Mean Teacher based SSL Framework for Indoor Localization Using Wi-Fi RSSI Fingerprinting

Conventional large-scale indoor localization based on Wi-Fi RSSI fingerprinting faces issues of time-consuming and labor-intensive labeled data collection, limited generalization of a model trained under a supervised learning (SL) framework due to its inability to leverage unlabeled data, and model performance degradation in dynamic scenarios with environmental variations. To address those challenging issues, we propose a comprehensive semi-supervised learning (SSL) framework for a deep neural network (DNN) localization model based on the Mean Teacher, which incorporates access point selection, model pre-training/cloning, and batch-level noise injection. The proposed SSL framework can not only efficiently use hybrid labeled/unlabeled databases for static training of a model during the offline phase, but also exploit unlabeled fingerprints from users of the indoor localization system deployed in the field for continuous retraining of the model during the online phase. We base the proposed SSL framework on the Mean Teacher because it can generate more stable target labels through an exponential moving average of model weights without incurring the high computational complexity of the Pi-Model and with better scalability for online learning than Temporal Ensembling, making it an optimal choice that strikes the right balance between performance and computational complexity in large-scale indoor localization. With the UJIIndoorLoc database, the proposed SSL framework reduces the mean 3D errors of the CNNLoc and SIMO-DNN models by 7.403% and 7.748%, respectively, compared with those under the conventional SL framework; with the XJTLU dynamic database, the maximum reduction in mean 2D error reaches up to 49.227% under a dynamic training scenario, demonstrating the substantial performance improvement achieved by the proposed SSL framework.