In this paper we address the issue of association fairness when Wi-Fi and LTEunlicensed (LTE-U) coexist on the same channel in the unlicensed 5 GHz band.Since beacon transmission is the first step in starting the association processin Wi-Fi, we define association fairness as how fair LTE-U is in allowing Wi-Fito start transmitting beacons on a channel that it occupies with a very largeduty cycle. According to the LTE-U specification, if a LTE-U base stationdetermines that a channel is vacant, it can transmit for up to 20 ms and turnOFF for only 1 ms, resulting in a duty cycle of 95%. In an area with heavyspectrum usage, there will be cases when a Wi-Fi access point wishes to sharethe same channel, as it does today with Wi-Fi. We study, both theoretically andexperimentally, the effect that such a large LTE-U duty cycle can have on theassociation process, specifically Wi-Fi beacon transmission and reception. Wedemonstrate via an experimental set-up using National Instrument (NI) USRPsthat a significant percentage of Wi-Fi beacons will either not be transmittedin a timely fashion or will not be received at the LTE-U BS thus making itdifficult for the LTE-U BS to adapt its duty cycle in response to the Wi-Fiusage. Our experimental results corroborate our theoretical analysis. Wecompare the results with Wi-Fi/Wi-Fi coexistence and demonstrate thatLTE-U/Wi-Fi coexistence is not fair when it comes to initial association sincethere is a much larger percentage of beacon errors in the latter case. Hence,the results in the paper indicate that in order to maintain associationfairness, a LTE-U BS should not transmit at such high duty cycles, even if itdeems the channel to be vacant.
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