1. CCK-OFDM Summary Steve Halford Mark Webster Jim Zyren Paul Chiuchiolo
May 2001
CCK-OFDM Summary
Steve Halford
Mark Webster
Jim Zyren
Paul Chiuchiolo
Intersil Corporation
S. Halford, et al Intersil Corporation

2. Why OFDM for High Rate? OFDM recognized as best solution for W-LAN
May 2001
Why OFDM for High Rate?
OFDM recognized as best solution for W-LAN
Selected by a & ETSI for W-LAN at 5 GHz
OFDM meets current & future needs:
Highest rates and backward compatibility
Meets consumer expectations set by a
High throughput with ultra-short preamble
New deployments & outdoor bridge applications
Share baseband with a
Dual band radios
Multiple baseband vendors
Best performance for complexity trade
Multipath & Bluetooth
S. Halford, et al Intersil Corporation

3. Overview of Intersil’s Proposal for 802.11g
May 2001
Overview of Intersil’s Proposal for g
S. Halford, et al Intersil Corporation

4. OFDM for High Rate Extension
May 2001
OFDM for High Rate Extension
Replace data portion of packet with OFDM modulation
Data rates 6, 9, 12, 18, 24, 36, 48 or 54 Mbps using 20 MHz symbol rate
Existing .11b radios will recognize preamble and header
Length field will be correctly decoded
CCA mechanisms maintained
Use reserve bits in b header for OFDM parameters
OFDM Proposal is compatible with b
S. Halford, et al Intersil Corporation

5. OFDM-Specific Fields Add OFDM-only sync and SIFs pad
May 2001
OFDM-Specific Fields
Add OFDM-only sync and SIFs pad
Reduces complexity of receiver & allows for flexible transmit filtering
Short Sync allows time for clock rate change
4 useconds duration is half duration of a
Allows time to switch rates
Can also use to refine time & frequency estimates
S. Halford, et al Intersil Corporation

6. May 2001
OFDM-Specific Fields
Long Sync provides training data for channel estimation
Provide 8 useconds of training data (same as a)
Do not need to rate-change b channel estimate
Can switch filters at transmitter for OFDM mode
SIFs pad extends the SIFs time to match a
802.11g receivers will see a 16 usec SIFs during OFDM operation
802.11b receivers will still see a 10 usec SIFs during OFDM operation
S. Halford, et al Intersil Corporation

7. Impact of OFDM-specific Fields
May 2001
Impact of OFDM-specific Fields
Simplify Radio design & add flexibility with added fields
OFDM Sync: Transition time & channel estimation
SIFs Pad: SIFs time compatibility between b & a
What is the impact on throughput?
Add 18 useconds of overhead
Reduction in Throughput
100 byte: 310 kbps
1000 byte: 500 kbps
2346 byte: 704 kbps
** Short Preamble option at 24 Mbps, No ACK
Throughput impact is negligible
S. Halford, et al Intersil Corporation

8. Ultra-Short Preamble Option
May 2001
Ultra-Short Preamble Option
Modulation & preambles identical to a
Reduces total preamble to 20 usecond
New deployments
Outdoor point-to-point links
Gives a route to a in the 2.4 GHz band
Compatible with mandatory g
Single PHY solution to high rate
S. Halford, et al Intersil Corporation

9. Radio Design Issues Baseband processor change only for 36 Mbps
May 2001
Radio Design Issues
Baseband processor change only for 36 Mbps
Current RF supports all rates up to 36 Mbps
OFDM preserves current channelization
3 channels spaced by 25 MHz (U.S. deployments)
48 & 54 Mbps supported with new RF
Same baseband
Higher density constellation has more stringent requirements on radio front end
Requirements are well from a designs
Design issues are well understood
S. Halford, et al Intersil Corporation

10. Forward Compatibility of CCK-OFDM
May 2001
Forward Compatibility of CCK-OFDM
S. Halford, et al Intersil Corporation

11. Advantages of compatibility
May 2001
Advantages of compatibility
CCK-OFDM provides .11a & .11g compatibility
Could also add HiperLAN2 compatibility
Marketplace will see single waveform as high rate wireless LAN solution
Introduction of a new waveform like PBCC will only fracture the marketplace
Lower cost dual band radios
Dual band a with PBCC requires one to build two complex basebands (OFDM & PBCC)
Multiple vendors provide basebands & IP for .11g
S. Halford, et al Intersil Corporation

12. Dual Band Radio Allow seamless transitions for laptop WLANs
May 2001
Dual Band Radio
Allow seamless transitions for laptop WLANs
Single low-cost card could provide support for .11b, .11a, and .11g
Auto-detect network or best connection type
S. Halford, et al Intersil Corporation

13. Further Advantages of Compatibility
May 2001
Further Advantages of Compatibility
Consider the cost of compatibility
Optional
Mandatory
TI Compromise
Proposal
.11a (OFDM-only) b(CCK) + .11g (PBCC)
.11b (CCK) g(OFDM)
Intersil
Proposal
inlcudes .11a(OFDM)
S. Halford, et al Intersil Corporation

14. Adding CCK Baseband to OFDM
May 2001
Adding CCK Baseband to OFDM
Adding CCK support is much easier than alternative
PBCC requires number of complex design efforts
PBCC-22 requires 30x ops/bit over CCK (00/384r1)
Must still support CCK
CCK receivers based on rake receiver
Implement with a channel matched filter & correlator
S. Halford, et al Intersil Corporation

15. Complexity Comparison between
May 2001
Complexity Comparison between
OFDM and PBCC
S. Halford, et al Intersil Corporation

16. PBCC:Reduced State Approach
May 2001
PBCC:Reduced State Approach
Suggested approach by PBCC proponents
Still higher complexity than OFDM
Minimum MF length will be 10 taps
Retain only 64 states out of 216 states at each update
8 symbols form the channel state
Each update, surviving state generate 4 new candidates
64 leads to 256 states
Retain the most likely 64 (requires sort)
Updates occur at symbol rate for this approach
S. Halford, et al Intersil Corporation

17. Compare PBCC & OFDM Compare Complexity of WMF with FFT & FEQ
May 2001
Compare PBCC & OFDM
Compare Complexity of WMF with FFT & FEQ
Compare Complexity of the two 64-state decoders
S. Halford, et al Intersil Corporation

18. Compare WMF w/ FFT & FEQ PBCC: WMF Complexity is driven by length
May 2001
Compare WMF w/ FFT & FEQ
PBCC: WMF Complexity is driven by length
Ignore the estimation problem
Needs 10 taps to handle 5 multipath rays
PBCC-22: Requires 880 x 106 real multiplies per second
PBCC-33: Requires 1980 x 106 real multiplies per second
Increase length to 15 to cover same delay spread
OFDM: Consider the FEQ & FFT
FEQ: 52 multiplies/symbol = 52 x 106 real multiplies per second
FFT: Radix 4, 96 multiplies/symbol = 96 x 106 real mps
Total: 148 x 106 real multiplies per second
Remains fixed for all data rates
S. Halford, et al Intersil Corporation

19. Compare 64-state Decoders
May 2001
Compare 64-state Decoders
Compare Trellis search approaches in terms of:
branch metrics calculations
path metric updates
compare selects
PBCC: 64 States  256 states  64-states for each symbol
256 branch metric calculations
256 path metric updates
Select 64 best of 256 states -- Variety of approaches
OFDM: 64 states  128 paths  64 states for each information bit
128 branch metric calculations
128 path metric updates
Compare each pair & select best compare-selects
S. Halford, et al Intersil Corporation

20. Compare Complexity: Summary
May 2001
Compare Complexity: Summary
Waveform
Real Multiplies
(x 106)
Branch Metric
Path Metric
PBCC-22
880
2816
PBCC-33
1980
4224
OFDM-24
148
3072
OFDM-36
4608
PBCC has much higher complexity due to matched filter
Equivalent OFDM operation (FFT & FEQ) have fixed complexity
OFDM & PBCC have nearly same complexity in trellis search (?)
PM & BM difference is proportional to data rate difference
S. Halford, et al Intersil Corporation

21. May 2001
Conclusions for g
S. Halford, et al Intersil Corporation

22. Conclusions PBCC relies on coding & sophisticated receiver
May 2001
Conclusions
PBCC relies on coding & sophisticated receiver
Non-standard code matched to 8-PSK signal
Different code than used for optional PBCC-11
No interleaver to help spread burst errors
Sensitive to burst errors like generated by Bluetooth
Cover code benefit never demonstrated
Why add unnecessary elements?
Requires complex decoder design to get adequate performance
3 years in development with no product or public demo
Single company provider ?
Already lags OFDM systems for data rate
S. Halford, et al Intersil Corporation

23. Conclusions OFDM is forward & backwards compatible
May 2001
Conclusions
OFDM is forward & backwards compatible
Uses existing long & short preamble for compatibility
802.11a modulation in place of CCK
OFDM-specific training data added for reduced complexity
Dual band radios possible
Offers a in the 2.4 GHz band through ultra-short preamble
OFDM offers the highest rates of all proposals
36 Mpbs with current radio (baseband only change)
48 & 54 Mbps possible with new radio design
Only proposal that meets consumer expectations
S. Halford, et al Intersil Corporation

24. Conclusions OFDM is ideal for W-LAN environment
May 2001
Conclusions
OFDM is ideal for W-LAN environment
Equalization split between transmitter & receiver for lower overall complexity
Guard Interval -- absorbs multipath without complexity
FFT & FEQ -- Low complexity & fixed for any rate
Nearly MLSE without complexity of PBCC
See Documents IEEE Submissions 01/153 & 01/060
Lower complexity error correction code
64 state code
Single code used for all code rates via puncturing
More robust to narrowband interference
Simple to remove known interference
S. Halford, et al Intersil Corporation

25. Conclusions OFDM now meets regulatory approval (5/10/01)
May 2001
Conclusions
OFDM now meets regulatory approval (5/10/01)
OFDM is now in the 2.4 GHz band
Higher rates than PBCC already being offered
IEEE should embrace & ensure network compatibility
OFDM (802.11a) development was collaborative
Multiple companies contributed ideas
Complexity & design is well known & proven
Many companies will offer products
S. Halford, et al Intersil Corporation