Real-time Generation of NHS-OFDM Signal for Direct-modulation and Direct-detection PON

Orthogonal frequency-division multiplexing-based passive optical networks (OFDM-PON) can provide high spectral efficiency and flexible two-dimensional bandwidth allocation by assigning subcarriers (SCs) and time slots. The direct-modulation and direct-detection (DM-DD) OFDM with Hermitian symmetry constraint has simple hardware implementation, low cost, and low power consumption compared to coherent detection and is more suitable for cost-sensitive PON. In such a Hermitian symmetric optical OFDM (HS-OFDM), only half of the SCs can be used to carry user data, and thus inverse fast Fourier transform (IFFT) and FFT sizes are inefficient in the viewpoint of hardware implementation. In this work, we use a single field programmable gate array and digital-to-analog converter to implement an HS-free OFDM (NHS-OFDM) transmitter without digital/analog up-conversion. We compare the on-chip resource overhead and power consumption between real-time HS-OFDM and NHS-OFDM transmitters under the same bandwidth granularity and data transmission rate. The registers, look-up tables and multipliers are saved up to 45%, 49.5% and 58.7%, respectively. Besides, the on-chip power consumption is also reduced by 15%. Moreover, the two real-time transmitters are experimentally investigated in a short-reach DM-DD transmission system. A similar bit error rate performance can be achieved after 20/50/70 km standard single-mode fiber transmission.