Hardware-efficient generation and reception of NHS-OFDM signal for direct-detection PON

Orthogonal frequency-division multiplexing-based passive optical network (OFDM-PON) with low-cost direct detection is the current research focuses of access networks. It can realize multi-dimensional bandwidth allocation based on subcarriers and time slots. In general, the inverse fast Fourier transform (IFFT) with Hermitian symmetric (HS) mapped symbols is applied to generate the real-valued baseband signal for intensity modulation. However, only half of the subcarriers can be employed to carry user data, while the other half are modulated by the corresponding conjugate data. Therefore, both IFFT and FFT are not fully utilized in HS-OFDM systems. Under the same bandwidth granularity (data subcarriers) and data transmission rate, the non-HS OFDM (NHS-OFDM) can reduce IFFT/FFT size by half. The hardware implementation and power consumption will be significantly reduced compared to the HS-OFDM. In this paper, we design and implement the baseband HS/NHS-OFDM transceivers based on field programmable gate array chips and data converters. On-chip resource usage and power consumption are analyzed and compared. It shows that the NHS-OFDM transmitter (receiver) can save up to 58% (58%) multipliers, 46.8% (41.7%) registers, and 48% (42.9%) look-up tables. About 11.9% (19.6%) of on-chip power consumption is reduced for the transmitter (receiver). In addition, the two real-time baseband transceivers are comparatively investigated in a directly-modulated laser-based direct detection transmission system. The bit error rate (BER) performance after 20 km standard single-mode fiber transmission is measured with both offline and real-time digital signal processing (DSP) algorithms. It exhibits that the NHS-OFDM system’s power penalty is negligible at the BER of 3.8e−3, compared to the HS-OFDM. Moreover, the real-time receivers have a similar BER performance to corresponding offline ones.