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  • A Study of the Next WLAN StandardIEEE 802.11ac Physical Layer

    NADER AL-GHAZU

    Master of Science ThesisStockholm, Sweden 2013

    XR-EE-SB 2013:001

  • A Study of the Next WLAN StandardIEEE 802.11ac Physical Layer

    NADER AL-GHAZU

    Master of Science Thesis performed atthe Signal Processing Group, KTH.

    January 2013

    Supervisor: Adrian SchumacherExaminer: Joakim Jalden

  • KTH School of Electrical Engineering (EE)Signal Processing

    XR-EE-SB 2013:001

    Nader Al-Ghazu, January 2013

    Tryck: Universitetsservice AB

  • Abstract

    This thesis studies the Physical Layer (PHY) of the new IEEE 802.11acWireless Local Access Network (WLAN) standard. The 11ac is built basedon the 11n successful standard. The standard is expected to be completedby the end of 2013. And it promises a Very High Throughput (VHT),and robust communication. In order to achieve that, the 11ac uses morebandwidth, it employs higher numbers of Multiple-Input Multiple-Output(MIMO) spatial streams, and higher orders of modulations. The 11ac PHYframe structure is studied in details. The transmitter and receiver blocks areexplained and implemented by MATLAB. Bit Error Rate (BER) and ErrorVector Magnitude (EVM) simulations of the PHY were run. The effectof different Modulation and Coding Scheme (MCS), and bandwidths werestudied. The performance of MIMO and Space-Time Block Coding (STBC)was investigated. The simulation results shows how the 11ac benefits fromthe new employed features. The created MATLAB simulation program canbe used for further tests and research.

    iii

  • Acknowledgements

    This work would not have been possible without the help, guidance, andpatience of my supervisor Mr. Adrian Schumacher. I would like to thankhim for giving me this opportunity to conduct my Master thesis under hissupervision at Rohde & Schwarz, and for all the time he dedicated to helpme.

    Many thanks to my academic examiner Dr. Joakim Jalden for his usefuladvice and support.

    And Finally, I am very grateful to my family for their continuous loveand support that they gave me.

    iv

  • Contents

    1 Introduction 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    1.1.1 Challenges and Hardware Requirements of 11ac . . . . 21.2 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    2 Background 42.1 IEEE 802.11 Amended Changes . . . . . . . . . . . . . . . . . 4

    2.1.1 802.11a . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 802.11b . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3 802.11g . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.4 802.11n . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.5 802.11ac . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.6 802.11ad . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2.2 Wi-Fi Alliance . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Orthogonal Frequency-Division Multiplexing (OFDM) . . . . 6

    2.3.1 OFDM vs. Single-Carrier . . . . . . . . . . . . . . . . 72.3.2 OFDM vs. Frequency Division Multiplexing (FDM) . 72.3.3 OFDM in IEEE 802.11ac . . . . . . . . . . . . . . . . 9

    2.4 MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4.1 Spatial Multiplexing . . . . . . . . . . . . . . . . . . . 112.4.2 STBC . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    3 IEEE 802.11ac 133.1 PHY Frame Structure . . . . . . . . . . . . . . . . . . . . . . 13

    3.1.1 Legacy Short Training Field (L-STF) . . . . . . . . . 143.1.2 Legacy Long Training Field (L-LTF) . . . . . . . . . . 143.1.3 Legacy Signal (L-SIG) . . . . . . . . . . . . . . . . . . 153.1.4 VHT Signal-A (VHT-SIG-A) . . . . . . . . . . . . . . 153.1.5 VHT Short Training Field (VHT-STF) . . . . . . . . 173.1.6 VHT Long Training Field (VHT-LTF) . . . . . . . . . 173.1.7 VHT Signal-B (VHT-SIG-B) . . . . . . . . . . . . . . 183.1.8 DATA field . . . . . . . . . . . . . . . . . . . . . . . . 19

    3.2 Transmitter Structure . . . . . . . . . . . . . . . . . . . . . . 20

    v

  • Contents vi

    3.2.1 PHY Padding . . . . . . . . . . . . . . . . . . . . . . . 203.2.2 Scrambler . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.3 Forward Error Correction Code (FEC) encoders . . . 213.2.4 Binary Convolutional Code (BCC) encoder parser . . 223.2.5 Stream parser . . . . . . . . . . . . . . . . . . . . . . . 223.2.6 Segment parser and segment deparser . . . . . . . . . 233.2.7 BCC Interleaver . . . . . . . . . . . . . . . . . . . . . 233.2.8 Constellation mapper . . . . . . . . . . . . . . . . . . 233.2.9 Pilot insertion . . . . . . . . . . . . . . . . . . . . . . 243.2.10 Tone rotation . . . . . . . . . . . . . . . . . . . . . . . 243.2.11 STBC encoder . . . . . . . . . . . . . . . . . . . . . . 243.2.12 Cyclic Shift Diversity (CSD) . . . . . . . . . . . . . . 253.2.13 Spatial mapper . . . . . . . . . . . . . . . . . . . . . . 253.2.14 IDFT, Guard Interval (GI) insertion and Windowing . 26

    3.3 Receiver Structure . . . . . . . . . . . . . . . . . . . . . . . . 263.3.1 Detection and Synchronisation . . . . . . . . . . . . . 263.3.2 Channel estimation . . . . . . . . . . . . . . . . . . . . 273.3.3 Equalizer . . . . . . . . . . . . . . . . . . . . . . . . . 293.3.4 Deparsing and Decoding . . . . . . . . . . . . . . . . . 30

    4 Simulation Environment 314.1 Settings and Parameters . . . . . . . . . . . . . . . . . . . . . 324.2 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3 Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.4 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.5 Performance Calculations . . . . . . . . . . . . . . . . . . . . 35

    5 Performance Analysis 375.1 BER Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 375.2 Single-Input Single-Output (SISO) vs. MIMO . . . . . . . . . 395.3 STBC Performance . . . . . . . . . . . . . . . . . . . . . . . . 395.4 Bandwidth Effect . . . . . . . . . . . . . . . . . . . . . . . . . 40

    6 Conclusion 426.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    References 44

  • List of Tables

    2.1 Number of OFDM subcarriers in 11ac. . . . . . . . . . . . . . 9

    3.1 Bandwidth rotation parameters. . . . . . . . . . . . . . . . . 133.2 Number of VHT-LTF fields. . . . . . . . . . . . . . . . . . . . 173.3 Modulation schemes normalizing factor. . . . . . . . . . . . . 233.4 CSD values for pre-VHT and VHT fields. . . . . . . . . . . . 25

    4.1 Important 11ac TXVECTOR fields. . . . . . . . . . . . . . . 324.2 Calculated parameters. . . . . . . . . . . . . . . . . . . . . . . 334.3 IEEE standards and the required SIGNAL fields. . . . . . . . 35

    5.1 MCSs parameters used in simulations. . . . . . . . . . . . . . 38

    vii

  • List of Figures

    2.1 Difference between FDM and OFDM. . . . . . . . . . . . . . 72.2 Orthogonal subcarriers in OFDM. . . . . . . . . . . . . . . . 82.3 OFDM Subcarriers in 20 MHz channel in IEEE 802.11. . . . 92.4 IEEE 802.11 Symbols GI. . . . . . . . . . . . . . . . . . . . . 102.5 A 3x3 MIMO system. . . . . . . . . . . . . . . . . . . . . . . 10

    3.1 VHT PHY frame fields. . . . . . . . . . . . . . . . . . . . . . 143.2 L-SIG field bit assignment. . . . . . . . . . . . . . . . . . . . 153.3 Constellations of L-SIG and VHT-SIG-A symbols. . . . . . . 163.4 VHT-SIG-A frame structure for Single-User. . . . . . . . . . . 163.5 VHT-LTF multiplication with PV HT LT F matrix. . . . . . . . 183.6 VHT-SIG-B for Sing-User and Multi-User. . . . . . . . . . . . 193.7 VHT-SIG-B for different bandwidths. . . . . . . . . . . . . . 193.8 The SERVICE field and its relation with VHT-SIG-B. . . . . 203.9 Transmitter block diagram . . . . . . . . . . . . . . . . . . . . 203.10 Data Scrambler . . . . . . . . . . . . . . . . . . . . . . . . . . 213.11 BCC encoder block diagram. . . . . . . . . . . . . . . . . . . 223.12 Tone rotation for different bandwidths. . . . . . . . . . . . . . 243.13 Receiver block diagram. . . . . . . . . . . . . . . . . . . . . . 263.14 A block diagram of the signal detection algorithm. . . . . . . 27

    4.1 Representing data bits in MATLAB matrices. . . . . . . . . . 314.2 Block diagram of the simulation. . . . . . . . . . . . . . . . . 324.3 Estimated 2 2 MIMO channel response. . . . . . . . . . . . 344.4 Constellation of a 16-QAM received signal. . . . . . . . . . . 364.5 80 MHz received signal spectrum. . . . . . . . . . . . . . . . . 36

    5.1 BERs for different MCSs . . . . . . . . . . . . . . . . . . . . . 385.2 EVM for different MCSs . . . . . . . . . . . . . . . . . . . . . 395.3 BER for different MIMO and SISO systems. . . . . . . . . . . 405.4 The effect of STBC on BER of a 2 2 MIMO system. . . . . 415.5 Effect of BW on BER. . . . . . . . . . . . . . . . . . . . . . . 41

    viii

  • List of Abbreviations

    3GPP 3rd Generation Partnership Project

    ADC Analog to Digital Converter

    ADSL Asymmetric Digital Subscriber Line

    AGC Automatic Gain Control

    AWGN additive white Gaussian noise

    BCC Binary Convolutional Code

    BER Bit Error Rate

    BPSK Binary Phase-Shift Keying

    CP Cyclic Prefix

    CRC Cyclic Redundancy Check

    CSD Cyclic Shift Diversity

    DC Direct Current

    DSP Digital Signal Processors

    DSSS Direct-Sequence Spread Spectrum

    EVM Error Vector Magnitude

    FDM Frequency Division Multiplexing

    FEC Forward Error Correction Code

    FFT Fast Fourier Transform

    FPGA Field Programmable Gate Array

    GI Guard Interval

    HDTV High-Definition Television

    ix

  • List of Abbreviations x

    HSPA+ High-Speed Packet Access plus

    HT-SIG High Throughput Signal

    ICI Inter Carrier Interference

    IDFT

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