Battery life has always been a concern in the wireless communications industry. We would like to avoid the need for frequent recharge of a portable device and the possible limitation in communications and processing capability caused by a depleted energy source. At the same time, we would also like to be able to power a device by means of a battery which is small, light and reasonably inexpensive. Unfortunately, the rate at which battery performance improves (in terms of available energy per unit size or weight) is fairly slow, despite the great interest generated by the booming wireless business. Aside from major breakthroughs, it is doubtful that significant reductions of the battery size and weight can be expected in the near future. It does therefore make sense to look for alternative strategies to be employed towards the goal of power savings and energy management. In other words, instead of trying to improve the amount of energy which can be packed into a power source, one could try to build devices which can perform the same functionalities and provide the same services while reducing the overall power consumption.
On the one hand, better and better performance are being achieved by the components which are traditionally considered the most power-consuming portions of the device (e.g., the RF circuitry), and this brings other functions (e.g., signal processing) to play a significant role in the power budget. On the other hand, the emergence of various applications and the need to support them in a wireless setting may open up new possibilities for energy saving strategies. For example, delay tolerant traffic may provide some degree of flexibility in that one does not have to fight bad channel conditions by just sending more power, but rather may defer transmission to better times by using channel-dependent scheduling.
The recognition that all levels of the communications device and protocol stack may contribute to this energy saving approach by intelligently using some knowledge about the behavior of the processes, the traffic, the channel and the network, has led to a somewhat new approach to improving energy efficiency. Even the classic concept of ``battery life'' (essentially based on a circuit-switching mindset) is to be revisited, since the amount of data which can be reliably sent using a certain energy allocation may be more important than the actual time that the battery is ``alive.''
Surprisingly, despite the practical interest that these issues have to both wireless customers and service providers and manufacturers, it has not been until very recently that articles have appeared in the literature addressing these alternative power saving approaches. This special issue is meant to make a step in that direction, and has a twofold objective. First, trying to collect in a single magazine issue a number of contributions on the topic, we hope to draw attention on such an important area and on the related research efforts. Secondly, by presenting articles which address the same problem at very different levels of the communications system, we try to convey the message that 1) there is room for efficient energy management at all levels in the system design, and 2) even though researchers who study those different levels may belong to different communities, there might be some value in integrating the knowledge and in looking for collaborative solutions towards truly energy efficient schemes.
Specifically, the issue contains six contributions. The first article, by L.E. Larson, is focused on the radio-frequency part of a communications device, which represents a considerable fraction of the battery drain of the whole unit. The most significant technical challenges encountered by the RF circuit designer are summarized, and possible directions for improved efficiency are described. The second article, by T.H. Meng et al., addresses the issues which arise in the design of signal processing algorithms. Through careful analysis of the energy consumption associated to various operations and its dependence on quantities such as supply voltage and clock speed, some low-power design principles can be identified and applied to algorithms, architectures, circuit design and fabrication technology.
The article by E. Biglieri et al. discusses coding and modulation schemes by taking into account the availability of a limited-energy source. In particular, trade-offs at the physical layer are described and the relationship between Quality-of-Service requirements and power/energy constraints is addressed. The fourth article, by H. Woesner et al., focuses on the power-saving mechanisms which are specified in the two recent standards for Wireless Local Area Networks (IEEE 802.11 and HIPERLAN), and discusses the impact of the system parameters on the energy efficiency of the schemes, showing the trade-offs involved and the potential for energy optimization.
Opportunities for optimization of power allocation are discussed in the article by N. Bambos, who proposes the concept of ``power-sensitive'' network architecture, in which power control is revisited in a resource allocation/optimization perspective and is shown to have significant potential for effective network and traffic management. The last article in this issue, by J.R. Lorch and A.J. Smith, provides a comprehensive description of how the operating system in a portable computer environment can achieve energy savings by appropriate management of low-power modes or rescheduling of tasks.
As one of the objectives of the issue was to encourage cross-stimulation and mutual understanding, the scope of the articles has been intentionally kept at some tutorial level in order to allow the non-experts to grasp the essence of the problems to be solved and their possible impact on one's own field of interest. On the other hand, presentation of up-to-date information and of some original material has also been sought so that even the experts can enjoy reading these papers and find them useful and informative.
As the editor of this issue, I believe that the objectives I had set for this effort have been met, thanks to the excellent job done by the authors, who have shown considerable flexibility and cross-disciplinary understanding.
Michele Zorzi (S'89-M'95-SM'98) was born in Venice, Italy, in 1966.
He received the Laurea Degree and the Ph.D. in Electrical Engineering
from the University of Padova, Italy, in 1990 and 1994, respectively.
During the Academic Year 1992/93, he was on leave at the University of
California, San Diego (UCSD), attending graduate courses and doing
research on multiple access in mobile radio networks. In 1993, he
joined the faculty of the Dipartimento di Elettronica e Informazione,
Politecnico di Milano, Italy. He is currently with the Center for
Wireless Communications at UCSD.
His present research interests involve performance evaluation in mobile
communications systems, and random access in mobile radio networks.
Dr. Zorzi is a Member of AEI, and currently serves on the Editorial
Boards of the IEEE Personal Communications Magazine and of
the ACM/URSI/Baltzer Journal of Wireless Networks. He is also
co-guest editor for the special issue on ``Multi-media Network Radios''
of the IEEE Journal on Selected Areas in Communications.
E-mail: zorzi@cwc.ucsd.edu
Michele Zorzi
Wed May 6 17:15:48 PDT 1998