# Optimization of Pico-eNB Tx Power and the Effects of Picocell Range Expansion in Multiband HetNet
## 4 Optimization of Pico-eNB Tx Power  
### User Throughput Versus Pico-eNB Tx Power  
In this section, we investigate the user throughput as a function of pico-eNB Tx power to determine the optimal pico-eNB Tx power using system-level computer simulations on the parameters provided in Table 1.  
Figure 4 shows the simulation outputs of macro- and pico-eNBs as well as the UE distribution in 19 macrocells at a given instant for a cluster size of 40 m, where the blue diamonds, green triangles, and red squares show the macro-eNBs, pico-eNBs, and UE, respectively. Figure 5 shows the average and 5-percentile user throughput as a function of pico-eNB Tx power, where the blue bars and orange bars show the average user throughput and the 5-percentile user throughput, respectively. The average user throughput increases as the Tx power increases up to approximately 37 dBm. However, Tx powers greater than 37 dBm slightly degrades the average user throughput. A wider signal bandwidth (i.e., large number of resource blocks for pico-eNBs) could be effectively used up to a Tx power of 37 dBm. However, efficiency is reduced for Tx powers greater than 37 dBm because the interference among pico-eNBs increases. For example, a Tx power of 37 dBm  
Figure 4: Simulation output of macro- and pico-eNBs layouts as well as UE distribution at a given instant. \begin{table}
\begin{tabular}{c c} \hline \hline
**Gain (dB)** & **Delay (ns)** \\ \hline \(-15.5\) & 0 \\ \(0.0\) & 10 \\ \(-5.1\) & 15 \\ \(-5.1\) & 20 \\ \(-9.6\) & 25 \\ \(-8.2\) & 50 \\ \(-13.1\) & 65 \\ \(-11.5\) & 75 \\ \(-11.0\) & 105 \\ \(-16.2\) & 135 \\ \(-16.6\) & 150 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: TDL model used in macrocells and picocells. was able to improve the average user throughput by approximately 12% compared with that for a Tx power of 31 dBm. The 5-percentile user throughput increases as the Tx power increases. Here, the 5-percentile user throughput is defined as the 5 th percentile point of the cumulative distribution function (CDF) of the user throughput. On the basis of these results, it was shown that a pico-eNB Tx power of 37 dBm was able to provide the best performance if the average user throughput was given priority. Figure 6 shows the CDF of the downlink effective SINR for all UE as a function of the three pico-eNB Tx powers, i.e., 28 (blue line), 37 (gray line), and 46 (red line) dBm. This is used to verify the results obtained in Figure 5. The SINR for the pico-eNB Tx power of 37 dBm provides the best performance among the three. Figure 7 shows the connection ratio of UE as a function of the pico-eNB Tx power, where the blue and the yellow bars show the ratios of the UE connected with pico-eNBs and macro-eNBs, respectively. Essentially, the ratio of the UE connected with pico-eNBs increases as the pico-eNB Tx power increases. When the pico-eNB Tx power is 37 dBm, the ratio of the UE connected with a pico-eNB reaches 43%. This means that 735 pieces of  
Figure 5: Average and 5-percentile user throughput as a function of pico-eNB Tx power. Figure 6: CDF of downlink effective SINR as a function of three pico-eNB Tx powers. UE are connected with pico-eNBs in the 19 macrocells and the rest (975 pieces of UE) are connected with macro-eNBs. Figures 8 and 9 analyze the average user throughput of all UE obtained in Figure 5. Figure 8 shows the average user throughput of a specific UE connected with pico-eNBs as a function of the pico-eNB Tx power. The average user throughput decreases as the pico-eNB Tx power increases because the number of UE connected with pico-eNBs increases. When the Tx power of pico-eNBs is 37 dBm, the average user throughput is approximately 120 Mbps. The throughput is so high because of the wide signal bandwidth. At the same time, the average user throughput of a specific UE connected with macro-eNBs is less than that of the specific UE connected with pico-NBs, as shown in Figure 9. For example, when the pico-eNB Tx power is 37 dBm, the average user throughput is approximately 2.2 Mbps.