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?

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• 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

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LTE MAC Vs. Wi-Fi MAC

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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?

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

What are LTE-U alternatives?

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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.

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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

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Coexisting (1/4) Medium Access Fairness

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• 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

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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.

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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

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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

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Paper by Babaei, 2015 mathematically modeled how LTE would

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

probability of Wi-Fi’s 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

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Other forms of LBT:

– Synchronous LBT

– Adaptive LBT (alternates between channels)

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

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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

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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

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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

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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

• [13] Qualcomm report, “R10-based LTE-U”, available at https://www.qualcomm.com/invention/research/projects/lte-unlicensed/r10-based-lte-u , Nov. 2014

• [14] Suna Choi, Seungkeun Park, “Co-existence analysis of duty cycle method with Wi-Fi in unlicensed bands” IEEE 2015

• [15] Jaewook Lee, Haneul Ko, Sangheon Pack, “Performance Evaluation of LTE-Unlicensed in Handover Scenarios” IEEE 2015

• [16] Anwer Al-Dulaimi, Saba Al-Rubaye, Qiang Ni, Elvino Sousa, “Pursuit of More Capacity Triggers LTE in Unlicensed Band” IEEE 2015

• [17] Federal Communications Commission, “Order and Second Memorandum Opinion and Order,” Jun. 2014.

• [18] Hao Song, Xuming Fang, “A Spectrum Etiquette Protocol and Interference Coordination for LTE in Unlicensed Band (LTE-U)” IEEE 2015

• [19] Fuad M Abinader, Jr., Erika P. L. Almeida, Fabiano S. Chaves, Andre’ M. Cavalcante, Robson D. Vieira, Rafael C. D. Paiva, Angilberto M. Sobrinho, Sayantan Choudhury, Esa 15 Tuomaala, Klaus Doppler, Vicente A. Sousa, Jr. “Enabling the Coexistence of LTE and Wi-Fi in Unlicensed Bands” IEEE 2014

• [20] Yang Xu, Rui Yin, Qimei Chen, Guanding Yu, “Joint Licensed and Unlicensed Spectrum Allocation for Unlicensed LTE” IEEE 2015

• [21] Abhijeet Bhorkar, Christian Ibars, Pingping Zong, “On the Throughput Analysis of LTE and WiFi in Unlicensed Band” IEEE 2014

• [22] Cisco White Paper, “Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2011–2016,” 2012.

• [23] O. Aboul-Magd, IEEE 802.11 HEW SG Proposed Project Authorization Request (PAR), IEEE 802 WG Std. IEEE 802.11-14/0165r1; https://mentor.ieee.org/802.11/dcn/14/11- 14-0165-01-0hew-802-11-hew-sg-proposedpar.docx

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