  Coexistence of Wi-Fi and LTE Unlicensed (LTE-U) in shared or unlicensed bands
has drawn growing attention from both academia and industry. An important
consideration is fairness between Wi-Fi and duty cycled LTE-U, which is often
defined in terms of channel access time, as adopted by the LTE-U Forum. Despite
many studies on duty cycle adaptation design for fair sharing, one crucial fact
has often been neglected: LTE-U systems unilaterally control LTE-U duty cycles;
hence, as self- interested users, they have incentives to misbehave, e.g.,
transmitting with a larger duty cycle that exceeds a given limit, so as to gain
a greater share in channel access time and throughput. In this paper, we
propose a scheme that allows the spectrum manager managing the shared bands to
estimate the duty cycle of a target LTE-U cell based on PHY layer observations
from a nearby Wi-Fi AP, without interrupting normal Wi-Fi operations. We
further propose a thresholding scheme to detect duty cycling misbehavior (i.e.,
determining if the duty cycle exceeds the assigned limit), and analyze its
performance in terms of detection and false alarm probabilities. The proposed
schemes are implemented in ns3 and evaluated with extensive simulations. Our
results show that the proposed scheme provides an estimate within +/- 1% of the
true duty cycle, and detects misbehavior with a duty cycle 2.8% higher than the
limit with a detection probability of at least 95%, while keeping the false
alarm probability less than or equal to 1%.
