In a communication medium including a first set A of n communication channels and a second set U of m communication channels, a method for selecting sensor channels in A for quantifying crosstalk from U, including operating A in a receive-only mode (FIG. 2 Step of Initialize Modem pool A. Work in receive-only mode.), choosing a subset S.sub.1 of A, estimating the expansion coefficients of A as a predefined function of subset S.sub.1 and signals received by A (FIG. 2 Step of Choose a set S1 of m modems in A and estimate the expansion coefficients g.sub.jj of modems in A relative to S1), choosing a candidate subset S.sub.2 of size m of A where the determinant of a matrix of the expansion coefficients corresponding to the subset S.sub.2 is greater than that of the expansion coefficients corresponding to any other subset of size m of A divided by a predefined bound D (FIG. 2 Step of Choose an initial candidate subset S2 where det (A.sub.S2) det (A.sub.S)/D), calculating a threshold alpha, choosing a final subset S.sub.2 that is an -amplifier of (FIG. 2Step of Test S2 until a final subset S2 is found which is an -amplifier for a defined), and employing the communications channels in the final subset S.sub.2 as sensor channels (FIG. 2 Step of Use the modems corresponding to the final subset S2 as the sensor set for the remaining modems m-n in A) for quantifying crosstalk from U.