# Optical OFDM for SiPM-Based Underwater Optical Wireless Communication Links
## 6 Performance Study of the UWOC Link
### Parameter Specification  
In this work, at the Tx, we consider a NICHIA NSPB510 AS LED with emitting wavelength \(\lambda=470\,\)nm and 3 dB cut-off frequency of \(10\,\)MHz [6]. A Lambertian emission pattern is considered for the LED with \(m\approx 45\), corresponding to a half-angle of \(\approx 10^{\circ}\)[6]. Recall the I-V characteristics of the LED from Figure 4. Without loss of generality, the imperfect conversion efficiency of the LED is neglected. Indeed, an important part of the electrical power consumption at the Tx is due to the conversion efficiency of the LED, typically about 80%, where the corresponding power loss is converted to thermal dissipation. At the Rx, consider a Hamamatsu C13366 3050 GA SiPM [49]. The thermal noise effect is reasonably neglected, compared to the SiPM shot noise [50]. The background noise effect is also neglected, assuming that the Rx operates in relatively deep waters [10; 51]. Hence, only the shot and dark noises of the SiPM are taken into account. Note that the low DCR of this component is obtained by maintaining the chip temperature at \(-10^{\circ}\) 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].  
Lastly, concerning the aquatic channel, the case of clear waters is considered with the diffuse attenuation coefficient \(K=0.08\,\)m\({}^{-1}\)[51].  
Table 1 contains the main parameters of the Tx, channel, and the Rx used in the simulations.  
In the presented study, unless otherwise mentioned, to compare the performances of the different transmission schemes, the same overall link data rate, \(R_{b}\), is considered. For DCO and ACO, the considered default signal constellations are 4-QAM and 16-QAM, respectively. For LACO, we will use two layers with, by default, 8-QAM and 4-QAM in the first and the second layers, respectively, which results in the same spectral efficiency of \(\eta\approx 1\,\)bps\(/\)Hz as for the two former schemes from (10). Note that no bit or power loading is considered at the Tx in order to better see the limitations of each transmission scheme. Additionally, the number of sub-carriers \(N=1024\) is set by default, which resultsin an effectively flat channel per sub-channel for the considered data rates. Depending on \(R_{b}\), the CP length \(N_{\rm CP}\) is set appropriately (i.e., large enough, compared to the aggregate channel delay spread) in order to avoid ISI. The BW of the aggregate channel is determined by those of the SiPM (\(\sim\)4 MHz), the LED (\(\sim\)10 MHz), and the aquatic channel. Lastly, for DCO-OFDM, unless otherwise specified, the DC bias is set by considering a clipping factor of \(\mathcal{K}_{\rm dB}=7\) dB, see Section 5.5. This value makes a good compromise between the scaling factor and the clipping noise level for a given \(P_{\rm Tx,e}\) (more discussion will be provided later in Section 6.8 and in the Appendix B). For the two other schemes, \(B_{\rm DC}\) is set to \(V_{\rm min}\) of the LED, i.e., 2.75 V, see Figure 4.