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Part 2 introduces energy saving techniques for wireless communication, and proposes a framework for energy-aware cross-layer radio management.
Introduction
With the advent of new nomadic devices (SmartPhones, PDAs, laptops) and multimedia applications, the requirements for wireless connections are shifting from simple data rate increases to complex and heterogeneous Quality of Service (QoS). And as these devices are battery-powered, the performance requirements come with a severe constraint on energy consumption. This results in a continuously growing gap between the available energy—constrained by the battery technology—and the steeply increasing energy requirements of emerging radio systems. Technology scaling, platform improvements and circuit design progress are not sufficient to bridge this gap; there is a need for holistic system-level strategies. In this article, we advocate a two-step approach. First, energy-scalability is introduced in the design of the radios. Secondly, intelligent run-time control is introduced to enable low power operation, by exploiting this scalability as well as the dynamics in the system. As a result, software defined radios (SDR) can be realized achieving a power consumption which can be comparable with dedicated radio implementations.
Enable flexibility through design
To enable the translation of functional flexibility into energy scalability, the reconfigurable radio (algorithms, architectures, components and circuits) should first be designed accordingly.
For the reconfigurable digital baseband engine, one has to carefully tradeoff flexibility and energy efficiency: flexibility should only be introduced where its impact on the total average power is sufficiently low or where it offers a broad range of control options that can be exploited effectively later in the control step (targeted flexibility). The required sub-functions of the wireless modem should be designed according to their nature (i.e., control or data processing) and flexibility/energy efficiency requirements. This calls for heterogeneous multi-processor system-on-chip (MPSoC) platforms.
For the reconfigurable analog front-end, architectures and circuits should be designed for a broad range of requirements in carrier frequency, channel bandwidth and noise performance with minimal penalty in power consumption, while also offering energy scalability. One way is to equip all building blocks in the RF front-end with configuration "knobs" that allow them to adjust their performance to the requirements of the considered standards, but also to scale their energy consumption to the actual requirements.
Possible designs for energy-scalable reconfigurable radio architecture are discussed starting on page 3.
Exploit flexibility through control
To exploit flexibility for saving energy, it is mandatory to control the reconfigurable radio system as function of the operation conditions. The key observation here is that wireless communications systems typically face very dynamic conditions (in terms of propagation environment and of application requirements). By carefully adapting the system to these dynamics at run-time, capitalizing on the energy scalability discussed above, much energy can be saved compared to conventional design. This problem has to be addressed from a cross-layer perspective, as measuring performance requires taking into account the characteristics of the protocol stack, whereas optimizing energy expenditure assumes detailed knowledge of the low-level radio hardware.
An energy-aware cross-layer radio management framework is proposed in Part 2 of this article series.
Exploit spectrum
Besides the energy constraint, spectrum is also becoming a major resource bottleneck, due to the accelerated deployment of broadband personal communication and the continuously increasing demand for higher data rates. New paradigms for efficiently exploiting the spectrum are clearly needed. A current trend is the evolution towards dynamic and open access to spectrum, motivated by the under-utilization of many licensed frequency bands. This has led to the concept of cognitive radio (CR) [20], [21]. Such CRs will extend the flexibility enabled by new reconfigurable radio architectures, by adding intelligent control solutions. Additional requirements will make energy-efficient implementations even more challenging: such CRs will have to couple energy savings and efficient spectrum use.
The design framework introduced above is clearly paving the way towards this challenging goal, as further explained in part 2.
Figure 1. IMEC's reconfigurable Scaldio chip – SDR analogue front-end.
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