Mobile phones have been undergoing a breathtaking evolution over the last decade starting from simple mobile phones with only voice services towards the transition of smart phones offering Internet access, localization information and even more. It seems that there are simply no limitations for mobile devices getting smaller, offering higher data rates, brighter displays.
Unfortunately this assumption is not correct. The limiting factor is known, namely the energy and power consumption of mobile devices being battery driven. As the complexity within the mobile device is increasing dramatically due to new services such as GPS modules, digital photo cameras, mp3 players and others, the improvement of battery capacity is quite moderate. The increase in complexity of the mobile device is related to the fact that mobile handset vendors need new services to market their products and therefore exploit all the computational power of the given hardware, which is following Moore’s law. Even the wireless air interfaces are getting more and more complex starting from simple TDMA systems towards the planned OFDMA/MIMO systems for the 4G wireless communication systems. The increased complexity has two key impacts. First, the enormous power consumption will lead to heating problems. Thus, the mobile device cannot cope with the heat using passive cooling anymore and active cooling would be necessary.
Secondly, the increased energy consumption would lead to lower stand-by times. Those stand-by times are limiting the time a customer can use its mobile device, which in turn makes the customer as well as the network operator unsatisfied. The customer cannot use any service and the network operator does not make any revenue. In other words, the degree on mobility for the customer depends on mobile device with low recharge cycles.
As 4G mobile phones will be even more complex than 3G phones, the question arises how the battery should cope with this new challenge. The increase in battery consumption by mobile phones is the reason that public battery chargers at airport and hotels are getting more diffused. Those public chargers have several cable plugs for different mobile phones to charge the device, but the user needs to pay several dollars for the service – energy has a price!
Besides the cellular air interfaces, mobile devices are equipped with several local area network technologies such as Bluetooth or WLAN 802.11. The different heterogeneous wireless technologies lead to energy savings if they are used properly. By the example of cooperative wireless networks, energy saving potentials are derived for different heterogeneous wireless technology combination. In contrast to cellular systems, where the mobile devices are only connected to the base station, in cooperative wireless networks, the mobile device, in addition to the cellular communication, will establish short range links to neighboring mobile devices within its proximity as illustrated in the figure here.
When cooperative clusters are formed, also referred to as wireless grid, they can offer each participating mobile device a better performance in terms of data rate, delay, robustness, security, and energy consumption in contrast to any stand alone device. The improved data rate, delay, and robustness come obviously by the accumulated cellular links with its inherent diversity. In a nutshell, as long as the energy per bit ratio is better in the short range connection than the cellular link, the cooperation will improve the energy consumption. This is the case for all available and future wireless technologies as the path loss is much smaller in short range communication (around 10 m) than cellular communication (several 100 m).
