  We present an analytical model for describing highly relativistic
acceleration of magnetically driven jets, within the framework of ideal MHD for
cold, stationary and axisymmetric outflows. Our novel procedure is to treat the
wind equation as an algebraic relation between the relativistic Alfv\'{e}n
Mach-number and the poloidal electric to toroidal magnetic field amplitudes
ratio $\xi$.
  This allows us to obtain easily the wind solutions for
trans-fast-magnetosonic flows, together with the required range of
  $\xi$. Then, to determine the spatial variation of $\xi$, we solve
approximately the Grad-Shafranov equation applied to a jet flow ejected with a
very large total specific energy $E$ and confined within a very small opening
angle. Our trans-fast-magnetosonic model provides a closed-form expression for
the transition from a magnetically dominated flow to a kinetic-energy dominated
one, which occurs in the sub-asymptotic region far beyond the light cylinder of
the radius
  $R_{\rm L}$. Importantly, we find that the equipartition between magnetic and
kinetic energies is realized at a cylindrical radius of order of
  $R_{\rm L}E/c^{2}$, and confirm that the further conversion of magnetic to
kinetic energy proceeds logarithmically with distance in the asymptotic region.
Finally, we discuss briefly the astrophysical implications of our model for
jets originating from active galactic nuclei.
