Robust Relay Selection for Large-Scale Energy-Harvesting IoT Networks
We consider the relay selection problem in largescale energy-harvesting (EH) networks. It is known that if channel state information (CSI) is available at EH relays, a diversity order equal to the number of relays can be obtained, however at the penalty of a feedback overhead (necessary to obtain accurate CSI) which is not suitable for energy-limited devices intended e:g: for internet-of-things (IoT) applications. In this paper, we therefore propose a new EH relay selection scheme which is based on the residual energy at each relay’s battery, and on information on the distribution of the channels between relays and the destination. The method thus minimizes both the outage probability and the feedback cost. Where previous work relay selection based on channel distribution information (CDI) consider only small-scale fading distribution, we employ a stochastic geometry approach to consider jointly the geometrical distribution (i:e:, large-scale fading) and small-scale fading yielding a simple relay selection criterion that furthermore utilizes only rough information on the relay’s location, i:e:, an ordinal number from the destination. The outage probability of the proposed relay selection scheme is analytically derived, and the achievable diversity order of the proposed approach is investigated. Computer simulations confirm our theoretical analyses and show that our approach is robust against errors in the estimation of the distances between nodes.
Typical wireless channels, however, suffer from multipath fading and shadowing, which significantly reduce communication capacity for a given average transmission power and hinder reliable transmission. Although an effective option is to use multiple antennas to obtain spatial diversity gain , in practice, it is difficult equip small IoT devices with multiple antennas due to their size, complexity, and cost. Hence, another concept has been proposed: when the source cannot reliably communicate directly with its destination, other nodes temporarily serve as relays in order to support the communication. This cooperative diversity approach allows devices to enjoy spatial diversity gain without the need to equip them with additional antennas , and it has been shown that if sufficiently many relays are available, opportunistic relying can attain a diversity order equal to the number of relays itself . In opportunistic relaying, the best relay among those available is chosen based on the perfect knowledge of the instantaneous channel state information (CSI), where best is defined in terms of the corresponding instantaneous signal-to-noise ratio (SNR) at the destination. However, in a cooperative diversity system, relays consume their own battery order to support other nodes’ communication, so that if multiple nodes drain their batteries at the same time, the network life-time or its topology may quickly deteriorate.
To overcome this challenge, opportunistic relaying methods avoiding the requirement for perfect CSI have been proposed –. In , a scheme relying on the average channel gains only was presented. To some extent, average CSI does indeed indirectly capture the contribution of network topology (specifically the distance between relays), but leaves out the contribution of fading. A complementary approach was proposed in , , where CSI knowledge was replaced by channel distribution information (CDI). This method was shown that this results in a better performance than when the selection is based on the mean. However, the approach (like its predecessor) requires that relays know the actual distance between nodes, which in fact defeats the very purpose of opportunistic relaying, since actual location information also implies a heavy overhead.
In this paper, we have proposed a new EH relay selection scheme that is based on a residual battery of relays and the CDI of both small-scale and large-scale fading. Furthermore, we derived a simple selection rule and a closed solution for the end-to-end outage probability. As discussed in Section III-C, this system can achieve a diversity order of one, even though each relay can exploit knowledge of the entire distribution between the destination and itself. These results show that if we wish to enhance the spatial diversity, it is important to obtain the instantaneous CSI. On the other hand, considering the practical limitations of IoT devices, our approach is still an attractive option that realizes low-power consumption and highly reliable communications.
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