
From:  Bastian Bloessl 
Subject:  Re: [Discussgnuradio] how does Doppler shift increment in flat fading channel GNU radio 
Date:  Sun, 6 Mar 2016 16:12:12 0800 
…looks like this topic isn’t too popular : ) Anyhow, I wanted to make another attempt as I would really appreciate if someone could comment on this thread or the threads at [1] or [2]. Unfortunately, I still fail to understand the implementation of the flat fader and particularly these lines [3] float s_q = scale_sin*cos(2*M_PI*d_fDTs*d_m*sin(alpha_n)+d_phi[n]); As far as I see, alpha_n is changing from sample to sample, which produces an amplified effect over time that changes autocorrelation properties. (It behaves as if the instantaneous Doppler frequency would have been present since the start and, thus, produces larger and larger phase shifts between subsequent samples.) I tried to show the effect with an iPython notebook It would be great if someone could point me to a paper/book with the algorithm that is implemented here. The only thing I could find is "Improved Models for the Generation of Multiple
Uncorrelated Rayleigh Fading Waveforms” by Yahong R. Zheng and Chengshan Xiao which proposes the parameter set, but not the random walk through the Doppler spectrum that seems to be an attempt to make simulations ergodic. Best, Bastian

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