Wi-Fi is a radio signal that can be used to connect various devices together. A connected router sends signals to nearby devices. For its signal, the 2.4 GHz or 5 GHz frequency bands are used by Wi-Fi. With dual-band devices, one may choose the frequency they want to use for their Wi-Fi network. The IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards cover Wi-Fi or wireless LAN. As a result, there are several subcategories of Wi-Fi protocols, including 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, and 802.11ax. Each Wi-Fi frequency band has several channels that devices may use to transmit and receive data [42,43]. Wi-Fi-based indoor positioning methods provide a number of benefits, including low cost, ubiquitous infrastructure availability, and the capacity for real-time location updates. As Wi-Fi signals may pass through walls and other obstructions, precise indoor localization is still achievable, even when there is no direct line of sight. Wi-Fi-based devices, however, might lose accuracy due to external variables, including signal attenuation and interference. Additionally, the necessity for several access points to cover a vast region might make their deployment and maintenance more difficult. Finally, modifications to the physical surroundings or network setup can have an impact on the precision of Wi-Fi-based location estimation [44].

This is a wireless standard-based technology that was created to support low-cost, low-power Machine-to-Machine (M2M) and Internet of Things (IoT) networks. As a result, low-data-rate and low-power applications use this open standard. The 2.4 GHz, 900 MHz, and 868 MHz unlicensed radio bands are used by the IEEE 802.15.4 physical board radio specification for the Zigbee communication standard. ZigBee provides outstanding adaptability and scalability for both developers and end-users. The wireless range of ZigBee is 400 m outside and 70 m inside. It supports both point-to-point and point-to-multipoint mesh networks, among other types [45].

The benefits of Zigbee include its low cost, low power consumption, and capacity for numerous concurrent users. Furthermore, Zigbee is appropriate for larger interior areas because it has a greater coverage area than Bluetooth. Zigbee’s negative aspects include its constrained capacity, which can result in decreased accuracy in high-density settings where several devices are vying for bandwidth. In addition, its location accuracy can be impacted by interference from other wireless devices in the surroundings, and Zigbee requires a more complicated network architecture than alternative localization methods, such as Wi-Fi or Bluetooth [46].

The two components of the wireless technology known as Radio-Frequency Identification are tags and readers. RFID uses radio waves to carry out AIDC (Automatic Identification and Data Capture technology) operations. An electrical device called a reader uses radio waves to send and receive signals from RFID tags. One or more antennas could be present. Either passive or active tags employ radio waves to transmit their identity and other data to nearby readers [47].

RFID has several benefits for interior localization, including high precision, affordability, and ease of deployment. RFID systems may be utilized for a variety of purposes and can function effectively under challenging conditions. The restricted read range of RFID systems, which can be impacted by ambient variables, such as radio signal interference from other devices, is one drawback of the technology. The cost of RFID tags can also be high, and their small size might make it challenging to find them in large interior environments [48].

In order to function at incredibly low power levels, Bluetooth Low-Energy (LE) radios have been designed. By delivering data over 40 channels in the 2.4 GHz unlicensed ISM frequency space, Bluetooth LE radios provide manufacturers with a substantial degree of flexibility to develop devices that meet the unique connectivity needs of their market. Moreover, they support a range of communication topologies, including point-to-point, broadcast, and, most recently, mesh, enabling Bluetooth technology to facilitate the creation of reliable, vast device networks. Bluetooth Low Energy is now being extensively used in the Internet of Things and the business advertising industry. Bluetooth LE is the best option for longer-lasting devices that only need to occasionally communicate small amounts of data because it slows down data transmission and uses 0.01 to 0.5 Watts of power [49,50].

UWB is a radio wave-based short-range wireless communication technology similar to Bluetooth or Wi-Fi. The fact that it runs at an extremely high frequency, however, makes it easily distinct. Using the broad-spectrum frequency, a UWB transmitter sends billions of pulses (UWB was once known as “pulse radio”). The real-time accuracy of UWB is enhanced by its capacity to transmit pulses at a rate of one per two nanoseconds. UWB can send a large amount of data from a host device to other devices up to around 30 feet away while using very little power because of its wide bandwidth (500 MHz) [45].

Other indoor positioning technologies cannot compete with UWB’s high precision, high dependability, and ability to function under challenging conditions. Walls and other obstructions cannot block UWB signals, making it possible to communicate effectively, even under difficult interior conditions. Additionally, UWB technology uses less power, resulting in a long battery life and little energy use. UWB technology, however, also has certain drawbacks. The high cost of UWB infrastructure and hardware is one of its key drawbacks, which may render it unusable for various applications. The precision of the positioning system may also be impacted by the interference that UWB technology may experience from other wireless signals, such as Wi-Fi and Bluetooth. Finally, some applications may be constrained by UWB technology’s restricted range, which normally operates over a distance of only a few meters [52].

This is a method of wireless modulation. LoRa devices are affordable and simple to integrate into a network because they have a long range and require little infrastructure. It enables chirp spread spectrum communication over vast distances. It limits interference from other devices by using specialized radios, which are uncommon in end-user devices. In comparison with other network technologies, it costs 20% less. LoRa uses unlicensed RF bands for operation. Moreover, LoRa employs Forward Error Correction (FEC) to lessen signal noise significantly [53,54,55].

LoRa’s low power requirements, long-range capabilities, and inexpensive cost make it a good choice for indoor localization. Systems for indoor localization that use LoRa technology are simple to set up and can function under a variety of conditions. However, compared with other technologies, LoRa’s biggest flaw is its poor precision, which makes it difficult to pinpoint a specific place. Furthermore, LoRa may be limited in places with significant levels of signal attenuation and may be subject to interference from other wireless devices [56].

The LPWAN family of technologies includes the Sigfox technology. Long-range wireless cellular communication is called Sigfox. Sigfox offers specialized solutions, particularly for low-throughput Internet of Things (IoT) and M2M applications, using its end-to-end IoT connection services and distinctive technology. The Sigfox network was created to make efficient communication possible while using minimum power. With the help of Sigfox, IoT devices can communicate over large distances and broadcast using few base stations. According to frequency and geographical region limits, the duty cycle of Sigfox technology ranges from 0.1% to 10% inside the transmitting spectrum. The client server hosting the application and the nodes and gateways that send communications from the node side to the Sigfox Cloud make up the architecture [57,58]. Figure 1 illustrates a general layout of a Sigfox System.

Sigfox is a LPWAN (Low-Power Wide-Area Network) system that can deliver long-distance communication with little power usage. However, Sigfox has a lower data rate and a smaller capacity than other LPWAN technologies, making it suitable for straightforward applications that do not need high-speed data transfer. In addition, Sigfox uses the unlicensed ISM band, which might lead to interference problems in congested areas. However, for low-power, low-bandwidth indoor localization applications that do not need great precision, Sigfox may be a financially viable option [59].

Data interchange between electronic devices is possible across a short range of up to 4 cm due to a group of radio transmission protocols known as Near-Field Communication (NFC) (1.6 in.). Devices can be physically touched or brought close enough to transmit data using this technology. Operating at a 13.56 MHz frequency, which is an unlicensed band, this method takes advantage of inductive coupling. NFC systems may function following three different methods: the NFC card emulation approach, P2P method, and the reader/writer method.

Two-way communication between electronic devices is made possible by NFC. Additionally, it has the capacity to write to an RFID chip. As a result, an NFC-enabled mobile phone and NFC reader may establish a bidirectional connection [60,61].

Owing to its low power usage, excellent precision, and low cost, NFC technology has several benefits for indoor localization. Owing to its short-range transmission, it is less sensitive to interference from outside signals and can function in locations where GPS signals are not accessible. It is simple to install and maintain NFC tags because they may be placed on items or walls. NFC does have certain drawbacks, however, such as its limited coverage due to its low communication range. Additionally, because line-of-sight communication is required for NFC, it cannot pass through barriers, such as walls. For vast and complicated interior situations, it might not be the best option [62].

A cellular network is made up of several cells, each of which covers a specific geographic area. Each cell also has a base station that functions similarly to an 802.11 AP in that it aids mobile users in connecting to the network, and each cell and base station have an air interface that combines the physical and link layer protocols. Cellular networks might be a useful choice for SMs and utilities to communicate with one another. The expansion of smart metering deployments into a large-scale setting is also made possible by cellular network technologies. The cellular communications technologies that are accessible to utilities for the implementation of smart meters include 3G and LTE [63,64].

The benefits include extensive coverage, which makes it simpler to find devices in various locations. Additionally, cellular networks may offer time-of-flight and signal strength metrics with high levels of precision. Cellular networks can, however, have drawbacks for interior localization, such as expensive implementation and maintenance costs. Another drawback is the poor accuracy in densely crowded or high-interference locations [65].

Table 1 presents a comparative summary in terms of the advantages and disadvantages of various communication technologies used in Radio Signals-Based Positioning.