Radio labeling with pre-assigned frequencies

A radio labeling of a graph G is an assignment of pairwise distinct, positive integer labels to the vertices of G such that labels of adjacent vertices differ by at least 2. The radio labeling problem (RL) consists in determining a radio labeling that minimizes the maximum label that is used (the so-called span of the labeling). RL is a well-studied problem, mainly motivated by frequency assignment problems in which transmitters are not allowed to operate on the same frequency channel. We consider the special case where some of the transmitters have preassigned operating frequency channels. This leads to the natural variants P-RL(l) and P-RL((*)) of RL with l pre-assigned labels and an arbitrary number of pre-assigned labels, respectively. We establish a number of combinatorial, algorithmical, and complexity-theoretical results for these variants of radio labeling. In particular, we investigate a simple upper bound on the minimum span, yielding a linear time approximation algorithm with a constant additive error bound for P-RL((*)) restricted to graphs with girth greater than or equal to 5. We consider the complexity of P-RL(l) and P-RL((*)) for several cases in which RL is known to be polynomially solvable. On the negative side, we prove that P-RL(*) is NP-hard for cographs and for k-colorable graphs where a k-coloring is given (k greater than or equal to 3). On the positive side, we derive polynomial time algorithms solving P-RL((*)) and P-RL(l) for graphs with bounded maximum degree, and for solving P-RL (l) for k-colorable graphs where a k-coloring is given.