Spectrogram Analysis of Multipath Fading Channels under Variations of the Mobile Speed

When modelling, analysing, and simulating mobile radio channels, it is generally assumed that the speed of the mobile station (MS) is constant. In practice, however, the mobile speed varies with time. Mobile speed variations result in nonwide- sense stationary multipath fading channels, which can be modelled by a sum-of-chirps (SOCh) processes. In this paper, we analyse the local Doppler power spectral density (PSD) of SOCh processes by using the concept of the spectrogram. The spectrogram provides an estimate of the local Doppler PSD and gives insight into how the Doppler PSD changes over time if the MS increases or decreases its speed. A closed-form solution is presented for the spectrogram of SOCh processes, which includes the spectrogram of sum-of-cisoids (SOCi) processes as a special case if the MS moves with constant speed. The results of this paper contribute to the modelling, analysis, and simulation of non-stationary mobile radio channels and provide the basis for the performance analysis of mobile communication systems under non- stationary conditions caused by mobile speed variations.