1. Closed versus Open Loop Comparisons
January 2005
doc.: IEEE /xxxxr0
January 2005
Closed versus Open Loop Comparisons
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John Ketchum, Qualcomm
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
Eigenvector steering (SS) or spatial spreading (SS)
Number of streams
Rates per stream
Throughput and Latency comparisons
Eigenvector Steering (ES) with feedback
Spatial Spreading (SS) with feedback
Spatial Spreading (SS) with no feedback
John Ketchum, Qualcomm

3. Simulation Conditions
January 2005
Simulation Conditions
Obtained Rate versus Range curves for
ES with feedback (ES)
SS with feedback (SS closed loop)
SS with no 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).
John Ketchum, Qualcomm

4. January 2005
Throughput versus SNR
John Ketchum, Qualcomm

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)
John Ketchum, Qualcomm

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
John Ketchum, Qualcomm

7. MSDU Delay CDF Target PHY PER = 2%
January 2005
MSDU Delay CDF Target PHY PER = 2%
John Ketchum, Qualcomm

8. MSDU Delay CDF Target PHY PER = 10%
January 2005
MSDU Delay CDF Target PHY PER = 10%
John Ketchum, Qualcomm

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.
John Ketchum, Qualcomm

10. January 2005
Conclusions
We have demonstrated throughput and latency benefits 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 bits at Data Rate
John Ketchum, Qualcomm