The main requirements for 5G and beyond connectivity include a uniform high quality of service, which can be attained in crowded scenarios by extra-large MIMO (XL-MIMO) systems. Another requirement is to support increasing connected users in (over)crowded machine-type communication (mMTC). In such scenarios, pilot assignment (PA) becomes paramount to reduce pilot contamination and consequently improve spectral efficiency (SE). We propose a novel quasi-optimal low-complexity iterative pilot assignment strategy for XL-MIMO systems based on a genetic algorithm (GA). The proposed GA-based PA procedure turns the quality of service more uniform, taking into account the normalized mean-square error (NMSE) of channel estimation from each candidate of the population. The simulations reveal that the proposed iterative procedure minimizes the channel estimation NMSE averaged over the UEs. The second procedure is the subarray (SA) selection. In XL-MIMO systems, commonly, a UE is close to a SA antenna subset such that a sufficient data rate can be achieved if only a specific SA serves that UE. Thus, a SA selection procedure is investigated to make the system scalable by defining the maximum number of UEs each SA can help. Hence, the SA clusters are formatted based on the PA decision. Furthermore, we introduce an appropriate channel model for XL-MIMO, which considers a deterministic LoS component with a distance-dependent probability of existence combined with a stochastic spatially correlated Rayleigh NLoS fading component. The developed simulations and analyses rely on this suitable channel model under realistic assumptions of pilot contamination and correlated channels.

翻译：第五代及未来通信连接的主要需求包括统一的高服务质量，这可以通过超大规模MIMO（XL-MIMO）系统在拥挤场景中实现。另一需求是支持大规模机器类型通信（mMTC）中日益增长的连接用户数量。在此类场景中，导频分配（PA）对于减少导频污染、进而提升频谱效率（SE）至关重要。我们提出一种基于遗传算法（GA）的新型准最优低复杂度迭代导频分配策略，适用于XL-MIMO系统。所提出的基于GA的PA程序通过考虑种群中每个候选者的信道估计归一化均方误差（NMSE），使服务质量更加均匀。仿真结果表明，所提出的迭代程序最小化了用户设备平均的信道估计NMSE。第二个程序是子阵列（SA）选择。在XL-MIMO系统中，用户设备通常靠近某子阵列天线子集，因此若仅由特定SA服务该用户，即可实现足够的数据速率。为此，我们研究了一种子阵列选择程序，通过定义每个SA可服务的最大用户设备数量，使系统具有可扩展性。因此，SA簇基于PA决策形成。此外，我们引入了一种适用于XL-MIMO的信道模型，该模型考虑了具有距离依赖性存在概率的确定性视距分量，并结合了随机空间相关瑞利非视距衰落分量。所开发的仿真与分析均基于该实际假设下的合适信道模型，考虑了导频污染及相关信道。