Coexistence of LTE-U with WiFi 802.11n at 5GHz Unlicensed Spectrum

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  • Coexistence of LTE-U with Wi-Fi 802.11n

    at 5GHz unl icensed spectrum S u r v e y

    Osama Askoura

    EECS6590 Project

    1

  • Roadmap

    Introduction

    Background

    Coexistence Problems

    Surveying Mechanisms

    Discussion/Future Work

    Conclusion

    2

  • What is the unlicensed spectrum?

    3

    Fig1: Ad-hoc Network Example. Source: peterpaulengelen.com

    Frequency bands at 5 GHz

    Band: 5150-5250 MHz

    Fig2: Available spectrums Source: Huawei

  • What is LTE and LTE-U?

    4

    Long-Term-Evolution (LTE) is a 4G

    technology.

    LTE-U (LTE in unlicensed spectrum) is

    proposed to be used by providers with no

    licensing and high data rates

  • What is LTE and LTE-U?

    5

    Fig3: LTE architecture. Sources: Google Images

  • What is LTE and LTE-U?

    6

    Fig3: LTE architecture. Sources: Google Images

    LTE Wi-Fi UE

    User Equipment

    Mobile Station

    eNodeB

    (evolved Node B)

    Access Point

    EPC

    (Evolved Packet Core)

    Router/Gateway

  • Coexistence of LTE-U with Wi-Fi 802.11n

    at 5GHz unl icensed spectrum

    7

  • LTE MAC Vs. Wi-Fi MAC

    8

    LTE Wi-Fi

    Collision

    Avoidance

    None

    Assumes reserved and scheduled channel access

    CSMA/CA with RTS/CTS

    Listen-before-speak

    Channel Access Centralized LTE scheduler that coordinates

    uplink/downlink and continuously transmit

    Distributed Coordination Function

    (DCF), contention based

    Channel Usage Continuous channel usage; Frames are contiguous

    (even when no data to send)

    Channel is occupied only when data

    packets need to be transmitted

    Maximum quiet

    period

    3ms DIFS + CWmax

    Coverage Range 2KM ~ 100m 1 KM

    UE can connect

    to multiple APs

    Yes No

  • Why is coexistence challenging?

    9

    Fig4: LTE-U WiFi Coexistence. Source: Babaei, 2014

  • What are LTE-U alternatives?

    10

    Fig5: LTE-U WiFi Coexistence. Source: Babaei, 2014

  • Background Companies (Intel, Huwaei & Qualcomm) have done

    studies about implementing LTE-U into their infrastructure Their simulation models are not published

    People have studied LTE-WiFi coexistence in a mathematical probabilistic model Probability of WiFi backoff delay < LTE-U periods

    Other schools have studied LTE-WiFi individually, but not together

    We need to study their coexistence and interference to address their problems. Literature is scarce.

    11

  • Coexistence Problems

    Broadly divide into following problems:

    Medium Access Fairness

    LTE/Wi-Fi hidden terminal problem

    Transmission Power (Channel sensing

    problem)

    LTE can affiliate with more than one AP/eNb

    12

  • Coexisting (1/4) Medium Access Fairness

    13

    WiFi 802.11 uses CSMA/CA at DCF-MAC listen before speak using RTS/CTS

    Devices wait DIFS before transmitting RTS or DATA. Adopts back-off delay mechanism

    LTE is a continuously transmitting protocol Periodically send control and reference signals, even when

    no data to transmit

    This period can be smaller than DIFS or backoff delay

    In China and Europe, a quiet period or listen before speak mechanism is mandatory for operation in 5GHz. In North America there is no such regulation.

  • Coexistence (2/4) LTE/Wi-Fi hidden terminal problem

    14

  • Coexistence (3/4) Transmission Power constraints

    The US FCC rules that unlicensed devices operating in licensed bands must be lower than 1W [17]. Unfortunately, with respect to unlicensed spectrum operation in 2.4GHz and 5GHz bands, there is no similar requirement.

    This means that LTE or other devices operating in unlicensed spectrum could jam channels for wider ranges and mute Wi-Fi.

    15

  • Coexistence (4/4) Multiple Affiliation

    LTE can affiliate with multiple eNodeB at the same time. As a user moves between ranges, handover between base stations occur.

    This handover affects LTE 6 throughput in case of employing a coexistence mechanism such as CSMA/CA [15] - since the LTE-U AP cannot occupy the unlicensed band right after handover due to the Listen-before-talk (LBT) mechanism

    16

  • Coexistence Mechanisms

    Broadly divide into:

    MAC Protocols

    1. LTE quiet period

    2. LTE LBT

    3. LTE-WiFi TDD

    4. LTE Duty Cycle ON/OFF

    Architecture Modifications

    17

  • MAC Protocols (1/4) Quiet Period Analysis Babaei, 2014

    18

    Paper by Babaei, 2015 mathematically modeled how LTE would

    behave if quiet period was added to it. They calculated the

    probability of Wi-Fis back-off delay is less than LTE-U quiet period

    Pure statistical approach. Eliminates PHY layer effects, and hidden/exposed

    terminal problems.

  • MAC Protocols (1/4) Quiet Period Analysis Babaei, 2014

    19 Fig6: Wi-Fi Channel Access Vs. LTE quiet period. Source: Babei, 2014

  • MAC Protocols (2/4) LTE LBT Bhorkar, 2015

    20

    Paper by Intel, Bhorkar, 2015 suggested a MAC scheme of listen

    before speak (LBT) added to LTE in supplemental downlink (SDL)

    mode

    Adds collision avoidance algorithms to LTE-U

  • MAC Protocols (2/4) LTE LBT Bhorkar, 2015

    21 Fig7: cdf of WiFi throughput; 60% of users have 0 throughput. Source: Bhorkar, 2015

  • MAC Protocols (2/4) LTE LBT Bhorkar, 2015

    22 Fig8: cdf of LTE throughput; degrades due to WiFi. Source: Bhorkar, 2015

  • MAC Protocols (2/4) LTE LBT Bhorkar, 2015

    23

    Other forms of LBT:

    Synchronous LBT

    Adaptive LBT (alternates between channels)

  • MAC Protocols (3/4) LTE-WiFi TDD Cano, 2015

    24

    Paper by Cano, 2015 suggested to divide Transmission burst times, T

    over the n Wi-Fi nodes and N LTE nodes. Each node gets tj

    Means that the Base stations must know n and N number of nodes of Wifi &

    LTE

    This is challenging if not nodes can overhear each other, and is left to future

    work

  • MAC Protocols (3/4) LTE-WiFi TDD Cano, 2015

    25 Fig7: Throughput analysis using fair allocation proposed by Cano, 2015

  • MAC Protocols (4/4) LTE ON/OFF Cano, 2015

    26 Fig7: Throughput analysis using fair allocation proposed by Cano, 2015

  • Architecture Modifications 802.11-LTE protocol fusion stack

    27 Fig8: 4 Operation for LTE devices access in the spectrum etiquette. Source: Song & Fang, 2015

  • Architecture Modifications 802.11-LTE protocol fusion stack

    28 Fig8: Operation for 802.11 Wi-Fi devices access in spectrum etiquette. Source: Song & Fang, 2015

  • Discussion/Future Work Adding LBT to LTE basically makes LTE, Wi-Fi?

    Takes away LTE advantage?

    Future physical modeling of coexistence to account

    for hidden terminal problems, Taylor series

    emissions

    Simulate all proposed mechanisms in same test-

    bed or model with same large number of nodes and

    parameters to evaluate which is better

    29

  • Conclusion Studying coexistence of LTE-U and Wi-Fi 802.11n in

    unlicensed spectrum of 5GHz

    Unlicensed spectrum offers for exploitation

    LTE is continuously transmitting and thus degrades WiFi throughput by 70%. Its throughput is only degraded by 4%

    Coexistence mechanisms divide broadly into MAC protocols for LTE and a modified 802.11-LTE protocol fusion stack

    Literature is scarce (2014-2015) and better mechanisms analysis might be needed

    30

  • References [1] Alireza Babaei, Jennifer Andreoli-Fang, Belal Hamzeh, On the impact of LTE-U on Wi-Fi performance IEEE, 2014

    [2] Huawei white-paper U-LTE: Unlicensed Spectrum Utilization of LTE, [online] www.huawei.com/ilink/en/download/HW_327803

    [3] Qualcomm Research white-paper LTE in Unlicensed Spectrum June 2014, [online] https://www.qualcomm.com/media/documents/files/lte-unlicensed-coexistence-whitepaper.pdf [4] Abhijeet Bhorkar, Christian Ibars, Apostolos Papathanassiou, Pinping Zong, Medium Access Design for LTE in Unlicensed Band, Intel Corporation (2015)

    [5] S. Abraham1 , A. Meylan and S. Nanda, 802.11n MAC Design and System Performance (2005)

    [6] Simulation Comparison between LTE and Wi-Fi in Networks [Online] Available: www.sfu.ca/~ckc29/ENSC427SP14G1/ENSC427_F.pdf

    [7] The Performance Analysis of LTE Network [Online] Available: www.ensc.sfu.ca/~ljilja/ENSC427/.../ENSC427_team6_report.pdf

    [8] Alcatel.Lucent white-paper The LTE Network Architecture [Online] Available: http://www.cse.unt.edu/~rdantu/FALL_2013_WIRELESS_NETWORKS/LTE_Alcatel_White_Paper.pdf

    [9] OPNET Simulator. [Online] Available: http://www.riverbed.com/products/steelcentral/opnet.html?redirect=opnet.

    [10] Cristina Cano, Douglas J. Leith, Coexistence of Wifi and LTE in Unlicensed band, a proportional Fair Allocation Scheme IEEE, 2015

    [11] Abhijeet Bhorkar, Christian Ibars, Apostolos Papathanassiou, Pingping Zong Medium Access Design for LTE in Unlicensed Band IEEE, 2015

    31

    http://www.riverbed.com/products/steelcentral/opnet.html?redirect=opnet

  • References [12] Qualcomm report, LTE-U/Wi-Fi Coexistence, available at

    https://www.qualcomm.com/invention/research/projects/lteunlicensed/lte-u-wi-ficoexistence , Nov. 2014