Spectrum overcrowding continues to present is a fundamental challenge for both military and commercial communications. Recent studies suggest that spectrum congestion is primarily due to inefficient usage rather than spectrum availability. Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) are two techniques being considered to improve spectrum efficiency and utilization. Interest in Cognitive Radio (CR) remains strong as the communications community strives to solve the spectrum congestion problem. In conventional CR implementations, interference to primary users is minimized using either overlay waveforms that exploit unused (white) spectrum holes or underlay waveforms that spread their power density over an ultra-wide bandwidth. In general, underlay approaches use more spectrum than overlay approaches and operate below the noise floor of primary users. We proposed a hybrid overlay/underlay waveform that realizes benefits of both waveforms and demonstrated its performance in frequency selective fading channels. This was done by extending the original Spectrally Modulated Spectrally Encoded (SMSE) framework to enable soft decision CR implementations that exploit both unused (white) and underused (gray) spectral areas. We analyze and evaluate performance of the overlay, underlay and hybrid overlay/underlay waveforms in frequency selective fading channels is presented and benefits discussed.