Timeliness-aware Anti-discrimination MAC Scheme for Wireless Passive Sensor Networks

Abstract

In a wireless passive sensor network, radio frequency (RF) sources wirelessly supply energy to sensor nodes. Against theoretical expectations of an abundance of energy, a wireless passive sensor network suffers from a scarcity of energy in practice. Such a scarcity compels the network to adopt a contending-type medium access control (MAC) scheme, which inevitably brings about a packet collision, and hence a capture phenomenon. The problem is that the packet collision may incur an intolerable delay in packet delivery, and furthermore, the capture phenomenon can cause discrimination in packet delivery against sensor nodes that are far from the sink node. In this paper, we thus confine our attention to a class of contending-type MAC schemes, rooted in framed and slotted ALOHA, which guarantee a degree of timeliness in packet delivery. In the timeliness-aware class, we then seek an anti-discrimination MAC scheme that prevents discrimination against distant sensor nodes by providing every sensor node with an equal opportunity to succeed in packet delivery, i.e., in the substantive sense of equality. In case studies, we construct an antidiscrimination MAC scheme in an explicit form. Also, we develop an exact expression of nodal throughput that can be attained by an anti-discrimination MAC scheme. Using the throughput formula, we then find an optimal anti-discrimination MAC scheme that attains maximum total throughput among all the anti-discrimination MAC schemes. Numerical examples corroborate that an optimal anti-discrimination MAC scheme not only always achieves stronger nodal evenness in throughput but also occasionally attains higher total throughput than a conventional MAC scheme.