Spatial multiplexing
In communications, MIMO means sending different data from each antenna so the receiver, knowing the channel, can separate them. In radar, MIMO means transmitting orthogonal waveforms from each antenna. The returns contain angle, Doppler and range information from every transmit-receive pair. One array does the work of many.
5G as radar
A 5G base station is already a massive MIMO array with 64+ antennas. It must track users to steer beams. Researchers are showing that the same signals, processed correctly, can detect pedestrians around street corners, map building occupancy, and even sense breathing and heart rate through walls. No extra hardware — just smarter signal processing.
Waveform orthogonality
The key is making each transmit channel distinct. Methods include frequency division (each antenna gets a slice), time division (rapid switching), or code division (unique phase codes per antenna). Up-and-coming: index modulation, where the active antenna itself carries information. The better the orthogonality, the cleaner the radar image.
From lab to deployment
Automotive MIMO radar prototypes resolve multiple targets in dense urban clutter. Military systems use MIMO to track swarms of drones. The IEEE 802.11bf standard, expected around 2025, will formalise Wi-Fi sensing. Your router will not just stream video — it will know when you walk into the room.