# Energy Efficiency Perspectives of PMR Networks +
## 5 Technical and Economic Considerations on Energy Efficiency of Next Generation PMR Systems
### Radio Resource Management Strategies for Energy Efficiency  
Looking at commercial cellular networks, the adoption of efficient radio resource management solutions is one of the most effective ways to reduce the overall energy consumption. As a matter of fact, network dimensioning is peak-load oriented. Therefore, most of the day the traffic is much lower than in peak hours, and a lot of energy gets wasted. The main goal of an energy efficient radio resource management scheme is to adapt the network energy consumption to the actual daily traffic load. The main way to adapt the radio resources to the users' requests is the introduction of base station sleep mode, which gives the system the possibility to put some devices of a base station in a low power state. Referring to the LTE system, several sleep mode techniques have been proposed [10]. In particular, frequency domain, system domain and time domain approaches are under investigation in current research about green wireless access networks. All the solutions focus on putting the RF power amplifiers on low consumption state. Frequency domain solutions are able to manage the available bandwidth at the base station by putting on sleep mode the relative carrier blocks. In order to maintain the same power spectral density, the reduced bandwidth requires less radiated power. Spatial domain solutions derive from the coexistence of multiple radio access technologies, which are allowed by the LTE standard: the global network energy efficiency can be improved by introducing cooperation schemes between the available access technologies. Other spatial domain solutions are reducing antenna number at the base station and dynamically configuring cells in a multicell scenario. Even if such approaches are an attractive solution for commercial LTE cellular networks, characterized by the densification of base stations and cell layers, they are not suitable for PMR networks like TETRA. Therefore, in order to design an energy efficient TETRA over LTE network, just the time domain approaches should be investigated. The most promising time domain solution is identified as cell discontinuous transmission (cell DTX), a hardware feature based on the deactivation of some components of a base station when there is no traffic, i.e., during the zero transmission time intervals [31]. As we observed before, if a base station with no traffic can be put into sleep mode, then the idle power consumption will be significantly reduced; in addition, cell DTX acts only when no data or signaling is transmitted, thereby the cell coverage is not affected. As shown in Figure 10, only a small fraction of each subframe must be transmitted even if the base station is not managing any traffic flow. In particular, a high cell DTX gain can be obtained using Multicast-broadcast single-frequency network (MBSFN) or extended cell DTX subframes instead of the normal unicast ones [10]. Unlike long term sleep solutions, cell DTX deactivates only some parts of the base station equipment, in order to ensure the immediate activation of the base station upon request. This approach hence allows the energy consumption to adapt to the variation of traffic in a very short time scale. Thanks to these characteristics, cell DTX is able to decrease the baseline power consumption of the base station to \(P_{s}=\delta P_{0}\), where \(0\leq\delta\leq 1\). The power consumption model presented in (2) then can be written as follows:  
\[P_{in}=N_{TRX}(\Delta_{p}P_{out}+(1-\delta)\eta P_{0}+\delta P_{0}). \tag{4}\]  
Here, \(\eta\) denotes the load of the base station, while \((1-\delta)\eta P_{0}\) represents the load dependent baseline power consumption, bound to the fast traffic adaptation property of cell DTX. Note that, in the case of \(\delta=1\), the base station does not have the DTX capability, and the power consumption model is the same as the one observed in Equation (2).