Abstract-The frequency assignment problem is to assign a frequency which is a nonnegative integer to each radio transmitter so that interfering transmitters are assigned frequencies whose separation is not in a set of disallowed separations. This frequency assignment problem can be modelled with vertex labelings of graphs. An ( 2 1 . In this paper, we develop a dramatically new approach on the analysis of the adjacency matrices of the graphs to estimate the upper bounds of -numbers of the four standard graph products. By the new approach, we can achieve more accurate results and with significant improvement of the previous bounds.Index Terms-Channel assignment, (2 1)-labeling, Cartesian product, lexicographic product, direct product, strong product.