Bluetooth: Why Broadcast Channels Must Stay Apart
The Airwaves at 2.4 GHz
Bluetooth lives in the 2.4 GHz Industrial, Scientific and Medical (ISM) band, a slice of spectrum shared by Wi-Fi, Zigbee, microwave ovens, baby monitors and countless proprietary gadgets. Within this busy neighbourhood, Bluetooth divides its playground into forty 2 MHz-wide channels. Only three of them—37, 38 and 39—are reserved for advertising packets, the short bursts that let beacons shout their presence. Keeping these three channels separated is not an oversight in the protocol; it is a deliberate survival strategy.
Collision Avoidance in a Crowded Band
Imagine forty tiny lanes merging into a single highway. If every vehicle picked the same lane, pile-ups would be inevitable. Bluetooth’s advertising channels act like staggered on-ramps. Channels 37 (2402 MHz), 38 (2426 MHz) and 39 (2480 MHz) sit at the very bottom, middle and top of the band. This spacing maximises spectral distance, reducing the probability that two advertisers will step on each other’s transmissions. A crowded café filled with phones, tablets and wireless headphones still allows a beacon on channel 37 to be heard while another on 38 or 39 remains clear.
Mitigating Wi-Fi Overlap
Wi-Fi channels 1, 6 and 11 are 20 MHz wide and sit almost exactly where Bluetooth’s advertising packets appear. Channel 1 overlaps with 37, channel 6 straddles the mid-band, and channel 11 encroaches on 39. By spacing the three advertising channels wide apart, Bluetooth ensures that even if Wi-Fi traffic saturates one region, the other two remain usable. A fitness tracker broadcasting on 2426 MHz (channel 38) can still reach a phone even when a nearby laptop is streaming Netflix on Wi-Fi channel 6.
Frequency-Hopping Resilience
Classic Bluetooth connections use adaptive frequency hopping (AFH) across seventy-nine 1 MHz channels, but advertisers do not hop; they stay put. Fixed yet widely separated channels give advertisers a fighting chance to be heard despite the hopping traffic around them. If interference spikes on 2402 MHz, the listener can still catch the beacon on 2426 MHz or 2480 MHz without waiting for the next hop cycle. This redundancy is built into the protocol rather than left to chance.
Regulatory and Power Constraints
Regulatory bodies impose strict limits on transmit power and duty cycle in the 2.4 GHz band. By spacing the advertising channels, Bluetooth achieves regulatory compliance while minimising power consumption. A beacon need only transmit three short packets on three separate channels instead of sweeping the entire band, saving battery life and staying within legal emission limits.
Real-World Consequences of Mis-spacing
Early proprietary beacon designs that used adjacent channels (for example, 2402 MHz, 2404 MHz and 2406 MHz) quickly discovered coexistence issues. In dense deployments, collision rates soared, causing missed detections and erratic ranging. Re-engineering those systems to adopt the official 37-38-39 spacing restored reliability overnight. The lesson was clear: spectral breathing room is not optional.
Future-Proofing Against New Threats
As the ISM band grows ever more crowded with IoT sensors, Thread networks and upcoming Wi-Fi 6E extensions, the fixed spacing of Bluetooth advertising channels offers a stable anchor. Engineers can design new features—mesh networking extensions, direction-finding packets, or low-energy audio—confident that the basic advertising layer will remain audible even when the spectrum feels like rush hour.
Conclusion
The deliberate separation of Bluetooth advertising channels is a textbook example of engineering foresight. By planting three beacons far apart in a noisy jungle of radio waves, the protocol buys resilience, coexistence and battery life in one elegant stroke. The next time you see a beacon chirping on 2402 MHz, remember: its companions at 2426 MHz and 2480 MHz are not arbitrary neighbours but strategic guardians, ensuring your device always finds its voice amid the cacophony.