Thanks to the large spectrum chunks available above 6-GHz bands, several centimeter-wave (cm-wave) and millimeter- wave (mm-wave) bands are being exploited for mobile communications to meet 5G high data speed and low latency requirements. The 5G standardization has been started with the channel modelling for the bands from 6 and up to 100 GHz, and the current models adapting the 3GPP stochastic cluster-based approach. This paper presents a recent measurement campaign and multipath cluster characterization of 15 and 28 GHz channels in an urban street canyon. The measurement environment and setup are targeted for small-cell backhaul scenarios, whose model parameters are currently lacking in current standardized channel models. Inter-cluster and intra-cluster characteristics are presented and compared between two measured frequency ranges. The results show that the number of MPCs per cluster is noticeable higher in 28 GHz, and that is mainly attributed by the higher measurement bandwidth in this band. The higher number of MPCs in 28 GHz leads to little wider cluster delay and angle spreads, while other model parameters are similar or just little varied with carrier frequency. The paper also provides other relevant cluster statistics required to establish 3GPP channel model for the scenario of interest in the 15 and 28 GHz bands.