I. Introduction
Channel modeling plays an important role in radio communications, since the performance of any practical system depends on channel characteristics. Recently, many radio channel models are based on the concept of clustered mutipath components (MPCs), such as the Saleh-Valenzuela model [2], COST 2100 [3] and 3GPP Spatial Channel Model [4], etc., where the cluster has been usually considered as a group of MPCs with similar channel parameters [5], [6]. Meanwhile, radio channels are generally time-varying due to the movements of scatterers and/or communication terminals, which cause significant challenges in time-varying channel characterizations.