# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 3 Wireless Technologies
### Bluetooth  
The classical Bluetooth technology is based on the IEEE 802.15.1 standard and is used for ad hoc communications. Similar to LoRa, SigFox, Wi-Fi, and ZigBee, Bluetooth operates in the unlicensed ISM band, with low-power, low-cost transceivers, and is designed for peer-to-peer communications. It was originally intended for replacing the wired communication between devices, with effective high data rates for audio and data streaming applications. It is a LPLAN technology with range coverage of less than 1000 m, which is divided into three classes, class 1 up to 100 m, class 2 up to 10 m, and class 3 up to 5 m [24]. Nevertheless, the majority of the Bluetooth devices operate in class 3. Compared to Wi-Fi, Bluetooth is widely available and embedded in almost every smart device, while it has a lower bit rate and a shorter transmission distance [20]. Bluetooth limitations include the restriction to one-to-one communication [10] and latency issues caused during the device discovery.  
The Bluetooth low energy (BLE), also called Bluetooth 4.0 [10], was introduced in 2011 as an extension featuring low energy and low transmission power [101]. It is the successor of the classic Bluetooth and operates on the unlicensed ISM spectrum band with a maximum signal range determined by the transmission power [25]. It divides the spectrum into 40 channels, 3 of them are dedicated for advertisements, which are connectionless and broadcast their signals periodically, and the rest are for data exchange. Compared to the classic Bluetooth, the BLE reduced the number of channels for preventing the signals from overlapping with other technologies. The low-cost BLE beacons (receivers) are powered by coin cell batteries, operating for several years without replacement [101]. BLE has shorter coverage and, compared to Wi-Fi, is more suitable for power constrained applications, such as the IoT. Compared to the classic Bluetooth, BLE supports one-to-many connections and periodically, within an advertising interval, broadcasts short packets. These packets are received by the BLE beacons and can be used for estimating their position, i.e., using the RSS measurements. The trilateration and fingerprinting localization algorithms are commonly used for BLE positioning.  
For instance, in [43], the BLE trilateration-based location tracking system utilized a smartphone's RSS measurements, which were analysed, and the closest BLE AP was selected as a reference point. Additionally, the smartphone's accelerometer sensor was used for detecting the pedestrian's movements, and the location tracking error was evaluated by touching a button on a smartphone application when approaching each BLE AP. It achieved an average location tracking error of 3.01 m, which was slightly decreased by applying the Kalman filter. Additionally, the iBeacon and Eddystone, introduced by Apple in 2013 and Google in 2015, respectively, are among the most popular IoT BLE beacons [3,25]. BLE beacon-based localization systems are also used in real-world environments, such as in the Gatwick International Airport, where, combined with augmented reality technology, they are used to aid passengers in navigating unfamiliar places, i.e., locating the check areas, departure gates, and baggage belts [25].