Wi-Fi shares its spectrum with other Wi-Fi networks and with devices that do not speak Wi-Fi at all. How it behaves depends on whether it can understand the signal, and the law decides how loud it may be.
1. DefinitionKinds of interference
- Co-channel interference: another Wi-Fi network on the same channel. Because Wi-Fi can decode it, the two share the airtime fairly: they take turns.
- Adjacent-channel interference: a network on a nearby but not identical channel. The radio cannot decode its preamble, so it counts as noise, and the transmissions cannot take turns. This is worse than co-channel.
- Non-Wi-Fi interference: Bluetooth, ZigBee, cordless phones, microwave ovens, baby monitors and poorly shielded electronics. They never take turns.
- Hidden nodes: stations that cannot hear each other (see the MAC topic).
- Clear channel assessment (CCA): the radio’s decision that the channel is busy, which is what makes it wait.
2. PurposeWhy sharing rules exist
The licence-free bands only work if devices of many kinds tolerate each other. Wi-Fi’s own rule is to listen before talking; the regulator’s rules limit power, require radar detection in some 5 GHz channels, and decide who may use what, where.
3. MechanismWhat shares 2.4 GHz
The 2.4 GHz band is the most crowded. Choose a ZigBee channel and see where it sits among Wi-Fi channels 1, 6 and 11, then switch Bluetooth and a microwave oven on.
A Wi-Fi receiver declares the channel busy when it detects the start of a Wi-Fi transmission at -82 dBm or stronger (for a 20 MHz channel). It also treats any signal 20 dB stronger, at -62 dBm, as busy even if it cannot decode it. Between those two levels a non-Wi-Fi signal does not stop Wi-Fi at all: it simply adds noise, so it damages frames instead of making stations wait.
4. VisualHow much interference a receiver tolerates
The standard also says how strong an interfering signal on a neighbouring channel a receiver must survive. A faster rate tolerates less:
| Modulation and coding rate | Adjacent channel rejection | Alternate adjacent channel rejection |
|---|---|---|
| BPSK 1/2 | 16 dB | 32 dB |
| QPSK 1/2 | 13 dB | 29 dB |
| 16-QAM 1/2 | 8 dB | 24 dB |
| 16-QAM 3/4 | 4 dB | 20 dB |
| 64-QAM 2/3 | 0 dB | 16 dB |
| 64-QAM 3/4 | -1 dB | 15 dB |
Receiver performance requirements for 20 MHz OFDM, from IEEE Std 802.11-2020, Table 17-18. At 64-QAM 3/4 the receiver need not tolerate an interferer even as strong as the wanted signal on the next channel; “alternate adjacent” means two channels away.
5. MathematicsOverlap and thresholds
- Channel spacing. 2.4 GHz channels are 5 MHz apart and 20 MHz wide, so channels 5 apart do not overlap; that gives 1, 6, 11.
- ZigBee. Channel k (11 to 26) is centred on 2405 + 5 × (k – 11) MHz. Channel 25 is at 2475 MHz, above Wi-Fi channel 11 (2462 MHz, 2452 to 2472 MHz).
- Bluetooth. A 20 MHz Wi-Fi channel covers about 21 of the 79 channels (27%). Adaptive frequency hopping lets Bluetooth skip them.
- Thresholds. Preamble detection at -82 dBm; energy detect at -82 + 20 = -62 dBm (20 MHz).
6. Real-world behaviourPractical rules
- Use 1, 6 and 11 in 2.4 GHz when you have to share. A deliberately overlapping plan makes everyone’s life worse than sharing a channel.
- Move to 5 and 6 GHz where there is more space, fewer neighbours and no household devices.
- Radar rules (DFS) in 5 GHz: a radar detected on a channel forces the network to leave it; the radar rules set a 60 second channel availability check before use, a 10 second window to move, and a 30 minute non-occupancy period.
- Wide channels multiply the problem. A 40 MHz channel in 2.4 GHz overlaps with everything; 160 MHz in 5 GHz leaves little room.
- Check what else is there with a spectrum analyser before blaming Wi-Fi.
7. Rules and regulationsUK first, then the EU and the US
Each domain starts with who sets the rules and then the limits. The tables open on the UK (Ofcom); switch to the EU (CEPT and ETSI) or the US (FCC). The region you choose is remembered across the lab.
2.4 GHz
5 GHz
6 GHz
Practical validationProve it in a capture and on a spectrum analyser
Reading about a band is one thing; seeing it in your own evidence is the skill. These checks work with any monitor-mode capture:
- Scan for neighbouring networks and note their channels and signal levels. Count how many share your channel and how many sit on overlapping channels.
- Read the channel utilisation in the BSS Load element of a beacon (access points that support it announce how busy they are).
- Run a spectrum analyser (a Wi-Fi adapter with spectrum capability, or a dedicated tool) and look for energy that does not look like Wi-Fi.
- Question to answer: your network is on channel 6 and you see another strong network on channel 4. Would you move to channel 6 co-channel, or leave them overlapping? Why?
Check your understandingQuick quiz
ReferencesSources and further reading
- IEEE Std 802.11-2020 (IEEE Xplore)
- Ofcom: UK Interface Requirement 2030 (IR 2030), current edition
- ETSI EN 300 328 (2.4 GHz harmonised standard)
- ETSI EN 301 893 (5 GHz RLAN harmonised standard)
- ETSI EN 303 687 (6 GHz WAS/RLAN harmonised standard)
- eCFR: 47 CFR 15.247 (US 2.4 GHz rules)
- eCFR: 47 CFR 15.407 (US 5 GHz U-NII rules)
- FCC 20-51: unlicensed use of the 6 GHz band (23 April 2020)
- Bluetooth Core Specification (Bluetooth SIG)
- IEEE 802.15 Working Group (802.15.4, used by ZigBee)
CCA levels (-82 dBm and -62 dBm for 20 MHz) are from clause 17.3.10.6 of IEEE Std 802.11-2020 and the adjacent channel rejection figures from Table 17-18. ZigBee channel numbering follows IEEE 802.15.4. Regulatory limits are shown by region with their sources in the rule tables; regulators change them, so check the current text.