A method of operating a communication device (106,108) to communicate as a secondary user in a portion of a frequency spectrum (202,204,206) that may be used by a primary user to transmit at least one primary signal to convey data to one or more primary recipients, the method comprising: in response to a detected primary signal, selecting a modulation scheme for a secondary signal for communicating in a channel within the portion of the frequency spectrum without disrupting decoding of the at least one primary signal, to obtain the data, by the one or more primary recipients, wherein selecting the modulation scheme comprises defining a derived modulation scheme, the derived modulation scheme comprising symbols selected from at least two of a plurality of modulation schemes (600,602); and selecting, for the derived modulation scheme, a set of symbols included in the at least two modulation schemes, the set of symbols comprising symbols having a low correlation to a primary set of symbols used for the primary signal, wherein the selecting of symbols to include in the derived modulation scheme includes detecting the primary signal in the portion of the frequency spectrum; determining a primary set of symbols used to transmit data on the primary signal; forming one or more subsets of symbols that are available for use in producing a new signal, wherein the one or more subsets of symbols are associated with one or more predefined constellations of symbols; correlating each of the symbols in each of the subsets to each of the symbols of the primary set of symbols; weighting the determined correlations for of each of the subsets; examining the accumulated weighted correlations; and selecting the subset with the lowest accumulated weighted correlation for use as the secondary signal. in response to a detected primary signal, selecting a modulation scheme for a secondary signal for communicating in a channel within the portion of the frequency spectrum without disrupting decoding of the at least one primary signal, to obtain the data, by the one or more primary recipients, wherein selecting the modulation scheme comprises defining a derived modulation scheme, the derived modulation scheme comprising symbols selected from at least two of a plurality of modulation schemes (600,602); and selecting, for the derived modulation scheme, a set of symbols included in the at least two modulation schemes, the set of symbols comprising symbols having a low correlation to a primary set of symbols used for the primary signal, wherein the selecting of symbols to include in the derived modulation scheme includes detecting the primary signal in the portion of the frequency spectrum; determining a primary set of symbols used to transmit data on the primary signal; forming one or more subsets of symbols that are available for use in producing a new signal, wherein the one or more subsets of symbols are associated with one or more predefined constellations of symbols; correlating each of the symbols in each of the subsets to each of the symbols of the primary set of symbols; weighting the determined correlations for of each of the subsets; examining the accumulated weighted correlations; and selecting the subset with the lowest accumulated weighted correlation for use as the secondary signal.