  This paper studies magnetic induction for wireless powering and the data
uplink of microsensors, in particular for future medical in-body applications.
We consider an external massive coil array as power source (1 W) and data sink.
For sensor devices at 12 cm distance from the array, e.g. beneath the human
skin, we compute a minimum coil size of 150 um assuming 50 nW required chip
activation power and operation at 750 MHz. A 275 um coil at the sensor allows
for 1 Mbit/s uplink rate. Moreover, we study resonant sensor nodes in dense
swarms, a key aspect of envisioned biomedical applications. In particular, we
investigate the occurring passive relaying effect and cooperative transmit
beamforming in the uplink. We show that the frequency- and location-dependent
signal fluctuations in such swarms allow for significant performance gains when
utilized with adaptive matching, spectrally-aware signaling and node
cooperation. The work is based on a general magneto-inductive MIMO system
model, which is introduced first.
