  In this letter, we propose an enhanced Access Reservation Protocol (ARP) with
a partial preamble transmission (PPT) mechanism for the narrow band Internet of
Things (NB-IoT) systems. The proposed ARP can enhance the ARP performance by
mitigating the occurrence of preamble collisions, while being compatible with
the conventional NB-IoT ARP. We provide an analytical model that captures the
performance of the proposed ARP in terms of false alarm, mis-detection and
collision probabilities. Moreover, we investigate a trade-off between the
mis-detection and the collision probabilities, and optimize the proposed ARP
according to the system loads. The results show that the proposed ARP
outperforms the conventional NB-IoT ARP, in particular at heavier system loads.
