Transceiver Imbalances Compensation and Monitoring by Receiver DSP

Self Cite

Transceiver imperfections become the primary source of impairment as baud rate and modulation order grow in advanced optical coherent communications. Thus, transceiver imperfections, both linear and nonlinear, need to be appropriately characterized, measured, and specified. Treatments for linear imperfections are relatively mature. This study reviews the transceiver's linear imperfection modeling, characterization, and measurement technologies. In practical applications, in-field measurement using the transceiver and a few low speed additional devices is preferred. In the case of nonlinear imperfections, the situation is complex. One important task is to estimate the nonlinear system performance from the nonlinear characteristics of the devices. In this study, we attempt to establish a connection between them by examining different technologies. Although the orthogonal component has a good correlation with nonlinear system performance, its measurement is prohibitively complex. In the nonlinear noise to power ratio measurement, a certain frequency component of the input signal is notched, and the re-growth component at the notch frequency is measured at the nonlinear device output. The ratio between the re-growth component power and the output signal power is the noise to power ratio. While this method is easy to carry out, it does not correctly estimate nonlinear impairment in general. The reason for this is that the signal incurs different nonlinear responses in two conditions, i.e., with or without a notch. This method accurately estimates the nonlinear impairment in some special but useful cases, such as Gaussian input signals and nonlinear systems whose dominant nonlinear term is the even order term.

Section: In-field Measurement After Servicementioning

confidence: 99%

Characterization, Measurement and Specification of Device Imperfections in Optical Coherent Transceivers

Tao

Fan

et al. 2022

Self Cite

“…Amplitude and phase imbalance generated at the receiver side can be monitored and mitigated before CD compensation by means of the Gram-Schmidt orthogonalization procedure (GSOP) [23]. At the same time, receiver IQ-skew can be handled together with channel equalization also in the presence of large accumulated CD by modifying the 2x2 equalizer into a 4x2 structure [24].…”

Section: Performance In Presence Of Transceiver Iq-imbalancesmentioning

confidence: 99%

“…At the same time, receiver IQ-skew can be handled together with channel equalization also in the presence of large accumulated CD by modifying the 2x2 equalizer into a 4x2 structure [24]. On the contrary, transmitter impairments can only be compensated after FOE and CPR [23]. This implies that transmitter impairments will strongly affect the signal at the equalization stage also after convergence of the equalizer.…”

Section: Performance In Presence Of Transceiver Iq-imbalancesmentioning

confidence: 99%

Likelihood-Based Selection Radius Directed Equalizer With Time-Multiplexed Pilot Symbols for Probabilistically Shaped QAM

Rosa

Richter

2021

Probabilistically shaped (PS) quadrature amplitude modulation (QAM) recently established itself as the solution to adopt for state-of-the-art coherent transponders. This in turn has drawn attention to the rework of conventional digital signal processing (DSP) algorithms, which are not optimal for QAM-PS. In this context, we propose a novel pilot-aided equalization technique based on the ubiquitous radius directed equalizer (RDE). Our solution employs both payload and time-multiplexed pilot symbols for the adaptive filters update. The payload symbols are treated on the basis of the likelihood of their correct blind assignment. We show that this approach is at the same time able to reduce the global pilot overhead (POH) required by the DSP chain and to provide greatly improved performance in the tracking ability of the equalizer in the event of fast state of polarization rotations. We describe a simulation environment in which we model accurately static and dynamic polarizationrelated impairments. We test our algorithm over different shaped modulation formats and in the presence of transceiver impairments and demonstrate its superior performance with respect to standard feed-forward implementations in all the scenarios considered.

“…Gram-Schmidt orthogonalization procedure (GSOP) [4] and Löwdin orthogonalization procedure (LOP) [5] are two common compensation algorithms of amplitude/phase imbalance and all these algorithms are based on statistical analysis principle. The orthogonalization matrix obtained by these algorithms can be further used to estimate amplitude/phase imbalance [7]- [8]. Among them, GSOP is an asymmetrical orthogonalization scheme with lower complexity but will amplify the quantization noise; while LOP is based on symmetric orthogonalization with higher complexity and is robust to quantization noise [5].…”

Section: Introductionmentioning

confidence: 99%

“…These schemes can monitor Rx and Tx imbalances before and after FO compensation, respectively. Liang et al uses GSOP and MIMO algorithms to estimate transceiver IQ amplitude/phase imbalance and skew [19]; our previous research utilizes the convex hull assisted ellipse correction and k-means clustering assisted blind phase search scheme to monitor transceiver IQ amplitude/phase imbalance [17]. However, these existing schemes can only separate the transceiver IQ imbalances and are unable to separate the transceiver XY imbalances.…”

Section: Introductionmentioning

confidence: 99%

Multi-Dimensional, Wide-Range, and Modulation-Format-Transparent Transceiver Imbalance Monitoring

Zhang

Yao

et al. 2021

We propose and experimentally demonstrate a dualpolarization (DP) transceiver imbalance monitoring scheme, which is applicable for the coherent system using square QAM signals. The scheme can separate/monitor various kinds of transceiver imbalances and has technical advantages of multidimension, wide monitoring range, and modulation-formattransparency. In the proposed scheme, a polarization scrambler is configured between transmitter (Tx) and receiver (Rx). After coherent detection, the proposed digital signal processing (DSP) modules are implemented to monitor transceiver imbalance based on the received signals. To simplify the DSP modules, the implementation sequence of DSP modules is in reverse order compared to the order in which the sequence of impairments is introduced. Firstly, Godard timing error detection (TED) and Gram-Schmidt orthogonalization procedure (GSOP) is used to monitor and compensate Rx imbalances. After compensating the Rx imbalances, Tx imbalance monitoring is implemented. Complex/real maximum likelihood independent component analysis (ML-ICA) and Godard-TED are used to monitor Tx imbalance. Finally, the amplitude ratios of the final output signal and Tx in-phase/quadrature (IQ) amplitude imbalance are fed back to mitigate the interaction of Tx imbalances on the signal power and improve monitoring accuracy. The transceiver IQ and x-polarization/y-polarization (XY) imbalances are separated for the first time by using the frequency offset (FO) naturally existing in the system and the continuous polarization rotation induced by a polarization scrambler. The separation method combined with the reverse algorithm design and amplitude ratio feedback mechanism makes the proposed scheme achieve the most multidimensional imbalance monitoring by far. The simulation and experimental results verify that the proposed scheme is modulation-format-transparent and can separate/monitor multidimensional transceiver imbalances within a wide range.