Latency and reliability analysis of cellular networks in unlicensed spectrum

In this paper, the achievable latency-reliability performance of a standalone cellular network over the 5 GHz unlicensed spectrum is analysed. Fulfilling strict latency-reliability requirements comes with significant challenges for unlicensed operation, especially due to mandatory channel access procedures. Using MulteFire as the reference system-model, an analysis of a highly realistic multi-cell network with bi-directional traffic shows that latency of 23 ms with a reliability level of 99.99% is achievable for low-loads, while latency is increased to 79 ms at high-loads. Different techniques are described to improve the system performance. First, a pre-emptive scheme to cope with continuous uplink listen before talk (LBT) failures for uplink control transmissions is proposed. It provides a latency reduction of 24% at low-loads with two transmission opportunities and 11% for high-loads with three opportunities. Secondly, the possibility of skipping LBT performance under given conditions is evaluated. This results in a lower uplink LBT failure rate which translates to a latency reduction of 8% for low-loads and up to 14% for high-loads, at 99.99% reliability. Thirdly, as an alternative to grant-based uplink, grant-free uplink is evaluated. Grant-free uplink achieves better performance than grant-based uplink at low-loads, offering 50% lower uplink latency. At high-loads, the gain of grant-free uplink decreases due to the high number of simultaneous transmissions.