A method for achieving the continuous-space electromagnetic channel capacity bound using loaded$N$-port structures is described. It is relevant for the design of compact multiple-input multiple-output (MIMO) antennas that can achieve channel capacity bounds when constrained by size. The method is not restricted to a specific antenna configuration and a closed-form expression for the channel capacity limits are provided with various constraints. Furthermore, using loaded$N$-port structures to represent arbitrary antenna geometries, an efficient optimization approach is proposed for finding the optimal MIMO antenna design that achieves the channel capacity bounds. Simulation results of the channel capacity bounds achieved using our MIMO antenna design with one square wavelength size are provided. These show that at least 18 ports can be supported in one square wavelength and achieve the continuous-space electromagnetic channel capacity bound. The results demonstrate that our method can link continuous-space electromagnetic channel capacity bounds to MIMO antenna design.

A systematic and general method for obtaining orthogonal far-field radiation patterns using any electronically steerable parasitic array radiator (ESPAR) embedded in a propagation environment with an arbitrary power angular spectrum (PAS) is described. The method is useful in designing single-RF multiple-input multiple-output (MIMO) systems where the orthogonal patterns can be utilized as a basis set for beamspace MIMO. The method utilizes the correlation matrix of the open-circuit radiation patterns and is termed the pattern correlation decomposition method (PCDM). PCDM is applicable to any antenna type (including realistic antennas with losses), array configuration, PAS and overcomes the limitations of previously proposed techniques. PCDM also provides closed-form solutions for the antenna currents to generate the orthogonal patterns. It also provides an estimate of the effective aerial degrees-of-freedom of the ESPAR. In addition, efficient optimization approaches to finding the optimal reactive loads that excite the required orthogonal radiation patterns are provided. Simulation examples using a 5-element and a 8-element planar inverted-F antenna array including material losses in various PAS are provided. These show that the proposed approach increases system capacity by up to 7 bits/s/Hz or equivalently reduces the required transmit power by up to 73% compared with previous approaches.

Ambient radio-frequency (RF) energy harvesting can extract energy from RF signals in the ambient environment and has potential for applications in the Internet of Things. However, the power density of the ambient RF energy is low and therefore methods to maximize the average output direct current (dc) power are required. In this work we show that average output dc power in ambient RF energy harvesting is nonlinearly dependent on antenna directivity and linearly dependent on antenna port number. To maximize average output dc power it is therefore necessary to utilize directional multiport rectennas. To demonstrate the enhancements possible, the design for a directional 4-port pixel patch rectenna system to harvest ambient RF energy from the GSM-1800 frequency band is provided. The design has an average antenna size for each port of 0.3λ×0.3λ, and realized gains of 5.5 dBi. Measurement results show that the proposed rectenna can increase average output dc power by up to 6.2 and 4.5 dB compared to omnidirectional and directional single-port rectenna designs of similar size, respectively. Measurement in a real ambient environment is also conducted, showing that the proposed rectenna can achieve an output dc power of 11.2 μW which is five times higher than the two reference rectennas.

A systematic method based on the Theory of Characteristic Modes (TCM) for finding orthogonal radiation patterns of any electronically steerable parasitic array radiator (ESPAR) is described. This method can be useful for designing single-RF front-end multiple-input multiple-output (MIMO) systems in which orthogonal patterns can be utilized as a basis set for space modulation (SM) or full multiplexed MIMO systems. The method is based on the$N$-port formulation of TCM and is not restricted to antenna type or array configuration. It can also provide closed formed solutions for the antenna element currents required to generate the orthogonal patterns. In addition an approach to finding the required load reactances in ESPAR for generating the orthogonal patterns is provided and is based on the quasi-Newton method utilizing a closed form expression for the initial approximate solution. Approximate estimation of the effective aerial degrees of freedom of the ESPARs is also discussed. Two simulation examples of ESPARs, a 4-element linear dipole array and an 8-element rectangular planar inverted-F antenna array, using SM as well as full multiplexed MIMO are provided, demonstrating the effectiveness of the proposed method.

In this paper a 4 times 4 MIMO channel testbed is proposed that is based on a straightforward and modular design. The proposed testbed is designed explicitly for MIMO antenna testing and characterization and is easily configurable to a variety of needs in terms of frequency band and M times N requirements. The testbed is easily constructed and can be maintained by non-RF specialists. The data received from the testbed are stored on hard disk and post-processed offline to obtain MIMO channel estimates and other useful parameter estimates such as capacity and antenna correlation.