## 1 Introduction
These types of systems consist of constellations of geo-referenced high orbit artificial satellites that continuously transmit location and timing data. There are four main GNSS systems currently in operation: GPS (U.S.), GLONASS (Russia), Beidou (China), and Galileo (E.U.). These systems, typically, present accuracies in the magnitude order of 1 to 10 m, and have been used in IoT applications that demand higher accuracies and low device autonomy [11]. However, they present some important drawbacks that make them inappropriate for general IoT/IIoT outdoor localization: (1) they consume a lot of power (IoT systems have power battery limitation); (2) they are expensive (IoT localization methods should be low cost), and (3) they provide location information only for the IoT device itself, meaning that additional data must be forwarded to the gateways consuming additional power and bandwidth. Based on range, network communication protocols can be divided into three groups: short-range (Bluetooth, RFID...), medium-range (ZigBee, Wi-Fi,...), and long-range (LowPower Wide-Area Network (LPWAN)) [12]. LPWAN provides long-range communication among IoT devices with low-power consumption and low cost. Different technologies have been considered to be LPWAN, which include LTE-M, NB-IoT, Sigfox, and LoRaWAN [13]. LPWAN provides low-power, low-cost, and long-range communication and its signals can be used for communication and localization simultaneously. The LPWAN technologieshave star topology. Typically, IoT nodes send uplink transmissions to LPWAN gateways as end nodes. The LPWAN gateway sends collected data from end nodes to the LPWAN network server throw UDP/IP protocol. These data can be used for end-node localization by the LPWAN network server. LPWAN network server sends MAC control command and downlink packets to end nodes throw LPWAN gateways. LPWAN Application Servers can be owned by different organizations for distinct purposes. There are different proposed localization methods that can be used to obtain the end nodes' relative location or for tracking purposes. Each category of these proposed methods has pros and cons that make them useful for specific IoT systems. Nevertheless, there are some limitations in proposed localization methods that need to be eliminated to meet the IoT ecosystem needs completely. Obviously, there is an urgent need for the development of novel intelligent solutions to improve outdoor localization methods for IoT systems based on their limitations and requirements. This has motivated this paper to provide a survey paper to review, at first, different features and requirements of IoT systems on outdoor localization and, in the following, different proposed methods on each IoT system. Furthermore, this paper empowers researchers to create their own efficient and useful models for outdoor IoT localization by showcasing the state of the art employing LPWAN methods for location estimation. To the best of the author's knowledge, this is the first survey that reviews the latest research efforts focused on GNSS-free outdoor localization methods for the IoT ecosystem, having in mind its specific requirements and limitations. The main contributions of this work can be summarized as follows:  
1. Definition of the main requirements and limitations of outdoor localization for the IoT ecosystem, having in mind the following application domains: smart healthcare; smart transportation; smart cities; smart manufacturing. 2. Review of the most relevant GNSS-free outdoor localization methods, with a focus on LPWAN technologies and their relationship with the newly 5G mobile network architectures;
3. Review of the most relevant methods for improving the localization accuracy in the IoT ecosystem. 4. Discussion on open challenges and future directions in the field. This research has been conducted between May and August 2021.