Electromagnetic interference at 2.4ghz

Electromagnetic interference at 2.4ghz can affect various devices.This article details the different users of the 2.4ghz band, how they cause interference to other users and how they are prone to interference from other users.

Phone

Many cordless telephones in the US and Canada use the 2.4GHz frequency, the same frequency at which Wi-Fi standards b, g and n 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 though, some simple, and some more complicated.
*Buy/Use wired phones.
*Buy 5.8GHz or 900MHz phones, commonly available today.
*Use VoIP/WiFi phones; these share the WiFi base stations and participate in the WiFi contention protocols.
*Test several different Wi-Fi channels 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.

Car alarm

Certain car manufacturers use the 2.4GHz frequency for their car alarm internal movement sensors. These devices broadcast on 2.45GHz (between channels 8 and 9) at a strength of 500mW. 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 susceptable to interference from other devices on the 2.4GHz band.

Microwave oven

Microwave ovens operate by emitting a very high power signal in the 2.4ghz band. Older devices have poor shielding, and often emit a very "dirty" signal, which has many spurious emissions over the entire 2.4ghz band.

This can cause considerable difficulties to Wifi and Video senders, resulting in reduced range or complete blocking of the signal.

Video sender/wireless CCTV camera

Video senders typically operate using an FM carrier to carry a video signal from one room to another (for example, satellite TV). These devices typically have 100% duty cycle, and low (10mW) transmit power. However, some devices, especially wireless cameras, operate with (often unauthorized) high power levels, and have high-gain antennas.

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 little switches inside the camera.

These devices are prone to interference from other 2.4GHz devices, due to the nature of an analog video signal showing up interference very easily. A carrier:noise ratio of some 20dB 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 Wifi, cause horizontal noise bars to appear on the screen, and can cause "popping" or "clicking" to be heard on the sound.

Wifi networks

Video senders can, in some areas, have very poor performance due to interference from Wifi networks. Particularly in the UK, whereby most broadband providers are giving away free routers with Wireless built in and enabled by default, it is possible that no clear channel can be found for the video sender, and that it is not possible to make it work.

Video Senders are a big problem for Wifi networks. Unlike Wifi, they operate with a 100% duty cycle, and also unlike Wifi, are typically only 10mhz 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 WiSP type environment is that clients (who cannot hear the video sender due to the "Hidden Node" effect) can hear the WiSP without any issues, but the receiver on the WiSP's access point is completely obliterated by the video sender, so is extremely deaf. Furthermore, due to the nature of video senders, they are not interfered with by Wifi easily, since the receiver and transmitter are typically located very close together, so the capture effect is very high, and Wifi has a very wide spectrum, so only typically 30% of the peak power of the Wifi actually affects the video sender. Also, the Wifi is not continuous transmit, so it is very difficult for the Wifi signal to interfere with the Video Sender. A combination of these factors - low power output of the Wifi compared to the Video Sender, the fact that typically the video sender is far closer to the receiver than the Wifi transmitter and the FM capture effect means that a video sender may cause problems to Wifi over a wide area, but Wifi causes little problems to the video sender.

EIRP

Many Video Senders on the market in the UK advertise a 100mW Equivalent isotropically radiated power (EIRP). However, the UK market only permits a 10mW EIRP limit. These devices cause far more interference across a far wider area, due to their excessive power. Furthermore, UK video senders are required to operate across a 20mhz bandwidth (not to be confused with 20mhz deviation, which is a different thing). This means that some foreign imported video senders are not legal since they operate on a 15mhz bandwidth or lower, which causes a higher spectral power density, increasing the interference. Furthermore, most other countries permit 100mW EIRP for video senders, meaning a lot of video senders in the UK have excessive power outputs.

Resolving interference

Normally interference is not too hard to find. Products are coming onto the market cheaply which act as spectrum analyzers and use a standard USB interface into a laptop, meaning that the interference source can be fairly easily found with a little work, a directional antenna and driving around to find the interference.

Channel change

Often solving interference is as simple as changing the channel of the offending device. Particularly with video senders, whereby plugging in the receiver with no transmitter attached will let you "see" the neighbour's video sender, this technique is considered part of the "Installation process". Where the channel of one system, such as a Wireless ISP cannot be changed, and it is being Interfered with by something such as a video sender, the owner of the video sender is normally very happy to assist with doing this, providing it is not too much work. However the problem comes when the interference is something such as a wireless CCTV camera which is mounted on a chimney and requires a long ladder to access. Such cameras, due to their height, cause serious problems across a wide area.

Jamming

One cure, although not entirely morally ethical, is to jam the signal. Legally, depending on how this is done, this is acceptable. An often used method is to program an 802.11 access point on the channel used, and to set the power to maximum and the beacon interval to something like 1ms. This causes little interference to the 802.11 network, typically far less than the offending device since the 802.11 Carrier sense multiple access (CSMA) mechanisms are still operational, but can cause great interference to the offending device. There are products coming on the market which are intended as 2.4GHz jammers, intended to jam a video sender without jamming the wireless network which is receiving the interference. [http://videojammers.weebly.com/]

Alternative product

Another cure is to offer an alternative product to the owner free of charge. Typically this would be a wired camera, which normally have far better performance than wireless cameras anyway, a cable to replace the video sender, or an alternative video sender which has been hard-wired to an alternative channel, with no means of changing it back to the offending frequency.

Parameter change

In extreme cases, where the interference is either deliberate or all attempts to get rid of the offending device have proved futile, it may be possible to look at changing the parameters of the network. Changing collinear antennas for high gain directional dishes normally works very well, since the narrow beam from a high gain dish will not physically "see" the interference. Often sector antenna have sharp "nulls" in their vertical pattern, so changing the tilt angle of sector antennas with a spectrum analyzer connected to monitor the strength of the interference can place the offending device within the null of the sector. High gain antennas on the transmitter end can "overpower" the interference, although their use may cause the Effective radiated power (ERP) of the signal to become too high, and so their use may not be legal.

ee also

* Electromagnetic interference

References