Paschalis C. Sofotasios

On the Performance of Visible Light Communication Systems With Non-Orthogonal Multiple Access

Visible light communication (VLC) has been proposed as a promising and efficient solution to indoor ubiquitous broadband connectivity. In this paper, non-orthogonal multiple access, which has been recently introduced as an effective scheme for fifth generation (5G) wireless networks, is considered in the context of VLC systems under different channel uncertainty models. To this end, we first derive a novel closed-form expression for the bit-error-rate (BER) under perfect channel state information (CSI). Capitalizing on this, we then quantify the effect of noisy and outdated CSI by deriving a simple and accurate approximation for the former and a tight upper bound for the latter. The offered results are corroborated by respective results from extensive Monte Carlo simulations and assist in developing useful insights on the effect of imperfect CSI knowledge on the overall system performance. Furthermore, it was shown that while noisy CSI leads to slight degradation in the BER performance, outdated CSI can cause considerable performance degradation, if the order of the users’ channel gains change due to the involved mobility.

Multi-user techniques in visible light communications: A survey

Visible light communication (VLC) is a recent proposed paradigm of optical wireless communication, in which the visible electromagnetic radiation is used for data transmission. The visible part of the spectrum occupies the frequency range from 400 THz to 800 THz, which is 10,000 times greater than the radio frequency (RF) band. Therefore, its exceptional characteristics render it a promising solution to support and complement traditional RF communication systems, and also overcome the currently witnessed scarcity of radio spectrum resources. To this end, in the last few years, there has been a rapid interest in multi-user processing techniques in VLC. Motivated by this, in this paper, we present a comprehensive and up-to-date survey on the integration of multiple-input multiple-output systems, multi-carrier modulations and multiple access techniques in the context of VLC.

The effects of I/Q imbalance on wireless communications: A survey

Radio frequency front-ends constitute a core part of both conventional and emerging communication systems. Yet, although hardware realizations practically suffer from several types of impairments that degrade the overall system performance, the corresponding effects are often neglected and transceivers are assumed ideal. This typically refers to effects that are typically related to amplier nonlinearities, phase noise and in phase and quadrature imbalance (IQI), with the latter being among the most critical ones. In this context, this work provides a thorough survey on the effects of IQI, aiming to highlight their important manifestations which depending on the considered scenario can result to destructive and occasionally constructive effects.

Error performance of NOMA VLC systems

Visible light communication (VLC) systems are expected to provide remarkably high speed indoor communications and effective ubiquitous connectivity. However, the key limitation of such systems is the narrow modulation bandwidth of the light sources. Based on this, non-orthogonal multiple access (NOMA) has been recently proposed as an effective method that can enhance considerably the spectral efficiency of indoor downlink VLC systems. In this context, the present work is devoted to the evaluation of the bit-error-rate (BER) performance of NOMA-based VLC systems. Specifically, a novel closed-form expression is first derived for the BER of the considered set up, by also taking into account the realistically incurred cancellation errors and interference terms. The validity of the derived expressions is verified through extensive comparisons with respective results from Monte Carlo simulations, while their algebraic representation is relatively simple, which renders them convenient to handle both analytically and numerically. This leads to meaningful insights on the behavior and performance gains achieved, thanks to the adoption of NOMA, which are particularly useful in future design and deployment of VLC systems.

Performance of differential modulation under rf impairments

Coherent detection requires exact knowledge of the channel state information, which is often a challenging task in demanding practical applications. Based on this, non-coherent detection of differentially modulated signals can be considered as an alternative method. The present paper investigates the effects of in-phase/quadrature-phase imbalance (IQI), which are known to degrade the performance of wireless communication systems. Specifically, we evaluate the effects of IQI on the bit error rate (BER) performance of differential quadrature phase shift keying (dQPSK) for ideal receiver (RX) with transmitter (TX) IQI, ideal TX with RX IQI and joint TX/RX IQI. Explicit analytic expressions are derived for the BER of both single-carrier and multi-carrier systems suffering from IQI at the TX and/or RX. Extensive Monte-Carlo simulation as well as offered analytic results show that realistic TX/RX IQI values can degrade the corresponding BER by over 30%. Likewise, it is shown that the detrimental effects of IQI are more considerable on DQPSK than on QPSK.

Error analysis of differentially modulated cooperative systems under generalized fading

This paper analyzes the average bit error rate performance of dual-hop cooperative systems over generalized multipath fading conditions. The considered system is differentially modulated and is assumed to operate based on the amplify-and-forward relaying protocol. Therefore, its main advantage is that it does not require any channel state information neither at the relay nor at the destination nodes. Novel closed-form expressions are derived for the end-to-end statistics for the case of generalized multipath fading conditions that follow the η-μ, distribution, which includes as special cases the well-known Nakagami-m, Rayleigh and Nakagami-g (Hoyt) fading models. Capitalizing on this, a closed-form expression is subsequently derived for the corresponding average bit-error-rate, which is given in terms of known elementary and special functions. The derived analytic expressions are then employed in evaluating the performance and behavior of the considered system and thus quantify the corresponding effects of generalized fading conditions on the achieved bit error rate performance. It is shown that the effect of multipath fading is detrimental at both high and low signal-to-noise ratio regimes as the corresponding deviations are often close to an order of magnitude. This indicates that accurate channel characterization is essential in effective design of cooperative communications as well as that differential modulation can be a suitable modulation scheme for relay systems since it can provide adequate performance at a reduced implementation complexity.