1. Heterogenous per-Client Doppler in MU-MIMO Scenarios
Month Year
doc.: IEEE yy/xxxxr0
August 2009
Heterogenous per-Client Doppler in MU-MIMO Scenarios
Date:
Authors:
Greg Breit, Qualcomm Incorporated
John Doe, Some Company

2. August 2009
Abstract
Individual clients in a multi-user MIMO (MU-MIMO) system are expected to experience unequal levels of Doppler. Simulations were performed to assess the impact on unequal per-client Doppler on beamforming performance in a downlink MU-MIMO system.
Greg Breit, Qualcomm Incorporated

3. August 2009
Context
Recent measurement campaigns indicate that the environmental Doppler level assumed in the 11n channel model is excessive
11n Model: 1.2 km/hr = 60ms channel coherence 5 GHz
DCN 784 (Intel)
ms median coherence time (0.10 – 0.15 km/hr)
ms 90%ile coherence time (0.16 – 0.60 km/hr)
DCN 828 (NTT)
1.2s coherence time at 90%ile (0.06 km/hr)
Results support a conservative assumption of ~400ms for 11ac channel model
Doppler will be unequal across MU-MIMO clients
What is the impact on beamforming performance?
Greg Breit, Qualcomm Incorporated

4. Simulation Overview August 2009 AP antennas: 8
Clients: 8 (1 Rx antenna per client)
Fully loaded channel most likely to exhibit loss of orthogonality between clients
Channel model
D-NLOS, TGac MU-MIMO model
Channel gain normalized per user
Doppler model w/~1kHz channel sampling time
Simulation spanned ~50 coherence times
Two Doppler scenarios
1) Coherence time = 400ms for all STAs
2) Coherence time = 400ms for STA1-7; 100ms for STA8
MMSE channel capacity calculated per user (averaged across carriers)
Multiple CSI feedback intervals modeled
10, 25, 100ms (approx.)
Greg Breit, Qualcomm Incorporated

5. Simulation Results August 2009
CSI Feedback Interval: 10ms
CSI Feedback Interval: 25ms
CSI Feedback Interval: 100ms
Coherence Time
STA1-8: 400ms
Coherence Time
STA1-7: 400ms
STA8: 100ms
Degraded beamforming performance of STA 8 (thick curve) has no effect on performance of STAs 1-7 (compare top and bottom plots)
Greg Breit, Qualcomm Incorporated

6. August 2009
Discussion
Variability of channel conditions across clients in a MU-MIMO system does not justify assuming an extreme Doppler value for all users
400ms coherence time is conservative based on recent measurements
Proposed text for TGac Channel Model:
Recent indoor channel measurements [4, 8, 9, 10] indicate that the magnitude of Doppler assumed in the TGn channel model is too high. TGac shall use the Doppler model specified in Section of the TGn channel model document, but at a magnitude consistent with a channel coherence time of 400ms or an RMS Doppler spread of 0.83Hz at a carrier frequency of 5GHz. This level of ambient Doppler corresponds to an equivalent environmental speed vo=0.18 km/h.
Based on recent channel measurement campaigns
Greg Breit, Qualcomm Incorporated