Using a single photon avalanche diode (SPAD) as a receiver in an optical wireless communications (OWC) system can effectively expand the transmission distance. However, the performance of the SPAD receiver is usually affected with the bit error rate (BER) lower bound determined by background light and the inter-symbol interference (ISI) distortion caused by dead time. In this paper, external time-gated technology is employed, and the SPAD is only activated within the set gate-ON time to alleviate the influence of background light and ISI distortion. The SPAD photon counting model and the communication BER model are established according to the external time-gated characteristics. Considering the functional relationship among the gate-ON time, signal light flux, background light flux and blocking time, we take the minimum BER as the optimization target, the optimal gate-ON time is derived. The numerical results show that whenever the signal light flux or the background light flux is higher, the BER performance of the time-gated mode is apparently better than the free-running mode. For example, when signal photons and background photons are 30 and 10 per symbol time, respectively, the BER of free-running SPAD converges to 0.1, while the BER of the time-gated scheme is about 10−6.
In visible light communication (VLC) system, the intensity of light emitting diode (LED) is modulated to transmit data. As a transmitter, the design of LED should consider the performance of lighting and communication simulataneously. However, the output flux of LED is jointly determined by the electrical and thermal characteristics, and is concave-related to the forward bias current. In order to maximize the use of LEDs, in this paper, the mutual restriction of optical, electrical and thermal mechanisms for the LED systems are considered, and a dynamic photoelectrothermal model is established. Then, the optimal bias current is presented according to the electrical-thermal relationship of LED. Modulating LED at this bias current can maximize the communication performance without sacrificing the light flux. Finally, a BER experiment to verify the optimal communication performance is conducted at this bias current.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.