2.4 Windowing (transmitter) (labview)

Input/outputs and help

The above figure shows the sub-VI snapshot of the rather simple module that windows the outgoing signal. This VI can be imperative in certainsystems. The reason it becomes critical is the inevitable discontinuity that will arise between OFDM symbols.

If this is unclear, make sure you understand just how OFDM symbols are synthesized. The signals are built from symbol placement in the frequency domain at harmonics ofthe fundamental. As a result, there is no time-domain cohesiveness between symbols (or at least it occurs rarely). This is the same property (essentially) that plaguesOFDM systems in the form of peak-to-average power ratios.

Block diagram layout

Pictured above is the LabVIEW block diagram implementation of the windowing process. Thankfully, almost all prominent windowfunctions are built into LabVIEW. The windowing process consists of a simple multiplication in the time-domain. Those familiar with Fourier decomposition willrecognize this corresponds to convolving in the frequency domain. Usually, practical windowing involves emphasizing the middle of the time domain signal, anddeemphasizing the beginning and the end of the signal.

The reason for this deemphasis stems from the fundamental (but covert) function the Discrete Fourier Transform (DFT) performs.While subtle, it's important to note that by taking the DFT of a signal, one is enforcing periodicty of that signal. That is, we are transforming the signal into thefrequency domain under the assumption that the input repeats in the time-domain indefinitely. As a result, the un-windowed signal (or equivalently, rectangularlywindowed signals) present serious flaws from this poor assumption. This is especially manifested in our situation in which discontinuities are almost guarunteed to occur.Therefore, deemphasizing the end points, and consequently the discontinuities, proves fruitful.

The only slight of hand in the LabVIEW implementation is the window selection method. After creating a case structure, simply naming the various windows withstrings and inserting them suffices. However, in the interest of user-friendliness, an effective interface is needed. By creating a menu-ring, the options easily presentthemselves, and a simple array index operation allows us to access the case structure with ease. For more information, check out the instructional tutorial below, the example video in Figure 4, or email theauthor.

Download This LabVIEW sub-VI