Vasileios M. Kapinas

MU-MIMO precoding for VLC with imperfect CSI

This paper investigates the performance of different precoding schemes for a multi-user MIMO VLC system with channel estimation errors, an assumption that is commonly neglected in the literature. In particular, dirty paper coding, channel inversion, and block diagonalization, are considered for interference mitigation under imperfect channel state information. The impact of the variation of the beam angles of the transmitters and the field of view (FOV) of the receivers on the system performance is also examined. Simulation results reveal that, dirty paper coding provides the best performance under perfect channel state information (CSI). However, under imperfect CSI, suboptimal linear precoding schemes will give better performance. Furthermore, tuning the transmitting angles and the FOVs can significantly improve the system performance.

I/Q-Imbalance Self-Interference Coordination

In this paper, we present a novel low-complexity technique, which improves the performance of single-antenna multi-carrier communication systems, suffering from in-phase and quadrature (I/Q)-imbalance (IQI) at the receiver. We refer to the proposed scheme as I/Q-imbalance self-interference coordination (IQSC). This technique not only mitigates the detrimental effects of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI in order to achieve second-order frequency diversity. However, these benefits come at the expense of a reduction in transmission rate. More specifically, IQSC is a simple transmit diversity scheme that improves the signal quality at the receiver by elementary signal processing operations across symmetric (mirror) pairs of subcarriers. Thereby, the proposed transmission protocol has a similar complexity as Alamoutis space-time block coding scheme and does not require extra transmit power nor any feedback. To evaluate the performance of IQSC, we derive closed-form expressions for the resulting outage probability and symbol-error rate. Interestingly, IQSC outperforms not only existing IQI compensation schemes but also the ideal system without IQI for the same spectral efficiency and practical target error rates, while it achieves almost the same performance as ideal (i.e., IQI-free) equal-rate repetition coding. Our findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

OFDM-IM vs FQAM: A comparative analysis

Recently, orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) as well as frequency and quadrature amplitude modulation (FQAM) have been proven to be promising techniques for the multicarrier transmission over frequency selective fading channels by successfully mitigating the induced interference due to multipath, while at the same time boosting the overall spectral efficiency. In this paper, we present a thorough study of OFDM-IM and FQAM schemes and conduct a comparison between them and the classic OFDM system. Different configurations are considered and evaluated in terms of their spectral efficiency and error performance, capitalizing on recent results from the literature.

A universal MIMO approach for 3GPP wireless standards

We propose a generalized framework for handling Multiple-Input Multiple-Output (MIMO) schemes defined within the context of the Third Generation Partnership Project (3GPP) physical layer specifications. A detailed review of all the multi-antenna transmission techniques adopted by the 3GPP so far is firstly presented, starting from the Global System for Mobile (GSM) communications and passing through the High-Speed Packet Access (HSPA) releases to the very recent Long Term Evolution (LTE)-Advanced standard. Motivated by the outcome of this overview, we introduce the Transmit Matrix (TRAM) concept, that can include as special cases several MIMO systems (e.g. transmit diversity, spatial-multiplexing, etc.) regardless of the presence or not of a feedback channel. Our approach is based on the adoption of a novel measure called Necessary Transmit Information (NTI) and on its one-to-one correspondence with the TRAM. The last allows for a convenient alternative representation of a TRAM-based MIMO system model and serves the straightforward construction of NTI-codebooks for a wide range of transmission schemes. The indicative applications provided reveal the universal nature of the proposed methodology.

On the Optimal Tone Spacing for Interference Mitigation in OFDM-IM Systems

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been recently proposed as an efficient technique to improve the error performance and enhance the spectral efficiency achieved by the classical OFDM. In this letter, we minimize the presence of intercarrier and intersymbol interference, experienced by OFDM-IM systems under mobility conditions and multipath scenarios, by selecting the appropriate tone spacing between adjacent subcarriers. Finally, we prove that the optimal value of tone spacing increases the system capacity, occupying only the necessary amount of bandwidth, and provide closed-form expressions for the interference power per active subcarrier.

Inter-band carrier aggregation in heterogeneous networks: Design and assessment

This paper deals with the performance assessment of the Long Term Evolution (LTE)-Advanced Release 12 physical downlink channel, emphasizing on the Carrier Aggregation (CA) technology and its recent advances, such as the challenging interband non-contiguous solution. By processing the LTE-Advanced waveforms in the time domain (instead of the more common baseband), we describe the underlying system model and the associated simulation setup in detail. The error performance of the system is evaluated under different physical layer parameters and CA scenarios, according to the latest updates of the Third Generation Partnership Project (3GPP) technical specifications. Our analysis reveals that the Heterogeneous Band (HetBand) non-contiguous CA technology can be efficiently applied to the design of next generation mobile broadband networks, given that the exploitation of both unlicensed and frequency dispersed bands might be a promising solution against the spectrum scarcity.

On the optimal timing of detection in molecular communication systems

In this paper, the error performance of a molecular communication via diffusion system in the absence of flow is analysed. Closed-form expressions for the bit error rate are presented and utilized to enhance the timing of the receivers observations, which are used in the detection of the transmitted information. More precisely, we specify the optimal time, when the observation process should start and finish, in order to achieve the minimum probability of error. Furthermore, in the case where the receiver counts the molecules at specific time instants, an advantageous sampling time has been proposed, along with the ideal number of samples that should be collected in order to minimize the error probability. Finally, closed-form expressions for the first sample and the total number of samples that should be considered, have been derived and validated through extensive simulations.

A Low-Complexity Detector for BPPM Systems Under Additive Gaussian Mixture Noise

We analyze the performance of the single-threshold detector (STD) for binary pulse position modulation in the presence of symmetric Gaussian mixture noise (GMN). We first derive a general closed-form expression for the average error probability that is later used to investigate the optimality of the STD with zero threshold, which is the simplest form of STD. We also establish conditions sufficient to guarantee minimum error rates with the low-complexity STD for a wide range of signal-to-noise ratio. The theoretical results are verified through simulations of GMN with various noise parameters.

Differential distributed space-time coding for vehicle-to-vehicle networks

Differential distributed space-time coding (DDSTC) has been proved to be suitable for wireless relay networks, since it can provide spatial diversity without the need for channel state information at neither the transmitter nor the receiver side. However, DDSTC suffers from significant error floor in fast-fading channel conditions with high Doppler frequencies due to rapid time variations. For this reason, multiple-symbol differential detection (MSDD) has been proposed in the past, where the detection process involves a larger window size of the received symbols. So far, differential detection for vehicle-to-vehicle (V2V) networks has been studied only for single Rayleigh channels. However, experimental and theoretical studies report that double Rayleigh can be considered as an appropriate fading channel model for V2V networks. In this paper, we assess the error performance of a DDSTC scheme operating in a V2V network using MSDD. Simulation results confirm that the error performance of such a system can be improved significantly with MSDD under different channel time-variation scenarios.

A theoretical limit for the ML performance of MIMO systems based on lattices

We employ the theory of multidimensional lattice constellations in order to evaluate the maximum-likelihood (ML) performance of a general multiple-input multiple-output MIMO system. We show that the lattice representation of the equivalent system model enables a mapping of the received signal set to a single point in a multidimensional constellation, with the generator matrix being equal to the equivalent channel matrix. With the aid of this geometrical interpretation, we derive a performance limit for a general MIMO system in terms of the symbol error probability, which can be written in closed form for various designs. Application of the proposed analysis to popular MIMO schemes reveals the tractability of the approach and validates that designs exploiting the available degrees of freedom in a better way approach the proposed performance limit.