Elsevier

Computer Communications

Volume 117, February 2018, Pages 164-174
Computer Communications

On the energy efficiency of rate and transmission power control in 802.11

https://doi.org/10.1016/j.comcom.2017.07.002Get rights and content

Abstract

Rate adaptation and transmission power control in 802.11 WLANs have received a lot of attention from the research community, with most of the proposals aiming at maximising throughput based on network conditions. Considering energy consumption, an implicit assumption is that optimality in throughput implies optimality in energy efficiency, but this assumption has been recently put into question. In this paper, we address via analysis, simulation and experimentation the relation between throughput performance and energy efficiency in multi-rate 802.11 scenarios. We demonstrate the trade-off between these performance figures, confirming that they may not be simultaneously optimised, and analyse their sensitivity towards the energy consumption parameters of the device. We analyse this trade-off in existing rate adaptation with transmission power control algorithms, and discuss how to design novel schemes taking energy consumption into account.

Introduction

In recent years, along with the growth in mobile data applications and the corresponding traffic volume demand, we have witnessed an increased attention towards “green operation” of networks, which is required to support a sustainable growth of the communication infrastructures. For the case of wireless communications, there is the added motivation of a limited energy supply (i.e., batteries), which has triggered a relatively large amount of work on energy efficiency [1]. It turns out, though, that energy efficiency and performance do not necessarily come hand in hand, as some previous research has pointed out [2], [3], and that a criterion may be required to set a proper balance between them.

This paper is devoted to the problem of rate adaptation (RA) and transmission power control (TPC) in 802.11 WLANs from the energy consumption’s perspective. RA algorithms are responsible for selecting the most appropriate modulation and coding scheme (MCS) to use, given an estimation of the link conditions, and have received a vast amount of attention from the research community (see e.g. [4], [5] and references therein). In general, the challenge lies in distinguishing between those loses due to collisions and those due to poor radio conditions, because they should trigger different reactions. In addition, the performance figure to optimise is commonly the throughput or a related one such as, e.g., the time required to deliver a frame.

On the other hand, network densification is becoming a common tool to provide better coverage and capacity. However, densification brings new problems, especially for 802.11, given the limited amount of orthogonal channels available, which leads to performance and reliability issues due to RF interference. In consequence, some RA schemes also incorporate TPC, which tries to minimise the transmission power (TXP) with the purpose of reducing interference between nearby networks. As in the case of “vanilla” RA, the main performance figure to optimise is also throughput.

It is generally assumed that optimality in terms of throughput also implies optimality in terms of energy efficiency. However, some previous work [6], [7] has shown that throughput maximisation does not result in energy efficiency maximisation, at least for 802.11n. However, we still lack a proper understanding of the causes behind this “non-duality”, as it may be caused by the specific design of the algorithms studied, the extra consumption caused by the complexity of MIMO techniques, or any other reason. In fact, it could be an inherent trade-off given by the power consumption characteristics of 802.11 interfaces, and, if so, RA-TPC techniques should not be agnostic to this case.

This work tackles the latter question from a formal standpoint. A question which, to the best of the authors’ knowledge, has never been addressed in the literature. For this purpose, and with the aim of isolating the variables of interest, we present a joint goodput (i.e., the throughtput delivered on top of 802.11) and energy consumption model for single 802.11 spatial streams in the absence of interfering traffic. Packet losses occur due to poor channel conditions and RA-TPC can tune only two variables: MCS and TXP.

Building on this model, we provide the following contributions: (i) we demonstrate through an extensive numerical evaluation that energy consumption and throughput performance are different optimisation objectives in 802.11, and not only an effect of MIMO or certain algorithms’ suboptimalities; (ii) we analyse the relative impact of each energy consumption component on the resulting performance of RA-TPC, which serves to identify the critical factors to consider for the design of RA-TPC algorithms; (iii) we experimentally validate our numerical results; and (iv) we assess the performance of several representative RA-TPC algorithms from the energy consumption’s perspective.

The rest of this paper is organised as follows. In Section 2, we develop the theoretical framework: a joint goodput-energy model built around separate previous models. In Section 3, we provide a detailed analysis of the trade-off between energy efficiency and maximum goodput, including a discussion of the role of the different energy parameters involved. We support our numerical analysis with experimental results in Section 4. Section 5 explores the performance of RA-TPC algorithms from the energy consumption’s perspective. Finally, Section 6 summarises the paper.

Section snippets

Joint goodput-energy model

In this section, we develop a joint goodput-energy model for a single 802.11 spatial stream and the absence of interfering traffic. It is based on previous studies about goodput and energy consumption of wireless devices. As stated in the introduction, the aim of this model is the isolation of the relevant variables (MCS and TXP) to let us delve in the relationship between goodput and energy consumption optimality in the absence of other effects such as collisions or MIMO.

Beyond this primary

Numerical results

Building on the joint model presented in the previous section, here we explore the relationship between optimal goodput and energy efficiency in 802.11a. More specifically, our objective is to understand the behaviour of the energy efficiency of a single spatial stream as the MCS and TXP change following our model to meet the optimal goodput.

Experimental validation

This section is devoted to experimentally validate the results from the numerical analysis and, therefore, the resulting conclusions. To this aim, we describe our experimental setup and the validation procedure, first specifying the methodology and then the results achieved.

On the performance of RA-TPC algorithms

So far, we have demonstrated through numerical analysis, and validated experimentally, the existence of a trade-off between two competing performance figures, namely, goodput and energy efficiency. This issue arises even for a single spatial stream in absence of interference. Furthermore, we have discussed in Section 3.6 some ideas about the kind of mechanisms that energy-aware RA-TPC algorithms may incorporate, to leverage the behaviour that we have identified in our analysis in these

Conclusions

In this paper, we have revisited 802.11 rate adaptation and transmission power control by taking energy consumption into account. While some previous studies pointed out that MIMO rate adaptation is not energy efficient, we have demonstrated through numerical analysis that, even for single spatial streams without interfering traffic, energy consumption and throughput performance are different optimisation objectives. Furthermore, we have validated our results via experimentation.

Our findings

Acknowledgements

This work has been performed in the framework of the H2020-ICT-2014-2 projects 5GNORMA (grant agreement no. 671584) and Flex5Gware (grant agreement no. 671563). The authors would like to acknowledge the contributions of their colleagues. This information reflects the consortium’s view, but the consortium is not liable for any use that may be made of any of the information contained therein.

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