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Enhancing BSS Load Balancing for MU-MIMO Environments

Explore BSS load balancing strategies for MU-MIMO scenarios, proposing a new control element for optimal STA association and improved throughput.

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Enhancing BSS Load Balancing for MU-MIMO Environments

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  1. BSS load balancing for MU-MIMO Authors: Date: 2010-11-09 Daewon Lee, LG Electronics

  2. BSS load balancing without MU-MIMO • Example scenario with two AP with only legacy STAs • If BSS load of AP1 is lower than AP2 than association with AP1 will give better throughput for the STA Daewon Lee, LG Electronics

  3. BSS load balancing with MU-MIMO • Example scenario with two AP with legacy STAs and MU-MIMO supported STAs • In case BSS load of AP1 similar to AP2 and MU-MIMO supported STAs associated with each AP is different, association with which AP will give more benefits to the STA? Daewon Lee, LG Electronics

  4. Example of BSS load status (1/3) • Example timeline for an AP with only legacy STAs • BSS load is calculated by ratio of idle and busy time in a given measurement window • BSS load status definition will still work for SU-MIMO Daewon Lee, LG Electronics

  5. Example of BSS load status (2/3) • Example timeline for an AP with only MU-MIMO capable STAs • BSS load status definition does not take into account under utilized spatial streams which is available by MU-MIMO STA pairing Daewon Lee, LG Electronics

  6. Example of BSS load status (3/3) • Example timeline for an AP with legacy STAs and MU-MIMO capable STAs • BSS load status definition only give partial information of actual load and does not take into account spatial stream utilization Daewon Lee, LG Electronics

  7. 802.11 BSS Load Element [3] • BSS load element contains ‘Channel Utilization’ • Channel Utilization = Integer((channel_busy_time/(dot11ChannelUtilizationBeaconIntervals × dot11BeaconPeriod × 1024)) × 255) • “channel_busy_time” is defined by CS busy time within the measurement window • No consideration of actual system capacity of MIMO-capable APs Daewon Lee, LG Electronics

  8. Issues with 11n BSS load element • Channel Utilization metric • does not take into account additional resource dimensionality the MU-MIMO capable AP has • resource dimensions { time, frequency, space } • Station Count • does not distinguish STAs with MU-MIMO support Daewon Lee, LG Electronics

  9. General observations and Suggestion • General observations • 802.11 BSS load element does not seem to adequately take into account particulars of MU-MIMO operation • BSS load balancing in MU-MIMO environment are especially important since MU-MIMO benefits high data rate transmissions and large number of active STA • Better mechanisms supporting MU-MIMO supported environments seems necessary • Potential solution is to introduce a new control element for MU-MIMO supported STA association Daewon Lee, LG Electronics

  10. MU-MIMO BSS load element (1/2) • Propose to adopt an additional BSS load element (MU-MIMO BSS load element) • Keep existing BSS load indication for legacy load balancing • Support extra BSS load element to benefit 11ac devices • Desired characteristics • Non duplicate information from legacy BSS load element • Information on utilized or under-utilized level of AP’s potential MU-MIMO capacity Daewon Lee, LG Electronics

  11. MU-MIMO BSS load element (2/2) • New BSS load element should strive to utilize medium characteristics of MU-MIMO transmission channel • e.g. Idle / Busy : SU-MIMO transmission (STA-to-AP, AP-to-legacy STA) / Partially Busy : MU-MIMO transmission (AP-to-11ac STA) • Exact Formulation needs further study Daewon Lee, LG Electronics

  12. 802.11ac BSS MU-MIMO Load Element • MU-MIMO load element • Proposed load element is entirely optional (Just as the original BSS load element) and only broadcasted if such capability is supported by AP (e.g. only implemented if AP is ‘MU-MIMO capable’ AND ‘dot11QBSSLoadOptionImplemented’ is true) • Does not replace the 802.11 BSS load element, is an addition to existing load element • MU-MIMO load element contains ‘TBD’ • TBD may compose of 1 or multiple information values Daewon Lee, LG Electronics

  13. Pre-Motion • Do you support to define an additional load information element target for MU-MIMO capable devices as shown in slide 12? • BSS load element for MU-MIMO contains 1 octet element ID field, 1 octet length field, and N octet TBD field, in which N is TBD • Yes : No : Abstain : Daewon Lee, LG Electronics

  14. References [1] Greg Breit, Nachiket Desai, “Multi-User AoD Diversity Measurements,” IEEE 802.11-09/0699r0 [2] Kwak, B.-J. et al., “Measured Channel Capacity and AoD Estimation for Multi-User MIMO Scenarios,” IEEE 802.11-09/0543r0 [3] “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, IEEE802.11-2007 Daewon Lee, LG Electronics

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