1. A system for cooperative modulation classification of multiple signals in cognitive radio networks, the system comprising: a plurality of cognitive radios, wherein each cognitive radio includes: an antenna that receives one or more unknown signals, and a modulation classifier executable to calculate fourth order cumulants of the received signals; and a centralized node that: receives a plurality of fourth order cumulants from the plurality of cognitive radios, and combines the received plurality of fourth order cumulants through a maximum likelihood-combining algorithm using the joint probability distribution of estimated fourth order cumulants of each transmitted signal.