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A method for resolving users' collision in random access schemes in 5G systems

One of the challenges facing the deployment of massive machine type communications in mobile networks is the overload in the radio access. When high number of devices attempt to access the system in a random access procedure, a collision happens and the transmitted data will be lost. As the base-station is not aware of how many users are colliding, it will not be able to resolve the collision. In this paper, we propose a method that enables the base-station to detect the number of colliding users in random access schemes. The proposed method capitalizes on the unique feature of frequency quadrature amplitude modulation (FQAM) that the number of active subcarriers in a FQAM symbol depends on the number of users transmitting on that symbol. Thus, in the proposed method, FQAM will be used by the users for the random access, and the base-station will estimate the number of colliding users based on the number of active subcarriers. We provide in details the channel structure, procedures and theoretical derivation of the average number of active subcarriers. Numerical simulations show that by using the proposed method the number of colliding users can be accurately detected at the base-station. Furthermore, it is shown that, with the knowledge of the number of colliding users, the base-station can successfully retrieve the users' data, and the performance of the random access schemes can be significantly enhanced.