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

2006/10/242 Outline ► Introduction ► Review of the OFDM System ► OFDM PHY ► Summary

2006/10/243 Introduction ► WiMAX  Worldwide Interoperability for Microwave Access  Replace last mile  Cost saving  Easy to deploy

2006/10/244 Basic WiMAX Network Architecture Subscribe Station (SS) Subscribe Station Wireless link Base Station (BS) Subscribe Station Core network Wired/wireless links Users

2006/10/245 Reference Model

2006/10/246 Physical Layer ► WirelessMAN-SC PHY ► WirelessMAN-SCa PHY ► WirelessMAN-OFDM PHY ► WirelessMAN-OFDMA PHY

2006/10/247 OFDM PHY ► Based on OFDM modulation.  256 subcarriers ► Designed for NLOS operation in the frequency band below 11 GHz.

2006/10/248 Outline ► Introduction ► Review of the OFDM System ► OFDM PHY ► Summary

2006/10/249 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.

2006/10/2410 The Applications of OFDM ► High-definition Television ► Wireless LANs  IEEE a/g  HIPERLAN2 ► IEEE (WiMAX) ► IEEE  Mobile Broadband Wireless Access (MBWA)  Group ’ s activities were temporarily suspended.

2006/10/2411 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.

2006/10/2412 Single carrier and Multicarrier Transmission S/P ∑ Single carrier transmissionMulticarrier transmission N oscillators are required

2006/10/2413 The Basic Principles of OFDM ► FFT-based OFDM system ► Modulation and mapping ► Orthogonality ► Guard interval and Cyclic Extension

2006/10/2414 FFT-based OFDM system

2006/10/2415 FFT-based OFDM system ► Generation of OFDM signal  Discrete/Fast Fourier Transform implementation.  No need for N oscillators to transmit N subcarriers. S/P ∑

2006/10/2416 Why FFT-based (1/3) ► A OFDM subcarrier signal can be expressed as ► Suppose there are N subcarrier signals amplitudephase

2006/10/2417 Why FFT-based (2/3) ► After sampling ► If

2006/10/2418 Why FFT-based (3/3) ► The definition of IDFT Identical

2006/10/2419 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

2006/10/2420 BPSK QPSK 16-QAM 64-QAM

2006/10/2421 An Example ► Input stream  ► Output stream (I, Q)  1, 1  -1, 1  1, -1  1, 1 QPSK

2006/10/2422 Orthogonality (1/5) ► Time domain ► Frequency domain

2006/10/2423 Orthogonality (2/5) ► Two signals

2006/10/2424 Orthogonality (3/5)

2006/10/2425 Orthogonality (4/5) Time DomainFrequency Domain

2006/10/2426 Orthogonality (5/5) Time DomainFrequency Domain

2006/10/2427 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.

2006/10/2428 Guard Interval ► To eliminate the effect of ISI  Guard interval is used in OFDM systems DATAGuard Interval

2006/10/2429 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).

2006/10/2430 Cyclic Extension Guard Interval (Cyclic Extension) COPY

2006/10/2431 OFDM symbol time

2006/10/2432 Outline ► Introduction ► Review of OFDM System ► OFDM PHY ► Summary

2006/10/2433 OFDM Symbol ► Time domain

2006/10/2434 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

2006/10/2435 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

2006/10/ subchannels

2006/10/2437 Channel Coding ► Channel coding is composed of three steps  Randomization  FEC  Interleaving Data to transmit RandomizerFECBit Interleaver Modulation

2006/10/2438 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.

2006/10/2439 ► PBRS (Pseudo-Random Binary Sequence) of randomization with generator 1+X 14 +X 15

2006/10/2440 Initialization vector ► Uplink ► For burst #1, the initialization vector is DIUC: Downlink Interval Usage Code

2006/10/2441 Initialization vector ► Downlink UIUC: Uplink Interval Usage Code

2006/10/2442 FEC ► Forward Error Correction  Concatenated Reed-Solomon-convolutional code (RS-CC) – Mandatory  Block Turbo Coding (BTC) – optional  Convolutional Turbo Codes – optional

2006/10/2443 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

2006/10/2444 Binary Convolutional Encoder

2006/10/2445 Puncturing Pattern ► “ 1 ” means a transmitted bit and “ 0 ” denotes a removed bit

2006/10/2446 An Example ► Code rate = 5/6 ► Input data = ► Output data will be 12 bits. ► All memory registers start with a value of 0.

2006/10/ Input Initial values of registers G1G1 G2G2 Y X Output Puncturing Pattern 1.Bitwise multiplication 2.Summation 011

2006/10/2448 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 abcdefgabcdefgabcdefgabcdefg abcdefgabcd bcdefgabcdefg aa_abbbbccccdddde_eef_ffg_gg De-interleaving Error-free transmission transmission with a burst error

2006/10/2449 Interleaveing (2/3) ► Let  k be the index of the coded bit before the first permutation.  m k be the index of the coded bit after the first and before the second permutation.  j k be the index after the second permutation.  N cpc be the number of coded bits per subcarrier. ► BPSK  1 16-QAM  4 ► QPSK  264-QAM  6

2006/10/2450 Interleaveing (3/3) ► The first permutation ► The second permutation

2006/10/2451 De-interleaveing ► Let  j be the index of a received bit before the first permutation.  m j be the index of that bit after the first and before the second permutation.  k j be the index of that bit after the second permutation.

2006/10/2452 De-interleaving ► First permutation ► Second permutation

2006/10/2453 Block Sizes of the Bit Interleaver

2006/10/2454 Outline ► Introduction ► Review of OFDM System ► OFDM PHY ► Summary

2006/10/2455 Summary (1/3) ► Advantages of the OFDM system  Better bandwidth usage than traditional FDM ► The subcarrier is keep orthogonality with overlap ► No guard band among subcarriers  Low complexity ► Using off-the-shelf DFT/FFT DSP technologies  Tolerate ISI and ICI ► Guard interval ► Cyclic extension

2006/10/2456 Summary (2/3) ► Disadvantages of the OFDM system  Cyclic prefix overhead  Frequency synchronization ► Sampling frequency synchronization ► Carrier frequency synchronization  Symbol synchronization ► Timing errors ► Carrier phase noise

2006/10/2457 Summary (3/3) MAC Layer PHY Layer MAC PDU Randomizer FEC Bit Interleaver Modulator IFFT

Backup Materials

2006/10/2459 Modulation and Mapping QPSK16-QAM

2006/10/2460 Example OFDM Uplink RS-CC Encoding (1/3)

2006/10/2461 Example OFDM Uplink RS-CC Encoding (2/3)

2006/10/2462 Example OFDM Uplink RS-CC Encoding (3/3)

2006/10/2463