Downlink Scheduling in 3GPP Long Term Evolution (LTE)

Downlink Scheduling in 3GPP Long Term Evolution (LTE) free pdf ebook was written by Tdikamba on March 21, 2011 consist of 73 page(s). The pdf file is provided by repository.tudelft.nl and available on pdfpedia since June 03, 2012.

wireless and mobile communication (wmc) group faculty of electrical engineering, mathematics..dl and 50 mb/s in the ul), low latency (10ms..to the system capacity (shannon capacity). the main contribution of this thesis...

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Downlink Scheduling in 3GPP Long Term Evolution (LTE) pdf




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Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 1
Wireless and Mobile Communication (WMC) Group Faculty of Electrical Engineering, Mathematics and Computer Science Delft University of Technology Downlink Scheduling in 3GPP Long Term Evolution (LTE) ASSIGNMENT SUPERVISOR DATE STUDENT STUDENTNUMBER : Master of Science Thesis : Dr. Anthony Lo : March 18 th , 2011 : Tshiteya Dikamba : 1377612
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Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 2
Abstract In this report an overview of the Long Term Evolution (LTE) is presented. LTE is the evolution of the Universal Mobile Telecommunications System (UMTS). It allows mobile users to access Internet through their devices (mobile telephones, laptop…). LTE intends to deliver high speed data and multimedia services to next generation. In the coming years LTE mobile broadband technology will be widely used by devices such as notebooks, smartphones, gaming devices and video cameras. The Long Term Evolution provides a high data rate and can operate in different bandwidths ranging from 1.4MHz up to 20MHz. LTE supports high peak data rates (100 Mb/s in the DL and 50 Mb/s in the UL), low latency (10ms round-trip delay), improves system capacity and coverage and reduces operating costs. Furthermore it supports Multiple Input Multiple Output (MIMO) and allows seamless integration with existing systems. A scheduler assigns the shared resources (time and frequency) among users terminals. In this master thesis the focus is on the downlink scheduling. The Best CQI scheduling algorithm and the Round Robin scheduling algorithm have been considered in this report. The implementation, the analysis and the comparison of these scheduling algorithms were done through simulations executed on a MATLAB-based downlink link level simulator from the Vienna University. I have examined the impact of the scheduling schemes on the throughput and I have investigated the fairness of each scheduling scheme. Furthermore the throughput results are compared to the system capacity (Shannon Capacity). The main contribution of this thesis work is to propose a new scheduling algorithm that can be a compromise between the throughput and the fairness. The novel scheduling scheme has been designed and tested to investigate whether it achieves its goal. Two ITU-channel types have been used: The Pedestrian B and the Vehicular A channel. I have studied the impact of the channel delay on the throughput. MIMO systems have been used to increase the throughput. 2
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 3
Acknowledgement This Master thesis project is the final step in obtaining my Master Degree in Electrical Engineering with the programme track Telecommunications at the Delft University of Technology. This project was carried out in the Department Wireless and Mobile Communication (WMC) of the Faculty Electrical Engineering from the Delft University of Technology (TU Delft, The Netherlands). I have been working on my Master thesis project from April, 2010 to March 2011. While undertaking this project I have had much encouragement from many people. I would like to express my sincere gratitude. First of all I am particularly indebted to Dr. Anthony Lo, my daily supervisor. He was been a great support when I had unexpected problems during my project and for the patience and flexibility with which he allowed me to work on my graduation project. Further, I want to express my deep respect to my supervisor, Prof. Ignas Niemegeers. Secondly, I would also like to thank my friends and all Telecommunications Master students who helped me and gave me suggestions. They also contributed to the improvement of the sphere of the laboratory practice where we worked. Further I would like to thank everybody who has helped me by proof reading my thesis. Finally, I want to thank my family, my parents Dr. Charles Dikamba and Marie Madeleine Meta, my brothers and sisters, Didier, Mireille, Gisele, Jean Patrick and Serge Raphaël for their unwavering support. 3
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 4
Table of Contents 1 Introduction...................................................................................................................... 5 1.1 Background ............................................................................................................... 5 1.2 Motivation and goal of the thesis.............................................................................. 7 1.2.1 Motivation.......................................................................................................... 7 1.2.2 Thesis goal ......................................................................................................... 7 1.3 Thesis Scope ............................................................................................................. 8 2 An Overview of LTE ........................................................................................................ 9 2.1 LTE requirements ..................................................................................................... 9 2.2 Orthogonal Frequency Division Multiplexing........................................................ 10 2.3 Spectrum Flexibility................................................................................................ 10 2.4 Downlink physical resource.................................................................................... 11 2.5 Multiple Input Multiple Output .............................................................................. 14 2.6 Theoretical channel capacity................................................................................... 17 3 Related work ................................................................................................................... 20 3.1 Scheduling Methods................................................................................................ 20 3.1.1 Best CQI scheduling ........................................................................................ 22 3.1.2 Round Robin Scheduling ................................................................................. 23 4 The proposed new scheduling algorithm ......................................................................... 25 4.1 Principle .................................................................................................................. 25 4.2 Illustration ............................................................................................................... 27 5 Simulation ....................................................................................................................... 31 5.1 LTE Link Level Simulator overview...................................................................... 31 5.2 MATLAB Code Problems ...................................................................................... 32 5.2.1 Round Robin scheduling.................................................................................. 32 5.2.2 Best CQI Scheduling........................................................................................ 38 5.3 Simulation Results and Analysis ............................................................................ 45 5.3.2 Simulation scenarios ............................................................................................ 46 6 Conclusions and future work........................................................................................... 67 6.1 Conclusion .............................................................................................................. 67 6.2 Future work............................................................................................................. 68 Abbreviations..................................................................................................................... 69 References.......................................................................................................................... 71 4
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 5
1 Introduction In this chapter, the 3GPP LTE will be introduced and all its relevant features. We will begin with the background information on the subject we present in section1.1. The motivation for our thesis project and its goals are stated in Section 1.2. The last section 1.3 gives the thesis outline. 1.1 Background In the recent years, the world was introduced to mobile broadband. Multimedia applications through the Internet have gathered more attention. Applications such as live streaming, online gaming, mobile TV require higher data rate. The Third-generation Partnership Project (3GPP) started to work on solutions to these challenges and came up with the HSPA. The HSPA is currently used in 3G phones for such applications. Later, the 3GPP has worked on the Long Term Evolution (LTE) and intends to surpass the performance of HSPA. Thus LTE will enhance applications such as online gaming and interactive TV. It is expected that in 2014, 80% of broadband users will be mobile broadband subscribers and they will be served by HSPA and LTE networks [1]. The 3GPP is the standards-developing body that specifies the 3G UTRA and GSM systems. LTE as defined by the 3GPP [2] is the evolution of the Third-generation of mobile communications, UMTS. LTE intends to create a new radio-access technology which will provide high data rates, a low latency and a greater spectral efficiency. The 3GPP has started with the RAN Evolution workshop in November 2004 [3]. A lot of research has been carried out and proposals have been presented on the evolution of the Universal Terrestrial Radio access Network (UTRAN). The specifications related to LTE are formally known as the evolved UMTS terrestrial radio access (E-UTRA) and evolved UMTS terrestrial radio access network (E-UTRAN), but are in general referred as project LTE. In December 2008, the LTE specification was published as part of Release 8. The initial deployment of LTE was expected in 2009. The first release of LTE namely release-8 supports peak rates of 300Mb/s, a radio-network delay of less than 5ms. Multiple Input Multiple Output (MIMO) have gathered a lot of attention recently. It allows the achievement of high peak data rates. Furthermore LTE operates both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) and can be deployed in different bandwidths. With TDD the uplink and downlink operate in same frequency band whereas with TDD the uplink and downlink operate in different frequency bands. 5
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 6
Orthogonal Frequency Division Multiplexing (OFDM) has been adopted as the downlink transmission scheme for the 3GPP LTE. A downlink is a transmission from the base station to the mobile station. OFDM divides the transmitted high bit-stream signal into different sub-streams and sends these over many different sub-channels. A base station (BS) is called an Evolved NodeB (eNodeB) in the Long Term Evolution and a mobile station (MS) is called a User Equipment (UE) in the Long Term Evolution. For the sake of simplicity we will use BS and MS to refer to eNodeB and UE respectively. The downlink physical resource is represented as a time-frequency resource grid consisting of multiple Resource Blocks (RB). A resource block is divided in multiple Resource Elements (RE). A scheduler is a key element in the BS and it assigns the time and frequency resources to different users in the cell. Thus a RB is the smallest element that can be assigned by the scheduler. Our research is focused on the Round Robin scheduling and on the Best CQI scheduling. The Best CQI scheduling assigns the resource blocks to the user with the highest CQI on that RB. To perform this scheduling the MS must feedback the Channel Quality Indication (CQI) to the BS. In Round Robin (RR) scheduling the terminals are assigned the resource blocks in turn (one after another) without taking the CQI into account. Thus the terminals are equally scheduled. The Round Robin scheduling and Best CQI scheduling [4] have been simulated in a MATLAB-based Downlink Link Level Simulator from the Vienna University. The performance of these scheduling algorithms in terms of throughput is analyzed. Furthermore the throughput results are compared to the channel capacity (Shannon Capacity). We have used the Pedestrian B channel and the Vehicular A channel. We have examined the impact of the channel delay on the throughput. The throughput of a MIMO (2x2) and MIMO (4x4) systems have been taken into consideration in this report. 6
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 7
1.2 Motivation and goal of the thesis 1.2.1 Motivation The motivation to work on this project comes from the fact that LTE is the future of mobile broadband. It is expected that in the future 80% of all mobile broadband users will be served by LTE [1]. The time and frequency are scared resources. The scheduler is a key element in the BS since it determines to which users the resource blocks should be assigned. Round Robin scheduling and Best CQI scheduling have been selected because of their characteristics. The Best CQI scheduling optimizes the user throughput by assigning the resource block to the user with the good channel quality and the Round Robin scheduling is fair in the long term since it equally schedules the Mobile Station (MS). In general cell- center users have a good channel quality compare to the cell-edge users. In order to find a trade-off between the throughput and the fairness a new scheduling algorithm has been proposed. The proposed new scheduling algorithm can be considered as a compromise between the Best CQI scheduling and the Round Robin scheduling. 1.2.2 Thesis goal The purpose of this thesis is to implement and simulate the downlink scheduling in LTE. We have also investigated the impact of the scheduling algorithms on the throughput and on the fairness. To achieve this goal two scheduling algorithms are considered: the Best CQI scheduling and the Round Robin scheduling. We have analyzed and compared the performance of these two scheduling algorithms in terms of throughput. Furthermore MIMO systems will be used for different scheduling algorithms. For the multipath channel, the ITU Pedestrian B and ITU Vehicular A are used. The effect of channel delay on the cell throughput will be examined. The main contribution of this thesis work is to propose a new scheduling algorithm that can be a compromise between the throughput and the fairness. The novel scheduling scheme has been designed and tested to investigate whether it achieves its goal. To perform simulations a MATLAB-based Link Level simulator from the Vienna University is used. 7
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 8
1.3 Thesis Scope This thesis is organized in 6 chapters. The rest of the chapters are organized as follows: Chapter 2 gives an overview of LTE. Chapter 3 explains the related work one the downlink scheduling algorithms in LTE. Chapter 4 introduces the proposed new scheduling algorithm. Chapter 5 presents the simulation results. Finally chapter 6 draws the conclusion and gives recommendations for future works. 8
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 9
2 An Overview of LTE In this chapter an overview of the 3GPP Long Term Evolution is provided. We will begin with LTE requirements. Then Orthogonal Frequency Division Multiplexing (OFDM) will be introduced. In section 2.4 the downlink transmission scheme is explained and in section 2.5 MIMO is reviewed. Finally, section 2.6 provides information on the theoretical channel capacity. 2.1 LTE requirements To achieve its goals, LTE must satisfy the following requirements [3]: Data rates LTE should support a data rate up to 100 Mb/s within a 20 MHz downlink spectrum allocation and 50 Mb/s within a 20 MHz uplink or, equivalently, spectral efficiency values of 5bps/Hz and 2.5 bps/Hz, respectively. Throughput The downlink average throughput per MHz is about 3 to 4 times higher than in the release 6. The uplink average user throughput per MHz is about 2 to 3 times higher than in the release 6. Bandwidth LTE allows bandwidth ranging from 1.4 MHz up to 20 MHz, where the latter is used to achieve the highest LTE data rate. Furthermore, LTE operates in both paired and unpaired spectrum by supporting both Frequency-Division Duplex (FDD) and Time- Division Duplex (TDD). Mobility The mobility is optimized for low terminals speeds ranging from 0 to 15 km/h. The connection should be maintained for very high UEs speeds up to 350 km/h or even up to 500 km/h. 9
Downlink Scheduling in 3GPP Long Term Evolution (LTE) - page 10
Coverage The above targets should be met for 5 km cells and some slight degradation in throughput and spectrum efficiency for 30 km cells. 100 km cells and up can’t meet the targets requirements. 2.2 Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing (OFDM) has gathered much attention in recent years and has been adopted as the downlink transmission scheme for the 3GPP LTE. OFDM is a multicarrier transmission scheme because it splits up the transmitted high bit-stream signal into different sub-streams and sends these over many different sub- channels [5]. In other words OFDM simply divides the available bandwidth into multiple narrower sub-carries and transmits the data on these carries in parallel streams. Each sub- carrier is modulated using different levels of modulation, e.g. QPSK, QAM, 64QAM and an OFDM symbol is obtained by adding the modulated subcarrier signals. 2.3 Spectrum Flexibility In LTE communication is available in different frequency bands, of different sizes. Furthermore the communication can take place both in paired and unpaired bands. Paired frequency bands means that the uplink and downlink transmissions use separate frequency bands, while unpaired frequency bands downlink and uplink share the same frequency band. In LTE downlink transmissions are grouped in (radio) frame of length 10 ms. One radio frame is formed of 10 subframes of 1ms duration. Therefore there are ten subframes in the uplink and ten frames in the downlink. Each subframe is divided into two slots of 0.5 ms duration. Each slot counts 6 or 7 OFDM symbols for normal or extended cyclic prefix used. The LTE frame structure is illustrated in the Figure 2.1 Figure 2.1 LTE Frame structure [6] 10
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