A first mobile node supports peer to peer communications but not cellular communications. Other mobile modes in the communication system support a cellular mode and may support a peer to peer communications mode. The communications system is such that spectrum may be dynamically reallocated between being designated to be used for peer to peer operations and being used primarily for cellular network based operations. Beacon signals are used to communicate a current mode of frequency spectrum usage designation. The first mobile node, being in an ongoing peer to peer communications session detects that the spectrum is being reallocated for cellular based operations. The first mobile node reduces its transmission power level in response to the detected spectrum reallocation. In some embodiments, the first mobile is allowed to continue its ongoing peer to peer session while the spectrum is primarily allocated for cellular communications, albeit at a lower transmission power level.