Abstract
A data clipping and normalization technique is employed to improve the performance of the overall direct detection optical orthogonal frequency division multiplexing (DCO-OFDM) system. A detailed analysis of clipping distortion introduced by digital clipping and normalization is provided. The normalization operation amplifies the clipped data signal to the maximum input amplitude of a digital-to-analog converter (DAC). Based on the analysis, a BER formula of the proposed scheme is derived over the AWGN channel and single fiber channel. Performance of an optical clipped OFDM with normalization is assessed through numerical simulations and Monte Claro simulation over the AWGN channel. Theoretical analysis and simulation results both show that the clipping and normalization scheme can greatly improve the BER of an optical OFDM. In particular, BER performance of the proposed transmission scheme was measured in a practical OFDM transmission platform. The measured experimental results show that the clipped and amplified OFDM signal exhibits superior performance in comparison with the conventional OFDM signal. The received sensitivity at a BER of 10−3 for a 4 Gsamples/s (2.6667 Gbits/s) clipped and normalized OFDM signal with clipping ratio of 4 after 100 km standard single-mode fiber (SMF) transmission was improved by 4.3 dB when compared with the conventional OFDM system. The measured results also showed that the clipped OFDM signal exhibits superior performance in comparison with the conventional OFDM signal. Therefore, a clipping and normalization at the transmitter is most effective, and a substantial performance improvement can be obtained by a simple normalization after clipping.
Highlights
In recent years, orthogonal frequency division multiplexing (OFDM) modulation has been proposed in optical communications due to its advantage of having robustness to fiber transmission impairments such as chromatic dispersion [1,2,3]
The clipping decreases the average power of the OFDM signal, and the bit error rate (BER) performance can be slightly improved if the loss average power of the signal can be compensated
E time-domain QPSK OFDM waveforms are rst generated in a Matlab program and uploaded onto an arbitrary waveform generator (AWG) operated at 4 GS/s to generate the corresponding OFDM analog signal with the peak-peak value of 1 V. e net bit rate was 4 Gs/s ∗ 192/2/256 ∗ 256/(256 + 32) ∗ 2 2.6667 Gbits/s. en, the analog OFDM signal is modulated via an external Mach–Zehnder modulator (MZM). e MZM is biased at the quadrate point with a biased voltage of 2.2 V
Summary
Orthogonal frequency division multiplexing (OFDM) modulation has been proposed in optical communications due to its advantage of having robustness to fiber transmission impairments such as chromatic dispersion [1,2,3]. The optical OFDM signal in both types of transmission systems has a disadvantage of high PAPR. E high peak-to-average power ratio (PAPR) can lead to larger nonlinear effects, which can cause optical signal intensity fluctuation and degrade the BER performance of systems [5, 6]. For single-channel DDO-OFDM systems, high PAPR gives rise to fiber nonlinearity such as self-phase modulation (SPM). High PAPR requires large dynamic range of nonlinear devices such as digital-to-analog converters (DACs), power amplifiers, and external modulators. A fiber nonlinearity equalizer, which is based on support vector classification for optical OFDM, has been
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