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.

Abstract-Recently, a technique called beam-space MIMO has been demonstrated as an effective approach for transmitting multiple signals while using a single RF-chain. In this work, we present novel design considerations and a compact antenna solution to stimulate the deployment of beam-space MIMO in future wireless applications. Targeting integration in small wireless devices, the novel antenna is made of a single integrated radiator rather than an array of physically-separated dipoles. It also drastically simplifies the implementation of variable loads and DC bias circuits for BPSK modulated signals, and does not require any external reconfigurable matching circuit. Finally, we show that this antenna system could be reconfigured by dynamic adjustment of terminating loads to preserve its beam-space multiplexing capabilities over a 1:2 tuning range, thereby promoting the convergence of MIMO and dynamic spectrum allocation via reduced-complexity hardware. A prototype achieving single-RF-chain multiplexing at a fixed frequency is designed and measured, showing excellent agreement between simulations and measurements.Index Terms-Beam-space MIMO, cognitive radio, dynamic frequency allocation, MIMO antenna, reconfigurable antenna, reduced complexity.

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The performance and design of the novel single-RF-chain beam-space MIMO antenna concept is evaluated for the first time in the presence of the user. First, the variations of different performance parameters are evaluated when placing a beam-space MIMO antenna in close proximity to the user body in several typical operating scenarios. In addition to the typical degradation of conventional antennas in terms of radiation efficiency and impedance matching, it is observed that the user body corrupts the power balance and the orthogonality of the beam-space MIMO basis. However, capacity analyses show that throughput reduction mainly stems from the absorption in user body tissues rather than from the power imbalance and the correlation of the basis. These results confirm that the beam-space MIMO concept, so far only demonstrated in the absence of external perturbation, still performs very well in typical human body interaction scenarios.Comment: 4 pages, 7 figures, 2 table