# Accounting for Energy Cost When Designing Energy-Efficient Wireless Access Networks
## 2 Materials and Methods
### Strategy 1: Simple Algorithms  
In this case, the methodology of [14] is used and extended. The considered network is the real one of the city center of Ghent. It is composed of eight macrocell base stations (blue points in Figure 1), each supporting four microcell base stations (orange points in Figure 1). For each time slot (one-hour long), the number of active users is determined, using users' predictions, built according to data provided by a mobile Belgian operator. Once the number of active users is known, the network can be determined: each user is associated with a base station, according to the algorithm used in [4], which aims at power consumption minimization (Step 1 in Figure 3). At this point, as shown by the red step in Figure 4, the algorithm computes the power consumption of the network (NPC of Step 3 in Figure 3) for the current time interval (indicated with \(t_{i}\) in Figure 3), using the power consumption model for macrocell and microcell base stations, described in [17]. Figure 3: Flowchart when using simple algorithms. NPC, network power consumption. The network's power consumption is compared with the available energy, given the energy in the battery and the energy produced by the PV panels (respectively energy_stored[j] and energy_produced[j] in Step 4 in Figure 3). If the network consumes more than the available energy, one of the energy-reducing actions proposed and analyzed in [14] is applied (Step 7 in Figure 3 and the purple step in Figure 4):  
1. _Undertake no action_: When using this, no adjustments are made at the network level. It represents the current situation. This is considered as the reference scenario, concerning the energetic aspect. 2. _Deactivate all microcell BSs_: When this is used, all microcell base stations are turned off. When a base station is deactivated, its power consumption is assumed to be negligible. All users connected to these microcell base stations are reconnected to a macrocell base station, if possible. 3. _Deactivate as much microcell BSs as needed_: This is a hybrid one; microcell base stations are turned off gradually. The network's energy consumption is computed when switching off 1, 2 or 3 microcell base stations per macrocell base station. As soon as the network's energy consumption becomes lower than the amount of available renewable energy, the necessary number of microcell base stations to be turned off is known. If turning off three microcell base stations is not sufficient, the previous strategy is applied. As in the previous case, all the users connected to the switched microcell base stations are reconnected, if possible. The last two actions can be combined with a prediction time window _TW_, expressed in hours. Without considering the time window, the network's energy demand and energy production for the current time interval are considered, and an energy action is applied in the case of energy shortage. When considering a time window, any energy shortage in the next few hours is taken into consideration. In particular, assuming a given time interval _TI_ and a given time window _TW_, the time window period (indicated as _TWPeriod_ in Figure 3) is defined as the period between _TI_ and _TI+TW_ (boundaries included). If an energy shortage is predicted during this, microcell base stations are switched off from _TI_, for the whole time window period duration. Once the energy reducing action is applied, if the available energy continues to be insufficient, the amount of energy that cannot be provided by the PV panels and by the storage is bought from the traditional power grid (Step 8 in Figure 3 and the blue step in Figure 4). Therefore, the necessary energy cost and the energy storages are updated (Step 9 in Figure 3). For this study, algorithms that aim at minimizing the total amount of money spent to buy the energy needed for the supply of base stations are introduced in the tool used in [14]:  
Figure 4: Flowchart of the research process. 1. _No action_: When this is used, the energy is bought as soon as it is needed.