Entering a crowded place, such as a train station during rush hour, can produce unexpected behaviour on your shiny Bluetooth headphones. The music may cut out at the best part or turn into a robotic sound like Neo going down the rabbit hole in The Matrix. These glitches can be explained by electromagnetic interference from other devices operating in the Bluetooth band, the quality of the gadgets, and their compliance with international standards on acceptable interference levels.
One possible source of interference is Wi-Fi, which shares the same 2.4 GHz “unlicensed” frequency band. Wi-Fi is ubiquitous in every train station, airport, and shopping mall, with these big spaces requiring higher transmission power. In addition, Wi-Fi channels are wider than those from Bluetooth, so one Wi-Fi channel can impact several Bluetooth channels.
To combat interference, Bluetooth technology may access 79 available channels with Frequency-Hopping Spread Spectrum (FHSS), which can select a better channel if the current is below the reception threshold.[1] When encountering interference, the device will “hop” between channels until it finds one that’s suitable. The more sources of interference, the more hops are likely, possibly interrupting the link between the mobile phone and the headphones and creating an unwelcome interruption for the user.
The quality of these gadgets can vary widely, and it is noticeable that some cheaper brands are designed with less protection against harmful interference. Batteries may not have the capacity to support the transmission power needed in interference environments, while build quality can have a detrimental effect on both performance and longevity. The result can be a headphone that loses connection just as your favourite song begins.
Regulations and standards restrict the maximum permitted transmit power levels, but the 2.4 GHz frequency band has become crowded. In addition to Wi-Fi, many Bluetooth-enabled wearables and IoT devices are incorporated into people’s houses, increasing the sources of interference and the chances of unpredictable operation.
Beyond entertainment, Bluetooth has applications in more critical technologies. One such application is the “digital bridge”, an implant that reads brain waves and sends instructions via Bluetooth to the spine of patients suffering from paralysing injuries.[2] Such sensitive, real-time, and vital applications will require robust and reliable connections to guarantee low latency, uninterrupted transmission, and protection against external attacks.
In summary, Bluetooth devices can be affected by other transmitters on the same frequency, powerful Wi-Fi services, and poor design or construction. While the standards define a robust design, manufacturers implement them in different ways, using a range of materials, antennas, and battery capacity. The bargain eBay purchase may therefore not perform to the same standard as a premium brand, as with most modern tech. At the critical end of the market, biomedical applications are becoming more common, leveraging the low-latency, reliable, and robust connections that the standards intend.
Technology does not stand still. Like Wi-Fi, manufacturers are looking to update Bluetooth technology to use higher frequencies that provide more channels and generate less interference.[3] Your next set of headphones may not suffer the same challenges on your daily commute.
[1] Bluetooth, “Bluetooth Technology Overview,” 2021. [Online]. Available: https://www.bluetooth.com/learn-about-bluetooth/tech-overview/. [Accessed 2 August 2023].
[2] D. J. Wong, “Mashable SE Asia,” 29 May 2023. [Online]. Available: https://sea.mashable.com/tech/23955/paralyzed-man-walks-again-after-using-bluetooth-to-connect-his-brain-to-his-spine#:~:text=His%20Spine%20-%20Tech-,Paralyzed%20Man%20Walks%20Again%20After%20Using%20Bluetooth%20To%20Connect%20His,Wireless%20motor%20impul. [Accessed 7 August 2023].
[3] Businesswire.com, “Bluetooth SIG Targets 6 GHz Frequency Band,” 2022. [Online]. Available: https://www.businesswire.com/news/home/20221115005551/en/Bluetooth-SIG-Targets-6-GHz-Frequency-Band. [Accessed 04 August 2023].
