## 2 Localization System Architecture  
### Front-End Architecture  
As shown in [16], the localization system is based on a constellation of anchors capable of exchanging communication data with the target node, while simultaneously collecting the corresponding set of RSSI. A block diagram of the anchor system architecture is given in Figure 1[15]. It is based on an 8-channel SBA, a SP-8T microwave switch, and a System-on-Chip (SoC), which implements a radio-frequency transceiver and a microcontroller unit (MCU). The hardware scheme proposed in Figure 1 is deliberately simplified to highlight the ease of implementation of the anchor architecture. The vast majority of Commercial Off The Shelf (COTS) SoCs are suitable to provide the required featuresfor this applications. The device chosen for this work is the Texas Instruments CC2530, which includes a 2.45 GHz transceiver supporting the IEEE 802.15.4 protocols and integrates a 8051 MCU with 8-kB of RAM. The additional feature required to the MCU consists of the calculation and the transmission of the Received Signal Strength Indicator (RSSI) to the localization engine. In this project the RSSI is transmitted by the Ethernet interface. The MCU provides a set of GPIOs, used also to control the single-pole eight-trough (SP-8T) RF switch that sequentially routes the transmitted or received signal to each of the antenna elements of the SBA. In this project, we adopted a HMC321, which is a low-loss SP-8T non-reflective switch capable to work in the DC-8 GHz bandwidth with less than 2.5 dB losses at the operative frequency. This bandwidth is much larger than what is required for this application; this is because this architecture is extremely flexible and modular, and the switch board has been used also for other experiments at different bandwidths. The firmware provides the proper synchronization between the RSSI data collection and the switch settling time, which is 150 ns, thus granting a bijective correspondence between the receiving antenna and the obtained RSSI by avoiding the spreading of a single packet through multiple antennas. An RSSI collection cycle is completed when a network packet is exchanged with the mobile node for each of the 8 antennas of a single SBA. In the case of the IEEE 802.15.4 radio access, the data transfer rate is about 250 kbit/s with a packet length of 60 bytes. The entire set of RSSI data for each anchor is collected in about 13.3 ms. An additional time has to be added to this value for the position algorithm execution, which is around 10 ms depending on the chosen algorithm and the power of the machine on which it is performed. An entire RSSI data set is collected at the receiving of 8 packets by each anchor node. The anchor network is able to optimize the packet flow by synchronizing packet readings between different anchor nodes, thus the mobile nodes have to transmit only 8 packets. Referring to CC2530 transceiver datasheet, when powered at 3 V supply voltage, the current absorption during active transmission is about 29 mA corresponding to a peak power consumption of 87 mW, while during radio receiving the current absorption is 24 mA (with a peak power consumption of 72 mW). According to the general need about which, mobile nodes transmission profile must be tailored to maximize battery life, they send their data as a burst of 8 repetitions of data packets, only at the required localization refresh according the radio channel is sensed to be free. Following this, their average power consumption decreases, while the localization refresh time increases. As an example, considering a burst of 8 packets, equal to 480 bytes or 14 milliseconds of transmission time, a refresh time of 1 second leads to an average power consumption of 1.2 mW. Because the anchor nodes are part of fixed network infrastructure, to deal with user nodes random packet transmissions, they must be always ready to receive data, so they are always in receiver enabled state. By this, their average power consumption exactly equals peak consumption of 72 mW.