A method of preventing interference of adjacent frequencies which selects and uses a carrier frequency which is not adjacent to another carrier frequency is disclosed. A gate electrode is separated from the channel by a gap through which a chemical to be sensed can reach the channel which comprises a continuous monocrystalline graphene layer arranged on an electrically insulating graphene layer substrate. A processor is operative with the at least one receiver and determines the location of the at least one proxy receiver based on one of querying a look-up table or extracting image features from the mapping system and determining the location of the transmitter based on the time of arrival and at least one of angle of arrival of signals from the at least one proxy receiver and transmitter. According to embodiments, a spectral transform of a PPG signal may be performed to identify a frequency region associated with a pulse rate of the PPG signal.