1. A method of radio signal identification using a layered radio signal classifier, the method comprising: receiving a plurality of training data samples of radio signals, wherein the plurality of training data samples includes radio signals from a plurality of radio signal classes; extracting a plurality of training features from each of the plurality of training data samples; determining mutual information between each training feature of the plurality of training features and a radio signal classification decision for the plurality of radio signal classes; ranking the plurality of training features based on the mutual information; designing a plurality of Gaussian classifiers using the plurality of training features based on the ranking of the plurality of training features; selecting, from the plurality of Gaussian classifiers, a Gaussian classifier with the lowest classification error rate for the plurality of training data samples as a reduced enhanced Gaussian classifier for a first layer; classifying the plurality of training data samples using the reduced enhanced Gaussian classifier for the first layer; generating a confusion matrix summarizing the radio signal classes that training data samples from each radio signal class are classified as; identifying a disjoint error subgroup of radio signal classes based on the confusion matrix, wherein a rate of misclassification of training data samples from each radio signal class of the disjoint error subgroup of radio signal classes is above a first threshold, and wherein a rate of misclassification between radio signal classes within the disjoint error subgroup and radio signal classes outside of the disjoint error subgroup is below a second threshold value; designing a different reduced enhanced Gaussian classifier in a lower layer for the disjoint error subgroup of radio signal classes; receiving a target radio signal to be classified; extracting a plurality of target features from the target radio signal; classifying the target radio signal as belonging to a radio signal class in a subgroup of correctly classified radio signal classes or in the disjoint error subgroup that needs further classification, using the reduced enhanced Gaussian classifier for the first layer and at least one of the plurality of target features; and if the target radio signal is classified as belonging to a radio signal class in the disjoint error subgroup, classifying the target radio signal using the different reduced enhanced Gaussian classifier in the lower layer for the disjoint error subgroup.