“Green ICT” means Information and Communications Technology friendly for the globe. To stop global warming and climate change, ICT, with low emission of carbon dioxide thus low consumption of energy, is required. When there was no constraint of low energy consumption in communications, one solution for making us happier and more comfortable from the information-transmission perspective was to have a “stupid network” in which communication entities can handle just higher data transmission rates with no intelligence. However, once the constraint is imposed, an “intelligent network” is required as another solution in which the entities can intelligently reduce their energy consumption as much as possible while satisfying users’ needs.
Let us pay attention to the link from the relay stations to the base station. When the relay stations forward the information from the mobile station to the base stations, in a conventional “incoherent” transmission, they transmit their signals with no care about their amplitudes and phases, so the power of the signals combined at the base station is just in proportion to the number of relay stations, namely, K. On the other hand, in a coherent transmission, they transmit their signals so that the signals combined at the base station can be coherently added in “amplitude,” so the combined signal power is in proportion to the squared number of relay stations, namely, K2. This fact clearly shows that the coherent transmission has the power gain of K; in other words, to have the same received power at the base station, as compared with the incoherent transmission, the coherent transmission can reduce the transmission power at each station by a factor of K; therefore, it can significantly reduce the energy consumption.
Coherent transmission is categorized into two classes, namely regenerative and non-regenerative. The pre-processing at relay stations which once demodulates the received signal before forwarding is called “regenerative transmission,” whereas the one which does not demodulate (just amplifies and forwards) the received signal is called “non-regenerative.”
Coherent transmission as a “Green Wireless” technique for stopping global warming and climate change can therefore be used to achieve not only lower energy consumption but also higher spectral utilization efficiency as compared with current mobile communication systems.
In wireless communication, a technique to reduce energy consumption is also required, which is called “Green Wireless.” Emission of wireless signal from antenna is dominant in energy consumption for wireless devices, so a shortcut for reducing energy consumption is to reduce the power consumption in wireless signal transmission. In principle, the power required for wireless signal emission is proportional to the distance between a pair of communicating transceivers, so this can be achieved by shortening the distance between them. Therefore, in a cellular system, one solution for reducing energy consumption is to put a lot of relay stations to forward information between a base station and mobile stations in each cell.
The “coherent transmission” concept, which can achieve much less energy consumption, has been proposed. The figure below shows the concept of a coherent transmission system which is composed of a mobile station, K relay stations and a base station.
