Optimum Power Allocation for Multiuser OFDM with Arbitrary Signal Constellations

This paper formulates power allocation policies that maximize the region of mutual informations achievable in multiuser downlink OFDM channels. Arbitrary partitioning of the available tones among users and arbitrary modulation formats, possibly different for every user, are considered. Two distinct policies are derived, respectively for slow fading channels tracked instantaneously by the transmitter and for fast fading channels known only statistically thereby. With instantaneous channel tracking, the solution adopts the form of a multiuser mercury/waterfilling procedure that generalizes the single-user mercury/waterfilling introduced in [1], [2]. With only statistical channel information, in contrast, the mercury/waterfllling interpretation is lost. For both policies, a number of limiting regimes are explored and illustrative examples are provided.