Signal-to-signal Beating Interference Research Articles

Joint signal-to-signal beat interference mitigation for the field recovery of symmetric carrier-assisted differential detection with low carrier-to-signal power ratio

As a combination of direct detection and coherent detection technologies, self-coherent detection has the advantages of low cost and optical field recovery ability. However, most of the self-coherent detection techniques are limited to single sideband (SSB) signals. Recently, carrier-assisted differential detection (CADD) has been proposed to realize complex-valued double sideband (DSB) signals, but it requires a high carrier-to-signal power ratio (CSPR) to mitigate the signal-to-signal beat interference (SSBI). Later, a more cost-effective symmetric CADD (S-CADD) has been proposed while the required CSPR is still high. In order to alleviate the high requirements of CSPR, we propose a scheme based on the joint of digital pre-distortion (DPD) at transmitter and clipping at receiver to further improve the S-CADD system performance. This joint processing can not only solve the problem of non-uniform distribution of subcarrier signal-to-noise ratio (SNR) caused by non-ideal transfer function, but also the error propagation problem caused by enhanced SSBI under low CSPR. After the validation of the 64 Gbaud 16-ary quadrature amplitude modulation (16-QAM) orthogonal frequency division multiplexing (OFDM) signal transmitted over 80 km standard single mode fiber (SSMF), the CSPR required by the proposed scheme to reach the 20% soft decision-forward error correction (SD-FEC) and 7% hard decision-forward error correction (HD-FEC) can be reduced by 1.3 dB and 2.8 dB, respectively, with a comparison of the conventional S-CADD. The results show the potential of the proposed scheme in the short-reach optical transmissions.

Physical prior inspired ensemble learning enables effective channel estimation of underwater visible light communication.

Deep neural networks have been applied to estimate the optical channel in communication systems. However, the underwater visible light channel is highly complex, making it challenging for a single network to accurately capture all its features. This paper presents a novel approach to underwater visible light channel estimation using a physical prior inspired network based on ensemble learning. A three-subnetwork architecture was developed to estimate the linear distortion from inter-symbol interference (ISI), quadratic distortion from signal-to-signal beat interference (SSBI), and higher-order distortion from the optoelectronic device. The superiority of the Ensemble estimator is demonstrated from both the time and frequency domains. In terms of mean square error performance, the Ensemble estimator outperforms the LMS estimator by 6.8 dB and the single network estimators by 15.4 dB. In terms of spectrum mismatch, the Ensemble estimator has the lowest average channel response error, which is 0.32 dB, compared to 0.81 dB for LMS estimator, 0.97 dB for the Linear estimator, and 0.76 dB for the ReLU estimator. Additionally, the Ensemble estimator was able to learn the V-shaped Vpp-BER curves of the channel, a task not achievable by single network estimators. Therefore, the proposed Ensemble estimator is a valuable tool for underwater visible light channel estimation, with potential applications in post-equalization, pre-equalization, and end-to-end communication.

2×2 MIMO Equalizer Enabled Transmitter Side IQ Imbalance Compensation for Optical Single Sideband Direct Detection System

Recently, optical single sideband (SSB) based direct detection (DD) has been considered as a promising candidate for inter data center interconnects (inter-DCIs). Kramers-Kronig (KK) receiver is always applied to eliminate the signal-to-signal beating interference (SSBI) caused by the square law-detection of a single-ended photodiode (PD) in SSB DD system. The electro-to optical (E/O) conversion of the complex signal can be realized by IQ modulator (IQM). However, IQ imbalance exists at the transmitter since the in-phase and quadrature-phase ports of IQM are driven by the real and imaginary parts of the signal, respectively. In this paper, we show the theoretical analysis of the transmitter side (Tx-side) IQ imbalance includes time delay, amplitude mismatch and phase mismatch. Tx-side IQ imbalance will seriously deteriorate the system performance. To deal with the impairment caused by Tx-side IQ imbalance, a 22 real-value multiple-input multiple-output (MIMO) equalizer is proposed and implemented, and the system with the impact of these three factors are investigated in simulation, respectively. Besides, a transmission of 180 Gb/s/ discrete Fourier transform spread (DFT-S) orthogonal frequency division multiplexing (OFDM) signal over 80-km single mode fiber (SMF) is experimentally demonstrated with bit error rate (BER) below the soft-decision forward error correction (SD-FEC) threshold of 210-2 enabled by the proposed 22 MIMO equalizer.

Carrier Assisted Differential Detection With Generalized and Simplified Receiver Structure

Coherent detection has the primary advantage of optical field recovery, which enables channel impairment compensation and advanced modulation formats for high-capacity transmission. On the other hand, direct detection (DD) is more cost-effective due to its local oscillator (LO)-free detection. To gain the field recovery advantage while preserving the LO-free detection for DD-based receivers, carrier assisted differential detection (CADD) has been proposed to retrieve the complex-valued double-sideband (DSB) signal. In this paper, to extend the concept of CADD, we propose a simplified CADD scheme with reduced hardware complexity and present signal-to-signal beating interference (SSBI) iterative mitigation algorithms with and without symbol decision. The required number of photodetectors and analog-digital converters (ADCs) for the simplified CADD receiver becomes the same as the coherent homodyne counterpart but without needing the expensive narrow-linewidth LOs. The performance of the proposed simplified CADD receiver is evaluated by 60-Gbaud 16-QAM DSB signals. Extensive performance comparisons are made between (i) the generalized CADD receivers with and without the simplified receiver structure, (ii) SSBI iterative mitigation algorithm with and without the symbol decision, and (iii) single carrier (SC) and OFDM-modulated DSB signals for the simplified CADD receiver.

Photonic Armstrong method enabled by direct detection for wideband electrical PM generation.

Wideband signal generation using frequency/phase modulation (FM/PM) is the key fundamental function for various applications such as radar and analog communication systems. It is well known that analog FM/PM communication systems can dramatically improve signal quality by spectral expansion. In classical communication theories, the Armstrong indirect method is one of the most popular methods for bandwidth expansion of FM signals. In the Armstrong method, a narrowband signal is converted to a broadband signal with the help of a nonlinear frequency multiplier. In this paper, we propose a photonic Armstrong method enabled by direct detection. By utilizing the nonlinearity caused by direct detection, we can increase bandwidth so that it is double that of the original signal in the photonic Armstrong method. Also, it completely eliminates signal-to-signal beat interference (SSBI) at the same time. We conducted an experiment to verify the concept and confirmed these advantages experimentally.

Optical Single Sideband Signal Reconstruction Based on Time-Domain Iteration

Due to its low cost, simple architecture and robustness to fiber dispersion, single sideband (SSB) transmission with direct detection (DD) system is an attractive solution for 80-km inter data center interconnects (DCIs). However, it will suffer performance penalty caused by the signal-to-signal beating interference (SSBI). Kramers-Kronig (KK) receiver has been extensively investigated for SSBI elimination by reconstructing the SSB signal. The nonlinear operations in KK algorithm require up-sampling to cope with spectral broadening, which results in high complexity for practical application. Optical signal phase retrieval method based on the minimum phase signal has also been investigated for SSB signal recovery, in which the SSB and DC-Value properties are iteratively imposed on the amplitude signal in frequency domain. In this paper, we propose a low complexity iterative algorithm for minimum phase signal recovery without up-sampling in time domain. Finite impulse response (FIR) filter is applied to iteratively generate the SSB signal and update the phase component. Based on the proposed scheme, the transmission of 30 GHz SSB 16-QAM discrete multitone (DMT) signal over 80 km single mode fiber (SMF) is successfully demonstrated with the bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3. The experimental results show that, the BER performance of KK scheme with up-sampling factor of 2, frequency-domain iterative scheme and our proposed scheme is almost the same. However, compared with the KK scheme, the proposed method can save the numbers of adders and multipliers by the factors of 29 and 7, while the factors are 5.5 and 4 comparing to the frequency-domain iteration scheme.

A Lower Frequency Shift Based on Mode Conversion for Optical Heterodyne Micro-Vibration Measurement

We experimentally obtain a lower frequency shift based on mode conversion by using a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG) for optical heterodyne micro-vibration measurement. Through efficient conversion between LP11 and LP01 core modes in a few-mode fiber, a mode conversion frequency shifter (MCFS) can be achieved and the magnitude of frequency shift is several hundred kHz, which is generally challenging for traditional bulk acousto-optic frequency shifter. Moreover, the signal-to-signal beating interference (SSBI) can be eliminated by using the MCFS without requirement of guard band. The scheme of optical heterodyne micro-vibration measurement based on the MCFS has a high-quality signal-to-noise ratio (SNR) over 85 dB. An all-fiber Bragg grating (FBG) scheme is also designed to improve the measurement sensitivity of heterodyne detection, which has a measurable vibration frequency range of 1 Hz to 300 kHz and minimal detectable amplitude of 0.019 nm. The all-fiber MCFS has the advantages of compactness, easy integration and can be applied directly to satellite ground station and laser feedback tomography.

Dual-Drive Mach-Zehnder Modulator-Based Single Side-Band Modulation Direct Detection System Without Signal-to-Signal Beating Interference

Kramers-Kronig (KK) receiver based optical single sideband (SSB) direct detection (DD) systems have been extensively investigated to cope with signal-to-signal beating interference (SSBI) for long-reach optical interconnect and 400G ZR. In this paper, we analyze the principle of dual-drive Mach-Zehnder modulator (DDMZM)-based SSB modulation due to its low cost compared with IQ modulator. We find that SSBI does not exist in back-to-back (BTB) after square-law direct detection, and electrical dispersion compensation (EDC) cannot be realized at the transmitter due to the nonlinear modulation of DDMZM. In our proposed scheme, Cartesian to Polar conversion (CPC) is used to achieve the linear modulation of DDMZM, then pre-EDC is achievable. Signal construction operation is applied to generate a target signal avoiding SSBI in DDMZM-based SSB modulation with direct detection, thus KK algorithm is not necessary both in BTB and after fiber transmission with pre-EDC, and the computation complexity of the receiver can be effectively reduced. In the simulation results of 25 GHz SSB Discrete Fourier transform spread discrete multi-tone (DFT-S DMT), the BER of our proposed scheme and conventional DDMZM scheme with optical signal-to-noise ratio (OSNR) of 26 dB are 1.2 × 10−3 and 1.7 × 10−2, respectively, after 80 km standard single mode fiber (SSMF) transmission with pre-EDC. The simulation results show that our proposed scheme is better than conventional DDMZM scheme. Although it is still inferior to the conventional IQ scheme with KK algorithm, the computation complexity of CPC is much lower than KK algorithm as it saves the numbers of adders and multipliers by the factors of 119 and 62, respectively, and reduces the memory size by a factor of 4.3.

SSBI-Free Direct-Detection System Employing Phase Modulation for Analog Optical Links

Analog optical transmission techniques such as radio-over-fiber (RoF) and multiple intermediate-frequency-over-fiber (multi-IFoF) have attracted a great deal of attention for use in 5G mobile fronthaul links. Recently, an IFoF transmission with a record CPRI-equivalent capacity of 1.53 Tb/s has been demonstrated by using the Kramers-Kronig (KK) receiver. However, one major issue associated with the KK receiver is its computational complexity. Also, since the minimum phase condition must be satisfied, the optimal carrier-to-signal power ratio (CSPR) should be large enough; however, this results in a low receiver sensitivity. To address these issues, we have recently proposed and demonstrated a signal-to-signal beat interference (SSBI)-free direct-detection (DD) system employing phase modulation (PM). Since a PM signal has a constant amplitude, its SSBI term becomes just a direct-current (DC) component. Therefore, the SSBI can be perfectly removed using a DC block without resorting to complicated digital-signal-processing (DSP)-based schemes such as the KK algorithm. In this article, we describe the principles of operation of the SSBI-free PM system in detail. We present a numerical study of performance comparisons between the SSBI-free and KK systems. The result reveals that the optimal CSPR is always 0 dB in the SSBI-free system, while it depends on optical-signal-to-noise-ratio (OSNR) levels in the KK system. The dispersion tolerance of the SSBI-free system is also investigated, showing that the SSBI-free system can support a 2-GHz 1024QAM-signal delivery in a typical deployment scenario of mobile fronthaul links. Finally, we present an experimental demonstration of a 2-GHz 1024QAM OFDM transmission with the SSBI-free system. To the best of our knowledge, this is the first demonstration of 1024QAM DD-based transmission for analog mobile fronthaul links.

400-Gb/s Single-Sideband Direct Detection Over 7-Core Fiber With SSBI Cancellation

In this letter, a signal-to-signal beating interference (SSBI) cancellation technique is proposed in direct detection transmission system based on balanced photodiodes. We use a pilot-supported single-sideband direct detection scheme to transmit up to 400-Gb/s signal in three branches through a 10-km 7-core fiber. Using the manner of transmitting the signal in one core and the combination of signal and pilot in another core, the SSBI terms can be efficiently mitigated in both 100-Gb/s and 400-Gb/s direct detection optical orthogonal frequency division multiplexing transmission systems. Compared to the conventional pilot-supported direct detection scheme, no guard band is required based on our proposed SSBI elimination technique. The efficiency of the receiver’s bandwidth is significantly enhanced.

Balanced Detection of 100-Gb/s Optical OFDM Signals With SSBI Cancellation Based on Fused-Type Fiber Mode-Selective Coupler

We propose a signal-to-signal beating interference (SSBI) cancellation scheme in two-mode fiber (TMF) transmission system based on balanced detection without requirement of a guard band. The SSBI can be eliminated based on the mode selectivity characteristic of mode coupler. The mode selectivity in mode coupler relies on the coupling behavior between the fundamental mode LP01 and the first-order mode LP11, which is highly dependent on the spatial orientation of the fiber modes. We design and fabricate a fused-type fiber mode coupler (FFMC) based on circular-core TMF. Based on the fabricated FFMC, we experimentally demonstrate a multi-band 100-Gb/s optical orthogonal frequency division multiplexing signal transmission over 10-m TMF with SSBI cancellation. Significant performance improvement is observed when compared with conventional direct detection scheme under the condition of insufficient bandwidth of a guard band.

Optical single side-band Nyquist PAM-4 transmission using dual-drive MZM modulation and direct detection.

We present the design and optimization of the optical single side-band (SSB) Nyquist four-level pulse amplitude modulation (PAM-4) transmission using dual-drive Mach-Zehnder modulator (DDMZM)modulation and direct detection (DD), aiming at the C-band cost-effective, high-speed and long-distance transmission. At the transmitter, the laser line width should be small to avoid the phase noise to amplitude noise conversion and equalization-enhanced phase noise due to the large chromatic dispersion (CD). The optical SSB signal is generated after optimizing the optical modulation index (OMI) and hence the minimum phase condition which is required by the Kramers-Kronig (KK) receiver can also be satisfied. At the receiver, a simple AC-coupled photodiode (PD) is used and a virtual carrier is added for the KK operation to alleviate the signal-to-signal beating interference (SSBI).A Volterra filter (VF) is cascaded for remaining nonlinearities mitigation. When the fiber nonlinearity becomes significant, we elect to use an optical band-pass filter with offset filtering. It can suppress the simulated Brillouin scattering and the conjugated distortion by filtering out the imaging frequency components. With our design and optimization, we achieve single-channel, single polarization 102.4-Gb/s Nyquist PAM-4 over 800-km standard single-mode fiber (SSMF).

Sparse Volterra model based on optical single side-band NPAM-4 direct-detection system

Abstract Signal-to-signal beating interference (SSBI) is one of the main drawbacks in direct-detection based optical transmission systems. Volterra filter is a common equalization method to mitigate the nonlinear distortion. However, the computational complexity may be unacceptable as the transmission capacity increases. In this paper, we propose a sparse Volterra model combining the feed forward equalization (FFE) and higher order terms of a modified Volterra filter with Schmidt orthogonal searching to mitigate the linear and nonlinear interference and reduce the complexity significantly in an optical single-side band (SSB) Nyquist pulse-shaped four-level pulse amplitude (NPAM-4) system. The experimental results show that the sparse Volterra filter and full Volterra filter have comparable performance, but the former only needs half kernels of the latter.

Single-Lane 112-Gbit/s SSB-PAM4 Transmission With Dual-Drive MZM and Kramers–Kronig Detection Over 80-km SSMF

We have experimentally demonstrated the signal-to-signal beating interference (SSBI) mitigation capability of Kramers–Kronig (KK) detection in a single sideband (SSB) four-ary pulse-amplitude-modulation (PAM4) direct detection (DD) system. A 112-Gbit/s (56-GBaud) SSB-PAM4 optical signal suitable for KK detection is generated by using a single commercial, low-cost dual-drive Mach–Zehnder modulator. The system bandwidth constraint is mitigated by Nyquist pulse shaping and bandwidth precompensation. The SSBI mitigation capability of KK algorithm is identified by comparing with two order Volterra filter. After 80-km dispersion-uncompensated standard single-mode fiber transmission, linear equalizer with KK algorithm and two order Volterra filter based nonlinearity equalizer show similar bit error rate (BER) performance, which are 3.2 × 10–3 and 2.8 × 10–3 , respectively. Both BERs are below 7% hard-decision forward error correction threshold of 3.8 × 10 –3. The experiment results present the excellent SSBI mitigation capability of KK detection in SSB-PAM4 DD system. Above all, our experiments reveal the potential of KK detection in DD system for data center interconnects.

Hilbert superposition and modified signal-to-signal beating interference cancellation for single side-band optical NPAM-4 direct-detection system.

We propose a Hilbert superposition and modified signal-to-signal beating interference (SSBI) cancellation scheme in an optical single side-band (SSB) modulation and direct-detection system. The optical SSB signal is generated by a relatively low-cost dual-drive Mach-Zehnder modulator (DDMZM). The two driving signals are a pair of Hilbert signals with Nyquist pulse-shaped four-level pulse amplitude modulation (NPAM-4). In addition to the transmitted baseband signal, both its Hilbert transform and the SSBI can also be detected by direct-detection, which introduce the interference to the transmitted signal. We use the first-stage Hilbert superposition cancellation (HSC) to cancel the unwanted Hilbert transform signal and a modified single-stage linearization filter which contains the second-stage HSC to deduct the SSBI in the receiver. We experimentally demonstrate that 40 Gb/s optical SSB NPAM-4 signal transmission over 80 km standard single mode fiber (SSMF).

Power budget of direct-detection ultra-dense WDM-Nyquist-SCM PON with low-complexity SSBI mitigation

Abstract The power budget (PB) of a direct-detection ultra-dense wavelength division/subcarrier multiplexing (SCM) passive optical network (PON) is assessed numerically for downstream, when a low-complexity iterative signal-to-signal beat interference (SSBI) mitigation technique is employed. Each SCM signal, inserted in a 12.5 GHz width optical channel, is comprised of two or three electrically generated and multiplexed 16-quadrature-amplitude-modulation (QAM) or 32-QAM Nyquist pulse-shaped subcarriers, each with a 7% forward error correction bit rate of 10.7 Gbit/s. The PB and maximum number of optical network units (ONUs) served by each optical line terminal (OLT) are compared with and without SSBI mitigation. When SSBI mitigation is realized, PB gains up to 4.5 dB are attained relative to the PB in the absence of SSBI mitigation. The PB gain enabled by the SSBI mitigation technique proposed in this work increases the number of ONUs served per OLT at least by a factor of 2, for the cases of higher spectral efficiency. In particular, for a SCM signal comprised of three subcarriers, the maximum number of ONUs served per OLT is between 2 and 32, and between 8 and 64, in the absence of SSBI mitigation, and when SSBI mitigation is employed, respectively, depending on the fiber length (up to 50 km) and order of QAM.

Joint Influence of the Optical Carrier-to-Sideband Ratio and Guard Band on Direct-Detection SSB-OOFDM System

The signal-to-interference-plus-noise ratio of the noised single-sideband optical orthogonal frequency-division multiplexing (SSB-OOFDM) signal in a direct-detection OFDM system is first deduced theoretically. The relationship of the carrier-to-sideband ratio (CSR) and guard band (GB) to the signal-to-signal beat interference (SSBI) is explored. According to our theoretical analysis, the degradation caused by SSBI on the SSB-OOFDM signal becomes worse as the GB reduces, whereas the increase in the CSR can loosen this degradation. Therefore, a tradeoff between the GB and CSR balances their joint influence on system performance. The simulation for the 20-km optical link with 40-Gb/s 16-quadrature amplitude modulation (16-QAM) noised SSB-OOFDM signal is conducted to confirm our theoretical results. It shows that, without any complex device or algorithm, the GB can be reduced to 40% only by increasing the CSR to $\sim$ 8 dB, and thus, the spectral efficiency of the DDO-OFDM link is improved after optimizing the CSR and GB of the SSB-OOFDM signal. In addition, the influence of the optical signal-to-noise ratio and the electronic filter on the optimum CSR of the system is further analyzed.

100.29-Gb/s direct detection optical OFDM/OQAM 32-QAM signal over 880 km SSMF transmission using a single photodiode.

We propose a novel guard-band-shared direct-detection (GBS-DD) scheme for a 100-Gb/s single-photodiode direct-detection transmission system. The 100.29Gb/s signal is successfully transmitted over 880km standard single mode fiber (SSMF) with Raman amplification under the 20% forward error correction (FEC) threshold within 52GHz optical bandwidth. The signal is modulated with orthogonal frequency-division multiplexing/offset quadrature-amplitude-modulation 32-ary QAM (OFDM/OQAM 32-QAM). The signal-to-signal beat interference (SSBI) terms fall and overlap in the same guard band. Adopting the proposed approach, the bandwidth usage efficiency of the photodiode is greatly enhanced, which brings benefits on the data rate and transmission performance.

Single photodiode direct detection system of 100-Gb/s OFDM/OQAM-64QAM over 80-km SSMF within a 50-GHz optical grid

We propose a novel guard-band-shared direct-detection (GBS-DD) scheme to improve the receiver spectrum efficiency (SE). The 100-Gb/s signal is modulated by 2 sub-bands, which are assigned onto two orthogonal polarizations. The central wavelengths of the two sub-bands are set as 10.84-GHz frequency space. The two sub-bands are then received simultaneously using a single conventional photodiode (PD) of 40-GHz bandwidth. Only one optical pilot carrier is inserted to beat with the 2 sub-bands on the two polarizations. When the 2 sub-band signal entering into the receiver, the signal-to-signal beat interference (SSBI) terms fall and overlap in the same guard band. As a consequence, the bandwidth usage of the PD is enhanced from 1/2 to 2/3. The 100-Gb/s signal is modulated using orthogonal frequency-division multiplexing based on offset quadrature-amplitude-modulation of 64-quadrature amplitude modulation (OFDM/OQAM-64QAM), and transmitted over 80-km standard single mode fiber (SSMF) within a 50-GHz optical grid. It is shown that the proposed GBS-DD scheme can be implemented by the current commercial optical/electrical devices.

Low-cost coherent receiver for long-reach optical access network using single-ended detection.

A low-cost coherent receiver using two 2×3 optical hybrids and single-ended detection is proposed for long-reach optical access network. This structure can detect the two polarization components of polarization division multiplexing (PDM) signals. Polarization de-multiplexing and signal-to-signal beat interference (SSBI) cancellation are realized by using only three photodiodes. Simulation results for 40 Gb/s PDM-OFDM transmissions indicate that the low-cost coherent receiver has 3.2 dB optical signal-to-noise ratio difference compared with the theoretical value.