1. An interference cancellation system comprising: (a) a main signal reception means for receiving a main signal of a desired digital signal and an undesired interference signal; (b) an auxiliary signal reception means for receiving said undesired interference signal; (c) a control means for providing a compensation signal having an amplitude and anti-phase equal to an amplitude and anti-phase of the undesired interference signal in the main signal; (d) a combiner for combining the main signal and an output of said control means to cancel the undesired interference signal in the main signal; (e) a quadrature phase detector for demodulating the main signal; (f) a phase detector for phase detecting the undesired interference signal, said phase detector being supplied with a clock signal obtained by said quadrature phase detector for the main signal; (g) a decision circuit coupled with an output of said quadrature phase detector, to provide a demodulated digital signal; (h) an error signal detector for providing a difference between a decision level of said desired digital signal detector and an output of said quadrature phase detector; (i) a correlation means for providing a correlation between an output of said error signal detector and an output of the phase detector for said undesired interference signal; and (j) said control means being controlled by an output of said correlation means.