IEEE 802.16 OFDMA PHY

IEEE 802.16 OFDMA PHY

Wen-bin Lin

[email protected]

08-09-2006

Proprietary of NTHU Communication SOC Lab, Copyright @ 2006

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


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

Puncturing block

Interleaver

A

B


1/3 CTC Encoder

Feedback branch:0xB , 1+D+D3

Y parity:0xD, 1+D2+D3

W parity:0x9, 1+D3


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))


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

(0 7)S


Subpacket Generation

• In order to achieve various coding rate, puncturing is introduced to the mother code


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

2 ( mod ) ( / )mk mT k J BRO k J


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

, ( ) mod(3 )k i k EPS F i N


A Simulation Result In 802.16d


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


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


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


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


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)


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


C15 C14 C13 C12 C11 C10 C9 C8 C7 C6

C5

C4C3C2C1C1C0inputs

HARQ (3/3)

• CRC CCITT16 generator

Cx 1-bit shift register

2-in, 1-out XOR

Polynomial = 0x1021

initial = 0xffff

Cycles = Length (M) + Length (P) - 1


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


UL ACK Channel (2/2)


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


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

PaddingCRC

additionFregmentation Randomization

CTCencoding


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


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]


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


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


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

48

A

ba

nd


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 ( ) 1offperS j

per PermBase mod 48off / 48 mod 49PermBase , an element of GF(72)


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


Receiver Requirement

• Receiver sensitivity– The BER measured after FEC(CC-1/2) must < 10-6

– Using standardized packets

– Using AWGN channel


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


• Non-adjacent rejection– Any channel other than adjacent channel or co-channel



• BER < 10-6

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IEEE 802.16 OFDMA PHY