How to do analysis of bispectrum results?

I am doing project on signal detection using bispectrum.In this i am takin a signal using two frequecies(like one frequency at 4k and another frequency at 7k.sampling freq-18k) and adding random noise to this signal.after implementing power spectral analysis and bispectral analysis to the signal, am able to detect the signal at the given frequencies for power spectrum but for bispectrum m not able to analyze the result at the given frequency.and also compared to power spectrum bispectrum strength is decreasing.why it is coming like this?plz help me.