The rapid progress of the deployment of IoT services pushes the evolution of wireless communication techniques. Because the number of IoT devices is much more than that of the human-held devices for traditional services. It introduces the random access issue in radio networks. In order to support massive IoT devices to transmit data in NB-IoT, the release 14 of 3 GPP provides the preambles in non-anchor carrier for random access. However, if more non-anchor carriers are provided for random access, the resource of uplink shared channel will be compressed. The use of non-anchor carrier for random access preambles shall be carefully allocated for effective resource utilization. In this paper, we propose the adaptive non-anchor allocation algorithm by referring to the collision report flag (CRF) from the user equipment. The proposed CRF algorithm considers the congestion status of uplink to adjust the number of non-anchor carriers in flexible way for better random access experience of huge random access attempts condition. The simulation results show that the proposed algorithm achieves high success access ratio and effective non-anchor carrier utilization when comparing to that of the fixed allocation schemes. The proposed scheme can save 5 - 10 numbers of non-anchor carriers for the number of UEs varies from 15,000 to 37,500 when comparing to the fixed 15 non-anchor carriers scheme under the similar successful access ratio.
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