Inter Symbol Interference
Communication can be very straightforward for non-technical people. At one end, we put a signal on a cable, fibre, or in the air. At the receiving end of the medium, we have a receiver who picks up the signal, and that’s like a walk in the park.
However, nothing could be further from the truth. The transmission medium (Channel) is never ideal and can affect the transmission in many bad ways. One harmful effect is Inter-Signal Interference, which we will try to explain in this article.
Inter-Symbol Interference (ISI) is a phenomenon that occurs in digital transmission due to imperfections in the transmission channel.
We can see two reasons why symbols transmitted one after the other still can interfere.
- Channel Bandwidth Limitation (bandwidth not high enough)
- Channel Delay Spread (different delays of the symbols)
Channel Bandwidth Limitation
The higher the digital symbol rate, the higher the bandwidth needed to transmit the symbols without distortion. The rule is that the provided bandwidth should follow the symbol rate.
In digital communications, the bandwidth needed is directly proportional to the symbol rate.
Ignoring this rule will soften the sharp beginning and end of the symbol (steep signals mean high bandwidth). As a result, the consecutive symbols will start to overlap, which can be seen as noise (unwanted signals). Decreasing the channel’s bandwidth can degrade the signal, making it more difficult to detect individual symbols at the receiver end.
The following figure is a trial to show the effect of the bandwidth on the signal. If the symbol rate increases and the channel’s bandwidth remains the same, symbols will be more complex to detect without errors.
Delay Spread
In broadcast transmission, many electromagnetic wave reflections can and will occur. The receiver will receive multiple signals with different delays. Symbols will start to overlap and interfere with each other. The greater the overlap, the more interference and difficulty there will be in detecting the individual symbols.
Again, the transmission rate of the symbols will play a significant role. The higher the symbol rate, the more the delay spread will play a significant role.
The above figure shows how Symbols that are too short in time/length can interfere when received by a vehicle receiver. Due to the different lengths of the paths, the symbols are shifted in time against each other.
Consecutive symbols are colored RED, ORANGE, GREEN, and BLUE. It shows that the delay between signals due to different paths can lead to the receiver capturing two different symbols simultaneously. This is a typical example of ISI in wireless transmissions.
Note:
It is also a phenomenon that occurs in waveguides and fibre optics. Both waveguides and fibre optical cables have a so-called “mode”. Fibre is very similar to RF-wave guides.
A multimode fibre is thicker, and reflections occur on the cladding (outside the fibre tube). Light signals can take different reflected paths, and ISI will occur. Therefore, transmission is limited to a few hundred meters. This type of fibre is also low in price.
However, a fibre that can transport signals over a great distance is called a mono-mode fibre. It is so thin that only one light beam can go straight through the fibre core. It generates almost no ISI and can be used for long-distance communication (undersea fibre cables).
The understanding diagrams show the difference between the two cables. ISI will be generated on the multimode fibre, so its length will be a showstopper. The longer the fibre, the greater the delay spread and decrease in bandwidth. The very short fibre cables (a few meters) we use in audio are multimode fibre.
