# Throughput Enhancement in Downlink MU-MIMO Using Multiple Dimensions
## 4 Simulation Results  
This section shows simulation results to demonstrate the throughput gain of the proposed scheme and compares the results with other conventional schemes. Simulation results also provide some considerations for the proposed scheme. The simulations are performed in seven multi-path Rayleigh fading and time-invariant channel model. OFDM overcomes the frequency selectivity of the wideband channel and multiple carriers enable the high rate data transmission [22; 23]. The OFDM symbol is composed of 128 FFT size, four pilots and the 108 data subcarriers based on specification for IEEE 802.11n. The remaining 16 subcarriers are zero padding and OFDM symbol duration is 4 microseconds. Figure 6: The proposed signal detection model. All the simulation are simulated with quadrature phase-shift keying (QPSK) modulation (data bits per subcarrier is 2). Figure 7a,b compare the sum throughput between the proposed schemes and the conventional schemes for the case of two users as a function of SNR. The throughput \(T\) is calculated as follows,  
\[ \begin{array}{l}T=N_{b}\times(1-E)^{L}\times K\div T_{s}\\ N_{b}=N_{s}\times O\times N_{r}\times s,\\ L=N_{s}\times O,\end{array}  \tag{17}\]  
where \(N_{b}\) is the number of transmit data bits and \(L\) is the number of data bits in one OFDM symbol. \(E\) is the BER for each user and \(N_{s}\) is the number of data subcarriers. \(O\) is the number of data bits per subcarrier and \(s\) is the number of symbols in one superposed signal. \(T_{s}\) is the OFDM symbol duration. In the simulation, proposed scheme and conventional BD are the MIMO system that BS and each user have multiple antennas. On the other hand, the conventional NOMA has one antenna at the BS and each user since multiple access is accomplished by the power domain. The proposed scheme and BD scheme are simulated in the case of \(N_{t}=4\), \(N_{r}=2\), \(K=2\)\((4,2,2)\) and NOMA scheme is \(N_{t}=1\), \(N_{r}=1\)\(K=2\)\((1,1,2)\). And in the proposed scheme, the number of symbols at each superposed signal is two \((N=2)\). Therefore, the total number of transmit symbols to each user is 8. The power of each symbol is allocated at a ratio of 8:2 from the total power \(P_{t}=1\). In MIMO system, the ML MIMO detection is applied before conducting SIC. The summary of the simulation parameters is shown in the Table 1.  
In the simulations, there are two types of simulation results: with and without SIC error. Figure 7a,b show the simulation result with SIC error and Figure 7c,d show the simulation result without SIC error. Both cases of the proposed scheme outperform the conventional schemes in terms of maximum throughput since the proposed scheme exploits both the spatial and power dimensions. The conventional BD shows better performance than the proposed scheme in terms of BER. However, the proposed scheme has higher throughput since the proposed scheme transmits more symbols. In Figure 7b, the BER performance of the proposed scheme is better than that of NOMA since the effect of MIMO detection and SIC decoding is improved at high SNR. Figure 7c,d show the impact of SIC error on the proposed scheme. Without SIC error, throughput and BER performance are better than when there is SIC error. If the strong symbol is wrongly decoded at low SNR, the weak signal is also wrongly decoded. As a result, the error propagation occurs. Therefore, the system needs to be designed to avoid error propagation and reduce SIC error. As the proposed scheme minimizes the impact of SIC error, the performance of the proposed scheme approaches the case where there is no SIC error. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Scheme** & \(N_{s}\) & \(O\) & \(N_{r}\) & \(s\) & \(K\) \\ \hline Conventional BD & 108 & 2 & 2 & 1 & 2 \\ Conventional NOMA & 108 & 2 & 1 & 1 & 2 \\ Proposed Scheme & 108 & 2 & 2 & 2 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters.