Spectral efficiency of direct detection (DD) orthogonal frequency division multiplexing (OFDM) transmission systems is studied in the context of passive optical networks (PONs).Impacts of limited roll-off factor of shaping filter and required guard-band (GB) are studied through numerical simulations, showing that the Carrier to Signal Power Ratio (CSPR) value plays a significant role in the systems performance. A heterodyne envelope (HE) receiver is proposed in which for a QPSK system with 10Gbps bitrate and for a bit error ratio (BER) quality threshold of 10-3 shows an improvement of 10.8dB in sensitivity and 25% in spectral efficiency by using optimum CSPR values as compared to standard unity CSPR. This is due to the fact that larger CSPR values reduce the level of Signal to Signal Mixing Interference (SSMI) products falling in the GB with respect to signal levels. Besides, remote heterodyne (RH) DD-OFDM system is evaluated and it is shown that total GB of RH-DD-OFDM system can be removed using optimum value of CSPR at the cost of 9 dB power Penalty with respect unity CSPR.