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WiMAX OFDM PHY Overview. Chen-Nien Tsai Institute of Computer Science and Information Engineering National Taipei University of Technology 2006.10.24. Outline. Introduction Review of the OFDM System OFDM PHY Summary. Introduction. WiMAX Worldwide Interoperability for Microwave Access

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## WiMAX OFDM PHY Overview

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**WiMAX OFDM PHY Overview**Chen-Nien Tsai Institute of Computer Science and Information Engineering National Taipei University of Technology 2006.10.24**Outline**• Introduction • Review of the OFDM System • OFDM PHY • Summary**Introduction**• WiMAX • Worldwide Interoperability for Microwave Access • Replace last mile • Cost saving • Easy to deploy**Core network**Wireless link Base Station (BS) Subscribe Station (SS) Subscribe Station Wired/wireless links Subscribe Station Users Basic WiMAX Network Architecture**Physical Layer**• WirelessMAN-SC PHY • WirelessMAN-SCa PHY • WirelessMAN-OFDM PHY • WirelessMAN-OFDMA PHY**OFDM PHY**• Based on OFDM modulation. • 256 subcarriers • Designed for NLOS operation in the frequency band below 11 GHz.**Outline**• Introduction • Review of the OFDM System • OFDM PHY • Summary**Review of the OFDM System**• OFDM stands for Orthogonal Frequency Division Multiplexing. • It was proposed in mid-1960s and used in several high-frequency military system. • It is a multicarrier transmission technique. • Divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream.**The Applications of OFDM**• High-definition Television • Wireless LANs • IEEE 802.11a/g • HIPERLAN2 • IEEE 802.16 (WiMAX) • IEEE 802.20 • Mobile Broadband Wireless Access (MBWA) • Group’s activities were temporarily suspended.**Single carrier and Multicarrier Transmission**• Single carrier transmission • Each user transmits and receives data stream with only one carrier at any time. • Multicarrier transmission • A user can employ a number of carriers to transmit data simultaneously.**S/P**∑ Single carrier and Multicarrier Transmission Single carrier transmission Multicarrier transmission N oscillators are required**The Basic Principles of OFDM**• FFT-based OFDM system • Modulation and mapping • Orthogonality • Guard interval and Cyclic Extension**S/P**∑ FFT-based OFDM system • Generation of OFDM signal • Discrete/Fast Fourier Transform implementation. • No need for N oscillators to transmit N subcarriers.**Why FFT-based (1/3)**• A OFDM subcarrier signal can be expressed as • Suppose there are N subcarrier signals amplitude phase**Why FFT-based (2/3)**• After sampling • If**Why FFT-based (3/3)**• The definition of IDFT Identical**Modulation and Mapping**• Modulation types over OFDM systems • Phase Shift Keying (PSK) • Quadrature Amplitude Modulation (QAM) • WiMAX OFDM PHY • BPSK • QPSK • 16-QAM • 64-QAM**BPSK**QPSK 64-QAM 16-QAM**An Example**QPSK • Input stream • 11 01 10 11 • Output stream (I, Q) • 1, 1 • -1, 1 • 1, -1 • 1, 1**Orthogonality (1/5)**• Time domain • Frequency domain**Orthogonality (2/5)**• Two signals**Orthogonality (4/5)**Time Domain Frequency Domain**Orthogonality (5/5)**Time Domain Frequency Domain**Guard interval and Cyclic Extension**• Inter-symbol interference (ISI) • The crosstalk between signals within the same subcarrier of consecutive OFDM symbols. • Caused by multipath fading. • Inter-carrier interference (ICI) • The crosstalk between adjacent subcarrier of frequency bands of the same OFDM symbols.**Guard Interval**DATA Guard Interval • To eliminate the effect of ISI • Guard interval is used in OFDM systems**Guard Interval**• The guard interval could consist of no signals at all. • Orthogonality would be violated. • The problem of ICI would arise. • Call for cyclic extension (or cyclic prefix).**COPY**Guard Interval (Cyclic Extension) Cyclic Extension**OFDM symbol time**OFDM symbol time**Outline**• Introduction • Review of OFDM System • OFDM PHY • Summary**OFDM Symbol**• Time domain**OFDM Frequency Description**• Frequency domain • Data subscarriers: For data transmission • Pilot subscarriers: For various estimation purposes • Null subscarriers: For guard bands, non-active subcarriers, and the DC subcarrier**OFDM Frequency Description**• Subchannel is a combination of data subcarriers. • Subcarriers in a subchannel can be adjacent or spread out. • 256 subcarriers per carrier • 1 DC subcarrier (index 0) • 55 Guard subcarriers • data subcarriers + pilot subcarriers = 200 subcarriers**Channel Coding**• Channel coding is composed of three steps • Randomization • FEC • Interleaving Randomizer FEC Bit Interleaver Data to transmit Modulation**Randomization**• Purpose: additional privacy • For each allocation of data block, the randomizer shall be used independently. • Each data byte shall enter sequentially into the randomizer, MSB first.**PBRS (Pseudo-Random Binary Sequence) of randomization with**generator 1+X14+X15**1**0 0 1 0 1 0 1 0 0 0 0 0 0 0 Initialization vector DIUC: Downlink Interval Usage Code • Uplink • For burst #1, the initialization vector is**Initialization vector**UIUC: Uplink Interval Usage Code • Downlink**FEC**• Forward Error Correction • Concatenated Reed-Solomon-convolutional code (RS-CC) – Mandatory • Block Turbo Coding (BTC) – optional • Convolutional Turbo Codes – optional**Binary Convolutional Encoder**• Each m-bit information to be encoded is transformed into an n-bit symbol • Code rate = m/n • To convolutionally encode data: • k memory registers (k = 6 in OFDM PHY) • Input bits are fed into the leftmost register • Output bits are generated by the generator polynomials and the existing values in the remaining registers**Puncturing Pattern**• “1” means a transmitted bit and “0” denotes a removed bit**An Example**• Code rate = 5/6 • Input data = 0100100100 • Output data will be 12 bits. • All memory registers start with a value of 0.**1**1 1 1 0 0 1 0 1 1 1 0 1 1 1 1 1 1 0 1 1 0 1 1 0 1 0 1 0 0 0 1 0 0 0 0 1 1 1 0 1 0 0 1 0 1 Initial values of registers Input • Bitwise multiplication • Summation 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 0 G1 G2 1 1 0 Puncturing Pattern X Y Output**Interleaveing (1/3)**• Why bother? • FEC codes are effective when transmission errors occur randomly in time. • In most cases, errors occur burstly. • Without interleaving • With interleaving aaaabbbbccccddddeeeeffffgggg aaaabbbbccc____deeeeffffgggg Error-free transmission transmission with a burst error De-interleaving abcdefgabcdefgabcdefgabcdefg abcdefgabcdbcdefgabcdefg aa_abbbbccccdddde_eef_ffg_gg**Interleaveing (2/3)**• Let • k be the index of the coded bit before the first permutation. • mk be the index of the coded bit after the first and before the second permutation. • jk be the index after the second permutation. • Ncpc be the number of coded bits per subcarrier. • BPSK 1 16-QAM 4 • QPSK 2 64-QAM 6**Interleaveing (3/3)**• The first permutation • The second permutation

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