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This section discusses several different possible implementations for achieving energy savings in wireless communications, as well as the following paramount topics regarding the wireless energy efficiency improvements:
Globally Resource-optimised and Energy-Efficient Networks – Green is a marketing term for the improvement of energy efficiency and energy independence of telecommunications without any notable impact on the QoS.
TANGO aims at the optimisation of the radio resources as well as the energy efficiency within, without impeding the coverage using the Green -framework.
Evaluations of the different wireless network cell types.
The electronic steering of the antenna array with the purpose of creating higher directionality and thus sensibility to a specific sector while minimising the directionality to another, unwanted sector.
The sharing of a computational problem on a wireless network of heterogenous and independent radio nodes.
BS sleep mode refers to switching the BS’s radio transmissions off whenever possible.
The BS’s sleep- and wake-up transients are called cell wilting and cell blossoming.
Cell zooming is a technique at the network layer that changes the cell size by adjusting the transmit power of the control signals.
Commonly referred to as MIMO-systems. The multiple antennas can be used to increase the data rates through multiplexing, or to improve performance through diversity (Goldsmith 2005).
Relay is a stand-alone device placed within the range of a wireless router, or an AP
or a BS. It serves as a two-way relay for wireless signals between the AP and the
remote clients unable otherwise to connect to the AP by themselves.
WMNs are rapidly deployable, robust and low cost. The end-to-end communication in WMNs can hop through multiple WMN nodes.
More details of networks of composite radio environments than can consist of multiple different RANs, such as Wi-Fi, 3G, DVB.
Power control can be a useful tool in ensuring energy efficient spatial reuse, while minimising the energy consumption. These functionalities are quite similar to those of cell zooming. Cell zooming is nevertheless quite different from power control in many ways. Whereas power control focuses on the link-level performance as well as transmit power consumption, cell zooming focuses on the performance and energy consumption reduction of the network as a whole. Power control does not actively change the cell size, whereas cell zooming does, by adjusting the transmit power of control signals. Various power control topics (fixed and variable signal-to interference- ratio, beamforming and scheduling algorithms) from the perspective of energy efficiency were analysed by researchers.
The energy efficiency at the wireless network infrastructure level can be divided into three levels:
Strictly from the environmental point of view, the goal of green radio communications is to lower the CO2 emissions. This can be achieved for instance by using renewable energy sources (e.g. solar or wind power) to supplement the energy received from the national power grid. Typically, the power generated to the national power grid is no produced from 100% renewable sources. Also, in the cold climates, the cold air can be used to aid the cooling of the BS equipment.
By adopting different sized cells in the wireless network, the network can adapt to
the mobility and to the increase of the users more efficiently and flexibly. Recent
wireless technologies, such as femtocells, have met this demand with the capability to extend the cell coverage more easily into densely built and congested city buildings. Femtocells operate with much less transmission power compared to a macrocell, which is also why their BSs consume less energy.
To take advantage of the spatial and temporal traffic load fluctuations by switching off some of the existing wireless resources when the traffic load is light, is known as dynamic network planning. Shutting down some of the resources can lead to increase in the transmission power of the active BSs in order to increase their cell radius to provide radio coverage for the shut down cells. This can also result in coverage holes if the maximum allowed transmission power of the remaining active BSs still cannot create necessary radio coverage for the shut down cells. This also leaves the network vulnerable to service disruptions as well as to intercell interference in these areas. The modulation schemes used in WCDMA/HSPA and LTE characteristically have highly volatile signal envelopes, where the PAPR1 (which is also known as the crest factor) can exceed over 10 dB. To boost the energy efficiency, signal conditioning algorithms, such as crest factor reduction (CFR) for decreasing the PAPR and digital pre-distortion to increase the PA’s linearity, must be utilised to enable the PA’s operation closer to saturation point Non-Orthogonal CDMA has been used in 2G and 3G systems like WCDMA, CDMA2000, and CDMA, and it is the dominant multiple access technique for present 3G wireless networks.
Compared with OFDMA2, CDMA excels in cancelling the intercell interference, it
is also resilient to channel fading, but not scalable for high data rate transmission in asynchronous transmission environments, and also its bandwidth is much larger than the data rate used to suppress the interference. In orthogonal modulations, the signals from different MTs are orthogonal to each other, and their cross correlation is zero. OFDMA is good especially for high data rates, but not efficient for inter-cell interference and low transmission power scenarios .
In order to improve the energy efficiency, the BS’s front end’s consumed power should be able to scale as much as possible to meet the amount of served traffic. To achieve this, for instance load adaptive CFR along with adaptive power supply (for variable input power) in the PA should be utilised. High energy efficiency power management is required for reconfigurable circuits as their key elements.
A low power front-end for different levels of transmit power with adjustable performance has been proposed in the work of Debaillie et al. (2006). In the article a solution where the driver and the PA were both digitally controlled and thus flexible in terms of their output power, linearity,
and DC power consumption, was presented. This type of flexibility is appealing, and thus desirable to be extended to cover the high power transceivers found in macrocell BSs as well. Techniques like DVS and frequency scaling are both able to adapt to the voltage and clock frequency of the platform, depending on the momentary load. This allows for scaling of the power consumed in the digital chain along with traffic.
The signal processing also accounts for a significant portion of the overall wireless network power consumption. Typically the extremely efficient transmission techniques also have a tendency to demand more intricate computations with a corresponding increase of processing power. The gains from advanced transmission techniques on the energy efficiency might thus get outweighed by the negative side-effects in other parts of the system – especially in small cells, where the wireless transmission power accounts for only a few percent of that of a macrocell. This is also why the baseband signal processing might end up dominating the overall energy consumption.
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