  In this paper, we study energy-based Carrier Sense Adaptive Transmission
(CSAT) for use with LTE-U and investigate the performance in Wi-Fi/LTE-U
coexistence using theoretical analysis and experimental verification using NI
USRPs. According to the LTE-U forum specification, if an LTE-U base station
(BS) finds a vacant channel, it can transmit for up to 20 ms and turn OFF its
transmission for only 1 ms, resulting in a maximum duty cycle of 95%. In a
dense deployment of LTE-U and Wi-Fi, it is very likely that a Wi-Fi access
point (AP) will wish to use the same channel. It will start transmission by
trying to transmit association packets (using carrier sense multiple access
with collision avoidance (CSMA/CA)) through the 1 ms LTE-U OFF duration. Since
this duration is very small, it leads to increased association packet drops and
thus delays the Wi-Fi association process. Once LTE-U, using CSAT, detects
Wi-Fi, it should scale back the duty cycle to 50%. We demonstrate in this
paper, using an experimental platform as well as theoretical analysis, that if
LTE-U is using a 95% duty cycle, energy based CSAT will take a much longer time
to scale back the duty cycle due to the beacon drops and delays in the
reception. Hence, in order to maintain association fairness with Wi-Fi, we
propose that a LTE-U BS should not transmit at maximum duty cycles (95%), even
if the channel is sensed to be vacant.
