Channel Interleaver for Convolutional Turbo Code Document Number

Channel Interleaver for Convolutional Turbo Code Document Number: IEEE C 802. 16 m-09/0141 r 1 Date Submitted: 2009 -01 -05 Source: Ciou. Ping Wu (ciouping. wu@mediatek. com), Peikai Liao (pk. liao@mediatek. com), Yu-Hao Chang (yuhao. chang@mediatek. com), Yih. Shen Chen (yihshen. chen@mediatek. com) and Paul Cheng (paul. cheng@mediatek. com) Media. Tek Inc. No. 1 Dusing Rd. 1 Hsinchu Science Park Hsinchu City 300 Taiwan. Venue: SDD: section 11. 13. 1. 3 FEC Encoding In response to IEEE 802. 16 m-08/052 “Call for Comments on Project 802. 16 m SDD”. Base Contribution: This is base contribution. Purpose: Propose to be discussed and adopted by TGm for the use in Project 802. 16 m SDD. Notice: This document does not represent the agreed views of the IEEE 802. 16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802. 16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: <http: //standards. ieee. org/guides/bylaws/sect 6 -7. html#6> and <http: //standards. ieee. org/guides/opman/sect 6. html#6. 3>. Further information is located at <http: //standards. ieee. org/board/pat-material. html> and <http: //standards. ieee. org/board/pat >.

Introduction • In SDD, the FEC encoder used in IEEE 802. 16 m includes CTC (convolutional turbo code) of code rate 1/3 defined in the IEEE 802. 16 e[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 be allocated onto different bit locations within symbols

Interleaver of IEEE 802. 16 e Convolutional Turbo code(1/2) • Channel interleaver for convolutional turbo code (CTC) used in IEEE 802. 16 e[1]

Interleaver of IEEE 802. 16 e Convolutional Turbo Code(2/2) • In IEEE 802. 16 e, the channel interleaver includes: – Symbol separation • • Sequentially distribute all encoded bits into six subblocks Subblocks A and B are information blocks Subblocks Y 1 and Y 2 are the parity blocks generated by the first convolutional encoder of CTC Subblocks W 1 and W 2 are the parity blocks generated by the second convolutional encoder of CTC – Subblock interleaving • • The six subblocks are interleaved independently The behavior of subblock interleaver can be described by the mathematical formula, which is a function of block size, m and J. For example, a tentative output address Tk in 802. 16 e 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 • Y 1, Y 2, W 1 and W 2 are bit-wise interleaved

Issues with IEEE 802. 16 e subblock interleaver (1/2) • Example: 16 QAM, data_len = 48 bytes, code_rate = ½ – Based on the previous output address formula Tk, the read address of subblocks A and B are shown as • In 16 QAM 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 IEEE 802. 16 e 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. 16 e 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

Issues with IEEE 802. 16 e symbol grouping • Symbol grouping for Y 1, Y 2, W 1 and W 2 are illustrated as below Issue #2: • When 16 QAM is considered, subblock Y 1 (W 1) and Y 2 (W 2) are mapped onto more and less robust bit location, respectively • This design may not combat burst error efficiently

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:

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. 16 e • Constellation-based address generator: – Produce a read address Ci which can insure that the adjacent coded 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 check 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 tentative address – 64 QAM • Step 1(grouping): Divide the whole tentative addresses Ai into three groups of the same size. • Step 2(1 st shifting): Perform one circular shift along the left or right direction on the second group by one tentative address • Step 3(2 nd shifting): Perform one circular shift along the same direction in step 2 on the last group by two tentative addresses

Proposed symbol grouping scheme • Goal – Change the bit multiplex order of subblocks Y and W so that not all the parity check 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 is 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 16 QAM (s = 2) 64 QAM (s= 3)

Simulation Result • Simulation parameters: AWGN – – Modulation order : 16 QAM Code rate : ½ Data block size (bytes): 48 bytes Code block size (bytes) : 96 bytes – – Modulation order : 64 QAM Code rate : 2/3 Data block size (bytes): 48 bytes Code block size (bytes) : 72 bytes

Conclusion • Since the proposed channel interleaver provide the frequency diversity and constellation diversity, the performance of proposed channel interleaver is better than original channel interleaver : – 16 QAM : • With target BLER = 0. 01, the performance of proposed channel interleaver is better than original channel interleaver 0. 2 d. B in AWGN channel. – 64 QAM : • With target BLER = 0. 01, the performance of proposed channel interleaver is better than original channel interleaver 0. 5 d. B in AWGN channel. • 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. 16 m-08/003 r 6 [Adopt the following modified text starting from line#16, page#123 in IEEE 802. 16 m-08/003 r 6[2]] … The encoder block depicted in Figure 53 includes sub-block interleavers. The interleaving design should be optimized to exploit frequency diversity and constellation diversity. The interleaving details are FFS.

Reference [1] [2] IEEE P 802. 16 Rev 2 / D 7, “Draft IEEE Standard for Local and Metropolitan Area Networks: Air Interface for Broadband Wireless Access, ” Oct. 2008. IEEE 802. 16 m-08/003 r 6, “The Draft IEEE 802. 16 m System Description Document”