# Accounting for Energy Cost When Designing Energy-Efficient Wireless Access Networks
## 2 Materials and Methods
### Strategy 3: Microcell BSs Networks  
In this method, energy expenditure is reduced by the reduction of the power consumption. The microcell base stations are lower power consumers than macrocell base stations, as claimed in [5]. Therefore, two new configurations, composed by microcell base stations only, are proposed.  
Figure 5: Flow chart when using the _Take action according to the energy price_ algorithm.  
The presented configurations are obtained by placing the microcell base stations in equispaced points on the considered area (base stations are assumed to be points), at a 1.5-m height:  
1. base stations are spaced by 50 m horizontally and vertically: for the considered area of 0.3 km\({}^{2}\) of the city center of Ghent, 126 microcell base stations are needed (Figure 6a);
2. base stations are spaced by 40 m horizontally and vertically: 187 microcell base stations are used (Figure 6b).  
The horizontal and vertical distances at which base stations are placed (50 m and 40 m) are chosen arbitrarily, taking into consideration the radio range of the used infrastructures, aiming at minimizing the uncovered area. The cell interference margin of the link budget (Table 1) is approximated as 4 dB and 5 dB for the configuration with base stations spaced by 50 m and 40 m, respectively.  
Once the number of active users is known, the network is built considering one of the two configurations previously described, using the algorithm developed in [4] (Step 1 in Figure 7). Then, the power consumption is computed, and it is compared with the available energy (stored and produced). In case the needed energy is greater than that available, one of the two following energy-reducing actions is applied:  
1. _Only Microcell BSs: No Action_: In this case, no action is performed. It is used to check the effects of the new proposed configurations. If the available energy is not sufficient to power the base stations, the missing amount is purchased from the electricity grid (Step 7 in Figure 7). The storage and the total necessary energy cost are updated (Step 8 in Figure 7).
2. _Only Microcell BSs: Deactivation of some BSs_: A part of the microcell base stations is turned off gradually in the event of an energy shortage. The energy consumption of the access network is computed when switching off 1, 2, 3,...,N microcell base stations. As soon as the network's energy consumption becomes lower than the amount of available renewable energy (stored and produced energy), the necessary number of microcell base stations to be turned off is known. Since the power consumption of a microcell base station is low, many of them should be switched off. In this way, the capacity of the network can be compromised. For this reason, base stations can be deactivated until a minimum percentage of users is covered. The solution is tested with 95% and 90% guaranteed user coverage (Step 5 in Figure 7). All the users connected to the switched microcell base stations are reconnected, if possible (Step 6 in Figure 7). If the available energy continues to be insufficient for the base stations' supply, the missing part is bought from the traditional power grid (Step 7 in Figure 7). The necessary energy cost and the energy storage are updated (Step 8 in Figure 7). The strategy can be combined with the time window _TW_: when an energy shortage is detected in a time slot belonging to the time window period, the necessary microcell base stations are deactivated.  
Figure 6: Configurations only with microcell BSs, spaced by 50 m (**a**) and by 40 m (**b**).