  We examined dipolar field effects on the microwave generation in the domain
wall spin-valve by solving simultaneously the Landau-Lifshitz-Gilbert and
Zhang-Levy-Fert diffusion equations. By numerically analyzing dipolar field
dependence, we showed that the microwave generation needs the dipole-dipole
interaction for a 180$^\circ$ domain wall and the amplitude of the microwave
voltage signal depends strongly on the exchange length. In order to design a
microwave generator using the domain wall spin-valve with high efficiency, we
propose that the materials with short exchange length are preferred.
