## 2 System Model  
An indoor millimeter wave system with WPT and SWIPT is considered for short distance. We utilize the ray-tracing method to compute the environment channel and the channel regularity feedback can be acquired as follows:  
\[\mathbf{H}(\mathbf{f})=\sum\
olimits_{i=1}^{\mathbf{N}_{\mathbf{IM}}}\mathbf{b}_{i}(\mathbf{f})e^{i\Phi _{i}(\mathbf{f})} \tag{1}\]  
where the frequency of sinusoidal wave is \(\mathbf{f}\) and the path index is \(\mathbf{i}\). Both \(\Phi_{i}(\mathbf{f})\) and \(\mathbf{b}_{i}(\mathbf{f})\) are the \(i\)-th phase shifts with receiving magnitudes for different frequencies. The total number of the receiving ray is \(\mathbf{N}_{\mathbf{IM}}\). The regularity feedback can transform to the time domain by utilizing the inverted fast Fourier transform. We can compose the time domain impulse feedback of the equivalent baseband here as follows:  
\[\mathbf{h}(\mathbf{t})=\sum\
olimits_{\mathbf{q}=1}^{\mathbf{Q}}\mathbf{\alpha_{q}}\mathbf{s}\mathbf{ \left(}\mathbf{t}-\mathbf{\tau_{q}}\mathbf{\right)} \tag{2}\]  
where we have the overall number of paths observed at time \(\mathbf{Q}\). \(\mathbf{\alpha_{q}}\) and \(\mathbf{\tau_{q}}\) are the channel gain and time delay for the \(q\)-th path, respectively. Regarding the system nodes, there are three type nodes in our system, which is shown in Figure 1. First, there are the transmitter nodes. There are N antennas in the antenna array to protect against the path loss in the millimeter wave and to focus the transmission power efficiently. Second is the SWIPTnode; this node uses one antenna to receive the signal information and harvest power. The third type of node is only used for harvesting power in downlink transmission. We made use of the power-splitting style in SWIPT node, as displayed in Figure 2. The received radio frequency (RF) signals are divided into two streams for the information receiver and RF power harvester with various power levels. A circular array with N millimeter wave dipole antennas is shown for the first type of node in Figure 3.  
Figure 1: System node description. SWIPT: simultaneous wireless information and power transfer; WPT: wireless power transfer. Figure 2: Receiver architecture for the SWIPT node. The array factor for the antenna array is as follows:  
\[\mathbf{AF}(\theta,\Phi,\mathbf{f})=\sum_{n=1}^{N}\mathbf{W}_{n}e^{ikasin\Theta cos(\Phi-\frac{ 2\pi}{N}(n-1))} \tag{3}\]  
where the \(\theta\) and \(\Phi\) is the spherical coordinate system, \(N\) is the total number of antennas in the circular array, and \(\mathbf{a}\) is the distance from the antenna position to coordinate center. \(\mathbf{W}_{n}=\mathbf{I}_{n}e^{i\mathbf{\alpha}_{n}(\mathbf{f})}\) includes the excitation current \(\mathbf{I}_{n}\) and phase delay \(\mathbf{\alpha}_{n}(\mathbf{f})\). \(\mathbf{k}=2\pi/\lambda\) is the wavenumber and \(\mathbf{\lambda}\) is the wavelength. \(\mathbf{\alpha}_{n}(\mathbf{f})\) can expressed as follows:  
\[\mathbf{\alpha}_{n}(\mathbf{f})=-\frac{2\pi\mathbf{f}}{c}\sqrt{\epsilon_{\mathbf{r}}}\cdot \mathbf{\ell}_{n} \tag{4}\]  
where \(\epsilon_{\mathbf{r}}\) is the relative permittivity of the feed line, \(\mathbf{\ell}_{n}\) is the feed length of line needed to adjust the phase delay, and \(c\) is the speed of light. In the second type of node, the power-splitting architecture in the system is used, as shown in Figure 2. The obtained RF signals are divided into two streams for the information receiver and RF power collector with various power levels. Let \(\eta\in[\mathbf{0},\mathbf{1}]\) signify the energy-dividing coefficient for the receiver (i.e., \(\eta\) is the portion of RF signals utilized for power gathering). The total energy collected from the receiver is modeled as:  
\[\mathbf{h}(\mathbf{t})=\sum\
olimits_{\mathbf{q}=1}^{\mathbf{Q}}\mathbf{\alpha_{q}}\mathbf{s}\mathbf{ \left(}\mathbf{t}-\mathbf{\tau_{q}}\mathbf{\right)} \tag{2}\]0  
In a SWIPT system, the quality of information is also important. Thus, we consider the BER and capacity as the information quality for the millimeter wave system.