Discrete Multitone Receiver Research Articles

64APSK constellation scheme for short-reach DMT with ISDD enabled SFO compensation

Sampling frequency offset between digital-to-analog converter (DAC) and analog-to-digital converter (ADC) always exists in asynchronous optical discrete multi-tone (DMT) systems. The overall bit error rate (BER) performance may be seriously deteriorated due to SFO-induced phase rotation. Inter-symbol differential encoding/detection (ISDE/ISDD) with decision-aided residual phase compensation (RPC) has been employed to compensate SFO. No extra training sequences or pilot subcarriers are required for SFO compensation. However, the increased phase noise induced by ISDD is still a problem and may degrade the BER performance of the DMT receiver, especially for high-order modulation formats. In this work, we propose the use of amplitude and phase shift keying (APSK) constellation scheme to against the ISDD-induced phase noise and then improve the BER performance. In our simulations, the 64QAM and two types of 64APSK modulation formats for DMT are studied in both additive white Gaussian noise (AWGN) channel and intensity-modulation/direct-detection optical fiber transmission system. The simulated results exhibit that the employed two 64APSK constellation schemes outperform the conventional 64QAM modulation format in the presence of ISDD-induced phase noise. By using the proposed 64APSK constellation scheme, the SNR performance can be improved by up to 3dB when the BER is 1E-3 in AWGN channel. Besides, up to 2dB improvement in the receiver sensitivity in terms of the received optical power can be also achieved, compared to the conventional 64QAM modulation format.

100-Gb/s DMT Transmission for Short-Reach Optical Interconnect With DAC Clock Leakage Compensation

Digital-to-analog converters (DAC) with a high sample rate and large bandwidth are preferred to achieve high data-rate signal generation in an optical discrete multi-tone (DMT) system. Clock leakage is one of the imperfections in high-speed DACs. One or more data subcarriers of the generated DMT signal may be interfered by the additional clock tone, which may cause error floor in bit error rate (BER) performance. In this work, the pre-equalized 16/32QAM-DMT signals with the net data rate up to 121 Gbit/s are generated by an 80 GSa/s DAC with a clock leakage at 20 GHz. Two simple DAC clock leakage compensation (CLC) methods are employed and investigated in the DMT receiver. The results exhibit the two CLC methods have a similar performance. After 2 km of standard single-mode fiber (SSMF) transmission, by using the two CLC methods, the BER performance can be improved by up to about an order of magnitude and below 3.8e-3 for 16QAM-DMT. The receiver sensitivity in terms of the received optical power can be improved by 2-dB in 32QAM case at the BER of 2e-2. Furthermore, the BER performance with two CLC methods has good stability over the measurement period.

Comparison of null-subcarriers reservation and adaptive notch filter for narrowband interference cancellation in intra-data center interconnect with DMT signal transmission.

In this paper, we experimentally compare the performance of two different narrowband interference suppression schemes in 120 Gb/s intensity modulation and direct detection (IM/DD) system with discrete multi-tone (DMT) signal transmission for intra-data center interconnect (Intra-DCI). Digital pre-equalization and DFT-spread techniques are applied for system bandwidth limitation induced signal distortion compensation and signal peak to average power ratio (PAPR) reduction, respectively. Null-subcarriers reservation (NSR) and adaptive notch filter (ANF) techniques are compared during the suppression of digital-to-analog convertor (DAC) clock leakage induced narrowband interference. 1.2 dB and 1.8 dB DMT receiver sensitivity improvements can be achieved at a bit-error rate of 3.8 × 10-3 in optical back-to-back (OBTB) transmission when optimized NSR and ANF schemes are applied for narrowband interference cancellation, respectively. After 2-km single mode fiber (SMF) transmission, the required received optical power (ROP) of DMT signal with optimized NSR and ANF for narrowband interference cancellation at BER of 3.8 × 10-3 are -6.5 dBm and -7.1 dBm, respectively. Obviously, ANF outperforms NSR scheme in narrowband interference cancellation for DMT system.

3 × 104 Gb/s Single-λ Interconnect of Mode-Division Multiplexed Network With a Multicore Fiber

In this paper, we study the use of silicon microring modulators in an integrated mode-division multiplexing (MDM) network for short-reach interconnects using discrete multitone (DMT) modulation. A fiber grating coupler array is used as a high-speed optical interface between the on-chip mode channels and off-chip multicore fiber (MCF). The whole link has a crosstalk level lower than –23.1 dB. The MDM network can work without the crosstalk compensation of digital signal processing. We present experimental results of the linearity of the silicon microring modulator in terms of spurious-free dynamic range (SFDR). The third-order intermodulation distortion SFDR is 93.4 dB/Hz 2/3 at 20 GHz. We can improve system performance by using adaptive nonlinear equalizer at the DMT receiver to mitigate transmitter and transmission impairments. An aggregate bitrate above 312 Gb/s is achieved with a bit error ratio of 3.8 × 10−3. With the addition of nonlinear equalizer, the line rates of the MDM network can be further increased to about 330 Gb/s. The experimental results demonstrate the feasibility of single wavelength DMT transmission in MDM network with MCF.

Adaptive bit rate maximizing time-domain equalizer design for DMT-based systems

The classical discrete multitone receiver as used in, e.g., digital subscriber line (DSL) modems, combines a channel shortening time-domain equalizer (TEQ) with one-tap frequency-domain equalizers (FEQs). In a previous paper, the authors proposed a nonlinear bit rate maximizing (BM) TEQ design criterion and they have shown that the resulting BM-TEQ and the closely related BM per-group equalizers (PGEQs) approach the performance of the so-called per-tone equalizer (PTEQ). The PTEQ is an attractive alternative that provides a separate complex-valued equalizer for each active tone. In this paper, the authors show that the BM-TEQ and BM-PGEQ, despite their nonlinear cost criterion, can be designed adaptively, based on a recursive Levenberg-Marquardt algorithm. This adaptive BM-TEQ/BM-PGEQ makes use of the same second-order statistics as the earlier presented recursive least-squares (RLS)-based adaptive PTEQ. A complete range of adaptive BM equalizers then opens up: the RLS-based adaptive PTEQ design is computationally efficient but involves a large number of equalizer taps; the adaptive BM-TEQ has a minimal number of equalizer taps at the expense of a larger design complexity; the adaptive BM-PGEQ has a similar design complexity as the BM-TEQ and an intermediate number of equalizer taps between the BM-TEQ and the PTEQ. These adaptive equalizers allow us to track variations of transmission channel and noise, which are typical of a DSL environment.

Combined RLS-LMS initialization for per tone equalizers in DMT-receivers

In discrete multitone receivers, the classical equalizer structure consists of a (real) time domain equalizer (TEQ) combined with complex one-tap frequency domain equalizers. An alternative receiver is based on a per tone equalization (PTEQ), which optimizes the signal-to-noise ratio (SNR) on each tone separately and, hence, the total bitrate. In this paper, a new initialization scheme for the PTEQ is introduced, based on a combination of least mean squares (LMS) and recursive least squares (RLS) adaptive filtering. It is shown that the proposed method has only slightly slower convergence than full square-root RLS (SR-RLS) while complexity as well as memory cost are reduced considerably. Hence, in terms of complexity and convergence speed, the proposed algorithm is in between LMS and RLS.

Constraints in channel shortening equalizer design for DMT-based systems

In discrete multitone receivers, a time domain equalizer (TEQ) is used to shorten the channel impulse response, so that the equalized channel impulse response is shorter than the inserted prefix. The aim of this paper is to show that the minimum mean square error (MMSE) channel shortening problem with two different energy constraints, remarkably, lead to the same TEQ coefficients, up to a scaling factor. Moreover, implying the two energy constraints together in the MMSE optimization again yields the same result and comes down to a canonical correlation analysis between the subspace spanned by the transmitted samples and the received samples, respectively. Hence, the TEQ obtained by these three distinct MMSE cases yields the same performance in terms of bit rate. Since the resulting problem can easily be reformulated as a maximization problem, an iterative procedure based on power iterations can be devised to reduce the computational complexity.

Turbo multiuser detection for coded DMT VDSL systems

In recent years, iterative processing techniques with soft-in/soft-out components have received considerable attention. Such techniques, based on the so-called turbo principle, are exemplified through turbo decoding, turbo equalization, and turbo multiuser detection. Turbo multiuser detection is applied to a discrete multitone (DMT) very-high-rate digital subscriber line system to combat crosstalk signals and to obtain substantial coding gain. The proposed iterative DMT receiver is shown to achieve an overall 7.0 dB gain over the uncoded optimum receiver at a bit error rate of 10/sup -7/ for a channel with severe intersymbol interference and additive white Gaussian noise and with one dominant crosstalk signal. Impulse noise is detrimental to the proposed scheme but can be overcome through erasure decoding techniques, as is shown by example.

Equalization for discrete multitone transceivers to maximize bit rate

In a discrete multitone receiver, a time-domain equalizer (TEQ) reduces the intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Current TEQ design methods such as the minimum mean-squared error (MMSE), maximum shortening SNR (MSSNR), and maximum geometric SNR (MGSNR) do not directly maximize bit rate. We develop two TEQ design methods to maximize the bit rate. First, we partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel SNR definition. Then, we derive a nonlinear function of TEQ taps that measures the bit rate, which the proposed maximum bit rate (MBR) method optimizes. We also propose a minimum-ISI method that generalizes the MSSNR method by weighting the ISI in the frequency domain to obtain higher performance. The minimum-ISI method is amenable to real-time implementation on a fixed-point digital signal processor. Based on simulations using eight different carrier-serving-area loop channels, (1) the proposed methods yield higher bit rates than MMSE, MGSNR, and MSSNR methods; (2) the proposed methods give three-tap TEQs with higher bit rates than 17-tap MMSE, MGSNR, and MSSNR TEQs; (3) the proposed MBR method achieves the channel capacity (as computed by the matched filter bound using the proposed subchannel SNR model) with a five-tap TEQ; and (4) the proposed minimum-ISI method achieves the bit rate of the optimal MBR method.