  We consider a multi-user two-ray ground reflection scenario with unknown
distances between transmitter and receivers. By using two frequencies per user
in parallel, we can mitigate possible destructive interference and ensure
ultra-reliability with only very limited knowledge at the transmitter. In this
work, we consider the problem of assigning two frequencies to each receiver in
a multi-user communication system such that the average minimum receive power
is maximized. In order to solve this problem, we introduce a generalization of
the quadratic multiple knapsack problem to include heterogeneous profits and
develop an algorithm to solve it. Compared to random frequency assignment, we
report a gain of around 6 dB in numerical simulations.
