  Crucial developments to the recently introduced signal-space approach for
multiplexing multiple data symbols using a single-radio switched antenna are
presented. First, we introduce a general framework for expressing the spatial
multiplexing relation of the transmit signals only from the antenna scattering
parameters and the modulating reactive loading. This not only avoids tedious
far-field calculations, but more importantly provides an efficient and
practical strategy for spatially multiplexing PSK signals of any modulation
order. The proposed approach allows ensuring a constant impedance matching at
the input of the driving antenna for all symbol combinations, and as
importantly uses only passive reconfigurable loads. This obviates the use of
reconfigurable matching networks and active loads, respectively, thereby
overcoming stringent limitations of previous single-feed MIMO techniques in
terms of complexity, efficiency, and power consumption. The proposed approach
is illustrated by the design of a realistic very compact antenna system
optimized for multiplexing QPSK signals. The results show that the proposed
approach can bring the MIMO benefits to the low-end user terminals at a reduced
RF complexity.
