In this paper, we investigate binary frequency shift keying (BFSK) over ambient OFDM signals. By cycling through a sequence of antenna loads providing different phase shifts at the tag, we are able to unidirectionally shift the ambient spectrum either up or down in frequency allowing the implementation of BFSK. We exploit the guard band and the orthogonality of the OFDM subcarriers to avoid both direct-link and adjacent channel interference. Different from energy detection based techniques which suffer from asymmetric error probabilities, the proposed scheme has symmetric error probabilities. Furthermore, we analyze the error performance of the optimal noncoherent detector and obtain an exact expression for the average probability of error. Finally, simulation results corroborate our analysis and show that the proposed scheme outperforms energy detection based schemes available in the literature by up to 3 dB.