Active parasitic arrays for low cost compact MIMO transmitters

Recently, electronically steerable passive array radiator antennas have been considered for beam-forming and spatial multiplexing, in order to reduce the size, cost and power consumption of smart antenna systems. However, the re-configurability of such antenna is limited, since parasitic elements were constrained to use only passive loads. If we are to integrate such systems to cost-sensitive devices (as for example in wireless sensor nodes for structural or urban microclimate monitoring), we should be able to fully emulate traditional smart antenna systems, using a single RF feed. This paper presents a method for generating parasitic loads with a negative real part using active circuit blocks. The proposed method could be used for increasing the modulation order of the aforementioned systems, decrease the required number of parasitic elements or even optimize the antenna efficiency in energy-constraint applications. Two alternative designs are presented: a consumer-off-the-shelf JFET based resistance circuit block, which could generate a load real part of up to -15 Ohm with power consumption of 40mW, and a CMOS based negative resistance circuit, which could generate negative resistances of up to -30 Ohm with power consumption around 1mW.