In this paper, we provide performance analysis for a pilot-channel-assisted log-likelihood-ratio selective combining (PCA-LLR-SC) scheme that is designed to support robust long-range low-rate UWB communications targeted by the IEEE 802.15.4a. Including the loss of orthogonality between data and pilot pulses caused by the UWB multipath channel, a closed-form bit-error-rate expression is derived, and verified by comparisons with simulation results. Our analytical result shows its accuracy in performance evaluation for the PCA-LLR-SC scheme and confirms that PCA-LLR-SC is able to support robust low-rate UWB communications in fast-fading multipath channels.