Abstract
Underwater optical wireless systems have dual requirements of high data rates and long ranges in harsh scattering and attenuation conditions. In this paper, we investigate the advantages and limitations of optical orthogonal frequency-division multiplexing (O-OFDM) signaling when a silicon photo-multiplier (SiPM) is used at the receiver in order to ensure high sensitivity. Considering a light-emitting diode (LED) transmitter and taking into account the limited dynamic range imposed by the transmitter and the SiPM receiver, we study the performance of three popular O-OFDM schemes, i.e., DC-biased, asymmetrically-clipped, and layered asymmetrically-clipped O-OFDM (DCO-, ACO-, and LACO-OFDM, respectively). We consider a constraint on transmit electrical power and take into account the required DC bias for the three considered schemes in practice, showing the undeniable advantage of ACO- and LACO-OFDM in terms of energy efficiency. For instance, for the considered SiPM and LED components, a spectral efficiency of ∼1 bps/Hz with a data rate of 20 Mbps, a link range of 70 m, and a target bit-error-rate (BER) of , ACO and LACO allow a reduction of about 10 and 6 mW, respectively, in the required , compared to DCO-OFDM. Meanwhile, we show that when relaxing the constraint, DCO-OFDM offers the largest operational link range within which a target BER can be achieved. For instance, for a target BER of and a data rate of 20 Mbps, and considering of 185, 80, and 50 mW for DCO-, LACO-, and ACO-OFDM, respectively, the corresponding intervals of operational link range are about 81, , and m. Lastly, we show that LACO-OFDM makes a good compromise between energy efficiency and operational range flexibility, although requiring a higher computational complexity and imposing a longer latency at the receiver.
Highlights
The increasing need to explore underwater resources has given rise to the development of high-performance underwater equipment and robotics with data transmission capability
Taking into account the effects of upper and lower signal clipping at the Tx, silicon photo-multiplier (SiPM) saturation at the Rx at relatively short ranges, and beam attenuation in water, we show that, overall, ACO-OFDM has an undeniable advantage over the two other schemes in terms of energy efficiency for low-to-moderate spectral efficiencies
For the case of an SiPM-based underwater wireless optical communications (UWOCs) system and in order to increase the link data rate, this work investigated the use of high spectral efficiency modulation schemes based on orthogonal frequency-division multiplexing (O-OFDM), namely the three schemes of DCO, ACO, and LACO-OFDM
Summary
The increasing need to explore underwater resources has given rise to the development of high-performance underwater equipment and robotics with data transmission capability. The primary challenges for UWOC systems include extending the range and enhancing the data rate of such links To address the former, silicon photo-multipliers (SiPMs), called multi-pixel photon counters (MPPCs), have recently drawn particular attention thanks to their high internal gain, allowing a high receiver (Rx) sensitivity, and operation over large distances. Given the limited DR of an SiPM, which primarily impacts Rx performance at relatively short ranges [6,7], the use of O-OFDM signaling could impose further limitations in practice. Taking into account the effects of upper and lower signal clipping at the Tx, SiPM saturation at the Rx at relatively short ranges, and beam attenuation in water, we show that, overall, ACO-OFDM has an undeniable advantage over the two other schemes in terms of energy efficiency for low-to-moderate spectral efficiencies.
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