Stimulated electromagnetic emission (SEE) observation has been used as a diagnostic tool in the study of wave-wave interaction in ionospheric heating experiments. The physical mechanism of the BSS (broad symmetric structure) phenomenon in SEE is investigated, and an attempt is made to develop a theory that can explain the 1989 Tromso experimental results and guide future experiments. Two parametric processes are considered: the parametric decay of the heater wave into a lower hybrid wave and an electron Bernstein wave and the parametric excitation of a purely growing density irregularity together with a pair of electron Bernstein sidebands. The electron Bernstein wave generated in the frist process then can be mode converted into EM emission through scattering of the short-scale irregularity excited in the second process. The emission forms the down-shifted side of BSS. Similarly, the EM emission responsible for the upshifted side of BSS can also be generated by scattering off the electron Bernstein sidebands of the second process by the density perturbations of the oppositely propagating lower hybrid waves generated in the first process. Thus, a symmetric spectrum structure with the shift-frequencies of the lower hybrid wave frequency can be obtained.