“…ML has been used in fingerprinting solutions since its infancy, where standard ML methods have been utilized as matching mechanism, e.g., [6], [315]. Since then it has been utilized in other aspects as well, for instance for feature extraction and radio-map construction [72], [72], [91], [94], [103], [138], [142], [154], [231], [297], radio-map updating [85], [122], [132], [181], hierarchical solutions [75], [151], [222], [256], [316], and robust matching [79], [86], [88]. This is expected because fingerprinting systems, similar to ML, are data-driven and both may 9 operate in a training (offline) phase, and an online phase.…”