Direct-detection Optical OFDM Systems Research Articles

Joint adaptive code rate technique and bit interleaver for direct-detection optical OFDM system

In this paper, we have theoretically analyzed that the existence of inter-subcarrier mixing interferences (ISMI) and frequency selective fading (FF) in the directed-detection optical orthogonal-frequency-division-multiplexing (DDO-OFDM) system will cause an unbalanced error distribution and degrade the system performance. Then we propose to employ the adaptive code rate technique (ACT) and bit interleaver in the DDO-OFDM system to combat ISMI and FF. The experimental results show that the receiver sensitivity of 5.9-Gb/s 64-ary quadrature-amplitude-modulation (64QAM) OFDM signal employing ACT(9.43Gb/s after encoding) is improved more than 2dB compared with the 5.9-Gb/s OFDM signal with 0.66 turbo coding rate (8.85Gb/s after encoding) at the bit-error ratio (BER) of 1e-4. When the coding rate is all 0.625, the OFDM signal employing ACT has more than 1-dB gain improvement compared with the OFDM signal encoded with fixed-rate turbo coding. And additional ∼1-dB receiver sensitivity improvement is enabled when the OFDM signal employ ACT and bit-interleaver simultaneously.

Hadamard Transform Combined With Companding Transform Technique for PAPR Reduction in an Optical Direct-Detection OFDM System

In this paper, we propose a novel method to reduce the high peak-to-average power ratio (PAPR) of the optical orthogonal frequency division multiplexing (OFDM) signal. The method is based on the technique of the Hadamard transform combined with a companding transform. We experimentally demonstrate that, in an optical intensity-modulation direct-detection OFDM transmission system, the hybrid method has better performance in reducing the PAPR compared with the case of applying only the Hadamard transform or only a companding transform.

An Improved Frame Synchronization for Optical OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is applied to optical communication widely because of its robustness against channel dispersion and high spectral efficiency. But the synchronization between transmitter and receiver is a critical factor for system performances. This paper proposes an improved frame timing synchronization algorithm for optical OFDM system to keep synchronization by delay time compensation. The simulation of 40 Gb/s single-side-band (SSB) direct-detection optical OFDM system is setup and investigated. The curve of timing metric and the constellation of four-quadrature amplitude modulation (QAM) are given in the paper. And the transmission distance using the proposed method is compared with Park method. The simulation results show that the improved frame synchronization method promote the performances of optical OFDM system greatly. The distance can be 1600 km without dispersion compensation under the bit error ratio of 10-4.

A PAPR Reduction Technique Based on a Constant Envelope OFDM Approach for Fiber Nonlinearity Mitigation in Optical Direct-Detection Systems

In this paper, we propose a new peak-to-average power ratio reduction technique based on a constant envelope orthogonal frequency division multiplexing (CE-OFDM) approach to mitigate fiber induced nonlinearities in direct-detection optical OFDM (DDO-OFDM) systems. Simulation results show that the proposed 10 Gbps DDO-CE-OFDM system using 16-quadrature amplitude modulation (16-QAM), 2.66 GHz signal bandwidth, and different values of electrical phase modulation index outperforms DDO-OFDM systems as it increases the fiber nonlinearity tolerance in fiber links without optical dispersion compensation. The bit error rate of the proposed transmission scheme is decreased by a factor of 1000 if compared to conventional DDO-OFDM systems, for 10 dBm of optical input power and considering a span of 960 km of standard single-mode fiber.

Bit Error Probability Evaluation in Optically Preamplified Direct-Detection OFDM Systems Using the Moment Generating Function

A novel method that allows the evaluation of the performance of an optically preamplified direct-detection baseband orthogonal frequency division multiplexing (OFDM) system with arbitrary optical and electrical filtering at the optical receiver is proposed. The method is based on the moment generating function of the symbol detected in each OFDM subcarrier after equalization and relies on the assumption that the noise samples at the fast Fourier transform (FFT) block input at the receiver side are practically uncorrelated. It is shown that, for typical filter bandwidths used in direct-detection optical OFDM systems (similar to or exceeding the OFDM signal bandwidth), the proposed method provides reasonably accurate estimates of the bit error probability for different electrical and optical filter types with various bandwidths, and for different numbers of OFDM subcarriers. For filter bandwidths smaller than the OFDM signal bandwidth, the proposed method becomes inaccurate due to the high correlation between the noise samples at the FFT block input.

Sensitivity improvement and carrier power reduction in direct-detection optical OFDM systems by subcarrier pairing

This paper introduces subcarrier pairing to optical OFDM systems and shows, using simulations, that the sensitivity of Direct-Detection Optical Orthogonal Frequency Division Multiplexed (DDO-OFDM) systems can be improved by 0.7 dB, without any coding overheads. Subcarrier pairing works because each subcarrier acquires a different electrical Signal to Interference plus Noise Ratio (SINR), which typically increases with the subcarrier's frequency. Pairing the good and bad subcarriers, so that information is split between them, improves the performance of the bad subcarrier more than it degrades the performance of the good subcarrier. This lowers the required Optical Signal to Noise Ratio (OSNR) for the system to give a certain Bit Error Ratio (BER).

Comparison of LMS-TEQ and DF-TEQ to reduce cyclic prefix length in direct detection optical OFDM system

This paper presents the comparison of using least mean square time-domain equalizer (LMS-TEQ) and decision feedback time-domain equalizer (DF-TEQ) to reduce cyclic prefix (CP) length for direct-detection of optical orthogonal frequency division multiplexing (O-OFDM) over 6960km of single mode fiber (SMF). Both TEQs are used immediately after the channel. Numerical modeling results show that they can cancel the residual inter symbol interference (ISI) and inter carrier interference (ICI) caused by both the group velocity dispersion (GVD) and the CP length being shorter than the channel impulse response (CIR). Using these TEQs allow the reduction of CP length, and consequently leading to system performance improvement. On the other hand, each of TEQs adds complexity to the system. Therefore, the aim of this paper is to analyze and compare the performance of LMS-TEQ and DF-TEQ while considering different CP length and complexity.