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much smaller latency, lower energy consumption, and increased range with higher data rates [49]. BLE adopts master–slave architecture, and role of a device (as master or slave) is defined in its MAC layer. To decrease channel sensitivity to interference and multipath fading, BLE employs TDMA-based MAC protocol and frequency hopping [82]. Additionally, the center frequency of channels has been assigned in a way to minimize interference with IEEE 802.11 channels [57]. Even though there are options to increase BLE’s data rate at the expense of range, the limited range of BLE makes it inept for extensive IIoT deployments. To address this problem, a Bluetooth mesh network is introduced to increase the number of relaying neighbors, making it applicable to industrial use cases such as alerting and logging systems [83]. Recently most wearables are equipped with BLE interfaces. Overall, considering its low power consumption, BLE could be utilized as a practical and effective wireless technology for IoT applications that require short-range communication [68, 81].

      1.4.1.3 IEEE 802.11 Standard (Wi-Fi)

Wi-Fi is a family of wireless network protocols regulated by the IEEE 802.11 standard [84], originally designed to offer high data rate connectivity and Internet access to a limited number of devices for WLAN. The early versions of the standard (IEEE 802.11b/g/n/ac) are limited by high power consumption and frame overhead. In addition, they do not fully support small and deterministic payloads required for mission-critical IoT applications. Thus, a set of modifications is proposed by IEEE 802.11 working group to amend WLAN to IoT scenarios. The first of such efforts was IEEE 802.11ah, known as Wi-Fi HaLow, which fulfills IoT requirements and could achieve denser deployment, lower overhead, and less energy consumption compared with legacy Wi-Fi networks [85]. The main features of the IEEE 802.11ah PHY layer are inherited from IEEE 802.11ac, which accommodates relatively narrow channel bandwidths. Its MAC layer has adopted some enhancements to improve power-saving features, to assist large number of devices, and to increase data rate. IEEE802.11ah supports machine-to-machine (M2M) communications for IoT services such as smart metering and industrial automation [86].

The IEEE 802.11ax standard, also called Wi-Fi6, is another effort that supports mission-critical data transmissions [87]. It underpins complex applications such as VR and robotic motion control [88]. To ensure distinct transmission scheduling for time-critical and delay-sensitive use cases, the MAC layer of Wi-Fi6 adopts orthogonal frequency division multiple access (OFDMA). Moreover, adaptive modulation and coding schemes are used in MAC protocol to tackle high levels of variations in wireless links [89].

      1.4.2 Long-Range Wireless Communication

      Long-range wireless networks are adapted for scenarios that require long-distance data transmission (up to 100 km). These wireless communications include very small aperture terminal (VSAT) technology, cellular networks (2G/3G/4G, LTE, and 5G), and low power wide area networks (LPWANs). Since VSAT

technology is based on satellite communication and employed in hard-to-reach places or remote sites, it is beyond the scope of this chapter. Since we will review cellular networks in Section 1.5, in this section we will focus on a number of main LPWAN technologies: long range (LoRa) [90], LTE-M [91], and narrowband Internet of things (NB-IoT) [92].

      1.4.2.1 Low Power Wide Area Network (LPWAN)

The term LPWAN, also known as low power wide area (LPWA) network or low power network (LPN), was primarily designed for M2M networking. LPWAN denotes energy-efficient, low-cost, and especially wide area coverage communication [93]. “Wide area” in LPWAN relates to radio links of over 1 kilometer range. The applications of LPWAN are limited to low bandwidth communication with low data rate and infrequent transmissions [94]. Therefore, it is well-suited for IoT services that require small data transmission over a wide area. For industrial IoT systems, LRWAN is suitable for retrieving data from field devices that transmit low traffic (a few bytes in the payload) over long distances for a short period of air time for each node per day.

Wireless LPWANs have emerged in licensed and unlicensed frequencies. They include open standards and proprietary options such as Huawei’s cellular IoT (CIoT) [95] and NWave [96] and differ in coverage area and other technical characteristics. A set of LPWAN standards has been specified by GSMA wireless industry association in 2015 to assist network operators to meet the requirements of IoT use cases in terms of coverage, energy consumption, and cost [97]. Consequently, LPWAN became a preferable choice for IoT use cases and gained significant attention as a complementary technology to the existing cellular networks. LPWAN comes in various ranges, sizes, and operational properties. In this section, we overview three LPWAN technologies with different features.

Long range (LORA). As a promising wireless standard for IoT use cases, LoRa is a physical layer technology for LPWAN developed by Semtech Corporation [98]. It operates in an unlicensed band and offers seamless connection with wide range deployment (several kilometers) and minimum investment and maintenance costs. LoRa has adopted enhanced modulation and optimum network protocols for finite energy sensors.

LoRa modulation achieves bidirectional communication through a proprietary spread spectrum technique in the sub-GHz frequency band: the chirp spread spectrum (CSS) scheme [99, 100]. This technique in LoRa generates a narrowband (NB) signal and spreads it over a wider channel bandwidth to provide a signal with wide bandwidth.⁴ This results in larger signal-to-noise ratio where the received signal is difficult to be jammed, making it resilient to communication channel degradation arising from Doppler effect, multipath fading, and a high level of noise [101, 102]. Importantly, LoRa modulation contributes to greater maximum coupling loss (MCL) compared with existing sub-GHz communications and enlarges the capacity of the network as well as extending the coverage distance [103]. LoRa modulation utilizes variable spreading factors (SF) to balance between the data rate and range: a lower SF offers higher data rate at the cost of shorter range; by contrast, a high SF provides a low data rate, implying a larger range. Depending on the SF and channel bandwidth, a LoRa network achieves the data rate of 22 bps–50 kbps. In addition to adaptive data rates, spreading enables the simultaneous transmission of multiple spread signals over identical frequency channel. Therefore, LoRa base stations could simultaneously receive the transmitted messages with different SFs [104]. The largest length of payload for every message is 243 bytes [93].

LoRa’s performance has been verified in multiple countries on smart meters, traffic monitoring, and smart health care [105, 106]. It also optimizes the protocol for power-restricted sensors and introduces three modes of operation for LoRa-based terminals in IoT environments. According to various application scenarios, these modes of operation identify data-receiving windows for each class of end devices and determine how these terminals communicate with the network. LoRa technology utilizes AES-128 for data encryption to ensure channel security.

LoRaWAN. As LoRa is mainly focuses on physical layer and its specification for connection of devices to infrastructures, LoRaWAN focuses on MAC protocol [107]. It is maintained by the LoRa Alliance and acts as the protocol that manages LPWAN communication from end devices to gateways. LoRaWAN is based on the pure Additive Links Online Hawaii Area (ALOHA) protocol [49] to increases the success rate of the reception of the messages. This is because in ALOHA all base stations within the range could receive each message sent by an end device.

NB-IoT. Standardized by the Third Generation Partnership Project (3GPP) as a narrowband IoT communication technology [92], the NB-IoT is built on the prevailing LTE functionalities and works on the licensed frequency bands. Since NB-IoT could coexist with GSM and LTE, its deployment is rather simple, particularly in the existing LTE networks. The protocol of NB-IoT is derived from the LTE protocol; however, many LTE functionalities are reduced to make it simple and more suited to IoT applications. Thus, from the perspective of a protocol stack, NB-IoT could be seen as a novel air interface built on LTE infrastructure. NB-IoT could deploy LTE backend systems and broadcast signals for all end apparatuses within a cell. To minimize battery (and resource) consumption of the end devices, the cell is designed for short and sporadic data messages. Additionally, properties such as monitoring the quality of channel, dual connectivity, and carrier aggregation

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