In order to cope with the exponential growth of mobile traffic, mobile operators need to access more spectrum resources. LTE in unlicensed spectrum (LTE-U) has been proposed to extend the usual operation of LTE in licensed spectrum to cover also unlicensed spectrum. However, this extension poses significant challenges especially regarding the coexistence between LTE-U and legacy systems like Wi-Fi. In case of LTE-U adopts Time-Division Multiplexing (TDM) schemes to share the spectrum with Wi-Fi, we expect performance degradations of Wi-Fi networks. In this paper, we quantify the impact of TDM schemes on Wi-Fi performance in a coexistence scenario. We provide detailed analytical models using two different random walk approaches to compute the probability of collision faced by Wi-Fi stations and their throughput performance. Besides, we derive the performance results using an exponential approximation which shows its insufficiency to capture the exact behavior. We implement the coexistence in the NS3 simulator and we show that the models estimate accurately the collision probability and the throughput experienced by Wi-Fi. The models are then used to study and compare different coexistence schemes showing for instance that the Wi-Fi frame size has a non-negligible impact on the performance of Wi-Fi users.

1. Modeling and Performance Analysis of
Wi-Fi Networks Coexisting with LTE-U
Amr ABDELFATTAH and Naceur MALOUCH
Atlanta, 1st May 2017
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2. Conclusion
Proposal
Preliminaries
Introduction
OUTLINE
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3. Introduction ConclusionPreliminaries Proposal
 What is LTE-U?
 Extend LTE operation in unlicensed spectrum, i.e. 300 MHz
 Aggregate licensed and unlicensed spectrum into unified network
 What are LTE-U requirements?
 Share fairly the spectrum with Wi-Fi “ Behave like Wi-Fi ”
 Not impact Wi-Fi services more than another coexisting Wi-Fi
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4. Introduction ConclusionPreliminaries Proposal
 What are the deficiencies of TDM protocol ?
 LTE-U does not perform a clear channel assessment
 LTE-U has a negative impact on Wi-Fi due to Collision
 LTE-U behaves like a master on the channel access
 LTE-U is not fair with Wi-Fi
 How does LTE-U share the spectrum?
Aggressive
TDM Protocol
LBT Protocol
CSMA/CA
Frame Based Equipment
Load Based Equipment
Duty Cycled LTE
CSAT-LTE
LTE-U
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5. Introduction ConclusionPreliminaries Proposal
 How does LTE-U Work?
 LTE-U transmission occurs during on-periods
 LTE-U offers off-periods to Wi-Fi channel access
Off-periods = 𝑇1, 𝑇2, … , 𝑇 𝐾
LTE-U ON/OFF pattern
 What are LTE-U configuration parameters?
1) Duty cycle period = 𝐓𝐝𝐜
2) Duty cycle percentage =
𝐓 𝐝𝐜 − 𝐤=𝟏
𝐊
𝐓 𝐤
𝐓 𝐝𝐜
3) Number of off-periods = 𝐊
 What is our objective?
 Measuring the negative impact of LTE-U on Wi-Fi
 Compensation through controlling the above parameters
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6. Introduction ConclusionPreliminaries Proposal
 Slot-by-Slot Random Walk
 Z: random variable represents the slot duration
 How does Wi-Fi work?
 CSMA/CA = Sensing + Back off + Frame transmission
 Wi-Fi communication is divided into time slots
𝑃 𝑍 = 𝑇 =
𝑃𝑏 𝑓𝑜𝑟 𝑇 = 𝑇𝑏
𝑃𝑖𝑑 𝑓𝑜𝑟 𝑇 = 𝑇𝑖𝑑
𝑃 𝐵𝐹 = 𝑗 =
1
η𝑊0
𝑓𝑜𝑟 𝑗 = 0
1
η
𝑃𝑖𝑑
𝑗
𝑃𝑏 𝑓𝑜𝑟 1 ≤ 𝑗 ≤ 𝑊 𝑚−1
 Frame-by-Frame Random Walk
 X: random variable represents the frame transmission duration
𝑿 = 𝜹 ∗ 𝐁𝐅 + 𝑻 𝒇 + 𝑫𝑰𝑭𝑺
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7. Introduction ConclusionPreliminaries Proposal
 How to find the pdf of Z and X?
 How to calculate Wi-Fi throughput ?
Markov chain model for Wi-Fi station
τ =
2(1−𝑝 𝑚+1)
𝑊0 1− 2𝑝 𝑚′+1 1−𝑝
1−2𝑝
+ 1−𝑝 𝑚′+1 +𝑊02 𝑚′
𝑝 𝑚′+1(1−𝑝 𝑚−𝑚′)
(1)
𝑝 = 1 − (1 − τ) 𝑛−1
(2)
 Now you can find
 Using Z : Γ =
Ps Tpl
E Z
(3)
𝑤ℎ𝑒𝑟𝑒 𝑇𝑝𝑙 𝑖𝑠packet payload duration
 Using X : Γ =
Ps Tpl
E X /(𝐸 𝐵𝐹 +1)
(3)
 Bianchi’s fixed point formulation
𝑃𝑖𝑑 = (1 − τ) 𝑛
, 𝑃𝑏 = 1 − (1 − τ) 𝑛
, 𝑃𝑠 = 𝑛 τ (1 − τ) 𝑛−1
, 𝑃𝑐 = 𝑃𝑏 − 𝑃𝑠
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8. Introduction ConclusionPreliminaries Proposal
 What does it change?
 And Wi-Fi throughput ?
τ =
2(1−𝑝 𝑚+1)
𝑊0 1− 2𝑝 𝑚′+1 1−𝑝
1−2𝑝
+ 1−𝑝 𝑚′+1 +𝑊02 𝑚′
𝑝 𝑚′+1(1−𝑝 𝑚−𝑚′)
(1)
 Using Z : Γ =
Ps Tpl
E Z
(3)
𝑤ℎ𝑒𝑟𝑒 𝑇𝑝𝑙 𝑖𝑠packet payload duration
 Using X : Γ =
Ps Tpl
E X /(𝐸 𝐵𝐹 +1)
(3)
𝑝 = 1 − 1 − 𝜏 𝑛−1
+ 𝟏 − 𝝉 𝒏−𝟏
𝒑𝒍𝒕𝒆 (2)
𝑝 = 1 − (1 − τ) 𝑛−1
(2)
𝒑𝒍𝒕𝒆
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9. Introduction ConclusionPreliminaries Proposal
 Slot-by-Slot Random Walk
Pr 𝑁𝑘 = 𝑛 = 𝑃𝑟
𝑗=1
𝑛−1
𝑧𝑗 < 𝑇𝑘 − 𝐷𝐼𝐹𝑆 − δ − 𝑃𝑟
𝑗=1
𝑛−1
𝑧𝑗 < 𝑇𝑘 − 𝐷𝐼𝐹𝑆 − δ
 Counting only the busy slots
𝐸 𝑁𝑘
𝑏
= 𝑃𝑏 𝐸 𝑁𝑘
 Assume that On-period always starts over a busy slot
 Wi-Fi random walk will be stopped at the end of each 𝑇𝑘
𝑝𝑙𝑡𝑒 =
K
k=1
K
E Nk
b
 Calculating the new Wi-Fi throughput
Γ1 = Γ (1 − 𝑝𝑙𝑡𝑒)
k=1
K
(Tk − DIFS − δ + E Rk )
Tdc
Approximate expression
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10. Introduction ConclusionPreliminaries Proposal
 Powerful model: Can be used to compute
 Capture effect
 Exponential Model for LTE-U
𝑝𝑐𝑎𝑝 =
𝑘
𝐾
(1 − 𝐵𝐸𝑅) 𝐶𝐵𝑅∗𝐸 𝑅 𝑘
𝐾
𝑝𝑙𝑡𝑒 = 1 − 𝑒ℜ𝑇 𝑓
𝐸 𝑅 𝑘 = 𝑃𝑏 𝑇𝑏 − 𝐸 𝑇𝑇𝑏 + 𝑃𝑖𝑑 δ − 𝐸 𝑇δ
Where ℜ =
𝐾
𝑘=1
𝐾 𝑇 𝑘
𝑝𝑙𝑡𝑒
′
= 𝑝𝑙𝑡𝑒 1 − 𝑝𝑐𝑎𝑝
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11. Introduction ConclusionPreliminaries Proposal
 Frame-by-Frame Random Walk
Pr 𝑀 𝑘 = 𝑚 = 𝑃𝑟
𝑗=1
𝑚−1
𝑥𝑗 < 𝑇𝑘 − 𝐷𝐼𝐹𝑆 − δ − 𝑃𝑟
𝑗=1
𝑚−1
𝑥𝑗 < 𝑇𝑘 − 𝐷𝐼𝐹𝑆 − δ
 Probability of each case
 Counting the collided and sended frames
 Wi-Fi random walk will be stopped at the end of each 𝑇𝑘
 Calculating the new Wi-Fi throughput
Pr 𝑐 𝑘, 𝑚 = Pr
𝑗=1
𝑚
𝑥𝑗 − 𝑇𝑓 ≤ 𝑇𝑘 ≤
𝑗=1
𝑚
𝑥𝑗 Similarily, Pr 𝑓𝑘, 𝑚 𝑎𝑛𝑑 Pr 𝑠 𝑘, 𝑚
𝑝𝑙𝑡𝑒 =
𝑘=1
𝐾
𝑃𝑟 𝑐 𝑘
k=1
K
E 𝑀 𝑘 − Pr 𝑓𝑘
Exact expression
Γ2 =
(𝑃𝑠/𝑃𝑏) 𝑘=1
𝐾
(𝐸 𝑀 𝑘 − 1 + 𝑃𝑟 𝑠 𝑘 )
𝑇𝑑𝑐 11

12. Introduction ConclusionPreliminaries Proposal
 Validation: Analytical models vs
5x0
3x2
4x1
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13. Introduction ConclusionPreliminaries Proposal
 Frame-by-Frame Random Walk Model
 Slot-by-slot Random Walk Model
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14. Introduction ConclusionPreliminaries Proposal
 Slot-by-slot Random Walk Model
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15. Introduction ConclusionPreliminaries Proposal
 Exponential dist. is exponential
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16. Introduction ConclusionPreliminaries Proposal
 Making LTE-U to be fair with Wi-Fi
Friendly
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17. Introduction ConclusionPreliminaries Proposal
 Introducing the notion of random walk in studying Wi-Fi network
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 Quantifying the negative impact of LTE-U on Wi-Fi performance
 Tying up between LTE-U configuration parameters and Wi-Fi performance
 Providing several solutions for making LTE-U to be Wi-Fi friendly
 The duty cycled LTE-U cannot be approximated by an exponential approximation as the PASTA property does not hold

18. 18
Questions