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

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