I. Introduction
According to the research by Gartner, the number of internet of things (IoT) devices will grow exponentially to reach 26 billion by 2020 [2]. A lot of IoT devices use WiFi or Bluetooth Low Energy (BLE) that work at the industrial, scientific and medical (ISM) 2.4 GHz band. However, WiFi and BLE protocols are not compatible with each other at the physical layer. Therefore, a gateway (i.e., a bridge equipped with multiple radios) is normally used to connect these IoT devices. However, the gateway-based approach has two major issues: i) the additional cost to purchase the gateway hardware and ii) traffic overhead from a BLE device to a gateway, and then from the gateway to a WiFi device (shown in Figure 1b). When the number of IoT devices exponentially increases, the huge amount of data traffic generated by these IoT devices and the extra traffic introduced by gateways will significantly affect the wireless spectrum usage rate and eventually lead to spectrum crisis.
Gateway-based model vs. model. (a) Gateway-based model, which needs two steps to enable the communication between a BLE device and a WiFi device. (b) Simplified model, which can support 3-way concurrent communications: W2W, B2W, and B2B.