There are several uses of the 2.4 GHz band. Interference may occur between devices operating at 2.4 GHz. This article details the different users of the 2.4 GHz band, how they cause interference to other users and how they are prone to interference from other users.
Many cordless telephones and baby monitors in the United States and Canada use the 2.4 GHz frequency, the same frequency at which Wi-Fi standards 802.11b, 802.11g and 802.11n operate.
This can cause a significant decrease in speed, or sometimes the total blocking of the Wi-Fi signal when a conversation on the phone takes place. There are several ways to avoid this however, some simple, and some more complicated.
Using wired phones, which do not transmit.
Using cordless phones that do not use the 2.4 GHz band.
Using the 5 GHz band.
DECT 6.0 (1.9 GHz), 5.8 GHz or 900 MHz phones, commonly available today, do not use the 2.4 GHz band and thus do not interfere.
VoIP/Wi-Fi phones share the Wi-Fi base stations and participate in the Wi-Fi contention protocols.
Several different Wi-Fi channels are available and it is possible to avoid the phone channels.
The last will sometimes not be successful, as numerous cordless phones use a feature called Digital Spread Spectrum. This technology was designed to ward off eavesdroppers, but the phone will change channels at random, leaving no Wi-Fi channel safe from phone interference.
Bluetooth devices intended for use in short-range personal area networks operate from 2.4 to 2.4835 GHz. To reduce interference with other protocols that use the 2.45 GHz band, the Bluetooth protocol divides the band into 79 channels (each 1 MHz wide) and changes channels up to 1600 times per second. Newer Bluetooth versions also feature Adaptive Frequency Hopping which attempts to detect existing signals in the ISM band, such as Wi-Fi channels, and avoid them by negotiating a channel map between the communicating Bluetooth devices.
The USB 3.0 computer cable standard has been proven to generate significant amounts of Electromagnetic interference that can interfere with any Bluetooth devices a user has connected to the same computer. Various strategies can be applied to resolve the problem, ranging from simple solutions such as increasing the distance of USB 3.0 devices from any Bluetooth devices to purchasing better shielded USB cables.
Certain car manufacturers use the 2.4 GHz frequency for their car alarm internal movement sensors. These devices transmit on 2.45 GHz (between channels 8 and 9) at a strength of 500 mW.
Because of channel overlap, this will cause problems for channels 6 and 11, which are commonly used default channels for Wi-Fi connections. Because the signal is transmitted as a continuous tone, it causes particular problems for Wi-Fi traffic. This can be clearly seen with spectrum analysers. These devices, due to their short range and high power, are typically not susceptible to interference from other devices on the 2.4 GHz band.
Microwave ovens operate by emitting a very high power signal in the 2.4 GHz band. Older devices have poor shielding, and often emit a very “dirty” signal over the entire 2.4 GHz band.
This can cause considerable difficulties to Wi-Fi and video transmission, resulting in reduced range or complete blocking of the signal.
The IEEE 802.11 committee that developed the Wi-Fi specification conducted an extensive investigation into the interference potential of microwave ovens. A typical microwave oven uses a self-oscillating vacuum power tube called a magnetron and a high voltage power supply with a half wave rectifier (often with voltage doubling) and no DC filtering. This produces an RF pulse train with a duty cycle below 50% as the tube is completely off for half of every AC mains cycle: 8.33 ms in 60 Hz countries and 10 ms in 50 Hz countries.
This property gave rise to a Wi-Fi “microwave oven interference robustness” mode that segments larger data frames into fragments each small enough to fit into the oven’s “off” periods.
The 802.11 committee also found that although the instantaneous frequency of a microwave oven magnetron varies widely over each half AC cycle with the instantaneous supply voltage, at any instant it is relatively coherent, i.e., it occupies only a narrow bandwidth. The 802.11a/g signal is inherently robust against such interference because it uses OFDM with error correction information interleaved across the carriers; as long as only a few carriers are wiped out by strong narrow band interference, the information in them can be regenerated by the error correcting code from the carriers that do get through.
Wireless AV senders typically operate using an FM carrier to carry a video signal from one room to another (for example, satellite TV or closed-circuit television). These devices typically operate continuously but have low (10 mW) transmit power. However, some devices, especially wireless cameras, operate with (often unauthorized) high power levels, and have high-gain antennas.
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Amateur Radio operators can transmit two-way Amateur television (and voice) in the 2.4 GHz band – and all ISM frequencies above 902 MHz – with maximum power of 1500 watts in the US if the transmission mode does not include spread spectrum techniques. Other power levels apply per regions. In the UK, maximum power level for a full privileged license is 400 watts. In other countries, maximum power level for non – spread spectrum emissions are set by local legistration.
Although the transmitter of some video cameras appears to be fixed on one frequency, it has been found in several models that the cameras are actually frequency agile, and can have their frequency changed by disassembling the product and moving solder links or dip switches inside the camera.
These devices are prone to interference from other 2.4 GHz devices, due to the nature of an analog video signal showing up interference very easily. A carrier to noise ratio of some 20 dB is required to give a “clean” picture.
Continuous transmissions interfere with these, causing “patterning” on the picture, sometimes a dark or light shift, or complete blocking of the signal.
Non-continuous transmissions, such as Wi-Fi, cause horizontal noise bars to appear on the screen, and can cause “popping” or “clicking” to be heard in the audio.
AV senders are a big problem for Wi-Fi networks. Unlike Wi-Fi they operate continuously, and are typically only 10 MHz in bandwidth. This causes a very intense signal as viewed on a spectrum analyser, and completely obliterates over half a channel.
The result of this, typically in a Wireless Internet service provider-type environment, is that clients (who cannot hear the AV sender due to the “hidden node” effect) can hear the Wi-Fi without any issues, but the receiver on the WISP’s access point is completely obliterated by the AV sender, so is extremely deaf.
Furthermore, due to the nature of AV senders, they are not interfered with by Wi-Fi easily, since the receiver and transmitter are typically located very close together, so the capture effect is very high. Wi-Fi also has a very wide spectrum, so only typically 30% of the peak power of the Wi-Fi actually affects the AV sender.
Wi-Fi is not continuous transmit, so the Wi-Fi signal interferes only intermittently with the AV sender. A combination of these factors – low power output of the Wi-Fi compared to the AV sender, the fact that typically the AV sender is far closer to the receiver than the Wi-Fi transmitter and the FM capture effect means that a wireless AV sender may cause problems to Wi-Fi over a wide area, but the Wi-Fi unit causes few problems to the AV sender.
802.11n Wi-Fi networks are proving to be a source of interference for other wireless data networks operating at 2.4 GHz.
Many AV senders on the market in the UK advertise a 100 mW equivalent isotropically radiated power (EIRP). However, the UK market only permits a 10 mW EIRP limit. These devices cause far more interference across a far wider area, due to their excessive power.
Furthermore, UK AV senders are required to operate across a 20 MHz bandwidth (not to be confused with 20 MHz deviation). This means that some foreign imported AV senders are not legal since they operate on a 15 MHz bandwidth or lower, which causes a higher spectral power density, increasing the interference. Furthermore, most other countries permit 100 mW EIRP for AV senders, meaning a lot of AV senders in the UK have excessive power outputs.