Modular exponentiation, or scalar multiplication, is core to today's main stream public key cryptographic systems. In this article we generalize the classical fractional wNAF method for modular exponentiation - the classical method uses a digit set of the form {1, 3, . . . , m} which is extended here to any set of odd integers of the form {1, d ₂ , . . . , d _n }. We propose a general modular exponentiation algorithm based on a generalization of the frac-wNAF recoding and a new precomputation scheme. We also give general formula for the average density of non-zero therms in these representations, prove that there are infinitely many optimal sets for a given number of digits and show that the asymptotic behavior, when those digits are randomly chosen, is very close to the optimal case.