In wireless access, whether in cellular networks or general ad-hoc networks, energy-efficiency is of paramount importance. Especially when energy is finite and non-renewable, the performance objectives (throughput, bit-volume, lifetime, delay, etc.) are intertwined and involve trade-offs that have not been clearly developed or understood yet. To obtain a good understanding of these trade-offs, we focus first on a simple version of wireless access. That is, we initially strip out all the complexities that multi-hop operation introduces (such as routing and coordination of transmissions over multiple hops for interference avoidance) and concentrate on a single cell with one receiver and multiple transmitters and revisit the well-studied issue of multiple access in this environment from the energy-efficiency perspective.
We consider a single communication channel shared by all uplink users and look at the basic form of random access that was proven to have the best stable throughput properties, namely the one based on conflict resolution. We focus on random, rather than scheduled, access because at some level (perhaps at the reservation subchannel level) some form of random access will be unavoidable in any wireless network.
The main idea in our paper is to base the criterion for splitting the number of interfering users not on ID or arrival time, as was done before, but on the amount of residual energy left at each node. This novel criterion produces surprising performance gains.
The premise of using residual battery energies as tree-splitting criteria for conflict resolution relies on accurate and precise monitoring of battery capacities by individual nodes. Advances in portable battery technology provide practical solutions for accurate measurements of battery energies, which are also theoretically supported by discrete-time stochastic battery models for high level power and accurate battery-lifetime estimation.
We consider both the simple classical collision channel model as well as the signal-to-interference-plus-noise-ratio (SINR)-based physical model for packet collision. We then allow mobility (which has significant effect on performance of the SINR-based model) and then, after showing the performance gain achieved, we outline an extension of our approach to multi-hop ad-hoc networks. We consider both the case of finite, non-renewable energy as well as the case of unlimited energy in which, however we require energy-efficient operation.
In this paper, we introduce several energy-efficiency concepts using conflict resolution algorithms as reliable means for random access. However, our objectives fall short of proposing a medium-access-control (MAC) protocol ready to use in wireless ad-hoc networks. We believe that energy-based conflict resolution promises energy-efficient solutions for random access and should be studied in detail before we can clearly understand how to utilize these ideas in a full-fledged MAC protocol.
The paper is organized as follows. Section 2 is devoted to the energy-limited case for a single cell with a finite number of nodes that have uniformly distributed amounts of initial battery energy. We describe our residual-energy-based tree-splitting (REBS) algorithm and compare its performance to the first-come-first-served (FCFS) algorithm as well as to the purely random tree-splitting (RS) algorithm. In this section, we also explore the dependence of performance measures on energy interval partitioning methods, battery energy measurement errors as well as feedback errors. In Section 3, we revise the REBS algorithm to make it applicable to the case of nodes with equal amounts of initial battery energy. We propose a hybrid approach that employs a quaternary interval partitioning method to start with FCFS algorithm and gradually switch to REBS algorithm. Section 4 introduces a new energy-efficiency criterion for conflict resolution in systems without hard energy constraints and describes a distance-based tree-splitting (DBS) algorithm. In Section 5, we analyze the interactions between collision resolution processes in multi-receiver systems and develop a partial decoupling method based on load-balancing among contending nodes. This is followed by the extension of our energy-based approach to multi-hop ad-hoc networks and a brief study of routing effects on the performance of the CRA’s in Section 6. Finally, we draw some conclusions and outline thoughts for future work in Section 7.
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