Achieving Proportional Fairness for LTE-LAA and Wi-Fi Coexistence in Unlicensed Spectrum

LTE Licensed Assisted Access (LTE-LAA) is a promising solution for harmonious coexistence with WiFi in unlicensed spectrum. Although LTE-LAA employs a listen-before-talk approach similar to the distributed coordination function in IEEE 802.11, it uses different parameters and varying transmission durations. As a result, achieving fair coexistence between LTE-LAA and WiFi (by any definition) remains an open question, which in a pragmatic framework, devolves to: how LTE-LAA should select its parameters. To address this issue, a multi-group model is proposed for LTE-LAA and WiFi coexistence, as a function of respective initial backoff window sizes, sensing durations, maximum backoff stages, retry limits and transmission opportunities. The network steady-state point in saturated conditions is obtained, based on which the node airtime and total network airtime are derived as functions of system parameters of LTE-LAA and WiFi networks. The analysis shows that LTE-LAA can maintain proportional fairness with WiFi by either tuning its initial backoff window size or sensing duration. In particular, the optimal initial backoff window size and number of sensing slots of LTE-LAA are both derived and verified by simulation. The significance of our analysis is two-fold: a) it exposes the result that the current standard-proposed parameter settings will not generically achieve fairness and thereafter b) suggests optimal settings whereby LTE-LAA and WiFi nodes can achieve equal per-node airtime. It is further revealed that the initial backoff window size tuning of LTE-LAA could be a preferable option for achieving fairness between LTE-LAA and WiFi in practical scenarios as it requires less system information and achieves better precision.