Network rate optimization

Hi All,

I am new to CVX. Could anyone help me ? My CVX code is described as follows:

rand(‘state’,1)
L = 20;
n = 10;
k = 7; %average links per flow
A = double(rand(L,n) <= k/L);
c = 0.9*rand(L,1)+0.1;

% Solve network rate problem
cvx_begin
variable x(n);
maximize(sum(-log(x)))
subject to
A*x <= c
cvx_end
primal_obj = cvx_optval;

When I run it, the error occur. The error is:

Error using cvxprob/newobj (line 57)
Disciplined convex programming error:
Cannot minimize a(n) concave expression.

Error in minimize (line 21)
newobj( prob, ‘minimize’, x );

Error in mcf (line 12)
minimize(sum(log(x)))

Best,
Jiaqi