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Hierarchical Federated Learning (HFL) has shown great promise over the past few years, with significant improvements in communication efficiency and overall performance. However, current research for HFL predominantly centers on supervised learning. This focus becomes problematic when dealing with semi-supervised learning, particularly under non-IID scenarios. In order to address this gap, our paper critically assesses the performance of straightforward adaptations of current state-of-the-art semi-supervised FL (SSFL) techniques within the HFL framework. We also introduce a novel clustering mechanism for hierarchical embeddings to alleviate the challenges introduced by semi-supervised paradigms in a hierarchical setting. Our approach not only provides superior accuracy, but also converges up to 5.11× faster, while being robust to non-IID data distributions for multiple datasets with negligible communication overhead 

Hyperdimensional computing (HDC) has drawn significant attention due to its comparable performance with traditional machine learning techniques. HDC classifiers achieve high parallelism, consume less power, and are well-suited for edge applications. Encoding approaches such as record-based encoding and N -gram-based encoding have been used to generate features from input signals and images. These features are mapped to hypervectors and are input to HDC classifiers. This paper considers the group-based classification of graphs constructed from time series. The graph is encoded to a hypervector and the graph hypervectors are used to train the HDC classifier. This paper applies HDC to brain graph classification using fMRI data. Both the record-based encoding and GrapHD encoding are explored. Experimental results show that 1) For HDC encoding approaches, GrapHD encoding can achieve comparable classification performance and require significantly less memory storage compared to record-based encoding. 2) The utilization of sparsity can achieve higher performance as compared to fully connected brain graphs. Both threshold strategy and the minimum redundancy maximum relevance (mRMR) algorithm are employed to generate sub-graphs, where mRMR achieves higher performance for three binary classification problems: emotion vs. gambling, emotion vs. no-task, and gambling vs. no-task. The corresponding AUCs are 0.87, 0.88, and 0.88, respectively.