Electronic Dispersion Compensation Research Articles

Enhanced Performance of Intensity Modulation with Direct Detection Using Golay Encoded Nyquist Pulses and Electronic Dispersion Compensation

Enhanced Performance of Intensity Modulation with Direct Detection Using Golay Encoded Nyquist Pulses and Electronic Dispersion Compensation

FSK/ASK Orthogonal Modulation System Based on Novel Noncoherent Detection and Electronic Dispersion Compensation for Short-Reach Optical Communications

We propose an FSK/ASK orthogonal modulation system based on a novel noncoherent detection (NCD) scheme, aimed at expanding the system capacity for short-reach optical communications cost-effectively. In the transmitter, the FSK optical signal is generated by simple frequency modulation through a directly modulated distributed feedback laser. Subsequently, by utilizing a Mach–Zehnder modulator for ASK modulation, the FSK/ASK optical signal is obtained. The novel and low-complexity NCD receiver consists of an intensity detection branch and a frequency detection branch. The frequency detection branch is composed of an optical differentiator, a photodetector, and frequency extraction circuits. Notably, the proposed NCD scheme overcomes the limitation of the traditional FSK/ASK-NCD receiver stemming from the trade-off between the detected signal quality of the amplitude and frequency. Furthermore, electronic dispersion compensation (EDC) is available. Through numerical simulations, our findings demonstrate that the proposed FSK/ASK-NCD system, assisted by EDC, achieves a remarkable 100 km transmission span for both 40 Gbps 2FSK/2ASK and 60 Gbps 2FSK/4ASK modulation formats, which surpasses the 2ASK-DD and the 4ASK-DD systems, where the maximum achievable spans are limited to less than 20 km. These results underscore the potential of the proposed system as a robust candidate for future passive optical access networks.

Reducing Computational Complexity of Neural Networks in Optical Channel Equalization: From Concepts to Implementation

In this paper, a new methodology is proposed that allows for the low-complexity development of neural network (NN) based equalizers for the mitigation of impairments in high-speed coherent optical transmission systems. In this work, we provide a comprehensive description and comparison of various deep model compression approaches that have been applied to feed-forward and recurrent NN designs. Additionally, we evaluate the influence these strategies have on the performance of each NN equalizer. Quantization, weight clustering, pruning, and other cutting-edge strategies for model compression are taken into consideration. In this work, we propose and evaluate a Bayesian optimization-assisted compression, in which the hyperparameters of the compression are chosen to simultaneously reduce complexity and improve performance. Next, this paper presents four distinct metrics (RMpS, BoP, NABS, and NLGs) that are discussed here that can be used to evaluate the amount of computing complexity required by various compression algorithms. These measurements can serve as a benchmark for evaluating the relative effectiveness of various NN equalizers when compression approaches are used. In conclusion, the trade-off between the complexity of each compression approach and its performance is evaluated by utilizing both simulated and experimental data in order to complete the analysis. By utilizing optimal compression approaches, we show that it is possible to design an NN-based equalizer that is simpler to implement and has better performance than the conventional digital back-propagation (DBP) equalizer with only one step per span. This is accomplished by reducing the number of multipliers used in the NN equalizer after applying the weighted clustering and pruning algorithms. Furthermore, we demonstrate that an equalizer based on NN can also achieve superior performance while still maintaining the same degree of complexity as the full electronic chromatic dispersion compensation block. We conclude our analysis by highlighting open questions and existing challenges, as well as possible future research directions.

Gerchberg-Saxton Based FIR Filter for Electronic Dispersion Compensation in IM/DD Transmission Part I: Theory and Simulation

Intensity-modulation and direct-detection (IM/DD) transmission over short-reach optical fiber links, require electronic dispersion compensation (EDC) at the transmitter and/or electronic equalization at the receiver. Recently, the iterative Gerchberg-Saxton (GS) algorithm was demonstrated for EDC in IM/DD systems, through treating the amplitude at the transmitter and the phase prior-to the direct detection receiver as a degree of freedom. In Part I of this work, three GS approaches using finite impulse response (FIR) filters for EDC in IM/DD systems are demonstrated. The first two are closely related and rely on a cascaded FIR structure, while the third offers a novel non-iterative EDC solution using a single GS optimized static FIR filter. This is achieved through decoupling pattern dependent aspects of transmission from the GS iterations by targeting a single impulse at the DD receiver. With every successive iteration an impulse response for the GS filter emerges and sets the FIR tap weights. It is also demonstrated that closed-form analytical expressions for the GS filter impulse response can be obtained through small-signal frequency-domain analysis. The FIR filter is simulated using 8-bit finite-precision arithmetic. An adaptive <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T$</tex-math></inline-formula> -spaced post feed-forward equalizer (FFE) is utilized for mitigating residual chromatic dispersion. It is shown, that a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T/2$</tex-math></inline-formula> -spaced pre-EDC FIR filter with 417 taps can support 56 Gb/s non-return-to-zero (NRZ) on-off keying (OOK) transmission over 80 km of single mode fiber (SMF) with a chirp-free Mach-Zehnder modulator (MZM). Part II, presents experimental demonstration of the non-iterative GS FIR filter proposed and simulated in this article.

Gerchberg-Saxton Based FIR Filter for Electronic Dispersion Compensation in IM/DD Transmission Part II: Experimental Demonstration and Analysis

In this article, the first experimental demonstration of a non-iterative electronic dispersion compensation (EDC) solution implemented at the transmitter using a finite impulse response (FIR) filter optimized with the Gerchberg-Saxton (GS) algorithm, is presented, for intensity-modulation and direct-detection (IM/DD) systems. The theoretical framework and preliminary simulations have been presented in Part I of this work. Here, the performance is compared between the GS FIR filter and the iterative GS algorithm in 56-Gb/s on-off keying (OOK) transmissions over 80-km single mode fiber (SMF) with a post feed-forward equalizer (FFE) for combating residual inter-symbol interference. Furthermore, the influence of the pulse shape (raised cosine or rectangular) and modulation format (return-to-zero (RZ) or non-return-to-zero (NRZ)) on the measured bit error ratio (BER) is investigated, while changing the number of FIR taps, post-FFE taps, and post-FFE samples-per-symbol. It is shown that within the range of the target digital extinction ratios (DERs) for which the original iterative GS algorithm offers benefit, both analytical and numerical methods for calculating the optimum FIR taps, outlined in Part I, produce similar BER performance as predicted. Hence, the former method is extended, here, through a non-recursive frequency response formula, which offers insight into the action of the GS filter with different pulse shaping and enables the derivation of the explicit GS impulse response. It is also shown that rectangular RZ exploits the full benefit of GS filtering through a uniform spectrum, achieving BER < 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> , with a 641-tap <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> /2-spaced pre-EDC FIR filter and a 3-tap adaptive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> -spaced post-FFE.

Electronic dispersion compensation and functional link neural network equalization for IM/DD links

Short-reach fiber optical links employing intensity modulation (IM) at the transmitter (Tx) and direct detection (DD) at the receiver (Rx), suffer from linear and nonlinear sources of impairments due to the interaction of chromatic dispersion (CD) with DD. In this article, joint electronic dispersion compensation (EDC) at the Tx, using two distinct Gerchberg–Saxton(GS) based approaches, and at the Rx, using a functional link neural network (FLNN) equalizer is demonstrated for IM/DD transmission. The first Tx approach utilizes the modified iterative GS algorithm, which partially mitigate linear and nonlinear sources of inter-symbol interference (ISI). The second Tx pre-EDC approach only pre-compensates for the linear power fading effect through implementing a GS based finite impulse response (FIR) filter. At the Rx, a T/2-spaced or T-spaced adaptive post-feed forward equalizer (FFE) is employed to fully compensate residual chromatic dispersion prior to attempting nonlinear equalization. Furthermore, a Volterra nonlinear equalizer (VNLE) is introduced to benchmark the performance and complexity of the FLNN, Subsequently, either a FLNN or a VNLE are utilized for nonlinear system identification and subsequent post-equalization mitigating uncompensated nonlinear sources of ISI. The FLNN nonlinear taps resulting from the functional expansion block are optimized using the recursive least square (RLS) algorithm. The Tx-FIR and Rx-FLNN enable 112 Gbit/s non-return to zero (NRZ) on–off keying (OOK) transmission over 20 km of single mode fiber (SMF) and 112 Gbit/s 4-level pulse-amplitude modulation (PAM-4) transmission over 10 km of SMF. It is shown that the use of Tx pre-EDC reduces the complexity of the required equalization at the Rx. In addition, the FLNN was found to offer a 74% reduction in computational complexity relative to the third-order VNLE.

Wideband Multichannel Nyquist-Spaced Long-Haul Optical Transmission Influenced by Enhanced Equalization Phase Noise.

Enhanced equalization phase noise (EEPN), generated from the uncompensated dispersion experienced by laser phase noises, can cause serious damage to the transmission quality of optical fiber systems. In this work, the performance of a wideband Nyquist-spaced long-haul nonlinear optical fiber communication systems suffering from EEPN is investigated and discussed through split-step numerical simulations and analytical models based on the perturbation analysis, in the cases of digital nonlinearity compensation (NLC) and electronic dispersion compensation (EDC). The efficiency and the accuracy of the analytical models were validated via simulations, considering the different symbol rates and modulation formats. The performance of the C-band transmission was comprehensively studied based on the model. Our results reveal that the growth of symbol rates and transmission distances aggravates the distortions in the C-band system.

Dispersion Compensation in Optical Fiber: A Review

A cylindrical-shaped dielectric waveguide is what an optical fiber is. The core-cladding interface confines light, as electromagnetic (EM) energy, within its surface and guides light through several internal reflections if the angle of incidence is larger than the C-critical angle. Dispersion is the spreading of light pulses as they travel along a fiber, resulting in a degradation of optical signal quality. The scattering of optical signs contorts both computerized and simple transmission across optical strands. When optical fiber transmission is widely employed and some sort of digital modulation is applied, dispersion mechanisms inside the transmitted light pulses broaden as they pass through the fiber and move along the channel. The waveguide dispersion is calculated using a simple curve-fitting method. Dispersion must require compensating. Dispersion compensation techniques are essential for the successful transmission of data over optical fiber. Dispersion compensation techniques are used to reduce the effects of dispersion and improve SNR. Dispersion compensation is the process of reducing or eliminating chromatic dispersion in an optical fiber. There are two primary methods of dispersion compensation electronic and optical. In electronic dispersion compensation, the distorted signal is converted into the electrical domain, and digital signal processing techniques are used to correct the dispersion-induced distortion. While this method is effective, it adds complexity and cost to the communication system. Hence, most modern-day systems employ optical dispersion compensation, which is a more efficient and cost-effective method. Optical dispersion compensation involves using specialized optical components, such as dispersion compensating fibers (DCFs) and fiber Bragg gratings (FBGs), to adjust the different wavelengths' arrival time. DCFs are designed to have the opposite dispersion characteristics of the transmission fiber, which allows them to compensate for the dispersion as the signal passes through. FBGs are periodic structures inscribed in the fiber that reflect specific wavelengths of light, effectively delaying their arrival time and compensating for the overall dispersion.

Extended Study on Equalization-Enhanced Phase Noise for High-Order Modulation Formats

Coherent optical receivers with electronic dispersion compensation (EDC) become ubiquitous for high-capacity and long-distance transmissions. However, their performance can be decimated by equalization-enhanced phase noise (EEPN). The existing reports have shown some unique features that deserve further studies, especially for high-order modulation formats. In this paper, we firstly show that the intra-symbol EEPN is dominated by the phase noise. Therefore, the probability density function (PDF) due to intra-symbol EEPN and inter-symbol EEPN are quite different. We derive the variances of EEPN-induced phase and amplitude noise for QPSK signals, and fully explain the elliptical PDF. Secondly, we show that the intra-symbol EEPN is scaled with the power of the different modulus, but the inter-symbol one, the average power of the constellation. Subsequently, we elucidate that for higher-order constellations with multiple moduli, the total EEPN, the EEPN-induced phase noise and amplitude noise, all distribute differently on the different moduli. Finally, we analyze the EEPN after the practical carrier phase recovery algorithms, also with a closed-form prediction. Our analysis of EEPN is readily extended to arbitrary modulation formats.

Optimizing DC restoration in Kramers-Kronig optical single-sideband receivers.

Kramers-Kronig optical single-sideband receivers remove the signal-signal beat interference (SSBI) that occurs when detecting a signal that has electrical signals mapped onto its optical field at the transmitter; such signals support electronic dispersion compensation without the need for a coherent receiver. To use the full range of the analog-to-digital converter's (ADC) range, it is best to a.c.-couple the photocurrent, to remove its DC content; however, the DC must be restored digitally before the KK algorithm is applied. Recent publications have concentrated on perfectly determining the restored DC's required level from the signal, with a view this is optimal for lowering error rates. In this paper, we investigate signal-signal beat interference (SSBI) cancellation in a single photodiode receiver using Kramers-Kronig receiver algorithm, with large variations in optical carrier-to-signal power ratio (CSPR) and DC offset level. Through simulations and experiments, we find a strategy to optimize the signal quality without the need of an extensive search for the DC offset value. We also find that a theoretically perfect determination of the original DC level does not provide best signal quality especially for low CSPRs; in order to achieve maximum cancellation of signal-signal beat interference, the level of the restored DC has an optimum value that depends on the optical CSPR. We define a digital CSPR, which is the value of the CSPR in the digital domain after DC restoration. Our measurements show that we simply need to bias the signal upwards and make the minimum signal above zero by 0.1% of the r.m.s. signal amplitude when the optical CSPR is low. For higher values of optical CSPR, the optimal digital CSPR is about 2-dB lower than the optical CSPR, and the optimal DC offset can be calculated from this digital CSPR. We find that the boundary between our low optical CSPR region and high optical CSPR region depends on the noise level in the system.

Analysis and Compensation of Chromatic Dispersion in Long-hauls Optical Coherent System

In this paper, we demonstrate the efficiency of Electronic Dispersion Compensation (EDC) for coherent optical systems based on Polarization Division Multiplexed Quadrature Phase Shift Keying (PDM-QPSK). The performance of the proposed system is tested using a pulse that has been recently used in the presence of nonlinear effects.The proposed system is compared to the 0.3RZ-PDM-QPSK system at the optimum launched power under different symbol-rates and lengths of transmission. The simulation results confirm that the proposed method enhances the system performance. In addition, it secures a low penalty that is below 0.6 dB. As a result, the feasible transmission distance is improved by 29 %, 20.15 %, and 26.7 %, at 14 GBaud, 28 Gbaud, and 56 Gbaud, respectively.

Optical fiber deformation and vibration monitoring at distinct data rates with distinct photo-detectors devices

The transmission of the high data rate and information over the optical fiber is restricted by various dispersion phenomenon which further results into intersymbol interference (ISI). This article proposes two different methodologies for optical fiber deformation aiding vibration monitoring one using the pin diode as a photo-detector device and another manifesting the avalanche photo-detector. The novelty of this article lies in optical signal conversion using photo-detector processing followed by low pass Gaussian filter in order to reduce the extra contortion. This work is approaching electronic dispersion compensation technique (EDC) for compensation of chromatic dispersion at two distinct bit rates of 25 and 30 Gbps over 120 km of single mode fiber. Both the diodes have tested for theory performance using factors like Q-factor, bit error rate and eye height. It is found that pin diode is exhibiting a quality factor of 67.150 and 40.5944 at 25 and 30 Gbps, respectively. When avalanche pin diode is used at the same data rates, quality factor of 73.0240 and 43.3523 are obtained, respectively. Hence, use of avalanche photodiode at high bit rates is utilized and the simulations showed the optimum photo-diode for efficacious dispersion compensation at such a long transmission distance.

Nonlinear Coherent Optical Systems in the Presence of Equalization Enhanced Phase Noise

Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN.

Analog Low-Latency Kramers-Kronig optical Single-Sideband Receiver

Kramers-Kronig (KK) receivers have been shown to be able to cancel the signal-signal beat interference that occurs upon direct-photodetection in optical single-sideband systems, where the transmitted optical field is proportional to the modulation signal (Field-Modulated Direct-Detection systems). They can support complex modulation formats, and being single-sideband, electronic dispersion compensation, without the need for a coherent receiver. All current research has used digital-signal processing (DSP) to implement the KK algorithm. Radio-frequency KK receivers using analog processing were first introduced in the 1960's, and offer an alternative to DSP, particularly in their approximate form. Analog processing is inherently real-time and offers the lowest latency (processing delay). In this paper, we show that with minor changes, analog processing can also be used with signals from a photodetector. We have built an experimental receiver, using standard parts that have functionality that approximates to the desired algorithm. In particular, we show that a differential pair of bipolar transistors can approximate a square-root function, that the Hilbert transform can be approximated by an 8-tap transmission-line delay, and that a commercial analog multiplier chip can be used for a squaring and subtraction function. The system is demonstrated with 16-QAM at 500 Mbit/s with the photocurrent emulated in software and the KK algorithm performed in hardware. Finally, software band-limits the signal, down-converts it to form a constellation, and calculates symbol error rates. We show that the analog KK reduces the symbol error rate by at least a factor of ten, enabling low carrier-to-signal power ratios to be used with standard forward error correction. Much higher data rates could be supported using custom-designed microwave ASICs, which could be incorporated in packaged receivers. This would enable miniaturized receivers using standard DSP to work with dispersion-limited links. Interestingly, half the benefit could be gained by simply re-biasing the differential stage of a standard photoreceiver.

Mitigating the Impact of Noise on Nonlinear Frequency Division Multiplexing

In the past years, nonlinear frequency division multiplexing (NFDM) has been investigated as a potentially revolutionary technique for nonlinear optical fiber communication. However, while NFDM is able to exploit the Kerr nonlinearity, its performance lags behind that of conventional systems. In this work, we first highlight that current implementations of NFDM are strongly suboptimal, and, consequently, oversensitive to noise: the modulation does not ensure a large minimum distance between waveforms, while the detection is not tailored to the statistics of noise. Next, we discuss improved detections strategies and modulation techniques, proposing some effective approaches able to improve NFDM. Different flavors of NFDM are compared through simulations, showing that (i) the NFDM performance can be significantly improved by employing more effective detection strategies, with a 5.6 dB gain in Q-factor obtained with the best strategy compared to the standard strategy; (ii) an additional gain of 2.7 dB is obtained by means of a simple power-tilt modulation strategy, bringing the total gain with respect to standard NFDM to 8.3 dB; and (iii) under some parameters range (rate efficiency η≤30%), the combination of improved modulation and detection allows NFDM to outperform conventional systems using electronic dispersion compensation.

56 Gbit/s OOK Signal in C-band Over 20 km Dispersion-Uncompensated Link Transmission With Receiver-Side EDC Algorithm

In this paper, we propose and experimentally demonstrate a receiver-side electronic dispersion compensation (EDC) algorithm for direct-detection 56 Gbit/s on-off keying signal in C-band over 20 km dispersion-uncompensated standard single-mode fiber transmission. The proposed receiver-side EDC algorithm includes the feed-forward equalizer, post noise-whitening filter and modified iterative detection algorithm, which is originally used in spectrally efficient frequency division multiplexing systems for inter-carrier interference cancellation. Only with the aid of the proposed EDC algorithm for the compensation of power fading effect caused by bandwidth limit and chromatic dispersion, bit-error ratio below the 7% hard-decision forward error correction limit is achieved. This scheme can keep the system cost-effective and flexible, showing potentials for optical interconnects.

Theoretical Analyses of Different Nonlinear Compensation Methods Based on Perturbation Theories in the Unrepeatered System With Raman Amplification

In this paper, the pre- and post-nonlinear compensation (NLC) methods based on regular perturbation (RP) theory are contrastively investigated with the original NLC as a bridge for analyses. Firstly, the numerical error functions of pre-NLC and original NLC are derived, revealing that the numerical error of pre-NLC is more severe due to the error accumulation. Secondly, we deduce the relevance of post-NLC and original NLC, which uncovers the essential difference is that the input condition of post-NLC possesses certain additional information, making the signal's intensity distribution after post-NLC more beneficial for the hard-decision. Meanwhile, the pre-, post- and original NLC methods based on enhanced regular perturbation (ERP) theory have also been discussed. Finally, the simulation is carried out with signal modulation of 16/32/64 QAM and baud-rate of 32 GBaud in an unrepeartered system with Raman amplification. The results agree with the analyses, the post-NLC is the best while the original NLC surpasses the pre-NLC. Additionally, we demonstrate an experiment with signal modulation of 16 QAM and baud-rate of 10 GBaud. The improvements of the Q2-factor of the RP-based and ERP-based post-NLC are about 0.6 dB and 1 dB compared with the electronic dispersion compensation (EDC).

Dual frame OFDM with optical phase conjugation for MIMO system in multimode fiber

For nonlinearity and dispersion mitigation in optical fiber, the authors propose a dual frame OFDM transmission, based on optical phase conjugation. The proposed dual frame OFDM is modification of asymmetrically clipped OFDM (ACO-OFDM) frame. The proposed OFDM frame consist of two positive frames, first one is time compressed ACO-OFDM frame and second one is optical phase conjugated version of first frame. This increases the bandwidth efficiency and total transmitted power for the system. For electronic dispersion compensation (EDC) in conventional OFDM, signal pre-processing is applied at transmitter side where it requires channel information. In case of OFDM and multiple input multiple output (MIMO) system setup for multi mode fiber, channel becomes almost unpredictable. The proposed dual frame OFDM does not require EDC at transmitter side. For received optical power $$-12$$ dbm conventional ACO-OFDM records bit error rate (BER) $$4.48*10^{-1}$$ and proposed dual frame ACO-OFDM with OPC records BER $$2.58*10^{-4}$$ . Simulation results for variable fiber length show BER $$2.20*10^{-1}$$ for phase conjugated subcarrier coding and $$2.24*10^{-3}$$ for proposed ACO-OFDM with OPC for multi-mode fiber length 500 m. It also allows different multiframe transmission based on time and wavelength diversity.

Single Sideband Transmission Employing a 1-to-4 ADC Frontend and Parallel Digitization

Distributed data-centers have emerged as a key architecture for future optical networks. This architecture relies 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-polarization 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 includes the simple transceiver architecture and the capability of electronic dispersion compensation. However, the required digitizer's bandwidth in a single-polarization SSB transmission is around four times higher than in those of a conventional dual-polarization coherent detection system offering the same data rate. This critical issue prevents the practical implementation of high-speed SSB digital signal processing (DSP) in CMOS ASIC. To address this problem, we consider in this work a 112 GSa/s SiGe HBT BiCMOS ADC frontend. Using this ADC front-end, we experimentally demonstrate a WDM SSB DD transmission at a net data rate of 200 Gb/s per channel over 80 km while using only 14 GHz of digitizer's bandwidth which is comparable to those of a full-coherent counterpart.

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.