Discrete Multitone Transmission Research Articles

Spectrally Efficient Non-Orthogonal Discrete Multi-Tone Transmission for Underwater Wireless Optical Communication With Low-Complexity High Performance ICI Mitigation

In this paper, a novel spectrally efficient non-orthogonal discrete multi-tone (NODMT) system is proposed for the bandwidth-starved underwater wireless optical communication (UWOC) to compensate for the insufficient bandwidth of components. Different from other non-orthogonal systems with complex-valued correlation matrices, the designed NODMT system has a real-valued correlation matrix, which is significant in reducing the complexity of decoders, e.g., the complexity of the proposed NODMT with iterative detection (ID) algorithm can be reduced by 50% as compared to the conventional non-orthogonal system with ID. However, in circumstances of higher spectrum efficiency (SE), the non-orthogonality is intensified and inter-carrier interference (ICI) becomes severer, leading to the sharp deterioration of ID decoding capability. The traditional sphere decoding (SD) algorithm cannot solve this issue because Cholesky decomposition only works with the positive definite matrix. Therefore, we propose a novel SD algorithm based on QR decomposition, named QRSD, which effectively compensates for the insufficient decoding capability of the ID algorithm in the case of higher SE. By combining ID and QRSD methods, over 40% complexity reduction and nearly identical BER performance with QRSD are achieved. Through a proof-of-concept UWOC experiment, a 3.31-Gbps NODMT transmission with up to 24.44% data rate promotion is demonstrated, and the feasibility / effectiveness of the proposed ID-QRSD algorithm in different turbidity water is verified. Moreover, we have experimentally realized 75-m/2.66-Gbps and 60-m/3.02-Gbps underwater transmission with 15% and 25% bandwidth savings, respectively. This UWOC system with high SE shows great potential in the bandwidth-limited UWOC and underwater internet of things (UIoT).

Multi-band DFT-S 100 Gb/s 32 QAM-DMT transmission in intra-DCI using 10 G-class EML and low-resolution DAC.

In this paper, 100 Gb/s/λ 32 quadrature amplitude modulation discrete multi-tone (QAM-DMT) transmission using 10 G-class electro-absorption modulated laser (EML) and 4/5-bit digital-to-analog converters (DACs) are experimentally demonstrated to meet the requirement of intra-datacenter interconnection (intra-DCI). Unequal length multi-band (ULM) discrete Fourier transform spread (DFT-S) precoding is investigated to alleviate the distortion induced by the high peak-to-average power ratio (PAPR) of DMT. The results show that the required computational complexity of ULM DFT-S precoding with 2-bands (k1=256, k2=64) decreases sharply compared to the traditional DFT-S technique with only about 0.5 dB receiver sensitivity penalty. In addition, compared to the equal length multi-band (ELM) DFT-S precoding, the ULM DFT-S precoding can bring about 2.5 dB receiver sensitivity improvement with slight added computational complexity. With the assistance of ULM DFT-S precoding and noise shaping (NS) technique, the bit-error ratio (BER) of 100 Gb/s 32 QAM-DMT signal generated by 5-bit DAC over 2-km single-mode fiber (SMF) transmission can reach the hard-decision forward error correction (HD-FEC) threshold with received optical power (ROP) of -6.5 dBm, with only additional 39.9% multiplier and 33.7% adder.

Symbol division multiplexing in optical fiber communication systems.

Modern emerging data services and applications have put forward an ever-increasing bandwidth requirement for fiber-optic communication channels. To this end, we propose a novel symbol division multiplexing technology (SDM) by multiplexing/de-multiplexing of multiple quadrature amplitude modulation (QAM) symbols onto one complex constellation point. In our SDM scheme, every 7-bit 128QAM symbol is multiplexed per complex valued signal sequentially according to the optimal many-to-one mapping law, forming a 32QAM in the constellation and achieving an extra 40% gain for symbol capacity in an optical discrete multi-tone transmission system. The experiments prove that the SDM-32QAM successfully mitigates the signal impairments induced by fiber chromatic dispersion and Kerr nonlinearity, thus leading to 3.91-dB superior receiver power sensitivity and 2-dB enhancement of systematic tolerance to fiber nonlinear effect. The results highly motivate a fundamental paradigm in multiplexing techniques for optical fiber communication systems.

Comparison of digital interpolation and phase compensation in asynchronous optical DMT transmission systems

The sampling frequency offset (SFO) will induce phase rotation, inter-symbol interference and inter-carrier interference, which may seriously deteriorate the receiver’s performance in asynchronous optical discrete multi-tone (DMT) systems. In this work, a high-accuracy training symbols-based SFO estimation (SFOE) method is studied in both the additive white Gaussian noise channel and short-reach directly-modulated laser-based DMT transmission system. Two general SFO compensation (SFOC) methods, i.e., digital interpolation (DI) and phase compensation (PC), are comparatively investigated through both simulations and experiments. The results indicate that the accuracy of the SFOE method can be within ±0.2 ppm under the SFO range of ±500 ppm and different received optical powers. With the high-accuracy SFOE method, the 4th order DI has the best compensation performance compared to PC, 2nd and 3rd order DI methods. Moreover, the negligible EVM performance penalty compared with the case of SFO-free can also be achieved in an optical DMT transmission system. Besides, the PC-based SFOC method has similar EVM performance to the 4th order DI in the case of small SFO within ±300 ppm.

Demonstration of >1Tbit/s WDM OWC with wavelength-transparent beam tracking-and-steering capability

Beam tracking-and-steering is crucial for the operation of high-speed, narrow beam, optical wireless communication (OWC) systems. Using a system based on two sets of low-cost cameras for continuous beam tracking and a set of mirrors for steering, we demonstrate here a high-capacity (>1Tbit/s) ten-channel wavelength-division multiplexed (WDM) OWC system based on discrete multitone transmission. The results, which are achieved over a 3.5-m perpendicular distance and across a lateral coverage up to 1.8 m, constitute to the best of our knowledge, the highest aggregate OWC capacity at this coverage.

Active Demultiplexer-enabled Directly Modulated DMT Transmission Using Optical Frequency Combs for Data Center Interconnects

In this paper, we experimentally demonstrate an optical frequency comb (OFC) based transmitter, employing directly modulated active demultiplexers, for data center interconnects. The results validate that the proposed transmitter has the potential to achieve aggregate data rates of 100 Gb/s (8 × 12.5 Gb/s) and 200 Gb/s (8 × 25 Gb/s) for systems employing 4- and 16- quadrature amplitude modulated (QAM) discrete multi-tone (DMT) modulation. An OFC based on an externally injected gain switched laser (EI-GSL) is used, providing excellent stability and flexibility in free spectral range (FSR). The OFC is followed by an injection locked active demultiplexer, which not only filters, but also amplifies individual comb tones, thus alleviating the need for an external optical amplifier to boost the low powered comb tones. Using the proposed configuration, we experimentally demonstrate a successful transmission of 12.5 Gb/s/λ 4-QAM DMT and 25 Gb/s/λ 16-QAM DMT signals over 40 km and 25 km of SSMF, respectively. In addition, we show that it is possible to filter and modulate comb lines that are 20 dB below the spectral peak, whilst achieving a BER below the hard decision (HD-) FEC limit of 3.8e-3. This gain flattening or comb expansion feature leads to a significant increase in the channel count, which in turn provides a reduction in the energy consumption and the footprint of the transmitter.

A Chaotic Encryption Scheme in DMT for IM/DD Intra-Datacenter Interconnects

In this letter, a novel physical layer encryption method based on digital chaos is proposed for the security in high-speed intensity modulation and direct detection (IM/DD) discrete multi-tone (DMT) transmission system. In the proposed encryption scheme, a random interleaving method is used, which further enhances the ability to encrypt the quadrature amplitude modulation (QAM) constellation and improves the transmission performance. In addition, probabilistic shaping (PS) technique is also cascaded into the encrypted transmission system for the first time. Adopting the proposed algorithm, we experimentally demonstrated the transmission of 100-Gbit/s encrypted 64QAM and PS-64QAM DMT signals over 2-km standard single-mode fiber (SSMF).

Performance-Enhanced DMT System With Joint Precoding and Probabilistic Constellation Shaping

The promising solution for short-reach optical communication, discrete multitone (DMT) system suffer from high peak-to-average power ratio (PAPR). It is easy for the DMT signal to enter the nonlinear region of the components in the systems, which distorts the signal and limits the modulation efficiency. It is well-known that the precoding technique can suppress the PAPR of DMT without loss of spectral efficiency. We found that, in the I/Q plane, the nonlinear distortion clusters in the constellation points with higher power due to the joint precoding and inverse fast Fourier transform. The principle of this phenomenon has been studied. To cope with the specially distributed nonlinear distortions induced by precoding, we adopted the probabilistic constellation shaping (PCS) technique to reduce the probability of constellation points with high power, resulting in the overall performance improvement. Joint precoding and PCS significantly improve the resistance to nonlinear distortions for DMT systems. Simulations and experiments of 16-QAM and 64-QAM DMT transmission systems were conducted to evaluate the performance. The experimental results show that at the NGMI threshold of 0.92, up to 4 dB enhancement of the receiver sensitivity has been achieved by the proposed scheme when 4-GBd 64-QAM signals are transmitted.

Experimental demonstration of advanced modulation formats for data center networks on 200 Gb/s lane rate IMDD links.

This work contributes experimental demonstrations and comprehensive comparisons of various modulation and coding techniques for 200 Gb/s intensity modulation and direct detection links including four-level pulse amplitude modulation (PAM-4), PAM-6, trellis-coded modulation (TCM) over PAM and discrete multi-tone (DMT) transmission. Both C-band Mach-Zehnder modulator and O-band electro-absorption modulated laser transmitters were examined for intra-data center applications based on state-of-the-art commercial components.

DMT Transmission in Short-Reach Optical Interconnection Employing a Novel Bit-Class Probabilistic Shaping Scheme

A 60-GBaud intensity modulation and direct detection discrete Fourier transform spread (DFT-S) discrete multi-tone (DMT) transmission system employing low-density parity check- (LDPC-) encoded probabilistic shaping 16-ary quadrature amplitude modulation (PS-16QAM) is proposed and experimentally demonstrated. In contrast to classical probability-oriented PS schemes such as arithmetic distribution matching (ADM) and constant composition distribution matching (CCDM), the proposed one is performed based on a novel bit-weighted distribution matching (BWDM) algorithm having the advantage of simpler operations along with lower computation and hardware complexity, which merely relies on bit-class processing. As the key content of the BWDM, the bit weight intervention operation is utilized to enlarge the probability of bit `0' in the first two out of four serial binary sequences before PS-16QAM mapping. Different from the labels indicating probability distribution in ADM and CCDM, the end product of the BWDM is a group of bits mapped from input data source, which can be seamlessly compatible with the following LDPC-based forward error correction. In our experiments, system performance is respectively investigated with three LDPC code rates (3/4, 5/6, and 9/10) as well as three PS parameter values (k = 4, 5, and 7), which leads to various symbol probability distribution. Compared with standard uniformly-distributed 16QAM, up to 0.407-dB shaping gain can be achieved to approach Shannon limit in the low signal-to-noise ratio region. Moreover, the experimental results show that appreciable receiver power sensitivity improvement can be obtained by the PS-16QAM DMT based on BWDM over 2-km standard single-mode fiber for short-reach optical interconnection. Additionally, the reference-based pre-equalization is equipped to compensate for the high frequency loss by ultra-wide signal bandwidth.

Comparison of Different Precoding Techniques for Unbalanced Impairments Compensation in Short-Reach DMT Transmission Systems

Channel independent precoding technique has been widely used in optical discrete multi-tone (DMT) transmission systems to compensate unbalanced impairments induced by bandwidth limitations and imperfect frequency responses of electrical/optical devices and various interferences. However, the comparison of different precoding techniques in terms of peak-to-average power ratio (PAPR) reduction, nonlinear distortion tolerance, implementation complexity, and bit error rate (BER) improvements has not been fully studied. In this article, we comparatively investigate seven most commonly used precoding techniques, i.e., discrete Fourier transform (DFT), orthogonal circulant matrix transform (OCT), constant amplitude zero autocorrelation sequence-based matrix transform (CAZACT), Zadoff-Chu matrix transform (ZCT), discrete cosine transform (DCT), discrete Hartley transform (DHT), and Walsh-Hadamard transform (WHT), through both numerical simulations and offline experiments. Simulations show that the ZCT can achieve the best PAPR reduction, and the OCT cannot reduce the PAPR. Besides, DFT, CAZACT, ZCT, DCT, and DHT precoded DMT signals have superior error vector magnitude performance after passing through nonlinear models. And the corresponding precoded QPSK-DMT signals have better BER performance than both OCT/WHT precoded and conventional ones in the distortion-limited scenarios. However, the precoded 16/64QAM-DMT signals, excluding OCT precoded one, are more sensitive to nonlinear distortions and provide minor BER improvement or even may degrade the BER performance. Complexity analysis exhibits the WHT precoding does not require multiplications and therefore has the lowest implementation complexity. In the inter-symbol interference-limited case, OCT precoding can still achieve a good signal-to-noise ratio (SNR) balance and provide the best BER performance. A simple timing synchronization point adjustment (TSPA) method is employed to enhance SNR balance. Simulated and experimental results exhibit that similar BER performance can be achieved by using these precoding techniques together with TSPA in noise-limited scenarios. Considering the implementation complexity, WHT precoding may be a good option to compensate unbalanced impairments in the short-reach DMT transmission system.

Compensation of DAC clock leakage in short-reach DMT system in the presence of sampling frequency offset

Digital-to-analog converter (DAC) with a high sampling-rate has been extensively applied in optical discrete multi-tone (DMT) system for high data-rate signal generation. However, there exist various imperfections during the manufacturing of DAC when its sampling rate is tens of giga-samples per second and beyond. Clock tone leakage (CTL) is a kind of interference in high-speed DACs. One or more data subcarriers of the generated DMT signal may suffer from additional clock tones, which may degrade the overall bit error rate (BER) performance of the optical DMT system. In the literature, several schemes have been proposed to compensate CTL for short-reach applications. Nevertheless, these compensation schemes may not work effectively in the asynchronous DMT system in the presence of sampling clock frequency offset (SFO) between DAC and analog-to-digital converter. In this paper, an effective CTL compensation (CLC) with the digital interpolation scheme is proposed and experimentally investigated in a short-reach 100G DMT transmission system with direct detection. The results indicate that the proposed CLC with a fourth-order interpolation (FOI) scheme can effectively compensate the CTL in the presence of an SFO up to ±1000 parts per million (ppm). What is more, we discuss the impact of SFO estimation error on the BER performance of the 64-ary quadrature amplitude modulation-encoded DMT system. It shows that the BER performance penalty can be ignored by shortening the DMT frame even though the estimation error is up to 2 ppm.

Frequency-domain averaging-based hybrid channel estimation for short-reach discrete multitone transmission systems

Channel estimation is a key digital signal processing algorithm in optical discrete multitone (DMT) transmission systems. The least-square (LS)-based channel estimation is a simple method, but it is very sensitive to various noises and other interference. The estimation accuracy can be improved using the LS-based intrasymbol frequency-domain averaging (ISFA) channel estimation method. However, there exists inaccurate channel estimation for the edge data subcarriers caused by chromatic dispersion and nonsymmetric averaging windows. We propose a hybrid channel estimation method with LS-based intersymbol frequency-domain averaging and modified ISFA to improve the estimation accuracy of edge data subcarriers and the stability of the bit error rate (BER) performance. The proposed hybrid channel estimation method is experimentally investigated in a short-reach 16-ary quadrature amplitude modulation-encoded DMT transmission system. After 20-km standard single-mode fiber transmission, with the help of the proposed hybrid channel estimation and at a BER of 3.8 × 10 − 3, the receiver sensitivity in terms of the received optical power can be improved by about 1.8 dB compared to that of the conventional ISFA. Furthermore, the proposed hybrid channel estimation has better BER stability.

Multi-Band Direct-Detection Transmission Over an Ultrawide Bandwidth Hollow-Core NANF

In this article, we report high-speed multi-band direct-detection (DD) transmission over a hollow-core nested antiresonant nodeless fiber (NANF). Thanks to the ultrawide bandwidth of the NANF, we demonstrate dual-band transmission across the O- and C-bands over a ∼1-km length of a hollow-core fiber for the first time. Eight wavelength-division multiplexed (WDM) channels were transmitted using 100-Gb/s/λ Nyquist 4-ary pulse-amplitude modulation (PAM4) signals, which to the best of our knowledge, is the highest aggregate capacity ever transmitted in a DD hollow-core fiber-based transmission system. Optical pre-amplification was adopted for signal reception in both bands, achieved using an in-house built bismuth-doped optical fibre amplifier (BDFA) and a commercial erbium-doped fiber amplifier (EDFA) in the O- and C-band, respectively. We further demonstrate beyond 100-Gb/s/λ adaptively-loaded discrete multitone (DMT) transmission over the S+C+L-bands using the same NANF, without the use of optical amplification. Our experiments show that apart from fiber loss, the use of the NANF did not introduce any additional transmission penalties. The demonstrated results validate the ultrawide bandwidth and excellent modal purity of the fabricated NANF, which allow beyond 100-Gb/s/λ penalty-free transmission over multiple bands, highlighting the potential of this fiber technology for high-speed short- to intermediate-reach applications.

MIMO Time Domain Equalizer Design for Long Reach xDSL MIMO Channel Shortening

In discrete multi-tone (DMT) transmission based digital subscriber line (DSL) systems, a cyclic prefix (CP) is added to each symbol before transmission, where the length of the CP is larger than the estimated channel impulse response (CIR) length. This ensures the elimination of inter-symbol interference (ISI) and inter-carrier interference (ICI) between the carriers of the same symbol, and allows for single tap frequency domain equalizers and crosstalk cancellation at the receiver. Recently, long reach xDSL (LR-xDSL) has been proposed to extend the reach of conventional DSL systems. With the extended loop lengths, the required CP length increases, in order to match the larger CIR length. The longer CP adds a large overhead and results in overall throughput loss. A more efficient way to deal with extended loop lengths is to use a channel shortening filter - commonly referred as a time domain equalizer (TEQ), to reduce the length of the CIR to the length of CP. This paper focuses on minimum mean square error (MMSE) based multiple input multiple output (MIMO) TEQ design for LR-xDSL MIMO channel shortening. Constraints are applied to the minimization problem to eliminate the trivial solution. This paper proposes two new constraints for the MMSE based MIMO TEQ design for upstream scenarios, which result in a lower complexity and provide better (or similar) performance compared to existing MMSE based MIMO TEQ design methods. Furthermore, a diagonal MIMO TEQ with lower memory requirement and lower computational complexity is presented based on the proposed constraints, which can be applied in upstream as well as downstream scenarios.

Experimental comparison of discrete Fourier transform-spread high-order quadrature amplitude modulation discrete multitone systems for optical interconnection

The performance of discrete Fourier transform-spread (DFT-spread) discrete multitone (DMT) transmission systems based on four high-order quadrature amplitude modulation (QAM) formats (4/16/32/64QAM) for optical interconnection is investigated and experimentally compared on the same platform. We first theoretically study and analyze the benefits of the implemented DFT spread for the peak-to-average power ratio (PAPR) deduction in DMT transmission systems, and then experimentally compare the receiver optical power sensitivities and transmission performance using flexible transceiver configuration in standard single-mode fiber (SSMF) link. Experimental results show that the power penalties are 1.2, 1.8, 2.2, and 2.7 dB for 10 Gbaud DFT-spread DMT signal employing 4/16/32/64QAM over 10-km SSMF transmission. There are, respectively, 0.3, 0.7, 0.9, and 1.1 dB transmission power penalty improvements by the implementation of the DFT-spread scheme, compared with the conventional DMT transmission systems. Furthermore, we experimentally compare the improved fiber nonlinear effect tolerance for the proposed DFT-spread DMT system, which shows the potential reach improvement and thus can provide an abundant system loss budget for leveraging legacy optical access networks. In addition, a comparison of the hardware implementation complexity of the DFT-spread scheme with several typical PAPR deduction schemes is also presented.

$\alpha$ -Fair Dynamic Spectrum Management for QRD-Based Precoding With User Encoding Ordering in Downstream G.Fast Transmission

In next-generation digital subscriber line networks such as G.fast, employing discrete multi-tone transmission in high frequencies up to 212 MHz, the crosstalk among lines reaches very high levels. To precompensate the crosstalk in downstream transmission, QRD-based precoding has been proposed as a near-optimal dynamic spectrum management (DSM) technique. However, the performance of QRD-based precoding is greatly affected by the user encoding ordering (UEO). Since current multi-tone UEO methods are rather heuristic in the way they approach fairness, we develop, in this paper, a set of novel DSM algorithms for joint power allocation and UEO that enforce a generalized $ \alpha $ -fairness policy. Since finding the globally optimal UEO entails a combinatorial optimization problem with excessive computational complexity, an iterative algorithm is proposed which uses per-tone exhaustive searches (PTESs) and provides near-optimal approximate solutions. To further reduce the computational complexity, two suboptimal methods are suggested to replace the expensive PTESs, leading to two additional $ \alpha $ -fair DSM algorithms that are tractable for large scenarios against little performance loss. Simulations of a G.fast cable binder show that the $ \alpha $ -fair DSM algorithms achieve an efficient trade-off between fairness and performance in contrast to current UEO methods.

Discrete multitone transmission for underwater optical wireless communication system using probabilistic constellation shaping to approach channel capacity limit.

Probabilistic constellation shaping (PCS) is utilized to approach the channel capacity limit in discrete multitone (DMT) transmission for underwater optical wireless communication (UOWC) system. A fixed quadrature amplitude modulation (QAM) format with various probabilistic distributions is individually allocated for different subcarriers to obtain achievable maximum channel capacity in accordance with the pre-estimated signal-to-noise ratio. By using a 450-nm directly modulated laser diode (LD) with an available modulation bandwidth of ∼2.75 GHz, DMT with PCS technique is experimentally realized with a net data rate of 18.09Gbit/s over 5m, 17.21Gbit/s over 25m, and 12.62Gbit/s over 35m underwater transmission, giving substantial capacity improvement of 32.22%, 30.03%, and 27.55%, respectively, in comparison with the widely used regular QAM formats in DMT with bit-power loading scheme. The figure of merit of the UOWC system in terms of entropy, generalized mutual information (GMI), and normalized GMI are also presented. To the best of our knowledge, this is the first time to employ PCS-QAM-DMT in a UOWC system, and it is also the highest data rate ever reported for a single LD in UOWC.

Non-orthogonal Discrete Multi-tone With Low Decoding Complexity Utilizing Symmetric Compression in Time–Frequency Space

Utilizing symmetric compression in time–frequency space, we can eliminate the crosstalk between real and imaginary components, and thus simplify the decoding complexity significantly in a non-orthogonal discrete multi-tone transmission system. The experiments are also demonstrated for an intensity-modulated direct detection system with the decoding complexity decreased to 1/4 for 13.8 Gbit/s QPSK and 1/8 for 27.6 Gbit/s 16-QAM with 1310 nm transmission over 40 km SMF.

Unique Word DMT Schemes Using Real-Valued Constellations for IM/DD Optical Systems

Discrete multitone transmission (DMT) schemes employing real-valued constellations such as PAM (PAM-DMT) are popular multi-carrier transmission schemes for optical systems using intensity modulation and direct detection. PAM-DMT employs a data dependent cyclic prefix (CP) to maintain cyclicity. In this letter, we propose a novel DMT scheme using real-valued modulation formats where the usual CPs are replaced by a unique word (UW) that is a known deterministic sequence. This is achieved by designing a code generator matrix in the frequency domain that introduces correlation among the data symbols. The proposed PAM UW-DMT scheme shows a superior bit error ratio performance as compared with PAM CP-DMT. Moreover, the UW can be used for other purposes, such as synchronization and channel estimation.