## 4 Use Cases
The need to model the non-idealities only at the transmitting side and not at the receiving side arises because they are not too significant at the BS and can easily be mitigated through offline designs before the BS is deployed. For the VLC system model, we have considered an indoor scenario with a room of dimensions 6 m \(\times\) 6 m \(\times\) 3 m, with plaster ceilings, walls, and a pinewood floor. There are \(k=1,2\ldots K\) VLC-supported access points mounted to the ceilings. The end users are distributed uniformly throughout the area and can move in any direction with equal probability. The channel parameters used in the simulations are given in Table 3.  
We consider adaptive direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM), which assigns data to all odd and even subcarriers as  
\[\mathbf{X}=[0\qquad x_{1}\quad\ldots\quad x_{N/2-1}\quad 0\quad x_{N/2-1}^{*} \quad\ldots\quad x_{1}^{*}] \tag{6}\]  
where n = 0, 1, 2... \(N-1\) is the number of subcarriers, an N-point inverse fast Fourier transformation (N-IFFT) is applied to \(\mathbf{X}\), which results in a real-valued waveform x at the output, whose nth element is given by  
\[x[n]=\frac{1}{\sqrt{N}}\sum_{k=0}^{N-1}X[k]e^{\frac{2 ink\pi}{N}} \tag{7}\]  
where n = 0, 1, 2... \(N-1\) and X[k] is the modulation symbol that is drawn from 2-PSK and M-QAM with M = 4, 8, 16, 32, 64, 128, and 256. To avoid inter-symbol interference (ISI), a cyclic prefix, of length \(N_{cp}\) is further added at the beginning of \(x[n]\). To achieve the unipolar constraint of IM/DD, the waveform is biased with a DC offset (\(A_{DC}\)). The resulting time domain waveform is then given as  
\[x(t)=\sum_{n=0}^{N+N_{cp}-1}x[n]\delta(t-nT_{s})+A_{DC} \tag{8}\]  
where \(T_{s}\) is the sampling interval. At the receiver's end, the signal y(t) is defined as (9)  
\[y(t)=R\sqrt{P_{x}}x(t)\otimes h(t)+n(t) \tag{9}\]  
where R is the responsivity of the photodetector (A/W), \(P_{x}\) is the total electrical power, h(t) is the channel impulse response, and \(n(t)\) is the additive white Gaussian noise (AWGN) with zero mean and variance of \(\sigma_{N}^{2}\). A series-to-parallel operation is performed on the received signal and the CP is removed. The fast Fourier transform (FFT) gives the complex-valued symbols in the frequency domain. The signal at the \(k\)th sub-carrier is now given as  
\[Y[k]=R\sqrt{P_{x}}X[k]H[k]+N[k] \tag{10}\] \begin{table}
\begin{tabular}{l l} \hline \hline
**Room Size** & **6 m \(\times\) 6 m \(\times\) 3 m** \\ \hline \multirow{3}{*}{Materials} & Walls: Plaster \\  & Ceiling: Plaster \\  & Floor: Pinewood \\ \hline \multirow{2}{*}{Object specifications} & Cell phone: Black gloss paint \\  & Human body: Absorbing \\ \hline \multirow{3}{*}{Luminaire specifications} & Number of luminaires: 9 \\  & Brand: Cree\({}^{\text{\textregistered}}\) CR6-800L \\ \cline{1-1}  & Half viewing angle: 40\({}^{\circ}\) \\ \cline{1-1}  & Power per each luminaire: 11 W \\ \hline \multirow{2}{*}{Receiver specifications} & Receiver area: 1 cm\({}^{2}\), POV: \\ \cline{1-1}  & 85\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Channel parameters for VLC simulations. The receiver selects the photodetector, which gives the highest data rate while satisfying a target bit error rate (BER). The data rate (in bits per second) is given as  
\[DataRate=\frac{1}{T_{s}}\frac{N/2-1}{L}log_{2}M \tag{11}\]  
where \(L=N+N_{cp}\) is the total length of the OFDM symbol. Similarly, for MCSs, a typical one-dimensional system with a transmitter and a receiver with a diffusive channel was considered. We constituted an MCvD-based nanonetwork, with \(k=1,2,3\ldots K\) nodes, where the information is encoded over messenger molecule concentration waves. The MCvD system consists of the following process: the molecules are first encoded using channel-coding techniques and then emitted in the channel.