On the Fundamental Limit of Distributed Learning With Interchangable Constrained Statistics

In the popular federated learning scenarios, distributed nodes often represent and exchange information through functions or statistics of data, with communicative processes constrained by the dimensionality of transmitted information. This paper investigates the fundamental limits of distributed parameter estimation and model training problems under such constraints. Specifically, we assume that each node can observe a sequence of i.i.d. sampled data and communicate statistics of the observed data with dimensionality constraints. We first show the Cramer-Rao lower bound (CRLB) and the corresponding achievable estimators for the distributed parameter estimation problems, and the geometric insights and the computable algorithms of designing efficient estimators are also presented. Moreover, we consider model parameters training for distributed nodes with limited communicable statistics. We demonstrate that in order to optimize the excess risk, the feature functions of the statistics shall be designed along the largest eigenvectors of a matrix induced by the model training loss function. In summary, our results potentially provide theoretical guidelines of designing efficient algorithms for enhancing the performance of distributed learning systems.