# Device-Free Localization and Identification Using Sub-GHz Passive Radio Mapping
## 2 Experimental Setup and Methodology
### Construction of a Sub-GHz Passive RF Sensing Network  
The RF sensing network we install consists of 15 regular nodes which are attached to tripods at a height of approximately 1.10 m. Each node consists of a plastic casing containing two battery-powered transceiver modules for respectively 433 MHz and 868 MHz. The transceivers for both frequency bands operate independently of each other and can therefore be considered to belong to two entirely separate networks which operate in an identical manner. Figure 1 shows a photograph of a node.  
Communication uses the DASH7 Alliance Protocol (D7A) [19] and is cycle-based. Each node is assigned a unique node id which determines the order in which they will broadcast a message to all other nodes. Upon receiving this message, a node will store the RSS with which it was received in an internal list at the index corresponding to the node id of the transmitting node. When a node's turn to broadcast has arrived, the message it transmits will contain this list. A special type of node called the controller continuously listens to all communication within the network and passes on all of the RSS data to a laptop to which it is connected. Once all nodes have broadcasted, the cycle begins anew.  
In addition to collecting and passing on the RSS measurements, the controller is also responsible for the timing of the communication described in the previous paragraph. At the beginning of each cycle, it will transmit a specialized start message and this will instruct each node that its moment to broadcast occurs at a specific time after receiving this message which is determined by the node id and the network parameter wtime. This parameter defines the period of time between transmissions of two subsequent nodes. In order to minimize the delay between an RSS measurement being performed and the data being passed on to the controller, it is important for wtime to be as small as possible. For the experiments described in this paper, wtime was equal to approximately 10 ms. The controller can also be instructed via the laptop to stop (or resume) periodically transmitting start messages and therefore halt (or resume) all network communication. Additionally, it can also instruct all network nodes to reset themselves and/or request new configuration parameters from the configurator. The configurator is a final, third type of node which contains all necessary parameters  
Figure 1: A photograph showing the inside of a regular network node. It contains two OCTA-mini transceivers for respectively 433 MHz and 868 MHz communication attached to two antennas and a 6600 mAh battery.  
related to network communication (e.g., wtime, frequency channel, network size and node ids for every node). This node consistently listens for configuration requests from regular nodes and responds with the required information. A regular node will send these requests shortly after boot--only after it has received the necessary parameters can it participate in regular communication cycles. All communication related to configuration occurs on a separate frequency channel so that it will not interfere with regular operation.  
The main advantage of this system architecture is the flexibility it provides. One only needs to change the relevant parameters in the configurator to add/remove nodes, increase/decrease the wtime parameter or change the frequency channel on which the cycle communication occurs. For our future development of the system, we are planning on incorporating the functionality of the configurator into the controller node.