Signals with unpredictable transmission properties enabling physically secure, unscheduled and interference-resistant machine-to-machine (M2M) communication are described. A physical dwell structure containing time slots and frequency channels is established, and a combination of unpredictable physical dwells, spread spectrum modulation with unpredictable spreading vectors, and unpredictable selection of intended receivers is described. These unpredictable transmission properties enable receivers to detect and extract transmissions intended for the receiver, and excise transmissions not intended for the receiver, even if they are received at much higher power levels than the intended transmissions. The system eliminates the ability for an adversary to predict and override M2M transmissions; allows reception of ad-hoc transmissions in dense environments without scheduling, carrier-sense multiple access with collision avoidance (CSMA/CA) protocols, or scheduling feedback paths; and allows macrodiverse reception of transmissions at networks of connected receivers, thereby providing additional efficiency and security improvements exploiting route diversity of the network.