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Researchers at Cal Tech Pasadena have built an active 1.5x1.5mm near-field phased array antenna, with secure communication by spatial encryption in mind.
The 61GHz radiator consists of an on-chip dipole surrounded by 10 reflectors, all built in the bottom three metal layers of a seven metal process.
Each reflector is interrupted by nine RF switches, implemented as a mosfet shorted by a length of transmission line resonating with the Fet's capacitance, making a matrix of 90 switches in total.
The reflectors re-radiate dipole energy, modifying the phase and amplitude of the overall radiated signal. Opening and closing switches allows the modification to be controlled - and in use all modulation is through the 90 switches, a CW (unmodulated apart from course control) signal being fed to the centre dipole.
Rapidly operating combinations of switches allows an arbitrary constellation of I and Q modulation to be transmitted in an arbitrary direction.
20 I-Q constellation points, said the researchers, can be generated by 20 switches.
As there are 90 switches, simultaneous transmission of different arbitrary data to multiple receivers is possible providing they are in different directions.
Security comes when data is to be sent to single receiver, because there are many sub-sets of the 90 switches that will produce the same I-Q constellation point in a single direction.
Each of these combinations sends a different I-Q point in every other direction.
If the combination from the sub-set is chosen randomly each time the same I-Q point is sent towards the chosen receiver, then the chosen receiver gets clear unencrypted data, and random data is sent in every other direction.
As it is implemented, the final 0.13µm SiGe BiCMOS chip is 2.5x5mm, including: antenna, switches, control circuit, VCO, transmitter and a test receiver.
Energy is radiated through the substrate which is backed with a hemispherical dielectric radiating lens.