COTA: Channel occupancy time adaptation for LTE in unlicensed spectrum

Kangjin Yoon, Taejun Park, Jihoon Kim, Weiping Sun, Sunwook Hwang, Ingab Kang, Sunghyun Choi

Abstract: LTE operation in unlicensed spectrum is emerging as a promising technology in achieving higher data rate with LTE since ultra-wide unlicensed spectrum, e.g., about 500 MHz at 5-6 GHz range, is available in most countries. Recently, 3GPP has finalized standardization of licensed assisted access (LAA) for LTE operation in 5 GHz unlicensed spectrum, which has been a playground only for Wi-Fi. LAA defines downlink transmission over 5 GHz spectrum, where channel sensing, i.e., listen before talk (LBT) operation similar to Wi-Fi medium access control (MAC) protocol, is performed before each transmission of eNodeB. LAA has a fixed 8 ms maximum channel occupancy time (COT), which is the maximum continuous transmission time after channel sensing, while Wi-Fi may transmit for much shorter time duration. As a result, when Wi-Fi coexists with LAA, Wi-Fi airtime and throughput can be much less than those achieved when Wi-Fi coexists with another Wi-Fi. To guarantee fair airtime and improve throughput of Wi-Fi, we propose a COT adaptation (COTA) algorithm, which observes Wi-Fi aggregate MAC protocol data unit (A-MPDU) frames and matches LAA's COT to the duration of A-MPDU frames. Moreover, COTA detects saturation of Wi-Fi traffic and adjusts COT only if Wi-Fi traffic is saturated. We prototype saturation detection algorithm of COTA with commercial off-the-shelf Wi-Fi device and show that COTA detects saturation of Wi-Fi networks accurately. Through ns-3 simulations, we demonstrate that COTA achieves up to 153% Wi-Fi throughput gain while providing airtime fairness between LAA and Wi-Fi.