# Measurement and Analysis of RSS Using Bluetooth Mesh Network for Localization Applications
## 4 Bluetooth Mesh Network for Experimental Studies of RSS Measurements  
### Specifications of BLE Signals  
Bluetooth was originally designed as a wireless technology for short-range cable replacement, and it was used mainly for streaming applications such as audio streaming and file transfers. BLE was introduced as a subsystem of Bluetooth for applications that require low power consumption and low cost to exchange small amounts of data periodically [21]. To differentiate, the original Bluetooth is now known as Bluetooth Classic. BLE provides similar communication range to that of Bluetooth Classic with a maximum transmission power of 10 dBm in Bluetooth 4 and 20 dBm in Bluetooth 5. BLE achieves reduced power consumption by keeping the radio off as much of time as possible and sending small amounts of data at low data rates. The low cost and low power consumption nature of BLE makes it possible for many devices to be deployed throughout a service area for long-term operations, and also makes it suitable for integration in other devices. Consequently, BLE has become extremely popular in power-constrained IoT and machine-to-machine communication applications [2, 4, 22].  
BLE operates over 40 channels that span the 2.4 GHz ISM band with a spacing of 2 MHz between adjacent channels as shown in Figure 1[5, 21]. Three of these channels are designated as primary advertising channels or simply advertising channels, including channel 37, 38, and 39, while the remaining 37 channels are data channels that are used for secondary advertisements and data packet transmission during connection. Three advertising channels are located at 2402, 2426, and 2480 MHz, respectively, which are selected to minimize the interference from the Wi-Fi systems that operate in the same 2.4 GHz ISM band. There are 11 overlapping channels defined for Wi-Fi at the 2.4 GHz band in the North America region, among which three non-overlapping channels, i.e., channel 1, 6 and 11 shown in Figure 1, are most widely used to avoid cross-channel interference.  
The BLE advertisinging channels are used to broadcast short packets for device discovery, connection establishment, and beacon service [21]. Data channels are used for bidirectional communication between connected devices. Advertisements always start with sending an advertising packet on all three or a subset of advertising channels, which allows fast discovery and connection. In comparison, Bluetooth Classic needs to scan 32 channels for discovery. Advertising packets are sent at advertising events that occur periodically with an advertising interval. Advertising interval can be set to any value between 20 ms and 10.24 s with a step size of 0.625 ms. To prevent advertising events of multiple devices from overlapping, an additional small random time between 0 and 10 ms is added between advertising events.  
BLE localization solutions mostly employ RSS measurements over the BLE advertising channels to implement ranging or location fingerprinting approaches [1, 3, 4, 5, 15, 16, 23]. The use of three advertising channels that are widely spaced in frequency brings challenging issues as discussed in [5, 14, 15], but it may also be possible to exploit frequency diversity offered by three channels. The repeated transmission of short bursts of advertising packet makes it possible to exploit time diversity in RSS measurements as the scanning device will be able to receive multiple copies of advertising packet over a period of time. When either or both of the devices are in motion, measurements over time also provide opportunities to exploit space diversity. In this paper, we present a RSS measurement system designed based on the Bluetooth mesh networking technology to exploit both space and time diversities in measurement, which can be used in channel measurement and modeling as well as in the RSS-based localization applications.