## 4 Performance Assessment of Optical Wireless Networks-on-Chip  
The performance of a communication link is often characterized in terms of bit error probability (BEP). In a binary, optical communication system, BEP can be defined as the probability that the received photocurrent transmitting 0 is detected above the decision threshold, or it is detected below the threshold while transmitting 1. In the literature related to Wireless NoC [4; 19; 22] BEP is usually experimentally assessed or estimated by means of analytical formulas for single link, which can be computed based on the estimated value of the received power after the photodiode. However, wireless network design always requires a careful study of cross-link interferences, which may occur among simultaneously links exploiting the same (optical) channel. In fact, the spatial reuse of optical wavelengths improves significantly the transport capacity in OWiNoC, since the number of simultaneous communications will not be limited by the number of available wavelengths. On the \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Geometrical** & \multicolumn{2}{c}{**Electromagnetic**} \\ \hline t\({}_{\text{down}}\) & 2 \(\mu\)m or 3 \(\mu\)m & frequency & 193.5 THz (\(\lambda_{0}\) = 1.55 \(\mu\)m) \\ t\({}_{\text{up}}\) & 2 \(\mu\)m or 3 \(\mu\)m & n\({}_{\text{down}}\) & 3.47 (Silicon, metal otherwise) \\ t & t\({}_{\text{down}}\) + t\({}_{\text{up}}\) & n\({}_{0}\) & 1.44 (Silica) \\ d & tens to hundreds of \(\mu\)m & n\({}_{\text{up}}\) & 1 (Air) \\  & & antenna & single/array of plasmonic Vivaldi antenna [9; 37] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main input parameters for the Ray Tracing simulations. Figure 5: Ray tracing evaluation of on-chip wireless propagation. other hand, wavelength reuse can trigger co-channel interference, which can harshly degrade the system performance, if not effectively handled and mitigated. Hence, the design of OWiNoC can greatly benefit from an interference-aware framework for BEP evaluation. By exploiting the analytical approach in [40,41] compliance of different link configurations with the target performance for the on-chip communication can be investigated. In a general OWiNoC scenario, I + 1 transmitters using the same optical wavelength can be present, with only one useful transmitter \(\mathrm{TX}_{0}\) for the desired receiver \(\mathrm{RX}_{0}\) and the other I transmitters (\(\mathrm{TX}_{1}\), \(\mathrm{TX}_{2}\),..., \(\mathrm{TX}_{\mathrm{I}}\)) causing interference to \(\mathrm{RX}_{0}\) while communicating with their corresponding receivers. The optical power (i.e., the power of the unmodulated optical carrier) of the received desired signal is here denoted by \(\mathrm{P}_{0}\), and the normalized optical power of the i-th received interfering signal is denoted by \(\mathrm{x}_{\mathrm{i}}=\mathrm{P}_{\mathrm{i}}/\mathrm{P}_{0}\), with i = 1, 2,..., I. The received power values depend on the radiation properties of the antennas, their spacing and orientation and on the propagation characteristics of links. In this paper, BEP performance is investigated for the following, two simple OWiNoC scenarios. In the former case (Figure 6a), two parallel links, separated by the distance \(\Delta\), carry data in the same direction up to a distance \(\mathrm{d}_{0}\). The optical signal from \(\mathrm{TX}_{0}\) to \(\mathrm{RX}_{0}\) is exposed to the interference of the signal coming from \(\mathrm{TX}_{1}\), which may therefore affect the communication BEP. In the latter scenario (Figure 6b), an additional, parallel, interfering link is introduced at a distance \(\Delta^{\prime}\) from the reference link \(\mathrm{TX}_{0}\)-\(\mathrm{RX}_{0}\). For each OWiNoC scenario, the received powers can be evaluated with FDTD or RT tools as discussed in the following Section 5.1.