areas (macrocells) will need

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to be complemented by a dense network of low power base stations providing very high- throughput coverage to reduced areas, thus creating small cells. The capability of a system to cover a given area by combining two or more infrastructures gives rise to the concept of multi-tier networks, an idea also known as Heterogeneous Networks (HetNets). The multi-tier concept has gained much attention recently with the appearance of femtocells. It is envisioned that massive femtocell deployment will result in a very significant reduction of the network operational cost, since they are deployed and maintained by the user, while having the potential to dramatically increase overall capacity. Consequently, the second part of this thesis concsiders two-tier HetNet analytical model combining an operator- managed infrastructure of Macrocell Base Stations (MBSs) with a user-deployed network of Femtocell Base Stations (FBSs). A worst-case scenario in terms of inter-tier interference is evaluated in which macrocell and femtocell tiers are assumed to be uncoordinated and co-channel deployed (full spectrum reuse). Based on a unified approach, the obtained analytical model allows the evaluation of the impact produced by the inter- as well as the co-tier interferences on either the macro-users or the femto-users.

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R ESUMEN

La cuarta generación (4G) de comunicaciones móviles se está desplegando en muchos lugares del mundo bajo el paraguas de los estándares Third Generation Partnership Project(3GPP),Long Term Evolution(LTE) y LTE-Advanced(LTE-A). Las versiones más avanzadas de estos sistemas ofrecen tasas de transmisión de hasta 100 Mbit/s y 1 Gbit/s para usuarios de alta y baja movilidad, respectivamente. Aunque estas cifras representan una mejora considerable respecto de la capacidad que ofrecían los anteriores estándares móviles de segunda y tercera generación (2G GSM y 3G WCDMA, respectivamente), es previsible que el aumento espectacular en la demanda de capacidad de transmisión de datos supere ampliamente las capacidades de los sistemas 4G en un futuro no muy lejano. El acceso múltiple por división de frecuencias ortogonales (OFDMA del inglés Orthogonal Frequency Division Multiple Access) ha jugado un papel crucial en el éxito de los sistemas celulares de 4G y un número cada vez mayor de participantes en la definición de los estándares de quinta generación (5G) apuestan por su continuación dada su elevada eficiencia espectral y su grado de flexibilidad. Las redes basadas en OFDMA no introducen interferencia intracelular. Sin embargo, debido al uso habitual de planes de reutilización de frecuencias universales, la interferencia intercelular (ICI) afecta de manera crítica a los usuarios ubicados cerca de los límites de las celdas.

En este contexto, se considera que las estrategias de control/coordinación de interferencia intercelular (ICIC del inglésIntercell Interference Coordination) juegan un papel clave en las redes multicelulares 5G basadas en OFDMA. Dos de las técnicas ICIC más repre- sentativas son la reutilización de frecuenciasfractionaly la reutilización de frecuencias soft(FFR y SFR, respectivamente), que ya han sido implantadas en despliegues celulares emergentes como una forma eficiente de mejorar el rendimiento que perciben los usuarios cercanos de los límites celulares. Por esta razón, la primera parte de esta tesis presenta un marco analítico que permite la evaluación del desempeño de estos planes de reutilización de frecuencias en redes multicelulares realistas basadas en OFDMA, donde se consideran tanto los despliegues regulares como los irregulares. Para ello, se ha desarrollado una nueva caracterización estadística de la relación señal-interferencia-más-ruido (SINR) a través de soluciones cerradas de la función de distribución acumulativa (CDF) de la SINR, así como expresiones matemáticamente tratables de la eficiencia espectral promedio de una determinada célula para diferentes políticas de distribución de recursos radio. Después nos hemos centrado en la optimización de los diseños relacionados con FFR/SFR para garantizar altas eficiencias espectrales en toda el área de cobertura y mejorar la capacidad del sistema, manteniendo al mismo tiempo la calidad de los servicios (QoS) y equidad entre los usuarios. Estos diseños óptimos permiten la compensación entre el rendimiento celular y la equidad entre usuarios mediante el adecuado dimensionamiento de las áreas del centro y del borde celular definidas por los esquemas FFR/SFR, optimizando la asignación

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de frecuencia a cada región y la potencia de transmisión correspondiente.

Aunque aún no hay cerrada ninguna especificación de 5G, parece claro que para hacer frente a las demandas de tráfico previstas es muy probable que se requiera un cambio radical en la arquitectura de la red móvil. Respecto a esto, el consenso entre la comunidad de investigadores sugiere que la arquitectura celular clásica basada en el despliegue de una red regular de estaciones base de alta potencia montadas en torres que cubran extensas áreas (macroceldas) deberá ser ampliamente complementada por una densa red de estaciones de baja potencia que proporcionen cobertura a áreas reducidas pero de muy alto rendimiento, creando así células pequeñas. La capacidad de un sistema para cubrir un área determinada mediante la combinación de dos o más infraestructuras da lugar al concepto de redes multinivel, una idea también conocida con el nombre de redes heterogéneas (HetNets del inglés Heterogeneous Networks). El concepto multinivel ha cobrado mucho interés con el despliegue de femto-celdas. Se prevé que el despliegue masivo de femto-celdas resultará en una reducción muy significativa en los costes operacionales de la red, ya que éstas son desplegadas y mantenidas por el usuario, a la vez que tienen el potencial de aumentar dramáticamente la capacidad total de la red.

Por ello, la segunda parte de esta tesis se centra en la definición de un modelo analítico HetNet de dos niveles que combina una infraestructura gestionada por el operador de estaciones base macro-celulares (MBS) con una red de estaciones base femto-celulares (FBS) desplegadas por el usuario. Se evalúa el peor escenario posible en términos de interferencia entre niveles, en el que se supone que los niveles macro-célula y femto-célula no están coordinados y emplean un despliegueco-canal(con reutilización de espectro completo). Basándose en un enfoque unificado, el modelo analítico obtenido sirve para evaluar el impacto producido por las interferencias entre los dos niveles tanto sobre los usuarios macro como sobre los femto-usuarios.

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R ESUM

La quarta generació (4G) de comunicacions mòbils s’està desplegant en molts indrets del món sota el paraigües dels estàndardsThird Generation Partnership Project(3GPP),Long Term Evolution(LTE) i LTE-Advanced(LTE-A). Les versions més avançades d’aquests sistemes ofereixen taxes de transmissió de fins a 100 Mbit/s i 1 Gbit/s per a usuaris d’alta i baixa mobilitat, respectivament. Tot i que aquestes xifres representen una millora considerable respecte de la capacitat que oferien els anteriors estàndards de comunicacions mòbils de segona i tercera generació (2G GSM i 3G WCDMA, respectivament), és previsible que l’ augment espectacular en la demanda de capacitat de transmissió de dades superi àmpliament les capacitats dels sistemes 4G en un futur no gaire llunyà. L’accés múltiple per divisió en freqüències ortogonals (OFDMA de l’anglèsOrthogonal Frequency Division Multiple Access) ha jugat un paper crucial en l’èxit dels sistemes cel·lulars de 4G i un nombre cada vegada més gran de participants en la definició dels estàndards de cinquena generació (5G) aposten per la seva continuació atesa la seva elevada eficiència espectral i el seu grau de flexibilitat. Les xarxes basades en OFDMA no introdueixen interferència intracel·lular. Tanmateix, degut a l’ús habitual de plans de reutilització de freqüències universals, la interferència intercel·lular (ICI) afecta de manera crítica als usuaris ubicats prop dels límits de les cel·les.

En aquest context, es considera que les estratègies de control/coordinació d’interferència intercel·lular (ICIC de l’anglèsIntercell Interference Coordination) ju- goran un paper clau en les xarxes multicel·lulars 5G basades en OFDMA. Dues de les tècniques ICIC més representatives són la reutilització de freqüènciesfractionali la re- utilització de freqüènciessoft(FFR i SFR, respectivament), que ja han estat implantades en desplegaments cel·lulars emergents com una forma eficient de millorar el rendiment que perceben els usuaris propers dels límits cel·lulars. Per aquesta raó, la primera part d’aquesta tesi presenta un marc analític que permet l’avaluació de el rendiment d’aquests plans de reutilització de freqüències en xarxes multicel·lulars realistes basades en OFDMA, on es consideren tant els desplegaments regulars com els irregulars. Per a això, s’ha desen- volupat una nova caracterització estadística de la relació senyal-interferència-més-soroll (SINR) a traves de solucions tancades de la funció de distribució acumulativa (CDF) de la SINR, així com expressions matemàticament tractables de l’eficiència espectral mitjana d’una determinada cèl·lula per a diferents polítiques de distribució de recursos ràdio.

Després ens hem centrat en l’optimització dels dissenys relacionats amb FFR/SFR per garantir altes eficiències espectrals en tota l’àrea de cobertura i millorar la capacitat del sistema, mantenint al mateix temps la qualitat de servei (QoS) i equitat entre els usuaris.

Aquests dissenys òptims permeten la compensació entre el rendiment cel·lular i l’equitat entre usuaris mitjançant l’adequat dimensionament de les àrees del centre i de la vora cel·lular definides pels esquemes FFR/SFR, optimitzant l’assignació de freqüència a cada

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regió i la potència de transmissió corresponent.

Tot i que encara no hi ha tancada cap especificació de 5G, sembla clar que per fer front a les demandes de trànsit previstes és molt probable que es requereixi un canvi radical en l’arquitectura de la xarxa mòbil. Respecte a això, el consens entre la comunitat d’investigadors suggereix que l’arquitectura cel·lular clàssica basada en el desplegament d’una xarxa regular d’estacions base d’alta potència muntades en torres que cobreixin extenses àrees (macrocel·les) haurà de ser àmpliament complementada per una densa xarxa d’estacions de baixa potència que proporcionin cobertura a àrees reduïdes però de molt alt rendiment, creant així cèl·lules petites. La capacitat d’un sistema per cobrir una àrea determinada mitjançant la combinació de dues o més infraestructures dóna lloc al concepte de xarxes multinivell, una idea també coneguda amb el nom de xarxes heterogènies (HetNets de l’anglèsHeterogeneous Networks). El concepte multi-nivell ha cobrat molt d’interès amb el desplegament de femto-cèl·les. Es preveu que el desplegament massiu de femto-cèl·les resultarà en una reducció molt significativa en els costos operacionals de la xarxa, ja que aquestes són desplegades i mantingudes per l’usuari, alhora que tenen el potencial d’augmentar dramàticament la capacitat total de la xarxa. Per això, la segona part d’aquesta tesi es centra en la definició d’un model analític HetNet de dos nivells que combina una infraestructura gestionada per l’operador d’estacions base macrocel·lulares (MBS) amb una xarxa d’estacions base femto-cel·lulares (FBS) desplegades per l’usuari. S’avalua el pitjor escenari possible en termes d’interferència entre nivells, en el qual se suposa que els nivells macro-cel·lulares i femto-cel·lulares no estan coordinats i empren un desplegamentco-canal(amb reutilització d’espectre complet). Basant-se en un enfocament unificat, el model analític obtingut serveix per avaluar l’impacte produït per les interferències entre els dos nivells tant sobre els usuaris macro com sobre els femto-usuaris.

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A CRONYMS

3G Third generation . . . 1 3GPP Third generation partnership project . . . 1 4G Fourth generation . . . 1 5G Fifth generation . . . 1 AMC Adaptive modulation and coding . . . 6 BS Base station . . . 2 CP Cyclic prefix . . . 20 CRA Continuous rate allocation . . . 6 CDF Cumulative distribution function . . . 5 DRA Discrete rate allocation . . . 6 FBS Femtocell base station . . . 3 FU Femtocell user . . . 138 FUE Femtocell user equipment . . . 174 FFR Fractional frequency reuse . . . 2

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HetNet Heterogeneous network . . . 3 ICI Inter-cell interference . . . 2 ICIC Inter-cell interference coordination . . . 2 IEEE Institute of Electrical and Electronics Engineers . . . 1 ITI Inter-tier interference . . . 3 JFI Jain’s fairness index . . . 102 KPI Key performance indicator . . . 85 LTE Long Term Evolution . . . 1 LTE-A LTE-Advanced . . . 1 MBS Macrocell base station . . . 3 MUs Macrocell users . . . 138 MUE Macrocell user equipment . . . 174 MSINR Maximum SINR . . . 3 MS Mobile station . . . 57 MCS Modulation and coding scheme . . . 22 MOP Multiobjective optimization problem . . . 99 OFDMA Orthogonal frequency division multiple access . . . 1 PPP Poisson point process . . . 9

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PDF Probability density function . . . 26 PF Proportional fair . . . 3 QoS Quality-of-services . . . 1 RB Resource blocks . . . 17 RR Round robin . . . 4 SFR Soft frequency reuse . . . 2 SINR Signal-to-interference-plus-noise ratio . . . 2 UMB Ultra Mobile Broadband . . . 1 UE User equipment . . . 17

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C

HAPTER

1

I NTRODUCTION

1.1 Context and motivation

Over the last decade, mobile communications have experienced an explosive growth fueled by the massive market penetration of smartphones, tablets and notebooks and the corresponding demand for a huge volume of data this has brought along. The current data deluge, mainly driven by broadband Internet access related to multimedia-oriented packet-data-based services, is not a pinnacle and in fact, it is likely to be further exacer- bated by new unforeseen applications that may materialize as early as in 2020. In such a context, mobile users’ expectations and demands result in unprecedented requirements in terms of coverage and system capacity, while often requiring the fulfilment of tight quality-of-services (QoS) constraints.

As an answer to this market demand, a plethora of new standards has been developed such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and 802.16 families of standards for local- and metropolitan wireless area networks, respectively, or the Long Term Evolution (LTE) and LTE-Advanced (LTE-A), introduced by third generation partnership project (3GPP), and the Ultra Mobile Broadband (UMB) system specified by 3GPP2, aiming at cellular deployments. These proposals have resulted in the transition from the third generation (3G) to the fourth generation (4G) of mobile communications supporting reliable transmissions with peak data-rates ranging from 100 Mbps to 1 Gbps for high- and low-mobility environments, respectively, and whose evolution is expected to catalyze the development of the new generation of mobile communications (so called fifth generation (5G) systems). All these standards rely on the use of orthogonal frequency division multiple access (OFDMA) as the main air-interface technology and there are good chances that OFDMA will still play an overriding role in the forthcoming 5G standard [1.1].

In modern cellular systems, a dense frequency reuse of the scarce radio spectrum is needed in order to maximize spectral efficiency, a solution that, inevitably brings along

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higher levels of interference. The interference has always been one of the most important performance-limiting factors in the context of cellular communications. In particular, cellular mobile communication systems typically suffer from two major classes of interference, namely, intra-cell interference and inter-cell interference (ICI). In the former, interference is caused among frequency channels within the same cell due to the power leakage from one channel to an adjacent channel when both are simultaneously used within a given cell. In the latter, interference is caused among neighbouring cells making use of the same frequency resources. In OFDMA-based networks, a wideband frequency-selective fading channel is decomposed into a set of orthogonal narrow-band subchannels and this orthogonality makes the intra-cell interference negligible. However, the use of universal frequency reuse schemes cause the OFDMA-based mobile networks to suffer from very high levels of ICI, particularly for users located in the cell-edge areas whose interference level might even exceed the desired signal power from the serving base station (BS). ICI limits the system capacity making inter-cell interference coordination (ICIC) techniques a critical mechanism towards the optimization of 4G/5G networks.

ICIC strategies can be implemented dynamically or statically. Dynamic ICIC schemes employ adaptive algorithms to efficiently manage radio resources. In this approach, the parameters are adjusted according to the instantaneous channel and traffic information of the network, allowing these strategies to promptly respond to non-homogeneous traffic load distributions and other time varying conditions. Unfortunately, dynamic ICIC schemes are only possible at the expense of a prohibitive complexity and impractical requirements on the available channel state information [1.2], thus precluding their deployment. In contrast, in static ICIC schemes, all parameters are configured in advance and remain constant over a certain period of time, thus implying the use of fixed and predefined frequency reuse patterns and power levels to different groups of users according to their average signal- to-interference-plus-noise ratio (SINR) or their received average power [1.3]. Applying different frequency reuse factors, as well as different transmit powers, to different groups of users depending on their average channel quality is the fundamental design principle exploited in static ICIC strategies. A myriad of ICIC strategies have been proposed [1.4], among which,staticfractional frequency reuse (FFR), soft frequency reuse (SFR) and all their variants have shown a good tradeoff between cell-edge throughput enhancement, provision of high spectral efficiency and implementation complexity [1.5].

FFR/SFR schemes allocate non-overlapping frequency bands to cell-center and cell- edge users, with a low frequency reuse factor for the cell-center users and a higher frequency reuse factor for the cell-edge users. More specifically, FFR splits the frequency bands assigned to cell-edge users into disjoint sets of sub-bands and applies a reuse factor greater than one on each of them. In this scheme, cell-center users do not share any spectrum with cell-edge users, thus reducing the interference for both center and edge users at the cost of partly sacrificing spatial reuse. SFR is a variation of FFR whereby the central region of each cell is also allowed to employ the frequency resources allocated to the edges of the neighboring cells, thus the whole system bandwidth is reused in every cell. Furthermore, different transmit powers can be allocated to the central and edge frequency bands in SFR-based cellular systems, with the aim of increasing the signal-to- interference-plus-noise ratio (SINR) experienced by those users located far away from the BS. A detailed description of FFR and SFR can be found in [1.6–1.9].

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1.1. CONTEXT AND MOTIVATION

Aiming at even higher area spectral efficiencies, modern cellular systems are transi- tioning from planned homogeneous (one-tier) macro-cellular networks to highly heterogeneous (multi-tier) deployments. In these systems, often referred to as heterogeneous networks (HetNets), high power nodes, such as macrocell base stations (MBSs), are deployed for extended coverage and high mobility support, while low power nodes, generally known as small cells (e.g., microcells, picocells and femtocells), are in charge of supplying very high data rates over certain small areas (e.g., offices, houses). By deploying addi- tional low power nodes within the local-area range and bringing users closer to the desired BSs, HetNets can potentially improve the spatial resource reuse and extend coverage, thus allowing them to achieve higher data rates with a lower energy consumption, yet retaining the uninterrupted connectivity and seamless mobility of cellular networks [1.10].

In particular, user-deployed femtocells have recently attracted significant more interest from academia and industry in comparison to other types of small cells [1.11]. These two-tier macro-femto networks have the potential to offload traffic from the macrocells while providing high-quality low-cost network access to indoor users [1.12]. Due to the large cost of licensed spectrum, operators typically choose to allocate the same frequency bands to both macrocells and femtocells. These co-channel deployments inevitably result in inter-tier interference (ITI) between the macro and femto layers, which is particularly damaging for femto-users connected to a femtocell base station (FBS) situated close to a MBS or for the macro-users far from the MBS who are roaming in the vicinity of an FBS [1.13]. A thourough review of the principles underpinning heterogeneous networks can be found in [1.14–1.16].

Regardless of the particular technique in use or the tier-dependent frequency plan deployed, in the downlink of OFDMA-based networks the time/frequency channel quality of the resources allocated to a given BS, typically measured in terms of the SINR, varies among the different users. Such variations in channel conditions can be exploited by using channel-aware schedulers able to allocate each resource to a user with favourable channel conditions at a given scheduling time slot. Opportunistic maximum SINR (MSINR) schedulers [1.17] make the most of the multiuser diversity by allocating the resources to the users experiencing the best channel conditions. Although applying this scheduling rule maximises the spectral efficiency of the system, it poses serious fairness issues as those users suffering from bad channel conditions over extended periods of time are prone to starvation, thus resulting in a dramatic QoS degradation. In order to provide a reasonable tradeoff between spectral efficiency and fairness, a proportional fair (PF) scheduling rule was proposed by Kellyet al.in [1.18] and then extended by Shakkottai et al.in [1.19]. In this case, scheduling decisions are based on a weighted version of the instantaneous channel behaviour, with the weighting coefficient for a given user being inversely proportional to the average channel behaviour during the time this user has gained access for transmission. In this way, the resources end up being allocated to users experiencing the relatively best channel conditions in comparison to their average channel state. Consequently, when using a PF scheduler, the possibility of a user with a very bad link suffering from long periods of starvation is drastically reduced.

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1.2 Challenges

In the context of OFDMA-based cellular communications, the analysis of optimal static FFR/SFR designs and the derivation of exact mathematically tractable analytical models for FFR/SFR-aided OFDMA-based heterogeneous cellular networks using channel-aware schedulers is a challenge of significant interest when there are different user requirements, channel conditions, and also, when designing optimal resource allocation strategies.

In this way, one of the major issues when dealing with FFR/SFR-based static ICI mitigation strategies is the design of long-term resource allocation algorithms aiming at the optimization of throughput-related utility functions with constraints on the degree of fairness among users arbitrarily located throughout the cell [1.20]. In order to tackle these optimization problems, it is desirable to obtain mathematically tractable analytical models for both the whole system and per-user throughput. The performance of long-term FFR/SFR-based resource allocation algorithms, however, is fully related to the short-term scheduling rules used to select the set of users on each time/frequency/energy resource, thus making the derivation of mathematically tractable models a challenging task. Gener- ally speaking, amongst many scheduling rules, such as MSINR or round robin (RR), it is well known that the PF scheduler has been successfully deployed in the latest genera- tions of wireless networks owing to the excellent trade-off it offers between fairness and spectral efficiency [1.18, 1.19]. However, the derivation of exact mathematically tractable analytical models for the system and user throughput of FFR/SFR-aided OFDMA-based cellular networks under PF scheduling is a challenging aim of significant interest when designing optimal long-term resource allocation strategies.

Additionally, the performance of any ICIC strategy is critically influenced by the large scale propagation characteristics affecting the desired and interfering signals, thus any simulation study must average over many user positions and multiple, possibly correlated, shadowing realizations. In particular, the derivation of analytical and optimal models for the FFR/SFR-aided OFDMA-based cellular system is a challenging issue when the spatially correlated shadowing effect is taken into account for the network design [1.21].

Moreover, the analytical characterization and optimization of both the cell and network performance are very demanding tasks when irregular multi-cellular deployments affected by correlated shadow fading are considered.

The previous set of challenges also prevail, and even in a more acute manner, in heterogeneous environments, where now, analytical performance models and optimal FFR/SFR-based designs involve: (i) the evaluation of the inter-tier interference given that macrocell and femtocell tiers are assumed to be uncoordinated and rely on a co-channel deployment, (ii) new classes of users, such as, indoor users only served by FBSs and outdoor users served by MBSs, (iii) more parameters need to be incorporated to the framework. Including, the degree of isolation provided by wall penetration losses, the urbanization factor or the number of FBSs per macrocell should all be taken into account. Several research studies have presented simulation-based performance analysis of OFDMA-based two-tier macro/femto networks considering selected scenarios [1.22–1.24], however, besides being usually conducted via computationally expensive, time-consuming, and proprietary system-level testbeds, simulation-based approaches neither serve to cap-

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1.3. OBJECTIVES

ture the joint interplay of system design parameters nor allow gaining insights behind the obtained performance results.

The aforementioned problems and corresponding challenges have been the driving forces of this thesis, whose main objectives are presented next.

1.3 Objectives

The main goal of this PhD thesis is the development of a unified theoretical framework for the management of resources in static partial frequency reuse-aided heterogeneous OFDMA-based multi-tier networks using channel-aware schedulers. In tackling this problem, this thesis focuses on the following specific objectives:

• The derivation of mathematically tractable analytical expressions that can serve as a basis for an abstraction of the physical layer (mathematical model that captures the main features of the system to be modeled), which can serve as a foundation stone for radio resource management schemes in the upper layers. In this manner, the framework has to contemplate the use of different rate allocation strategies, thus allowing the assessment of the network behaviour under ideal (capacity-based) or realistic (goodput-based) conditions. This framework should also allow the analytical evaluation of performance metrics such as coverage, outage probability, network capacity, spectral efficiency, user throughput cumulative distribution function (CDF) or the 5th-percentile of users rate in the context of FFR/SFR-aided OFDMA networks. Furthermore, an analytical characterization of multi-cellular networks needs to consider the specific network topology. In particular, it has to consider the irregular deployment of base stations that leads to a non-regular tessellation of the network coverage area.

• The design of a unified physical layer model taking into account the multi-tier nature of the system. In the considered model, the macrocell and femtocell tiers are assumed to be uncoordinated and co-channel deployed, thus representing a worst- case scenario in terms of inter-tier interference. In this way, the heterogeneous design has to allow the theoretical evaluation of the impact produced by both inter- and co-tier interferences on the network performance.

• The development of resource management techniques and scheduling policies that are able to agilely react to changes in the environment, such as channel variations, network load fluctuations or changes of system requirements. The performance evaluation results should be obtained as a function of the scheduling policy, the density of users per area unit, the SINR and throughput characterization. In doing so, it is important to incorporate the main environmental factors, such as, the characteristics of both the fast multipath fading and the spatially correlated slow shadow fading.

• The optimization of FFR/SFR-related parameters, using convex optimization techniques together with results from information theory and stochastic geometry. To this end, optimization strategies have to be devised aiming at determining the size of the reuse scheme-related spatial and the frequency and power partitions maximizing

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the average cell throughput while satisfying the corresponding operator-defined system constraints.

When jointly considered, the fulfillment all of these objectives results in a cross- layer design (PHY-MAC) whose parameters can be calculated when considering an heterogeneous environment where users from any tier (macro or femto) have potentially different QoS.

1.4 Contributions

The contributions collected in this PhD thesis, as a compendium of articles, are explained regarding groups of papers (chapters) that together represent a structural thematic unit on the analysis, performance evaluation of the spectral efficiency, and optimization in the downlink of static FFR/SFR designs in homogenous and heterogeneous OFDMA-based multi-cellular networks.

The first group of papers comprises Chapters 2 through 6 and deals with homogenous OFDMA-based networks. In Chapters 2 and 3, a statistical analysis of static FFR/SFR schemes using channel-aware scheduling is presented. In particular, Chapter 2 introduces the optimization of benchmark FFR-based designs, such as, Fixed-spectrum-allocation- factor Design (FxD), Area-proportional Design (ApD) and QoS-constrained Design (QoScD). Chapter 2 also incorporates different rate allocation strategies, like continuous rate allocation (CRA), and discrete rate allocation (DRA), suitable to examine the performance of systems based on adaptive modulation and coding (AMC) strategies. Chapter 3 introduces the static SFR scheme in the analytical framework. Chapter 4 is concerned with the channel impairments, where a characterization of the SINR is carried out taking into account the spatially correlated shadowing. Additionally, a tractable analytical model for irregular cellular deployments is fully described when exploring both the FxD-based FFR and the QoScD-based FFR optimal designs. Chapter 5 tackles the throughput characterization allowing the analytical evaluation of the users throughput CDF. This chapter also introduces a novel FFR-based design, namely, the 5th-percentile user rate constrained design (R5pD) that is able to guarantee fairer QoS levels throughout the cell. Chapter 6 presents a multi-layer FFR-scheme, which splits the cells into inner, middle and outer layers in order to improve the fairness among users located in different layers without sacrificing the overall cell throughput. The last group of papers comprises Chapters 7 and 8, which deal with two-tier HetNets, where a worst-case scenario in terms of inter-tier interference is evaluated (i.e., uncoordinated co-channel deployment). More specifically, Chapter 7 presents an in-depth discussion about optimal FFR-based designs allowing the evaluation of the impact produced by the inter- as well as the co-tier interferences on different classes of users. Chapter 8 tackles the use of channel-aware scheduling in OFDMA-based heterogeneous cellular networks. The final chapter closes with a general discussion, the main outcomes of the entire document and provides the concluding remarks and some hints on future research avenues.

Next, the contributions of the individual chapters that constitute the main body of this thesis are summarized.

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1.4. CONTRIBUTIONS

Chapter 2: "Performance Analysis and Optimisation of FFR-Aided OFDMA Net- works using Channel-Aware Scheduling". This chapter introduces a novel analytical framework allowing the throughput performance evaluation of an FFR-aided OFDMA-based homogenous cellular network using a PF scheduling policy. The framework shows how the performance of both RR and MSINR scheduling rules can be derived as special cases of the PF scheduler. This framework is able to accommodate different rate allocation strategies, such as, CRA or DRA, examining the impact of multi-rate systems based on adaptive modulation and coding schemes.

In the analysis, an SINR estimate is obtained at the receiver of each user and it is then fed back to the base station so that the transmission mode, comprising a modulation format and a channel code, can be adapted in accordance to the instantaneous channel characteristics. One of the strengths of this chapter is that it turns out to extend the applicability of the analysis to the optimization of the FFR in the spatial and frequency domains using different optimization criteria, such as, FxD, ApD and QoScD.

Chapter 3: "Statistical Analysis and Optimization of FFR/SFR-aided OFDMA-based Multi-cellular Networks". An SFR scheme-based analytical framework is considered in this chapter. This works develops a novel statistical characterization of the average throughput of an OFDMA-based multi-cell network employing PF scheduling that is shown to be valid when using either SFR or FFR. The statistical performance model focuses on one particular design, the QoS-constrained design, that strikes a balance between the cell-edge and cell-center throughputs, hence improving fairness. The proposed analytical framework allows the performance of both SFR and FFR to be assessed and compared, and hence, revealing under what conditions one frequency reuse technique is preferable over the other. Although results are obtained for an FFR/SFR-aided deployment, this analytical framework opens the door to the theoretical spectral efficiency evaluation of OFDMA-based cellular networks using other ICIC techniques, as well as to the assessment of cellular multi-tier heterogeneous networks.

Chapter 4: "Analysis and Design of FFR-aided OFDMA-based Irregular Cellular Networks with Shadowing". This chapter presents an analysis and design of an FFR-aided OFDMA-based irregular cellular network using resource allocation algorithms when the shadowing effect is present. A novel statistical characterization of the SINR is introduced and discussed that takes into account the effects of a spatially correlated lognormal shadowing among the base stations. This SINR statistical characterization is used to obtain the cell-specific and network spectral efficiency when using either RR or MSINR scheduling strategies. The mathematical description of the cell association process in irregular deployments is fully characterized for those realistic cases where a mobile station associates to the base station that, first, belongs to the set of neighbouringbase stations and, second, provides the mobile station with the highest average received power. Related to the previous contribution, once a mobile station has been associated to a given BS, the FFR-based resource allocation unit has to decide whether this mobile station is classified as cell-center or cell-edge user. In this work, the cell association and user’s classification processes are based on the average received power (or equivalently,

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on the average SINR). The proposed analytical framework allows the derivation of an optimal design regarding the size of the FFR-related spatial domain parameters maximizing the average spectral efficiency of the system.

Chapter 5: "Statistical Analysis and Optimization of a 5th-Percentile User Rate Constrained Design for FFR/SFR-aided OFDMA-based Cellular Networks".

In this chapter, the users throughput CDF is tackled in the context of FFR/SFR schemes. Remarkably, the users throughput CDF is used to analytically derive the 5th-percentile of users rate, an important fairness metric to consider, since modern cellular networks are increasingly required to provide high data-rate and also guaranteed QoS throughout the cell. In the proposed analysis, different SFR/FFR optimal designs, namely, the fixed spectrum-allocation-factor FFR-design and the fixed power-control-factor SFR-design, are studied and compared. Additionally, a novel analytical optimization approach, which we term R5pD, is introduced whereby a cell throughput-based utility function maximization is conducted subject to a constrained 5th-percentile user rate. In order to deal with the R5pD-based design, a global characterization of throughput is fully described for both SFR- and FFR-aided OFDMA-based cellular networks using PF scheduling. Furthermore, results for the R5pD-based strategy are compared to those obtained using well- known optimal designs, namely, FxD and QoScD, whose optimization criteria strike a balance between the cell-edge and cell-center throughput leading to different network performance behaviors.

Chapter 6: "Multi-layer FFR-aided OFDMA-based Networks using Channel-Aware Schedulers". An analysis and optimization of multi-layer FFR-aided OFDMA- based cellular networks is introduced in this chapter. Based on the statistical channel characterization and the cell throughput expressions, an analytical framework is provided. This can be used to evaluate the impact any of the FFR layers induces on the average cell throughput, the average throughput per layer or the average throughput experienced by theworstMSs in each layer. In order to select the size of the FFR-related spatial and spectral partitions, two multi-layer FFR-based optimal designs, namely the ApD and the free-design (FrD), are introduced. Furthermore, for both ApD and FrD approaches, an optimization problem is posed aiming at maximizing the max-min throughput fairness among users located in different layers.

Chapter 7: "Analysis and Optimization of FFR-aided OFDMA-based Heterogeneous Cellular Networks". The performance evaluation of FFR-aided OFDMA-based two-tier HetNets is introduced in this work. The proposed framework comprises a planned FFR-based macro-cellular network in the first tier, overlaying a randomly deployed second-tier of femtocells. This unified approach allows the evaluation of the impact produced by both inter- and co-tier interferences on either the macro-user- equipment or the femto-user-equipment. Such an approach can then be regarded as a hybrid model in between a deterministic two-tier deployment where the locations of all base stations are knowna priori, and a stochastic two-tier deployment where

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1.4. CONTRIBUTIONS

both MBSs and FBSs are distributed using Poisson Point Processes (PPPs). As a major benefit of the hybrid approach proposed here, our model allows the analytical evaluation of two-tier HetNets that incorporate specific network parameters typically designed for planned macrocell networks (e.g., optimal threshold distance designs for different scheduling schemes in FFR-based OFDMA networks), while capturing the random nature of unplanned FBS deployments. The analytical model proposed in this chapter allows the optimization of the average throughput provided by macro- or femto-base-stations.

Chapter 8: "Channel-aware Scheduling in FFR-aided OFDMA-based Heterogeneous Cellular Networks". In this work, the framework of Chapter 7 is extended to the performance evaluation of FFR-aided OFDMA-based HetNets using a PF scheduling policy, widely used in practice by current networks. Again, it is shown how the performance of both RR and MSINR scheduling can be derived as special cases of the PF scheduler, but now, for the HetNet case. Moreover, this chapter shows that multiuser diversity is also seen to play a key role in increasing the femtocell throughput under MSINR and PF schedulers even when considering a small number of femto-users per femtocell.

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C

HAPTER

2

P ^ERFORMANCE A NALYSIS AND

O PTIMISATION OF FFR-A ^IDED OFDMA N ETWORKS USING C HANNEL -A WARE

S ^CHEDULING

Jan García-Morales, Guillem Femenias, and Felip Riera-Palou

This paper has been published in Mobile Networks and Applications, Springer, 2017, vol. 22, no. 6, pp. 1068-1082.

Part of this work was presented in the paper "On the Analysis of Channel-Aware Schedulers in OFDMA-Based Networks using FFR", at the 11th IEEE International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob), October 2015.

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2.1. ABSTRACT

2.1 Abstract

Modern cellular standards typically incorporate interference coordination schemes allowing near universal frequency reuse while preserving reasonably high spectral efficiencies over the whole coverage area. In particular, fractional frequency reuse (FFR) and its variants are deemed to play a fundamental role in the next generation of cellular deployments (B4G/5G systems). This paper presents an analytical framework allowing the downlink performance evaluation of FFR-aided OFDMA networks when using channel-aware scheduling policies. Remarkably, the framework contemplates the use of different rate allocation strategies, thus allowing to assess the network behaviour under ideal (capacity- based) or realistic (throughput-based) conditions. Analytical performance results are used to optimise the FFR parameters as a function of, for instance, the resource block scheduling policy or the density of user equipment (UE) per cell. Furthermore, different optimisation designs of the FFR component are proposed that allow a tradeoff between throughput performance and fairness by suitably dimensioning the FFR-defined cell-centre and cell-edge areas and the corresponding frequency allocation to each region. Numerical results serve to confirm the accuracy of the proposed analytical model while providing insight on how the different parameters and designs affect network performance.

2.2 Introduction

Orthogonal frequency division multiple access (OFDMA) has been adopted as the downlink multiple access scheme for state-of-the-art cellular communications standards such as Long-Term Evolution (LTE) and LTE-Advanced (LTE-A) [2.1]. In OFDMA, a wideband frequency-selective fading channel is decomposed into a set of orthogonal narrow-band subchannels. These subchannels are jointly used with a time-slotted frame pattern to provide a set of frequency/time resources (also known as resource blockss (RBs)), which are distributed among cells based on predefined frequency reuse plans. The orthogonality among RBs makes the intra-cell interference negligible. However, the use ofaggressive high spectral efficiency universal frequency reuse plans, with all cells using the same set of RBs, cause the OFDMA-based networks to suffer from very high levels of inter-cell interference (ICI), particularly affecting the UEs located in the cell-edge areas. With the aim of mitigating ICI experienced by the cell-edge users while still achieving high spectral efficiencies, a myriad of ICI control (ICIC) strategies have been proposed [2.2], among whichstaticfractional frequency reuse (FFR) and all its variants show a good tradeoff between cell-edge throughput enhancement, provision of high spectral efficiency and implementation complexity [2.3].

In the downlink of FFR-aided OFDMA-based networks, the time/frequency channel quality of the set of RBs allocated to a given base station (BS), typically measured in terms of the signal-to-interference-plus-noise ratio (SINR), varies for different UEs. Such variations in channel conditions can be exploited by using channel-aware schedulers able to allocate each RB to a UE with favourable channel conditions at a given scheduling time slot. Opportunistic maximum SINR (MSINR) scheduler [2.4] makes the most of the multiuser diversity by allocating the RBs to the UEs experiencing the best channel conditions. Although applying this scheduling rule aims at maximising the spectral efficiency of the system, it raises a serious fairness problem, with UEs suffering from

Analysis, desing and optimization of partial frequency reuse-aided OFDMA-based heterogeneous cellular networks

DOCTORAL THESIS 2018

Doctoral Programme of Information and Communications Technology

ANALYSIS, DESIGN AND OPTIMIZATION OF PARTIAL FREQUENCY REUSE-AIDED

OFDMA-BASED HETEROGENEOUS CELLULAR NETWORKS

Jan García Morales

Director/Advisor: Guillem Femenias Nadal

Director: Felip Riera Palou

A CKNOWLEDGEMENTS

Funding

L IST OF MANUSCRIPTS

C ONTENTS

A BSTRACT

R ESUMEN

R ESUM

A CRONYMS

C

1

I NTRODUCTION

1.1 Context and motivation

1.2 Challenges

1.3 Objectives

1.4 Contributions

R EFERENCES

C

2

P ERFORMANCE A NALYSIS AND

O PTIMISATION OF FFR-A IDED OFDMA N ETWORKS USING C HANNEL -A WARE

S CHEDULING

2.1 Abstract

2.2 Introduction

P ^ERFORMANCE A NALYSIS AND

O PTIMISATION OF FFR-A ^IDED OFDMA N ETWORKS USING C HANNEL -A WARE

S ^CHEDULING