INTERFERENCE: OTHER WIRELESS TRANSMISSIONS AFFECT MINE
Prof. James Gross
In the last blog post of 2018, we explained the concept of collision, the challenges that wireless networks face when collisions happen, and how EchoRing™ efficiently deals with them. It is very important to remember that a collision occurs whenever two devices of the same network transmit at the same time, causing a reception failure at the receiver. In this blog post, we dig into a new topic: interference. Interference is nothing but a disturbance at the receiver that is caused by an electromagnetic emission originating from a source that does not belong to the considered network. Just as in the previous blog entries, we explain the impact that interference has as well as the different potential solutions to address this challenge.
The engineer (transmitter), the robot (receiver) and the businessman (interfering station)
Interference, a disturbance from other electromagnetic sources
An interfering signal is nothing but an unwanted electromagnetic wave that operates/occurs in the same frequency band as the network of interest and where the corresponding source of the electromagnetic wave does not belong to the network of interest. In the previous blog post, we used the analogy of two persons interrupting each other to explain the concept of a collision. If we stress this analogy again, interference could be understood as the sound coming from loud construction machinery working close to where the conversation takes place. Of course, this source of noise is unwanted and makes the conversation more difficult. Going back to wireless communications, when interference manifests at the receiving antenna, it distorts the signal of interest, more or less in the same way as in the case of a collision. The resulting received signal is a is a combination of the signal-of-interest and the interfering signal.
We distinguish two types of interference. In the first case, the interfering signal belongs to a different type of (digital) radio system, i.e. the electromagnetic wave is generated with the intent to convey information. This type of interference is known as cross-technology interference, but for the sake of simplicity, we will only refer to it as interference in this blog post. An excellent example of this type of interference is the situation in which Bluetooth headsets interfere with Wi-Fi transmissions and vice versa. Both systems are digital radio transmission systems, but on the other hand, they cannot communicate directly with each other due to a technology gap (one can stress the analogy of two persons speaking different languages). A second example for interference is the electromagnetic emission of a microwave oven: It is well known that the operation of a microwave oven can interfere with Wi-Fi transmissions in the 2.4 GHz band, where the interference pattern of the oven has a very distinct pattern. The key issue though is that this emission is not intended to convey information. Instead, it is just a by-product of the heating process.
The RX node receives a combined signal from the TX node and the interfering node
Impact of Interference
The impact of the interference is the to some extent the same as that of the collisions, which we explained in detail already in the previous blog post. Thus, we can ultimately view interference as just another noise source which makes the correct reception of the signal of interest more difficult. Recall from the previous blog posts that typically a reliable reception can only happen as long as the signal of interest is much stronger than the combined noise sources, for instance, 100 times stronger. Just as in the case of collisions, the closer the source of interference to the receiving station, the higher the impact of the interference. If this source is close and happens to start emitting during an ongoing reception of a signal of interest, the consequence is that there is a sudden drop in the quality of the signal leading to a packet loss.
To analyse the duration of the interference we need to distinguish between the two sources stated in the previous section. In the case of interference coming from a different type of system, let’s say a Wi-Fi system interfering with another Wi-Fi system, the duration would be similar to the one experienced during a collision: the interfering Wi-Fi station might actually only try to transmit a very short packet, such that the unfortunate transmission situation changes rapidly with a few tens of microseconds or even faster. Whereas in a cross-technology interference situation the time span of the interference can be much greater. For example, a microwave oven might emit interference in bursts of several 10s of milliseconds.
Again, just as in the case with collisions, shadowing and fading, we cannot rely on predictions to overcome interference, as interference is a random process.
Solutions to overcome interference
To overcome interference, the key question relates to the nature of the interfering source. For instance, a short RF interference burst from a welding machine might only be present for a few microseconds. This certainly can interfere with a single packet transmission, but a subsequent transmission is not affected. In contrast, a permanent on-off interference pattern as emitted from a microwave oven permanently makes reliable communication nearly impossible. In this case, the best solution is to detect such an interference pattern and vacate the channel to a different one, which is not impacted by interference for the time being. Such a scheme is known as frequency-hopping and is well accepted as an effective mitigation scheme for interference. For instance, Bluetooth changes the working channel in a recurring and extremely fast way, however, systems that do not belong to the Bluetooth network cannot determine the hopping pattern. The lack of information of the current status of the channels and the fact that Bluetooth operates in the 2.4 GHz band, nowadays a very crowded band, makes this solution not the most suitable one to guarantee a reliable enough connection for time-critical industrial automation processes.
EchoRing™, intelligent frequency-hopping
Two distinct features in EchoRing™ add up to a powerful interference mitigation technique. Firstly, in EchoRing™ a channel is only changed if it becomes subject to significant interference. Then, reliable mechanisms lead to a channel switch even if interference is that strong that no communication between the EchoRing™ nodes is possible at all. Second, EchoRing™ nodes continuously exchange information about a pool of channels that are candidates to be used. In such a case, in the event of hopping it can be ensured that reliable communication will be possible on the new channel. This requires some of the stations to check on the pool of available channels from time to time, during which they are not able to receive payload information. In the case of a token-passing protocol like EchoRing™, the most efficient way to implement such a sensing scheme on different channels is to add a certain “scanning time” when the token has passed through all stations that are used to analyse to scan the pool of channels previously stated. This can also be introduced dynamically at run-time of the system, for instance when the load of the network is low.
EchoRing™ cleverly changes channel depending on the interference
Doing the maths, this technique obviously adds some latency to the communication. However, it is extremely powerful to tackle the challenge of interference. In the end, the time invested in scanning the channels pays off as long as there are potential interfering sources in the surroundings.