The authors report on a strongly filtered direct detected non-return-to-zero differential quadrature phase shift keying (NRZ-DQPSK) system, based on a multicarrier transmitter with a tunable free spectral range. Transmission of a 7×21.4 Gb/s multichannel NRZ-DQPSK signal over a dispersion compensated optical link (legacy optical infrastructure) on a 15 GHz grid is investigated experimentally and by simulations. Further improvement of the spectral efficiency is demonstrated by simulating the same 7 × 21.4 Gb/s system on a 12.5 GHz grid. Finally, the scalability of the proposed system is investigated by simulating the transmissionof a Tb superchannel, composed of a 10-carrier polarization division multiplexed NRZ-DQPSK operating at 28 GBaud on 33 GHz channel spacing, over 320 km standard single-mode fiber. © 2014 OSA.