Direct-detection Optical OFDM Research Articles

Low-Complexity Receiver Using Undersampling for Guard-Band SSB-DDO-OFDM

A guard-band-based and single-side-band (SSB) modulation scheme is usually employed to improve receiver sensitivity and avoid power fading for the direct-detection optical orthogonal frequency-division multiplexing (DDO-OFDM). In this paper, we take advantages of analog-to-digital converter (ADC) undersampling to achieve a low-complexity receiver for guard-band SSB-DDO-OFDM. By using an electrical bandpass filter, the signal-to-signal beating interference is removed from the guard-band, and the desired OFDM signal is sampled at a rate twice its signal bandwidth rather than its highest frequency component. The guard-band will be filled with the aliased components which can be used to fully recover the transmitted data, and the discrete Fourier transform size at the receiver side is reduced by more than half. As a result, the implementation complexity of the receiver can be greatly reduced with the lower ADC sampling rate and reduced digital signal processing complexity. The feasibility of the proposed low-complexity receiver is investigated by numerical simulations. When the quantization noise is not dominant, the simulations show that the proposed low-complexity guard-band SSB-DDO-OFDM has similar error vector magnitude performances with the conventional one over different bias voltages and single-mode fiber links.

Suppression of laser phase noise in direct-detection optical OFDM transmission using phase-conjugated pilots

Due to the unique phase noise (PN) characteristics in direct-detection optical OFDM (DDO-OFDM) systems, the design of PN compensator is considered as a difficult task. In this paper, a laser PN suppression scheme with low complexity for DDO-OFDM based on coherent superposition of data carrying subcarriers and their phase conjugates is proposed. Through theoretical derivation, the obvious PN suppression is observed. The effectiveness of this technique is demonstrated by simulation of a 4-QAM DDO-OFDM system over 1000 km transmission length at different laser line-width and subcarrier frequency spacing. The results show that the proposed scheme can significantly suppress both varied phase rotation term (PTR) and inter-carrier interference (ICI), and the laser line-width can be relaxed with up to 9 dB OSNR saving or even breakthrough of performance floor.

Polarization-Tracking-Free IFoF Mobile Fronthaul With Adaptively Modulated PDM Multiband DDO-OFDM

This letter proposes a polarization-tracking-free polarization division multiplexing (PDM) in an intermediate frequency over fiber mobile fronthaul system based on multiband direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) with adaptive modulation. The use of two orthogonally polarized optical carriers with a simple optical filter at each radio access unit is sufficient to perform passive demultiplexing of the corresponding multiband signals in PDM schemes without transmission efficiency loss in training signal, which is commonly found in conventional PDM demultiplexing schemes. We experimentally achieved a multiband OFDM signal with adaptive modulation, which maximizes signal capacity, in a PDM scheme with the data rate of 49.64 Gb/s, which doubles that of corresponding single polarization schemes, over a 25-km single mode fiber. We also achieved a 6-dB power margin which is sufficient for the wireless transmission part.

BOTDA using channel estimation with direct-detection optical OFDM technique.

A novel Brillouin optical time-domain analysis (BOTDA) system without frequency sweep operation is proposed using intensity-modulated direct detection optical orthogonal frequency division multiplexing (IM-DD-OOFDM) probe signal. The influence of peak to average power ratio (PAPR) of OFDM probe signal on the recovery of Brillouin gain spectrum (BGS) is analyzed in theory and experiment. The complex BGS is reconstructed by channel estimation algorithm and Brillouin frequency shift (BFS) is located by curve fitting of intensity spectrum. The IM-DD-OOFDM BOTDA is demonstrated experimentally with 25 m spatial resolution over 10 km standard single mode fibers within much less measurement time.

Three-Dimensional Adaptive Modulation and Coding for DDO-OFDM Transmission System

We propose a 3-D adaptive loading algorithm (ALA), which can individually allocate proper modulation formats, power levels, and the forward error correction codes to each orthogonal frequency division multiplexing (OFDM) subcarrier according to the estimated channel state information (CSI). The introduced look-up table operation can reduce the iterations and improve the convergence speed. To evaluate the performance of the proposed algorithm, we tailor the system parameters of direct detection optical OFDM (DDO-OFDM) to investigate the difference of effective data rate (EDR) between fixed schemes and proposed adaptive modulation and coding (AMC) schemes. When the fiber length is set to 90 km and received optical power (ROP) is −10 dBm in simulation, the AMC scheme achieves 7.86 Gb/s EDR, which is 1.93 times of the maximum EDR the fixed schemes can achieve under the bit error rate (BER) constraint. Compared with the Levin−Campello algorithm, the 3-D ALA improves about 3% and 6% data rates in poor and good CSI, respectively. In experiments, when the fiber length and ROP are 50 km and −18 dBm, respectively, the AMC scheme achieves the EDR of 4.96 Gb/s, which is 2.84 times of the maximum EDR of the fixed schemes under the BER constraint.

Comparison Study of Long-haul 100-Gb/s DDO-OFDM and CO-OFDM WDM Systems

In this paper, for the first time, the transmission performances of long-haul 100-Gb/s direct detection optical OFDM (DDO-OFDM) and coherent optical OFDM (CO-OFDM) wavelength division multiplexing (WDM) systems are compared by simulation. It provides specific guides for system parameter selection to get a high-performance and cost-effective OFDM WDM system. Specifically, the comparison involves three aspects: launched power is investigated to achieve better system performance; laser linewidth is numerically investigated to choose cost-effective laser; system dispersion tolerances with different laser linewidths are analyzed to further reveal the advantages and disadvantages of these two detecting methods, direct detection and coherent detection, in long-haul OFDM WDM system.

Channel Estimation on Individual Subcarrier Using Image Processing in Optical OFDM System

In an optical orthogonal frequency division multiplexing (OFDM) system, channel impairments, such as residual chromatic dispersion, residual synchronization error, and clock frequency offset, induce phase shifts over subcarriers. We propose a method of using image processing for phase channel estimation with negligible overhead, which is applicable to various subcarrier modulation formats. We conduct the experiment of 32-, 40-, and 48-Gbit/s direct detection optical OFDM with 16QAM, 32QAM, and 64QAM subcarrier modulation and simulation of 112-Gb/s coherent optical OFDM to verify this method and compare the results with those of the conventional method. Then, we investigate the effects of the numbers of required test phases and OFDM symbols with experimental data. We also present an approach of removing the inner part of constellation points to reduce the complexity.

Asymmetric peak to average power ratio impacts on direct‐detected optical OFDM transmission link

ABSTRACTIn this article, the impact of asymmetric peak to average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal is investigated in direct detection OFDM transmission for the first time. The asymmetric PAPR is produced due to the positive definiteness of direct optical OFDM modulation. It gets bigger and bigger due to the generation of different time delay of each OFDM subcarrier, which is caused by chromatic dispersion. The performance of direct detection optical OFDM link in the presence of asymmetric PAPR is evaluated measuring error vector magnitude (EVM) of quadrature phase shift keying (QPSK) and 16‐qudrature amplitude modulation (QAM) symbols according to the optical transmission length and the transmission rate; 14% EVM (bit error rate of 10−3) of QPSK symbol is observed at the optical transmission length of 400 km despite the usage of channel equalization as well as the insertion of cyclic prefix when the asymmetric PAPR of input optical OFDM signal is above 25 dB. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2662–2665, 2016

Next-generation 400 Gb/s Ethernet PMD over SMF at 1310 nm via DD-OFDM with electro-absorption modulator-based transmitters

The technical feasibility of an unamplified 4 x 100 Gb/s wavelength division multiplexed direct-detection optical OFDM system that uses electro-absorption modulators (EAM) as transmitters is numerically demonstrated for up to 10-km reach of single-mode fiber (SMF) in O-band and 800 GHz-channel spacing. Based on the use of currently available optical components, most of which were developed for the current long-reach Ethernet standard, operating around 1300 nm, error-free operation (pre-FEC BER less than 3.8E-5) is achieved. The architecture here put forward represents a backward compatibility alternative that can be used to define next-generation 2- and 10-km 400 Gb/s Ethernet standard.

Impact of Optical Power in the Guard-Band Reduction of an Optimized DDO-OFDM System

A multiobjective optimization on the parameters of direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems in short-range links is proposed. Based on genetic algorithms, the optimization process takes into account the influence of optical power in the guard-band reduction of such multicarrier systems with 4- and 16-quadratic-amplitude modulation (QAM) subcarrier mapping and for propagation in $40$ km of standard single-mode fiber (SSMF). Simulation results show that the parameters optimization is responsible for a gain of approximately $2.5$ dB [ $\approx 1.8$ dB] in the required optical power for bit-error-rate ${\le }10^{-3}$ , for DDO-OFDM system transmission over $40$ km of uncompensated SSMF, with a guard band $B_G$ equals to $50\%$ of the signal bandwidth $B_w$ and 16-QAM [4-QAM] subcarrier mapping. For an extremely reduced guard band of only $1\%$ of $B_w$ , the gain reduced to $\approx 0.5$ dB and $\approx 1.5$ dB for 16- and 4-QAM mapping, respectively. The optimization process is experimentally validated under the transmission of the optimized DDO-OFDM systems in optical back-to-back (B2B) configuration and $40$ km of uncompensated SSMF. A fair performance comparison is discussed in the experimental results considering $B_G/B_w=0.5$ and $B_G/B_w=0.01$ in the range of the optimized optical powers. It is shown that a maximum optical power penalty of approximately $2$ dBm is registered according to a guard-band reduction from $50\%$ to $1\%$ of the useful bandwidth, for transmission over the 40 km of SSMF, considering 16-QAM subcarrier mapping.

Experimental Demonstration of Real-Time High-Level QAM-Encoded Direct-Detection Optical OFDM Systems

In this paper, high-level quadrature amplitude modulation (QAM)-encoded real-time orthogonal frequency division multiplexing (OFDM) transceivers are implemented with two field programmable gate arrays and high-resolution digital-to-analog converter (DAC) and analog-to-digital converter (ADC). Some key digital signal processing (DSP) algorithms for real-time direct-detection optical OFDM (DDO-OFDM) system are presented and described in detail. To improve the effective number of bits of ADC and reduce quantization noise, the DAC operates at 5 GS/s with an oversampling factor of 2. Meanwhile, the optimal digital clipping ratio at the transmitter is also investigated by numerical simulation to optimize the performance of the real-time transmitter. The results show that the real-time measured BERs after 10-km SSMF are below the hard-decision forward error correction threshold of $3.8 \times 10^{-3}$ . For comparison, the off-line BER performance is also analyzed using off-line DSP approaches. It shows that there is a negligible power penalty between the offline and real-time processing results. To the best of our knowledge, we have achieved the highest modulation format (1024-QAM) for real-time optical OFDM systems.

A PAPR reduction scheme based on a new spreading code in optical direct detection OFDM system

In this paper, we propose and experimentally demonstrate a peak-to-average power ratio (PAPR) reduction scheme based on a new spreading code in direct detection optical orthogonal frequency division multiplexing (OFDM) system. The new spreading code with low cross correlation and high auto-correlation can support $$2N+1$$2N+1 users. Thus, $$2N+1$$2N+1 users or data symbols can be transmitted over only N subcarriers. The experimental results show that, after transmission over 70 km single-mode fiber, at the bit error rate of $$10^{-3}$$10-3, with fiber launch power of 2.75 dBm, the receiver sensitivity can be improved 2.1 dB by using the proposed scheme based on new spreading code. The PAPR can be reduced about 4.6 dB, compared with the original OFDM signal at a complementary cumulative distribution function of $$10^{-4}$$10-4.

Real-Time LR-DDO-OFDM Transmission System Using EML With 1024-Point FFT

In this letter, we experimentally demonstrate a complete Field Programmable Gate Array (FPGA)-based real-time point-to-point long-reach (LR) direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) transmission system using a cost-effective electroabsorption modulated laser with 1024-point fast Fourier transform (FFT). To the best of our knowledge, it is the so-far reported largest FFT size used in real-time DDO-OFDM system. Based on the system, in an asynchronous way, an adaptively 16-quadratic-amplitude modulation/quadrature phase shift keying/binary phase shift keying modulated real-time optical OFDM LR transmission has been experimentally demonstrated without sampling frequency offset compensation, which is achieved by utilizing the effective symbol timing synchronization and the pilot-aided and linear-interpolated channel estimation scheme. Moreover, the bit-error rate of below than 1e-3 can be successfully achieved after 150-km standard single mode fiber transmission.

Experimental Demonstration of Improved Equalization Algorithm for IM/DD Fast OFDM

We experimentally investigate the fast orthogonal frequency division multiplexing (OFDM) scheme with improved performance in compensating asymmetric channel impulse response using single-tap equalizers. We demonstrate for the first time 18 Gb/s intensity modulation and direct detection optical fast OFDM over both 80 km standard single-mode fiber at $1.55~ \mu \text{m}$ and 1.15 km hollow-core photonic bandgap fiber at $2~\mu \text{m}$ . The results show that the proposed algorithm has better performance and lower complexity than the previous schemes under asymmetric channel impulse responses. In addition, the results validate that the emerging 2 $\mu \text{m}$ fiber-optic system can achieve the performance close to that of the 1.55 $\mu \text{m}$ system.

Cost-Effective Direct-Detection All-Optical OOK-OFDM System With Analysis of Modulator Bandwidth and Driving Power

Optical orthogonal frequency-division multiplexing (OFDM) with advanced modulation format [e.g., quadrature amplitude modulation (QAM) and quadrature phase-shift keying (QPSK)] provides many transmission advantages. As the OFDM data rate is limited by the electronic digital-to-analog (DAC) and analog-to-digital (ADC) integrated circuits, an all-optical technique (i.e., all-optical OFDM) is used to construct an OFDM symbol; hence, it can provide a significant improvement in transmission capacity. In this work, we propose and demonstrate a cost-effective 275-Gb/s direct-detection all-optical OFDM system for access or data center networks. As all wavelength channels (11 wavelengths in the experiment) are produced by one laser source, only the temperature control of one master laser is enough. In addition, each wavelength channel in the all-optical OFDM signal is encoded using on-off keying (OOK) modulation format; the traditional generation and detection circuits used in the transmitter (Tx) and receiver (Rx) of the present passive optical network can still be used. We also numerically analyze the requirements of the opt-comb-MOD (modulator used to generate the optical comb source) and the data-MOD (modulator used to generate the OOK data) in the system.

Over 210 Gb/s PDM multiband DDO-OFDM LR-PON downstream with simple self-polarization diversity.

A simple polarization division multiplexed (PDM) multiband direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) long reach passive optical network (LR-PON) with net data rate over 210 Gb/s on single wavelength channel is proposed and experimentally demonstrated with self-polarization diversity technique. The proposed self-polarization diversity function is realized at a powered remote node with all passive components to achieve cost-effectiveness and simultaneously double both the channel capacity and subscriber numbers. Meanwhile, this architecture retains the simplicity of direct-detection single receiver-end without any hardware or software modification at the optical network units. The measured power penalty of the proposed PDM multiband DDO-OFDM LR-PON is 0.8 dB over 100 km transmission with respect to that of the ordinary single polarization scheme at a specified forward error correction threshold.

A Pilot-Correlated PMD Monitoring Scheme for Direct Detection Optical OFDM System

A polarization mode dispersion monitoring scheme is proposed and demonstrated for direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems. A pair of data-assisted pilot subcarriers is inserted at two spectral edges of the OFDM signal spectrum. Simple data correlation technique is used to estimate the powers of the pilot subcarriers and thus retrieves the DGD value at the monitoring unit, without the need of demodulation of the pilot subcarriers. Experiments show a DGD monitoring range of 0 to 45 ps in a typical radio-frequency tone-assisted DDO-OFDM system. The robustness of the proposed scheme to optical signal-to-noise ratio and accumulated chromatic dispersion is further experimentally investigated. Besides, simulations and experiments are performed to demonstrate the feasibility of using photodiodes with narrow bandwidth. By inserting one more data-assisted pilot subcarrier, the proposed scheme becomes insensitive to the input angle of the state of polarization. The proposed scheme provides an effective monitoring solution for the optical OFDM signals across intermediate nodes in flexible optical network.

SFO compensation by pilot-aided channel estimation for real-time DDO-OFDM system

In this paper, we experimentally demonstrated a pilot-aided and linear interpolated channel estimation technique in the real-time direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) system using a cost-effective directly modulated laser (DML). It has been verified that the pilot-aided and linear interpolated channel estimation technique can help to compensate the sampling frequency offset (SFO) effect. The experimental results show that, based on the pilot-aided and linear interpolated channel estimation technique, even at a SFO of 170ppm, a 16-QAM-OFDM signal can be successfully transmitted over 100-km SSMF under the hard-decision forward-error-correction (HD-FEC) threshold with a bit error rate of 3.8×10−3. And the effect of up to ~25ppm SFO can be negligible.

Pilot-Aided Optical Signal-to-Noise Ratio Estimation for Direct-Detection OFDM Systems

We propose an optical signal-to-noise ratio (OSNR) estimation method for direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems by calculating the electrical SNR of one pilot subcarrier based on correlation. Simulation and experiment are conducted to verify this OSNR estimation method in a 15-Gb/s DDO-OFDM system. The maximum OSNR estimation error is 0.8 dB within an OSNR range of 8-28 dB for 100- and 200-km transmission distances.

Two orthogonal carriers assisted 101-Gb/s dual-band DDO-OFDM transmission over 320-km SSMF.

We propose a novel fading-free direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) scheme for 100-Gb/s medium-reach transmission. In the proposed scheme, we adopts two bands spaced at 100-GHz to accommodate the same complex-valued OFDM signal. However, the signals are coupled with a pair of orthogonal optical carriers. By doing so, real and imaginary parts of the complex-valued OFDM signal can be recovered from the two bands, respectively. We also propose a cost-effective scheme to generate such DDO-OFDM signal using an optical 90-degree hybrid and an optical I/Q modulator. The advantage of the proposed method is that it is fading-free, and the electrical spectral efficiency (SE) is doubled compared to traditional direct-detection method. Finally, we experimentally demonstrated a 101-Gb/s dual-band transmission over 320-km SSMF within only 30-GHz electrical bandwidth, which is highly competitive in both capacity and cost.