New Collision Detection Method for Fair LTE-LAA and Wi-Fi Coexistence

Abstract

Growing requirements for cellular network capacity made 3GPP consortium introduce License-Assisted Access (LAA), which implies unlicensed spectrum exploitation. Since these frequencies are already occupied by other technologies, such as Wi-Fi, it is vital to assure fair and efficient sharing of the unlicensed bands by LAA and Wi-Fi devices. Hence, LAA uses Listen Before Talk, the principle of which is similar to CSMA/CA implemented in Wi-Fi. Specifically, similar to Wi-Fi, an LAA base station performs the backoff procedure before every transmission attempt. However, the base station can only start LAA data transmission at licensed spectrum slot boundaries, which results in an extremely long vulnerable period from the end of backoff procedure till the slot boundary. During such a vulnerable period other devices, e.g., Wi-Fi stations, can occupy the channel, canceling the LAA transmission. To protect LAA transmissions, many developers propose sending a reservation signal during the whole vulnerable period, which in turn blocks Wi-Fi stations, occupying the channel with the useless signal and resulting in the channel resource sharing unfairness. To solve this problem, this paper designs a novel mechanism, which allows the LAA base station to detect collisions of Wi-Fi transmissions with the reservation signal. An analytical model for various LAA implementations is developed. Using the model, it is shown how LAA performance can be improved with the proposed mechanism without Wi-Fi performance degradation.