Estimate collision probability and throughput impact when WiFi, Bluetooth, Zigbee, LoRa, or other protocols share the same ISM band. Enter duty cycles, channel counts, and power offset.
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Formula
Pcollision=100DC1×100DC2×Fshared
DC₁, DC₂— Duty cycles of each protocol (%)
F_shared— Fraction of shared channel bandwidth
How It Works
<p>The 2.4 GHz ISM band is shared by WiFi (802.11b/g/n/ax), Bluetooth/BLE, Zigbee (802.15.4), Thread, Z-Wave (at 2.4 GHz), and microwave ovens. Each protocol uses different channel plans, modulation, and duty cycles, leading to interference when their transmissions overlap in time and frequency.</p><p>Collision probability depends on three factors: <strong>duty cycle</strong> (what fraction of time each device transmits), <strong>channel overlap</strong> (how many channels are shared), and <strong>capture effect</strong> (a receiver can lock onto the stronger signal if the power difference exceeds ~3 dB, reducing effective collisions).</p><p>Practical coexistence strategies: <strong>frequency hopping</strong> (BLE and Bluetooth Classic hop across 79 channels to avoid persistent interference), <strong>adaptive frequency hopping</strong> (AFH in Bluetooth detects occupied channels and avoids them), <strong>time-domain coexistence</strong> (Packet Traffic Arbitration / PTA chips coordinate WiFi and BT transmission windows), and <strong>sub-GHz migration</strong> (LoRa, Sigfox, and 802.15.4g at 915/868 MHz avoid the congested 2.4 GHz band entirely).</p>
Worked Example
Smart home gateway: Zigbee coordinator + WiFi AP, both at 2.4 GHz. WiFi duty cycle 40% (streaming video), 3 channels. Zigbee duty cycle 2%, 16 channels. Shared channel fraction = 2/(3+16) = 10.5%. Raw collision = 0.40 × 0.02 × 0.105 = 0.084% — very low. But WiFi TX power is 100 mW (20 dBm) vs Zigbee 1 mW (0 dBm) = 20 dB offset. Capture effect reduces Zigbee→WiFi interference to near zero, but Zigbee receiver still sees WiFi as noise floor elevation of +5 dB, reducing effective range ~40%.
Common Mistakes
✗Thinking 'different channels = no interference' — 802.11b/g 22 MHz-wide channels overlap with Zigbee 2 MHz channels even when center frequencies differ by 5 MHz
✗Ignoring near-far problem — a WiFi AP 1 m away can desensitize a Zigbee receiver even when transmitting on a 'different' channel
✗Not accounting for receiver blocking/desensitization — a strong out-of-band signal can raise the noise floor of an adjacent receiver by 10–20 dB
✗Assuming duty cycle is constant — WiFi traffic is highly bursty; peak duty cycle during video streaming can reach 80%+
Frequently Asked Questions
Zigbee channels 15, 20, 25, and 26 fall in the gaps between WiFi channels 1, 6, and 11. Channel 26 (2480 MHz) is completely outside WiFi channel 11 (2462 MHz center). In practice, Zigbee channel 25 or 26 gives the best coexistence with a WiFi 1/6/11 deployment.
Sub-GHz (915 MHz in Americas, 868 MHz in Europe) is almost always better for range-critical IoT. Path loss is 10–15 dB lower at 915 vs 2.4 GHz, the band is far less congested, and LoRaWAN duty cycle limits (1% in Europe) prevent channel saturation. The only advantage of 2.4 GHz LoRa is global frequency availability — no regional band plan issues.
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