Different from a long-term evolution (LTE) system that monopolizes a licensed channel, the standalone LTE system in an unlicensed spectrum (SA LTE-U) is not allowed to occupy the unlicensed channel uninterruptedly, the starting point of its channel occupancy cannot be determined in advance, and its maximum channel occupancy time is restricted. Therefore, user equipments (UEs) may be backlogged during the channel unoccupancy time in their random access (RA) procedure, which causes severe collision during the channel occupancy time and unnecessary retransmissions of preambles in an SA LTE-U system. To address the aforesaid issues, we propose a user grouping and effective window size based RA protocol. In this protocol, the UEs are divided into several groups, and at any time only one group is activated and allowed for its UEs to send RA attempt, which avoids the intergroup UEs' collision. The backlogged UEs of the activated group are scattered over multiple RA slots for them to send RA attempt, which alleviates the intragroup UEs' collision. In addition, to avoid the unnecessary retransmissions of preambles caused by the channel uncertainty, the transmission of RA attempt and the reception of RA response are performed on the effective channel that consists of the channel occupancy periods only. The expressions of success and dropping rates, cumulative distribution function of preamble transmission times, utilization of carrying RA slots, and RA delay are derived. An optimization problem is formulated to maximize the utilization of carrying RA slots to obtain the optimal number of carrying RA slots. Simulation results demonstrate that the proposed protocol outperforms the baseline protocol in SA LTE-U systems.
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