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Radio Resource Management in Ofdma Networks DESIGN AND PERFORMANCE EVALUATION OF RADIO RESOURCE MANAGEMENT IN OFDMA NETWORKS Javad Zolfaghari Institute for Theoretical Information Technology RWTH Aachen University DESIGN AND PERFORMANCE EVALUATION OF RADIO RESOURCE MANAGEMENT IN OFDMA NETWORKS Javad Zolfaghari A thesis submitted to the Department of Signal Theory and Communications, Technical University of Catalonia-Barcelona Tech, reviewed by Dr. Michael Reyer and Dr. Anna Umbert on 14th December 2013 in partial fulfillment of the requirements for the degree of master in Information and Communications Technology Master of Science in Electrical Engineering, Information and Communications Technology Institute for Theoretical Information Technology RWTH Aachen University I assure the single handed composition of this master’s thesis only supported by declared resources. Information derived from those resources has been acknowledged in the text and references are given in the bibliography. Aachen, 14th December 2013 Abstract Inter-cell interference is of great interest and one of the essential issues for wireless oper- ators who want to provide full coverage within their service area and guarantee a certain Quality of Service (QoS) to all users. In this thesis the resource allocation problem in an OFDMA based multi-cell network is formulated. Since no algorithm with polynomial running time exists due to non-linearity and combinatorial nature of underlying problem we adopted a heuristic approach with low computational cost and acceptable performance loss. We address specifically sharing available radio resources among users in terms of bandwidth allocation in order to suppress inter-cell interference. Besides, cell assignment and power allocation is foreseen. Performance of proposed schemes were evaluated by simulation network properly aligned with 3GPP LTE network. vii Acknowledgements I would like to express my gratitude and special thanks to my supervisor, Dr. Micheal Reyer for his enthusiasm, patience and kindness. I have learned many aspects of carry- ing out research in the field of wireless communications in the Institute of Theoretical Technology from his comments, advises and suggestions, and from our fruitful discussions. Without his encouraging and enlightening guidance, knowledge, persistent support this work would not have been successful. Many thanks are given to Dr. Anna Umbert from Technical University of Catalonia who reviewed this thesis and all staffs who have helped me in one way or another and made the time of my master thesis pleasurable and memor- able at the institute of Ti. Finally, I would like to give my heartfelt gratitude to my parents and specially my brother for his endless love, encouragement and support throughout my life. ix Contents 1 Introduction 1 1.1 Background . .1 1.2 Motivations . .2 1.3 Organization of Thesis . .2 2 Long Term Evolution (LTE) 5 2.1 Overview . .5 2.2 Multiple access technology in the downlink: OFDM and OFDMA . .6 2.2.1 OFDM Technique . .6 2.2.2 OFDM Transmitter and Receiver . .7 2.2.3 Comparison of OFDM and Code Division Multiple Access (CDMA)8 2.2.4 OFDMA Technique . 10 2.3 Multiple access technology in the uplink: SC-FDMA . 11 2.4 Multiple antenna techniques . 13 2.5 Radio access modes . 13 2.5.1 Transmission bandwidths . 14 2.5.2 FDD and TDD LTE frequency bands . 14 2.6 LTE Air Interface Protocol Aspects . 16 2.6.1 Physical channels and modulation . 17 2.6.2 Frame structure . 17 2.6.3 Resource element and resource block . 18 2.7 System Architecture Evolution (SAE) . 19 2.8 Self Organizing Network (SON) . 20 3 Literature review of radio resource allocation in OFDMA networks 23 3.1 Single cell multi-user system model . 23 3.2 Rate Adaptive (RA) Radio Resource Allocation (RRA) . 25 3.2.1 Schemes for single-cell OFDMA systems . 25 3.2.2 Distributed RA RRA schemes in OFDMA relay networks . 27 3.3 Margin adaptive (MA) radio resource allocation . 27 3.4 Adaptive resource allocation in cellular networks . 28 3.5 Network Optimization . 29 3.5.1 Single user water-filling . 29 3.5.2 Multi-user water-filling . 30 3.5.3 Rate region maximization . 31 xi Contents 4 Resource allocation in downlink multi-cell OFDMA networks 33 4.1 Problem formulation and system model . 33 4.1.1 System model . 36 4.2 Cell assignment and initialization . 38 4.3 Frequency Reuse Schemes . 38 4.4 Subcarrier Assignment . 39 4.5 Modifications in pre-assigned subcarriers . 44 4.5.1 Decrease pre-assigned subcarriers . 44 4.5.2 Cost benefit analysis of adding subcarriers . 44 4.5.3 Increase pre-assigned subcarriers . 47 4.6 Release subcarriers . 51 5 Performance evaluation 53 5.1 Simulation Framework . 53 5.1.1 User Profile . 54 5.2 Numerical results . 55 6 Conclusion and Future work 61 6.1 Future work . 62 6.1.1 UE Mobility . 62 6.1.2 Load balancing . 62 6.1.3 Femtocell and Macrocell Deplpyment . 62 Bibliography 63 xii 1 Introduction 1.1 Background Increasing demand of advanced mobile services such as high-speed internet access, multi- media online gaming, mobile TV, wireless Digital Subscriber Line (DSL), and integration of voice, video, text in recent years is motivation for technological evolution in mobile communication. Indispensably, delivery of wireless broadband services will become po- tential and widespread in today’s wireless communication systems. Therefore, one of the challenges for designing next-generation wireless systems is to provide wireless broadband at better cost and performance, while maintaining seamless mobility, service control and Quality of Service (QoS) provisioning [4]. Furthermore, the globalization of markets, in- creasingly serious competence of vendors and especially universality of IEEE 802 wireless technologies in wireless communication domain are also considered as essential motivations for technical evolution in wireless communication systems. As a result, 3GPP, which is a collaboration agreement between a number of telecommunic- ations standards bodies and wireless manufacturers, has already launched the project Long Term Evolution (LTE) regarding the evolution of the existed Third Generation (3G) sys- tems. The stated targets of the project LTE include support for high peak data rates, low latency, improved system capacity and coverage, reduced operating costs, multi-antenna support, flexible bandwidth operations and easy integration to existing systems. LTE is also referred to as EUTRA (Evolved Universal Terrestrial Radio Access) or E-UTRAN (Evolved Universal Terrestrial Radio Access Network). LTE employs multiple access technologies on the air interface: specifically, Orthogonal Frequency Division Multiple Access (OFDMA) in downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in uplink. OFDM technique that can be combined with multiple access using time, frequency or coding separation of the users are referred to as OFDMA technique. The OFDM technology has become a common advanced tech- nology for wide-band digital communication and also been considered one of prime trans- mission technologies of the next generation networks, e.g., LTE Advanced. The basic idea of OFDM is to use a large number of closely spaced orthogonal subcarriers and these subacrriers are used in parallel. The key advantages of employing OFDM over single- carrier schemes are its robustness against multi-path delay spread and frequency selective fading, elimination of the Inter-Symbol Interference (ISI). All aforementioned advantages 1 1 Introduction demonstrates that OFDM technique is chosen as a greatly promising candidate for next generation networks. 1.2 Motivations Interference mitigation in OFDM systems is one of the major challenges, particularly for cell edge users. Although intra-cell interference inside a cell can be eliminated basing on the orthogonal characteristic among the subcarriers in the performance of OFDM technology, inter-cell interference exists and hinges seriously on the performance of system in an OFDM based multi-cell wireless network. More specifically, inter-cell interference in OFDM based network occurs when the same subcarriers are re-used in neighbor cells. It decreases the Signal to Interference and Noise Ratio (SINR) on these subacarriers. Obviously, interference has the greatest impact on cell-edge users. Hence, the inter-cell interference is of great interest and one of the essential issues for wireless operators who want to provide full coverage within their service area and guarantee a certain Quality of Service (QoS) to all users including cell-edge users regardless of their positions inside a cell. In the scope of this thesis, we explore the problem of resource allocation in an OFDMA based multi-cell network or in other words, how to share the available radio resources among users in terms of bandwidth allocation in order to suppress inter-cell interference, enhance throughput of the cell-edge user and spectral efficiency. 1.3 Organization of Thesis The rest of the thesis is organized as follows: Chapter 2 we present an overview of 3GPP LTE networks because the simulation frame- work that we will work on, in the next chapters is well aligned with the current standards of the LTE. We focus on the description of LTE multiple access technologies such as Ortho- gonal Frequency Division Multiplexing (OFDM), Orthogonal Frequency Division Multiple Access (OFDMA) for downlink and also address Single Carrier Frequency Division Mul- tiple Access (SC-FMDA) for LTE uplink. Frequency bandwidth and different radio access modes such as FDD and TDD that LTE supports will be presented in this chapter. LTE air interface protocol with the Radio Resource Control (RRC) layer which is responsible for setting up and management of radio resource blocks will be elaborated. We will ad- dress self organizing networks as new approaches to the network structure of which LTE is taking advantage. Chapter 3 In this chapter we focus on resource allocation in wireless cellular networks employing OFDMA as a promising air-interface for wireless systems. We present an over- 2 1.3 Organization of Thesis view of previous work in the context of radio resource allocation in OFDMA networks.
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