Feng Lu, Geoffrey M. Voelker, and Alex C. Snoeren, University of California, San Diego
Abstract:
As manufacturers continue to improve the energy efficiency of battery-powered wireless devices, WiFi has become one of—if not the—most significant power draws. Hence, modern devices fastidiously manage their radios, shifting into low-power listening or sleep states whenever possible. The fundamental limitation with this approach, however, is that the radio is incapable of transmitting or receiving unless it is fully powered. Unfortunately, applications found on today’s wireless devices often require frequent access to the channel.
We observe, however, that many of these same applications have relatively low bandwidth requirements. Leveraging the inherent sparsity in Direct Sequence Spread Spectrum (DSSS) modulation, we propose a transceiver design based on compressive sensing that allows WiFi devices to operate their radios at lower clock rates when receiving and transmitting at low bit rates, thus consuming less power. We have implemented our 802.11b-based design in a software radio platform, and show that it seamlessly interacts with existing WiFi deployments. Our prototype remains fully functional when the clock rate is reduced by a factor of five, potentially reducing power consumption by over 30%.
USENIX is committed to Open Access to the research presented at our events. Papers and proceedings are freely available to everyone once the event begins. Any video, audio, and/or slides that are posted after the event are also free and open to everyone. Support USENIX and our commitment to Open Access.
BibTeX
@inproceedings {180309,
author = {Feng Lu and Geoffrey M. Voelker and Alex C. Snoeren},
title = {{SloMo}: Downclocking {WiFi} Communication},
booktitle = {10th USENIX Symposium on Networked Systems Design and Implementation (NSDI 13)},
year = {2013},
isbn = {978-1-931971-00-3},
address = {Lombard, IL},
pages = {255--258},
url = {https://www.usenix.org/conference/nsdi13/technical-sessions/presentation/lu},
publisher = {USENIX Association},
month = apr
}
SloMo is an interesting approach to further reduce smartphone energy consumption by operating the WiFi chipset at a (much) lower clock rate. The work is related to a recent work, E-MiLi, which saves energy by ``down clocking" the WiFi radio during idle listeningperiods. Different from E-MiLi, SloMo applies the concept of down-clocking all the way to the entire WiFi operation, including both data transmission and reception. While lowering clock rate during data transmission might not sound beneficial as during reception, theauthors argue that it is necessary in practice because the granularity of WiFi channel access is much faster than clock rate adaptation, so transmission and reception must operate at the same clock rate. With this motivation in mind, the authors proposed two neat ideas to enable down-clocking on 802.11b: compressive sensing during packet receptionto signal down-sampling during transmission. Both leverage the fact that excess channel capacity offers plenty of room to reduce clock rate without affecting network performance. The authors implemented a prototype on the Sora testbed, where detailed experiments show that slowing the clock by a factor of 5 does not lead to visible performance differences. Using network traces on 7 popular applications and existing power models, the authors also show that SloMo achieves 37.5% improvement over WiFi's current power saving method, and 10% improvement over E-MiLi.
This work is perhaps the first to show that WiFi chips can be fully down-clocked in all their operations. The authors have designed the SloMo system carefully to address several practical issues, including symbol synchronization and interaction with legacy devices. While the current design only applies to 802.11b with direct-sequence spread spectrum (DSSS) modulation, it would be very interesting to see whether this can be done for OFDM based wireless technologies, which is widely used in many recent wireless technologies, including WiFi itself.
connect with us