  Federated learning achieves effective performance in modeling decentralized
data. In practice, client data are not well-labeled, which makes it potential
for federated unsupervised learning (FUSL) with non-IID data. However, the
performance of existing FUSL methods suffers from insufficient representations,
i.e., (1) representation collapse entanglement among local and global models,
and (2) inconsistent representation spaces among local models. The former
indicates that representation collapse in local model will subsequently impact
the global model and other local models. The latter means that clients model
data representation with inconsistent parameters due to the deficiency of
supervision signals. In this work, we propose FedU2 which enhances generating
uniform and unified representation in FUSL with non-IID data. Specifically,
FedU2 consists of flexible uniform regularizer (FUR) and efficient unified
aggregator (EUA). FUR in each client avoids representation collapse via
dispersing samples uniformly, and EUA in server promotes unified representation
by constraining consistent client model updating. To extensively validate the
performance of FedU2, we conduct both cross-device and cross-silo evaluation
experiments on two benchmark datasets, i.e., CIFAR10 and CIFAR100.
