Why is there a timing ignore (TIG) for the top-level "sigma" path in the AWGN ".ucf" file?
The "sigma" path comes from a block RAM-based lookup table that maps the Signal to Noise Ratio (SNR) in dB to a linear scale (the standard deviation). Most users set the SNR to a value and then run the core for many cycles. Therefore, the SNR input is treated as static and the timing from the "sigma" path is not critical. That is why it is ignored using the TIG constraint.
For a detailed list of LogiCORE Additive White Gaussian Noise Generator (AWGN) Release Notes and Known Issues, see (Xilinx Answer 30171).