Konstantinos N. Maliatsos

Reconfigurable Orthonormal Basis Patterns Using ESPAR Antennas

Beamspace multiple-input-multiple-output (BS-MIMO) systems with reduced hardware complexity have been recently considered in the literature. So far, BS-MIMO research effort focuses on the development of efficient single-RF MIMO transmission schemes based on parasitic antenna arrays. Single-RF BS-MIMO systems can provide both multiplexing and beamforming capabilities by exploiting the available aerial degrees of freedom of the radiated field while maintaining low space and complexity requirements. Previous studies used electronically steerable passive array radiators (ESPARs) for the multiplexing of multiple data streams utilizing an ESPAR-specific set of orthonormal basis patterns. However, these basis patterns ensure orthogonal transmission of the multiple streams in idealistic rich scattering environments. In this letter, a novel technique is developed for the adaptive selection of the most effective basis patterns in both transmitter and receiver sides based on the actual wireless channel propagation conditions maximizing the system capacity.

Elaborate analysis and design of filter-bank-based sensing for wideband cognitive radios

The successful operation of a cognitive radio system strongly depends on its ability to sense the radio environment. With the use of spectrum sensing algorithms, the cognitive radio is required to detect co-existing licensed primary transmissions and to protect them from interference. This paper focuses on filter-bank-based sensing and provides a solid theoretical background for the design of these detectors. Optimum detectors based on the Neyman-Pearson theorem are developed for uniform discrete Fourier transform (DFT) and modified DFT filter banks with root-Nyquist filters. The proposed sensing framework does not require frequency alignment between the filter bank of the sensor and the primary signal. Each wideband primary channel is spanned and monitored by several sensor subchannels that analyse it in narrowband signals. Filter-bank-based sensing is proved to be robust and efficient under coloured noise. Moreover, the performance of the weighted energy detector as a sensing technique is evaluated. Finally, based on the Locally Most Powerful and the Generalized Likelihood Ratio test, real-world sensing algorithms that do not require a priori knowledge are proposed and tested.

Interference versus filtering distortion trade-offs in OFDM-based cognitive radios

Cognitive radio (CR) and opportunistic radio access came as a solution for the underutilisation of radio resources. An expected consequence is that the interference management procedures obtain high priority in CR research. While the vast majority of studies focus on interference phenomena towards the licensed primary system (PS), this paper analyses the effects of adjacent primary user (PU) signals to a CR system that uses orthogonal frequency division multiplexing (OFDM) modulation. The lack of orthogonality in OFDM receivers increases out of band interference. The ‘near PU–far CR’ scenario is introduced, an inverse equivalent to the ‘hidden terminal’ problem that may cause serious degradation in the receiver operation. An analytical interference model is developed, and direct filtering of the conventional OFDM waveform is investigated as a means to control interference from PU signals. Closed-form expressions of the induced distortion are extracted. The results are used to evaluate a filtered-based OFDM receiver via simulation and to prove that high-rank filters at CR receivers may be quite beneficial even though OFDM modulation generally avoids their use. Copyright

The Bivariate Double Rayleigh Distribution for Multichannel Time-Varying Systems

The bivariate double-Rayleigh distribution with non-identically distributed parameters is introduced and analyzed. Novel series expressions for the joint probability density, cumulative distribution, and the moments are derived. Based on this new distribution, a multichannel communication scenario is studied, where correlation exists between the actual channel and its corresponding estimate, under time varying channel assumption. In this context, the outage performance of the system under consideration is analytically evaluated, while for higher values of the average signal-to-noise ratio, closed-form results are derived. Finally, an analysis for total outage probability in varying-correlation conditions is also provided.

Adaptive Basis Patterns Computation for Electronically Steerable Passive Array Radiator Antennas

Single-RF front-end MIMO systems have drawn the attention of the research community due to their considerable advantages against the conventional MIMO systems. MIMO multiplexing capabilities are implemented in the beamspace domain using parasitic antenna arrays with one active and many parasitic elements, like the Electronically Steerable Passive Array Radiator (ESPAR) antenna. The multiplexing is achieved in terms of linear combination of diverse symbols with orthonormal basis patterns. Initially, an analytical method was proposed to derive the basis patterns, which remain orthogonal in rich scattering environments. Recently, the numerical calculation of channel-aware orthonormal patterns was introduced. In this paper, we extend the presented method in 3-dimensional channels and radiation patterns. The proposed technique is based on the singular value decomposition of the wireless channel, as seen in the beamspace domain, and it adaptively computes 3D orthonormal basis patterns at both transmitter and receiver sides. It is expected that the 3D analysis will facilitate the compensation of the body effects in the handheld terminal performance.

Channel estimation and link level evaluation of adaptive beamspace MIMO systems

Beamspace MIMO (BS-MIMO) systems have been recently proposed as a means to address the two key weaknesses of conventional MIMO systems: the antenna size and the need for multiple RF chains. Based on Electronically Steerable Passive Array Radiators (ESPAR), the research effort on BS-MIMO focuses on the development of functional MIMO transmission schemes with efficient multiplexing and beamforming capabilities with the use of a single RF chain while maintaining extremely small antenna size. Previous studies have shown that for small-sized antenna arrays, BS-MIMO systems clearly outperform conventional systems in terms of system capacity. However, until now research is limited to the ESPAR antenna properties and theoretical results. This paper makes the first step to practical system design and focuses on BS-MIMO channel estimation. Basic estimators are applied in a BS-MIMO system with adaptive pattern reconfiguration. Finally the first, fundamental link level evaluation results are produced from simulation and system performance is compared vs. equivalent conventional MIMO.

V-BLAST reception for beamspace MIMO systems with limited feedback

Beamspace MIMO (BS-MIMO) is a newly proposed transmission technique that can successfully overcome the two significant disadvantages of conventional MIMO. More specifically, BS-MIMO using Electronically Steerable Passive Array Radiators (ESPAR) achieves similar multiplexing and beamforming capabilities using a single RF chain. Moreover, it is proved that BS-MIMO benefits from the use of small sized antenna arrays outperforming in terms of capacity conventional MIMO systems that operate poorly due to spatial correlation. However, the open issues towards an implementable, fully functional BS-MIMO system are still plenty. This study is part of a series of research work that focuses on real-world BS-MIMO system design. The application of limited feedback techniques in BS-MIMO is introduced and the performance of the developed algorithm is evaluated. These results are also used to derive conclusions regarding the adaptivity requirements for the parasitic elements of the ESPAR antenna. Moreover, a practical V-BLAST reception scheme using receiver radiation pattern adaptation is also presented. Finally the performance of this reception scheme is compared with ideal reception in BS-MIMO.

V2V Communication Systems under Correlated Double-Rayleigh Fading Channels

In this paper, the correlated double-Rayleigh distribution is introduced and analyzed. This distribution has been found to provide an accurate statistical description of the vehicle-to- vehicle multipath propagation. Novel infinite series expressions for the joint probability density and the cumulative distribution functions are derived, while the joint moments are also obtained in closed form. Although the new statistical metrics can be employed in a wide range of applications, this paper focuses on the performance of dual-branch selection diversity receivers. In this context, the outage probability (OP) and the average bit error rate (ABER) of dual- branch selection diversity operating over double- Rayleigh fading channels are analytically evaluated. Moreover, an asymptotic analysis is also included, resulting to simpler closed-form expressions for the OP and ABER. Finally, the applicability of the analytical results were verified using the measurements based WINNER2 models.

Full-duplex communications with the use of parasitic array radiators

Full duplex (FD) communications have been brought into the center of the research activity as an effective way to increase spectral efficiency. This study introduces a novel antenna cancellation technique with the use of a parasitic antenna array at the FD transceiver. Elaborate Multiple-Input-Multiple-Output (MIMO) techniques that use parasitic arrays, such as Beamspace MIMO, have been recently proposed. These techniques use the variable reactances of the loads of the parasitic elements to increase channel capacity with the use of compact antennas and limited number of Radio Frequency (RF) chains. This study proposes a technique that can achieve high isolation of the transmitting and receiving elements of the FD transceiver and simultaneously beamform in order to improve signal reception with dynamic adaptation of the parasitic element load values. The performance of the technique was evaluated with simulation. Results show that the use of parasitic arrays in FD transceivers can drastically reduce the self-inflicted interference, improve overall reception and provide radio channels with FD operation support.