A novel method for blind detection of users' activity in grant-free access for 5G networks

Grant-free access represents one of the promising access schemes in 5G systems to meet the restrict power and delay requirements of the new usage scenarios. In grant-free access, the users transmit to the base-station without prior scheduling permission from the network. One of the challenges is that the base-station is not aware of how many users are transmitting at a given time. In this paper, we propose a method that enables the base-station blindly detect the number of active users in grant-free access. 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 grant-free transmission, and the base-station will estimate the number of active 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 active users can be accurately detected at the base-station. Furthermore, it is shown that the performance of the method depends on the number of active users and the number of radio resources used to estimate the users' activity.