Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

1. Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence EvgenyKhorov, PolinaKutsevol, VyacheslavLoginov, Andrey Lyakhov Institute for Information Transmission Problems, Russian Academy of Sciences (IITP RAS)

2. Channel Access Methods in LAA and Wi-Fi • Have much in common • Both LBT in LAA and EDCA in Wi-Fi are CSMA/CA based • 4 access categories • Parameter values (AIFSN, CWmin, CWmax, backoff slot size) • But have a lot of differences • ED/PD rules • Single thresholdscheme in LAA, double threshold scheme in Wi-Fi • TXOP (MCOT) duration • Best Effort traffic: LAA – 8ms, Wi-Fi – 2.5ms • Possible data frame start positions • Wi-Fi STA may start data frame in an arbitrary moment of time thanks to frame preamble • LAA eNB have to wait slot boundary in primary component carrier in licensed band Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

3. Reservation signal in LAA End of LBT backoff Possible frame start positions Possible solutions: 1. Keep silent before the slot boundary • Other LAA/Wi-Fi station may occupy the channel • Low LAA performance when coexisting with Wi-Fi stations 2. Send reservation signal (RS) • Big overhead (up to 500us of airtime wasted) • Unfair channel resource sharing • Asymmetric collisions Unlicensed CC DATA time What should eNB do? Licensed CC slot slot time subframe subframe LBT Slot = 9 us, LTE Slot = 500 us Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

4. Performance Evaluation [1] Two scenarios • Reference: N+1 Wi-Fi STAs • Coexistence: N Wi-Fi STAs and one LAA BS Traffic: saturated UDP sent with AC_BE KPI: per device throughput gain in the coexistence scenario comparing to the reference scenario Coexistence is assumed fair if throughput gain for both LAA and Wi-Fi is positive [1] PolinaKutsevol, VyacheslavLoginov, EvgenyKhorov, Andrey Lyakhov. Analytical Study of License-Assisted Access in 5G Networks. // In proc. of IFIP Networking 2019, Warsaw, Poland, 20-22 May, 2019 Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

5. Numerical Results • With LAA reservation signal, channel resource sharing is unfair from the Wi-Fi point of view • LAA has longer transmissions • Asymmetric collisions when RTS is used • Without LAA reservation signal, channel resource sharing is unfair from the LAA point of view • High probability that some Wi-Fi STA occupies the channel during Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

6. Asymmetric Collisions • LAA eNB detects collision only if the data is corrupted • If Wi-Fi transmission is shorter than LAA reservation signal, LAA eNB does not understand that the collision has happened and almost always successfully transmits its data • The problem is more noticeable when Wi-Fi uses RTS, the duration of which is small (~40us) comparing to the reservation signal (up to 500us) RTS Wi-Fi Reserv.Signal Data LAA time Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

7. Proposed Wireless Collision Detection for LAA • When backoff procedure is finished, LAA eNB transmits short reservation signal (Pre-RS) for us • equals the duration of the Legacy Short Training field (L-STF) in the Wi-Fi frame preamble • is enough for not-transmitting stations to detect that the channel becomes busy • After Pre-RS, the LAA eNB listens the channel for SIFS • If the channel is busy, LAA eNB detects the collision, doubles CW and repeats the backoff procedure • If the channel is free, then LAA eNB transmits RS until the slot boundary in licensed carrier followed by data transmission (legacy LAA eNB behavior) Data Wi-Fi Pre-RS RS Pre-RS Data SIFS SIFS LAA time Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

8. Performance Evaluation [2] Two scenarios • Reference: N+1 Wi-Fi STAs • Coexistence: N Wi-Fi STAs and one LAA BS Traffic: saturated UDP sent with AC_BE – probability that in case of collision of one Wi-Fi frame and Pre-RS receiving Wi-Fi STA successfully decodes L-STF • In case of collision of one Wi-Fi frame and Pre-RS, L-STF (and, as a consequence, whole Wi-Fi frame) may be successfully decoded because of capture effect [3] KPI: per device throughput gain in the coexistence scenario comparing to the reference scenario Coexistence is assumed fair if throughput gain for both LAA and Wi-Fi is positive [2] PolinaKutsevol, VyacheslavLoginov, EvgenyKhorov, Andrey Lyakhov. New Collision Detection Method for Fair LTE-LAA and Wi-Fi Coexistence // In proc. of IEEE PIMRC 2019, Istanbul, Turkey, 8-11 September, 2019 [3] E. Khorov, A. Kureev, I. Levitsky, and A. Lyakhov, “Testbed to study the capture effect: Can we rely on this effect in modern Wi-Fi networks,” in 2018 IEEE International Black Sea Conference on Communications and Networking (BlackSeaCom). IEEE, 2018, pp. 1–5. Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

9. Numerical Results. Without RTS/CTS • Proposed Wireless Collision Detection improves fairness of channel resource sharing • LAA throughput is slightly decreased (but still high) • Wi-Fi throughput is closer to the one in reference scenario (which corresponds to 0% gain) • The results are better if capture effect feature is implemented in Wi-Fi devices Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

10. Numerical Results. With RTS/CTS • Proposed Wireless Collision Detection solves the problem of asymmetric collisions • Wi-Fi throughput is almost doubled Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

11. Conclusion • We have proposed wireless collision detection (WCD) which is aimed at improving fairness of channel resource sharing in Wi-Fi/LTE-LAA coexistence scenario • Using analytical modeling, we have shown that the proposed solution solves the problem of asymmetric collisions when Wi-Fi stations use RTS/CTS and improves fairness of channel resource sharing between Wi-Fi and LTE-LAA stations • We have shown that the proposed solution also works when Wi-Fi stations do not use RTS/CTS Wireless Collision Detection for Fair LAA/Wi-Fi Coexistence

12. Questions VyacheslavLoginov loginov@iitp.ru The research was done at IITP RAS and supported by the Russian Government (Contract No 14.W03.31.0019)