Juan F. Valenzuela-Valdés

Evaluation of True Polarization Diversity for MIMO Systems

MIMO systems where multipath fading is only partially correlated could use polarization diversity to provide a higher diversity gain. Recent letters have proposed the use of tri-polarized antennas and a novel true polarization diversity (TPD) scheme. In this paper, the full potential of TPD is evaluated with both simulations and measurements and compared to conventional orthogonal polarization diversity (OPD). MIMO system performance with respect to capacity and diversity gain is obtained through the use of multimode-stirred chambers for both isotropic and non-isotropic environments. Simulated and measured results in over 591 different MIMO systems show that TPD outperforms conventional OPD for reduced volumes. Likewise, it has been demonstrated that TPD can be effectively combined with spatial diversity to nearly double the diversity gain and MIMO capacity for the same available handset volume.

Emulation of MIMO Nonisotropic Fading Environments With Reverberation Chambers

Some recent publications have extended the emulating capabilities of reverberation chambers. While polarization imbalance has been removed and Ricean-fading environments are now properly emulated, these chambers are still limited to isotropic nonline of sight (NLOS) scattering. By controlling the power received, number of resolvable multipath components (MPC), angular spread (AS), and angle of arrival (AoA), the emulation of real-propagating environments with both isotropic and nonisotropic scattering are demonstrated in this letter using a reverberation chamber with several multiple-input multiple-output (MIMO) arrays.

The Influence of Efficiency on Receive Diversity and MIMO Capacity for Rayleigh-Fading Channels

A previously published guideline for MIMO antenna arrays is refuted. The influence of radiation efficiency on diversity gain and MIMO capacity of wireless communications systems is investigated through simulations and measurements using a reverberation chamber. Integrated antennas on a portable device have efficiencies low enough to disallow typical inter-element correlation assumptions. Both diversity gain and MIMO capacity depend on the number of antennas, SNR and efficiency in a complex way. When the efficiency of antennas is considered, certain system capacity losses are predicted and measured. These losses may be recovered through using more receive elements than commonly recommended or through the addition of a smaller number of more efficient antennas.

Advances in mode-stirred reverberation chambers for wireless communication performance evaluation

Reverberation chambers (RC) are a popular tool for laboratory wireless communication performance evaluation, and their standardization for Over-The-Air (OTA) measurements is underway. Yet, the inherent limitations of single-cavity RCs to emulate isotropic Rayleigh-fading scenarios with uniform phase distribution and high elevation angular spread put their representation of realistic scenarios into jeopardy. Recent advances in the last few years, however, have solved all these limitations by using more general mode-stirred reverberation chambers (MSC), wherein the number of cavities, their stirring and coupling mechanisms, and their software postprocessing algorithms is far from simple, representing a new era for wireless communications research, development, and over-the-air testing. This article highlights recent advances in the development of second-generation mode-stirred chambers for wireless communications performance evaluation.

Diversity Gain and MIMO Capacity for Nonisotropic Environments Using a Reverberation Chamber

Several physical parameters of realistic nonisotropic environments have been recently emulated with reverberation chambers. In this letter, the different performance in terms of diversity gain and multiple-input multiple-output (MIMO) capacity of the same linear MIMO array in different nonisotropic propagating scenarios is demonstrated with a reverberation chamber (RC) for the first time. This could be useful for designing antenna arrays in handset MIMO.

Emulation of MIMO Rician-Fading Environments With Mode-Stirred Reverberation Chambers

Some recent publications have extended the emulating capabilities of mode-stirred reverberation chambers, which are now able to emulate Ricean-fading and non-isotropic environments. Either the need to physically modify existing chambers or multiple sets of measurements is required for these enhancements. In this paper a novel post-processing hybrid tool is presented for the transformation of a Rayleigh-fading emulated environment into a Rician one. The model is tested and compared to outdoor measurements and simulations through the K-factor, correlation, diversity gain and MIMO capacity. Results show an excellent matching performance with no hardware modifications of mode-stirred chambers with just one set of measurements. The method is patent protected by EMITE Ing.

Accurate Estimation of Correlation and Capacity for Hybrid Spatial-Angular MIMO Systems

In this paper, several novel functions for accurately estimating the correlation and the multiple-input-multiple-output (MIMO) capacity of combined spatial and true polarization diversity (TPD) schemes are proposed for the first time. Minimum error estimation of the correlation for a hybrid spatial-TPD linear scheme previously reported cannot be obtained by simple nonlinear least-square estimators, and the use of genetic algorithms (GAs) provides an accurate solution. Channel matrix coefficients and MIMO capacities are analyzed for the diverse optimization strategies employed, which aim to identify the best prediction of MIMO performance. The novel functions are used with previously reported results and validated for Rayleigh fading scenarios with isotropic scattering using a multimode-stirred chamber (MIMO Analyzer). An accurate prediction of high correlation values is concluded to be of extreme importance for the final MIMO performance estimation. This has been found to be particularly important for estimating MIMO capacity with achieved prediction accuracies of 1.1% at SNR = 15 dB. In contrast, the accurate prediction of low correlation values has been found to have a less-important effect on the final capacity-predicting performance.

Sample-Selection Method for Arbitrary Fading Emulation Using Mode-Stirred Chambers

Mode-stirred chambers (MSCs) consist of one or more resonant cavities coupled in some way in order to allow the measurement of different antenna parameters such as antenna efficiency, correlation, diversity gain, or multiple-input multiple-output (MIMO) capacity, among others. In a single-cavity MSC, also known as a reverberation chamber (RC), the environment is isotropic and the amplitude of the signal is Rayleigh distributed. Real environments, however, rarely follow an isotropic Rayleigh-fading scenario. Previous results have shown that a Rician-fading emulation can be obtained via hardware modification using an RC. The different methods lack from an accurate emulation performance and are strongly dependent upon chamber size and antenna configurations. With the innate complexity of more-than-one-cavity MSC, the coupling structure generates sample sets that are complex enough so as to contain different clusters with diverse fading characteristics. This letter presents a novel method to accurately emulate a more realistic Rician-fading distribution from a Rayleigh-fading distribution by selecting parts of the sample set that forms different statistical ensembles using a complex two-cavity multi-iris-coupled MSC. Sample selection is performed using a genetic algorithm. Results demonstrate the potential of MSCs for versatile MIMO fading emulation and over-the-air (OTA) testing. The method is patent protected by EMITE Ing., Murcia, Spain.

Estimating correlation functions for dipoles in correlated Rician-fading scenarios

While the spatial correlation function between two dipoles is well known for Rayleigh fading, that for Rician fading is not available in the literature. Here, a Rician-fading correlation function is proposed for two dipoles separated by a given distance. The existing models for Rayleigh fading are based on the distance between elements. The new idea is to penalize the distance between elements in different ways as the degradation of the environment increases (with increasing Rician -factor). The resulting functions are validated against previously reported results.

Saving energy in WSNs with beamforming

Energy efficiency is a critical issue in Wireless Sensor Networks (WSNs) as sensor nodes has a limited power supply. In order to address this issue, this paper elaborates on using beamforming to perform more efficient wireless communications. The results with different randomly deployed WSNs considering different energy-consuming sensors have shown that this technique allows the lifetime of the sensors to be increased substantially (more than one order of magnitude).