In this paper, we investigate how to embed small cells into a macro cell to enhance the performance of two-tier heterogeneous networks. Conventionally, overlay and underlay schemes are adopted by small cells, where the former enables a small cell to use the macro cell idle bands with transmitter detection, while the latter enables a small cell to use the macro cell busy bands with a certain access probability. To enhance the performance of small cells, we propose to exploit more busy band opportunities for small cells through passive receiver detection, i.e., identifying the location of an active macro user that occupies a certain band. Then, a small cell may access the busy band when the active macro user is far away from the small cell. In our method, the small cell uses the energy of the received signal from the macro cell as the test statistic. We obtain the closed-form distribution of the test statistic and design two detectors with one and two thresholds, respectively. Our results demonstrate that the proposed detectors achieve about 100% to 300% more transmission opportunities than the conventional energy detector in typical two-tier heterogeneous networks without requiring any prior information of the macro user signal.