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Backwards compatibility

Backwards compatibility

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Backwards compatibility

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  1. 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. The PAR • Some of the modes of operation defined in the HT amendment shall be backwards compatible and interoperable with 802.11a and/or 802.11g. Jan Boer, Agere Systems

  3. 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

  4. 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 • 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

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

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

  8. GI LT1 LT2 MIMO notification TX1 ST 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 802.11a 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. GI LT1 LT2 Rate and Length field TX1 ST 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. Mimo Signal detection Rate valid Length valid Knows duration Mimo notification 11a Signal detection Rate valid Length valid Knows duration Ignores R4 GI LT1 LT2 3x3 Mimo Rx vs 11a Rx signal TX1 ST Signal Jan Boer, Agere Systems

  11. 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 GI LT1 LT2 3x3 Mimo Rx vs 11a Rx sign2 TX1 ST Signal Sign2 # Tx antennas Additional Mimo info Jan Boer, Agere Systems

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

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

  15. 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 • 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. GI GI GI LT1 LT1 LT1 LT2 LT2 LT2 Mimo notification, Length, Rate TX1 ST 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 TX2 ST Signal TX3 ST Signal Jan Boer, Agere Systems

  18. 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) GI GI GI LT1 LT1 LT1 LT2 LT2 LT2 Mimo vs 11a/g reception TX1 ST Signal TX2 ST Signal TX3 ST Signal Jan Boer, Agere Systems

  19. Diagonally loaded preambleBackwards 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: • 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 • 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. 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 162 Mbit/s (40μs preamble) 35 % = 59 Mbit/s 39% = 65 Mbit/s 62% = 101 Mbit/s Throughput overview@1500 bytes frames 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 Datarate = 54Mbit/s, Ack rate = 24 Mbit/s Jan Boer, Agere Systems

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

  25. 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. 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