Multipath Interference Noise Research Articles

Bias-aided DD-LMS equalizer for optical multipath-interference-impaired high-speed PAM4 transmission systems

Multipath interference (MPI) noise induces drastic fluctuations in high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) systems, severely degrading the transmission performance. Here, we propose a bias-aided decision-directed least mean square (DD-LMS) equalizer to eliminate the MPI noise. In the simulation, the proposed bias-aided DD-LMS equalizer could adeptly track and compensate the MPI-impaired PAM4 signal, markedly improving the bit error rate (BER) performance under different MPI levels and laser linewidths. Within a 112 Gbps PAM4 transmission system, the proposed equalizer achieves a MPI tolerance over 4 dB at the KP4-forward error correction (FEC) threshold (2.4 × 10−4), outperforming existing MPI suppression techniques.

Low-complexity adaptive multipath interference and channel noise mitigation scheme based on optimized detection for bandwidth-limited IM/DD systems.

In this Letter, we propose a low-complexity adaptive multipath interference (MPI) and channel noise mitigation (AMCM) scheme in the receiver digital signal processing (DSP) for bandwidth-limited intensity modulation and direct detection (IM/DD) transmission systems. Following channel equalization, MPI and channel noise are distributed in the low- and high-frequency parts, respectively, and exhibit the characteristics of bandstop filtering. The proposed AMCM is designed based on optimized detection, which incorporates an adaptive bandpass filter (BPF) and a log-maximum a posteriori estimation with a lookup table-based fixed number of surviving states (LUT-based FS-MAP) decoder. The adaptive BPF is capable of mitigating the MPI and channel noise based on spectral distribution. Moreover, the LUT-based FS-MAP decoder can eliminate intersymbol interference (ISI) introduced by the BPF. The proposed AMCM is implemented in an O-band 56-Gbaud IM/DD optical 4-level pulse amplitude modulation (PAM-4) system with a 10.7-GHz bandwidth over a 10-km standard single-mode fiber with different linewidths. The results demonstrate that the proposed AMCM scheme can enhance signal-to-interference ratio (SIR) tolerance to 11 dB with only three real-valued multiplications per symbol, achieving a 7% hard-decision forward error correction threshold. To the best of our knowledge, for the first time, this represents the inaugural instance of optimized detection being employed for MPI mitigation.

Adaptive decision threshold for an optical multipath-interference-impaired short-reach 50-Gbps PAM4 transmission.

The short-reach optical transmission systems based on intensity modulation and direct detection (IM/DD) are gradually evolving into the networks with complex link topologies and connections, especially inside the data center. Multipath interference (MPI) introduces irregular fluctuations in the 4-level pulse amplitude modulation (PAM4) signals and therefore affects the transmission performance. This Letter proposes an adaptive decision threshold (ADT) scheme to dynamically update the decision threshold, which can track the signal fluctuations in real time and mitigate the impact of MPI noise on the transmitted PAM4 signals. Numerical simulation results present that the proposed ADT scheme can significantly improve the bit error rate (BER) performance of the MPI-impaired PAM4 transmission system considering different MPI levels and laser linewidths. In a 50-Gbps PAM4 transmission system, the ADT can improve the MPI tolerance by more than beyond 6 dB when the BER reaches the KP4-forward error correction (FEC) criterion (2.4 × 10-4), presenting a better denoising performance than the existing MPI-mitigation algorithms A1 and A2. Moreover, the ADT scheme offers a lower computation complexity compared with A1 and A2, making it more practical for implementation.

Enhanced Backscatter Fibers for Sensing in Telecom Networks

We discuss the application of enhanced backscattering fiber in telecom networks. Such fibers can greatly increase the potential of telecom networks to be used as sensors of network health and its surrounding environment. We describe the effects of increased attenuation and multipath interference (MPI) observed in such fibers as the fiber length and backscattering enhancement increases. For MPI we apply both a well known, approximate three bounce treatment as well as a full solution using coupled mode equations, and we verify the validity of the approximate solution. Our analysis allows us to obtain a sensor reach as a function of the backscattering enhancement level and apply this to interrogation schemes for which the minimum required signal power level and the maximum allowable MPI noise level are known. To show that the backscattering enhancement in our fiber does not interfere with telecom signals, we measure the penalty for signals propagating within and near the enhancement bandwidth in a 10 km length of continuously enhanced backscattering fiber. We find that signals only a few nm away from the enhancement bandwidth can propagate without OSNR penalty.

Optical Multipath Interference Mitigation for High-Speed PAM4 IMDD Transmission System

Multipath interference (MPI) noise suppression is crucial for high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) transmission systems. In this paper, two algorithms are proposed to eliminate the MPI noise, and these algorithms can be realized in the receiver digital signal processing (DSP) module without changing the existing IMDD system architecture. The first algorithm (A1) suppresses the MPI noise by removing the fluctuation of MPI-impaired PAM4 signals. The second algorithm (A2) can estimate the MPI noise which is intensity-dependent, and then the estimated MPI noise is subtracted from the impaired signal. The complexity of the proposed algorithms is lower than that of the traditional high-pass filter (HPF) scheme. Simulation results indicate that the proposed schemes can effectively suppress the MPI noise and improve the transmission performance under different signal-interference ratios (SIR), different laser linewidth, and random optical delay of the reflected signal. Besides, the cascade of A1 and A2 can further improve the transmission performance, and it has a higher tolerance for laser linewidth compared to the traditional HPF scheme. With the help of the proposed scheme, 56 Gb/s PAM4 signals are experimentally transmitted over 15.5 km standard single-mode fiber under the SIR of 18 dB and the laser linewidth of 100 kHz, and the measured bit error rate (BER) can be below the 7% overhead hard-decision forward error correction limit. When the laser linewidth is increased to 4 MHz, the traditional HPF scheme becomes less effective and may degrade the transmission performance, but the proposed schemes are still effective and the SIR tolerance can be improved by about 3 dB. 100 Gb/s PAM4 under optical back-to-back transmission with 24 dB SIR can be achieved, and the BER performance can be increased from 4.63e-3 to 2.06e-3 with the help of the proposed schemes.

Low-Noise Graded-Index Plastic Optical Fiber Achieved by Specific Copolymerization Process

Ultra-high-definition (UHD) technologies have recently received attention on account of their practical applications in consumer electronics. UHD devices require uncompressed video transmission at a data rate exceeding 100 Gb/s; thus, an optical fiber connection is essential. In consumer applications, optical fibers are very short. Moreover, their connections must permit variations in the fiber alignment, which accommodates rough handling by consumers. Under this condition, the transmitted signal quality is significantly degraded owing to noise and instabilities that strongly depend on the fiber alignment conditions in optical modules and connectors. Therefore, graded-index plastic optical fibers (GI POFs) are promising optical cables for consumer applications because of their flexibility, safety, and high bandwidth. Recently, the authors experimentally demonstrated that GI POFs can reduce interferometric noise, such as modal noise and multipath interference noise, in a multimode fiber link based on a vertical-cavity surface-emitting laser (VCSEL). This noise reduction effect results from strong mode coupling of GI POFs, which is closely related to microscopic heterogeneous structures of a core polymer matrix. In this paper, a control method of mode coupling using the copolymerization process for fiber core materials is proposed. The formation of composition fluctuations by copolymerization increases the mode coupling in the GI POF core. It thereby enables highly stable and robust data transmission with a large fiber misalignment tolerance in a VCSEL-based multimode fiber link. The proposed method for mode coupling control is expected to contribute to household optical communication systems in the upcoming UHD era.

RF-chirp phase dither for MPI mitigation in RoF-based 5G mobile fronthaul networks.

The multipath interference (MPI) noise is one of the most important limiting factors on the performance of the mobile fronthaul network (MFN) based on the radio-over-fiber (RoF) technology. Recently, it has been proposed to suppress this MPI noise by using the Gaussian phase dither. However, it broadens the optical spectrum significantly and, as a result, increases its vulnerability to the chromatic dispersion. To overcome this problem, we propose to suppress the MPI noise by using the RF-chirp dither instead of the Gaussian dither. The results show that, due to the narrow optical spectrum achieved by the RF-chirp dither, we can increase the transmission distance of the RoF-based MFN operating in the 1.5-µm region by three times.

BER Penalty Induced by Coherent MPI Noise in FTTH Optical Links

We measure the coherent multipath interference noise and the associated bit error rate (BER) penalty affecting an optical link in which a fraction of the signal is scattered from the fundamental into the first-order mode of a bend insensitive fiber, to mimic a situation that can be encountered in fiber to the home deployments. Our set up allows changing the relative weights and the phase difference between the two interfering modes, and measuring the BER in stationary and dynamical settings, namely with the interference conditions kept fixed or continuously varied during the measurement cycle, respectively. We show that the BER penalty detected in the dynamical measurements can be entirely ascribed to the stochastic variations of the received intensity, and we demonstrate a good agreement between the experimental data and the results of our numerical simulations.

Simultaneous monitoring technique for ASE and MPI noises in distributed Raman Amplified Systems

We develop a new technique for simultaneously monitoring the amplified spontaneous emission (ASE) and multi-path interference (MPI) noises in distributed Raman amplified (DRA) systems. This technique utilizes the facts that the degree-of polarization (DOP) of the MPI noise is 1/9, while the ASE noise is unpolarized. The results show that the proposed technique can accurately monitor both of these noises regardless of the bit rates, modulation formats, and optical signal-to-noise ratio (OSNR) levels of the signals.

Design of Double-Pass Dispersion-Compensated Raman Amplifiers for Improved Efficiency: Guidelines and Optimizations

In this paper, an intensive theoretical and experimental investigation is conducted on the dispersion-compensating Raman amplification module configured in double-pass geometry. An analytical model is developed to study the gain/pump efficiency improvement and the noise performance in a novel double-pass discrete-Raman-amplifier (DCRA) system. This paper demonstrates that the gain and pump efficiency can be improved significantly by using the double-pass configuration and the pump light reflector. The impairments of multipath-interference (MPI) noise due to the Rayleigh scattering and the external reflection are analyzed in order to balance the high gain efficiency and the enhanced MPI noise in the double-pass DCRA. Furthermore, the nonlinear penalty in the DCRA system is investigated and a 10-Gb/s nonreturn-to-zero (NRZ) signal transmission in a 4 /spl times/ 100-km fiber system is simulated. The signal quality and bit-error rate are examined, and the optimization is done with reference to the input signal level and Raman gain; hence, the impact of double-pass DCRA on the dispersion-compensated optical communication systems is explored.

Impact of reflecting residual first-order pump power on discrete higher order pumped Raman amplifier

A discrete higher order pumped Raman amplifier served for dispersion compensation and amplification with a fiber Bragg grating (FBG) reflecting residual first-order pump power back into it is demonstrated. The on–off Raman gain differences are analyzed between with and without FBG. Noise performance analysis shows that the higher order pumped Raman amplifier with FBG is less susceptible to multipath interference noise than without FBG, reducing system penalties at high on–off Raman gain. At the same time, the Q-factor-lowering problem is alleviated when the FBG is tailed to reflect residual first-order pump power.

Impact of double Rayleigh scattering noise in distributed higher order Raman pumping schemes

We analyze the equivalent optical noise figure (NF) performances and double Rayleigh-scattering-induced noise characteristics of distributed Raman amplifiers based on higher order pumping schemes. We confirm that higher order counterpumping configurations improve the amplifier equivalent NF but also increase multipath interference noise, inducing system penalties at high ON-OFF Raman gain. A Q-factor analysis allows a quantitative evaluation of the real benefits provided by higher order pumping schemes.

Impact of Multipath Interference on Raman Amplifiers Based System Performance

A novel multipath interference (MPI) model that is independent on modulation format, MPI crosstalk level, optical and electrical filter choice is proposed in this paper. MPI and ASE noise impact on receiver performance is considered taking into account the transfer functions of optical and electrical filters, intersymbol interference and receiver noise. ASE noise components are assumed to be colored Gaussian with power spectral density determined by EDFA output filter, while the components of multipath interference is colored Gaussian with the power spectral density determined by the signal spectrum. The comparison with widely used Gaussian crosstalk model is made and shown that Gaussian crosstalk model gives satisfying results just for low crosstalk level values. Also Gaussian crosstalk model is valid just for NRZ, rectangular shape of optical filter transfer function and integrate and dump electrical filter.

A method for reducing multipath interference noise

One source of noise in optical communication systems is multipath interference (MPI)-intensity noise caused by multiple discrete reflections or double Rayleigh backscatter (DRB). MPI converts phase noise to intensity noise, which can cause significant impairments in analog lightwave systems transmitting CATV signals. We have found that by dithering the laser frequency by several gigahertz at kilohertz frequencies this noise can be reduced. We have demonstrated this technique using a DFB laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurements and simulation of multipath interference for 1.7-Gb/s lightwave transmission systems using single- and multifrequency lasers

Power penalties due to multipath interference (MPI) have been measured for 1.7-Gb/s lightwave systems that use single-frequency (SF) or multifrequency (MF) lasers. Systems that use SF lasers potentially exhibit worse degradation than those using MF lasers. Bit-error-rate (BER) floors occur only under the worse-case conditions of poor receiver margin and large multiple reflections. The use of optical isolation to reduce laser feedback is ineffective in reducing multipath interference, and in many cases may worsen the penalty. It is shown that for a typical transmission system, these degradations are reduced if optical interconnection reflections are maintained below -20.5 dB. The experimental study is in good agreement with theoretical predictions using an analytic expression of the MPI noise power spectral density and with computer simulations using multimode laser rate equations. >