IEEE 802.16 OFDMA PHY

1. IEEE 802.16 OFDMA PHY Wen-bin Lin g925629@oz.nthu.edu.tw 08-09-2006

2. Convolutional Turbo Codes • 8.4.9.2.3 • CTC encoder • Use a double binary Circular Recursive systematic convolution code • Can be used for supported hybrid ARQ (HARQ) • The encoder is fed by blocks of k bits or N couples • k: multiple of 8 • N: multiple of 4 • Advantage of the code • Better convergence • Better performance, especially low SNR and high data rate • Circular state transition property eliminates the need for tail ending data and hence achieving higher data rate

3. CTC Encoder • First, we feed the bits to be encoded to A and B • The encoding bits are separate into six sub-block, and send into the sub-block interleaver • Finally, according to the request data rate, combine the punctured sub-block into a sub-frame 1/3 CTC encoder A B Interleaver Puncturing block

4. 1/3 CTC Encoder Feedback branch: 0xB , 1+D+D3 Y parity: 0xD, 1+D2+D3 W parity: 0x9, 1+D3

5. CTC Interleaver • The interleaver requires the parameters P0, P1, P2, P3 • Parameters are shown in table 326, and 327 for HARQ • Two steps interleaver • Switch alternate couples • For time index is odd, swap (A, B) as (B, A) • Let the resulting sequence be u1 • The function P(j) provide the address of the sequence form step 1 • for j = 0:N-1 • switch j mod 4 • case 0: P(j) = (P0*j + 1)modN • ... • Let the resulting sequence be u2, where u2(j) = u1(P(j))

6. CTC Initialization • The state of the encoder is denoted S • Initialization step • Encode the sequence in natural order with S = 0, the final state is S0N-1 • According to N and S0N-1 to look up table to determine Sc1 • Encode the sequence in interleaved order with S = 0, the final state is S0N-1 • According to N and S0N-1 to look up table to determine Sc2

7. Subpacket Generation • In order to achieve various coding rate, puncturing is introduced to the mother code

8. Subblock Interleaving • The six subblocks are interleaved seperately • A “symbol” based interleaving • Symbols are written in to an array at address 0~N-1, and read out in a permuted order with address ADi • Determine ADi • Determine interleaving parameter according Table 329 • Initial I and k to 0 • Calculate a tentative address Tk • If Tk is less than N, ADi = Tk, otherwise discard Tk • Repeat untill all address are generated

9. Grouping and Selection • Symbol grouping • Subblock A and B are not multiplexed • Y1 and Y2 are multiplexed by one Y1 symbol follow a Y2 symbol • W1 and W2 are multiplexed by one W1 symbol follow a W2 symbol • Symbol selection • The puncturing block is referred as symbols selection in the view point of subpacket generation • Symbols in subpacket are formed by selecting specified sequences of symbols from CTC encoder output • Select by the formula

10. A Simulation Result In 802.16d

11. Automatic Repeat Request • Introduction of ARQ • Messages are encoded with error detection code • If there is any error during transmission, a retransmission is issued • There are three conventional types of ARQ • Stop-and-wait ARQ • Go-back-N ARQ • Selective-repeat ARQ • Introduction to hybrid-ARQ • Type I HARQ • Using a simultaneous error correction and detection code • Transmit / retransmit whole codeword • Type II HARQ • The parity check bits for error correction are sent when they are needed • Retransmit different parity check bits may introduce diversity gain

12. Stop-and-Wait ARQ • Simplest ARQ procedure • The transmitter sends a codeword to the receiver and wait a response • Acknowledgement (ACK): transmits next message • Negative Acknowledgement (NACK): retransmits current message • In-efficient because of the idle time

13. Go-Back-N ARQ • Transmission continuously until NACK is received • Transmitter does not wait for ACK after sending codeword • A retransmission length N is identical to round trip delay • N-1 codewords followed are also retransmitted • Receiver sends at least N NACKs whether the codeword is correct or not

14. Selective-Repeat ARQ • Transmission continuously until NACK is received • Only retransmits messages with NACK • A buffer must be provided in the receiver • Store error free codewords following a message in error • May cause buffer overflow

15. HARQ(1/3) • 8.4.15 • 802.16e support 3 optional HARQs • Chase combining for all coding schemes • Retransmission according to AI_SN filed • CRC16-CCITT is appended to MAC data after padding • Mobile station may support • Incremental redundancy for convolutional code • Similar to Chase HARQ • An SPID filed is supplied to indicate the puncture pattern • SS, MS may support • Incremental redundancy (IR) for convolutional turbo code (CTC)

16. HARQ(2/3) • CRC16-CCITT check • Polynomial = 0x1021 • Initial value = 0xffff • Basically stop-and-wait protocol (retransmission) • NACK signal receiving • ACK is not received within the duration of “HARQ ACK Delay for UL/DL burst”, which are specified in DCD message

17. C15 C14 C13 C11 C10 C8 C7 C12 C9 C6 C5 inputs C1 C0 C1 C2 C3 C4 Polynomial = 0x1021 initial = 0xffff HARQ(3/3) • CRC CCITT16 generator Cx 1-bit shift register Cycles = Length (M) + Length (P) - 1 2-in, 1-out XOR

18. UL ACK Channel (1/2) • 8.4.5.4.13 • Provides feedback for Downlink HARQ • One ACK channel occupies half subchannel by three OFDMA symbols • 3 pieces of 4x3 uplink tile in the case of PUSC • 3 pieces of ex3 uplink tile in the case of optional PUSC • 1 for NACK, while 0 for ACK (ACK encoding) • If 0, than transmit vector 0,0,0 on ACK channel • If 1, than transmit vector 4,7,2 on ACK channel • ACK Channel • Orthogonal modulated with QPSK symbol • Even and odd half subchannel

19. UL ACK Channel (2/2)

20. CC Supported HARQ • Chase HARQ, 8.4.9.2.1.1 • Incremental HARQ • For each transmission, the coded block is not the same • Different puncture patterns are used to create HARQ packets identified by SPID • Combination is performed at receiver • SPID • SPID = 0, puncture pattern us the same as the mandatory one • SPID = 1, the left cyclic shift of the one from SPID = 0

21. CTC Support HARQ • Chase HARQ, 8.4.9.2.3.5 • IR HARQ • Define special channel coding procedure • Modulation is chosen by some parameters • Number of encoding bits • Number of allocated slots Padding CRC addition Fregmentation Randomization CTC encoding

22. Padding & CRC • The basic channel coding unit is MAC PDUs • If the length of MAC PDU is not include in allowed set, ‘1’s are padded at the end of MAC PDU • Padding until the smallest allowed length not less than the length of MAC PDU • 16 bits CRC-CCITT defined in ITU-R Recommendation X.25 • The packet size shall belong to set

23. Fragmentation & Randomization • If the size after padding and CRC encoding is lager than 4800 bits, fragmentation is needed • Encoding separately by block of 4800 bits and concatenated as the same order before modulation • The allowed number of the bits in and encoder packet • The randomization is performed on each encoder packet • 1+X14+X15 generator polynomial • Preamble are not randomized • Initial value: [LSB] 0 1 1 0 1 1 1 0 0 0 1 0 1 0 1 [MSB]

24. Modulation Order • The randomized codeword is than modulated according to number of bits to be encoded (NEP) and allocated slots (NSCH) • The NEP is encoded by 4-bits in HARQ MAP, every NEP has its associated encoded NSCH

25. AMC • 8.4.6.3 • A BS may change from distributed subcarrier permutation to adjacent subcarrier permutation • From non-AAS to AAS-enables traffic • Return distributed permutation at the beginning of a new DL subframe • The pilot and data subcarriers are assigned fixed positions • Bin Structure • A set of nine contiguous subcarriers • 8 data and 1 pilot carriers • Basic allocation unit in DL/UL

26. 48 AMC • AMC allocation can be made by two mechanisms • Subchannel index reference in DL / UL MAP • A slot is defined as N bines by M symbols • NxM = 6, N = 2 and M = 3 • Subchannel allocation in a band using HARQ map • A group of 4 rows of bins is called a band • A slot consists of 6 contiguous bins in a band A band

27. Symbol Allocation • Numbering the traffic subcarrier in a slot • From 0~47 • Subcarrier first, then the bin • The j-th symbol in a slot is mapped onto the -th subcarrier per PermBase mod 48 off , an element of GF(72)

28. Transmitter Requirements • Power level control • Monotonic power level control of 45 dB minimum • 1 dB minimum step size • Spectral flatness • Absolute difference between adjacent subcarriers < 0.1 dB • Average energy of constellation • Power at DC subcarrier shall not exceed -15 dB relative to total transmitted power • Constellation error • Relative constellation RMS error, averaged over subcarriers, frames, and packets shall not exceed specified values

29. Receiver Requirement • Receiver sensitivity • The BER measured after FEC(CC-1/2) must < 10-6 • Using standardized packets • Using AWGN channel

30. Channel Rejection • Measured by setting transmitting power 3dB larger than the minimum receiver sensitivity • Adjacent channel rejection • Conforming OFDMA signal • At least 11 dB power above than desired signal when 16-QAM-3/4 • At least 4 dB power above than desired signal when 64-QAM-2/3 • Non-adjacent rejection • Any channel other than adjacent channel or co-channel • At least 30 dB power above than desired signal when 16-QAM-3/4 • At least 23 dB power above than desired signal when 64-QAM-2/3 • BER < 10-6