Estimation of subspace occupancy

The ability to identify unoccupied resources in the radio spectrum is a key capability for opportunistic user in a cognitive radio environment. This paper draws upon and extends geometrically based ideas in statistical signal processing to develop estimators for the rank and the occupied subspace in a multi-user environment from multiple temporal samples of the signal received at a single antenna. These estimators enable identification of resources (i.e., the orthogonal complement of the occupied subspace) that may be exploitable by an opportunistic user.