Estratégias distribuídas para a coexistência de redes de banda larga sem fio em bandas não-licensiadas

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UNIVERSIDADEFEDERALDO RIO GRANDE DO NORTE

UNIVERSIDADEFEDERAL DORIOGRANDE DO NORTE

CENTRO DETECNOLOGIA

PROGRAMA DEPÓS-GRADUAÇÃO EMENGENHARIAELÉTRICA E

DECOMPUTAÇÃO

Distributed Strategies for the Coexistence of

Wireless Broadband Access Networks in

Unlicensed Bands

Fuad Mousse Abinader Junior

Advisor: Prof. Dr. Adaildo Gomes D’Assunção

PhD Thesis presented to the Electrical and Computer Engineering Post-Graduation Program (PPgEEC) (concentration area: Telecommunications) from the Federal University of Rio Grande do Norte (UFRN) as part of the requirements for obtaining the title of Philosophy Doctor (PhD) in Sciences.

Número de ordem PPgEE: D150

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Abinader Junior, Fuad Mousse.

Estratégis distribuídas para a coexist encia de redes de banda larga sem fio em bandas não-licensiadas - Natal, RN, 2015

133 f : il

Orientador: Prof. Dr. Adaildo Gomes D’Assunção

Tese (doutorado) - Universidade Federal do Rio Grande do Norte. Centro de Tecnologia. Programa de Pós-Graduação em Engenharia Elétrica e de Computação.

1. Engenharia de comunicação - Tese. 2. Sistema de telecomunicação - Tese. 3. Banda larga - Tese. 4. Wi-Fi - Tese. 5. Sistemas distribuídos - Tese. 6. Coordenação de interferência - Tese. I. DŠAssunção, Adaildo Gomes. II. Sousa Junior, Vicente Angelo. III. Universidade Federal do Rio Grande do Norte. IV. Título.

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Distributed Strategies for the Coexistence of

Broadband Wireless Networks in Unlicensed

Bands

Fuad Mousse Abinader Junior

Tese de Doutorado aprovada em 21 de Agosto de 2015 pela banca examinadora composta pelos seguintes membros:

Prof. Dr. Adaildo Gomes D’Assunção (orientador) . . . DCO/UFRN

Prof. Dr. Vicente Angelo de Sousa Junior . . . DCO/UFRN

Prof. Dr. José Patrocinio da Silva . . . DEE/UFRN

Prof. Dr. André Noll Barreto . . . ENE/UnB

Prof. Dr. Tarcísio Ferreira Maciel . . . DETI/UFC

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Estratégias Distribuídas para a Coexistência de

Redes de Banda Larga Sem Fio em Bandas

Não-Licensiadas

Fuad Mousse Abinader Junior

Tese de Doutorado aprovada em 21 de Agosto de 2015 pela banca examinadora composta pelos seguintes membros:

Prof. Dr. Adaildo Gomes D’Assunção (orientador) . . . DCO/UFRN

Prof. Dr. Vicente Angelo de Sousa Junior . . . DCO/UFRN

Prof. Dr. José Patrocinio da Silva . . . DEE/UFRN

Prof. Dr. André Noll Barreto . . . ENE/UnB

Prof. Dr. Tarcísio Ferreira Maciel . . . DETI/UFC

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Acknowledgments

I would like to start thanking professor Adaildo D’assumpção, from Federal University of Rio Grande do Norte (UFRN), for all the trust, support and inspiration he has been providing me since I started my PhD, and believing that a Computer Science bachelor could do well on a Electrical Engineering post-graduation course. I would likewise to express my gratitude to the Amazonas State Research Support Foundation (FAPEAM), for the scholarship grant which allowed me to finance my first year at UFRN.

My gratefulness also go to David Gallegos and Robson Domingos, from Institute of Technological Development (INDT), for believing in my potential and allowing me to develop my research theme as part of my work activities. My great thanks also go to other three INDT co-workers forming the "Fabulous Four" quartet with me. Fabiano Chaves, Erika Almeida and André Cavalcante, you provided me with the best years I had so far in terms of learning and research experience. I would like to acknowledge researchers Sayantan Choudhury, Esa Tuomaala and Klaus Doppler from Nokia Research Center (NRC) in Berkeley (United States) for the pleasure to collaborate in this research and evolve as a researcher in the process.

Last, but not least, I would like to praise professor Vicente Souza Research Group on Fast Prototyping for Solutions in Communications (GppCom) at UFRN, who was not only a great co-advisor, but fundamentally a friend, who guided me and supported me beyond reasonable throughout the experience of doing a PhD, from the moment I decided to do it until its conclusion. I credit to Vicente the discovery of my passion for research.

Numerous other people helped me accomplishing this PhD work. This includes professors Luiz Gonzaga, Luis Felipe, Márcio and Cristhianne from GppCom at UFRN, classmates Alynne, Daniel, Aluísio and Leandro from Post-Gratuation Program on Electrical and Computer Engineering (PPgEEC) at UFRN, undergraduate students Rhenan, Yuri and Hélio from Telecommunications Department (DCO) at UFRN, and INDT co-workes Rafael Paiva, Felipe Consta and Angilberto Muniz.

On the personal side, I could not forget the help and support provided by my family. For my wife, Amire, who supported me on my decision all the way since the beginning until the conclusion, and specially for the sacrifice in the name of the conclusion of this work since our baby boy Fuad was born, my deep love, recognition and appreciation. Also, my parents Ilza and Fuad and my brothers Bruno and Bianca were always most supportive.

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Resumo

A crescente demanda por tráfego de dados em redes de acesso de banda larga sem fio à Internet requer tanto o desenvolvimento de novas tecnologias de acesso mais eficientes quanto a alocação de novas faixas de frequência do espectro eletromagnético para este fim. A introdução de um grande número de small cells em redes celulares aliado à adoção de forma complementar de tecnologias de Wireless Local Area Network (WLAN) em faixas de espectro não-licensiadas tem se verificado como um dos conceitos mais promissores. Uma das alternativas é a agregação de espectro não-licensiado Industrial, Science and Medical (ISM) de 5 GHz à bandas licensiadas, utilizando padrões de redes definidos tanto pelo Institute of Electrical and Electronics Engineers (IEEE) como pelo Third Generation Partnership Project (3GPP). Enquanto redes baseadas no padrão IEEE 802.11 (Wi-Fi) são agregadas à redes Long Term Evolution (LTE) via LTE / WLAN Aggregation (LWA), temos que em propostas como o Unlicensed LTE (LTE-U) e License-Assisted Access LTE (LAA-LTE) a própria interface aérea LTE é utilizada. A tecnologia Wi-Fi já se encontra bastante difundida e operando nessa faixa de espectro, o que pode acarretar problemas de desempenho derivados da coexistência com LTE na mesma faixa de espectro. Além disso, existe a necessidade de melhorar a operação do Wi-Fi para que possa suportar cenários com um grande número de redes vizinhas, cada uma com um grande número de nós (conhecido como "implantação densa"). É sabido que o desempenho global das redes Wi-Fi cai de forma acentuada com o aumento de número de nós compartilhando o canal, e portanto mecanismos para aumentar sua eficiência espectral se fazem necessários.

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é muito pouco afetado. Isso se dá porque nas redes LTE a transmissão ocorre o tempo todo de forma ininterrupta, enquanto nas redes Wi-Fi cada terminal deve primeiro escutar o canal pra se certificar de que o mesmo esteja livre antes de transmitir. Dessa forma, as transmissões LTE inibem o acesso ao canal para terminais Wi-Fi. Para viabilizar a coexistência entre redes LTE e Wi-Fi, foi concebido um arcabouço de coexistência onde a rede LTE aloca periodicamente recursos para a coexistência com redes Wi-Fi. Resultados de avaliação via simulação sistêmica indicam que o mecanismo garante que o desempenho das redes Wi-Fi melhore consideravelmente quando coexistindo com redes LTE, demonstrando-se assim uma alternativa viável para a coexistência de ambas as redes em espectro não-licensiado. O arcabouço de coexistência também contempla outros mecanismos, como preâmbulo de coexistência e controle de potência, devidamente descritos e indicados como direção de trabalhos futuros.

Já os resultados da campanha de simulação comparando o uso de diferentes mecanismos de acesso ao meio para redes Wi-Fi indicaram que o uso de Hybrid Controlled Channel Access (HCCA) e Power-Save Multi-Poll (PSMP) proporcionou ganhos de desempenho quando comparados com o uso do mecanismo padrão, Distributed Coordination Function (DCF). A principal causa desses ganhos é a diminuição de contenção via centralização das decisões de alocação no ponto de acesso, embora ainda haja espaço para melhorias se o problema da interferência de redes vizinhas for atacado. A partir do estudo desses resultados, foi concebido um arcabouço de coordenação distribuída de interferência entre redes Wi-Fi que possui três elementos: (a) mecanismo de sensoreamento de interferência de rede vizinha, (b) mecanismo de negociação de operação coordenada para mitigação de interferência de rede vizinha e suporte a dispositivos legados, e (c) mecanismo de operação livre de contenção com mitigação de interferência de rede vizinha. O desempenho do mecanismo proposto, chamado Mutual Exclusion Groups / Mutual Exclusion Operational Window (MEG/MEOW), foi avaliado via simulação sistêmica, e os resultados indicam um ganho substancial no desempenho do mecanismo proposto frente ao Enhanced Distributed Channel Access (EDCA). Os resultados indicam que a principal causa dos ganhos foi a diminuição da interferência média nos terminais que sofrem mais interferência das redes vizinhas, o que demonstra a eficácia do mecanismo proposto. Cenários não explorados e melhorias são apontados como trabalhos futuros.

Com base nos resultados numéricos das avaliações de desempenho via simulação sistêmica das soluções de coexistência tanto para o cenário com múltiplas tecnologias quanto com múltiplas redes da mesma tecnologia, é possível afirmar que as soluções concebidas proporcionam ganhos significativos ante uma situação de coexistência sem coordenação distribuída. Desta forma, este trabalho contribuiu significativamente por meio da identificação e validação de novas direções a serem exploradas para resolver o problema da coexistência entre redes usando espectro não-licensiado.

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Abstract

The increasing demand for Internet data traffic in wireless broadband access networks requires both the development of efficient, novel wireless broadband access technologies and the allocation of new spectrum bands for that purpose. The introduction of a great number of small cells in cellular networks allied to the complimentary adoption of Wireless Local Area Network (WLAN) technologies in unlicensed spectrum is one of the most promising concepts to attend this demand. One alternative is the aggregation of Industrial, Science and Medical (ISM) unlicensed spectrum to licensed bands, using wireless networks defined by Institute of Electrical and Electronics Engineers (IEEE) and Third Generation Partnership Project (3GPP). While IEEE 802.11 (Wi-Fi) networks are aggregated to Long Term Evolution (LTE) small cells via LTE / WLAN Aggregation (LWA), in proposals like Unlicensed LTE (LTE-U) and LWA the LTE air interface itself is used for transmission on the unlicensed band. Wi-Fi technology is widespread and operates in the same 5 GHz ISM spectrum bands as the LTE proposals, which may bring performance decrease due to the coexistence of both technologies in the same spectrum bands. Besides, there is the need to improve Wi-Fi operation to support scenarios with a large number of neighbor Overlapping Basic Subscriber Set (OBSS) networks, with a large number of Wi-Fi nodes (i.e. dense deployments). It is long known that the overall Wi-Fi performance falls sharply with the increase of Wi-Fi nodes sharing the channel, therefore there is the need for introducing mechanisms to increase its spectral efficiency. This work is dedicated to the study of coexistence between different wireless broadband access systems operating in the same unlicensed spectrum bands, and how to solve the coexistence problems via distributed coordination mechanisms. The problem of coexistence between different networks (i.e. LTE and Wi-Fi) and the problem of coexistence between different networks of the same technology (i.e. multiple Wi-Fi OBSSs) is analyzed both qualitatively and quantitatively via system-level simulations, and the main issues to be faced are identified from these results. From that, distributed coordination mechanisms are proposed and evaluated via system-level simulations, both for the inter-technology coexistence problem and intra-technology coexistence problem. Results indicate that the proposed solutions provide significant gains when compare to the situation without distributed coordination.

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List of Figures

1.1 Global Growth of Devices and Traffic. . . 1

1.2 Taxonomy of the Coexistence of Wireless Broadband Access Networks. . 3

1.3 Overview and dependency between chapters, publications and research questions. . . 10

2.1 5 GHz spectrum allocations in Europe. . . 13

2.2 Existing and proposed Federal Communications Commission (FCC) part 15 rules for 5 GHz unlicensed spectrum. . . 13

2.3 Wi-Fi Clear Channel Access (CCA) and backoff procedures. . . 16

2.4 Wi-Fi Request-To-Send (RTS)/Clear-To-Send (CTS) exchange for hidden node protection. . . 17

2.5 Wi-Fi Enhanced Distributed Channel Access (EDCA) procedure with Transmission Opportunity (TXOP) and Block ACK Request (BAR)/Block ACK (BA). . . 17

2.6 Wi-Fi Hybrid Controlled Channel Access (HCCA) allocations. . . 18

2.7 Wi-Fi Power-Save Multi-Poll (PSMP) burst. . . 19

2.8 LTE Resource Block (RB) structure. . . 22

2.9 Unlicensed Spectrum LTE Secondary Serving Cell (SCell) Aggregation. . 24

2.10 LTE Listen-Before-Talk (LBT) operation. . . 25

2.11 LTE-U "duty-cicle" operation. . . 25

2.12 Illustrative scenario with three OBSSs. . . 28

2.13 Mean user throughput performance for single-floor scenario. . . 30

2.14 10thpercentile throughput performance for single-floor scenario. . . 31

2.15 Block Error Rate (BLER) for single-floor scenarios. . . 32

2.16 Percentage of time at Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) states for single-floor scenarios. . . 33

2.17 Mean user throughput performance for multi-floor dense deployments. . . 33

2.18 User throughput performance for single-floor scenario. . . 37

2.19 CSMA/CA states usage for single-floor scenario. . . 38

2.20 Signal over Noise plus Interference Ratio (SINR) performance. . . 39

2.21 Cumulative Distribution Function (CDF) of the Uplink (UL) SINR. . . . 39

2.22 User throughput performance for multi-floor scenario. . . 40

3.1 Generalized Collaborative Coexistence Algorithm. . . 44

3.2 Example of subframe allocation considering different coexistence times. . 48

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Stations (STAs) per technology in a single-floor scenario, and for different

LTE Blank Subframe configurations. . . 52

3.5 Wi-Fi performance gains and LTE performance decrease with different LTE Blank Subframe configurations for the single-floor 10 APs, 10 STAs scenario. . . 54

3.6 Time at Wi-Fi ACTIVE and CCA states, as a function of the LTE Blank Subframe patterns, for the single-floor 10 APs/ 10 STAs scenario. . . 54

3.7 Wi-Fi performance gains and LTE performance decrease with different LTE Blank Subframe configurations for the single-floor 10 APs, 25 STAs scenario. . . 55

3.8 Wi-Fi performance gains and LTE performance decrease with different LTE Blank Subframe configurations for the single-floor 4 APs, 10 STAs and 4 APs, 25 STAs scenarios. . . 56

3.9 Multi-floor results with different configurations of LTE Blank Subframes. 57 4.1 Proposed OBSS-coordinated contention-free operation. . . 65

4.2 Proposed Bulk OBSS Frame Request frame. . . 69

4.3 Example of an OBSS Sensing procedure. . . 70

4.4 CF Legacy Support IE. . . 74

4.5 Scenario with three APs in a "hidden node" configuration. . . 75

4.6 Synchronization between three OBSSs. . . 76

4.7 Mutual Exclusion Group (MEG) Frame format. . . . 78

4.8 Example of a Mutual Exclusion Groups / Mutual Exclusion Operational Window (MEG/MEOW) scenario. . . 80

4.9 Mutual Exclusion Operational Window (MEOW) Scheduling example. . . 81

4.10 MEG/MEOW Coordination example. . . 82

4.11 MEG/MEOW mean and 5th-percentile throughput gains over EDCA. . . . 89

4.12 Downlink (DL) and UL MEOW Blockage Rate. . . 90

5.1 Unexplored Research Directions on Inter-Technology Coexistence. . . 103

5.2 Unexplored Research Directions on Intra-Technology Coexistence. . . 104

A.1 Time and frequency granularity for LTE and Wi-Fi in SUPO. . . 120

A.2 LTE/Wi-Fi mutual interference calculation in SUPO. . . 121

A.3 Single-floor/multi-room indoor scenario composed of 2 rows of 10 rooms, each measuring 10m x 10m x 3m. . . 123

A.4 Multi-floor/multi-room indoor scenario composed of 5 floors, each with 2 rows of 10 rooms measuring 10m x 10m x 3m. . . 124

A.5 Received signal strength using IEEE 802.11ac Task Group (TGac) channel model B for unlicensed bands. . . 125

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List of Tables

2.1 FCC § 15.247 rules for 5 GHz operation in United States. . . 14

2.2 Regulatory Restrictions in the 5 GHz band in Brazil. . . 15

3.1 Taxonomy for LTE/Wi-Fi Coexistence Enablers. . . 45

3.2 Comparison between the proposed mechanism, License-Assisted Access LTE (LAA-LTE) and LTE-U. . . 61

4.1 Mean and 5th-percentile of throughput for EDCA and MEG/MEOW [Mbps]. . . 88

4.2 Performance comparison between EDCA and the best MEG/MEOW configuration for the proposed scenario, i.e. SINR target of 20dB and a maximum of 4 OBSS STAs per AP. . . 91

A.1 Deployment Scenario. . . 122

A.2 Channel Models. . . 124

A.3 LTE Physical (PHY)/Media Access Control (MAC) Parameters. . . 126

A.4 Wi-Fi PHY/MAC Parameters. . . 127

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List of Acronyms

11aa IEEE 802.11aa 11ac IEEE 802.11ac 11n IEEE 802.11-2007 3G Third Generation

3GPP Third Generation Partnership Project 4G Fourth Generation

5G Fifth Generation

A-MPDU Aggregated MPDU A-MSDU Aggregated MSDU ABS Almost Blank Subframe ACK Acknowledgement AP Access Point

APLA A Priori Link Adaptation APSD Automatic Power Save Delivery ASA Authorized Shared Access AttC Attendance Call

AttR Attendance Reply

AWGN Additive White Gaussian Noise

BA Block ACK

BAR Block ACK Request BLER Block Error Rate

BOFR Bulk OBSS Frame Request

BRAN Broadband Radio Access Networks BS Base Station

BSS Basic Subscriber Set

BSSID Basic Subscriber Set Identifier

BW Bandwidth

CAP Controlled Access Period CCA Clear Channel Access

CDF Cumulative Distribution Function

CEPT European Conference of Postal and Telecommunications Andministrations

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cmW centimeter-wave

Coex TG Coexistence Task Group

CoMP Coordinated Multipoint Transmission and Reception

CP Cyclic Prefix

CQI Channel Quality Indicator

CS Carrier Sense

CSI Channel State Indicator

CSMA/CA Carrier Sense Multiple Access with Collision Avoidance

CTS Clear-To-Send

CW Contention Window

CxBeacon Coexistence Beacon

DCF Distributed Coordination Function DFS Dynamic Frequency Selection DIFS DCF Inter Frame Space

DL Downlink

DSA Dynamic Spectrum Access

E-UTRAN Evolved UMTS Terrestrial Radio Access Network

ECC European Electronic Communications

Committee ED Energy Detection

EDCA Enhanced Distributed Channel Access

eICIC Enhanced inter-Cell Interference Coordination EIRP Effective Isotropic Radiated Power

EMF Electromotive Force eNodeB Enhanced Node B EPC Evolved Packet Core

ETSI European Telecommunications Standards Institute

FBE Frame-Based Equipment

FCC Federal Communications Commission FDD Frequency-Division Duplex

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FSA Flexible Spectrum Access

FWBA fixed Wireless Broadband Access

GRAP Group Randomly Addressed Polling

HARQ Hybrid Automatic Repeat Request HCCA Hybrid Controlled Channel Access HCF Hybrid Coordinator Function

HEW High Efficiency WLAN

HT Enhancements for High Throughput

IE Information Element

IEEE Institute of Electrical and Electronics Engineers IEEE-SA IEEE Standards Association

IMT-Advanced International Mobile Telecommunications-Advanced

ISM Industrial, Science and Medical ITS Intelligent Transport Systems ITU-R ITU Radiocommunication Sector

KPI Key Performance Indicator

LA Link Adaptation

LAA-LTE License-Assisted Access LTE

LBE Load-Based Equipment

LBT Listen-Before-Talk

LCCS Least-Congested Channel Search

LOS Line-of-Sight

LP Legacy Period

LTE Long Term Evolution

LTE-A LTE Advanced LTE-H LTE-Hetnet LTE-U Unlicensed LTE

LWA LTE / WLAN Aggregation

MAC Media Access Control

Mbps Megabit per second

MCS Modulation and Coding Scheme MEG Mutual Exclusion Group

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mmW milimeter-wave

MPDU MAC Protocol Data Unit MU-MIMO Multi-User MIMO

NAV Network Allocation Vector NLOS Non Line-of-Sight

NTIA U.S. National Telecom & Information Administration

O-DCF Optimal DCF

OBSS Overlapping Basic Subscriber Set

OFDM Orthogonal Frequency Division Multiplexing OFDMA Orthogonal Frequency-Division Multiple Access OFR-TTD OBSS Frame Report Transmission Time

Duration

PBCH Physical Broadcast Channel PCell Primary Serving Cell

PDCP Packet Data Convergence Protocol PF Proportional Fair

PHY Physical

PIFS PCF Interframe Space

PLCP Physical Layer Convergence Protocol PRB Physical Resource Block

PSD Power Spectral Density PSMP Power-Save Multi-Poll

PSMP-DTT PSMP DL Transmission Time PSMP-UTT PSMP UL Transmission Time PSS Primary Synchronization Signal

PU Primary User

QoS Quality-of-Service

RA Receiver Address

RAN2 Radio Layer 2

RB Resource Block

RE Resource Element

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RLC Radio Link Control

RM Radio Measurement

RRM Radio Resource Management RS Reference Signal

RTS Request-To-Send

Rx Receive

SC-FDMA Single-Carrier Frequency-Division Multiple Access

SCell Secondary Serving Cell SIFS Short Inter Frame Space

SINR Signal over Noise plus Interference Ratio SISO Single-Input, Single-Output

SON Self Organizing Networks

SSS Secondary Synchronization Signal

STA Station

SU Secondary User

TA Transmitter Address TDD Time-Division Duplex

TDWR Terminal Doppler Weather Radar TGac IEEE 802.11ac Task Group TGax IEEE 802.11ax Task Group TPC Transmission Power Control

TSG R1 Specification Group Radio Access Network TTI Transmission Time Interval

TVWS TV Whitespaces

Tx Transmit

TXOP Transmission Opportunity

U-NII Unlicensed National Information Infrastructure UDN Ultra Dense Network

UE User Equipment terminal

UL Uplink

VoIP Voice-over-IP

WAS Wireless Access Systems WFA Wi-Fi Alliance

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Chapter 1 Introduction

Over the last two decades, mobility in devices accessing the Internet has changed. Devices used to be "static" (e.g. desktop computers), but technological advances, economies of scale and market demands made devices to become "nomadic" (e.g. laptops), and finally "mobile" (e.g. smartphones). Likewise, wireless broadband access technologies such as Wireless Local Area Network (WLAN) and Fourth Generation (4G) data cellular networks ascended as the primary Internet connectivity enabler for mobile devices, in a trend that will continue for the coming years. According to a recent mobile Internet connectivity forecast report [Cisco 2015], global mobile data volume grew by 69% solely in 2014 (i.e. from 1.5 exabytes to 2.5 exabytes), and is expected to expand tenfold by 2019 (i.e. ~24.3 exabytes). Also, the traffic distribution profile has been changing, as the so-called "smart" devices and connections will increase from 26% in 2014 to up to 59% of the total of connections and devices by 2019. Additionally, the top 1% of mobile data subscribers generated 18% of mobile data traffic, down from 52% at the beginning of 2010, and the tendency is that this distribution becomes even "flatter", i.e. not only the demand for wireless Internet is growing, but also is becoming less concentrated in "power users". Figure 1.1 summarizes such growth forecasts for both devices and traffic.

Figure 1.1: Global Growth of Devices and Traffic.

(a) Data Traffic Forecast by Region

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There are some challenges for attending such growth in mobile data traffic demand. With wired technologies (e.g. Ethernet and optical fiber), it is always possible to lay new cables if the demand increases. In contrast, wireless technologies face the scarcity of available radio spectrum as an impeding factor. Wireless signal propagation effects (i.e. pathloss, fading and multipath) are far more challenging than those faced in wired signal propagation. Hence, the volume and scale of the demand for wireless Internet connectivity allied to physical and economical challenges for expanding wireless connectivity put pressure on operators and equipment manufacturers for providing technological solutions to attend such demands with reasonable costs [iGillottResearch 2014].

Increasing the spectrum efficiency of wireless technologies is one possibility to overcome these challenges. With the use of channel coders like "turbo coder", single-link capacity in Additive White Gaussian Noise (AWGN) channels is near to the Shannon limit [Dohler et al. 2011]. Orthogonal Frequency-Division Multiple Access (OFDMA) techniques allowed making an efficient usage of available spectrum for transmitting multiple data streams for multiple users at the same time. Hence, further improvements are expected to come from the use of an increasing number of transmitter and receiver antennas for implementing Multiple-Input, Multiple-Output (MIMO) techniques, and making technologically and economically viable to explore spectrum portions at centimeter-wave (cmW) and milimeter-wave (mmW) frontiers.

An important "topological" trend among operators worldwide is switching traffic from the macro cells to a large number of smaller cells, which allows both cost reduction and capacity increase [Markendahl and Makitalo 2010, Frias and Perez 2012]. According to [Cisco 2015], 46% of total mobile data traffic was already offloaded from macro cells to smaller cells in 2014, with a forecast of this becoming up to 62% by 2019. This means an increment on network deployments of the IEEE solution for WLAN, known as Wi-Fi, which operates at both 2.4 GHz and 5 GHz ISM bands in unlicensed spectrum, in special the latest high-performance IEEE 802.11ac standard [IEEE 2013]. From the 3GPP LTE cellular networks perspective, this means an increasing focus on the adoption of micro-, femto-, pico- and small cells utilizing licensed spectrum.

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1.1. PROBLEM TAXONOMY 3

In order to enable the continued growth of mobile traffic offloading deployments in unlicensed spectrum, understanding the potential problems that Wi-Fi and LTE may face when coexisting becomes of great relevance. This thesis aims at contributing for enabling the coexistence of heterogeneous wireless broadband access networks in unlicensed spectrum by the means of both qualitative and quantitative analysis of coexistence issues, and the conception and evaluation of a number of distributed mechanisms for enabling the efficient coexistence of heterogeneous networks.

The rest of this chapter is as follows. Section 1.1 presents a brief taxonomy of the problems studied in this PhD thesis. Section 1.2 presents the research questions guiding our investigations. Then, Section 1.3 presents the thesis outline, describing chapters and indicating where research questions are addressed. In Section 1.4 we list the main tangible products of this thesis, i.e. congress and journal papers and applied patents. Finally, in Section 1.5 we provide an overview on the thesis structure.

1.1 Problem Taxonomy

In this section, a brief taxonomy of the problem of coexistence of wireless broadband access networks is provided. The objective is to allow the reader to understand the general scope and goals of this thesis, detailed in the following sections. Figure 1.2 summarizes such taxonomy, with the selected problems for this thesis in highlight.

Figure 1.2: Taxonomy of the Coexistence of Wireless Broadband Access Networks.

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Source: the author.

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Another dimension to classify this problem is regarding the diversity of networks coexisting in unlicensed spectrum. According to the classification used for the purpose of this thesis, coexisting networks may be classified either as homogeneous or heterogeneous, depending on the administrative domains involved. Homogeneous networks are necessarily sharing the same backhaul infrastructure. As such, coexistence mechanisms may be implemented via out-of-bound communication channels. As for heterogeneous networks, they do not share a common out-of-bound communication channel for coexistence coordination. This means that some form of inbound coexistence coordination needs to be implemented in the same communication channel where data is exchanged with mobile terminals (i.e. over-the-air). Overall, homogeneous networks using out-of-bound communication channels allow increasing spectrum efficiency as the data communication channel is not used for coexistence coordination as it is heterogeneous networks using inbound communication for coordination. However, in contrast homogeneous networks introduce a dependency on a shared backhaul infrastructure for the coexistence mechanism to work, which benefits heterogeneous networks relying only on the data communication channel itself.

An additional dimension for classifying the issue of coexistence is regarding the technologies of the coexisting networks, which influence the nature of the coexisting mechanisms. According to the classification used for the purpose of this thesis, the coexisting networks are either from the same technology, i.e. intra-technology coexistence, or from different technologies, i.e. inter-technology coexistence. The main difference between the two forms of coexistence mechanisms is how the coordination is performed among the networks. Devices using the same technology may very likely exchange signaling messages among themselves, even though they may be from different administrative domains. As such, Intra-technology coexistence mechanisms may rely on a simple signaling exchange for performing the necessary steps for coordination. In contrast, differences in terms of modulation techniques, symbol time, sampling rate, subcarrier bandwidth, synchronization and others may limit the possibilities for networks from different technologies to exchange signaling messages directly. This makes inter-technology coexistence more challenging than intra-technology coexistence.

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1.2. RESEARCH QUESTIONS 5

1.2 Research Questions

According to the taxonomy classification in the previous section, the scope of this thesis includes: (i) to study the coexistence of heterogeneous wireless broadband access networks in unlicensed spectrum for both intra-technology and inter-technology; and (ii) to propose and evaluate distributed solution mechanisms. The author claims this scope defines a realistic set of deployment scenarios for the foreseen future, and introduces an interesting set of open research questions to be answered on the course of its PhD thesis.

Specifically, the objectives of this PhD thesis are (a) characterizing the main problems with the coexistence of wireless broadband access networks in 5 GHz ISM unlicensed bands, and (b) proposing and evaluating solutions for solving such problems for networks using different technologies (i.e. LTE/Wi-Fi coexistence) and single technology (i.e. multiple Wi-Fi OBSSs coexistence). Moreover, the thesis’ focus is on developing solutions that explore distributed coexistence mechanisms. By achieving those objectives, we claim this PhD thesis shall provide significant contributions to the challenge of attending the increasing demand for mobile traffic.

Based on the defined scope and objectives, a number of research questions were identified. These questions guided the development of this PhD thesis, and are listed below:

RQ #1: What is the state-of-the-art in coexistence of wireless broadband access networks in unlicensed bands?

RQ #2: Are there any performance issues for the coexistence between networks using different technologies, e.g. LTE and Wi-Fi, in unlicensed bands?

1. What is the scale of performance penalties for both networks?

2. Which technology is the most affected in terms of performance penalties?

RQ #3: What can be done to improve inter-technology coexistence performance?

1. Is there a distributed method for improving inter-technology coexistence performance?

RQ #4: Are there any performance issues for the coexistence between networks of the same technology, e.g. Wi-Fi, in unlicensed bands?

1. What is the scale of performance penalties for both networks?

RQ #5: What can be done to improve intra-technology coexistence performance?

1. Is there a distributed method for improving intra-technology coexistence performance?

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1.3 Thesis Outline

By providing answers to the research questions listed in the previous section, this thesis aims to contribute with the development of efficient ways to allow the coexistence of heterogeneous wireless broadband area networks in unlicensed spectrum. In order to answer the research questions in structured and organized way, we list in this section the thesis’ chapters, indicating their objectives as well as the research questions they are aiming at providing with answers.

Chapter 2: Coexistence in Unlicensed Bands

This chapter provides an explanation for the relevance of unlicensed spectrum for sustaining future mobile traffic expansion, as well as an overview of (a) the regulatory situation of unlicensed spectrum worldwide, (b) the wireless broadband access networks that are envisioned to operate on such spectrum bands, and (c) the problems that may derive from their coexistence in the same spectrum, identified via both qualitative and quantitative characterizations, for both inter-technology coexistence (i.e. LTE and wifi) as well as intra-technology coexistence (i.e. multiple neighbor Wi-Fi networks). Chapter goal is to answer research questions RQ#1, RQ#2 and RQ#4.

Chapter 3: Inter-Technology Coexistence Solutions

This chapter discusses the different types of solutions that may be applied for allowing the coexistence of different technologies in the same unlicensed spectrum, and provides the detailed description of a novel coexistence mechanism devised for allowing inter-technology coexistence between LTE and Wi-Fi in a distributed manner. Numerical performance evaluation results are also provided for indoor dense deployment scenarios, and future research directions not explored in the course of this thesis are identified. Chapter goal is to answer research question RQ#3, as well as provide partial answers to RQ#6 in respect to the inter-technology coexistence problem.

Chapter 4: Intra-Technology Coexistence Solutions

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1.4. THESIS PUBLICATIONS 7

scale of performance gains, its limitations and future research directions not explored in the course of this thesis. Our goal with this chapter is to provide an answer to research question RQ#5, as well as provide partial answers to research question RQ#6 in respect to the intra-technology coexistence problem.

Chapter 5: Conclusions and Final Remarks

This chapter reviews the main thesis achievements in terms of issues identified in the coexistence of heterogeneous wireless broadband access networks operating in unlicensed spectrum, as well as the performance gains and limitations of the proposed distributed solutions. This chapter also presents all future work proposals into a structured taxonomy, providing concrete future research guidelines for the continuation of such studies. Another goal of this chapter is to summarize the answers to research question RQ#6.

Appendix A: LTE/Wi-Fi System Level Simulator

This chapter describes the system-level simulation tool utilized for evaluation of issues and solutions proposed for inter-technology and intra-technology coexistence in the unlicensed spectrum. A description of simulation architecture, assumptions and how simulation results are calculated is provided. The goal of this chapter is to provide the necessary guidance for acknowledging confidence on the simulation results reported in previous chapters.

Appendix B: Parallel Bibliographic Production

This chapter provides a listing of parallel bibliographic productions in terms of scientific articles, patent applications and presentations which were conducted by the thesis author during the course of this PhD thesis. These bibliographic productions are not necessarily related to the specific topics covered by this PhD thesis, but allowed the author to acquire a broader knowledge in the Telecommunications field. The goal of this chapter is to allow understanding the other topics covered by the author during the pursuit of his PhD.

1.4 Thesis Publications

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To start, a number of conference papers were published in the course of the development of this thesis, containing performance evaluation for intra-technology and inter-technology coexistence scenarios and solutions, and are listed below:

CP #1: In "Performance Evaluation of LTE and Wi-Fi Coexistence in Unlicensed Bands" [Cavalcante et al. 2013], we evaluated the performance of LTE and Wi-Fi when coexisting in unlicensed spectrum, and identified major Wi-Fi performance penalties. The main conclusion was that LTE transmissions "killed" Wi-Fi performance due to the CCA mechanism implemented within the CSMA/CA method on Wi-Fi STAs and APs.

CP #2: In "Enabling LTE/Wi-Fi coexistence by LTE blank subframe allocation" [Almeida et al. 2013], we introduced one coexistence solution for LTE/Wi-Fi coexistence which extends LTE’s Almost Blank Subframe (ABS) concept for inter-technology coexistence, and presented initial performance results. Main results indicate that by making LTE rescinding from transmission on a few "blank subframes", Wi-Fi performace improves by a large factor.

CP #3: In "LTE/Wi-Fi Coexistence: Challenges and Mechanisms" [Chaves, Cavalcante, Abinader Jr. and others 2013], we reviewed the main challenges identified on LTE/Wi-Fi coexistence, and present an overview on the solutions being devised for overcoming such challenges.

CP #4: In "Performance Evaluation of IEEE 802.11n WLAN in Dense Deployment Scenarios" [Abinader Jr., Almeida, Choudhury and others 2014], we provided a study on the performance of the two main Wi-Fi mechanisms for increasing performance, namely HCCA and PSMP, in the presence of a dense deployment scenario, i.e. with a large number of APs and STAs in the neighbor OBSSs.

Then, as a means to consolidate the works performed on the two "fronts" (i.e. inter-technology and intra-technology coexistence), we published one journal paper on the inter-technology coexistence issue and proposed solution, and submitted another journal paper on the intra-technology coexistence issue and proposed solution, both of which are listed below:

JP #1: In "Enabling the coexistence of LTE/Wi-Fi in unlicensed bands" [Abinader Jr., Almeida, Chaves and others 2014], we put together the results of all inter-technology studies we conducted, including the proposal of an overall coexistence framework which includes inter-technology detection, negotiation and actual coexistence, and reviewed the coexistence solutions we provided.

JP #2: In "Distributed Interference Coordination for Contention-Free Operation in Dense Deployment Scenarios" [Abinader Jr. et al. 2015, under review], we present the performance problems of Wi-Fi operation in dense deployment scenarios, propose a comprehensive solution named MEG/MEOW for distributed interference coordination and evaluate its performance on an indoor dense deployment scenario.

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1.5. THESIS OVERVIEW 9

submitted that cover specific aspects of the solutions conceived for the problems of coexistence in unlicensed spectrum, and are listed below:

PA #1: In "Null subframe indication for coexistence between different network types" [Kim et al. 2014], we provided a method for LTE/Wi-Fi coexistence extending ABS to a "blank subframe" where Wi-Fi nodes can transmit while LTE nodes stay "silenced". We also provided a method for easing inter-technology coexistence via the transmission of "special preambles" that LTE base station sends indicating to Wi-Fi nodes that the proposed mechanism is in place.

PA #2: In "Interference avoidance and power savings for coexistence among different radio access technologies" [Abinader Jr., Almeida, Domingos and others 2014], we provide a method for increasing Wi-Fi efficiency in re-using "null subframes" allocated by the LTE network, where the AP determines whether each LTE subframe is available for Wi-Fi usage or not. If so, it indicates to its associated STAs that the channel is available to Wi-Fi usage for a certain number of LTE subframes. We also provide a method for optionally allowing the AP to check with a subset of the associated STAs whether there is any STA that has detected LTE transmissions, as a means to enhance efficiency.

PA #3: In "Performing Measurements in Wireless Network" [Abinader Jr., Chaves, Almeida and others 2015], we provided a method for performing OBSS sensing efficiently for dense deployment scenarios, thus allowing the AP to assemble an interference matrix denoting the level of the interference received by each associated STA from neighbor OBSS STAs.

PA #4: In "Interference Avoidance between Overlapping Wireless Networks" [Abinader Jr., Choudhury, Chaves and others 2015], we provided a method for performing distributed interference coordination among neighbor networks such that although all neighbor networks transmit at the same time in synchronized window opportunities, highly-interfering terminals from neighbor networks are avoided to transmit at the same time.

PA #5: In "Improving Communication Efficiency" [Almeida et al. 2015], we provided a method for performing distributed coordination for negotiating and implementing synchronized CFPs among supporting OBSSs, as well as providing support for "legacy" OBSS networks coexisting in the neighborhood.

1.5 Thesis Overview

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Figure 1.3: Overview and dependency between chapters, publications and research questions.

Chapter 2 –Coexistence of Heterogeneous Wireless Broadband Access Networks in Unlicensed Bands

Chapter 5 –Conclusions and Final Remarks RQ #1

RQ #2 RQ #4

RQ #3 RQ #6 (partial)

RQ #5 RQ #6 (partial)

RQ #6

CP #2: Performance Evaluation of IEEE 802.11n WLAN in Dense Deployment Scenarios CP #1: Performance Evaluation of LTE and Wi-Fi Coexistence in Unlicensed Bands

Chapter 3 –Solutions for Inter-Technology Coexistence in Unlicensed Bands

CP #3: Enabling LTE/WiFi coexistence by LTE blank subframe allocation

CP #4: LTE/Wi-Fi Coexistence: Challenges and Mechanisms

JP #1: Enabling the coexistence of LTE and Wi-Fi in unlicensed bands

PA #1: Null subframe indication for coexistence between different network types

PA #2: Interference avoidance and power savings for coexistence among different radio access technologies

Chapter 4 –Solutions for Intra-Technology Coexistence in Unlicensed Bands

PA #3: Performing Measurements in Wireless Network

PA #4: Interference Avoidance between Overlapping Wireless Networks

PA #5: Improving Communication Efficiency

JP #2: Distributed Interference Coordination for Contention-Free Operation in Dense Deployment Scenarios

Conference Paper (CP) Journal Paper (CP) Pattent Application (PA) Journal Paper (CP)

under review

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Chapter 2 Coexistence in Unlicensed Bands

The increasing adoption of smartphones as the primary Internet access device creates a demand for spectrum bandwidth to be used by wireless broadband access technologies. In order to handle this demand, both the allocation of more spectrum for such applications as well as novel techniques for making efficient spectrum usage will be required.

Modern wireless broadband access technologies like LTE [Sesia et al. 2011] and Wi-Fi [Perahia and Stacey 2008] already aim at making an efficient use of the available spectrum. However, available spectrum is a finite resource, though some researchers long advocate that spectrum scarcity is not necessarily an issue due to available technologies like relaying, Orthogonal Frequency Division Multiplexing (OFDM) and space-time coding which allow increasing spectrum usage efficiency [Staple and Werbach 2004]. Nevertheless, spectrum scarcity is already a serious issue faced by operators worldwide [Deloitte 2012]. Some analysts believe that spectrum refarming (i.e. reuse of under-utilized spectrum) is one solution to this problem [Stefanski 2012], but even if refarming is implemented in large scale, it will be expensive and will take time to be implemented in a worldwide scale. For instance, the U.S. National Telecom & Information Administration (NTIA) reported recently that the refarming of the 1755-1850 MHz band in United States would take 10 years to be implemented and would demand around 18 billion dollars [NIST 2012].

Many mobile operators worldwide already foresee the reuse of portions of unlicensed spectrum for offloading mobile traffic from macro cells operating in licensed spectrum, in special the ISM 2.4 GHz and 5 GHz bands [Al-Dulaimi et al. 2015a]. As such, understanding the main issues regarding coexistence of heterogeneous wireless broadband access networks in unlicensed spectrum is of uttermost relevance for sustaining future mobile traffic expansion. This chapter investigates these issues, providing both quantitative and qualitative indications of their effects and causes.

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2.1 ISM Unlicensed Spectrum Worldwide

Regulatory agencies like FCC in the United States or European Telecommunications Standards Institute (ETSI) in Europe allocate spectrum for wireless networks into two classes of spectrum bands. Licensed spectrum are reserved for organizations that have been granted exclusive license rights, where license holders may expect to operate without interference. Legal protection and enforcement of exclusive usage rights over the same frequency in the same geographic area are also provided. Unlicensed spectrum are designated for usage by an unspecified number of organizations and individuals, without exclusive usage rights. Different types of requirements are imposed for operation in such bands, including maximum allowed transmitted power, Dynamic Frequency Selection (DFS), Transmission Power Control (TPC), coexistence rules, out-of-band emission limits, among others.

One example of unlicensed spectrum are the ISM bands at 2.4 GHz and 5 GHz, where technologies like Wi-Fi and Bluetooth are used for wireless broadband access. Another example is the reuse of TV channels not used by licensed services, also known as Primary Users (PUs), at a particular location and time, referred to as TV Whitespaces (TVWS). Unlicensed wireless devices, also known as Secondary Users (SUs), may use TVWS spectrum portions provided they not interfere with PUs.

ISM spectrum in 2.4 GHz is already crowded by many different kind of services, like microwave ovens, digital voice and video communicators, and different wireless network technologies. On the other hand, the 5 GHz ISM spectrum is not only wider but also less crowded, with mainly Wi-Fi networks utilizing it. However, 3GPP started investigating the introduction of a standard for LTE for complimentary carrier aggregation of 5 GHz ISM unlicensed spectrum, known as LAA-LTE [3GPP 2015]. Understanding the state of 5 GHz ISM regulatory allocation in key markets is very important, and as such the following subsections detail allocations in Europe, United States and Brazil, respectively.

2.1.1 5 GHz ISM band in Europe

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2.1. ISM UNLICENSED SPECTRUM WORLDWIDE 13

Figure 2.1: 5 GHz spectrum allocations in Europe.

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Source: Figure 4.1.1-1 in [3GPP 2015].

The European Commission has recently submitted to European Conference of Postal and Telecommunications Andministrations (CEPT) a mandate to study the conditions for the extension of the 5 GHz range designated for WAS/RLANs [RSCOM 2013] in order to allow the use by WAS/RLANs of the whole 5150-5925 mHz band.

TPC is a mechanism to be used by the RLAN device to ensure a mitigation factor of at least 3 dB on the aggregate power from a large number of devices. ETSI also mandates the usage of DFS in some bands, while a LBT mechanism is requested independently of whether the channel is occupied or not. Both the requirements are valid for Frame-Based Equipment (FBE) and Load-Based Equipment (LBE). LBT is similar to the CCA procedure in Wi-Fi, but is under review and is expected to be defined by the end of 2015. No LBT requirement is requested for the FWBA band.

2.1.2 5 GHz ISM band in United States

The use of unlicensed 5 GHz spectrum in United States is governed by FCC part 15 regulations [FCC 2015]. In February 2013, potential new rules were proposed in FCC 13-22 [FCC 2013]. Figure 2.2 summarizes the relevant part 15 rules for 5 GHz unlicensed spectrum usage, where U-NII-x bands denote frequency bands for Unlicensed National Information Infrastructure (U-NII) devices usage that are governed by § 15.407 of FCC [FCC 2015]. It can be seen that there is also an overlapping ruling of § 15.247 from 5.725 to 5.85 GHz. A device could choose to follow either U-NII rulings or § 15.247 rulings when operating within the frequency range.

Figure 2.2: Existing and proposed FCC part 15 rules for 5 GHz unlicensed spectrum.

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Table 2.1 specifies the rules in § 15.247 for point-to-multipoint communications.

Table 2.1: FCC § 15.247 rules for 5 GHz operation in United States.

Rule Definition

Transmission Bandwidth 500 kHz for minimum dB bandwidth

Maximum Transmit Power Peak conducted output power shall not exceed 1 W for antennas with directional gains that do not exceed 6 dBi. If directional gains are greater than 6 dBi, output power shall be reduced by the amount in dB that the directional gain of the antenna exceeds 6 dBi.

Out-of-Band Emission In any 100 kHz bandwidth outside the frequency band, the radio frequency power that is produced by the intentional radiator shall be at least 20 dB below that in the 100 kHz bandwidth within the band that contains the highest level of the desired power, based on either an RF conducted or a radiated measurement. If the transmitter complies with the conducted power limits based on the use of RMS averaging over a time interval, the attenuation required under this paragraph shall be 30 dB instead of 20 dB. Power Spectral Density

(PSD)

The power spectral density conducted from the intentional radiator to the antenna shall not be greater than 8 dBm in any 3 kHz band during any time interval of continuous transmission. The same method of determining the conducted output power shall be used to determine the power spectral density.

Source: [3GPP 2015].

Additionally, § 15.407 rules for U-NII devices specifies different maximum transmit power, PSD and out of band emission requirements, as well as DFS requirements for radar detection. In parallel, FCC is continuing its work to develop long-term equipment authorization test procedures that will ensure that the devices comply with the rules that protect the Terminal Doppler Weather Radar (TDWR) operations.

2.1.3 5 GHz ISM band in Brazil

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2.2. WIRELESS TECHNOLOGIES OPERATING IN 5 GHZ UNLICENSED SPECTRUM15

Table 2.2: Regulatory Restrictions in the 5 GHz band in Brazil.

From (mHz)

To

(mHz) Service Restriction

5150 5350 Restricted radiation

Indoor use only.

Effective Isotropic Radiated Power (EIRP) limited to 200 mW. EIRP spectral

power density limited to 10 mW/mHz. DFS mandated between 5250-5350 mHz. 5350 5470 Unregulated

5470 5650 Restricted radiation

DFS mandated.

Max transmitter output power limited to 250 mW. EIRP limited to 1 W. EIRP spectral power density limited to 50 mW/mHz

5650 5725 Restricted radiation or amateur radio

DFS mandated.

Max transmitter output power limited to 250 mW. EIRP limited to 1 W.

EIRP spectral power density limited to 50 mW/mHz

5725 5875 Restricted radiation (ISM Band)

Max transmitter output power limited to 1 W. Max EIRP Electromotive Force (EMF) density of 50,000 mV/m (3 meter distance)

Source: Section 4.2.3 in [3GPP 2015].

2.2 Wireless

Technologies

Operating

in

5 GHz

Unlicensed Spectrum

In this section, we describe the major wireless broadband access technologies operating in 5 GHz ISM unlicensed spectrum, namely Wi-Fi and LTE.

2.2.1 IEEE 802.11 (Wi-Fi)

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DCF contention is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), where nodes listen to the channel prior to transmission in a procedure known as Clear Channel Access (CCA). The listening period is composed by a DCF Inter Frame Space (DIFS) and a random Contention Window (CW), drawn from a maximum CW size that increases exponentially on each retransmission. A node in CCA mode may receive transmissions coming from other nodes, making the channel to be understood as occupied, and hence, deferring transmission for a random backoff time. After each data frame, an Acknowledgement (ACK) frame is transmitted Short Inter Frame Space (SIFS) after by the receiver to indicate correct reception and decoding of the data frame. Figure 2.3 illustrates CCA and backof procedures in a BSS with 4 STAs.

Figure 2.3: Wi-Fi CCA and backoff procedures.

Source: Figure 7.1 in [Perahia and Stacey 2008].

When Wi-Fi nodes contend among themselves for the channel, two well known issues can arise, specially in Overlapping Basic Subscriber Set (OBSS) dense deployment scenarios: The exposed node problem and the hidden node problem. The exposed node problem occurs when a STA is prevented from sending packets to other STAs in its own BSS due to a neighboring OBSS transmitter. The hidden node problem, on the other hand, occurs when a STA is visible from its AP, but not from other STAs communicating with that AP. The hidden node problem is an intra-BSS issue, while exposed node is a inter-BSS issue.

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Figure 2.4: Wi-Fi RTS/CTS exchange for hidden node protection.

Source: Figure 7.10 in [Perahia and Stacey 2008].

IEEE 802.11-2007 (11n) introduced in 2007 a novel channel access mode named Hybrid Coordinator Function (HCF). The major changes in relation to DCF are the Quality-of-Service (QoS) support for voice and video traffic. In HCF, data traffic is classified into one of five QoS classes (i.e. Voice, Video, Best Effort, Background and Legacy), each with its own data queue with different priorities.

For implementing contention-based QoS support, HCF introduced the Enhanced Distributed Channel Access (EDCA), an extension to DCF that modifies CCA by setting a different maximum CW size to each QoS class. This makes each QoS class to have a different channel access probability, and these are chosen such that voice and video have higher priority than other types of traffic, in this order.

HCF also introduced three novel concepts for improving Wi-Fi performance over DCF. The first is Transmission Opportunity (TXOP), a fixed-size period of time in which a STA may transmit multiple frames without needing to contend for the channel. There is also frame aggregation with Aggregated MSDU (A-MSDU) and Aggregated MPDU (A-MPDU), which diminishes signaling overhead by making multiple data packets and multiple MAC frames to be aggregated into a single Wi-Fi PHY frame, respectively. Finally, Block ACK Request (BAR) and Block ACK (BA) frames allow to request and acknowledge multiple data frames with a single frame exchange, respectively. Figure 2.5 presents an illustration of EDCA procedure with TXOP and BAR/BA.

Figure 2.5: Wi-Fi EDCA procedure with TXOP and BAR/BA.

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DCF and its improved version, EDCA, are effective in ensuring long term fair access for all STAs sharing the same channel. However, they do not provide an spectrally-efficient channel usage, since a significant amount of time is wasted on CCA and backoff. It is long known that CCA and exponential backoff decrease probability of transmission collisions in Wi-Fi, at the cost of lower channel utilization [Kleinrock and Tobagi 1975]. To worsen the situation, dense deployments scenarios have a large number of STAs and/or APs collocated in the same geographical area, further increasing performance depreciation.

An “optimal” DCF was proposed in literature [Yun et al. 2012], whose performance vary depending on traffic pattern (saturated or not), channel access timing (synchronous vs. asynchronous), modeling assumptions (continuous vs. discrete), channel models (time-varying vs. static channels) and time-scale separation. A promising idea is Optimal DCF (O-DCF) [Lee et al. 2012], which modifies adaptation rules for backoff and transmission length based on a function of the demand-supply differential of link capacity captured by local queue length. Other mechanisms include joint PHY/MAC optimization including Modulation and Coding Scheme (MCS) and payload adaptation to maximize throughput and/or minimize packet error rate [Choudhury and Gibson 2006, 2007].

There are standardized mechanisms for providing QoS support while minimizing performance depreciation due to excessive CCA and exponential backoff. In these, the AP issues Contention-Free Periods (CFPs) for its associated STAs in the BSS, during which contention for the channel is not allowed and the AP itself determines who transmits or receives data. Examples of such mechanism include Hybrid Controlled Channel Access (HCCA) and Power-Save Multi-Poll (PSMP) [IEEE 2013].

In HCCA, a CFP Repetition Period is divided into two sub-periods, CFP and Contention Period (CP). Within these, Controlled Access Periods (CAPs) can be declared by the AP using Beacon frames (in CFP) or Poll frames (in CP). While in CAPs, STAs set their NAV timers to avoid contending for the channel while waiting for AP transmissions of Contention-Free (CF)-Poll frames (e.g. a QoS Null subframe or a QoS Data frame). A CF-Poll frame allocates a TXOP for a given STA in either Downlink (DL) or Uplink (UL). Figure 2.6 illustrates a HCCA allocation.

Figure 2.6: Wi-Fi HCCA allocations.

(45)

PSMP is a 11n feature that extends Automatic Power Save Delivery (APSD) by allowing STAs to operate on a group schedule rather than individually. PSMP was conceived for improving Wi-Fi performance for traffic patters utilizing periodic, jitter-sensitive traffic like Voice-over-IP (VoIP). A special frame, named PSMP Frame, creates a CFP while carrying a series of scheduling decisions taken by the AP on which STAs may transmit or receive, and when they do it. There are two types of PSMP scheduling decisions for DL and UL transmissions, named PSMP DL Transmission Time (PSMP-DTT) and PSMP UL Transmission Time (PSMP-UTT) respectively. Each PSMP-DTT or PSMP-UTT is scheduled for a specific STA associated to the AP, where a TXOP is created for the transmission or reception of data frames. A sequence of PSMP-DTTs and PSMP-UTTs may be scheduled within the duration of a PSMP Sequence. In its turn, a sequence of PSMP Sequences can be concatenated one after the other, forming then a PSMP Burst. Figure 2.7 illustrates a PSMP burst, composed by a series of PSMP Sequences, each containing PSMP-DTTs and PSMP-UTTs allocations.

Figure 2.7: Wi-Fi PSMP burst.

PSMP Sequence #2 PSMP Sequence #3 PSMP Sequence #N PSMP Sequence #1

PSMP Burst = 1 or more consecutive PSMP Sequences

SIFS SIFS

PSMP Frame

PSMP-DTT for STA #1

PSMP-DTT for STA #2

PSMP-DTT for STA #N

PSMP-UTT for STA #1

PSMP-UTT for STA #2

PSMP-UTT for STA #N

AP

STA #1

STA #2

STA #N

PSMP-DTT for STA #1

PSMP-DTT for STA #2

PSMP-DTT for STA #N

SIFS SIFS aDTT2UTTTime SIFS

PSMP-UTT for STA #1

PSMP-UTT for STA #2

PSMP-UTT for STA #N

PSMP Duration (maximum of 8184 ❈s ❉ 1023 Wi-Fi OFDM symbols)

PSMP Sequence

Estratégias distribuídas para a coexistência de redes de banda larga sem fio em bandas não-licensiadas

Distributed Strategies for the Coexistence of

Wireless Broadband Access Networks in

Unlicensed Bands

Fuad Mousse Abinader Junior

Número de ordem PPgEE: D150

Distributed Strategies for the Coexistence of

Broadband Wireless Networks in Unlicensed

Bands

Fuad Mousse Abinader Junior

Estratégias Distribuídas para a Coexistência de

Redes de Banda Larga Sem Fio em Bandas

Não-Licensiadas

Fuad Mousse Abinader Junior

Acknowledgments

Resumo

Abstract

Contents

List of Figures

List of Tables

List of Acronyms

Chapter 1

Introduction

1.1

Problem Taxonomy

1.2

Research Questions

1.3

Thesis Outline

Chapter 2: Coexistence in Unlicensed Bands

Chapter 3: Inter-Technology Coexistence Solutions

Chapter 4: Intra-Technology Coexistence Solutions

Chapter 5: Conclusions and Final Remarks

Appendix A: LTE/Wi-Fi System Level Simulator

Appendix B: Parallel Bibliographic Production

1.4

Thesis Publications

1.5

Thesis Overview

Chapter 2

Coexistence in Unlicensed Bands

2.1

ISM Unlicensed Spectrum Worldwide

2.1.1

5 GHz ISM band in Europe

2.1.2

5 GHz ISM band in United States

2.1.3

5 GHz ISM band in Brazil

2.2

Wireless

Technologies

Operating

in

5

GHz

Unlicensed Spectrum

2.2.1

IEEE 802.11 (Wi-Fi)