We propose a novel guard-band-shared direct-detection (GBS-DD) scheme to improve the receiver spectrum efficiency (SE). The 100-Gb/s signal is modulated by 2 sub-bands, which are assigned onto two orthogonal polarizations. The central wavelengths of the two sub-bands are set as 10.84-GHz frequency space. The two sub-bands are then received simultaneously using a single conventional photodiode (PD) of 40-GHz bandwidth. Only one optical pilot carrier is inserted to beat with the 2 sub-bands on the two polarizations. When the 2 sub-band signal entering into the receiver, the signal-to-signal beat interference (SSBI) terms fall and overlap in the same guard band. As a consequence, the bandwidth usage of the PD is enhanced from 1/2 to 2/3. The 100-Gb/s signal is modulated using orthogonal frequency-division multiplexing based on offset quadrature-amplitude-modulation of 64-quadrature amplitude modulation (OFDM/OQAM-64QAM), and transmitted over 80-km standard single mode fiber (SSMF) within a 50-GHz optical grid. It is shown that the proposed GBS-DD scheme can be implemented by the current commercial optical/electrical devices.