This paper investigates a grant-free non-orthogonal multiple access (GF-NOMA) system in the presence of carrier frequency offsets. We propose two schemes for enumerating active users in such a GF-NOMA system, which is equivalent to estimating the sparsity level. Both schemes utilize a short common pilot and the eigenvalues of the sample covariance matrix of the received signal. The two schemes differ in their treatment of noise variance: one exploits known variance information, while the other is designed to function without this knowledge. Simulation results demonstrate the effectiveness of the proposed schemes in terms of the normalized root-mean-squared error.

翻译：本文研究了存在载波频偏的免授权非正交多址接入系统。针对此类GF-NOMA系统中的活跃用户枚举问题（等效于稀疏度估计），我们提出了两种方案。两种方案均利用短公共导频及接收信号样本协方差矩阵的特征值。其区别在于对噪声方差的处理方式：一种方案利用已知的方差信息，另一种则设计为无需该先验知识即可工作。仿真结果表明，所提方案在归一化均方根误差指标上具有优越性能。