Association fairness in Wi-Fi and LTE-U coexistence

Abstract

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

Wi-Fi/LTE-U Coexistence: Real-Time Issues and Solutions

“…In the case of Wi-Fi occupying a channel that LTE-U wishes to use, the problem is simple since it is expected that LTE-U will simply start transmitting with a 50% duty cycle, however the reverse situation is more complicated and will be the one we address in this paper (in-terms of association, latency, and throughput). In our past work [16], we performed theoretical analyses and extensive measurements with Wi-Fi and LTE-U in realistic deployments to understand the Wi-Fi beacon transmission/reception behavior when LTE-U operates on the same channel. The motivation is to understand if LTE-U should be operating with its maximum allowed duty cycle of 95% when it is operating on an empty channel.…”

“…Each LTE frame consists of 10 subframe with the total duration of 10 ms (where, 1 LTE Frame = 20 slots). During the OFF state, LTE-U BS can detect the Wi-Fi AP signals that is operating on the same channel and adjust its duty cycle accordingly (i.e., when one AP is detected, duty cycle must be set at 50%, when two APs are detected, duty cycle is set at 33% [16]). According to the LTE-U specification, the cycle can set a maximum of 20 ms ON time and a minimum of 1 ms OFF time, which leads to a 95% duty cycle as shown in Fig.…”

“…In case 3a, while beacon packets may not be received successfully, it can still be recovered using error correction coding given a large portion of the frame is valid. Our works will quantify both these type of events as a function of duty cycle and period using theoretical analysis [16] and experimental study.…”

“…If there is no deterioration in successful beacon transmission and reception as compared to baseline Wi-Fi/ Wi-Fi performance, then the Wi-Fi/LTE-U deployment can be considered to be ''fair''. We study [16], both theoretically and experimentally, the effect that such a large LTE-U duty cycle can have on the association process, specifically Wi-Fi beacon transmission and reception. We demonstrate using carefully designed experiments in realistic settings that a significant percentage of Wi-Fi beacons will either not be transmitted in a timely fashion or will not be received at the LTE-U BS thus making it difficult for the LTE-U BS to adapt its duty cycle in response to the Wi-Fi usage.…”

Analysis of CSAT Performance in Wi-Fi and LTE-U Coexistence

“…The overlap of Wi-Fi beacon with the LTE-U ON period (as the LTE-U in not sensing for transmission) is the reason for the beacon drop. This is explained in detail in Section III 3a in [9]). We assume that any partial overlap of LTE-U with Wi-Fi beacon results in the beacon packet collision.…”