Energy-Delay Evaluation and Optimization for NB-IoT PSM With Periodic Uplink Reporting

The narrowband Internet of Things (NB-IoT) is a new wireless protocol proposed by the 3rd Generation Partnership Project intending for low data rate IoT applications. The general objectives of the NB-IoT include supporting massive connections, enhanced coverage, reduced cost and complexity, ultra-low power consumption, and flexible delay characteristics. To lower energy consumption while providing reliable connections, extended discontinuous reception and power saving mode (PSM) mechanism are applied in the NB-IoT. To evaluate the energy consumption and delay performance under periodic uplink reporting, which is common among cellular IoT applications, this paper develops a semi-Markov chain with four states, namely, PSM, idle, random access (RACH), and transmission (Tx) states. RACH and Tx states are introduced from the well-known CONNECTED STATE to account for the extra power consumed due to increased access collisions under massive synchronous connections. Furthermore, an optimization model is introduced to find the best PSM duration, which is configured to minimize energy consumption and average delay according to user's preference. The numerical results show that setting higher limits for the number of possible RACH request transmission can make the user equipment (UE) more tolerant to delay and energy consumption in massively deployed concurrent communication UEs. Extending the PSM duration to longer period will cause excessive increase in delay without much impact on energy saving improvement.