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WiMAX OFDM PHY Overview

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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  1. WiMAX OFDM PHY Overview Chen-Nien Tsai Institute of Computer Science and Information Engineering National Taipei University of Technology 2006.10.24

  2. Outline • Introduction • Review of the OFDM System • OFDM PHY • Summary

  3. Introduction • WiMAX • Worldwide Interoperability for Microwave Access • Replace last mile • Cost saving • Easy to deploy

  4. Core network Wireless link Base Station (BS) Subscribe Station (SS) Subscribe Station Wired/wireless links Subscribe Station Users Basic WiMAX Network Architecture

  5. Reference Model

  6. Physical Layer • WirelessMAN-SC PHY • WirelessMAN-SCa PHY • WirelessMAN-OFDM PHY • WirelessMAN-OFDMA PHY

  7. OFDM PHY • Based on OFDM modulation. • 256 subcarriers • Designed for NLOS operation in the frequency band below 11 GHz.

  8. Outline • Introduction • Review of the OFDM System • OFDM PHY • Summary

  9. 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.

  10. 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.

  11. 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.

  12. S/P ∑ Single carrier and Multicarrier Transmission Single carrier transmission Multicarrier transmission N oscillators are required

  13. The Basic Principles of OFDM • FFT-based OFDM system • Modulation and mapping • Orthogonality • Guard interval and Cyclic Extension

  14. FFT-based OFDM system

  15. S/P ∑ FFT-based OFDM system • Generation of OFDM signal • Discrete/Fast Fourier Transform implementation. • No need for N oscillators to transmit N subcarriers.

  16. Why FFT-based (1/3) • A OFDM subcarrier signal can be expressed as • Suppose there are N subcarrier signals amplitude phase

  17. Why FFT-based (2/3) • After sampling • If

  18. Why FFT-based (3/3) • The definition of IDFT Identical

  19. 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

  20. BPSK QPSK 64-QAM 16-QAM

  21. An Example QPSK • Input stream • 11 01 10 11 • Output stream (I, Q) • 1, 1 • -1, 1 • 1, -1 • 1, 1

  22. Orthogonality (1/5) • Time domain • Frequency domain

  23. Orthogonality (2/5) • Two signals

  24. Orthogonality (3/5)

  25. Orthogonality (4/5) Time Domain Frequency Domain

  26. Orthogonality (5/5) Time Domain Frequency Domain

  27. 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.

  28. Guard Interval DATA Guard Interval • To eliminate the effect of ISI • Guard interval is used in OFDM systems

  29. 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).

  30. COPY Guard Interval (Cyclic Extension) Cyclic Extension

  31. OFDM symbol time OFDM symbol time

  32. Outline • Introduction • Review of OFDM System • OFDM PHY • Summary

  33. OFDM Symbol • Time domain

  34. 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

  35. 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

  36. 16 subchannels

  37. Channel Coding • Channel coding is composed of three steps • Randomization • FEC • Interleaving Randomizer FEC Bit Interleaver Data to transmit Modulation

  38. 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.

  39. PBRS (Pseudo-Random Binary Sequence) of randomization with generator 1+X14+X15

  40. 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

  41. Initialization vector UIUC: Uplink Interval Usage Code • Downlink

  42. FEC • Forward Error Correction • Concatenated Reed-Solomon-convolutional code (RS-CC) – Mandatory • Block Turbo Coding (BTC) – optional • Convolutional Turbo Codes – optional

  43. 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

  44. Binary Convolutional Encoder

  45. Puncturing Pattern • “1” means a transmitted bit and “0” denotes a removed bit

  46. An Example • Code rate = 5/6 • Input data = 0100100100 • Output data will be 12 bits. • All memory registers start with a value of 0.

  47. 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

  48. 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

  49. 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

  50. Interleaveing (3/3) • The first permutation • The second permutation

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