For example, LTE supports different channel bandwidth options from 1.4 MHz to 20 MHz, which require FFT lengths of 128 to 2048 respectively. The required FFT length for OFDM modulation and demodulation for these standards varies with bandwidth option. Many popular standards like WLAN, WiMax, digital video broadcast (DVB), digital audio broadcast (DAB), and long term evolution (LTE) provide multiple bandwidth options. This example includes two models VariableSizeFFTHDLExample and VariableSizeFFTArbitraryValidPatternHDLExample that show variable-size FFT implementations for different input valid patterns. In this case, we can use a mixer (running at the low sample rate of the decimated signal) to center the desired band to zero Hertz.This example shows how to implement a variable-size FFT using a single FFT core. In fact, the center frequency Fc will be translated to : In general, if Fc cannot be expressed in the form k*Fs/D (where K is an integer), then the shifted, decimated spectrum will not be centered at DC. If we pass the signal through a complex (one-sided) bandpass filter centered at Fc = (F1+F2)/2 and with bandwidth BW = F2 - F1, and then downsample it by a factor of D = floor(Fs/BW), we will bring down the desired band to baseband. Bandpass SamplingĪn alternative zoom FFT method takes advantage of a known result from bandpass filtering (also sometimes called under-sampling): Assume we are interested in the band of a signal with sampling rate Fs Hz. the next section presents an alternative, more efficient, zoom FFT approach. The complex-valued mixer adds an extra multiplication for each high-rate sample, which is not efficient. The Mixer Approachīefore discussing the algorithm used in dsp.FFT, we present the mixer approach, which is a popular zoom FFT method.įor the example here, assume you are only interested in the interval. The next sections will discuss the zoom FFT algorithm in more detail.
This is intuitive: for a decimation factor of D, the new sampling rate is Fsd = Fs/D, and the new frame size (and FFT length) is Ld = L/D, so the resolution of the decimated signal is Fsd/Ld = Fs/L.ĭSP System Toolbox offers zoom FFT functionality for MATLAB and Simulink, through the dsp.ZoomFFT System object and the zoom FFT library block, respectively. The savings come from being able to compute a much shorter FFT while achieving the same resolution. The shorter signal comes from decimating the original signal. The idea behind zoom FFT is to retain the same resolution you would achieve with a full-size FFT on your original signal by computing a small FFT on a shorter signal. Suppose you have an application for which you are only interested in a sub-band of the Nyquist interval.
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