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Table 1 Classification and assessment of the different energy-efficient solutions reviewed

From: An overview and classification of research approaches in green wireless networks

Research Issue Energy savings/enhancements Consequences on planning and operation stage Challenges
CLA (1/3): cell breathing and switching-off schemes Energy saving around (25-50)% according to [25, 26, 28] Energy efficiency improves with smaller cells and denser deployments. However, it increases infrastructure costs Further work must be done in the mechanisms that permit the coordination of distributed/clustered mechanisms at large-scale networks. This necessity is even more critical considering that future cell-breathing mechanism will work in heterogeneous networks combining macro/femtocells and relays
The utilisation of centralised network cell-breathing mechanisms could limit scalability and introduce single point failure nodes
CLA (2/3): macro/femtocells networks Energy savings up to 60% combined with sleep modes [8] Provides granularity to service provision and coverage by tailoring the type of access device and resources to the real traffic needs.
Operating a macroBS during 1 year can cost 60000 Euros/year, whereas a femtoBS only 200 Euro/year [33].
Careful planning needed: too many femtoBS may imply overprovisioning (if no sleep mode used), increasing unnecessarily the energy consumption [8], i.e. OPEX [34]
There are open aspects on coordination and management mechanisms to handle the network heterogeneity: e.g. different levels of resources in function of BS type; different environments, i.e. outdoor, indoor; interference, spectrum management,
CLA (3/3): relays Results in [41] shows savings of around (5-20%) compared to classic single BS-MS point-to-point transmission The relays are useful as energy-efficient mechanism only if the power consumed by relaying is sufficiently low compared to a direct BS-MS transmission [41, 42] The need of protocols and mechanisms in order to have cooperative schemes of relaying is still an open topic to explore. Additionally, further work must be done in bidirectional relaying [18]
RRM, mechanisms and energy-efficient transmission There is a diversity of approaches in this domain. Therefore, it is difficult to give a representative figure With RRM algorithms and transmission techniques, it exist always the possibility of taking advantage of trade-offs. However, to find the optimal operation point is a complex task Develop new mechanisms considering all different trade-offs is not an easy matter. The task becomes more complex if we consider that future networks are going moving forward to cooperative schemes in these two domains
CR CR is a tool that works along with the RRM and the transmission mechanism. It is difficult to give a representative figure New technologies like SDR [53] are providing flexibility and low cost, which facilitates the introduction of CR in modern communication systems.
The CR is a very advantageous tool in environment where it exists conflict of interest and information is not shared among nodes
It must continue the work on flexibility of software and hardware in order to enhance the features that CR may bring to the energy-efficient approaches [53].
It exist open topics to study like the combination of backhauls protocol for information exchange with CR for having communications devices with more accurate information from outside world
Component approach The main concern is the PA, which nowadays (e.g. Doherty-Pre-distorted, Class AB) has PA eff. around 50% [22, 57]. The Class J amplifier is aimed to achieve PA eff. of 85-90% [14] the SMPA although theoretically could reach 100% according to [22], so far it has reached PA eff. of 55% [56] Having energy-efficient components is the basis before considering the other approaches for energy-efficient wireless networks Achieve higher components efficiencies. New prototypes like Class J Amplifier [14] and SMPA [56] are candidates for next generation PAs. It is critical for future BSs to have this kind of components.
Further work must be done in optical networks and photonics in order to reduce transmission losses at the backhaul and components interconnection