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Doubling Number of VHT-LTFs in MU-MIMO Operation

Doubling Number of VHT-LTFs in MU-MIMO Operation. Authors:. Date: 2010-10-15. Abstract. MU-MIMO is more sensitive to channel estimation accuracy than SU-MIMO Increasing the number of LTFs is a straightforward means to improve channel estimation accuracy

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Doubling Number of VHT-LTFs in MU-MIMO Operation

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  1. Doubling Number of VHT-LTFs in MU-MIMO Operation Authors: Date: 2010-10-15 Nir Shapira et al, Celeno Communications

  2. Abstract • MU-MIMO is more sensitive to channel estimation accuracy than SU-MIMO • Increasing the number of LTFs is a straightforward means to improve channel estimation accuracy • Last meeting [1] was presented. Straw poll passed on the principle of increasing number of VHT-LTFs in sounding frames • In this submission we show the benefit of increasing the number of LTFs in data-frames as well. Nir Shapira et al, Celeno Communications

  3. Increase number of LTFs in Sounding and/or Data Frames • Opt 1 – increase just in NDPs intended for MU sounding • CSI accuracy for precoding calculation is improved • Low overhead, NDPs are relatively infrequent • Can signal LTF increase in NDP announcement • Opt 2 –increase LTFs also in DL-MU data frames • Improved performance in case of resolvable LTFs • User has better estimate of interference channel (cannot improve interference channel estimate using decision directed methods) • Large overhead in case of short packets => necessitates per frame control in SigA Nir Shapira et al, Celeno Communications

  4. Simulation Results • 8 AP antennas, 4 users, each with 3 antennas. Channel model D. 20ms aging • We compare the case of using 1,2,3,4 or ∞ (perfect CSI) LTFs per stream in sounding frames • We compare the case of adding LTFs just in NDP (right graph), to adding LTFs also in data frames in resolvable LTF mode (left graph) Nir Shapira et al, Celeno Communications

  5. Results Analysis • There are significant gains from increasing number of LTFs in data frames • Use of 2 LTFs reduces degradation (vs. perfect CSI) by more than 50% • Use of 2 LTFs per stream improves throughput by 20% in 25dB SNR • Conclusion: Opt2 of doubling number of LTFs in each frame (with a per-frame control) is better - Will still enable the use of 1 LTF in case of short packets Nir Shapira et al, Celeno Communications

  6. Additional Benefit of Improved Equalization • It is a well known fact (from 11n/a/g) that the impact of FEQ based on imperfect CSI (in high SNR, single stream) is an additive noise of variance • Hence legacy 2 LP symbols have 1.3 dB advantage relative to 1 HT-LTF in 11n • This is a different noise component to that generated from the interference of other streams (as discussed in previous slides) • Using 2 LTFs per stream will further improve both SU and MU performance Nir Shapira et al, Celeno Communications

  7. Detailed Suggestion • Add bit in SigA to indicate usage of 1 or 2 VHT-LTFs per stream • Each VHT-LTF is duplicated. Waveform of duplicated VHT-LTF is identical to original VHT-LTF • For the case of n streams and one VHT-LTF per stream, we have: • For the case of n streams and two VHT-LTFs per stream, we have: • Averaging of LTFs can be done on time domain samples on the fly, no need for large buffers … VHT-LTF1 VHT-LTF2 VHT-LTFn … VHT-LTF1 VHT-LTF1 VHT-LTF2 VHT-LTF2 VHT-LTFn VHT-LTFn Nir Shapira et al, Celeno Communications

  8. Straw Poll • Do you support adding a bit in SigA to control the doubling of VHT-LTFs, and add to the specifications framework document (IEEE 802.11-09/0992) the description given in slide 7? Nir Shapira et al, Celeno Communications

  9. References [1] Nir Shapira, et. al., Improving CSI Estimation Accuracy for MU-MIMO, TGac, IEEE 802.11-10/0804r0 Nir Shapira et al, Celeno Communications

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