The described techniques relate to improved methods, systems, devices, and apparatuses that support modem control using millimeter wave (mmW) energy measurement. Generally, the described techniques provide for wireless device (e.g., wireless repeater) power savings in the absence of an attached (e.g., connected) user equipment (UE). For example, a wireless repeater may perform relay operations (e.g., amplification and forwarding operations) for communications between a base station and a UE in some channel (e.g., in a mmW channel). The wireless repeater may use energy measurements in the channel (e.g., analog mmW measurements) to control or configure a digital interface for monitoring out of band control information (e.g., in a sub-6 gigahertz (GHz) channel). For example, a wireless repeater may use energy measurements of signals transmitted by devices in a mmW band to configure (e.g., turn on) a digital interface of another modem to monitor for control information in a sub-6 GHz band.