The classification of discrete series is one important subject with numerous application in the harmonic analysis and in the theory of automorphic forms. The classification consists of two steps, whose first step is the classification of strongly positive representations. With Ivan Matic (University of Osijek, Croatia), we obtain the first step, i.e., classification of strongly positive representations of a quasi-split similitude unitary groups GU(n, n) defined with respect to a quadratic extension of non-archimedean local fields. This is second result of the project "classification of discrete series of all similitude classical groups". If time permits, we are going to explain briefly the second step of the classification, which are in progress.