How do I solve this

e Fig. 1 shows a cross 8-QAM constellation where the inner four points are QPSK on a circle of radius r, and the outer four points are QPSK on a circle of a larger radius R. The points on the outer circle are rotated by 7/2 relative to the points on the inner circle. What is the best ratio of R/r so that the cross 8-QAM is most efficient? In other words, subject to a fixed symbol energy, what would be the largest minimum distance by changing the ratio of R/r? 0 9 9 1 % -2F 3 4 a 3 2 1 o 1 2 3 4 Figure 1: The cross 8-QAM constellation