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Inprocomm PHY Proposal for IEEE 802.11n: MASSDIC-OFDM

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. Please refer to 04-1002 for technical specifications and 04-1018 for description of the LDPCC parity check matrices. Content.

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Inprocomm PHY Proposal for IEEE 802.11n: MASSDIC-OFDM

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  1. 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. Please refer to 04-1002 for technical specifications and 04-1018 for description of the LDPCC parity check matrices. Kim Wu et al., MASSDIC-OFDM

  2. 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 • PLCP Frame format • Preamble • FEC Coding • Compatibility to 802.11a • Simulation Results • Summary Kim Wu et al., MASSDIC-OFDM

  3. 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. • The proposal shall have another portion of MAC enhancement proposal. It can be amended later. Kim Wu et al., MASSDIC-OFDM

  4. Main features of the proposal 1/3 • 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 • Good performance by bit-interleaved convolutional coded modulation • This proposal uses MIMO (2x2 Mandatory) architecture to double the capacity. 4x4 and 3x3 configurations are optional • This proposal uses variable guard intervals to optimize the data rate against different channel delay spread • The operation bandwidth is 20 MHz. Kim Wu et al., MASSDIC-OFDM

  5. Main features of the proposal 2/3 • This proposal uses 256-QAM to boost bandwidth efficiency • The number of subcarriers in an OFDM symbol is increased from 64 to 128 to gain guard interval 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 coding • Only PHY data rates more than 53 Mbps are newly defined • For HT devices transmitting legacy data rates, space-time block coding (STBC) schemes are used to enhance system performance. Kim Wu et al., MASSDIC-OFDM

  6. Main features of the proposal 3/3 • No more overhead is needed for the PHY header relative to legacy frame format • New preamble structures are designed to minimize the overhead without sacrificing performance. • Only one OFDM symbol time interval is used for channel estimation • Long training symbols transmit higher power than other fields. • The maximum data length is extended from 4096 bytes to 65536 bytes. • This proposal uses modern powerful error control code: extended irregular repeat-accumulated (eIRA) low density parity check (LDPC) code to improve performance • Coding rates of 1/2, 2/3, 3/4 are separately designed with codeword length 2667 to optimize performance. • A new scheme to shorten codewords with hybrid code rate combination is proposed. 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 for 2x2 configuration Linear constellation Precoding Q QAM mapping p FEC Scrambler • Nt=2 ( 3, 4 optional), # of Tx antennas • Nc=128, the number of subcarriers per antenna • p: bit level interleaver (optional) • Q : a 4x4 unitary matrix • L-CP: 1200 or 800 ms cyclic prefix • Spatial processing: spatial multiplexing or space time block coding • FEC: eIRA LDPC code (2676, 2007), (2676, 1784), (2676,1338) RF L-CP Nc-IFFT Subcarrier grouping Spatial Processing Nc-IFFT RF L-CP Kim Wu et al., MASSDIC-OFDM

  9. Linear constellation precoding matrix Q - 4x4 case (optional) Kim Wu et al., MASSDIC-OFDM

  10. … NT=3 Group 1 Group 75 … NT=2 Group 1 Group 50 … … … … … … … … … … … d0 d74 d75 d99 d100 d149 d150 d199 d200 d224 d225 d299 d0 d49 d50 d99 d100 d124 d125 d174 d175 d199 Antenna 1 Antenna 2 Antenna 3 Antenna 1 Antenna 2 Subcarrier Grouping in LCP for 2x2 and 3x3 Kim Wu et al., MASSDIC-OFDM

  11. … NT=4 Group 1 Group 99 … … … … … d0 d49 d50 d99 d200 d224 d225 d274 d275 d299 Antenna 1 … Antenna 3 Group 2 Group 100 … … … … … d100 d149 d150 d199 d300 d324 d325 d374 d375 d399 Antenna 2 Antenna 4 Subcarrier Grouping in LCP for 4x4 Kim Wu et al., MASSDIC-OFDM

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

  13. System Parameters Kim Wu et al., MASSDIC-OFDM

  14. System Parameters Nt=2(Other data rates are the same as those in 802.11a) * 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. Kim Wu et al., MASSDIC-OFDM

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

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

  17. 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

  18. PLCP Frame format Kim Wu et al., MASSDIC-OFDM

  19. 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. 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

  20. Repeated Header • Using pad bits to convey a repeated header information • If the number of pad bits NPAD > , then a 16-QAM modulated repeated header is sequentially inserted into the following 32 data subcarriers • {d53, d59, d65, d71, d78, d84, d90, d96, d3, d9, d15, d21, d28, d34, d40, d46} of the 2nd antenna, • and {d53, d59, d65, d71, d78, d84, d90, d96, d3, d9, d15, d21, d28, d34, d40, d46} of the 1st antenna. • Then the number of pad bits is recalculated Kim Wu et al., MASSDIC-OFDM

  21. 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 antennas • For 802.11n Tx-Rx in 4x4 case, the mapped information is encoded as complex space-time code in the 4 Tx antennas • 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

  22. PLCP Preamble • N mode preamble structure • An access mode that is intended for a scenario that all devices are 802.11n. • LN mode preamble structure • An access mode that is intended for a scenario that some of the devices are legacy and others are 802.11n. Kim Wu et al., MASSDIC-OFDM

  23. N mode 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

  24. Kim Wu et al., MASSDIC-OFDM

  25. Generate time domain signals s1 and s2 from frequency domain signals S1 and S2 respectively via 128-IFFT. • Take the first 16 samples of s1 and s2, namely u1 and u2. • Get unity v1 and v2 via Gram-Schmidt procedure from u1 and u2. • Set and Kim Wu et al., MASSDIC-OFDM and

  26. N mode 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

  27. N mode 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

  28. 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

  29. Tone Interleaving for LTSs ( 2x2 case) Kim Wu et al., MASSDIC-OFDM

  30. PLCP Preamble • N mode preamble structure • An access mode that is intended for a scenario that all devices are 802.11n. • LN mode preamble structure • An access mode that is intended for a scenario that some of the devices are legacy and others are 802.11n. Kim Wu et al., MASSDIC-OFDM

  31. LN mode Preamble structure 2X2 case L-STS: The legacy short training symbolsLTSs are the same as those in N mode Kim Wu et al., MASSDIC-OFDM

  32. LN mode Preamble structure 3X3 case LTS1 is times the LTS1 in N mode. LTS2 and LTS3 are 800ns and 1200ns cyclic shifts of LTS1 respectively. Kim Wu et al., MASSDIC-OFDM

  33. LN mode Preamble structure 4X4 case LTSs are the same as those in N mode Kim Wu et al., MASSDIC-OFDM

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

  35. Bit-level Interleaver for 2x2 case Kim Wu et al., MASSDIC-OFDM

  36. 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

  37. 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

  38. 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

  39. 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

  40. 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

  41. 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 Kim Wu et al., MASSDIC-OFDM

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

  43. Packet Encoding • For a short block, it is not efficient if we encode it in the original code rate. Hence, we choose higher code rate to decrease the number of parity check bits. • If the information block size is smaller than , we change the original code rate to a higher one according to the following table. The threshold ( ) Kim Wu et al., MASSDIC-OFDM

  44. 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

  45. 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

  46. Codec Complexity Analysis • There are two kinds of operations in the decoding process: addition and check node operation (*). • The two operations have comparable complexity. • 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

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

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

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

  50. 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

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