Rate outage probability is an important performance metric to measure the level of quality of service (QoS) in the 4th Generation (4G) broadband access networks. Thus, in this paper, we calculate a closed form expression of the rate outage probability for a given user in a down-link multi-hop OFDMA-based system encountered as a result of links' channel variations. The channel random behavior on different subcarriers allocated to a given user are assumed to follow independent non-identical Nakagami-m distributions. Besides the rate outage probability formulas, we also derive a novel closed form formulas for the moment generating function, probability distribution function (pdf), and the cumulative distribution function (cdf) of a product of independent non-identical Gamma distributed random variables (RVs). These RVs are functions of the attainable signal-to-noise power ratio (SNR) on the allocated group of subcarriers.