1. Backwards compatibility
Sept 2003
Backwards compatibility
How to make a MIMO-OFDM system backwards compatible and coexistence with 11a/g at the link level.
Jan Boer, Bas Driesen and Pieter-Paul Giesberts, Agere Systems
Jan Boer, Agere Systems

2. Sept 2003
doc.: n
Sept 2003
The PAR
Some of the modes of operation defined in the HT amendment shall be backwards compatible and interoperable with a and/or g.
Jan Boer, Agere Systems
Jan Boer, Agere Systems

3. Sept 2003
doc.: n
Sept 2003
Meaning for MIMO-OFDM
Any higher order MIMO-OFDM system (with n Rx antennas) can receive a signal from a lower order MIMO/SISO transmitter (< n Tx antennas, SISO = 11a or 11g)
Detection of preamble, interpretation of the header:
Determining the number of transmit antennas (number of data streams) and switch Rx accordingly
Any higher order MIMO-OFDM transmitter (n Tx antennas) can transmit a signal that a lower order MIMO/SISO receiver can receive
Switch back to (ultimately) 11a or 11g
* order of MIMO system is dependent on # Tx antennas
Jan Boer, Agere Systems
Jan Boer, Agere Systems

4. Coexistence requirement
Sept 2003
Coexistence requirement
Any lower order system (with n Rx antennas) that cannot receive data of a transmitter (with more than n antennas) defers while this transmitter is sending, because it is capable to detect the start of this transmission and retrieve the length (duration) of this transmission.
Defer not on power only
Detection of the preamble and interpretation of the length field,
Using existing multirate capabilities of the current standard(s)
Jan Boer, Agere Systems

5. Backward Compatible Preambles
Sept 2003
doc.: n
Sept 2003
Backward Compatible Preambles
11a/g preamble structure must be maintained
Two examples are given of preamble structures that can be made backward compatible and coexistent:
Repetition preamble
Diagonally loaded preamble
3rd way: use protection mechanisms as defined in 11g
Jan Boer, Agere Systems
Jan Boer, Agere Systems

6. Sept 2003
1. Repeating preamble
TX1 ST GI
LT1
LT2
Signal
Sign2
TX2 GI
LT1
LT2
TX3 GI
LT1
LT2
802.11a Long Training Symbols (including Guard Interval) repeated on every transmit antenna, separated in time
Channel training length equal to n times length a training
Following is an example for 3 antenna MIMO Tx:
Jan Boer, Agere Systems

7. 3x3 Mimo Rx vs 11a Rx training
Sept 2003
3x3 Mimo Rx vs 11a Rx training
TX1 ST GI
LT1
LT2
3x3 Mimo
Detects short training symbols
Channel estimation (3 paths), etc
11a
Detects short training symbols
Channel estimation
Jan Boer, Agere Systems

8. Sept 2003
MIMO notification
TX1 ST GI
LT1
LT2
Signal
The Reserved bit 4 in the signal field is used to signal a MIMO transmission (R=1?)
11a-1999 standard says:
Bit 4 shall be reserved for future use
The a standard does not prescribe the value (all other reserved bits shall be set to zero for 11a compliance)
This most probably means that R is ignored by a 11a/g implementation
Jan Boer, Agere Systems

9. Sept 2003
Rate and Length field
TX1 ST GI
LT1
LT2
Signal
Rate: as defined in 11a (6,9,12….54Mbit/s) per antenna
Length in Bytes as defined in 11a per antenna
Includes
Payload
Sign2 (see next)
Additional training symbols
Padding bits
Jan Boer, Agere Systems

10. 3x3 Mimo Rx vs 11a Rx signal Mimo 11a Signal detection Knows duration
Sept 2003
3x3 Mimo Rx vs 11a Rx signal
TX1 ST GI
LT1
LT2
Signal
Mimo
Signal detection
Rate valid
Length valid
Knows duration
Mimo notification
11a
Signal detection
Rate valid
Length valid
Knows duration
Ignores R4
Jan Boer, Agere Systems

11. 3x3 Mimo Rx vs 11a Rx sign2 Mimo 11a Detects # Tx antennas
Sept 2003
3x3 Mimo Rx vs 11a Rx sign2
TX1 ST GI
LT1
LT2
Signal
Sign2
# Tx antennas
Additional Mimo info
Mimo
Detects # Tx antennas
Additional Mimo info:
E.g. correction on # bytes per antenna
11a
Tries to demodulate payload
Does not recognize service field
Starts to decrement lengthfield
Jan Boer, Agere Systems

12. 3x3 Mimo Rx vs 11a Rx Mimo 11a Trains other paths Decrements length
Sept 2003
3x3 Mimo Rx vs 11a Rx
TX1 ST GI
LT1
LT2
Signal
Sign2
TX2 GI
LT1
LT2
TX3 GI
LT1
LT2
Mimo
Trains other paths
Starts demod Mimo signal
11a
Decrements length
Defers during duration of mimo transmission (CCA busy)
Jan Boer, Agere Systems

13. Repeating preamble Backwards compatibility and coexistence
Sept 2003
Repeating preamble Backwards compatibility and coexistence
Higher order mimo can switch back to lower order using less antennas and adapt the training accordingly
Lower order mimo can be received by higher order
Mimo notification + # of transmit antennas
Lower order defers based on length and rate interpretation
Jan Boer, Agere Systems

14. 2. Diagonally loaded preamble
Sept 2003
2. Diagonally loaded preamble
TX1 ST GI
LT1
LT2
Signal
TX2 ST GI
LT1
LT2
Signal
TX3 ST GI
LT1
LT2
Signal
802.11a Training sequence made orthogonal by diagonally loading subcarriers onto the transmit antennas
Training length equal to a training length
Jan Boer, Agere Systems

15. Mimo channel estimation
Sept 2003
Mimo channel estimation
Mimo:
Step 1: Adding the long training sequences to gain in SNR
Step 2: Transformation to frequency domain
Step 3:Demodulation of the combined long training sequences
Step 4: Interpolation of the subcarriers, to get the full channel estimate:
Problem of the edge subcarriers
Interpolation of the outer subcarriers cannot be done because these subcarriers have subcarriers only at one side
These subcarriers can only extrapolated
Extrapolation error is bigger than interpolation error
Solution
Adding extra subcarriers at the edges of the spectrum
These carriers can be used to transfer information
Jan Boer, Agere Systems

16. 11a/g channel estimation on diagonally loaded preamble
Sept 2003
11a/g channel estimation on diagonally loaded preamble
Combination of diagonally loaded sequences is 11a preamble, on which 11a can train.
Different Tx antennas will be reflected in the channel estimation.
Averaging techniques over carriers in frequency domain or reducing the channel impulse response length in the time domain might disrupt the estimation: possible compatibility issue.
Jan Boer, Agere Systems

17. Mimo notification, Length, Rate
Sept 2003
Mimo notification, Length, Rate
TX1 ST GI
LT1
LT2
Signal
TX2 ST GI
LT1
LT2
Signal
TX3 ST GI
LT1
LT2
Signal
Use R4 in signal field for MIMO notification
Rate as defined in 11a per antenna
Length in Bytes per antenna
Extra subcarriers at edge of spectrum can signal the number of transmit antennas
Jan Boer, Agere Systems

18. Mimo vs 11a/g reception Mimo 11a/g ST GI LT1 LT2 Signal ST GI LT1 LT2
Sept 2003
Mimo vs 11a/g reception
TX1 ST GI
LT1
LT2
Signal
TX2 ST GI
LT1
LT2
Signal
TX3 ST GI
LT1
LT2
Signal
Mimo
Detect short training symbols
Channel estimation
Signal detection, rate, length
# antennas in edge subcarriers
Decode mimo signal
11a/g
Detect short training symbols
Channel estimation
Signal detection, valid rate, valid length
Tries to demodulate, no service field
Decrements length
Defers during duration of mimo transmission (CCA busy)
Jan Boer, Agere Systems

19. Diagonally loaded preamble Backwards compatibility and coexistence
Sept 2003
Diagonally loaded preamble Backwards compatibility and coexistence
Higher order mimo can switch back to lower order using less antennas and adapt the training accordingly
Lower order mimo can be received by higher order
Mimo notification + # of transmit antennas
Lower order defers based on length and rate interpretation
Jan Boer, Agere Systems

20. 3. Protection Mechanism As in 11g:
Sept 2003
3. Protection Mechanism
As in 11g:
Precede Mimo transmission with 11a or 11g (RTS/)CTS to reserve medium
Advantage: Mimo preamble can be dedicated
Disadvantage: overhead
Jan Boer, Agere Systems

21. Preamble and throughput
Sept 2003
Preamble and throughput
Longer preamble effects throughput negatively
If longer preambles are necessary (performance, backwards compatibility,…) the effect on throughput should be clear.
Next slides show examples of preamble overhead compared to other overheads for 54Mbit/s and 162Mbit/s
Jan Boer, Agere Systems

22. Throughput overview @1500 bytes frames
Sept 2003
Throughput bytes frames
Efficiency & throughput
Regular
3 frame burst
3 frame aggregation
54 Mbit/s
66% = 36 Mbit/s
69% = 38 Mbit/s
84% = 45 Mbit/s
162 Mbit/s
(20μs preamble)
40% = 67 Mbit/s
45% = 75 Mbit/s
66% = 107 Mbit/s
(40μs preamble)
35 % = 59 Mbit/s
39% = 65 Mbit/s
62% = 101 Mbit/s
Effect of preamblelength is not neglegable but for the boost of throughput
other techniques must be applied such as frame bursting or aggregation
Jan Boer, Agere Systems

23. 54 Mbit/s Sept 2003 Datarate = 54Mbit/s, Ack rate = 24 Mbit/s
Jan Boer, Agere Systems

24. 162 Mbit/s (20 µs preamble+header, diagonally loaded)
Sept 2003
162 Mbit/s (20 µs preamble+header, diagonally loaded)
Datarate = 162 Mbit/s,
Ack rate = 54 Mbit/s (both frames on MIMO speed)
Jan Boer, Agere Systems

25. 162 Mbit/s (40 µs preamble+header, repeating)
Sept 2003
162 Mbit/s (40 µs preamble+header, repeating)
Datarate = 162 Mbit/s,
Ack rate = 24 Mbit/s (Ack on regular .11a speed)
Jan Boer, Agere Systems

26. Sept 2003
Conclusion
A MIMO-OFDM system can be made (fairly simple) backwards compatible and coexistent with 11a/g on the link level.
Preamble based on 11a/g preamble structure
Repeated preamble
Diagonally loaded preamble
11g protection mechanisms
Criteria for decision:
Throughput overhead
Performance
Complexity
Jan Boer, Agere Systems