Probabilistic Shaping Based Modulation Schemes for Spectral-Efficient VLC systems

Overview

The increasing number of Internet of Things (IoT) devices and the growing demand for bandwidth-hungry applications have significantly increased wireless data traffic in radio access networks. Consequently, the overcrowded radio frequency (RF) spectrum has compelled researchers to explore the optical spectrum for communication, giving rise to visible light communication (VLC). In VLC, light emitting diodes (LEDs) transmit information using a process known as intensity modulation (IM), while photo-detectors (PDs) at the receiver recover the transmitted information through direct detection (DD). Despite the tremendous unlicensed bandwidth available in VLC, the narrow modulation bandwidth of LEDs limits the development of VLC systems with high achievable data rates. As a result, numerous advanced modulation schemes have been investigated to enhance the spectral Eefficiency (SE) of VLC systems. Nevertheless, a common limitation of existing schemes is their reliance on uniform symbol distributions, which leads to a loss in achievable data rate. This rate loss can be mitigated by optimizing the symbol distribution, a technique known as probabilistic shaping (PS).

First, to enhance the SE of multi-antenna VLC systems, a PS-based spatial modulation (SM) scheme is proposed, where each antenna consists of a single-color LED. The SE is improved by probabilistically shaping both spatial and constellation symbols. Second, to improve the SE of multiuser VLC systems, a PS-based nonorthogonal multiple access (NOMA) scheme is proposed, where each user employs a single-color LED for data transmission. In this scheme, the constellation symbols of each user are probabilistically shaped to enhance the overall SE. Finally, a PS-based color shift keying (CSK) modulation scheme is proposed, in which a multicolored quadrichromatic LED (QLED) is used for data transmission. Probabilistic shaping of four-dimensional constellation symbols efficiently improves the SE. In the proposed schemes, channel coding rates are also optimized to further enhance the SE. For each scheme, an algorithm is provided to compute the capacity-approaching (optimized) symbol distribution. The performance of the proposed schemes is evaluated in terms of SE and frame error rate (FER) under different optical signal-to-noise ratio (OSNR) levels and is compared with existing schemes. For instance, the results demonstrate that the proposed PS-based CSK scheme achieves a 20% improvement in SE compared to the uniform-distribution-based scheme. This work concludes with the design of a multi-color VLC testbed aimed at measuring the FER as a function of OSNR. The proposed testbed enables a more accurate experimental validation of the performance of the proposed schemes.