An approach for transmitting multiple signals using a single switched parasitic antenna (SPA) has been recently reported. The idea there is to map the signals to be transmitted onto a set of basis functions that serve as 'virtual antennas' in the beamspace (i.e. wavevector) domain. In this work, we generalize the derivation of the antenna pattern basis functions regarding a 3-element SPA of arbitrary radiating elements, within a symmetric array topology, for multiplexing signals in the wavevector domain (using different beampatterns) rather than in the hardware antenna domain with multiple feeding ports. A fully operational antenna system example is modeled, optimized regarding its return loss and the power imbalance between the basis functions, and finally realized. The measurements of the SPA show good agreement with the simulated target values, revealing an accurate design approach to be adopted as a fast SPA prototyping methodology. The SPA has been successfully employed for multiplexing two BPSK datastreams over-the-air, thus paving the way for practically compact and highly efficient MIMO transceiver designs.
Abstract-The paper sheds light on the beamforming (BF) performance of large (potentially unconstrained in size) as well as dense (but physically constrained in size) antenna arrays when equipped with arbitrarily many elements. Two operational modes are investigated: Single-layer BF and multi-layer BF. In the first mode, a realistic BF criterion namely the average BF gain is revisited and employed to understand the far-field and the near-field effects on the BF performance of large-scale antennas above a clutter. The diminishing throughput returns in a single-layer BF mode versus the number of antennas necessitate multi-layering. In the multi-layer BF mode, the RF coverage is divided into a number of directive non-overlapping sectorbeams in a deterministic manner within a multi-user multi-input multi-output (MIMO) system. The optimal number of layers that maximizes the user's sum-rate given a constrained antenna array is found as a compromise between the multiplexing gain (associated with the number of sector-beams) and the inter-beam interference, represented by the side lobe level (SLL).
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.
Index TermsBeam-space MIMO, reduced-complexity MIMO, reconfigurable antenna, phase shift keying (PSK), single-radio MIMO.
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