Reliable and prompt identification of active users is critical for enabling random access in massive machine-to-machine type networks which typically operate within stringent access delay and energy constraints. In this paper, an energy efficient active user identification protocol is envisioned in which the active users simultaneously transmit On-Off Keying (OOK) modulated preambles whereas the base station uses non-coherent detection to avoid the channel estimation overheads. The minimum number of channel-uses required for active user identification in the asymptotic regime of total number of users $\ell$ when the number of active devices k scales as $k = \Theta(1)$ is characterized along with an achievability scheme relying on the equivalence of activity detection to a group testing problem. A practical scheme for active user identification based on a belief propagation strategy is also proposed and its performance is compared against the theoretical bounds.

翻译：在机器对机器大规模网络中，可靠且快速地识别活跃用户对于实现随机接入至关重要，此类网络通常运行在严格的接入延迟和能量约束下。本文提出了一种节能的活跃用户识别协议，其中活跃用户同时传输基于开关键控（OOK）调制的导频序列，而基站采用非相干检测以避免信道估计开销。在总用户数ℓ渐近且活跃设备数k满足k = Θ(1)的尺度下，本文刻画了活跃用户识别所需的最小信道使用次数，并基于活跃检测与群体测试问题的等价性，提出了一种可实现方案。此外，本文还提出了一种基于置信传播策略的实用活跃用户识别方案，并将其性能与理论界限进行了对比。