This paper revisits the identity detection problem under the current grant-free protocol in massive machine-type communications (mMTC) by asking the following question: for stable identity detection performance, is it enough to permit active devices to transmit preambles without any handshaking with the base station (BS)? Specifically, in the current grant-free protocol, the BS blindly allocates a fixed length of preamble to devices for identity detection as it lacks the prior information on the number of active devices $K$. However, in practice, $K$ varies dynamically over time, resulting in degraded identity detection performance especially when $K$ is large. Consequently, the current grant-free protocol fails to ensure stable identity detection performance. To address this issue, we propose a two-stage communication protocol which consists of estimation of $K$ in Phase I and detection of identities of active devices in Phase II. The preamble length for identity detection in Phase II is dynamically allocated based on the estimated $K$ in Phase I through a table lookup manner such that the identity detection performance could always be better than a predefined threshold. In addition, we design an algorithm for estimating $K$ in Phase I, and exploit the estimated $K$ to reduce the computational complexity of the identity detector in Phase II. Numerical results demonstrate the effectiveness of the proposed two-stage communication protocol and algorithms.

翻译：本文重新审视了大规模机器类通信（mMTC）中现有免授权协议下的身份检测问题，并提出以下疑问：为获得稳定的身份检测性能，是否只需允许活跃设备在无需与基站（BS）进行任何握手的情况下直接发送前导码？具体而言，在当前免授权协议中，由于缺乏关于活跃设备数量$K$的先验信息，基站会盲目地为设备分配固定长度的前导码用于身份检测。然而，在实际场景中，$K$随时间动态变化，导致身份检测性能下降，尤其是在$K$较大时。因此，现有免授权协议无法保证稳定的身份检测性能。为解决该问题，我们提出一种两阶段通信协议：第一阶段（Phase I）估计$K$，第二阶段（Phase II）检测活跃设备的身份。Phase II中用于身份检测的前导码长度根据Phase I中估计的$K$通过查表方式动态分配，使身份检测性能始终优于预设阈值。此外，我们设计了Phase I中估计$K$的算法，并利用估计的$K$降低Phase II中身份检测器的计算复杂度。数值结果验证了所提两阶段通信协议与算法的有效性。