Channel Interleaver for Convolutional Turbo CodeDocument Number: IEEE C802.16m-09/0141Date Submitted: 2009-01-05Source: CiouPing Wu (ciouping.wu@mediatek.com), Peikai Liao (pk.liao@mediatek.com), Yu-Hao Chang (yuhao.chang@mediatek.com), YihShen Chen (yihshen.chen@mediatek.com) and Paul Cheng (paul.cheng@mediatek.com)

MediaTek Inc. No. 1 Dusing Rd. 1 Hsinchu Science Park Hsinchu City 300 Taiwan.

Venue:SDD: section 11.13.1.3 FEC EncodingIn response to IEEE 802.16m-08/052 “Call for Comments on Project 802.16m SDD”.

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Introduction

• In SDD, the FEC encoder used in IEEE 802.16m includes CTC (convolutional turbo code) of code rate 1/3 defined in the IEEE 802.16e[1] standard and the channel interleavers.

• Channel interleaving is employed to average the burst channel errors over one data package so as to improve performance.

• The channel interleaver design in OFDM system should meet the following two conditions:– Frequency diversity gain:

• When frequency domain mapping is considered, adjacent bits should be allocated to nonconsecutive subcarriers so as to maximize the exploited frequency diversity gain

– Constellation diversity gain: • When symbol constellation mapping is considered, adjacent bits should be b

e allocated onto different bit locations within symbols

Interleaver of IEEE802.16e Convolutional Turbo code(1/2)

• Channel interleaver for convolutional turbo code (CTC) used in IEEE 802.16e[1]

Interleaver of IEEE 802.16e Convolutional Turbo Code(2/2)

• In IEEE 802.16e, the channel interleaver includes:– Symbol separation

• Sequentially distribute all encoded bits into six subblocks• Subblocks A and B are information blocks• Subblocks Y1 and Y2 are the parity blocks generated by the first convolutional encoder of CTC• Subblocks W1 and W2 are the parity blocks generated by the second convolutional encoder of CT

C

– Subblock interleaving• The six subblocks are interleaved independently• The behavior of subblock interleaver can be described by the mathematical formula, which is a fun

ction of block size, m and J. For example, a tentative output address Tk in 802.16e is given as

• If the tentative output address Tk is greater than N or equal to N, the tentative output address Tk is discarded, where N = ½ * (the number of the information bits in a coded block)

– symbol grouping• A and B are block-wise interleaved

• Y1, Y2, W1 and W2 are bit-wise interleaved

)/()mod(2 JkBROJkT mm

k

Issues with IEEE802.16e subblock interleaver (1/2)

• Example: 16QAM, data_len = 48 bytes, code_rate = ½– Based on the previous output address formula Tk, the read address of subbloc

ks A and B are shown as

• In 16QAM constellation,– Blue indexing bits are mapped onto the bit positions with higher level

of robustness– Black indexing bits are mapped onto the bit positions with lower level

of robustness

Issues with IEEE802.16e subblock interleaver (2/2)

Observation:If we reorder the bit in the previous address table according to its indices from low to high as shown in the below figure, we find that 802.16e subblock interleaver will map contiguous bits onto the bit location with the same level of robustness in 16 QAM constellation

Issue #1: • Contiguous coded bits are mapped onto the bit location with the same level

of robustness in the symbol constellation• This kind of design fails to combat the channel burst error due to the lack of

constellation diversity

128 ...129 160 ...161 191...9663 ...64 95...3231...0

: More robust bits : Less robust bits

Issues with IEEE802.16e symbol grouping

• Symbol grouping for Y1, Y2, W1 and W2 are illustrated as below

Issue #2:• When 16 QAM is considered, subblock Y1 (W1) and Y2 (W2) are mapped o

nto more and less robust bit location, respectively• This design may not combat burst error efficiently

A Subblock

Y1 Subblock

Y2 Subblock

...

B Subblock

... ...

S Y

W1 Subblock

W2 Subblock

...

W

Proposed Channel interleaver(1/2)

• In order to obtain frequency and constellation diversity gain, we propose a modified version of subblock interleaving and symbol grouping for channel interleaver:

Bit separationSubblock

interleaveringsymbol grouping

Tentative address generator

Constellation-based address generator

Bit mapper

Modulation orderData block size

Ai

Ci

Proposed Channel interleaver(2/2)

– The proposed subblock interleaving includes• Tentative address generator:

– Produce a tentative address Ai which maps adjacent coded bits onto nonconsecutive subcarriers

– The behavior of tentative address generator is the same as the subblock interleaving used in IEEE 802.16e

• Constellation-based address generator:– Produce a read address Ci which can insure that the adjacent c

oded bits are mapped alternatively onto bit location with different levels of robustness in the constellation

– The behavior of the constellation-based address generator is described in following slides

– The proposed symbol grouping:• Multiplex the coded bits which are in six subblocks• The proposed symbol grouping can ensure that not all the parity che

ck bits in one subblock are mapped onto either more or less robust bit position.

Proposed Constellation-based address generator • Goal :

Interleave the tentative address to generate the read address which can alternatively map the adjacent coded bits onto less or more robust bits

– 16 QAM• Divide the whole tentative addresses Ai into two separated parts and then

perform one circular shift to the left on the second group by one read address

– 64 QAM• Step1(grouping): Divide the whole tentative addresses A i into three groups of the

same size.• Step2(1st shifting): Perform one circular shift along the left or right direction on

the second group by one tentative address• Step3(2nd shifting): Perform one circular shift along the same direction in step 2

on the last group by two tentative addresses

...

N/2 N/2

...

N/3 N/3

...

N/3

Proposed symbol grouping scheme

• Goal– Change the bit multiplex order of subblocks Y and W so that not all the parity che

ck bits in one subblock are mapped onto either more or less robust bit position

• Approach– Step 1(partition): The coded bits are divided into couples of units, each of which i

s with s contiguous bits where s = ½*(the number of the bits per modulated symbol)

– Step 2(unit-wised multiplexing): The parity check subblocks are unit-by-unit multiplexed

16QAM (s = 2)

64QAM (s= 3)

A Subblock

Y1 Subblock

Y2 Subblock

...

B Subblock

... ...

S Y

W1 Subblock

W2 Subblock

...

W

A Subblock

Y1 Subblock

Y2 Subblock

...

B Subblock

... ...

W1 Subblock

W2 Subblock

...

Y W

Simulation environment

Parameters Assumptions

Subcarrier spacing 10.9375kHz

FFT size 1024

CP configuration 1/8Tu (=11.42us)

Permutation (Subchannelization) PUSC

Number of Tx/Rx antennas 2Tx-2Rx

Channel model VehA 30km/h

Modulation and coding rate QPSK 1/12 and 1/16

Channel coding Convolutional turbo code

Channel estimation Perfect

Receiver structure MMSE

Multi-antenna transmission format Spatial multiplexing

Simulation Result(1/2)

• ProposedA channel interleaver: Only insert the constellation-based address generator in the channel interleaver used in 802.16e

• Simulation parameters:– Modulation order :16QAM– Code rate : ½– Data block size (bytes):48 byte

s – Channel : AWGN

Simulation Result(1/2)

• Simulation parameters:– Modulation order :16QAM– Code rate : ½– Data block size (bytes):48 bytes

Simulation Result(2/2)

• Simulation parameters:– Modulation order :64QAM– Code rate : ½– Data block size (bytes) : 72 bytes

Conclusion

• In AWGN:– The proposedA channel interleaver performs better than original channel interleav

er• Alternatively mapping the adjacent coded bits onto different bit locations within symbols

in the constellation can exploit the constellation diversity gain

– The proposed channel interleaving performs better than the channel interleaving used in 16e and the proposedA channel interleaver in high SNR region

• Alternatively mapping the coded bits which are in the whole subblock onto more or less robust bits locations will increase the performance of the convolutional turbo code.

• In fadding channel:– 16QAM :

• the performance of proposed channel interleaver is much better than original channel interleaver.

– 64QAM : • the performance of proposed channel interleaver is better than original channel interlea

ver 2dB more or less with target BLER = 0.001.(576,1/2,64QAM)

• The channel interleaver design in OFDM system should meet the following two considerations:

– Adjacent bits are allocated to nonconsecutive subcarriers– Adjacent bits are allocated onto different bit locations within symbols

Proposed text on IEEE 802.16m-08/003r6

[Adopt the following modified text starting from line#16, page#123 in IEEE 802.16m-08/003r6[2]]

… The encoder block depicted in Figure 53 includes sub-block interleavers. The interleaving design should insure that the adjacent coded bits are mapped onto nonadjacent subcarriers and they are alternatively mapped onto less or more robust bits of the symbol constellation. The interleaving details are FFS.

Reference

[1] IEEE P802.16 Rev2 / D7, “Draft IEEE Standard for Local and Metropolitan Area Networks: Air Interface for Broadband Wireless Access,” Oct. 2008.

[2] IEEE 802.16m-08/003r6, “The Draft IEEE 802.16m System Description Document”