Multihop Transmission for Time Correlation in Energy Harvesting Networks

Abstract

We consider a system of IoT nodes powered completely by energy harvesting.This work focuses on achieving the time correlation of data measurements ina network of energy harvesting sensor nodes. Time correlation is achieved byhopping a message through the whole network. This message wakes up all thenodes and lets them perform a measurement. Measurement data is added tothe transmitted message and is collected by a gateway at the end. The nodesharvest energy from a Radio Frequency (RF) source and store it in a capacitor.When the capacitor has sufficient energy, the nodes can turn on their system.A low power Wake-Up Receiver (WURx) is turned on and the nodes fall asleepwhile waiting for the incoming request message. Communication is done usingactive transmissions and the ultra-low-power WURx for data reception. Nodesconsume a continuous power of less than 2 µW in sleep mode while the WURx isturned on. The receiving sensitivity is -40 dBm, which limits the communicationrange. The request message hops through the network to overcome distancelimitation. Collisions are avoided with Clear Channel Assessment (CCA) usingthe WURx. The hidden node problem is overcome by toggling an operationalamplifier during CCA. Distance limitation is overcome by a novel network layeralgorithm. The network layer algorithm finds a directed acyclic graph (DAG)based on all nodes, starting in a single special source node and ending in agateway. Data from all the nodes are gathered in a round, where each node cantransmit one message around. The timing interval between the data collection ischosen to be bigger than the required energy divided by the minimal harvestedpower. In this way, all nodes will have sufficient energy in every time interval.The found DAG represents all important links where the nodes should wait forbefore measuring and transmitting. Other data from previous nodes are addedto the transmission of the nodes. In this way, the gateway will receive datafrom all nodes with the minimal time difference between their measurements.Simulations show that a correct gateway oriented DAG solution is always foundfor random networks. In > 92 % of the cases, all nodes are taken into account inthis solution and in 6% of the cases, just one node is missing. Nodes have beendesigned and evaluated. We can power the nodes with a minimal RF input of-15 dBm. The receiving range is found to be 8 m from a 10 dBm On-Off Keying(OOK) transmission. With 6 µW harvested energy, data from all the nodes canbe gathered every 15 minutes.