RESUMO
The complexities of common equalizer schemes are analytically analyzed in this paper in terms of complex multiplications per bit. Based on this approach we compare the complexity of mode-division multiplexed digital signal processing algorithms with different numbers of multiplexed modes in terms of modal dispersion and distance. It is found that training symbol based equalizers have significantly lower complexity compared to blind approaches for long-haul transmission. Among the training symbol based schemes, OFDM requires the lowest complexity for crosstalk compensation in a mode-division multiplexed receiver. The main challenge for training symbol based schemes is the additional overhead required to compensate modal crosstalk, which increases the data rate. In order to achieve 2000 km transmission, the effective modal dispersion must therefore be below 6 ps/km when the OFDM specific overhead is limited to 10%. It is concluded that for few mode transmission systems the reduction of modal delay is crucial to enable long-haul performance.
RESUMO
Spatial-division multiplexing in the form of few-mode fibers has captured the attention of researchers since it is an attractive approach to significantly increase the channel capacity. However, the optical components employed in such systems introduce mode-dependent loss or gain (MDL) due to manufacturing imperfections, leading to significant system impairments. In this work the impact of MDL from optical amplifiers in few-mode fibers with either weak or strong mode coupling is analyzed for a 3x136-Gbit/s DP-QPSK mode-division multiplexed transmission system. It is shown that strong mode coupling reduces the impact of MDL in a similar manner as that polarization-dependent loss is reduced in single mode fibers by polarization-mode dispersion.
RESUMO
We demonstrate a 93.8-Gb/s real-time optical OFDM transmitter with 1024-point IFFT using polarization-multiplexing and 4-QAM modulation. This is the highest IFFT size implemented for OFDM to our knowledge. The limited resources of FPGA make it challenging to place and route such a high size IFFT. The implementation penalty of the real time transmitter compared to the case where FPGAs are used as an arbitrary waveform generators increases up to 2 dB for BER of 7x10(-4). An optical back-back measurement showed required OSNR of 26.5 dB for a BER of 10(-3).
RESUMO
Transmitter and receiver IQ imbalance causes image interference that degrades performance in high capacity and high spectral efficiency optical orthogonal frequency division multiplexing (OFDM) schemes. Digital compensation is an attractive method to relax component specifications. In this paper we report the details of a hybrid compensation method for IQ imbalance compensation, comprising of orthogonal training symbol-based method for transmitter-side compensation and an iterative image reduction-based method for receiver-side imbalance compensation. We demonstrate performance improvement using the hybrid method in presence of frequency dependent imbalance by both simulation and back-to-back direct detection optical OFDM experiment. We report on the tolerable limit of transmitter IQ imbalance under presence of carrier frequency offset.
