This paper proposes a systematic coexistence scheme for a new radio communication system (secondary system), which is additionally introduced to the service area being operated by an existing communication system (primary system). The same frequency band is allocated for both systems. Our proposal is to protect the primary system from interference, on the basis that the primary and secondary systems work cooperatively with each other. For this objective, we propose the deployment of signal detectors in the operating area of the primary system and also the gathering of information regarding the signal detection. Such information is used for estimating the area, where an additional radio system is allowed to operate. A signal detection scheme with high sensitivity is also proposed that has the ability to detect a weak interference signal under the condition of mixed primary and secondary signals, to confirm that the secondary system does not causes harmful interference to the primary system. By using the concept of non-detection probability of the secondary system, we address the systematic scheme to introduce an additional radio system in the operating area of an existing radio communication system.