NetworkWorld has a post on IoT sensors must have two radios for efficiency. The idea is that IoT radios should only be on when there’s something to send and a weaker radio listens for a wake-up signal.
“The problem now is that these [existing] devices do not know exactly when to synchronize with the network, so they periodically wake up to do this even when there’s nothing to communicate,”
The article quotes researchers at University of California, San Diego and Stanford University working on wake-up solutions.
The current way to solve this problem is to use gateways. Permanently connected, mains powered gateways, listen for many low power battery-powered sensors with shorter range wireless such as Bluetooth, Z-Wave and Zigbee.