Abstract
In visible light communications (VLCs) relying on intensity-modulation and direct detection (IM/DD), the conversion from electrical signals to optical signals and the limited dynamic range of the light-emitting diodes (LEDs) constitute the fundamental impediments in the way of high-integrity communications, especially when orthogonal frequency-division multiplexing (OFDM) is employed. In IM/DD VLCs, only real-valued positive signals are used for signal transmission. However, the Fourier transform of OFDM systems is operated in the complex domain. In order to meet the requirements of the IM/DD VLCs, the complex-to-real conversion is achieved at the cost of reducing the bandwidth efficiency. Moreover, OFDM signals experience a high peak-to-average power ratio; hence, typically clipping is used for confining the positive-valued signals within the LED’s dynamic range. However, hard clipping leads to the loss of orthogonality for optical OFDM (O-OFDM) signals, generating inter-carrier interference. As a result, the performance of the clipping-based O-OFDM systems may be severely degraded. In this paper, the concept of piecewise companding transform (CT) is introduced into the O-OFDM system advocated, forming the CTO-OFDM arrangement. We first investigate the general principles and design criteria of the piecewise CTO-OFDM. Based on our studies, three types of piecewise companders, namely, the constant probability sub-distribution function, linear PsDF (LPsDF), and the non-LPsDF-based CT, are designed. Furthermore, we investigate the nonlinear effect of hard clipping and of our CT on O-OFDM systems in the context of different scenarios by both analytical and simulation techniques. Our investigations show that the CTO-OFDM constitutes a promising signaling scheme conceived for VLCs, which exhibits a high bandwidth efficiency, high flexibility, high reliability, as well as a high data-rate, despite experiencing nonlinear distortions.
Highlights
Visible light communication (VLC) has been recognized as a compelling technique of alleviating teletraffic congestion in the near future
This implies that a tradeoff has to be struck between the performance gain and complexity cost for designing efficient CTO-orthogonal frequency-division multiplexing (OFDM) systems
Piecewise companding transform (CT) schemes have been conceived for optical OFDM (O-OFDM) systems, in order to constrain the transmitted signals within the limited dynamic range of light-emitting diodes (LEDs)
Summary
Visible light communication (VLC) has been recognized as a compelling technique of alleviating teletraffic congestion in the near future. In order to render the classic OFDM scheme suitable for the IM/DD systems, a range of optical OFDM (O-OFDM) solutions have been proposed [12,13,14] In these approaches, the Hermitian symmetry is imposed on the frequency-domain (FD) subcarriers, in order to obtain real-valued baseband signals. Due to the simplicity of implementation, the clipping-based optical OFDM (CO-OFDM) solutions, which include the DCO-OFDM and ACO-OFDM systems, have attracted considerable research attention in VLCs. As the investigations in [13,14,15] show, the ACO-OFDM has a higher optical power efficiency than the DCO-OFDM, when a low or moderate constellation size is used [13]. Due to the antisymmetric property [13], clipping the negative parts of ACOOFDM signals does not introduce any loss of information, whilst clipping of the high positive peaks of ACO-OFDM signals results in nonlinear distortion, which degrades the system performance.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
