  The federated learning (FL) paradigm emerges to preserve data privacy during
model training by only exposing clients' model parameters rather than original
data. One of the biggest challenges in FL lies in the non-IID (not identical
and independently distributed) data (a.k.a., data heterogeneity) distributed on
clients. To address this challenge, various personalized FL (pFL) methods are
proposed such as similarity-based aggregation and model decoupling. The former
one aggregates models from clients of a similar data distribution. The later
one decouples a neural network (NN) model into a feature extractor and a
classifier. Personalization is captured by classifiers which are obtained by
local training. To advance pFL, we propose a novel pFedSim (pFL based on model
similarity) algorithm in this work by combining these two kinds of methods.
More specifically, we decouple a NN model into a personalized feature
extractor, obtained by aggregating models from similar clients, and a
classifier, which is obtained by local training and used to estimate client
similarity. Compared with the state-of-the-art baselines, the advantages of
pFedSim include: 1) significantly improved model accuracy; 2) low communication
and computation overhead; 3) a low risk of privacy leakage; 4) no requirement
for any external public information. To demonstrate the superiority of pFedSim,
extensive experiments are conducted on real datasets. The results validate the
superb performance of our algorithm which can significantly outperform
baselines under various heterogeneous data settings.
