# 3D Void Handling Geographic P2P-RPL for Indoor Multi-Hop IR-UWB Networks
###### Abstract  
IETF has standardized the point-to-point RPL (P2P-RPL) to ensure reliable and optimal P2P route discovery for low-power and lossy networks (LLNs). P2P-RPL propagates route discovery packets to all nodes in the network, which results in high routing communication overheads. Recently, other RPL-based P2P routing algorithms have been proposed to reduce such overheads, but still, quite an amount of overheads occur due to their flooding-based approach. In real life 3D environments, a larger number of nodes should be deployed to guarantee the full network connectivity, and thus the flooding strategy incurs higher overheads. In effort to alleviate high overheads, geographic routing is an attractive solution that exploits the nodes' geographic locations in its next-hop routing selection. However, geographic routing inherently suffers from the local minimum (void) problem following greedy next-hop selection. Local minima occur more often in 3D space, and therefore, a reliable 3D void handling technique is required. In this paper, we propose greedy forwarding and void handling point-to-point RPL with adaptive trickle timer (GVA-P2P-RPL), which is a novel RPL-based P2P routing protocol that quickly discovers energy-efficient and reliable P2P routes in 3D networks. In GVA-P2P-RPL, P2P-RPL is modified to greedily forward routing packets when it is possible. IR-UWB-based 3D multi-hop self-positioning is conducted in advance to obtain the geographical location of each node. When local minima are encountered, routing packets are temporarily broadcast just like in the traditional P2P-RPL. A new trickle algorithm called adaptive trickle timer (ATT) is also presented to reduce route discovery time and provide better collision avoidance effects. The performance of GVA-P2P-RPL is compared with that of P2P-RPL, partial flooding-based P2P-RPL (PF-P2P-RPL) and ER-RPL. It shows significant improvements in route discovery overheads and route discovery time against these state-of-the-art RPL-based P2P routing methods in 3D environments. Performance evaluation in the special network case where a huge 3D void volume exists in the center is also presented to show the strong void recovery capability of the proposed GVA-P2P-RPL in 3D environments. P2P-RPL; geographic routing; 3D void handling; adaptive trickle algorithm 3D Void Handling Geographic P2P-RPL for Indoor Multi-Hop IR-UWB Networks [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [20] [21] [  
deployed in 3D environments, where a larger number of nodes have to be deployed to support the full communication coverage and maintain the full network connectivity [4]. Consequently, the flooding strategy incurs a larger number of routing packet overheads in 3D environments. To ease the high overhead issue, the use of 3D geographic routing in P2P-RPL is a desirable solution since geographic routing only exploits local location information to greedily select the next hop node [5]. Despite the merits, geographic routing inherently suffers from the local minimum (void) problem where no one-hop neighbor node closer to the destination exists. The local minimum problem occurs more often when the network is not sufficiently dense [6]. In 3D environments, the network density can be much lower and the inter-node connectivity is weaker compared with 2D environments, leading to much more local minimum situations [7]. To overcome the local minima, a reliable 3D void recovery mechanism is required in response. In this paper, we propose a reactive 3D geographic P2P routing protocol called greedy forwarding and void handling point-to-point RPL with adaptive trickle timer (GVA-P2P-RPL) for indoor 3D IR-UWB (impulse radio ultra-wideband) networks. GVA-P2P-RPL combines the advantage of the high successful delivery rate of P2P-RPL's flooding and the low overhead of geographic routing's greedy forwarding.