How to deal t*2^{u/t} in cvx?

As shown in following figure, t_m, a^{TDMA} and u_m are nonnegative variables, we have proven that this constraint is convex, however, when we try to accomplish it in CVX with matlab platform, the error occured with “Disciplined convex programming error: Cannot perform the operation: {real affine} ./ {real affine}”