## 2 Low-Power Wide-Area Networks  
Applications such as smart cities and smart meters need to cover a large area with low power consumption communication to realize long-term operation without maintenance. Traditional wireless local area networks (WLANs) such as Wi-Fi and cellular networks are not suitable to meet this requirement. To meet the requirement, various standards for IoT networks have already been developed over the last decade, such as LoRa, Sigfox, and Wi-SUN. This section introduces these major standards of the IoT networks.  
LoRa is a unique chirp spread spectrum modulation technique optimized for long-range low-power communications. The data rate of the LoRa devices mainly depends on the used bandwidth, spreading factor (SF), and forward error correction (FEC) rate. The bandwidth is typically set to 125 kHz or 250 kHz for the uplink and 500 kHz for the downlink. The SF values can range from 7 to 12, and the FEC rate can vary from 4/8 to 4/5. Setting a larger SF value can improve receiver sensitivity and wider coverage; however, the data rate is consequently reduced. LoRaWAN is the most widely used protocol stack for LoRa networks and has 240 million devices in 170 countries [17], End LoRa devices connect to one or more gateways through a single hop. A LoRa gateway can process up to nine channels in parallel by combining different sub-bands, and SF [18]. LoRa has a capture effect that makes it possible to recover a LoRa signal, provided that the desired signal is at least one dB above the interference level.  
Sigfox is a standard originating in France and currently has 75 regions and countries with more than 19 million devices [19]. Sigfox utilizes unlicensed ISM bands and differential-BPSK (D-BPSK) modulation. The message is sent with a fixed bandwidth of 100 Hz and a speed of 100 bps for the uplink, and 200 Hz and 600 bps for the downlink, respectively. This modulation technique belongs to the ultra-narrow band (UNB) modulation. The advantages of using D-BPSK modulation are its high efficiency in the spectrum medium access and ease of implementation. A low bit-rate enables the use of low-cost transceiver components. Sigfox transmits data by changing the frequency three times for each data to ensure data transmission. Sigfox technology has a duty cycle whose restrictions vary within the transmission band from 0.1% to 10%, depending on regional regulations [20].  
Wi-SUN is a standard based on IEEE 802.15.4g/4e and deployed in 46 countries, and has more than 100 million devices [21]. IEEE 802.15.4g is an amendment to the IEEE 802.15.4 standard, focusing on SUN communications that play an essential role in the smart grid [22]. The standard specifies several modes that operate in different bands, including the sub-GHz industrial science and medical (ISM) bands. Multirate frequency-shift keying (MR-FSK) with 2-FSK or 4-FSK is the main modulation technique used in Wi-SUN devices. The data rate varies from 2.4 to 200 kbps, depending on the region and frequency band. The mandatory configuration for all regions is 2-FSK, which operates at 50 kbps, implying a channel spacing of 200 kHz. More than 50 million smart meters in Japan that can collect electricity consumption data using this standard have already been deployed. Their number is expected to increase in the future.  
The IoT standards described above coexist in the same frequency band called the ISM band [23]. Thus, an increase in the number of IoT devices leads to a significant decrease in network performance because devices following each standard may affect other devices, which will bring collisions to the massive heterogeneous IoT networks [24]. By the MAB-based channel assignment methods that will be present in this paper, approximately 10,000 IoT devices can be accommodated in such coexistence IoT networks.