Energy efficiency, which directly affects battery life and portability, is perhaps the single most important design metric in hand-held computing devices capable of mobile networking over wireless radio links. By virtue of their being relatively thin clients, a high fraction of the power consumption in portable wireless computing devices is accounted for by the transport of packet data over the wireless link [Stemm96]. In particular, the error con-. trol strategy (e.g. convolutional and block channel coding for forward error correction (FBC), ARQ protocols, hybrids) used for wireless link data transport has a direct impact on battery power consumption. Error control has traditionally been studied by channel coding researchers from the perspective of selecting an error control scheme to achieve a desired level of radio channel performance. We instead study the problem of error control from a perspective more relevant to battery operated devices: the amount of battery energy consumed to transmit bits across a wireless link. This includes both the physical transmission of useful and redundancy data, as well as the computation of the error control redundancy. We first describe a novel error control where the most battery energy efficient hybrid combination of an appropriate FBC code and ABQ protocol is chosen, and adapted over time, for each stream (ATM virtual circuit or IP/RSVP flow). Next, we present analysis and simulation results to guide the selection and adaptation of the most energy efficient error control scheme as a function of quality of service, packet size, and channel state.
Current wireless terminals are limited to voice terminals such as cellular and PCS phones, and traditional laptop computers and PDAs configured with wireless modems and network interface cards. However, the current wireless networks, which are by and large wireless extensions of the circuit-switched voice networks, are being replaced by emerging wireless networking technologies that are intrinsically designed to support packet data and multimedia services. This will lead to novel networked applications and services, which in turn will require wireless terminals capable of exploiting these services. What shape will these next-generation wireless terminals take? The answer, based on the much talked about notion of "convergence," would appear to be a marriage of the laptop or PC with a wireless phone in the same package, leading to terminals such as the Nokia 9000 [1] or Bell Laboratories' wireless handset [2]. We argue that such a complex one-size-fits-all voice-data integrated wireless terminal will, at best, be a point solution. Rather, with the availability of cheap radio and computing hardware and ubiquitous low-cost indoor and outdoor wireless networking infrastructures, the capability to access a wireless network will soon be embedded into a variety of devices, gadgets, and appliances with specialized functions in our environment. In this article we describe the technological challenges and identify potential solutions in designing these myriad future "wireless terminals" that will handle diverse data types, have limited battery resources, and operate in environments that are unplanned, insecure, and time-varying, and have context-dependent services.
The quality of wireless links suffers from timevarying channel degradations such as interference, flat-fading, and frequency-selective fading. Current radios are limited in their ability to adapt to these channel variations because they are designed with fixed values for most system parameters such as frame length, error control, and processing gain. The values for these parameters are usually a compromise between the requirements for worst-case channel conditions and the need for low implementation cost. Therefore, in benign channel conditions these commercial radios can consume more battery energy than needed to maintain a desired link quality, while in a severely degraded channel they can consume energy without providing any quality-of-service (QoS). While techniques for adapting radio parameters to channel variations have been studied to improve link performance, in this paper they are applied to minimize battery energy. Specifically, an adaptive radio is being designed that adapts the frame length, error control, processing gain, and equalization to different channel conditions, while minimizing battery energy consumption. Experimental measurements and simulation results are presented in this paper to illustrate the adaptive radio's energy savings.
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