January 2001 doc IEEE 802 11 01060 Coding

January 2001 doc. : IEEE 802. 11 -01/060 Coding and Equalization for High Rate Extensions Steve Halford, Ph. D. Mark Webster Paul Chiuchiolo Intersil Corporation Palm Bay, FL Submission 1 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Performance & Complexity • W-LAN performance is dominated by multipath – Power consumption dominated by receiver complexity – W-LAN systems spend 90% of time in receive mode • OFDM is designed for both AWGN and multipath – Error correcting code for AWGN – Use guard interval to absorb ISI – Use FFT/IFFT and block structure to simplify receive equalizer • PBCC is optimized for AWGN – Error correcting for AWGN OFDM is lesscode complex for W-LAN – Multipath performance depends entirely on receiver Submission 2 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 PER fo 11 Mbps Multipath with No Equalizer • CCK outperforms PBCC-11 • PBCC requires equalizer • Scramble code does not help multipath ? CMF Receiver w/o Equalization Exponential Fading channel with 1 sample/chip Submission 3 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 Equalizers Types doc. : IEEE 802. 11 -01/060 • Linear Equalizer: – Linear filter that inverts the multipath channel – Length of filter depends on number of significant multipath rays • Decision Feedback Equalizer (DFE) – Subtracts interference from past data symbols • Uses hard decisions on received symbols prior to error correction – Uses linear equalizer subsection for pre-cursor taps – Can have reasonable complexity and performance for high SNR cases • Viterbi Equalizer (Maximum likelihood sequence estimate or MLSE) – Optimum equalizer (minimizes the bit error rate) – Finds the most likely sequence of transmitted symbol based Submission 4 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp. on channel

January 2001 doc. : IEEE 802. 11 -01/060 Linear Equalizer: Complexity for W- LANs • Linear Equalizer complexity is driven by length • Simulations indicate a 15 -20 tap equalizer is req’d • Equalizer taps are typically found by matrix inverse proportional to length of filter • Very high complexity -- once per packet • Adaptive estimation can reduce complexity in theory • Convergence is too slow for Wireless LAN systems • Each symbol also requires L complex multiplies & L-1 Linear Equalizers arecomplex too complexadds for adequate performance Submission 5 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Linear Equalizer vs. OFDM: Receiver Complexity IFFT/FFT for OFDM Equalization • 64 point FFT using radix-4 requires 96 complex multiplies • For each symbol, must perform an FFT at receiver • After receiver FFT, equalizer then requires 52 complex multiplies (1 per tone) • Perform once every symbol -- 80*(1/20 x 106) = 4 x 10 -6 seconds • Equivalent to (4 x (96 + 52))/(4 x 10 -6 ) = 148 x 106 real multiplies per second Single Carrier Linear Equalizer Complexity • Linear Equalizer of length L requires L complex multiplies per symbol • Number of real multiplies = (4*L*11 x 106 ) = L * (44 x 106 ) • Length L must be less than (148/44) = 3. 4 to match complexity of IFFT/ FFT combinations • Using pulse shaping makes this worse due to presence of matched filter! • Not required for OFDM • Doesn’t include the complexity of estimating the equalizer taps • Matrix inverse proportional to L OFDM use of FFT make it less complex than Linear Eq. ** Based on R. Van Nee & R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, Boston, MA, 2000. Submission 6 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Decision Feedback Equalizers Equalized Symbols Received Data DFE Whitened Matched Filter Removes ISI from channel precursor + Symbol Slicer (Hard decisions) Channel Post-cursor Filter • Uses WMF and symbol decision feedback to remove ISI – Post cursor filter is only as long as channel • Unlike linear equalizer which is 4 -10 times longer than channel • Does not require a matrix inverse to compute! – Whitened Matched Filter length depends on pre-cursor Submission 7 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp. and pulse shaping

January 2001 doc. : IEEE 802. 11 -01/060 Decision Feedback Equalizers: Performance • DFE performs well when: – High SNR since symbol decision must be correct • Incorrect decision lead to burst of equalizer generated noise • This will limit range of W-LAN more than AWGN performance – Post-cursor filter is sufficiently long • Possible to use decisions after error correction – Would allow operation at lower SNR won’t work due todelay high SNR complexity of WMF –DFE Long processing and&very high complexity Submission 8 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 MLSE/Viterbi Equalizer Received Data Whitened Matched Filter Viterbi Equalizer Equalized Symbols MLSE • Equalizer estimates the most likely sequence based on knowledge of the channel and the received data – Optimum bit error rate performance – No matrix inverse required – Only need to estimate channel • Equalization is similar to decoding a convolutional code – Searches a trellis of possible paths to find the most likely • For adequate performance, MLSE is the most likely equalizer for PBCC-11, PBCC-22, and higher – Need to track 4 or more paths for adequate performance 9 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp. Submission

January 2001 doc. : IEEE 802. 11 -01/060 MLSE: Complexity Considerations • Complexity is similar to convolutional decoder • Number of states depends on constellation size and number of multipath rays being tracked Number of States in MLSE ** Example Track 4 rays for 8 -level PSK (PBCC-22) Number of states = 83 = 512 states Eight times as complex as the 64 state PBCC 11/OFDM decoder& only 4 rays are being tracked! ** See pg. 590, J. G. Proakis, Digital Communication, 3 rd Ed. , Mc. Graw-Hill, 1995. Submission 10 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 OFDM is nearly MLSE • OFDM uses a guard interval to absorb multipath interference • Outside the guard interval, signal is multipath free – Multipath causes individual tones to fade • After FFT, each tone is multipath free – Relative fade is known from channel estimation • Viterbi Decoder of error correction code gives MLSE in multipath – Reliability of each soft-decision is weighted by known fade – Optimum receiver is realized with only a FFT – True provided multipath is entirely inside guard interval • Path delay less than 800 n. Secs Submission 11 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 OFDM Multipath Tolerance • OFDM proposal includes 800 n. Secs Guard Interval • Equivalent to 800 e-9 x 11 e 6 = 8. 8 paths at PBCC symbol rate • Multipath tolerance equivalent to tracking 8 paths • FFT complexity is approximately half the complexity of a 64 state decoder Equivalent SC MLSE Complexity This is 8192 times the complexity of the 256 state decoder! Submission 12 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Convolutional Coding & Complexity • Encoding process is relatively low complexity – Consists of shift registers and combiners • Decoding complexity depends on code properties – Decoders are based on Viterbi algorithm • Viterbi algorithm finds best path into each possible state – Complexity depends on the number of states in decoder • Number of states determines size of the trellis searched by VA • PBCC-11 & OFDM use a 64 -state decoder • PBCC-22 uses a 256 -state decoder – Trellis size (& complexity) is 4 x the equivalent OFDM decoder OFDM has a less complex error correction code Submission 13 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 256 state code vs. 64 state code 0. 8 d. B Advantage for PBCC-22 at 1% PER 1. 0 d. B Advantage for PBCC-22 at 10% PER 0. 8 -1. 0 d. B Coding Gain with 4 x Decoder Complexity Is 1 d. B worth the increased complexity? Submission 14 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Scramble Code • PBCC-11 & PBCC-22 include a scramble code – Changes the constellation mapping on a per symbol basis • Purpose has never been demonstrated Doesn’t change AWGN performance Submission Doesn’t change multipath performance 15 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Scramble Code with Equalizer • Does the scramble code help when used with an equalizer? Submission 16 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Interleavers for W-LAN • Burst Errors will occur in a W-LAN environment – Microwave ovens, photocopiers, & Blue. Tooth will generate bursts of interference – Duration typically 220 n. Secs (photocopier) & 16 ms for microwave***1 • Burst Errors also occur due to MLSE equalizer – Duration depends on # of paths & channel distance • Error burst can overwhelm an FEC code***2 – Interleaver necessary when bursts duration is > 2 symbols – OFDM includes an interleaver to help mitigate PBCC-11 & PBCC-22 need an interleaver! *** 1 K. Blackard, T. Rappaport, & C. Bostian, “Measurements and Models of Radio Frequency Impulsive Noise for Indoor Wireless Communications, ” IEEE Journal on Selected Areas in Communications, Vol. 11, No. 7, pg. 991 -1001, September 1993. ***2 See page 366 of G. C. Clark and J. B. Cain, Error-Correction Codeing for Digital Communications, Plenum Press, New York, NY, 1981. Submission 17 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Approaches to Higher Data Rates • OFDM provides a known, defined path to achieving higher rates • Higher data rates can be achieved by: – Increasing the constellation size and/or decrease code rate • Used by OFDM to give rates of 6 Mbps to 54 Mbps • PBCC-22 uses 8 -psk with rate 2/3 code to go from 11 Mbps (QPSK with rate 1/2) to 22 Mbps – Increasing symbol rate • PBCC-33 uses 1. 5 times higher clock speed to go from 22 Mbps to 33 Mbps • Increasing the data rate increases the required SNR for AWGN channels – More sensitive to implementation & tracking What is the impact on the equalizer performance and complexity of higher rates? Submission 18 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Higher Data Rates: Equalizer Complexity • OFDM Equalizer has fixed complexity for all proposed rates – Higher rates does impact performance due to fading of tones • Guard interval however reduces the impact independent of rate • MLSE complexity will grow exponentially when constellation size increases – Higher rates will impact performance • No guard interval to protect from increased ISI sensitivity – Example: Track 4 paths -- Number of states = (constellation size)4 -1 • 22 Mbps (8 -PSK) requires 83 = 512 states (8 x the PBCC-11 decoder) • 33 Mbps (16 -QAM) will require 163 = 4096 states (64 x the PBCC-11 decoder) 3 = 262144 states (4096 x the • 44 Mbps (64 -QAM) will require 64 Extending PBCC to higher rates by increasing constellation PBCC-11 decoder) is not practical Submission 19 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 doc. : IEEE 802. 11 -01/060 Higher Data Rates: Equalizer Complexity • OFDM uses a fixed symbol rate for all data rates – Guard interval protection is same for all rates • PBCC-33 is PBCC-22 at a higher symbol rate – Pulse shaping used to keep same spectral width • Increasing symbol rate impacts performance – Increasing timing accuracy requirements • Increasing rate increase number of equalizer paths – Example: 8 -PSK -- Number of states = 8(number of paths -1) • 22 Mbps (11 Mhz, 4 paths) -- 84 -1 = 512 states (8 x the PBCC-11 decoder) • 33 Mbps (16. 5 Mhz, 6 paths) -- 86 -1 = 32, 768 states (512 x PBCC-11 decoder) • 44 Mbps (22 Mhz, 8 paths) -- 88 -1 = 2, 097, 152 states (32, 768 x PBCC-11 decoder) Extending PBCC to higher rates by increasing symbol rate is not practical Submission 20 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 Conclusions doc. : IEEE 802. 11 -01/060 • Equalization is required for W-LAN systems – Receiver complexity is critical to successful systems • Linear and DFE equalizers are not practical for proposed high rate single carrier systems • MLSE is only viable equalizer for proposed systems – Complexity can grow exponentially • PBCC-22 FEC is 4 x as complex as industry standard – Yields less than 1 d. B improvement • PBCC-22 inherited the scramble code – Appears to be un-necessary • PBCC-22 lacks an interleaver – Poor performance with burst errors Submission 21 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.

January 2001 Conclusions doc. : IEEE 802. 11 -01/060 • OFDM provides near to MLSE performance – Guard interval absorbs multipath – Complexity is small fraction of MLSE for PBCC – Complexity remains fixed for proposed data rates! • OFDM uses industry standard FEC code • OFDM includes an interleaver • OFDM is less complex than PBCC for W-LAN – No hidden complexity details • OFDM gives access to higher than 22 Mbps rates – PBCC complexity grows exponentially Submission 22 S. Halford, M. Webster, P. Chiuchiolo, Intersil Corp.