doc.: ieee 802.11-01/154 submission march 2001 s. halford, m. webster, & j. zyren, intersil...
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March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 1
doc.: IEEE 802.11-01/154
Submission
OFDM as a High Rate Extension to the CCK-based
802.11b Standard
Steve Halford, Ph.D.
Mark Webster
Jim Zyren
Intersil Corporation
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 2
doc.: IEEE 802.11-01/154
Submission
Why OFDM for High Rate?• OFDM recognized as best solution for W-LAN
Selected by 802.11a & ETSI for W-LAN at 5 GHz• Intersil’s proposed OFDM waveform offers:
Fully backwards compatible with 802.11bProvides forward compatibility with 802.11aMeets data rate needs & expectations set by
802.11aBest complexity versus performance tradeGood performance in real world W-LAN Well-known & proven technology
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 3
doc.: IEEE 802.11-01/154
Submission
Overview of Intersil’s Proposal for 802.11g
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 4
doc.: IEEE 802.11-01/154
Submission
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 802.11a• Maintain the same 2.4 GHz channels
– 25 MHz center frequency spacing (wider than 802.11a)• Use 802.11a clock rates (20 MHz) for OFDM mode
– Data rates identical to 802.11a (6,9,12,18,24,36,48,54 mbps)– Originally proposed using 802.11b clock of 22 MHz– Now feel acceptance would be faster with 802.11a rates
• Technical differences are very small• Rate change circuitry is common in low power IC
– Open to changes from the group
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 5
doc.: IEEE 802.11-01/154
Submission
Packet Structure: Backwards Compatible
PREAMBLE/HEADER(Barker Words -- 802.11b)
802.11g LONG (Short) Preamble Packets
192 usecs (Long)96 usecs (Short)
12 usecs
PSDU SELECTABLEOFDM Symbols
@ 6, 9, 12, 18, 24, 36, 48 or 54 Mbps
OFDMSYNC
6 usecs
SignalExtension
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 802.11bOFDM Proposal is compatible with 802.11b
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 6
doc.: IEEE 802.11-01/154
Submission
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
16Samples
64Samples
64 pt. IFFT of coded dataGuardInterval
time
4 usecs OFDM Symbol
Preceding Symbol
Multipath will cause preceding symbol to “bleed” into current symbol. Guard
interval absorbs this interference
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 7
doc.: IEEE 802.11-01/154
Submission
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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 8
doc.: IEEE 802.11-01/154
Submission
Performance of OFDM with Prism II Radio
9 10 11 12 13 14 15 1610
-4
10-3
10-2
10-1
100
PER vs. Eb/No for 39.6 Mbps Prism II Radio
Eb/No (dB)
PE
R
baseline backoff=6dBbackoff=7dBbackoff=8dBbackoff=9dB
• 10% PER -- 2.7 dB• 1% PER -- 4.1 dB
36 Mbps Mode**
24 Mbps Mode**• 10% PER -- 1.5 dB• 1% PER -- 2.1 dB
Current radio provides Current radio provides sufficient quality to operate at sufficient quality to operate at
rates up to 36 Mbpsrates up to 36 Mbps** see notes pages for more details
Implementation loss due to radio(Loss relative to ideal OFDM performance)
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 9
doc.: IEEE 802.11-01/154
Submission
Preambles and Throughput
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 10
doc.: IEEE 802.11-01/154
Submission
0 500 1000 1500 2000 25000
5
10
15
20
25
30
35
40
45
Data Length in Bytes
Max
imum
Eff
ectiv
e T
hrou
ghpu
t (M
bps)
Throughput without ACK using short preamble
OFDM-12 SP OFDM-24 SP OFDM-36 SP OFDM-54 SP CCK-11 SP OFDM-24 w/o OS
Throughput with added OFDM sync
Througput without added OFDM sync
Throughput Impact of OFDM Sync
• Proposed OFDM Sync in addition to 802.11b & SIFS pad– Up to 18 usecs of additional overhead
– Simplifies the receiver design
– Allows future flexibility
• What is the impact on throughput?
Decrease in throughputDecrease in throughput100 byte: 310 kbits/sec
1000 byte: 500 kbits/sec2346 byte: 704 kbits/sec
Throughput impact is negligible
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 11
doc.: IEEE 802.11-01/154
Submission
Complexity and Performance for 802.11g
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 12
doc.: IEEE 802.11-01/154
Submission
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
ConvolutionalEncoder
ConvolutionalEncoder PuncturePuncture InterleaveInterleave
ConstellationMapping(bits to
symbols)
ConstellationMapping(bits to
symbols)
64-ptInverse
FFT
64-ptInverse
FFT
UncodedInformation
Bits
CyclicExtension
CyclicExtension
To RF TransmitterTransmitter
Only added items compared to single carrier system like PBCC
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 13
doc.: IEEE 802.11-01/154
Submission
TimingAdjust
TimingAdjust
CNCOCNCO
Frequency Correction
Carrier/Timing Correction
Carrier/Timing Correction
TrimGuard Interval
TrimGuard Interval
FFT &FEQ52 tones
FFT &FEQ52 tones
Frequency Domain Equalizer:Multiply each tone by inverse gain &
phase of the channel
Soft-Decisionson Bits
(symbol to bits)
Soft-Decisionson Bits
(symbol to bits)
De-interleave &De-puncture
De-interleave &De-puncture
Extract 4 PilotTones
Extract 4 PilotTones
Viterbi Decoder
Viterbi Decoder
Compute BranchMatrix
Compute BranchMatrix
De-ScramblerDe-ScramblerTo MAC
From A-to-D
Receiver
• Major difference is use of FFT to simplify equalizer• Reduce tracking complexity with pilot tones
OFDM Receiver Structure
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 14
doc.: IEEE 802.11-01/154
Submission
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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 15
doc.: IEEE 802.11-01/154
Submission
0.8 dB Advantage for PBCC-22 at 1% PER1.0 dB Advantage for PBCC-22 at 10% PER
0.8-1.0 dBCoding Gain
relative to punctured industry
standard code. Requires Trellix 4x
larger...
If AWGN performance is needed, better codes could be developed for
OFDMIs 1 dB worth greatly increased complexity?
PBCC 256 state code vs. Industry Standard
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 16
doc.: IEEE 802.11-01/154
Submission
Multipath & Equalization for 802.11g
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 17
doc.: IEEE 802.11-01/154
Submission
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 OFDM is less complex than PBCC for W-LAN environment for W-LAN environment
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 18
doc.: IEEE 802.11-01/154
Submission
• Linear Equalizer -- Invert the channel with linear filter– Length of filter depends on number of multipath rays (15- 20 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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 19
doc.: IEEE 802.11-01/154
Submission
• 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
FilterViterbi
Equalizer
Viterbi Equalizer
Equalized Symbols
Received Data
MLSE/Viterbi Equalizer
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 20
doc.: IEEE 802.11-01/154
Submission
MLSE: Complexity Considerations• Complexity is similar to convolutional decoder• Number of states depends on constellation size
and number of multipath rays being tracked
1Number of states
: Constellation Size
: Number of rays tracked by equalizer
LM
M
L
Number of States in Joint DecoderNumber of States in Joint Decoder
ExampleExample
Track 4 rays for 8-level PSK (PBCC-22)Number of states = 83 = 512 states
Eight times as complex as Eight times as complex as the 64 state the 64 state
PBCC-11/OFDM PBCC-11/OFDM decoder& only 4 rays are decoder& only 4 rays are
being tracked!being tracked!** See pg. 590, J. G. Proakis, Digital Communication, 3rd Ed., McGraw-Hill, 1995.
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 21
doc.: IEEE 802.11-01/154
Submission
Joint Decoder MLSE: Complexity Considerations
• Possible to use single “super-trellis” for decoder• Includes both multipath and FEC memory
Number of States in SuperTrellisNumber of States in SuperTrellis
ExampleExample
Track 4 rays for 8-level PSK with 256 stateConv. Code (PBCC-22 or 33)
Number of states = 256 x 83 = 217 = 131072
Over 2000 times as Over 2000 times as complex as the 64 state complex as the 64 state
PBCC-11/OFDM decoderPBCC-11/OFDM decoder
Number of States in SuperTrellis = S ML-1
M: Constellation Size, L: Number of paths tracked, S: number states in FEC
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 22
doc.: IEEE 802.11-01/154
Submission
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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 23
doc.: IEEE 802.11-01/154
Submission
• 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 ToleranceOFDM Multipath Tolerance
Equivalent SC MLSE ComplexityEquivalent SC MLSE Complexity1
8-1
Recall Number of States = where is the constellation size &
is number of paths
For PBCC-22, 8 Number of states = 8 2,097,152 states
LM M
L
M
This is 215 (over 32,000) times the complexity of the 64 state decoder!
OFDM: MLSE performance w/o complexity
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 24
doc.: IEEE 802.11-01/154
Submission
Impact of interference on 802.11g
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 25
doc.: IEEE 802.11-01/154
Submission
Interference in 2.4 GHz band• 2.4 GHz spectrum is a shared resource
– BlueTooth & other FH systems generate in-band interference on 802.11b & 802.11g radios
– Other sources of interference include microwave ovens
• Higher data rates specified by 802.11g 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 802.11g
PBCC is sensitive to real world interference sources
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 26
doc.: IEEE 802.11-01/154
Submission
PBCC performance is sensitive to BlueTooth
0
2
4
6
8
10
12
1 10 100 1000
WLAN Receiver to Transmiter distance (M)
WL
AN
th
rou
gh
pu
t (M
bp
s)
8PSK with Random Tone Jammer
OFDM with Random Tone Jammer
8PSK without Interference
OFDM without Interference
E. Zehavi, et al (IEEE documents IEEE802.11-
01/061r0 & IEEE802.15-01/066r0)
showed that the throughput of coded 8-PSK w/o an interleaver
was very sensitive to the presence of a
BlueTooth- like interferer.
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 27
doc.: IEEE 802.11-01/154
Submission
Extending PBCC to higher rates (>22 Mbps)
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 28
doc.: IEEE 802.11-01/154
Submission
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 -- OFDM equalizer complexity is same for all rates -- What is the impact on the PBCC receiver? What is the impact on the PBCC receiver?
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 29
doc.: IEEE 802.11-01/154
Submission
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)4-1
• 22 Mbps (8-PSK) requires 83 = 512 states (8x the PBCC-11 decoder)• 33 Mbps (16-QAM) will require 163 = 4096 states (64x the PBCC-11 decoder)• 44 Mbps (64-QAM) will require 643 = 262144 states (4096x the PBCC-11
decoder)
Extending PBCC to higher rates by increasing constellationis not practical
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 30
doc.: IEEE 802.11-01/154
Submission
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) -- 84-1 = 512 states (8x the PBCC-11 decoder)• 33 Mbps (16.5 Mhz, 6 paths) -- 86-1 = 32,768 states (512x PBCC-11 decoder)• 44 Mbps (22 Mhz, 8 paths) -- 88-1 = 2,097,152 states (32,768x PBCC-11 decoder)
Extending PBCC to higher rates by increasing symbol rateis not practical
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 31
doc.: IEEE 802.11-01/154
Submission
Conclusions on OFDM for 802.11g
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 32
doc.: IEEE 802.11-01/154
Submission
Summary of Data Rates & Summary of Data Rates & ParametersParameters
Data Rate (Mbps) Code Rate Constellation Bits/Symbol6.0 1/2 BPSK 249.0 3/4 BPSK 36
12.0 1/2 QPSK 4818.0 3/4 QPSK 7224.0 1/2 16-QAM 9636.0 3/4 16-QAM 14448.0 2/3 64-QAM 19254.0 3/4 64-QAM 216
Modulation OFDMFundamental Sample Rate 20 Mhz
Data Symbol Length 4.0 Sec (80 samples @ 20 Mhz)Guard Interval Length 800 nSec (16 samples @ 20 Mhz)FFT Window Length 3.2 Sec (64 samples @ 20 Mhz)
Number of Data Tones 48Number of Pilot Tones 4
Frequency of Operation 2.4 GHz ISM band
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 33
doc.: IEEE 802.11-01/154
Submission
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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 34
doc.: IEEE 802.11-01/154
Submission
ConclusionsOFDM 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
March 2001
S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 35
doc.: IEEE 802.11-01/154
Submission
ConclusionsOFDM 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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