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Doc.: IEEE 802.11-01/154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 1 OFDM as a High Rate Extension to the CCK-based.

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Presentation on theme: "Doc.: IEEE 802.11-01/154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 1 OFDM as a High Rate Extension to the CCK-based."— Presentation transcript:

1 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 1 OFDM as a High Rate Extension to the CCK-based b Standard Steve Halford, Ph.D. Mark Webster Jim Zyren Intersil Corporation

2 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 2 Why OFDM for High Rate? OFDM recognized as best solution for W-LAN Selected by a & ETSI for W-LAN at 5 GHz Intersils proposed OFDM waveform offers: Fully backwards compatible with b Provides forward compatibility with a Meets data rate needs & expectations set by a Best complexity versus performance trade Good performance in real world W-LAN Well-known & proven technology

3 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 3 Overview of Intersils Proposal for g

4 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 4 OFDM for 2.4 Ghz band Use long & short preamble for backward compatibility – Ultra-short preamble possible for certain CCA modes Replace current CCK data with OFDM – Data modulation identical to a Maintain the same 2.4 GHz channels – 25 MHz center frequency spacing (wider than a) Use a clock rates (20 MHz) for OFDM mode – Data rates identical to a (6,9,12,18,24,36,48,54 mbps) – Originally proposed using b clock of 22 MHz – Now feel acceptance would be faster with a rates Technical differences are very small Rate change circuitry is common in low power IC – Open to changes from the group

5 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 5 Packet Structure: Backwards Compatible PREAMBLE/HEADER (Barker Words b) g LONG (Short) Preamble Packets 192 usecs (Long) 96 usecs (Short) 12 usecs PSDU SELECTABLE OFDM 6, 9, 12, 18, 24, 36, 48 or 54 Mbps OFDM SYNC 6 usecs Signal Extension Existing.11b radios will recognize preamble and header Length field will be correctly decoded Use reserve bits in header to denote switch Add OFDM Sync after.11b header to simplify design Add signal extension for SIFS compatibility Uses OFDM Modulation OFDM Proposal is compatible with b

6 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 6 OFDM Symbol Structure OFDM uses industry standard R=1/2, K=7 code –Known performance, complexity, and IP issues OFDM symbols are formed by IFFT of symbol block –Maps the coded data onto narrow carriers –IFFT block includes 4 pilot/training signals –Carriers retain orthogonality in multipath OFDM symbols include guard intervals for multipath –Provides buffer to absorb ISI 16 Samples 64 Samples 64 pt. IFFT of coded data Guard Interval time 4 usecs OFDM Symbol Preceding Symbol Multipath will cause preceding symbol to bleed into current symbol. Guard interval absorbs this interference

7 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 7 Radio Design Issues Requires a change in baseband processor only –Current RF gives adequate performance up to 36 Mbps OFDM preserves current channelization –3 channels spaced by 25 MHz (U.S. deployments) Power requirements are same as present products For 48 Mbps & 54 Mbps, new RF design is required –Standard would be in place & spur development –Design issues are well understood

8 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 8 Performance of OFDM with Prism II Radio 10% PER dB 1% PER dB 36 Mbps Mode** 24 Mbps Mode** 10% PER dB 1% PER dB Current radio provides sufficient quality to operate at rates up to 36 Mbps ** see notes pages for more details Implementation loss due to radio (Loss relative to ideal OFDM performance)

9 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 9 Preambles and Throughput

10 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 10 Throughput Impact of OFDM Sync Proposed OFDM Sync in addition to b & SIFS pad –Up to 18 usecs of additional overhead –Simplifies the receiver design –Allows future flexibility What is the impact on throughput? Decrease in throughput 100 byte: 310 kbits/sec 1000 byte: 500 kbits/sec 2346 byte: 704 kbits/sec Throughput impact is negligible

11 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 11 Complexity and Performance for g

12 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 12 OFDM Transmitter OFDM distributes equalization between the transmitter & receiver – Single carrier proposals relies on receiver for multipath protection – W-LAN systems are in receive mode 90% of time so reducing receive complexity is critical for power savings OFDM adds IFFT and cyclic extension operations to transmitter – Simplifies the equalizer in the receiver Data Scrambler Data Scrambler Convolutional Encoder Convolutional Encoder Puncture Interleave Constellation Mapping (bits to symbols) Constellation Mapping (bits to symbols) 64-pt Inverse FFT 64-pt Inverse FFT Uncoded Information Bits Cyclic Extension Cyclic Extension To RF Transmitter Transmitter Only added items compared to single carrier system like PBCC

13 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 13 Timing Adjust Timing Adjust CNCO Frequency Correction Carrier/Timing Correction Carrier/Timing Correction Trim Guard Interval Trim Guard Interval FFT &FEQ 52 tones FFT &FEQ 52 tones Frequency Domain Equalizer: Multiply each tone by inverse gain & phase of the channel Soft-Decisions on Bits (symbol to bits) Soft-Decisions on Bits (symbol to bits) De-interleave & De-puncture De-interleave & De-puncture Extract 4 Pilot Tones Extract 4 Pilot Tones Viterbi Decoder Viterbi Decoder Compute Branch Matrix Compute Branch Matrix De-Scrambler To MAC From A-to-D Receiver Major difference is use of FFT to simplify equalizer Reduce tracking complexity with pilot tones OFDM Receiver Structure

14 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 14 Error Correction Coding for High Rate Encoding process is relatively low complexity Decoding complexity depends on code properties Decoders are based on Viterbi algorithm VA searches trellis at each step for most likely state sequence 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 is 4x the equivalent all OFDM decoders Trace-back depth is larger than OFDM-24 OFDM has a less complex error correction code

15 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide dB Advantage for PBCC-22 at 1% PER 1.0 dB Advantage for PBCC-22 at 10% PER dB Coding Gain relative to punctured industry standard code. Requires Trellix 4x larger... If AWGN performance is needed, better codes could be developed for OFDM Is 1 dB worth greatly increased complexity? PBCC 256 state code vs. Industry Standard

16 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 16 Multipath & Equalization for g

17 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 17 Performance & Complexity Trades W-LAN performance is dominated by multipath OFDM is designed for both AWGN and multipath – Error correcting code to provide AWGN – Use guard interval to absorb ISI (0.96 dB AWGN loss) – Use pilot tones for improved tracking (0.34 dB AWGN loss) PBCC is optimized for AWGN only – Error correcting code for AWGN – Multipath performance depends entirely on receiver – Tracking depends entirely on receiver implementation OFDM is less complex than PBCC for W-LAN environment

18 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 18 Linear Equalizer -- Invert the channel with linear filter – Length of filter depends on number of multipath rays ( taps) – Matrix Inverse required for each packet – More complex than FFT based equalizer for OFDM Decision Feedback Equalizer (DFE ) -- Subtracts interference – Uses hard decisions on received symbols prior to error correction – May need a whitened matched filter (matrix inverse to compute) Viterbi Equalizer – Maximum likelihood sequence estimate or MLSE – Performance depends on number of paths tracked – May require whitened matched filter (# of taps ?) – Finds the most likely sequence of transmitted symbol based on channel Similar complexity & implementation to decoding a convolutional code Neither Linear nor DFE equalizers make sense for PBCC Equalizers for PBCC

19 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 19 Equalizer estimates the most likely sequence based on knowledge of the channel and the received data – Viterbi itself requires only a channel estimate – Matrix inverse may be required for WMF Can include in Viterbi -- affects the observed channel Similar to decoding a convolutional code – Searches a trellis for best path between states MLSE is the likely equalizer for PBCC-11 & 22 – Need to track 4 or more paths for adequate performance Whitened Matched Filter Whitened Matched Filter Viterbi Equalizer Viterbi Equalizer Equalized Symbols Received Data MLSE/Viterbi Equalizer

20 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 20 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 Joint Decoder Example Track 4 rays for 8-level PSK (PBCC-22) Number of states = 8 3 = 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, 3rd Ed., McGraw-Hill, 1995.

21 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 21 Joint Decoder MLSE: Complexity Considerations Possible to use single super-trellis for decoder Includes both multipath and FEC memory Number of States in SuperTrellis Example Track 4 rays for 8-level PSK with 256 state Conv. Code (PBCC-22 or 33) Number of states = 256 x 8 3 = 2 17 = Over 2000 times as complex as the 64 state PBCC-11/OFDM decoder Number of States in SuperTrellis = S M L-1 M: Constellation Size, L: Number of paths tracked, S: number states in FEC

22 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 22 OFDM gives MLSE type performance 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 nSecs

23 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 23 OFDM proposal includes 800 nSecs Guard Interval Equivalent to 800e-9 x 11e6 = 8.8 paths at PBCC symbol rate Multipath tolerance equivalent to tracking 8 paths FFT complexity is approximately twice the complexity of a 64 state decoder OFDM Multipath Tolerance Equivalent SC MLSE Complexity This is 2 15 (over 32,000) times the complexity of the 64 state decoder! OFDM: MLSE performance w/o complexity

24 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 24 Impact of interference on g

25 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 25 Interference in 2.4 GHz band 2.4 GHz spectrum is a shared resource – BlueTooth & other FH systems generate in-band interference on b & g radios – Other sources of interference include microwave ovens Higher data rates specified by g will be more sensitive to interference – Errors generated by presence of interference source can greatly influence the throughput BlueTooth enabled devices will proliferate at same time as g PBCC is sensitive to real world interference sources

26 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 26 PBCC performance is sensitive to BlueTooth E. Zehavi, et al (IEEE documents IEEE /061r0 & IEEE /066r0) showed that the throughput of coded 8- PSK w/o an interleaver was very sensitive to the presence of a BlueTooth- like interferer.

27 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 27 Extending PBCC to higher rates (>22 Mbps)

28 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 28 Approaches to Higher Data Rates OFDM provides a known path to 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 1.5x clock speed to go from 22 Mbps to 33 Mbps Increasing the data rate increases the required SNR OFDM equalizer complexity is same for all rates -- What is the impact on the PBCC receiver?

29 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 29 Are Higher Data Rates Possible? 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) Mbps (8-PSK) requires 8 3 = 512 states (8x the PBCC-11 decoder) 33 Mbps (16-QAM) will require 16 3 = 4096 states (64x the PBCC-11 decoder) 44 Mbps (64-QAM) will require 64 3 = states (4096x the PBCC-11 decoder) Extending PBCC to higher rates by increasing constellation is not practical

30 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 30 Are Higher Data Rates Possible? 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: – Example: 8-PSK -- Number of states = 8 (number of paths -1) 22 Mbps (11 Mhz, 4 paths) = 512 states (8x the PBCC-11 decoder) 33 Mbps (16.5 Mhz, 6 paths) = 32,768 states (512x PBCC-11 decoder) 44 Mbps (22 Mhz, 8 paths) = 2,097,152 states (32,768x PBCC-11 decoder) Extending PBCC to higher rates by increasing symbol rate is not practical

31 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 31 Conclusions on OFDM for g

32 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 32 Summary of Data Rates & Parameters

33 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 33 Conclusions OFDM is forward & backwards compatible –Uses existing long & short preamble for compatibility –802.11a signaling used in place of CCK –Minor impact on throughput of added headers OFDM offers the highest rates of all proposals – 36 Mpbs with current radio (baseband only change) –48 & 54 Mbps possible with new radio design –PBCC complexity grows exponentially

34 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 34 Conclusions OFDM is ideal for W-LAN environment –Equalization split between transmitter & receiver for lower overall complexity –Lower complexity error correction code –Nearly MLSE without complexity –PBCC Joint Decoder approach requires RSSE Complexity vs. Performance ? OFDM is robust to narrowband interference –PBCC seems to have an inherent problem with BT

35 doc.: IEEE /154 Submission March 2001 S. Halford, M. Webster, & J. Zyren, Intersil Corporation Slide 35 Conclusions OFDM will meet regulatory approval –All high rate waveforms possible under new rules (?) –OFDM will be in this band -- IEEE should ensure network compatibility OFDM has been developed in an open process –No hidden details –Complexity of PBCC never adequately described –Complexity and design is well known & proven


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