1. Closed versus Open Loop
January 2005
doc.: IEEE /xxxxr0
January 2005
Closed versus Open Loop
Date:
Authors:
Notice: This document has been prepared to assist IEEE It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures < ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE Working Group. If you have questions, contact the IEEE Patent Committee Administrator at
Sanjiv Nanda
John Ketchum, Qualcomm

2. Closed vs. Open Loop Quantify the benefits of closed loop
January 2005
Closed vs. Open Loop
Quantify the benefits of closed loop
Feedback on number of streams and rates
Throughput and Latency comparisons
Eigenvector Steering (ES) with Rate Feedback
Spatial Spreading (SS) with Rate Feedback
Spatial Spreading (SS) with no Rate Feedback
Sanjiv Nanda

3. Simulation Conditions
January 2005
Simulation Conditions
Obtained Rate versus Range curves for
ES with rate feedback (ES)
SS with rate feedback (SS closed loop)
SS with no rate feedback (SS open loop)
Simulation conditions
Qualcomm rate set
2x2. Up to 2 spatial streams
5 GHz
SGI-52 (shortened guard interval, 52 data tones)
Offered traffic: 1 UDP flow from AP-STA. Packet size = 1500 B
HCF, i.e., AP polls STA for Block Ack.
Block Ack. No Immediate Ack for BA or BAR.
Two cases: Target PHY PER = 2% and 10%
Open Loop Rate Control
Transmitter decreases “SNR Estimate” by D for each frame that is NAK-ed, and increases by d for each frame that is ACK-ed.
D and d are chosen to obtain the desired PHY packet error rate (PER).
Sanjiv Nanda

4. January 2005
Throughput versus SNR
Sanjiv Nanda

5. MAC Throughput Comparisons
January 2005
MAC Throughput Comparisons
Closed Loop (ES or SS)
Little throughput benefit by increasing the PHY PER from 2%-10%
Open Loop
Significant throughput benefit by increasing the PHY PER from 2%-10%
However, high PER results in unacceptable latency tails as shown next.
At 23 dB
SS open loop (PHY PER = 2%): 50 Mbps
SS open loop (PHY PER = 10%): 61 Mbps. But, note increased latency.
SS closed loop: Mbps
ES: Mbps
At 100 Mbps
Closed loop SS gains over open loop SS.
2.5 dB at 10% PHY PER
9 dB at 2% PHY PER
ES offers 8 dB gain over SS (closed or open loop)
Sanjiv Nanda

6. January 2005
MSDU Latency
Set offered load = 90% of achievable throughput at SNR=27 dB (corresponding to 20 m)
At PHY PER = 2% offered UDP load
ES: Mbps
SS closed loop: 68.0 Mbps
SS open loop : 55.4 Mbps
At PHY PER = 10% offered UDP load
ES : Mbps
SS closed loop : 71.2 Mbps
SS open loop : 64.8 Mbps
Sanjiv Nanda

7. MSDU Delay CDF Target PHY PER = 2%
January 2005
MSDU Delay CDF Target PHY PER = 2%
Sanjiv Nanda

8. MSDU Delay CDF Target PHY PER = 10%
January 2005
MSDU Delay CDF Target PHY PER = 10%
Sanjiv Nanda

9. Latency Comparisons January 2005 Long delay tails with open loop.
By setting PHY PER = 2% (instead of 10%)
25-30% loss in throughput.
We can pull in the 90th percentile MSDU latency from 45 ms to less than 20 ms.
But, the 95th and 99th percentile exceed 50 ms even at PHY PER = 2%.
Reason for long tail.
Open loop rate selection: the Tx Rate is dropped because of frame errors.
The queue service rate is decreased at the time of increased offered load due to retransmissions.
This results in significantly higher tails of the delay distribution.
We have demonstrated
Significant throughput and latency benefit of closed loop.
Significant throughput and latency benefit of ES.
Sanjiv Nanda

10. Benefit of Data Rate Feedback in TGn Simulation Scenarios
January 2005
Benefit of Data Rate Feedback in TGn Simulation Scenarios
Comparison:
Closed Loop: Rate selection and MIMO mode (ES or SS) selection
Open Loop: SS only
Substantial throughput gains:
2x2, standard symbols
Scenario 1: 50%
80.4 Mbps versus 53.6 Mbps
Scenario 6: 38%
81.9 Mbps versus 59.5 Mbps
These results were provided in San Antonio
Sanjiv Nanda

11. Benefit of Eigensteering with Closed Loop
January 2005
doc.: IEEE /xxxxr0
January 2005
Benefit of Eigensteering with Closed Loop
Comparison:
Closed Loop: Rate selection and MIMO mode selection
Closed Loop: SS only
Sample Gains:
2x2, SGI-52 symbols
30% throughput gain in Scenario 1.
40% in Scenario 4.
These results were provided in San Antonio
Sanjiv Nanda
John Ketchum, Qualcomm

12. January 2005
Conclusions
We have demonstrated throughput and latency benefit of closed loop feedback.
MIMO Mode feedback: Eigenmode steering versus spatial spreading
Stream feedback: number of spatial streams
Rate feedback: rates per spatial stream
Significant benefits with very little overhead.
16-bit at Data Rate specifies up to four preferred rates.
Tx PHY rate is maximized after single ACK received
Accurate PHY rate tracking for time varying channels
Sanjiv Nanda