WLAN Interference

Various types of interference can greatly affect the performance of an 802.11 WLAN. Interfering devices may actually prevent an 802.11 radio from transmitting.

If another RF source is transmitting with strong amplitude, an 802.11 radio can sense the energy during the clear channel assessment (CCA) and defer transmission entirely.

The other typical result of interference is that 802.11 frame transmissions become corrupted. If frames are corrupted due to interference, there will be excessive retransmissions and therefore throughput will be reduced significantly.

There are several different types of interference:

  • Physical interference Although physical interference is not technically a source of RF interference, physical obstructions can indeed disrupt and corrupt an 802.11 signal.

An example of this would be the scattering effect caused by a chain-link fence or safety glass with wire mesh. The signal is scattered and rendered useless. The only way to eliminate physical interference is to remove the obstruction or add more APs.

  • Narrowband interference A narrowband RF signal occupies a smaller and finite frequency space and will not cause a denial of service (DoS) for an entire band such as the 2.4 GHz ISM band.

A narrowband signal is usually very high amplitude and will absolutely disrupt communications in the frequency space in which it is being transmitted.

Narrowband signals can disrupt one or several 802.11 channels. The only way to eliminate narrowband interference is to locate the source of the interfering device with a spectrum analyzer.

To work around interference, use a spectrum analyzer to determine the affected channels and then design the channel reuse plan around the interfering narrowband signal.

  • Wideband interference A source of interference is normally considered wideband if the transmitting signal has the capabilities of disrupting the communications of an entire frequency band.

Wideband jammers exist that can create a complete DoS for the 2.4 GHz ISM band. The only way to eliminate wideband interference is to locate the source of the interfering device with a spectrum analyzer and remove the interfering device.

  • All-band interference The term all-band interference is normally associated with frequency hopping spread spectrum (FHSS) communications that disrupt HR-DSSS and/or ERP-OFDM channel communications.

FHSS constantly hops across an entire band intermittingly transmitting on very small subcarriers of frequency space. A legacy 802.11 FHSS radio, for example, transmits on 1 MHz hops.

While hopping and dwelling, an FHSS device will transmit in sections of the frequency space occupied by an HR-DSSS or ERP-OFDM channel.

Although a FHSS device will not cause a denial of service, the frame transmissions from the HR-DSSS and ERP-OFDM devices can be corrupted from the allband transmissions of the FHSS interfering radio.

Corruption results in retransmissions, which of course results in decreased throughput. Bluetooth (BT) is a short distance RF technology defined by the 802.15 standard. Bluetooth uses FHSS and hops across the 2.4 GHz ISM band at 1,600 hops per second.

Older Bluetooth devices were known to cause all-band interference. Newer Bluetooth devices utilize adaptive mechanisms to avoid interfering with 802.11 WLANs.

A now-defunct WLAN technology known as HomeRF also used FHSS; therefore HomeRF devices can potentially cause all-band interference. Some other all-band interferers are FHSS cordless phones and FHSS cordless headsets.

The only way to eliminate narrowband interference is to locate the source of the interfering device with a spectrum analyzer and remove the interfering device.

  • Inter-symbol interference Multipath can cause inter-symbol interference (ISI), which causes data corruption.

Because of the difference in time between the primary signal and the reflected signals known as the delay spread, along with the fact that there may be multiple reflected signals, the receiver can have problems demodulating the RF signal’s information.

The delay spread time differential results in corrupted data. Many of the negative effects of multipath, including inter-symbol interference, can be compensated for with the use of antenna diversity.

Using unidirectional antennas in areas such as hallways, long corridors, and where metal racks are present can cut down on reflections and hopefully reduce mutipath. ERP-OFDM technology is also more resistant to multipath than DSSS.