Performance of Autonomous Uplink Transmissions in Unlicensed Spectrum LTE

The immediate availability of Uplink (UL) time-frequency resources for transmission of new data is a key factor in reducing latency and improving UL throughput in cellular systems. This is particularly important for unlicensed spectrum operation where access to the channel is subject to Listen-Before-Talk (LBT), and scheduling UL transmissions with a preceding Downlink (DL) transmission as in Long-Term Evolution (LTE) can be inefficient. Therefore, this work proposes and studies autonomous UL transmissions (AUL) for LTE-based systems in unlicensed spectrum, where UL transmissions are allowed without requiring a prior scheduling request or an explicit scheduling grant from the Evolved Node B (eNB). The implications of AUL transmissions for the design of hybrid automatic repeat request (HARQ) and link adaptation mechanisms are investigated. Detailed performance evaluations in the 5 GHz unlicensed band show that dynamic switching between scheduled and autonomous UL transmissions offers the best throughput performance, while also coexisting fairly with co-channel Wi-Fi networks.