WIRELESS PROPAGATION EFFECTS AND THEIR IMPACT ON RELIABLE TRANSMISSIONS
Dr. Mathias Bohge
Prof. James Gross
As mentioned in the introduction blog entry of last week, we start this series of blog posts by explaining the random effects occurring purely from the so-called wireless propagation environment itself. These random effects have a big impact on every signal sent by any wireless technology, which challenges reliable communications. After explaining the basics of these challenges, we introduce cooperative communication as a powerful alternative to standard failure correction methods – as the latter ones are rarely of use for industrial networks – and present EchoRing™ as the most advanced cooperative communication system on the market.
The challenge of wireless communications
In wireless communications, a transmitter conveys information (a succession of bits forming a packet) to a destination encoding it through an electromagnetic wave. Among all the effects that the electromagnetic wave suffers while travelling, the two most important phenomena that can be observed are the following:
The receiving node gets a plethora of “copies” of the originally sent electromagnetic wave. This is due to the transmit antenna sending the original wave into many different directions, in which they then bounce off objects in the propagation environment through reflections and scattering.
A human (sender) transmits to the robot (receiver) which receives multiple signals
As the waves travel from the transmitter to the receiver, regardless of the specific interaction with the objects in the propagation environment, the waves carry less and less energy.
Assuming a typical path-loss of an electromagnetic wave, which shows how
much the signal strength decreases with distance
Both phenomena are well known from everyday life:
- Multi-Path-Propagation: if sound (for instance from a singing person) is reflected many times before it reaches the destination (a human listening to it), we perceive an echo.
- Attenuation: Furthermore, it is clear that the further away the source is positioned from the destination, and in particular the more objects are in between the source and destination, the more silent will the received sound be.
Coming back to wireless transmissions, the consequence of the two phenomena described above are two random effects that can significantly reduce the received power for wireless transmissions: shadowing and fading.
To learn the basics of Shadowing and Fading, please download the free PDF at the bottom of this blog post.
As a consequence of shadowing and Fading, only a tiny fraction of the transmitted energy actually arrives at the receiver. However, to ensure a successful communication, the received signal power needs to be stronger than the noise power (typically 100 times stronger is enough). If we assume a received signal strength of 1.000 times the noise power, we can tolerate now a drop of the received signal strength by a factor of 10. Unfortunately, shadowing as well as fading are entirely capable of incurring such a drop or even a stronger one.
Standard solutions vs. cooperative communication in industrial wireless communication systems
Simple solutions to correct transmission errors in wireless include:
- Data retransmissions via automatic repeat request (ARQ) schemes on the medium access control (MAC) layer
- Forward error corrections on the physical (PHY) layer
These standard solutions are not feasible for the application in industrial wireless settings – instead, we need more sophisticated approaches, cooperative systems being one among them.
In its simplest form, cooperative communication is given by a transmitter, a receiver, as well as a further single transceiver somewhere in the transmission range of both transmitter and receiver. Given this set-up and the knowledge of the existence of the relay at the transmitter, the transmitter conveys the data of interest both to the relay and the receiver, while afterwards, the relay retransmits this data in case it has been receiving the original transmission correctly.
The cooperative schemes can be extremely effective in overcoming the impact of shadowing and fading, if a sufficient number of stations is in the communication range of the transmitter/receiver pair and, up-to-date channel state information is available.
To learn more about why standard solutions do not, but cooperative solutions do work in automation wireless communications systems, please download the free PDF at the bottom of this blog post.
EchoRing™ cooperative scheme
EchoRing™’s cooperative works in a 2-step process:
- The transmitter sends the information to the intended receiver and to the relay (or buddy station). Here, remember that the information is sent via an electromagnetic wave to all directions, which means that there is no need to send the information two times
- If for whatever reason, the information does not reach the receiver correctly, the buddy station will relay the information automatically to the receiver
The hypothetical case in which the direct transmission fails and the buddy station relays
the information from the transmitter to the receiver for a successful communication.
With this cooperative approach, EchoRing™ overcomes the main challenges given by shadowing and fading, reaching a cable-like reliability of 1 packet loss out of 100 million (10-8). This performance makes this technology suitable for the reliability demands of most industrial automation applications. The main advantages of using cooperative communications are already mentioned in the previous section and, of course, also apply to EchoRing™’s cooperative scheme.
In the next blog entry, Blog Post II “Avoiding Collisions: Adding Determinism in My Network” we will introduce how EchoRing™ deals with latency problems and collisions by adding determinism to the network.