function [u] = second2(N,ooN,u,symbol,idt,alpha)

                mag = alpha*u.*conj(u);
                u = exp(idt*mag).*u;

                uh = fft(u)*ooN;
                udh = symbol.*uh;
                u = ifft(udh)*N;

                mag = alpha*u.*conj(u);
                u = exp(idt*mag).*u;

