Full Characterization of embedding relations between alpha modulation spaces

In this paper, we consider the embedding relations between any two $\alpha$% -modulation spaces. Based on an observation that the $\alpha$-modulation space with smaller $\alpha$ can be regarded as a corresponding $\alpha$% -modulation space with larger $\alpha$, we give a complete characterization of the Fourier multipliers between $\alpha$-modulation spaces with different $\alpha$. Then we establish a full version of optimal embedding relations between $\alpha$-modulation spaces. As an application, we determine that the bounded operators commuting with translations between $\alpha$-modulation spaces are of convolution type.