# Location-Aware Point-to-Point RPL in Indoor IR-UWB Networks
## 1 Introduction  
Wireless multi-hop ad hoc routing should provide the reliable interoperability in low-power and lossy networks (LLNs). The IETF standardized the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) [1]. RPL constructs and maintains a tree-structured topology called Destination-Oriented Directed Acyclic Graphs (DODAG), and always forwards packets over pre-identified links to improve the energy efficiency of battery-dependent devices. The configuration of DODAG aims to minimize routing costs between the root and each node in the network. RPL is optimized for multipoint-to-point (MP2P) communication. RPL has adopted a tree-based proactive routing scheme for the low-power operation, and inherently suffers from the inefficiency of point-to-point (P2P) routing. When two non-root nodes need to communicate with each other, the packets must be passed upward until they reach a common ancestor that knows the path to the destination node. This simple routing strategy may result in long end-to-end delay and low packet delivery ratio due to significantly suboptimal P2P paths and heavy traffic congestion near the root. To compensate for these shortcomings, IETF standardized a new routing protocol, reactive discovery of point-to-point Routes in Low-Power and Lossy Networks (P2P-RPL) [1; 2; 3; 4]. P2P-RPL constructs a temporary DAG that is solely for P2P route discovery to find efficient new paths to one or more nodes within the RPL framework. As a reactive routing protocol, P2P-RPL finds the best on-demand path in a lossy network. Since P2P-RPL is based on a broadcasting approach, it could lead to seriousmedium contention known as the broadcast storm problem [4; 5]. It is very important to achieve high reliability and consume less energy at the same time in P2P-RPL approaches. IEEE 802.15 Working Group established a Impulse Radio Ultra WideBand (IR-UWB) based physical layer standard (IEEE802.15.4a) with precise ranging capability [6]. Impulse Radio Ultra-WideBand (IR-UWB) system has bandwidth wider than 500 MHz and uses narrow pulses to transmit and receive information. The short duration of UWB pulse enables the identification of multipath components at high resolution, that makes IR-UWB as promising technology for various highly accurate positioning applications [7]. In this paper, we introduce Location-Aware P2P-RPL (LA P2P-RPL), which achieves energy-efficient P2P data delivery without compromising reliability. The ranging capability offered by UWB can be exploited to build a network coordinate map, enabling the location-aware multi-hop routing in upper layer protocols [8; 9; 10; 11]. Smartphone based Inertial Navigation System (INS) with particle filtering is used to increase the localization accuracy in indoor multi-hop networking environments. The LA P2P-RPL protocol uses knowledge of location to reduce the search space for the desired route discovery. Limiting the search space results in fewer route discovery messages, and this feature improves the route discovery overhead significantly especially when the distance between the source and the destination node is short. We first introduce the UWB-based cooperative multi-hop positioning algorithm for location-aware routing. Since the positional state of a node inevitably contains some degree of inaccuracy, we use the positioning algorithm that can infer the location error level of individual nodes to reflect the location uncertainty in the routing. Then, we demonstrate how to perform the energy-efficient point-to-point routing by using the estimated location knowledge of the individual node. The experiment results show the performance improvement of the Location-Aware P2P-RPL algorithm in multi-hop LLN environments. LA P2P-RPL algorithm is modified on the RPL framework, and compared with the existing solutions such as P2P-RPL and ER-RPL in terms of route discovery overhead, energy consumption, and reliability. The remainder of this paper is organized as follows. We begin with Section 2 by stating related research works for P2P-RPL problems.