The document discusses coexistence challenges between LTE-U and Wi-Fi networks operating in the 5GHz unlicensed spectrum. It identifies problems such as unfair medium access, hidden terminals, different transmission power constraints, and LTE's ability to connect to multiple access points. The document reviews several proposed coexistence mechanisms, including adding a quiet period to LTE, using listen-before-talk for LTE, and time-division duplexing of transmissions. It concludes that more analysis of proposed solutions is needed to effectively address coexistence issues between the technologies.

1. Coexistence of
LTE-U with Wi-Fi 802.11n
at 5GHz unlicensed spectrum
S u r v e y
OsamaAskoura
EECS6590Project
1

2. Roadmap
• Introduction
• Background
• Coexistence Problems
• Surveying Mechanisms
• Discussion/Future Work
• Conclusion
2

3. 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

4. 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

5. What is LTE and LTE-U?
5
Fig3: LTE architecture. Sources: Google Images

6. 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

7. Coexistence of
LTE-U with Wi-Fi 802.11n
at 5GHz unlicensed spectrum
7

8. 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

9. Why is coexistence challenging?
9
Fig4: LTE-U WiFi Coexistence. Source: Babaei, 2014

10. What are LTE-U alternatives?
10
Fig5: LTE-U WiFi Coexistence. Source: Babaei, 2014

11. 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

12. 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

13. 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.

14. Coexistence (2/4)
LTE/Wi-Fi hidden terminal problem
14

15. 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

16. 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

17. 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

18. 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-Fi’s back-off delay is less than LTE-U quiet period
Pure statistical approach. Eliminates PHY layer effects, and hidden/exposed
terminal problems.

19. MAC Protocols (1/4) Quiet Period Analysis
Babaei, 2014
19Fig6: Wi-Fi Channel Access Vs. LTE quiet period. Source: Babei, 2014

20. 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

21. MAC Protocols (2/4) LTE LBT
Bhorkar, 2015
21Fig7: cdf of WiFi throughput; 60% of users have 0 throughput. Source: Bhorkar, 2015

22. MAC Protocols (2/4) LTE LBT
Bhorkar, 2015
22Fig8: cdf of LTE throughput; degrades due to WiFi. Source: Bhorkar, 2015

23. MAC Protocols (2/4) LTE LBT
Bhorkar, 2015
23
Other forms of LBT:
– Synchronous LBT
– Adaptive LBT (alternates between channels)

24. 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

25. MAC Protocols (3/4) LTE-WiFi TDD
Cano, 2015
25Fig7: ThroughputanalysisusingfairallocationproposedbyCano,2015

26. MAC Protocols (4/4) LTE ON/OFF
Cano, 2015
26Fig7: ThroughputanalysisusingfairallocationproposedbyCano,2015

27. Architecture Modifications
802.11-LTE protocol fusion stack
27Fig8: 4 Operation for LTE devices access in the spectrum etiquette. Source: Song & Fang, 2015

28. Architecture Modifications
802.11-LTE protocol fusion stack
28Fig8: Operation for 802.11 Wi-Fi devices access in spectrum etiquette. Source: Song & Fang, 2015

29. 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

30. 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

31. 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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32. 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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