  This paper proposes a power allocation scheme with co-channel allocation for
a femto base station (BS) that maximizes the sum-rate of its own femto mobile
stations (MSs) with a constraint that limits the degradation of quality of
service (QoS) of macro MSs. We have found a closed-form solution for the upper
limit on the transmission power of each sub-channel that satisfies the
constraint in a probabilistic sense. The proposed scheme is practical since it
uses only the information easily obtained by the femto BS. Moreover, our scheme
meets the constraint with minimal degradation compared to the optimal sum-rate
of the femto MSs achieved without the constraint.
