Enabling Technologies for Green Networking

As an integrated and comprehensive research area, green networking covers all layers of the protocol stack or the network architectures. The target of green networking is to balance the fundamental trade-offs between the energy saving aspect and the overall system per- formance [41]. To address the challenge, it is essential to consider various techniques such as energy-efficient BS deployment, energy-efficient protocol and architecture design, oppor- tunistic network access, energy-efficient RRM, cross-layer design, cognitive and cooperative network deployment, HetSNet deployment, smart grids [38, 42]. In the following context, the green technologies related to the scope of the thesis are introduced.

Energy Saving Potential in BSs

To meet the rapidly growing data traffic, the number of BSs has increased tremendously in the past years and will keep growing continuously. As aforementioned, the power consumption of BSs is the main contributor to the total operating power consumption of a network. This

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accounts for the significant increase of energy cost or OPEX of mobile operators so that it is crucial to improve the energy efficiency of BSs. This aspect relates to the parameters of the power consumption models in Section 3.2.7. The main approaches for improvements include the minimisation of the hardware energy consumption of BSs and the application of advanced power management algorithms, such as sleep mode control algorithms. Renewable energy resources can also be adopted in some scenarios if they are financially worth investing. One of the energy saving approaches at the BS side is to improve the energy efficiency of BS hardware modules, e.g. to make power consumption more linear to the BS load. Fig. 2.12 presents a typical structure of a BS site. A BS contains power amplifiers (PAs), RF modules, feeders and the correspondingly served antenna interfaces (AIs), which are all connected to a BBU. They are powered by a DC-DC power supply and the whole site is powered by a main supply (AC-DC unit). There is a cooling system sometimes depending on the BS type for the purpose of climate control. Among all the hardware modules, PAs account for most of the total power consumption, which is dependent on the frequency band, the used modulation scheme and the operating environment [43]. Most of the power consumed in the currently adopted PAs is wasted as heat, which requires an additional cooling system and therefore costs more energy. The efficiency of PAs ranges from 5% to 20%. It is expected to reach around 70% based on a switch-mode PA architecture and more efficient modulation schemes [44], which are promising approaches to enhance the overall hardware energy efficiency. It is worth mentioning that there is no cooling system for small-size and low-power BSs (such as FBSs). They are cooled down by the natural environment, which is an advantage of small cell BSs in terms of power consumption.

Another approach is to use network level power management schemes. Specifically, sleep modes can be designed to exploit the dynamic network traffic loads varying in space and time. By adopting transceiver sleep modes, one or multiple transceivers of a BS can be switched off when it is not necessary to have so many transceivers serving a relatively low- level local traffic load. If the traffic load of a cell is even lower, it is possible to switch the whole BS to a sleep mode without its radio function activated and its coverage area can be compensated for by its neighbour BSs. The power management schemes can either be realised in a centralised or a distributed cooperative way. Other power management schemes that control component power consumption at various load levels can also be implemented. More details about on-demand radio resource provisioning and sleep mode operation are presented in the later context.

42 Chapter 2. Literature Review Main supply (AC-DC unit) Cooling DC-DC power supply BBU RF RF RF PA PA PA AI Feeder AI Feeder AI Feeder

Figure 2.12: Hardware modules of a typical base station site [7].

Energy-efficient Network Architecuture

As introduced before, the deployment of HetSNets is an effective way to deliver ultra-high capacity density wireless services. From the perspective of power consumption, HetSNets also contribute to energy saving at a network level. In network densification, the deployment of high power and big size MBSs is both cost-inefficient and energy-inefficient. Instead, small cell BSs, as low power entities, can reduce the average operating power consumption due to their sizes while providing the same amount of radio resources. For example, a typical FBS only draws 5 W compared with 5 kW needed for a MBS [42]. However, it is over-qualified to deploy additional small cell BSs even at areas with low traffic demands, e.g. rural areas. In this scenario, MBSs or medium power BSs can be chosen to cover wide areas and ensure the availability of wireless services. Hence, the deployment of HetSNets with mixed BSs becomes a promising solution to boosting capacity while achieving energy efficiency [45, 46]. As a result, it adds more complexity into cell planning, where the strategy of choosing appropriate types of BSs for different locations becomes a vital part.

Considering the SCN architecture of a single layer of a HetSNet, C-RAN is given much expectation for the reduction in network power consumption. As a kind of small cell BS, RRHs also feature the low operating power and low transmit power. Moreover, they can also be fully switched off when there is a low local traffic level by more efficient and optimised centralised schemes. Efficient cooling systems can be assembled at centralised BBU pools to save energy [47]. The BBU pools can also be implemented with a management scheme of the baseband processing capability, where the total number of active processing units can be

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reduced for energy saving when the network load is low [48]. The architecture of C-RAN is considered with schemes proposed in Chapter 7.