The riddle of OFDM
Like I said in the introduction on the home page, it was way back in the spring of 2005, when a friend of mine asked me to explain why in OFDM communication links, on the side of the transmitter, the signal was first subjected to inverse Fast Fourier Transform (FFT), and then on the side of the receiver it underwent forward FFT. My friend, until then, had assumed that in all practical communication systems (such as those for transmission of compressed speech or video) we always started with performing the forward transform first at the transmitter and then ended with restoring the signal back to its [nearly] original form at the receiver by performing the inverse transform...
...But not in OFDM channels, where we started with the inverse FFT at the transmitter and ended with the forward FFT at the receiver.
Well, what would you explain in an answer to such a question?
Precisely! The question called for some explaining to be done of the basics, the difference between channel coding and information coding, to start with. I wanted to make the answer uncomplicated and unconvoluted, and to provide useful knowledge and understanding of the principles in order to enable the reader to study further this subject using other, more serious, publications. And the answer had to demonstrate that there was nothing unique or specific to OFDM and that exactly the same principle would apply to any orthogonal decomposition of the signal, such as with wavelets for example.
This is how I decided to create the following simulations of various communication channels based on the same principle: an inverse transform in the transmitter, transmission through noisy and frequency-shifting media, and finally a forward transform in the receiver. I wanted to make these simulations as graphic as possible, also to make them accessible and editable by their students; and I wanted not to limit them to the professional (and excellent) simulation environments such as ns2/3, Matlab and OMNeT++. This is how I chose Microsoft Excel® and Visual Basic for Applications® (VBA). Whether the results merit the effort, you be the jury.
Thanks for taking a look around and please feel free to download the Excel® workbooks and watch the simulations in action (they do use VBA code).
Yours truly,
Plamen Grozdanov
P.S. I remember well the spring of 2005 -- those were the days pre-LTE. The ideas of OFDM and OFDMA were ripe, well appreciated, and already were implemented both in WiMAX and Wi-Fi. But single carrier OFDM (SC-OFDM) was not yet as widely known as it is today, many years later. So I had not created a simulation for it in 2005, but perhaps one day I might.
Hint: each simulation is presented on its own page, navigate to them from the links shown below or using the panel on the left side.