Hi there!
Let's check using the cyclic prefix to solve the OFDM multipath fading problem.
In the sending side, and when a multiplexing problem occurs, the same signal can arrive more than once and with different capacities, which is the so-called Multipath
To solve this problem, the cyclic prefix has been developed in the OFDM system by taking the last part of the signal and adding it to the next signal.
The time of this cyclic prefix is determined with respect to the maximum propagation delay time, which is determined by the channel,
The maximum propagation time is defined as the maximum time that the same signal can arrive duplicated by multipath to the receiver.
And this time is determined by the surrounding environment (buildings, cars ... and what causes the reflection of the signal)
We distinguish here several models of multipath fading according to the relationship between the maximum propagation delay and the symbol time.
When the propagation delay is smaller than the symbol period, the flat fading problem occurs
When the propagation delay is greater than the symbol period, we obtain the so-called selective frequency fading
Frequency Selective Fading
The cyclic prefix is added at the sender and removed at the receiver.
It serves two purposes:
Protection period: With this, the effect of multipath ISI interference can be excluded
Returning the end of the previous symbol allows for the possibility of performing a folding spell at the receiver between the received signals and thus reproducing the original signal.
It can also be used for synchronization purposes to achieve these goals.
The length of the cyclic prefix must be at least equal to the propagation delay of the channel as long as the cyclic prefix is longer than the propagation delay.
We can exclude all the problems resulting from multipath: whether it is the interference of the ISI symbols with each other or the interference of the frequency carriers with each other ICI
But if the propagation delay is greater than the cyclic prefix time, there will still be some ICI / ISI problems, noting that it will be only partial in the sense that only a small part of the delay will cause interference.
This causes a decrease in performance due to a loss of energy.
However, this loss is proportional only to the power of the portion resulting from the delay (receiving cyclic prefix intersecting the next symbol period) relative to the total OFDM symbol power.
The cyclic prefix problem can constitute the cyclic prefix 10-25% of the total code period and this causes a waste in the speed of the data as the resources are consumed without actually sending them data.
Thank you.
