Code Acquisition in Direct Sequence Spread Spectrum Communication Systems Using an Approximate Fast Fourier Transform

This paper deals with code acquisition in communication systems with large frequency offsets. In such a situation, the receiver has to perform a 2-dimensional search in a time-frequency plane to detect a known signal. In previous publications it has been proposed to use a fast Fourier transform (FFT) to implement the search in frequency direction. We would here like to examine the effect of using approximate FFTs. For this purpose, we restrict the angles of the complex multiplications that are used in the FFT to four or eight states. This helps to reduce the total number of real multiplications. Further, there is no need to store trigonometric values. The drawback is that the approximations lead to a degradation at the FFT output. We will show, however, that in a two-stage acquisition scheme, where the FFT is only used to obtain an estimate of the frequency offset, approximate FFTs achieve almost the same performance as a conventional FFT