How can I linearise this constraint $d_{u,c}\sigma \le ||{\bf f}_{u,c}||^2\le Td_{u,c}$\ $\sigma$ is a very small number based on scale of $f$ $T>0$, ${\bf f}_{u,c}$ is optimisation variable, a complex vector. $d_{u,c}$ is also an optimization variable which in binary.