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Project: IEEE P802.11 Working Group for Wireless Lacal Area Networks (WLANs)

Project: IEEE P802.11 Working Group for Wireless Lacal Area Networks (WLANs) Submission Title: Inprocomm PHY Proposal for IEEE 802.11n: MASSDIC-OFDM Date Submitted: Aug 13 th , 2004 Source: Kim Wu et al., Inprocomm, Inc. Address 9F, No. 93, Shuei-Yuan St. Hsinchu, 300 Taiwan, R.O.C

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Project: IEEE P802.11 Working Group for Wireless Lacal Area Networks (WLANs)

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  1. Project: IEEE P802.11 Working Group for Wireless Lacal Area Networks (WLANs) Submission Title:Inprocomm PHY Proposal for IEEE 802.11n: MASSDIC-OFDM Date Submitted: Aug 13th, 2004 Source:Kim Wu et al., Inprocomm, Inc. Address 9F, No. 93, Shuei-Yuan St. Hsinchu, 300 Taiwan, R.O.C Voice: +886 572 5050, FAX: +886 572 6060, E-Mail: kimwu@inprocomm.com Re:[Response to Call for Proposals] Abstract: We present a partial proposal for a high-data-rate physical layer of a Local Area Network, using MASSDIC-OFDM transmission. The air interface is based on 2x2 MIMO-OFDM, using up to 256-QAM for the modulation. Advanced FEC coding using modern LDPC code is proposed with signal-space diversity coding to significantly improve the performance. The proposal is extendable to 3x3 MIMO and 4x4 MIMO. Purpose:[Proposing a PHY-layer interface for standardization by 802.11n] Notice: This document has been prepared to assist the IEEE P802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.11. Kim Wu et al., MASSDIC-OFDM

  2. Inprocomm PHY Proposal for IEEE 802.11n: MASSDIC-OFDM Kim Wu, Chao-Yu Chen, Tsung-Yu Wu, Racy Cheng, Chi-chao Chao, Mao-Ching Chiu Inprocomm, Inc. Kim Wu et al., MASSDIC-OFDM

  3. Content • Assumptions in the proposal • Main features of the proposal • Proposed Multiple-Antenna Signal Space DIversity Coded OFDM (MASSDIC-OFDM) PHY System architecture • Modulation, precoding, FEC, proposed receiver structure • Frame format • Preamble • FEC Coding • Compatibility to 802.11a • Simulation Results • Summary Kim Wu et al., MASSDIC-OFDM

  4. Assumptions in the proposal • The proposal is targeted at the physical layer • A MAC efficiency of 60% is assumed • To reach the 100 Mbps MAC Goodput, a minimum of 167 Mbps is required. Kim Wu et al., MASSDIC-OFDM

  5. Main features of the proposal 1/2 • This proposal inherits the good features of 802.11a OFDM standard • Spectrally efficient, robust against narrowband interference • Low complexity in channel equalization • No ISI and intercarrier interference (ICI) if channel max delay is less than the guard interval • Powerful FEC code by bit-interleaved convolutional coded modulation • This proposal uses MIMO (2x2 Mandatory) architecture to double the capacity. And the MIMO is extendable to 4x4 • This proposal uses variable guard interval to optimize the data rate against different channel delay spread • The operation bandwith is targeted at 20 MHz, the same as that in 802.11a. Kim Wu et al., MASSDIC-OFDM

  6. Main features of the proposal 2/2 • This proposal uses 256-QAM to boost bandwidth efficiency • This proposal explores the signal and space diversity without sacrificing BW via linear constellation precoding technique (LCP) (optional) • With low rate (e.g. 3/4) FEC, OFDM is shown to be inferior to single-carrier transmission due to loss of multipath diversity • This problem is resolved by artificially making ICI among independent subcarriers • The LCP is a kind of signal-space diversity code • This proposal use modern powerful error control code: extended irregular repeat-accumulated (eIRA) low density parity check (LDPC) code to improve performance Kim Wu et al., MASSDIC-OFDM

  7. Proposed PHY:Multiple-Antenna Signal Space DIversity Coded OFDM (MASSDIC-OFDM) Kim Wu et al., MASSDIC-OFDM

  8. MASSDIC-OFDM Tx Architecture Linear constellation Precoding Q QAM mapping p1 FEC Scrambler • Nt=2 ( 3, 4 optional), # of Tx antennas • Nc=128, the number of subcarriers per antenna • p1: bit level interleaver (optional) • Q : a 4x4 unitary matrix • L-CP: 1200 or 800 ms cyclic prefix • FEC: eIRA LDPC code (2676, 2007), (2676, 1784), (2676,1338) RF L-CP Nc-IFFT Subcarrier grouping Nc-IFFT RF L-CP Kim Wu et al., MASSDIC-OFDM

  9. Linear constellation precoding matrix Q Kim Wu et al., MASSDIC-OFDM

  10. Performance improvement with LCP (ML-soft output sphere decoding) Kim Wu et al., MASSDIC-OFDM

  11. Subcarrier index Nc=128, K=4 and the subcarrier grouping for LP Antenna 1 y0 y2 95 96 127 0 1 2 Antenna 2 y1 y3 128 225 161 162 224 Kim Wu et al., MASSDIC-OFDM

  12. System Parameters Nt=2(lower bit rates are the same as 802.11a Nt=1 case) * Rate X is dedicated for the header. The K=7, CC encoded data is 16-QAM modulated and inserted into the pilot tones of the first OFDM symbol of the first 2 antennas while those of the other antennas are nulled. Kim Wu et al., MASSDIC-OFDM

  13. System Parameters Nt=3 (Optional) Kim Wu et al., MASSDIC-OFDM

  14. System Parameters Nt=4(Optional) Kim Wu et al., MASSDIC-OFDM

  15. Proposed Receiver Structure for 2x2 case RF DU1 MIMO Equalization (MMSE, DEF, ML-S) LCP-decoding (ML-S) RF DU2 Information bits De-Scrambler FEC-decoder QAM Demapping De-int. Demodulation Unit ( DU ) Channel Estimation, Equalization FFT Sym. Detection CP Remove Synch. Kim Wu et al., MASSDIC-OFDM

  16. 802.11n Frame format If Pad bits > 64, then a repeated PHY header is substituted for the first 64 bits of the pad bits. The FCS only checks for payload. First OFDM symbol PLCP preamble Repeated PHY Header PHY Header FCS CRC-16 Pad bits Payload: 0-65536 bytes Data field K=7, R=1/2 CC encoded and inserted into the pilot tones of the first OFDM Symbol of the fist two antennas while those of other antennas are nulled. Rate 6 bits Reserved 2 bits LCP 1 bit Service 16 bits Length 16 bits Interleaver 1 bit Tail 6 bits HCS 16 bits Kim Wu et al., MASSDIC-OFDM

  17. Operation for lower transmission rates • For 802.11n Tx-Rx in 2x2 case operating in the lower transmission rate mode (6~54Mbps), the mapped information is encoded as complex 2x2 Alamouti space-time code in the 2 Tx antenna • For 802.11n Tx-Rx in 4x4 case, the mapped information is encoded as complex 4x4 Alamouti space-time code in the 4 Tx antenna • For 802.11n Tx-Rx in 3x3 case, the third Tx antenna is turned off while the others are thesame as the case in 2x2 Kim Wu et al., MASSDIC-OFDM

  18. Preamble structure 2X2 case 5.6 ms 2.4 ms X X X X X -X X -X -X -X Y Y Y Y Y -Y Y -Y -Y -Y TX 1 Data 1 S1 L1 CP TX 2 S2 Data 2 L2 CP 1.6 ms 6.4 ms Si’s: Short Training Symbols, X is the same as 802.11a, Y is orthogonal to X. Li’s: Long Training Symbols CP: Cyclic Prefix Kim Wu et al., MASSDIC-OFDM

  19. Preamble structure 3X3 case 2.4 ms 5.6 ms X X X X X -X X -X -X -X Y Y Y Y Y -Y Y -Y -Y -Y Z Z Z Z Z -Z Z -Z -Z -Z TX 1 S1 L1 Data 1 CP TX 2 S2 Data 2 L2 CP TX 3 S3 Data 3 L3 CP 6.4 ms 1.6 ms X ,Y, Z orthogonal. Kim Wu et al., MASSDIC-OFDM

  20. Preamble structure 4X4 case 2.4 ms 5.6 ms X X X X X -X X -X -X -X Y Y Y Y Y -Y Y -Y -Y -Y Z Z Z Z Z -Z Z -Z -Z -Z W W W W W -X W -W -W -W TX 1 S1 L1 Data 1 CP S2 Data 2 L2 CP Data 3 S3 L3 CP TX 2 L4 S4 Data 4 CP 6.4 ms 1.6 ms X ,Y, Z W orthogonal. Kim Wu et al., MASSDIC-OFDM

  21. More on the Long Training Symbols in the frequency domain with • The long training symbols are designed such that the Mean squared error(MSE) is minimized when ML-estimation is used • The long training symbols Li’s have the following properties: • The Li’s are orthogonal • The Li’s are nearly circular-shift orthogonal • Low PAPR Kim Wu et al., MASSDIC-OFDM

  22. Data Scrambler • Use the same scrambler as 802.11a Kim Wu et al., MASSDIC-OFDM

  23. Bit-level Interleaver Kim Wu et al., MASSDIC-OFDM

  24. Details of theInterleaver • Denote byk the index before permutation and j the index after permutation. ( : the number of coded bits per OFDM symbol) • First permutation: • Second permutation: where . is obtained by concatenating two OFDM symbols that have been bit-interleaved by the above interleaver. Kim Wu et al., MASSDIC-OFDM

  25. Pad bits The number of OFDM symbols, ; the number of bits in the DATA field, ; and the number of pad bits, , are computed in terms of the length of the PSDU (LENGTH) by: Kim Wu et al., MASSDIC-OFDM

  26. FEC Coding • Using extended irregular repeat-accumulate (eIRA) code (a kind of LDPC code) • For R=3/4: (2676, 2007) • For R=2/3: (2676, 1784) • For R=1/2: (2676,1338) • For information length other than 2007 (1784 or 1338) use code shortening • Header is CC encoded with R=1/2 and K=7 (the same as 802.11a) Kim Wu et al., MASSDIC-OFDM

  27. FEC encoder structure Kim Wu et al., MASSDIC-OFDM

  28. Packet size accommodation with concatenation and shortening 2007 (1784, 1338) bit data field 669 (892, 1338) bit parity 2676 bit codeword For long packets, codewords are concatenated 669 (892, 1338) bit parity 2676-N bit zero pad N bit data field For short blocks, codeword shortening is adapted. The zero pad will not be transmitted. Kim Wu et al., MASSDIC-OFDM

  29. Details of the proposed FEC code • In general, the parity check matrix H of an (n,k) eIRA code could be written as the following form : , where is a random spare matrix, is the inverse of a lower-triangular matrix. Kim Wu et al., MASSDIC-OFDM

  30. We decide the column and row weight distribution of through Gaussian Approximation. • Then, we put 1’s on randomly and avoid the length-4 cycle. Kim Wu et al., MASSDIC-OFDM

  31. The generator matrix is in the format of • Usually, it costs large complexity to calculate . However, the which is a full upper-triangle matrix can be implemented by a differential encoder in the above figure. Kim Wu et al., MASSDIC-OFDM

  32. Packet Encoding • For a short block, it is not efficient if we encode it in the original code rate. Hence, we choose the other code rate to decrease the number of parity check bits. • If the block size is on the intervalof the table shownbelow, we change the code rate. Kim Wu et al., MASSDIC-OFDM

  33. Decoder Structure • For a LDPC code, it can be described as a Tanner graph with variable nodes , check nodes and edges. Kim Wu et al., MASSDIC-OFDM

  34. Each check node represents a parity check equation. • Check node j is connected to a variable node i if and only if the element hji in the parity check matrix H is a 1 • Through the Tanner graph, belief propagation algorithm is used to decode the eIRA code. Kim Wu et al., MASSDIC-OFDM

  35. Codec Complexity Analysis • There are two kinds of operations in the decoding process: addition and check node operation (*). • The complexity of the two operations are similar. • The degree of a node means the number of edges connected to the node. • We define and respectively as the number of variable and check nodes of degree ; and as the maximum degree of variable and check nodes. *: Kim Wu et al., MASSDIC-OFDM

  36. There are additions and check node operations in one iteration. Kim Wu et al., MASSDIC-OFDM

  37. The weight distributions of column and row are: Kim Wu et al., MASSDIC-OFDM

  38. In our eIRA code, the numbers of operations per iteration are as follows: Kim Wu et al., MASSDIC-OFDM

  39. TheViterbi decoding complexity for Convolutional code (802.11a) with code word length 2676 (*): Here, we ignore the calculation of branch metric and the memory trace-back process. And, we assume thatthe complexity of comparing is equal to that of addition. Kim Wu et al., MASSDIC-OFDM

  40. Compatibility to 802.11a • The proposed 802.11n is compatible to 802.11a by defining the same PHY and MAC as that of 802.11a in low data rate mode (6Mbps~54Mbps mode). • A 802.11n device can distinguish between 802.11a and 802.11n packets by detecting different format of packet preambles. • Upon detection of 802.11a packets, the 802.11n device turns to operate in the 802.11a mode. Kim Wu et al., MASSDIC-OFDM

  41. Simulation Results • PER vs. Eb/N0 performance for fading channel model B, C, D, E and AWGN channel. • PER vs. SNR performance for fading channel model channel B, C, D, E and AWGN channel. • Throughput vs. SNR performance for channel B, C, D, E and AWGN channel. Kim Wu et al., MASSDIC-OFDM

  42. 2X2 MIMO AWGN Channel PER vs. Eb/No for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  43. 2X2 MIMO Channel B PER vs. Eb/No for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  44. 2X2 MIMO Channel C PER vs. Eb/No for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  45. 2X2 MIMO Channel D PER vs. Eb/No for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  46. 2X2 MIMO Channel E PER vs. Eb/No for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  47. 2X2 MIMO AWGN Channel PER vs. SNR for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  48. 2X2 MIMO Channel B PER vs. SNR for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  49. 2X2 MIMO Channel C PER vs. SNR for Different Data Rates Kim Wu et al., MASSDIC-OFDM

  50. 2X2 MIMO Channel D PER vs. SNR for Different Data Rates Kim Wu et al., MASSDIC-OFDM

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