Signal-signal Beat Interference Cancellation Research Articles

Single-Ended Coherent Receiver

Commercial coherent receivers utilize balanced photodetectors (PDs) with high single-port rejection ratio (SPRR) to mitigate the signal-signal beat interference (SSBI) due to the square-law detection process. As the symbol rates of coherent transponders are increased to 100 Gbaud and beyond, maintaining a high SPRR in a cost-effective manner becomes more and more challenging. One potential approach for solving this problem is to leverage the concept of single-ended coherent receiver (SER) where single-ended PDs are used instead of the balanced PDs. In this case, the resulting SSBI should be mitigated in the digital domain. In this paper, we show that SSBI can be effectively mitigated using various low-complexity techniques, such as the direct filed reconstruction (DFR), clipped iterative SSBI cancellation (CIC) and gradient descent (GD). In addition, we present a self-calibration technique for SERs which can be extended for characterizing the optical-to-electrical (O/E) response of a conventional balanced coherent receiver (BR). Using the developed techniques, we then experimentally demonstrate a 90 Gbaud probabilistically constellation shaped 64-QAM (PCS-64QAM) transmission using a SER, achieving a net data rate of 882 Gb/s over 100 km of standard single mode fiber (SSMF). The sensitivity penalty compared to the BR is below 0.5 dB. We expect that when the symbol rate is increased further, a SER can potentially outperform a BR, especially when applied to cost-sensitive commercial pluggable coherent transceivers.

Polarization Multiplexed Optical OFDM System With a Beat Interference Cancellation Receiver

A novel beat interference cancellation receiver (BICR) is proposed to cancel the signal-signal beat interference (SSBI) for polarization division multiplexed (PDM) optical orthogonal frequency division multiplexing (OOFDM) system, where two single sideband (SSB) OOFDM signals are polarization multiplexed along with their offset optical carriers (OCs) located at opposite sides. At receiver, the PDM SSB-OOFDM signal is equally power split into four beams with one or two OCs filtered out, and then fed into two balanced photodiode pairs for recovering the RF-OFDM signals with SSBI cancellation. The PDM SSB-OOFDM system is insensitive to fiber polarization mode dispersion since the proposed BICR is implemented without polarization-sensitive devices. Moreover, the spectral efficiency is greatly improved because of PDM and the small guard band since the SSBIs are eliminated. The proposed BICR scheme is validated with a simulation link of 112 Gbps 16-QAM PDM SSB-OOFDM signals. After 120km SSMF transmission, the EVM keeps below forward error correction limit of 16.3%.

Multichannel 16-QAM Single-Sideband Transmission and Kramers–Kronig Detection Using a Single QD-MLL as the Light Source

A 12-channel single-sideband transmission of 16-QAM signals at 18 GBd is experimentally demonstrated for the first time using a single quantum-dot mode-locked laser (QD-MLL) as the light source for both the modulated carriers and the continuous wave components. The Kramers–Kronig field reconstruction algorithm was used at the direct detection receiver for signal-signal beat interference cancellation. A total net data rate of >800 Gb/s is achieved over 78 km fiber of 1680 ps/nm accumulated chromatic dispersion with BER ≤ 2 × 10−3, which is lower than the threshold of hard-decision FEC with 7% overhead. Numerical simulations were also performed to assess the impact of noises from the laser source and the receiver front-end on the system performance.

Power-Efficient Single-Sideband Transmission With Clipped Iterative SSBI Cancellation

Distributed data-center networks rely on power and cost-efficient high-speed fiber optical connections over distances up to 80 km which can be densely wavelength-division multiplexed (WDM) in the C-band. Recently, single-sideband (SSB) direct detection (DD) has been considered as an attractive transmission scheme for achieving data rates beyond 100 Gb/s per channel over 80 km. The advantages of SSB DD transmission include its simple transceiver architecture and its capability of electronic dispersion compensation. However, SSB transmissions require a carrier-to-signal power ratio (CSPR) of around 10 dB, even when signal-signal beat interference (SSBI) cancellations are applied. This high CSPR value reduces the power efficiency of the system and limits the total number of WDM channels and accordingly the total system capacity due to power limitation of optical amplifiers. In this article, we propose an iterative SSBI cancellation scheme, which can be effectively applied without digital upsampling at low CSPR values. Using this technique, we have experimentally demonstrated a 30 Gbaud 128 QAM SSB direct detection transmission over 80 km with a low CSPR of 5 dB, showing 4.6 dB performance improvement compared to the Kramers–Kronig scheme operated without digital upsampling.

SiP-Based SSBI Cancellation for OFDM

We propose for the first time to use a silicon photonics (SiP) solution for a passive optical network to both reduce signal-signal beat interference (SSBI) and recuperate a part of the downlink carrier for use in the uplink. The Kramers-Kronig (KK) receiver for direct detection of advanced modulation formats overcomes SSBI at the cost of a moderate carrier to signal ratio (>6 dB) and high oversampling (4×). We propose an optical SSBI solution that achieves better performance than KK and requires only standard sampling and low (3 dB) carrier to signal power ratio. The receiver is conceived for the downlink in passive optical networks, where carrier signal must be husbanded for re-use in the uplink. Using cost effective and power efficient SiP, the receiver filters the incoming signal, suppresses SSBI, and routes a portion of the carrier for use in the uplink. We experimentally examine the SSBI suppression in this paper. While previous demonstrations used bulky, discrete components, we achieve significant Q-factor improvement with a simple SiP solution. We examine the optimal frequency offset between the carrier and the microring resonator center frequency. The robustness to frequency drift, as well as the impact of imperfect filtering, is discussed and quantified.

Beyond 400 Gb/s Direct Detection Over 80 km for Data Center Interconnect Applications

Due to the growing demand for cloud services with high availability, high connection speed and low latency, distributed data-centers have emerged as a key architecture for future optical networks. This architecture relies on power and cost-efficient solutions for 400 Gb/s client interfaces over distances up to 80 km which can be densely wavelength-division multiplexed (WDM) in the C-band. Recently, single side band (SSB) direct detection (DD) has been considered as an attractive transmission scheme for achieving data rates beyond 100 Gb/s per channel due to its capability of electronic dispersion compensation. However, as SSB DD schemes utilize only a single polarization for data transmission, achieving 400 Gb/s per channel requires a baudrate beyond 80 Gbaud, which might reduce the effectiveness of the commonly used signal-signal beat interference (SSBI) cancellation techniques such as iterative SSBI cancellation or Kramers-Kronig algorithm due to the imperfection of Tx drivers, modulator and Rx front-end. In this paper, through effective Tx calibration and Rx DSP, we have achieved for the first time a net data rate per channel above 400 Gb/s with a 64 QAM signal at 85 Gbaud using a single photodetector (PD) at the receiver. In addition, a WDM transmission consisting of 5 channels at 100 GHz spacing was successfully conducted. This result indicates that SSB DD is an effective transmission technique for high capacity data center interconnect (DCI) applications.

Investigation of single- and multi-carrier modulation formats for Kramers–Kronig and SSBI iterative cancellation receivers

The Kramers-Kronig (KK) receiver has recently attracted significant attention due to its capability of field recovery with direct detection. Under minimum phase condition, the KK receiver may use either single- or multi-carrier modulation formats. In this Letter, we investigate the appropriate modulation formats for both KK and signal-signal beat interference (SSBI) iterative cancellation (IC) receivers. It is shown that for the KK receiver, the single-carrier modulation format is superior to orthogonal frequency division multiplexing (OFDM), because the multi-carrier nature of OFDM signals increases the peak-to-average power ratio, which causes a violation of minimum phase condition. For the IC receiver, SSBI cancellation is more effective when the OFDM modulation format is adopted; thus, OFDM is the better fit for IC receivers than single carrier.

1.6 Tb/s Virtual-Carrier Assisted WDM Direct Detection Transmission Over 1200 km

Single-sideband (SSB) self-coherent detection (SCD) has attracted a lot of attention recently as a promising technology for short and medium reach optical transmission systems, such as data center interconnect (DCI) and metro applications, due to the low cost, the compact module size, and the possibility of electrical dispersion compensation. In addition, it has been shown that the SSB SCD can be effectively realized using a “virtual carrier,” which is generated digitally together with the information-bearing signal to significantly simplify the hardware implementation. In this paper, we present an experimental demonstration of an 8 × 256 Gb/s WDM SSB SCD transmission with virtual carriers over a record distance of 1200 km with Corning TXF fiber. To achieve this performance, two digital linearization techniques have been considered and compared, namely the Kramers–Kronig (KK) field reconstruction and iterative signal-signal beat interference cancellation. Furthermore, we introduce a novel scheme called dc-offset KK to minimize the performance penalty associated with filtering and resampling process of the conventional KK scheme.

Direct detection of polarization multiplexed single sideband signals with orthogonal offset carriers.

We propose a novel direct detection (DD) scheme for polarization division multiplexed (PDM) single sideband (SSB) signals with two orthogonal carriers located at the opposite sides. Polarization diversity is realized with a pair of optical filters that are used to suppress the unwanted orthogonal carrier component. A PDM-SSB DD receiver is thus constructed without polarization de-rotation. The intra-polarization signal-signal beat interference (SSBI) can be mitigated by Kramers-Kronig detection or iterative SSBI cancellation. For inter-polarization SSBI mitigation, we propose a joint iterative SSBI cancellation method. The proposed PDM-SSB DD scheme is validated with a principle experiment of 40Gbaud PDM-SSB 16-ary quadrature amplitude modulation (16-QAM) signals. After 80km standard single-mode fiber (SSMF) transmission, the bit-error rates (BERs) achieve 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10-2. The performance of iterative SSBI cancellation, Kramers-Kronig detection, and joint iterative SSBI cancellation are evaluated for PDM-SSB signals with different carrier-to-signal ratios (CSPRs) through numerical simulations. Moreover, a multi-input-multi-output (MIMO) equalization scheme is proposed and validated with numerical simulation, which can suppress the linear inter-polarization crosstalk and relax the sharpness requirement of optical filter edges.

A polarization-division multiplexing SSB-OFDM system with beat interference cancellation receivers

In this paper, we have proposed a polarization-division multiplexing (PDM) single-sideband optical orthogonal frequency division multiplexing (SSB-OOFDM) scheme with signal-signal beat interference cancellation receivers with balanced detection (ICRBD). This system can double channel capacity and improve spectrum efficiency (SE) with the reduced guard band (GB) due to the PDM. Multiple input multiple output (MIMO) technique is used to solve polarization mode dispersion (PMD) associated with channel estimation and equalization. By simulation, we demonstrate the efficacy of the proposed technique for a 2×40 Gbit/s 16-QAM SSB-PDM-OOFDM system according to the error vector magnitude (EVM) and the constellation diagrams.

Comparison of digital signal-signal beat interference compensation techniques in direct-detection subcarrier modulation systems.

Single-polarization direct-detection transceivers may offer advantages compared to digital coherent technology for some metro, back-haul, access and inter-data center applications since they offer low-cost and complexity solutions. However, a direct-detection receiver introduces nonlinearity upon photo detection, since it is a square-law device, which results in signal distortion due to signal-signal beat interference (SSBI). Consequently, it is desirable to develop effective and low-cost SSBI compensation techniques to improve the performance of such transceivers. In this paper, we compare the performance of a number of recently proposed digital signal processing-based SSBI compensation schemes, including the use of single- and two-stage linearization filters, an iterative linearization filter and a SSBI estimation and cancellation technique. Their performance is assessed experimentally using a 7 × 25 Gb/s wavelength division multiplexed (WDM) single-sideband 16-QAM Nyquist-subcarrier modulation system operating at a net information spectral density of 2.3 (b/s)/Hz.

Signal-signal beat interference cancellation in spectrally-efficient WDM direct-detection Nyquist-pulse-shaped 16-QAM subcarrier modulation.

An experimental demonstration of direct-detection single-sideband Nyquist-pulse-shaped 16-QAM subcarrier modulated (Nyquist-SCM) transmission implementing a receiver-based signal-signal beat interference (SSBI) cancellation technique is described. The performance improvement with SSBI mitigation, which compensates for the nonlinear distortion caused by square-law detection, was quantified by simulations and experiments for a 7 × 25 Gb/s WDM Nyquist-SCM signal with a net optical information spectral density (ISD) of 2.0 (b/s)/Hz. A reduction of 3.6 dB in the back-to-back required OSNR at the HD-FEC threshold was achieved. The resulting reductions in BER in single channel and WDM transmission over distances of up to 800 km of uncompensated standard single-mode fiber (SSMF) achieved are presented.