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Long-Range Wide-Area Networks (LoRaWAN) allow the transmission of data via radio link from sensors, which are potentially isolated or difficult to access, to gateways and servers that are connected to cellular networks for data processing, exchange, or relay, with low transmission power. This concept employs Long-Range (LoRa) modulation and has led to the emergence of many applications for the monitoring and tracking of objects.
- localization
- positioning
- LoRa
- LoRaWAN
- Ultra-Wide Bandwidth
- UWB
1. Introduction
The connectivity of objects to gateways using the LoRaWAN protocol has allowed the emergence of many monitoring applications, such as intelligent resource management, predictive maintenance, supply and stock tracking, object and animal tracking and personal medical monitoring [1]. This suggests infinite monitoring possibilities depending on the physical variable detected via the sensor. Besides these monitored variables, it is often necessary for gateways to monitor the location of the sensor itself [2], especially if it is integrated into a mobile object or tag, for example, if this latter feature is moving inside an indoor environment. The task of localizing a mobile sensor could be challenging for the LoRa working scheme, as this type of communication was not originally optimized for such an application [3], especially if the localization is required to be of high accuracy or expected to accommodate both indoor and outdoor scenarios. In contrast, the main advantage of LoRa communication in LoRaWAN networks is that the sensing information can be transmitted along long ranges in the order of kilometers, typically ranging from 5 km in urban areas up to 15 km in remote areas, from the sensor to the gateways [4,5]. Indeed, the LoRa technique is used to ensure deep-in communication among a large number of devices that have low power requirements and collect and transmit small amounts of data [5,6]. Furthermore, LoRaWAN networks have a high capacity and can handle millions of messages from thousands of gateways. However, despite these advantages, this technology is still not suitable for gateways to locate mobile sensors precisely and as often as is necessary [7]. Indeed, to ensure the low consumption of power, LoRa sensors send data with low packet rate [8,9], that is, typically one or two packets are sent per day. This is not suitable if the object monitored is mobile in its environment and requires real-time monitoring, or at least partial real-time monitoring, which cannot be achieved with the relatively low data rate of the LoRa technique. Furthermore, if the monitored object needs to be located with high accuracy and/or if it were in an indoor environment, we would require radio communications with wide frequency bandwidths to resolve multipath problems [10]; however, this is not the case for narrowband LoRa communications, which have bandwidths of a few MHz.
Conversely, Ultra-Wide Bandwidth (UWB) is the pervasive technology nowadays when it comes to locating objects or tracking assets or any type of targets, especially those in complex indoor environments, such as inside buildings, industrial infrastructures, hospitals, airports, construction sites [11,12], etc. It is effectively used in time-based ranging and localization techniques such as one-way or two-way ranging with Time-of-Flight (ToF) and Time-Difference of Arrival (TDoA) [13]. It allows high-accuracy distance and position estimates, notably due to the transmission of the ranging information along a large frequency bandwidth of 500 MHz or more between a reader and a target. Its constraint is that it is considered a short-range communication technology despite having reading ranges of typically 100 to 200 m in Line-of-Sight applications [14], which are mostly enough for indoor use cases but cannot adapt in networks deployed in highly remote outdoor areas, where objects can be spaced with more than those distances, i.e., such as in LoRaWAN networks.
Researchers propose combining both LoRa and UWB technologies as complementary Internet of Things (IoT) schemes into one transceiver board to exploit both of their principal features. This combination enables LoRa gateways to locate the sensors that belong to it if they are mobile in their environment, with the high-accuracy and real-time availability of their position information. It also allows for UWB targets to be located at long-range data links such as those of LoRa links. This solution consists of employing UWB technology in the mobile target LoRa sensor to allow LoRaWAN gateways to track it in real time with high accuracy. The result is a multi-standard UWB-LoRa transceiver, which can work as a sensor-tag or a reader depending on where it is placed in the communication chain. For this reason, we propose equipping the mobile LoRa sensor with UWB technology (LoRa-UWB sensor-tag) and placing the proposed UWB-LoRa reader device as an intermediate node between the target object and the gateway. The reader receives the location information from the sensor-tag in real time through UWB ranging with ToF and sends it to the gateway through LoRa signals.
2. Long-Range Wide-Area Networks
Previous research has realized localization attempts via LoRa technology, mostly using the Time Difference of Arrival (TDoA) method [7,15,16], which requires at least three gateways to infer the location of the object. For example, in [16], a number of sets of messages sent via the target sensor were used to calculate the TDoAs of these messages and perform location estimation at the gateway level, the authors assessed the efficiency of the performance of the localization and the results indicated that the localization error was greatly affected by the noise of the received timestamps at the base stations. In [17], a similar procedure was followed to implement a LoRaWAN tracking system that was capable of exploiting transmitted packages to calculate the current position, and using LoRa signals and applying a multilateration algorithm on the timestamps received at the gateways, the results demonstrated that it can be feasible to locate a device in a static spot with an accuracy of around 100 m, though this accuracy is not good enough for indoor scenarios.
The reason that the times of arrival were not precise enough is the combination of undesired multipath signals at the receiving gateways, with these signals being due to the environment and unavoidable, especially because LoRa signals’ widths in time (~142 ns for the 863–870 MHz channel) are not narrow enough for the multipath to be distinguished from the desired path signal. In addition to problems related to low accuracy, the previously cited studies all employed three or more gateways to infer the positions of the sensors via the TDoA method, which represents another constraint, as the already deployed LoRaWAN networks do not always geographically provide this condition.
Other studies employed the ToF method to measure the distance between one gateway and the object [2]; however, the server can only calculate this distance based on the packet metadata provided by the gateway, and these metadata are only sent with low data rates, which make real-time monitoring difficult to achieve. Furthermore, in [18], researchers proposed a ToF-based localization method using a fingerprint map to handle the accuracy issues and reduce the localization error caused by the noise and multipath; however, the fingerprinting method depends on the environment and needs to be updated for the location estimation to work accordingly, which makes it effort and time consuming.
Other methods include the using Received Signal Strength Indicator (RSSI) to infer the distance. For example, in [19], researchers measured the RSSI in an indoor environment with a short distance under both LOS and NLOS conditions, and the results showed the occurrence of power loss by the received signal, despite the short distances of measurement, and these losses were more important in the NLOS conditions compared to those observed in the LOS conditions. In [20], the authors investigated the accuracy of LoRa positioning using RSSI measured at the gateways, and, in realistic conditions where the power attenuation caused by the radio link was not known, the work reported accuracy errors of up to 588 m. Another recent work [21] proposed an extensive position estimation algorithm to minimize the posteriori RSSI error for multi-anchor cooperative estimation scenarios, and the results showed that the location can be estimated with an accuracy less than 7 m; however, this system was only tested in an outdoor scenario, which tends to be less challenging than indoor scenarios. Indeed, localization via the RSSI method is based on signal power, which makes it very sensitive to multipath, and cannot provide high accuracy information, as the correlation between the received power and the distance is significantly influenced by the environment, which means that it cannot reliably infer the target’s location information [22,23].
This entry is adapted from the peer-reviewed paper 10.3390/electronics12214487
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