Multiplier(complex signal i, q data*fft windowing coefficients(2048)) IP >>>>>>> FFT IP
the output of 2 multipliers(each complex i * fft windowing coefficients(2048) & complex q * fft windowing coefficients)) goes into input of xilinx FFT IP. the operating clock of multipliers & FFT IP is same as 15.36MHz.
When the frequency of complex data is 0Hz, the result of fft is good. But, when it is 4 or 5 Mhz, fft output appears spectral leakage of FFT.
After a product of signals you get the sum and difference of frequencies. typically the sum is discarded by low-passing after the product. If you feed an FFT with that, if the HF component is above the Nyquist frequency it will appear aliased in your spectrum.
Also bear in mind that multipliers normally take more than one cycle, let's say 4, in that case your maximum data rate would be 15 MHz/4 = 3.75 MHz, anything above you'd be screwing it up (dropping points to be precise). For a multiplier to produce data at 15 MHz, you may need to clock it faster.