Nyquist Wavelength Division Multiplexing Research Articles

Nyquist-WDM With Low-Complexity Joint Matched Filtering and Adaptive Equalization

In this letter, we investigate the application of jointly processing the matched filtering and blind adaptive equalization functions in a receiver for a Nyquist wavelength division multiplexing (Nyquist-WDM) system employing a root-raised-cosine (RRC) pulse shaping filter. We show that blind equalization is able to also provide the matched filtering function, if the number of filter taps is increased. In addition, if implemented in the frequency domain, this form of blind equalization is more computationally efficient than when a separate RRC is used. It is also verified that this method is not sensitive to frequency offset. We demonstrate a 16-channel 19-GBd Nyquist-WDM system with QPSK and 16-quadrature-amplitude-modulation formats, and find the frequency domain blind equalizer can replace the RRC filter without adding a performance penalty.

Digital Nonlinear Compensation Based on the Modified Logarithmic Step Size

In this paper, we investigate the digital backward propagation (DBP) nonlinear compensation (NLC) based on the modified logarithmic step size distribution. Different from regular constant step size, we use the logarithmic non-constant step size distribution in DBP. The compensation performance is investigated with Nyquist wavelength division multiplexing (NWDM) system. The modified logarithmic step size distribution is proposed and studied by introducing an attenuation adjusting factor k. As a proof of the concept, the optimal k factor is studied by simulation results with the noise figure, the input power, the transmission distance, the fiber attenuation, the span length, the step number per span, the dispersion value, the baud rate, the calculated sub-channels, the pulse shape and the modulation formats. Reduced complexity and improved performance is observed by using the modified logarithmic step size distribution. Compared with constant step nonlinear compensation, the improved bit-error-ratio (BER) performance and Q-value for our scheme is demonstrated by 3 × 50-Gb/s NWDM polarization division multiplexing quadrature phase shift keying (PDM-QPSK) signal with 1120-km single-mode fiber-28 (SMF-28) transmission and Erbium-doped fiber amplifier (EDFA)-only amplification.

Joint Digital Preequalization for Spectrally Efficient Super Nyquist-WDM Signal

We propose a joint digital preequalization scheme for a Nyquist wavelength division multiplexing (N-WDM) system to compensate for the intersymbol interference caused by a limited channel bandwidth or aggressive prefiltering. A digital raised-cosine pulse filter is implemented before the reference-based preequalization scheme to enhance the tolerance of the signal to the crosstalk from adjacent channels. The experiment illustrates eight-channel digital-analog-converter generation of the polarization division multiplexing quadrature phase shift keying system at 112 Gb/s per channel on a 25-GHz grid, adopting the reference-based digital preequalization considering narrow-band optical filtering effect at the transmitter. The experimental results show that the proposed preequalization scheme allows for an ultradense channel spacing of 0.89 times of the baud rate in the super N-WDM system with a as large as 5-dB optical signal-to-noise ratio improvement.

Decision-Feedback Equalization of Bandwidth-Constrained N-WDM Coherent Optical Communication Systems

Nyquist Wavelength division multiplexing (N-WDM), is a promising scheme in order to enhance the spectral efficiency of future coherent optical communication systems. In N-WDM systems, the channel bandwidth and spacing are selected to maximize the spectral efficiency while maintaining acceptable levels of inter-carrier and inter symbol interference. To further increase the spectral efficiency, bandwidth constrained N-WDM, where baudrate is higher than channel bandwidth can be considered. We propose to combine the bandwidth-constrained N-WDM scheme with a decision feedback equalizer (DFE) designed according to the minimum mean square error (MMSE) criterion. We show that the enhanced resilience to the inter symbol interference offered by DFE provides an effective gain on spectral efficiency. We compare system benefits of DFE and maximum a posteriori sequence detection (MAP) for coherent optical receivers, and show that DFE is as efficient as MAP in mitigating inter symbol interference at a considerably lower complexity.

Robust 9-QAM digital recovery for spectrum shaped coherent QPSK signal

We propose 9-ary quadrature amplitude modulation (9-QAM) data recovery for polarization multiplexing-quadrature phase shift keying (PM-QPSK) signal in presence of strong filtering to approach Nyquist bandwidth. The decision-directed least radius distance (DD-LRD) algorithm for blind equalization is used for 9-QAM recovery and intersymbol interference (ISI) compression. It shows the robustness under strong filtering to recover 9-QAM signal rather than QPSK. We demonstrate 112 Gb/s spectrum shaped PM-QPSK signal by wavelength selective switch (WSS) in a 25-GHz channel spacing Nyquist wavelength division multiplexing (NWDM). The final equalized signal is detected by maximum likelihood sequence decision (MLSD) for data bit-error-ratio (BER) measurement. Optical signal-to-noise ratio (OSNR) tolerance is improved by 0.5 dB at a BER of 1x10(-3) compared to constant modulus algorithm (CMA) plus post-filter algorithm.

Dual-polarization OFDM-OQAM for communications over optical fibers with coherent detection

In order to improve the spectral efficiency of coherent optical communication systems, it has recently been proposed to make use of the orthogonal frequency-division multiplexing offset quadrature amplitude modulation (OFDM-OQAM). Multiple optical channels spaced in the frequency domain by the symbol rate can be transmitted orthogonally, even if each channel overlaps significantly in frequency with its two adjacent channels. The solutions proposed until now in the literature unfortunately only address a single polarization communication, and therefore do not benefit from the capacity gain reached when two polarizations are used to transmit independent information signals. The aim of the present paper is to propose a receiver architecture that can decouple the two polarizations. We build an equalizer per channel at twice the symbol rate and optimize it based on the minimum mean square error (MMSE) criterion. We demonstrate the efficiency of the resulting system compared to the Nyquist wavelength-division multiplexing (N-WDM) system both in terms of performance and complexity. We also assess the system sensitivity to transmit synchronization errors and show that system can even work under significant synchronization errors.

Transmission of 200 G PDM-CSRZ-QPSK and PDM-16 QAM With a SE of 4 b/s/Hz

We have realized the first field transmission of 8 × 216.8-Gb/s Nyquist wavelength-division-multiplexing (N-WDM) signals over 1750-km G.652 fiber consisting of 950-km real and 800-km lab fibers with erbium-doped fiber amplifier (EDFA)-only amplification. The average loss per span is 21.6 dB. Each channel is modulated with 54.2-Gbaud (216.8-Gb/s) polarization-division-multiplexing carrier-suppressed return-to-zero quadrature-phase-shift-keying (PDM-CSRZ-QPSK) data on a 50-GHz grid giving a record spectral efficiency (SE) of 4 b/s/Hz. Digital post filtering and 1-bit maximum likelihood sequence estimation (MLSE) are introduced into the offline digital signal processing (DSP) at the receiver to suppress noise, linear crosstalk and filtering effects. We have also investigated the co-transmission of 200 G PDM-CSRZ-QPSK and 200 G PDM 16-ary quadrature-amplitude-modulation (PDM-16 QAM) signals on a 50-GHz grid, and found that PDM-CSRZ-QPSK signals have better bit-error-rate (BER) performance for both back-to-back and 700-km transmission cases. Meanwhile, PDM-16 QAM signals are subject to larger crosstalk from the neighboring Nyquist QPSK channels.

6$\,\times\,$128-Gb/s Nyquist-WDM PDM-16QAM Generation and Transmission Over 1200-km SMF-28 With SE of 7.47 b/s/Hz

The Nyquist wavelength-division multiplexing technique enable a solution to achieve high spectral efficiency in long-haul transmission system. Compare to polarization division multiplexing quadrature-phase-shift-keying, even high-level modulation scheme such as polarization division multiplexing 16-quadrature-amplitude-modulation is much more sensitive to intra-channel noise and inter-channel linear crosstalk caused by Nyquist wavelength-division multiplexing. We experimentally demonstrated the generation and transmission of the 6 × 128-Gb/s Nyquist wavelength-division multiplexing sixteen quadrature amplitude modulation signal over 1200-km single-mode fiber (SMF)-28 with Erbium-doped fiber amplifier only amplification with a net SE of 7.47 b/s/Hz, which is so far, to our knowledge, the highest SE for the signal with the bit rate above 100-Gb/s using the polarization-division multiplexing 16-quadrature-amplitude-modulation format. This experiment was successfully enabled by digital-signal-processing (DSP) pre-equalization of transmitter-side impairments and DSP post-equalization of the channel and receiver-side impairments. Nyquist-band is considered in the pre- equalization to enhance the tolerance of the polarization-division multiplexing 16-quadrature-amplitude-modulation to the aggressive spectral shaping. The bit error rate for each of the 6 channels is smaller than the limitation of the forward-error-correction limit of 3.8 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> after 1200-km SMF-28 transmission.

100G Transmission Over Transoceanic Distance With High Spectral Efficiency and Large Capacity

In this paper, we review recent advances in undersea 100G system research with high spectral efficiency and large capacity. Several different techniques are covered including spectral shaping, Nyquist wavelength division multiplexing, orthogonal frequency division multiplexing, optical superchannel, tight optical filtering with multisymbol detection using maximum a posteriori probability or maximum likelihood sequence estimation, and coded modulation. In particular, we review several recent experiments transporting >; 10 Tb/s with >; 3 b/s/Hz spectral efficiency over transoceanic distance.

Performance Assessment of Noise-Suppressed Nyquist-WDM for Terabit Superchannel Transmission

This paper focuses on terabit superchannel transmission with enhanced network reach by using an emerging noise-suppressed Nyquist wavelength division multiplexing (NS-N-WDM) technique for polarization multiplexing quadrature phase shift keying subchannels at different symbol rate to subchannel spacing ratios up to 1.28, and for the first time, it was compared experimentally with the transmission capability of no-guard-interval coherent optical orthogonal frequency division multiplexing (NGI-CO-OFDM) on the same testbed. At 2 × 10-3 bit error ratio, the maximum reachable distance is 3200 and 2800 km SMF-28 with erbium-doped-fiber-amplifier-only amplification for NGI-CO-OFDM and NS-N-WDM terabit superchannels, respectively, at 100 Gb/s/ch. For 11 ntn112 and 11 × 128 Gb/s/ch NS-N-WDM transmission under assumption of different coding gain with hard-decision and soft-decision forward error correction, their maximum achievable distance was found to be equivalent, which are 2100 and 2170 km, respectively, both were achieved by using digital noise filtering and 1-bit maximum likelihood sequence estimation at the receiver DSP. In addition, the back-to-back characteristics of NS-N-WDM superchannel such as analog-to-digital converter bandwidth requirement and its tolerance to unequal subchannel power were experimentally evaluated and studied.

WDM transmission of 1084-Gbaud PDM-QPSK signals (40 × 4336-Gb/s) over 2800-km SMF-28 with EDFA-only

We experimentally demonstrated the transmission of 40 × 433.6-Gb/s Nyquist wavelength-division-multiplexing (N-WDM) optical time-division-multiplexing (OTDM) over 2800-km single-mode fiber (SMF)-28 with Erbium-doped fiber amplifier (EDFA)-only amplification, adopting polarization-division-multiplexing carrier-suppressed return-to-zero quadrature-phase-shift-keying (PDM-CSRZ-QPSK) modulation as well as post filter and 1-bit maximum likelihood sequence estimation (MLSE). Each channel occupies 100 GHz, yielding a spectral efficiency of 4.05 b/s/Hz. The bit-error ratio (BER) of all channels is less than the pre-forward-error-correction (pre-FEC) limit of 3.8 × 10(-3) after 2800-km SMF-28 transmission.

Building up low-complexity spectrally-efficient Terabit superchannels by receiver-side duobinary shaping

Recently, an increasing interest has been put on spectrally-efficient multi-carrier superchannels for beyond 100G. Apart from orthogonal frequency-division multiplexing (OFDM) and Nyquist wavelength-division multiplexing (WDM), another low-complexity WDM approach based on transmitter-side pre-filtering and receiver-side duobinary shaping is proposed to build up multi-carrier superchannels. This approach is referred to as receiver-side duobinary-shaped WDM (RS-DBS-WDM). Generation and transmission of a 1.232-Tbit/s 11-carrier superchannel is experimentally demonstrated. The superchannel signal can be well fit inside the passband of multiple 300-GHz reconfigurable optical add and drop multiplexers (ROADMs). In the superchannel scenario, the proposed RS-DBS-WDM is qualitatively compared with OFDM and Nyquist-WDM in terms of implementation complexity. In sum, the proposed RS-DBS-WDM approach features high transceiver analog-bandwidth efficiency, high spectral-efficiency, the absence of specific spectral manipulation, compatibility with conventional WDM technologies and coherent detection algorithms, and comparable implementation penalty.

Performance Limits of Nyquist-WDM and CO-OFDM in High-Speed PM-QPSK Systems

We compare by simulation the performance of two techniques for generating polarization-multiplexed quadrature phase-shift keying with a high spectral efficiency (SE). The first is based on coherent optical orthogonal frequency-division multiplexing (CO-OFDM). The second, which we call Nyquist wavelength-division multiplexing (N-WDM), is based on the use of optical pulses having an “almost” rectangular spectrum, with bandwidth ideally equal to the Baud-rate. We show that the two techniques have the same sensitivity and SE under idealized assumptions. However, we found that CO-OFDM requires a much larger receiver bandwidth and proportionally faster speed of the analog-to-digital converters. We also tested CO-OFDM and N-WDM over long-haul nonlinear links and found N-WDM to outperform CO-OFDM in this case, too.