Silicon-Photomultiplier-Based Underwater Wireless Optical Communication Using Pulse-Amplitude Modulation

Abstract

Emerging maritime applications arising from the continued growth of the marine economy have an inherent need for high data rate underwater wireless links. Within this context, underwater wireless optical communication is known as a promising technology for data transmission over shortto-medium ranges; the current available technology provides a transmission span of about 100 m. In view of extending the transmission range, silicon photo-multipliers (SiPMs) have recently emerged as a photo-detection solution offering high receiver (Rx) sensitivity together with operational flexibility. In this paper, we introduce the use of pulse amplitude modulation (PAM) together with frequency-domain equalization (FDE) at the Rx to boost the communication rate beyond the bandwidth (BW) limitation of the optoelectronic components. For instance, for a link BW limited to 2 MHz and 2-PAM transmission with a target BER of 10 −4 , the link becomes nonoperational for data rates larger than ∼ 8 Mbps without equalization, whereas much higher data rates can be attained using FDE, e.g., 20 and 50 Mbps with maximum ranges of 28 and 10 m, respectively, in clear waters for the SensL MicroSB-30020 SiPM and an average transmit optical power of 0.6 W only. Meanwhile, the nonlinear distortion of the SiPM is shown to limit the modulation order and thus the data rate in relatively short ranges. We also propose appropriate processing for PAM modulation and demodulation, given the quantum-noise-limited Rx when using an SiPM. We show that the use of non-binary PAM is undeniably advantageous for moderate data-rates (symbol rate a few MHz larger than the overall link BW) when no channel equalization is performed at the Rx. However, when employing FDE, only for very high data rates (e.g., symbol rate ten times larger than the link BW), where the link frequency response becomes highly frequency selective, the non-binary PAM becomes practically interesting, outperforming 2-PAM.

Optical OFDM for SiPM-Based Underwater Optical Wireless Communication Links

“…Note that the low DCR of this component is obtained by maintaining the chip temperature at −10 • C by means of a thermoelectric cooler. Thanks to this low DCR, as we will show, the attainable operation ranges are quite larger than those with an older generation of SiPMs, e.g., [52], see for instance the numerical results presented in [6,9].…”

“…First, notice a high BER for very short link ranges (e.g., Z 8 m for OOK), which is due to SiPM saturation [6,9]. For relatively large link ranges (e.g., Z 45 m for OOK at R b = 10 Mbps), the BER reasonably increases due to decreased SNR as a result of channel attenuation.…”

“…In order to push the data rate beyond the limited BW of the opto-electronic components, two common approaches are serial transmission with channel equalization at the Rx, and the use of high spectral efficiency modulation techniques such as multiple sub-carrier modulation [8]. For instance, the use of on-off keying (OOK), pulse-amplitude modulation (PAM), and pulse-position modulation (PPM) with frequency-domain equalization (FDE) was considered in [9,10]. Several works also have considered the use of O-OFDM for UWOC links, with the advantages of using a simple single-tap equalizer to equalize the frequency-selective aggregate channel.…”

“…An important point here concerns QAM signal demodulation since the Rx is clearly shot noise limited due to the use of an SiPM. Indeed, for a multi-level modulation scheme, the corresponding signal-dependent noise should be processed carefully, e.g., by performing the so-called square-root transformation [9,39] to avoid a degradation of the Rx performance. However, it has been shown in [39] that for the case of O-OFDM signaling, although the time-domain signal r n at the Rx (before the FFT block in Figure 1) is affected by signal-dependent noise, for the frequency-domain signal (i.e., R k at the output of the FFT block) the noise is in practice nearly independent of the signal [39].…”

“…The transmit power for "On" symbols is set so as to result in an average transmit electrical power of 125 mW. At the Rx side, signal demodulation is based on optimal thresholding, as considered in [9].…”