Wigner Functions for Arbitrary Quantum Systems
  The possibility of constructing a complete, continuous Wigner function for any quantum system has been a subject of investigation for over 50 years. A key system that has served to illustrate the difficulties of this problem has been an ensemble of spins. Here we present a general and consistent framework for constructing Wigner functions exploiting the underlying symmetries in the physical system at hand. The Wigner function can be used to fully describe any quantum system of arbitrary dimension or ensemble size. 
Todd Tilma, Mark J. Everitt, John H. Samson, William J. Munro and Kae   Nemoto
quant-ph math-ph math.MP