Given discrete subsets $\Lambda_j\subset { \Bbb R}^d$, $j=1,...,q$, consider the set of windowed exponentials $\bigcup_{ j=1}^{ q}\{ g_j(x)e^{ 2\pi i <\lambda,x>}: \lambda\in\Lambda_j\}$ on $L^2(\Omega)$. We show that a necessary and sufficient condition for the windows $g_j$ to form a frame of windowed exponentials for $L^2(\Omega)$ with some $\Lambda_j$ is that $m\leq \max_{ j\in J}|g_j|\leq M$ almost everywhere on $\Omega$ for some subset $J$ of $\{ 1,..., q\}$. If $\Omega$ is unbounded, we show that there is no frame of windowed exponentials if the Lebesgue measure of $\Omega$ is infinite. If $\Omega$ is unbounded but of finite measure, we give a sufficient condition for the existence of Fourier frames on $L^2(\Omega)$. At the same time, we also construct examples of unbounded sets with finite measure that have no tight exponential frame.
Categories: math.FA