Bluetooth® Low Energy (LE)

Overview of Bluetooth® LE

Bluetooth® LE operates as an adaptive frequency-hopping (AFH) spread spectrum system within the globally available ISM bands at 2.4 GHz. It employs Gaussian frequency shift keying (GFSK) modulation with a symbol rate of 1 or 2 Msym/s (LE 1M, LE 2M). This frequency-hopping mechanism spans across 37 general-purpose RF channels, ensuring robust and efficient communication.

For extended-range applications, Bluetooth® 5.0 introduced LE coded channels in conjunction with LE 2M. These channels offer two optional coding schemes: S=8, where eight symbols represent a single bit, and S=2, where two symbols represent a single bit. The S=8 scheme is specifically applied to the header part of both LE coded channels.

In addition to supporting advertising communication through primary and secondary advertising channels, Bluetooth® LE also supports asynchronous communication links (ACL). The purpose of these links is controlling signaling and best effort traffic. Moreover, Bluetooth® LE introduces connected isochronous streams (CIS) and broadcast isochronous streams (BIS), which are both specifically dedicated to LE audio applications.

Bluetooth® LE testing - physical layer

Several factors play a crucial role in determining overall communication performance:

• Antenna design
• Power supplies
• Coexistence with other wireless communication chips, such as LTE and WLAN
• Final housing

Throughout and following the design phase, it is important to conduct thorough tests to ensure the accuracy of the transmitted information and achieve the desired performance benchmarks.

There are RF testing methods for Bluetooth® LE devices. One approach involves utilizing a vendor-specific control interface, commonly employed for fully automated tests in research, development and production.

For compliance testing of Bluetooth® LE devices, the Bluetooth® Special Interest Group (SIG) has standardized the direct test mode (DTM). This mode enables testing of the Bluetooth® LE transmitter and receiver, with control facilitated through a dedicated wired test interface.

The direct test mode requires a cable connector on the implementation under test (IUT) for test control. Two specified options include a USB or RS232 connection to the host control interface (HCI) of the IUT or a dedicated 2-wire UART connection to the IUT. This test setup ensures highly reliable and secure control of the IUT.

Bluetooth® LE physical layer test items typically encompass:

• Transmitter tests, which cover aspects such as transmit power, power density spectrum, spurious emission and modulation accuracy
• Receiver tests, which assess sensitivity and blocking performance
• Introduction of new features, such as direction-finding

The industry is exploring options to facilitate convenient over-the-air (OTA) testing for Bluetooth® LE devices, which often lack the provision for a wired connection. Rohde & Schwarz address this by offering two possibilities:

• Utilizing the primary advertising channels for both RX and TX measurements
• Employing a standard Bluetooth® LE connection for testing - especially useful for evaluating the performance of applications such as audio/video streaming or data transfer

We have also developed a dedicated test solution to further enhance the testing capabilities over a Bluetooth® channel. It supports DTM-like test control through an adapted Bluetooth® link layer protocol, a methodology that will be supported by Bluetooth® SIG in an upcoming release.