Complexity reduction for time domain H matrix feedback

1. Complexity reduction for time domain H matrix feedback Date: 2010-11-09 Authors:

2. Time domain H matrix compressed feedback • Time domain H matrix feedback is without any objection the best method for feedback compression for 802.11ac • Combines feedback compression and improvement of channel estimation • Time domain feedback with TiDFT presents • the best CSI feedback accuracy (6dB MSE over uncompressed, way more over legacy compressed V matrix): • the best compression gain (4x or 8x) • Arguments against this technique: • not a legacy scheme • complexity Slide 2

3. Time domain H matrix compressed feedback: discussion around legacy • Objective: reduce the technological step between 11n (beamforming) and 11ac (MU-MIMO) • option 1 (ACcord): choose compressed V-matrix for 11n (beamforming) and push for compressed V-matrix in 11ac (beamforming and MU-MIMO) • this would lead to the non-selection of the best compression scheme for 11ac, which would potentially reduce the use of MU-MIMO (average by the bottom) • option 2: choose H-matrix feedback for 11n (beamforming) and choose time domain H matrix for 11ac (MU-MIMO and beamforming) • H matrix already favored by a majority of 11n client companies • selection of the best compression scheme for 11ac to have the best MU-MIMO Slide 3

4. Time domain H matrix compressed feedback: discussion around complexity • time domain H feedback with TiDFT or DCT has a reasonable complexity: • transfer matrix multiplication in STA for TiDFT (only 16kB ROM storage for all matrices), simple fast DCT for DCT • FFT or iDCT in AP (FFT already implemented in chipsets) • No additional channel estimation improvement schemes needed • Compressed V matrix also have an important complexity in receiver • Note that the compressed V matrix process is so long that after SIFS, the process isn’t completed: the preamble time of the CSI feedback is needed to end the process • In this condition, there is no way that better channel estimation will be performed on top of compressed V matrix (stay with LS estimation): that means that the quality of CSI feedback will be of the worst quality (see Celeno’s results in annex) • For all these reasons, we believe time domain H matrix feedback should be added to the spec framework. • and we show in this presentation that the complexity can even be further reduced Slide 4

5. Proposition to reduce the complexity of time domain H feedback • Instead of generating the impulse response of the complete channel (all subcarriers) • generate time domain response based on the all Np subcarriers • feedback of the CP first elements in the time domain • We propose to generate multiple partial impulse responses corresponding to multiple groups (B groups) of subcarriers • generate B time domain responses each based on a group of NFFT/B subcarriers • The feedback includes multiple fields of the B partial impulse responses, each constituted of CP/B elements (note that we have demonstrated that all the usefull energy is gathered in the CP/B first elements) • The two next slides illustrate the classical and the new proposition with B=2 groups of subcarriers. Slide 5

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7. Two fields in CSI feedback Slide 7

8. Conclusion • Time domain H matrix provides the best compression with the best quality of CSI feedback • especially when considering both compression and channel estimation • Improve the efficiency of MU-MIMO and reduce the overhead • Technological gap between 11n and 11ac and between beamforming and MU-MIMO is not higher with time-domain H matrix if the feedback issue is solved by taking into account 11n and 11ac jointly. • The complexity of the time domain H matrix can be even reduced by applying the process to 2 groups of subcarriers with TiDFT : • it enables to reduce by a factor of 2 the complexity of the compression scheme and the size of the storage for TiDFT matrices, both in STA and in AP • Note that this proposition can also be applied to the DCT-based time domain feedback (which provides a lower compression gain). in this condition, up to 8 groups can be defined with overlapping between subcarriers groups Slide 8

9. Pre-motion • Do you support updating the spec framework to require that time domain H matrix feedback, be one feedback format for both VHT SU beamforming and DL MU-MIMO? • Yes • No • Abstain

10. References [1] Hongyuan Zhang and all, 11ac explicit sounding and feedback, IEEE 802.11-10/1105r0, Sept 2010. [2] L. Cariou and M. Diallo, Time Domain CSI report for explicit feedback, IEEE 802.11-10/0586r1, May 2010. [3] K. Ishihara and al., CSI Feedback Scheme using DCT for Explicit Beamforming, IEEE802.11-10/0806r1, july 2010. [4] Nir Shapira, Yaron shany, Doubling Number of VHT-LTFs in MU-MIMO Operation, IEEE802.11-10/1113r0, Sept 2010. Slide 10

11. Annex • 1 Illustration of the process with DCT-based time domain feedback • 2 Simulation results from Celeno showing the performance degradation using compressed-V matrix with LS estimation instead of time domain H matrix. Slide 11

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13. Simulation Results from Celeno [4] • 8 AP antennas, 4 users, each with 3 antennas. Channel model D. 20ms aging • Compare the case of 1,2,3,4 or ∞ (perfect CSI) LTFs per stream in NDP • Results reflect impact of the quality of the feedback (independent of the channel estimation during Data transmission) • Results equivalent to comparing a channel estimation noise reduction scheme of 0dB, 3dB, 4.5dB, 6dB and ∞ (perfect CSI) Channel estimation resultsembeded in Time domaincompressed feedback • Curve in blue: LS channel estimation (equivalent to what is feasible with compressed V matrix feedback) • Curve in red: equivalent to channel estimation improvements embedded in time domain compressed CSI feedback starting from LS channel estimation Channel estimation resultswith LS channel estimation (only estimation feasiblewithcompressed V matrix) Slide 13