  Future wireless systems are expected to support mission-critical services
demanding higher and higher reliability. In this letter, we dimension the radio
resources needed to achieve a given failure probability target for
ultra-reliable wireless systems in high interference conditions, assuming a
protocol with frequency hopping combined with packet repetitions. We resort to
packet erasure channel models and derive the minimum amount of resource units
in the case of receiver with and without collision resolution capability, as
well as the number of packet repetitions needed for achieving the failure
probability target. Analytical results are numerically validated and can be
used as a benchmark for realistic system simulations
