4 min read

INTERFERENCE: HOW UNRELATED TRANSMISSIONS AFFECT EACH OTHER

Featured Image

Authors
Xavier Bush
Jesper Lindström
Prof. James Gross

In our previous entry, we explained the concept of collisions, the challenges they present to wireless networks and how EchoRing delivers an effective solution. This entry covers a related topic: interference.

 

INTERFERENCE, A DISTURBANCE FROM OUTSIDE ELECTROMAGNETIC SOURCES

An interference is an electromagnetic wave that originates from outside a network and disrupts communication on said network’s frequency band. In our previous post, we used the analogy of two people interrupting each other to explain collisions. Expanding on this analogy, interference would be noisy construction machinery near where the discussion takes place. Obviously, this noise is unwanted and requires the participants to speak louder and repeat themselves.

In the context of wireless communications, when interference manifests at a network’s receiver antenna it, like collisions, creates a scrambled mashup of both the intended and interfering signal.

Bild1

An operator (transmitter), a robot (receiver) and a passer-by on a mobile phone (interference source)

Interference, a Disturbance from Outside Electromagnetic Sources

An interference is an electromagnetic wave that originates from outside a network and disrupts communication on said network’s frequency band. In our previous post, we used the analogy of two people interrupting each other to explain collisions. Expanding on this analogy, interference would be noisy construction machinery near where the discussion takes place. Obviously, this noise is unwanted and requires the participants to speak louder and repeat themselves.

In the context of wireless communications, when interference manifests at a network’s receiver antenna it, like collisions, creates a scrambled mashup of both the intended and interfering signal.

InterferingNode1The Receiving node receives a combined signal from the Transmitting node and the Interfering node

 

Interference can further be divided into two categories:

In the first, the interfering signal belongs to another digital communications system (i.e. its electromagnetic wave is meant to convey information) unrelated to the affected network. This is known as cross-network interference. A good example is when Bluetooth headsets interfere with Wi-Fi transmissions and vice versa – they cannot communicate with each other directly due to platform incompatibility (imagine two people speaking different languages).

The second type of interference is known as cross-technology interference, where non-communications technology interferes with a wireless signal. A good example is when waves from a microwave oven scramble Wi-Fi transmissions on the 2.4 GHz band.

The Impact of Interference

The impact of interference on a network is also mostly identical to collisions. Thus, interference can be categorized as another type of noise that compromises signal reception. The second entry in this series stated that reliable network reception is only ensured when a signal is at least 100 times stronger than its surrounding noise. Once more, just like collisions, interference’s impact is higher the closer its source is to the receiver. A sharp drop in signal quality occurs within a certain distance, resulting in a packet loss.

Interference’s duration depends on its source. Cross-network interference, such as one Wi-Fi network interfering with another, has a duration similar to collisions. The interfering Wi-Fi station may only attempt to transmit a very short packet, therefore the interference would only last a few tens of microseconds or even shorter. The duration of cross-technology interference can be far longer, however. A microwave oven can trigger interference bursts in the tens of milliseconds – quite significant in the context of time-critical applications.

Lastly, it’s important to note that - just like collisions, shadowing and fading - interference is a random process. This means predictions are an unreliable solution.

Overcoming Interference

The key to overcoming interference depends on its source. For instance, a short electromagnetic burst from a welding machine may only last a few microseconds, which would only interfere with a signal’s initial packet transmission and not subsequent re-transmissions. A constant on-off interference pattern like that emitted from a microwave oven meanwhile would make reliable transmission nearly impossible. The ideal solution in this latter case is to monitor the network for these interference patterns and “hop” to a different, non-affected channel whenever they appear. This process is known as frequency hopping, which has long been accepted as an effective countermeasure to interference.

Bluetooth is one wireless technology that makes use of frequency hopping, yet its effectiveness is limited by being unable to analyze adjacent channel status to create a reliable hopping pattern. Combined with the fact that Bluetooth operates in the 2.4 GHz band – a very crowded band at present – Bluetooth is insufficient for industrial wireless applications.

EchoRing: Intelligent Frequency-Hopping

Two key features of EchoRing create a powerful channel-hopping solution.

Firstly, built-in handover mechanismsnsure reliable frequency-hopping, even when interference is strong enough to disrupt all communication between stations.

Secondly, EchoRing nodes continuously update and exchange information on a pool of channel-hopping candidates, ensuring at least one unaffected channel is always available. This feature requires certain EchoRing stations to periodically gather info on all available channels, during which time they cannot send or receive the EchoRing token (also explained in our previous entry) or any information packets.

The most effective way to implement a channel-sensing protocol in a token network like EchoRing is to set a “scanning period” to begin immediately after the token has cycled through all the network’s scanning nodes. Scanning periods can also begin dynamically based on the system’s runtime, such as whenever the network load is low.

FrequencyHopping2

EchoRing’s intelligent channel-hopping protocol

Lastly, it’s important to note that while EchoRing’s channel-hopping technique does require some additional latency, it remains an extremely powerful solution to avoid interference. Time invested in channel-scanning is far lower than delays resulting from interference sources in the network’s vicinity.

For more information or feedback, please write to us at sales[at]r3coms.com and we’ll be happy to assist.