The next generation wireless broadcasting systems combining with MIMO technology has drawn much attention recently. Considering the coexistence of receivers equipped with different numbers of antennas in these systems, there exists the special requirement to maximize the transmission rate for receivers having more antennas, while guaranteeing the normal rate for receivers having less antennas. In this letter, superposition coding is proposed to fulfill this requirement and the concept of broadcast cluster is introduced, wherein the optimized power allocation parameters are derived. The BER simulations for multiple services are provided to verify the significant SNR performance gap between receivers with various numbers of receive antennas.