Signal Pre-Equalization in a Silicon Photomultiplier-Based Optical OFDM System

In this article, the performance of a new time domain signal pre-equalization method for use with optical orthogonal frequency division multiplexing (OFDM) and a silicon photomultiplier (SiPM) based receiver is studied. A SiPM contains a large array of microcells and each microcell is able to detect single photons. Therefore, a SiPM can be used to create arguably the most sensitive optical receiver, which can detect light intensity signals by counting the number of arriving photons within each signal sampling period. However, each photon detection triggers an avalanche-and-quenching process and the related microcell becomes inactive for a recovery time of several nanoseconds. Consequently, any photons arriving during this period cannot be detected. This effect can cause a non-linear distortion of the received signal and, when the OFDM sampling period is short, also introduces interference between signal samples. In this article, a new signal pre-equalization method is specifically designed to compensate for the impact of the finite recovery time. In this method, the number of active microcells during the transmission of each OFDM signal sample is first estimated. Then, the amplitude of the time domain signal sample is pre-adjusted based on the predicted fraction of microcells that are active. Using this approach, the negative impacts of the recovery time of the microcells are significantly reduced. The results that are presented show that when this new form of pre-equalization is used the bit error rate (BER) performance of the system is improved for a wide range of irradiance levels.