IEEE 802.11bc introduces enhanced broadcast service (EBCS) to improve data broadcast scheduling and authentication performance in wireless local network (WLAN). The essential design in the EBCS is introducing a central scheduling scheme for data broadcasting, and this makes that the conventional distributed coordination function (DCF) based broadcast performance analysis does not apply. Thus, in this paper, we investigate the frame collision between DCF-based unicast data and scheduling-based broadcast data within the framework of IEEE 802.11bc, which has not been explored yet. Specifically, we study the tradeoff among station arrival intensity, broadcast and unicast frame collision, and broadcast control frame transmission interval, which directly determines the broadcast overhead. We derive an analytical model to quantity the above performance tradeoff. Based on the analytical model and simulation test, we optimize the broadcast control frame transmission interval to balance the broadcast overhead and data collision.