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Comments on LAA EVM

This document provides recommendations for evaluating the coexistence of Licensed-Assisted Access (LAA) and Wi-Fi in unlicensed spectrum. It includes recommendations for evaluating LBT requirements, simulating different load densities, incorporating VoIP and other traffic types, and considering 256.QAM, LDPC, and RTS/CTS. The document also discusses the use of truncated exponential backoff in LBT requirements.

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Comments on LAA EVM

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  1. Comments on LAA EVM Authors: Date:2015-01-14 Notice:This document has been prepared to assist IEEE 802.19. 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. Alireza Babaei, CableLabs

  2. Abstract • This document provides recommendations related to Annex A (Evaluation methodology) of the 3GPP draft study item document on Licensed-Assisted Access to Unlicensed Spectrum: 3GPP TR 36.889 v0.1.0 (2014-11). • The purpose of this contribution is to include these recommendations in an upcoming liaison from 802 to 3GPP on its ongoing LAA study item Alireza Babaei, CableLabs

  3. Summary of Recommendations • Incorporate Truncated Exponential Backoff in LBT requirements and simulate its effect. • For a complete understanding of LAA impact on Wi-Fi, a range of load densities should be simulated. • Include VoIP and other traffic types as mandatory traffic models and evaluate corresponding performance metrics. • Consider 256 QAM, LDPC and RTS/CTS mandatory for simulation. Alireza Babaei, CableLabs

  4. LBT • Listen-Before-Talk (LBT) based on European regulations (ETSI 301 893 v1.7.1) is mainly considered as a coexistence mechanism in 3GPP contributions. • LBT, as defined in the above ETSI document, has two forms : • Load Based and Frame Based Equipment (LBE and FBE) • 3GPP contributions indicate majority opinion is tending towards LBE • Concerns: • Fixed linear backoff window for extended CCA (q is fixed for a given device) • No increase with collision (q is fixed during operation) Extended CCA CCA 20 µs CCA 20 µs Channel Busy Extended CCA = N * 20 µs N ∈ {0, 1, … , q} and q ∈ {4, 5, … , 32} Slide 4 Alireza Babaei, CableLabs

  5. LBT Simulation Results Transmission Success Rate Gain/Loss 1.68X 1.53X • Assume Load Based Equipment LBT, q = 32 • Tx Success Rate: probability of any client sending successful burst • Steps to Tx Success: • Client must pass CCA • Client must Tx without collision • Results relative to the Tx Success Rate when both operators use Wi-Fi • 1X applies to any client count 1X Poor fairness with Wi-Fi 0.23X 0.12X Simulation Assumptions: Full buffer traffic, all nodes within CCA range of each other, single 20 MHz channel, co-channel operation, clients and APs/eNBs are stationary, Wi-Fi STAs use EDCA, BE AC (AIFS = 3, CWmin = 15) Conclusion: Version 1.7.1 of ESTI LBT rules are not sufficient for fair Coexistence of LTE and Wi-Fi Slide 5 Alireza Babaei, CableLabs

  6. What version of LBT to use? • 802.11 has proven success in enabling sharing between independent systems in a wide variety of use cases an load scenarios. • The basis of this success is EDCA which is based on CSMA/CA; CSMA/CA is an LBT mechanism using truncated exponential backoff. • The use of a similar mechanism by LAA should provide the basis for fair sharing between LAA and Wi-Fi (and also between LAA and LAA) • The Wi-Fi Alliance made a proposal to ETSI BRAN that defines LBT with exponential backoff for LAA (See [3]) Recommendation: Incorporate Truncated Exponential Backoff in LBT requirements and simulate its effect John Doe, Some Company

  7. System Load • Section A.1.1 lists the parameters for indoor LAA coexistence evaluation. • Only 10 LAA UEs or Wi-Fi clients is assumed per unlicensed band carrier for DL-only LAA coexistence evaluation. • The impact of LAA (using EU LBT) on Wi-Fi clients is more evident at high system load particularly when number of nodes is large • See the backup slides. Recommendation: For a complete understanding of LAA impact on Wi-Fi, a range of load densities should be simulated Alireza Babaei, CableLabs

  8. Traffic model • Wi-Fi and LAA have to operate in unlicensed spectrum carrying a variety of traffic types including voice, video, FTP, etc. • However, the simulations evaluating the fairness of LAA with Wi-Fi is currently limited to FTP. • Once initial simulations showing basic fairness between Wi-Fi and LAA are complete, further simulations will be required to ensure fairness and functionality are maintained using a variety of traffic types in a diversity of use and load scenarios. Recommendation: Include VoIP and other traffic types as a mandatory traffic models and evaluate corresponding performance metrics. Alireza Babaei, CableLabs

  9. Assumptions on Wi-Fi Parameters • Section A.2.1 lists Wi-Fi system evaluation assumptions. • 256 QAM and LDPC are considered optional. • Use of lower order modulation (when SINR is sufficient for 256 QAM) means unnecessarily longer frame duration. Longer frame duration increases the backoff period and decreases the channel utilization for other Wi-Fi clients. • RTS/CTS is considered optional. • The hidden node behavior of two wireless systems is key to coexistence. RTS/CTS is optional but commonly used in congested environments. Recommendation: Consider 256 QAM, LDPC and RTS/CTS mandatory for simulation Alireza Babaei, CableLabs

  10. References [1] Study on Licensed-Assisted Access to Unlicensed Spectrum, 3GPP TR 36.889 v0.1.0 (2014-11) [2] 802.19-14/0082r0: Overview of EU LBT and its Effectiveness for Coexistence of LAA LTE and Wi-Fi [3] ETSI BRAN(14)000102 Alireza Babaei, CableLabs

  11. Backup Alireza Babaei, CableLabs

  12. Simulation Parameters • Overall Simulation Assumptions: • All nodes are within CCA range of each other • Single channel access scenario • Full buffer traffic • 802.11 EDCA channel access modeled • Best Effort access category (AIFSN = 3, CWmin = 15) • Stationary nodes • Monte Carlo simulations with 1,000,000 TXOPs evaluated per data point • ETSI LBT Assumptions: • Always use extended CCA • LBE rules Implemented by LAA nodes Slide 12 Alireza Babaei, CableLabs

  13. Successful Packet Transmission • ETSI LBT performs poorly compared to Wi-Fi channel access • Lower q value = worse performance • Poor fairness behavior when sharing and >5 clients/operator • LAA clients get significantly higher P(success) compared to Wi-Fi clients i.e. black lines are all above the red lines. • At a certain client count, LAA clients nearly starve Wi-Fi clients i.e. P(success) approaches zero • This value varies with q, best case shown Slide 13 Alireza Babaei, CableLabs

  14. Collisions in the Shared Channel • ETSI LBT performs poorly compared to Wi-Fi channel access • Collision rate increases rapidly to 1 for low q values • Even for q = 32, collision rate dominates after 12 transmitting clients per operator • Inefficient channel usage for low q and/or scenarios with ~25 or more transmitting nodes. • Attributed to non-increasing backoff range Slide 14 Alireza Babaei, CableLabs

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