Figure 8
A graphical interpretation of the solutions of (28). The gradient vector 
depends on 
. Provided that the step-size 
is small enough, (28) yields two possible values for 
for which the updated precoder 
satisfies the power constraint. The two possible solutions for the updated precoder are given by 
and 
. The use of smaller 
yields the desired updated precoder 
for which the gradient descent converges.