A signal multiplexing apparatus, comprising: an injection level controller configured to generate a power-reduced enhanced layer signal by reducing power of an enhanced layer signal; a combiner configured to generate a multiplexed signal by combining a core layer signal and the power-reduced enhanced layer signal; a power normalizer configured to generate a power-reduced multiplexed signal by reducing power of the multiplexed signal to a power level corresponding to the core layer signal; a time interleaver configured to generate a time-interleaved signal by performing interleaving that is applied to both the core layer signal and the power-reduced enhanced layer signal in the power-reduced multiplexed signal after combining the core layer signal and the power-reduced enhanced layer signal; and a frame builder configured to generate a broadcast signal frame using the time-interleaved signal and L1 signaling information, wherein the time interleaver is shared for both a core layer corresponding to the core layer signal and an enhanced layer corresponding to the power-reduced enhanced layer signal, wherein the injection level controller reduces the power of the enhanced layer signal corresponding to a scaling factor and the power normalizer reduces the power of the multiplexed signal corresponding to a normalizing factor, wherein the scaling factor is applied to the enhanced layer signal by the injection level controller, and the normalizing factor is applied to both the core layer signal and the power reduced enhanced layer signal by the power normalizer, wherein the scaling factor decreases as a power reduction corresponding to the injection level controller becomes larger, and the normalizing factor increases as the power reduction corresponding to the injection level controller becomes larger, wherein the L1 signaling information includes injection level information of the injection level controller. an injection level controller configured to generate a power-reduced enhanced layer signal by reducing power of an enhanced layer signal; a combiner configured to generate a multiplexed signal by combining a core layer signal and the power-reduced enhanced layer signal; a power normalizer configured to generate a power-reduced multiplexed signal by reducing power of the multiplexed signal to a power level corresponding to the core layer signal; a time interleaver configured to generate a time-interleaved signal by performing interleaving that is applied to both the core layer signal and the power-reduced enhanced layer signal in the power-reduced multiplexed signal after combining the core layer signal and the power-reduced enhanced layer signal; and a frame builder configured to generate a broadcast signal frame using the time-interleaved signal and L1 signaling information, wherein the time interleaver is shared for both a core layer corresponding to the core layer signal and an enhanced layer corresponding to the power-reduced enhanced layer signal, wherein the injection level controller reduces the power of the enhanced layer signal corresponding to a scaling factor and the power normalizer reduces the power of the multiplexed signal corresponding to a normalizing factor, wherein the scaling factor is applied to the enhanced layer signal by the injection level controller, and the normalizing factor is applied to both the core layer signal and the power reduced enhanced layer signal by the power normalizer, wherein the scaling factor decreases as a power reduction corresponding to the injection level controller becomes larger, and the normalizing factor increases as the power reduction corresponding to the injection level controller becomes larger, wherein the L1 signaling information includes injection level information of the injection level controller.