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11ac Explicit Sounding and Feedback

11ac Explicit Sounding and Feedback. Date: 2010-09-14. Authors:. Outline. Overview Explicit Sounding Exchange NDP Sounding Compressed Feedback Format Summary Appendix: Compressed V FB Simulations Compressed V FB Angle extensions CSI and Compressed V size comparisons Straw Polls.

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11ac Explicit Sounding and Feedback

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  1. 11ac Explicit Sounding and Feedback Date: 2010-09-14 Authors:

  2. Outline • Overview • Explicit Sounding Exchange • NDP Sounding • Compressed Feedback Format • Summary • Appendix: • Compressed V FB Simulations • Compressed V FB Angle extensions • CSI and Compressed V size comparisons • Straw Polls

  3. 1. Overview • MU-MIMO is an important optional feature of 11ac • This generally requires the channel feedback for beamformer to communicate with multiple STAs simultaneously. • The feedback format for SU-beamforming is well defined in 11n • It is preferred to have common sounding and feedback mechanism for MU-MIMO and SU-BF. • Multiple options to implement a feature creates interoperability issues. • Propose a sounding and feedback mechanism based on IEEE 802.11n SU-BF sounding and feedback • TGac PAR is for 5GHz only, dual band devices may support IEEE 802.11n SU-BF sounding and feedback for 2.4GHz • Desirable to have similar implementation for operation in TGac mode.

  4. Background: Multiple Options in 11n TxBF • Multiple Modes for 11n TxBF, including: • Implicit BF, + CSI FB for calibration. • Explicit feedback format: CSI, compressed V, Non-compressed V. • Implicit BF, explicit BF, and calibration with either Staggered sounding, or NDP . • NDP announcement frame: regular data packet, QoS null frame, management frame, control wrapper. • Immediate-SIFS feedback, Immediate-Aggregated feedback, and delayed feedback. • It is highly desirable to trim down to only one option for TGac SU-BF and MU-MIMO.

  5. 2. Explicit Sounding Exchange • 11n SU-BF has multiple options: • Implicit Beamforming. • Explicit Beamforming (Immediate Feedback). (Steered) … BFMer TRQ Data Sounding BFMee (Steered) BFMer Sounding Data IFB BFMee

  6. Issues with Implicit Sounding in 11ac • Sounding feedback cannot be collected from a BFee that has less TX antennas than RX antennas.  • Implicit sounding requires calibration exchange in each channel: • It is desirable to define only one sounding exchange in 11ac, but the calibration responder still needs to support explicit FB according to 11n calibration process.

  7. Calibration Issues—cont’d • Imperfect calibration at the transmitter is less tolerable in MU than in SU-BF: lead to interference leakage that may not be completely cancelled by the Rx. • Uncalibrated clients is less tolerable in MU than in SU-BF. • Refer to Appendix I for the simulation of imperfect calibrations. • In 11n Implicit BF, BFMee is not required to do calibration. • For MU, uncalibrated RF imbalance at the clients may also introduce larger interference leakage for some MU precoder designs. • May require calibration supports at both AP and clients. • Example for a 2-client case:

  8. Propose Explicit Sounding Exchange as the Only 11ac Sounding Protocol • Provides the best possible channel estimation quality for accurate MU precoder design. • A unified SU/MU sounding protocol. • Also propose to define one type of explicit feedback format, and only allow immediate-SIFS feedback, to further unify among the multiple choices of 11n. • Recall: 11n defines three explicit feedbacks: CSI, Noncomp V, Compressed V. • Recall: 11n defines three types feedback timing: immediate-SIFS, immediate-aggregated, and delayed. • Immediate-SIFS FB is also important for MU overhead reduction. • Explicit sounding sequence and feedback format refer to the subsequent slides.

  9. 3. NDP Sounding • 11n defines two types of sounding format: Staggered and NDP. • NDP is more “friendly” for immediate FB at the receiver side. • NDP Announcement preceding NDP contains RA, STA information, and all other necessary information for preparing the feedback. • Rx knows exactly what to do (or whether need to do anything) before the NDP comes. • NDP does not require ACK. • A “clean” immediate FB frame SIFS following NDP is possible, and no frame aggregation is needed. • May not need multiple possibilities of immediate feedbacks (i.e. immediate SIFS, and immediate aggregated) like in 11n BF. • NDP is also more “friendly” for the Tx side: • AP/BFMer expects a single “clean” immediate SIFS feedback. • Q matrix for the sounding packet could always be set to identity matrix without power loss. • Not require complex number matrix multiplication to determine the final steering matrices. • NDP simplifies preamble design: • No additional NESS bits required in VHTSIG • No special VHTLTF design and handling (same as VHTLTF for normal multi-stream data packet).

  10. NDP PPDU Format • NDP PPDU format should be the same as the VHT preamble. • VHTSIGA subfields are defined as a SU Packet. • The SU-Nsts field implies the number of VHTLTFs • VHTSIGB has fixed bit pattern (TBD). To reduce PAPR VHT-SIGB should not have large number of zeros. … L-STF L-LTF L-SIG VHT-SIG-A (Symbol 1) VHT-SIG-A (Symbol 2) VHT-STF VHT-LTF1 VHT-LTFN VHT-SIG-B

  11. L-LENGTH in NDP • No “Non-Sounding” bit required in VHTSIGA. • To indicate an NDP packet (Sounding indication). • Rx: Assert NDP, if

  12. NDP Sounding Sequence—Single User • Not allow the 11n delayed or aggregated feedbacks. • Detailed sequence refer to [1].

  13. NDP Sounding Sequence—MU • Feedback happens SIFS after NDP, or SIFS after polling frames. • Detailed sequence refer to [1].

  14. 4. Compressed Feedback • Compressed V matrix FB is a good candidate for both SU and MU. • Unified format between SU and MU. • Unified format between 11ac in 5GHz and 11n in 2.4GHz and 5GHz. • Reduced overhead (important for MU) compared with CSI feedback, by using quantized angles in V to replace raw I/Q values in H. • Overhead reduction from compressed V FB was largely discussed back in 11n—refer to [2]. • Refer to Appendix III for size comparisons. • Enables feeding back partial rank, in LOS or ill-conditioned channels. • Feeding back V matrix in MU performs similarly as feeding back CSI matrix, for a wide range of precoder designs. • Refer to the Appendix I for simulations.

  15. Other Compressed FB Options • Regarding the time domain compressed CSI FB proposed in [3][4]. • It introduces a new feedback approach different from 11n, one more feedback mode for devices supporting both TGac feedback and 11n feedback(s). • Higher complexity, memory size and power consumption by using FFT engine to generate and decode the feedback, especially for immediate feedback. • Feeding back time domain CSI doesn’t allow reduced-rank feedbacks, which is definitely sufficient in SU, and somewhat sufficient in MU (e.g. in LOS or ill-conditioned channels).

  16. Compressed V FB Proposal—1 • Propose to define compressed V matrix FB, which is based on the 802.11n subclause20.3.12.2.5 and 7.3.1.29, with appropriate changes for 11ac: • Extend the table of angles up to 8 streams (refer to Appendix II) • Define the feedback reports for 80 and 160MHz • Tone-grouping, and tone mapping are TBD (recall: pilots are introduced in VHTLTF) • Number of bits used to quantize the angles is TBD. • MU is more sensitive to the angle quantization than SU-BF, so more simulations need to be run to determine this. • A SU or MU feedback request is signaled in the NDP sequence. • Whether MU feedback requires additional information compared with SU feedback is TBD. • Example, per-tone SNR may be needed for MU.

  17. Compressed V FB Proposal—2 • Propose to define the 11ac compressed V FB frame for SU and MU be the Action-No-ACK format based on 7.4.10.8, with appropriate extensions: • Category: VHT • Action: Compressed Beamforming • Define VHT MIMO Control Field, with TBD subfields. • Need a 1-byte sounding sequence number (refer to [1]) • Other subfield examples: Nc, Nr, BW, Ng, Codebook Info, SU/MU FB Indication, etc

  18. 5. Summary • Propose to define explicit sounding and feedback as the only sounding exchange protocol for 11ac SU and MU. • Propose to define NDP as the only sounding format in 11ac. • Propose the NDP sequence with SIFS feedback after NDP and/or Polling frames, and no aggregated or delayed feedbacks are allowed. • Propose to define compressed V matrix feedback as the only feedback format for 11ac SU and MU.

  19. Appendix I: Simulations

  20. Simulation Settings • SU-BF: • Ntx=4, Nrx=2, DNLOS 80MHz, MCS15, Compressed V FB with b_phi=4, b_psi=2, Ng=1,2. • DL-MU: • AP-Ntx=4, 2 Clients, each Nrx=2  (4,2,2) • 40MHz, DNLOS channels, equal path loss for two clients. • Nss=1 per user (MCS7), full-rank feedbacks. • In CV FB, b_phi = 8, b_psi = 6. • Precoding with Channel Nulling. • PER of user 1 is measured, each receiver conducts full multiuser interference cancellation. • Focus only on high SNR regime (20~35dB).

  21. SU-BF: DNLOS 80MHz, MCS15, 4x2, b_phi=4, b_psi=2

  22. MU:DNLOS 40MHz, (4,2,2) Nss=1 per user, b_phi=8, b_psi=6

  23. MU System Simulation: 8Tx AP, 3Rx Client • Tables below show throughput comparison of compressed V with tone grouping of one and four and with angle quantization relative to perfect channel feedback • Further overhead reduction can be achieved with rank one feedback

  24. MU System Simulation: 4Tx AP, 2Rx Client

  25. Explicit vs Implicit FB • Same MU scenario, comparing explicit CV FB, and implicit sounding with perfect channel estimation, but imperfectly calibrated AP and uncalibrated clients. • At AP, assume a +-0.5dB gain uncertainty, and +-5 degree phase uncertainty. • At clients, assume +-1dB gain uncertainty, and arbitrary phase uncertainty. • Input SINR (@10% outage) of user 1 is compared.

  26. MU SINR: DNLOS 40MHz, (4,2,2) Nss=1 per user

  27. Appendix II: Compressed V FB Angle extensions for Nr>4

  28. Angle Table—Nr=5 Extended table of the order of angles in the compressed beamforming report field: Slide 28

  29. Angle Table—Nr=6 Extended table of the order of angles in the compressed beamforming report field: Slide 29

  30. Angle Table—Nr=7 Extended table of the order of angles in the compressed beamforming report field: Slide 30

  31. Angle Table—Nr=8 Extended table of the order of angles in the compressed beamforming report field: Slide 31

  32. Appendix III: Comparing Sizes of CSI and CV Feedbacks

  33. CSI Feedback Size (Bytes) • Follow 11n CSI FB Definition. • 40MHz, Ng=1, Nb=8 Nc Nr

  34. Compressed V Feedback Size (Bytes) • Follow 11n CV FB Definition, extend to Nr>4 in Appendix II. • 40MHz, Ng=1, b_phi=8, b_psi=6 Nr Nc

  35. Discussions • The tables are mainly for MU FB (high bit-width required). • Overhead savings are big, e.g. • 4x2: %47 reduction • 4x4: %68 reduction • 8x4: %41 reduction • Overhead saving for SU-BF is more (see [2]). • With reduced rank CV feedback, the saving could be more.

  36. Straw Poll #1 • Do you support updating the spec framework to require that only explicit sounding and feedback be supported for VHT SU beamforming and DL MU-MIMO? • Yes • No • Abstain

  37. Straw Poll #2 • Do you support updating the spec framework to require that NDP be the only VHT sounding format? • Yes • No • Abstain

  38. Straw Poll #3 • Do you support updating the spec framework to define the VHT NDP format as follows: • Same format as the VHT PPDU but with no data portion • VHT-SIG-A indicates SU packet • VHT-SIG-B carries a fixed TBD bit pattern and adding the following figure? • Yes • No • Abstain … L-STF L-LTF L-SIG VHT-SIG-A (Symbol 1) VHT-SIG-A (Symbol 2) VHT-STF VHT-LTF1 VHT-LTFN VHT-SIG-B

  39. Straw Poll #4 • Do you support updating the spec framework to require that the explicit sounding feedback be sent SIFS after the NDP or Polling frame with no option for the delayed or aggregated feedback as defined in 11n? • Yes • No • Abstain

  40. Straw Poll #5 • Do you support updating the spec framework to require that compressed V matrix feedback, as defined in subclause 20.3.12.2.5, be the only feedback format for both VHT SU beamforming and DL MU-MIMO? • Yes • No • Abstain

  41. Straw Poll #6 • Do you support updating the spec framework to define the VHT compressed V feedback frame as an Action No ACK format with • Category = VHT • Action = Compressed Beamforming • Containing a VHT MIMO Control field with 1-byte Sounding sequence number, and other TBD subfields • Containing a Compressed Beamforming Report field and adding the following figure? • Yes • No • Abstain

  42. Straw Poll #7 • Do you support updating the spec framework to define Compressed Beamforming Report Field as shown in the next slide? • Yes • No • Abstain

  43. SP7—Proposed Spec Framework Text • Insert a new section 6.x describing the Compressed Beamforming Report Field, by adopting the 11n subclause 7.3.1.29, and: • With extending Table 7-25i to the angle ordering to up to 8 streams (refer to Appendix II) • With TBD extensions to 80MHz and 160MHz • With TBD extensions or modifications of 7-25f, on tone-grouping, and tone mapping. • With TBD extensions or modifications of Table 7-25j, on the number of bits used to quantize the angles.

  44. References [1]11-10-1091-00-00ac-Protocol-for-SU-and-MU-Sounding-Feedback [2] 11-07-0666-00-000n-BEAM-LB97-CID2969 [3] 11-10-0332-00-00ac-csi-report-for-explicit-feedback-beamforming-in-downlink-mu-mimo [4] 11-10-0586-01-00ac-time-domain-csi-report-for-explicit-feedback

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