Recently, in multiple beam mobile satellite systems, onboard frequency domain switching(FDS) is applied enabling flexible large capacity routing. In this paper, channel transmission characteristics are investigated for FDS. At first, overlap and save operation and concept of FDS are explained. Selecting a channel is performed by selecting frequency samples. Transmission characteristics and required number of samples par channel are investigated. It was shown that out of band characteristics are determined by the truncation time window. In this procedure, in order to improve out of band characteristics, additional weighting by cosine roll-off time window matched with the overlap and save operation is proposed. It is shown that by the proposed window, which does not affect signal transmission, out of band characteristics can be conspicuously improved. Finally QPSK transmission characteristics are also investigated.