Elpiniki Tsakalaki

Beamforming via Large and Dense Antenna Arrays Above a Clutter

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 sector-beams in a deterministic manner within a multi-user multi-input multi-output (MIMO) system. The optimal number of layers that maximizes the users 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).

Concurrent Communication and Sensing in Cognitive Radio Devices: Challenges and an Enabling Solution

Cognitive radios (CRs) need to continuously monitor the availability of unoccupied spectrum. Prior work on spectrum sensing mainly focused on time-slotted schemes where sensing and communication take place on different time periods in the same frequency. This however leads to a) limited CR throughput as data transmissions need to be interrupted for the sensing task, and b) unreliable detection performance since sensing happens in specific confined time durations. The paper describes the basic design challenges and hardware requirements that restrain CRs from simultaneously and continuously sensing the spectrum while transmitting in the same frequency band. The paper then describes a novel approach based on spatial filtering that promises to empower CRs with concurrent transmission and sensing capabilities. The idea is to equip the CR with redundant transmit antennas for forming an adaptive spatial filter that selectively nulls the transmit signal in the sensing direction. By doing so, a wideband isolation level of ~ 60 dB is obtained by the antenna system. Finally, by following the spatial filtering stage with active power cancellation in the radio-frequency stage and in the baseband stage, a total isolation in excess of a 100 dB required for enabling concurrent communication and sensing can be obtained.

Spatial spectrum sensing for wireless handheld terminals: design challenges and novel solutions based on tunable parasitic antennas [Dynamic Spectrum Management]

This article addresses the major design challenges of antenna systems intended for cognitive transceivers under compactness constraints. The first part of the article provides an overview of the design challenges with regard to sensing over different frequencies and angular directions, referred to as spatial spectrum sensing. The second part describes a novel approach based on parasitic antenna theory aiming at making spatial spectrum sensing feasible for portable lightweight terminals. The idea is to replace the wideband antenna required for sensing over a large bandwidth by a tunable narrowband antenna for both sensing and communication purposes. The flexibility of the proposed antenna system lies in its capability to scan both the frequency and spatial resource dimensions simultaneously via a single RF chain within a miniaturized antenna system. This is done by properly tuning a set of reactive loads connected to a group of parasitic elements closely coupled to the driven (active) element. By doing so, the operational frequency subband leaps to another subband (frequency tuning). Moreover, at every subband, circular permutations of the reactive loads rotate the narrowband beam pattern to different angular positions, giving the cognitive transceiver the capability of sensing over various segments of the space.

Spatial spectrum sensing for wireless handheld terminals

This article addresses the major design challenges of antenna systems intended for cognitive transceivers under compactness constraints. The first part of the article provides an overview of the design challenges with regard to sensing over different frequencies and angular directions, referred to as spatial spectrum sensing. The second part describes a novel approach based on parasitic antenna theory aiming at making spatial spectrum sensing feasible for portable lightweight terminals. The idea is to replace the wideband antenna required for sensing over a large bandwidth by a tunable narrowband antenna for both sensing and communication purposes. The flexibility of the proposed antenna system lies in its capability to scan both the frequency and spatial resource dimensions simultaneously via a single RF chain within a miniaturized antenna system. This is done by properly tuning a set of reactive loads connected to a group of parasitic elements closely coupled to the driven (active) element. By doing so, the operational frequency subband leaps to another subband (frequency tuning). Moreover, at every subband, circular permutations of the reactive loads rotate the narrowband beam pattern to different angular positions, giving the cognitive transceiver the capability of sensing over various segments of the space.

On the beamforming performance of large-scale antenna arrays

The paper investigates the beamforming (BF) performance of large, potentially unconstrained in size antenna arrays when equipped with arbitrarily many elements. A realistic BF criterion namely the average BF gain is revisited and employed to understand the BF performance of large-scale antennas above a user clutter. Simulation results show diminishing average BF gain returns as the number of antennas increases in low angle spread outdoor channels.

Non Cooperative Space-Time Communication for Energy Efficiency in Sensor Networks

Space-time coded cooperative transmission has been proposed as a means to prolong the battery lifetime of conventional single-antenna energy-constrained wireless sensor nodes. However, newly proposed smart-antenna sensor prototypes enable the existence of more than one antenna within a single-radio transceiver architecture. Exploiting such capability, this paper proposes simple non-cooperative space-time techniques for single-RF switched-antenna systems that reduce significantly both the transmission and the circuit energy consumption. The energy savings of the suggested schemes are shown not only against conventional SISO systems, but also against cooperative diversity schemes in Rayleigh and log-normal shadowed Rayleigh fading channels with distance-based path loss.

Enhanced selection combining for compact single RF user terminals in multiuser diversity systems

The downlink performance of systems exploiting multiuser diversity has been shown to improve by using multiple antenna receivers and diversity combining techniques. However, the cost and power restrictions of the user mobile terminals prohibit the use of multiple antennas whereas the size limitations can result in correlated and coupled receive antenna elements downgrading the effectiveness of the applied schemes. In this paper, we propose a simple beam selection combining (SC) technique using a miniaturized single radio receive antenna system assisted by low cost parasitic antennas. The system is optimized for both antenna efficiency and diversity thus made capable of forming four practically uncorrelated beampatterns. The computer simulations show that the proposed beam SC technique outperforms classical antenna SC as well as a previously reported beam combining approach in terms of the average throughput especially for small user populations.

Spatial spectrum sensing for cognitive radios via miniaturized parasitic antenna systems

The paper describes an antenna system for portable lightweight cognitive radios employing opportunistic spectrum access. Our approach comprises a frequency-agile narrowband antenna that is capable of sensing over a wide bandwidth and over different spatial directions within a miniaturized single-radio system assisted by parasitic antennas.

Analogue orthogonal precoding using reduced-complexity transceivers

Orthogonal transmission toward N undesired receivers via conventional digital beamforming techniques requires at minimum (N + 1) active transmit antennas. As the radio frequency (RF) complexity increases linearly with the number of active antennas, orthogonal transmit precoding becomes impractical for simple user terminals. The paper proposes an alternative solution utilizing a single-radio reactance-assisted antenna system, featuring low-power consumption, low cost and ease of fabrication. The orthogonal precoding technique to be proposed emulates classical digital orthogonal precoding for suppressing the interference toward a number of users, but at highly reduced RF complexity.

Reduced-Complexity Radio Architectures for Enhanced Receive Selection Combining in Multiuser Diversity Systems

Although antenna selection is a simple and efficient technique for enhancing the downlink performance of multiuser diversity systems, the large antenna interelement spacing required for achieving spatial diversity is prohibitive for user terminals due to size restrictions. In order to allay this problem, we propose miniaturized switched beam receiver designs assisted by low-cost passive reflectors. Unlike conventional spatial receive diversity systems, the proposed angular diversity architectures occupy a small volume whereas the antenna system properties are optimized by controlling the strong reactive fields present at small dimensions. The systems are designed for maximum antenna efficiency and low interbeam correlation, thus yielding N practically uncorrelated receive diversity branches. The simulation results show that the proposed enhanced diversity combining systems improve the average throughput of a multiuser network outperforming classical antenna selection especially for small user populations and compact user terminal size.