Orthogonal frequency-division multiplexing (OFDM) systems have been widely implemented in guided and non-guided communications. Even though they are becoming a standard for popular technologies such as WiFi, their implementation in newly emerging ones is still in evolution. As the number of connected devices increases, the radio frequency (RF) channel gets crowded. Here is where the visible light communication (VLC) links gain interest. VLC offers a wireless alternative to RF with a trade-off between the necessity of line of sight with the offer of a free, highly secure, and immune to electromagnetic interference communication channel. Therefore, the characterization and implementation of new techniques that can solve this kind of channel's particularities will help the technology achieve the maximum of its capacities. This paper proposes a programmable software implementation of an OFDM system using a novel phase correction technique for VLC channels. The new equation takes into account two types of phase shifts, one that will be non-deterministic (has a random variation) and another that will be deterministic and, therefore, can be estimated and corrected. This second can be split into two: one that comes from the clock differences and has more impact on higher subcarriers, and the second that appears from the electronics of the link and will directly impact the symbols' phase. The main difference with current phase correction techniques is the identification and estimation of the deterministic noise specific to VLC systems. The deterministic noise impact all subcarriers equally and must be subtracted before the measure of the shift induced by the differences between the transmitter and receiver clocks. Another difference is that the analysis and tests have been done with data collected in an actual hardware implementation.

Visible light communications (VLC) have been proposed for several applications beyond the traditional indoor scenarios, from vehicular to underwater communications. The common element in all these applications is the use of light-emitting diodes (LEDs) in which the forward current that flows through each LED plays a major role. Therefore, knowing the electrical equivalent of the LEDs is a useful tool for the proper design of the VLC systems. Currently, some measurement instruments exist, such as the LCR (inductance, capacitance, and resistance) meters or impedance analyzers to characterize the main parameters of the LEDs. However, these instruments and measurement procedures are subject to satisfying some requirements, such as a minimum value of the input impedance or the maximum forward current. In this work, the LED LXHL-BW02 is used to obtain its equivalent circuit, using different measurement methods and traditional methods of measurement with our proposed method. The equivalent model is implemented on the simulation tool LTSPICE. Our alternative method can be used for determining the electrical equivalent circuit of LEDs subject to high current variations at very high frequencies, in the MHz range, i.e., in an operating range for VLC applications.

The main contribution of this work is the proposal and hardware implementation of an algorithm for the time synchronization of orthogonal frequency division multiplexing (OFDM) modulation with Hermitian symmetry for visible light communications (VLC) systems, using only the real component of the transmitted signal. OFDM systems with Hermitian Symmetry are able to satisfy the high data rates demanded by many VLC systems, as well as deal with inter-symbol interference (ISI). However, these systems are characterized by being very vulnerable to synchronization problems in both time and frequency. Time synchronization being the most critical. In this paper, a novel time synchronization technique for OFDM systems with Hermitian Symmetry is proposed. This technique is based on the inclusion of a post-FFT synchronization stage to achieve time synchronization with one-sample precision. For this purpose, the relationship that exists between an offset in the frequency domain with an offset in the time domain is used, and with this information the correction of the frequency offset from the time domain is performed. An original and novel algorithm has been designed and implemented in hardware, which uses the cyclic prefix (CP) elimination stage to perform the time correction. The performance validation of the proposed technique is done with the implementation, using the ZYBO ™ development board, of a complete OFDM system applied to a real VLC system.INDEX TERMS Visible light communication (VLC), orthogonal frequency division multiplexing (OFDM), time synchronization, Hermitian symmetry.

No abstract