Multivariable Power law model

I want to regress multidimensional data (x1,x2....xn) using a power series model like:

where i ranges from 1 to n, n being number of input variables. a and m are the parameters of the power law model to be regressed.

I dont think there is an in-built function in MATLAB to support multivariate power law modeling. Can someone suggest me a simple approach to build such a model and regress the parameters.

Thanks!