Performance Analysis of License Assisted Access LTE with Asymmetric Hidden Terminals

Abstract: License Assisted Access (LAA) LTE (LTE-LAA) is a new type of LTE that aggregates the licensed LTE bands with the unlicensed bands via carrier aggregation. To operate in unlicensed bands, LTE-LAA adopts the listen-before-talk policy and designs its channel access mechanism similar to WLAN's DCF. In this paper, we consider an LAA network consisting of an LTE-LAA eNB coexisting with a Wi-Fi STA, and capture the {\em asymmetric hidden terminal problem} where the eNB recognizes the STA while the opposite is not true, which is caused by the asymmetric CCA thresholds between them. We model the network as a joint Markov chain (MC) consisting of two individual MCs, and derive its steady-state probabilities, throughput, and channel access delay along with other key metrics like transmit, busy, collision, and doubling probabilities. Through extensive evaluation, we confirm that the proposed model well predicts the dynamics of the LAA network, and identify important design guidelines for fair coexistence between LTE-LAA and WLAN as follows. First, LTE-LAA should design its contention window (CW) doubling policy by considering Wi-Fi's packet duration and subframe-dependent collision probabilities. Second, there exists a tradeoff between throughput and channel access delay, according to which the CW doubling policy should be adapted.