Vlasis I. Barousis

Beamspace-Domain Analysis of Single-RF Front-End MIMO Systems

It is known that the remarkable implementation complexity of multiple-input-multiple-output (MIMO) architectures limits their wide application to modern communication systems. To facilitate the inclusion of MIMO transceivers in devices with strict cost and size constraints, this paper presents a MIMO scheme built on a single-radio-frequency (RF) chain that uses parasitic antenna structures. After presenting the radiation characteristics of such antennas at the beamspace domain, the functionality and the performance evaluation of the proposed single-RF MIMO architecture is illustrated in detail.

A Stochastic Beamforming Algorithm for ESPAR Antennas

Beamforming capabilities of ESPAR antenna structures have attracted considerable attention in recent literature. In this letter, we consider a different beamforming approach, presenting an efficient optimization algorithm which enables such antennas to create desired transmit radiation patterns in real time, rather than patterns aiming at signal to interference ratio maximization. The proposed algorithm may be used in any beamforming, diversity or multiuser system. In order to demonstrate the algorithms efficiency, we investigate a scenario with mobile handsets equipped with ESPAR antennas, considering a beamspace multiple inputmultiple output architecture. It is shown that the algorithm may enforce even cost and size sensitive devices to efficiently operate in closed loop communication environments.

MIMO over ESPAR with 16-QAM Modulation

MIMO systems have become an indispensable part of modern wireless standards, e.g. LTE advanced. However, in applications with strict energy and size constraints, an alternative MIMO scheme with reduced hardware complexity would be attractive. Towards this direction, parasitic antennas with a single feeding port have been proposed to emulate MIMO transmission with PSK signaling. In order to support higher order constellations, this letter presents a smart loading scheme that enables the multiplexing of two 16-QAM signals over the air. Accompanying simulations show that this can be achieved by using a single feeding port and parasitic antennas.

Arbitrary Precoding with Single-Fed Parasitic Arrays: Closed-Form Expressions and Design Guidelines

Single-fed compact arrays known as electronically steerable parasitic antenna radiators (ESPARs) have recently emerged as a new paradigm for spatial multiplexing that requires only a single radio-frequency (RF) chain and a few easy-to-implement analog tunable loads. Besides the remarkable hardware savings, the ESPAR capabilities are still limited as it is not possible to find appropriate loadings that multiplex any signaling format over the air. Indeed, commonly the loading values are obtained through time-consuming iterative algorithms, e.g. exhaustive search, and can only emulate MIMO transmission of signals emerging from low-order constellations. This paper constitutes a clear step forward in solving this problem, as it defines the necessary guidelines for the design of single-fed ESPAR systems that are able to support an arbitrary precoding scheme. To alleviate the need for iterative processes, the proposed design methodology uses complex tunable loads and introduces new closed-form expressions for the exact computation of the loads and the feeding voltage.

AERIAL DEGREES OF FREEDOM OF PARASITIC ARRAYS FOR SINGLE RF FRONT-END MIMO TRANSCEIVERS

The beamspace domain of parasitic antenna arrays is explored in this paper, providing the aerial degrees of freedom available for use in Multiple Input-Multiple Output (MIMO) systems. The beamspace representation allows for the design of an alternative MIMO architecture based on single radio-frequency (RF) chains, and facilitates the inclusion of MIMO transceivers in devices with strict size limitations. A three dimensional orthogonal expansion is performed on the beamspace domain providing the basis patterns used for mapping of the transmitted symbols and for sampling at the receiver. The expansion is based on the Gram-Schmidt orthonormalization procedure and can be generalized for any parasitic antenna array. The multiplexing capability of ESPAR antennas is presented as a means for supporting future performance demanding communication systems. Performance evaluation results are illustrated in detail.

A new signal model for MIMO communication with compact parasitic arrays

Significant research effort has been drawn over the last few years to reduce the hardware complexity and size of Multiple Input - Multiple Output (MIMO) systems and to push this promising technology even to lightweight and small portable devices. Among other architectures, single-fed MIMO systems with compact parasitic arrays are a possible candidate toward this goal. To facilitate the study and design of parasitic arrays for MIMO applications, this paper presents an alternative signal model considering the currents at the ports of the transmitting array as the input to the system. Based on this model, a novel parasitic array is designed that is able to multiplex 16-QAM signals with the aid of a single radio-frequency (RF) source. The proposed signal model can also be useful for the evaluation of large parasitic arrays in massive MIMO regime, with significantly reduce hardware burden.

Load modulated arrays: a low-complexity antenna

This article describes promising recent progress in the area of massive antenna array architectures with low front-end hardware complexity. The presented technology enables the design and implementation of antenna arrays with large numbers of elements, while obtaining significant front-end hardware savings as compared to the conventional solutions. This newly appearing design approach could be used in order to design either massive arrays with complexity that would be prohibitive with the current technology, or smaller arrays that offer high spatial degrees of freedom and are suitable for future small yet powerful cell nodes. RF hardware architectures with a single RF chain are reviewed, compared, and found superior to conventional MIMO implementations in terms of cost, dissipated heat, and physical size. The proposed improvements on the RF side allow the merging of the two dominant cellular technologies of virtual (distributed) and massive (centralized) MIMO into a hybrid approach of antenna arrays that is suitable for both large base stations and small (possibly cooperative) units such as remote radio heads.

A Single RF MIMO Loading Network for High-Order Modulation Schemes

Recently, a novel MIMO transmitter architecture has been introduced that requires only a single radio-frequency (RF) chain and is built on parasitic antenna arrays. MIMO transmission in this case is achieved by shaping directly the radiation pattern with the aid of analog tunable loads attached to the parasitics. As it has been shown, such a single RF MIMO system can support all PSK modulation formats with purely imaginary loading values. This paper extends its capabilities and proposes a novel architecture for parasitic antennas that is able to provide complex loading values, even with a negative real part. Definitely, this will extend the flexibility of parasitic antennas to multiplex successfully over the air more complex signaling formats, for example, QAM signals. The bit error rate evaluation shows that the proposed architecture could be very promising in emerging devices and also illustrates its robustness under possible loading perturbations that might have been raised due to nonidealities in the design.

Precoding for multiuser MIMO systems with single-fed parasitic antenna arrays

Transmitter (TX) cooperation at various levels has been shown to increase the sum throughput of multiuser multiple-input multiple-output (MIMO) systems. In this paper we consider a k-user MIMO system where TXs have only global channel state knowledge. It has been theoretically shown that interference alignment (IA) achieves the K/2 degrees of freedom of this K-user MIMO interference channel. However, results on IA and all proposed transceiver techniques for this channel up to date, assume conventional antenna arrays at the transceivers with multiple radio-frequency (RF) chains, each connected to a different antenna element. To reduce the consequent hardware burden and power dissipation imposed by such arrays, we propose in this paper the utilization of compact single-RF electronically steerable parasitic (passive) array radiators (ESPARs) at the cooperating TXs. A signal model capable of capturing the characteristics of the considered antenna arrays is first described and then a general precoding design methodology for the tunable parasitic loads at the TXs ESPARs is introduced. Specific precoding techniques and an indicative ESPAR design are presented for a 3-user 2×2 MIMO system with one ESPAR TX, and the obtained performance evaluation results show that the gains of TX cooperation are still feasible.

A limited feedback technique for beamspace MIMO systems with single RF front-end

Recently, a novel beamspace multiple input multiple output (BS-MIMO) transmission technique has appeared, which increases the spectral efficiency of open-loop communication systems while using compact antenna structures with a single radio-frequency (RF) front-end at the transmitter. In this paper, we extend the aforementioned architecture proposing an efficient limited feedback technique for capacity optimization, considering electronically steerable parasitic array radiator (ESPAR) antennas at the transmitter. The performance of the resulting closed-loop BS-MIMO is evaluated against equivalent traditional MIMO systems that require a much larger number of active antenna elements to operate. The results are very promising, paving the way for integrating the proposed system in cost and size sensitive wireless handheld devices such as mobile terminals and mobile personal digital assistants.