# Design of an Energy Efficient Future Base Station with Large-Scale Antenna System
## 2 System Model
### Large-Scale Multiple-Input Multiple-Output Model  
In this subsection, we summarize the LS-MIMO model since the LS-MIMO would be a very important characteristic of a future BS. An LS-MIMO is an antenna system with a large number of TX antennas. It was first introduced by [PERSON] [29], and after that, much research has been carried out [19; 20; 28]. Due to its desirable improvements in throughput (TP) and EE, it has already been discussed by the important standardization body, i.e., the the 3 rd Generation Partnership Project (3 GPP), as one of the core technologies for 5 G. In 3 GPP, the LS-MIMO is usually referred to as full-dimension MIMO (FD-MIMO). Therefore, it is unquestionable for the LS-MIMO to be a key technology for future BS. The antenna structure of a BS equipped with LS-MIMO could be of various shapes depending on the implementation. One example of a high-level conceptual figure of an LS-MIMO system is shown in Figure 2.  
Each antenna has its own RF component, as shown in Figure 3, and the structure in Figures 2 and 3 can be used to generate a very sharp beam. The designer can choose to put more antennas in the vertical direction or horizontal direction. If we put more antennas in the vertical direction, the resolution of the vertical beam separation is very good, and thus, users who are located on each floor of a building can benefit. If we put more antennas in the horizontal direction, it can provide a horizontal separation effect, and thus, users located in different horizontal angles can benefit. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Description** & **Power Consumption** \\ \hline \(B\) & Bandwidth & 10 MHz \\ \(T_{sl}\) & Slot length & 0.5 ms \\ \(T_{p}\) & Pilot length in one slot & 0.214 ms \\ \(T_{s}\) & Symbol duration & 71.4 μs \\ \(T_{g}\) & GI & 4.7 μs \\ \(T_{u}\) & Symbol without GI & 66.7 μs \\ \(T_{d}\) & Delay spread & 4.7 μs \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Baseband (BB) power consumption parameters. GI: guard interval. Figure 1: Radio frequency (RF) power consumption model. However, there are some obstacles to realize LS-MIMO. First, the antenna spacing should be at least \(\lambda/2\), which means that it requires a quite large space for installation and use with cellular bands. Even though higher frequency bands could be a good option, the poor channel condition should be overcame. Second, it requires many RF chains, which are very expensive. Good antenna selection algorithms can be very helpful to reduce the number of RF chains [16]. Thus, implementing LS-MIMO with a low complexity and cost is a big research topic. To introduce the operation of the LS-MIMO system, let us think of a single isolated cell with one BS and Kuser terminals. The BS has \(N_{t}\) TX antennas, and each user terminal has one RX antenna. The received signal vector at the terminals can be represented as follows:  
\[\mathbf{y} = \sqrt{P_{tx}}\mathbf{H}\mathbf{s}+\mathbf{n} \tag{8}\]  
where \(\mathbf{y}\) is the \(K\times 1\) received signal vector for \(K\) user terminals, \(P_{tx}\) is the total TX power for the downlink, \(\mathbf{H}\) is the \(K\times N_{t}\) channel matrix between \(N_{t}\) BS antennas and K terminals, \(\mathbf{s}\) is the \(N_{t}\times 1\) TX  
Figure 3: An implementation example of the antenna and RF in an LS-MIMO system. Figure 2: An example of a high level conceptual figure of a large-scale multiple-input multiple-output (LS-MIMO) system. signal vector and \(\mathbf{n}\) is the \(K\times 1\) noise (AWGN) vector at the terminals. The element of the channel matrix between the \(m\)-th BS antenna and the \(k\)-th terminal, \(h_{km}\) can be decomposed as:  
\[h_{km}=g_{km}\cdot\chi_{k} \tag{9}\]  
where \(g_{km}\) is the zero-mean and unit variance i.i.d. Rayleigh fading channel coefficient and \(\chi_{k}\) is the path loss component. We only consider the narrowband signal, since the OFDM can successfully change the wideband signal into a narrowband signal. The TX is assumed to have perfect CSI.