Discrete Multitone Transceivers Research Articles

High Speed DMT for 224 Gb/s and Faster Wireline Transmission

This paper presents a high data rate wireline Discrete Multitone (DMT) transceiver capable of operating at 252 Gbps and higher for short range chip-to-chip and chip-to-module applications. The use of DMT allows for higher spectral efficiency than PAM SERDES systems, using similar bandwidth as proposed 224 Gb/s PAM-4 systems but achieves higher data rates. Data rates have improved over other published wireline DMT designs with the use of narrow sub-channels with both adaptive bit- and power-loading along with a combined transmit and receive side equalization. Simulation and measured results show that the transmit side pre-compensation has improved the BER by a factor of 104 at 250 Gb/s. The system is measured over a channel with a loss of 18 dB at 50 GHz, showing a data rate of 252 Gb/s with a BER of 10-6, and a data rate of 400 Gb/s with a BER of 10-2.

A Study of Discrete Multitone Modulation for Wireline Links Beyond 100 Gb/s

To overcome the severe losses beyond 28 GHz in low-cost electrical channels, 4-level pulse-amplitude modulation (4-PAM) wireline links targeting 112 Gb/s incorporate resource- or power-intensive equalization schemes such as decision-feedback equalizers (DFE) with many taps. Alleviating the timing constraints that cause DFEs to balloon in size and power, discrete multitone (DMT) modulation involves independent sub-channels that can be equalized in the frequency domain without feedback. DMT allows flexibility in assigning bits to each sub-channel, thereby potentially avoiding lossy parts of the frequency spectrum. This article presents behavioural modeling results and an experimental setup used to study DMT transceivers. Our simulations show 200 Gb/s operation at a bit error rate (BER) of less than ${10^{-5}}$ over an IEEE P802.3ck channel with 21 dB of loss at 50 GHz and assuming $150~\mathrm{fs_{rms}}$ of jitter, $1.26~{\mathrm{ mV}}_{\mathrm{ rms}}$ of noise at the receiver’s input, and 7-bit 80 GS/s data converters. An updated bit-loading algorithm led to BER values 1–2 orders of magnitude below our previous results. The estimated power and area of the associated digital signal processing are comparable to those of DFE. We also present an experimental DMT setup achieving 61.6 Gb/s at a BER of ${8.6 \times 10^{-4}}$ over a physical channel with severe notches that is very challenging for 4-PAM.

Demonstration of a 2.34 Gbit/s Real-Time Single Silicon-Substrate Blue LED-Based Underwater VLC System

We develop a real-time discrete multi-tone (DMT) transceiver based on field programmable gate array (FPGA) chips for a single chip silicon-substrate light-emitting diode (LED) based underwater visible light communication (UVLC). On-chip resource usages are analyzed and discussed. To improve bit error rate (BER) performance, a novel channel estimation technique utilizing hybrid inter-symbol frequency-averaging (Inter-SFA) and intra-symbol frequency-averaging (Intra-SFA) is proposed and investigated. The real-time DMT transceiver is experimentally verified in a silicon substrate blue LED-based UVLC system with a 1.2 m underwater link. By using the enhanced channel estimation, a gross bit rate of 2.34 Gbit/s real-time DMT signal over 1.2 m underwater transmission can be achieved with the BER of 3.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . What's more, multiple-symbol interleaved Reed-Solomon (RS) codes are employed to further improve BER performance. The real-time measured post-FEC BER of the DMT-UVLC with multiple-symbol interleaved RS (255, 191) codes can be improved by more than six orders of magnitude. As a result, error-free (less than 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> ) transmission is observed in our real-time experiment. Furthermore, 1.485 Gbit/s 720p high-definition video underwater transmission is successfully demonstrated. To the best of our knowledge, it is the first time to demonstrate a real-time LED-based UVLC system with DMT modulation beyond Gbit/s.

DWMT transceiver equalization using overlap FDE for downlink ADSL

Discrete wavelet multitone (DWMT) modulation is a wavelet transform based technique implemented using perfect reconstruction filter banks. It has been recently proposed for various wireline channels such as digital subscriber loops (DSLs) as a solution to the problems posed by a discrete multitone (DMT) transceiver including interblock interference (IBI) and lower spectral efficiency due to the employment of a cyclic prefix (CP) in the guard interval (GI) for DMT symbols. The greater side lobe attenuation offered by wavelet filter banks results in improved spectral containment and lower IBI in DWMT transceivers. However, no standard equalization technique exists for a DWMT based transceiver so as to remove the effect of channel on the transmitted signal in DWMT systems. This paper proposes the application of overlap frequency domain equalization (OFDE) in DWMT modulated systems and compares the bit error rate (BER) performance with time domain equalization (TDE) technique. It is shown through simulation results that minimum mean square error (MMSE) based OFDE can be applied as an equalization technique for a downlink asymmetric DSL (ADSL) channel with lower computational complexity and BER performance comparable to that of TDE.

Time-domain equalisation for discrete multi-tone transceivers: new results and performance analysis

The time-domain equaliser (TEQ) is a commonly used device to shorten the channel impulse response in a discrete multitone (DMT) receiver. Many methods have been proposed to design the TEQ with a capacity maximisation criterion. An implicit assumption used by existing methods is that circular convolution can be conducted for the noise signal and the TEQ. This assumption is not valid because the noise vector, observed in a DMT symbol, does not have a cyclic prefix. A similar assumption is also made for the residual inter-symbol interference (ISI) signal. Because of these invalid assumptions, the TEQ-filtered noise and residual ISI powers in each subcarrier were not properly evaluated. As a result, optimum solutions derived are not actually optimal. This paper attempts to resolve this problem. We first analyse the statistical properties of the TEQ-filtered noise signal and the residual ISI signal in detail, and derive precise formulae for the calculation of the TEQ-filtered noise and residual ISI powers. Then, we re-formulate the capacity maximisation criterion to design the true optimum TEQ. Simulations show that the proposed method outperforms the existing ones, and its performance closely approaches the theoretical upper bound.

A Novel Uniform Discrete Multitone Transceiver with Power-Allocation for Digital Subscriber Line

A novel Uniform Discrete Multitone (DMT) transceiver is proposed, utilizing a wavelet packet based filter bank transmultiplexer in conjunction with a DMT transceiver. The proposed transceiver decomposes the channel spectrum into subbands of equal bandwidth. The objective is to minimize the bit error rate (BER), which is increased by channel-noise amplification. This noise amplification is due to the Zero-Forcing equalization (ZFE) technique. Quantization of the channel-noise amplification is presented, based on post-equalization signal-to-noise ratio (SNR) and probability of error in all subbands of the Uniform DMT system. A modified power loading algorithm is applied to allocate variable power according to subband gains. A BER performance comparison of the Uniform DMT with variable and uniform power-loading and with a conventional DMT system in a Digital Subscriber Line (DSL) channel is presented.

Biwindowed Discrete Multitone Transceiver Design

A family of biwindowed discrete multitone (DMT) transceivers that provide both subchannel spectral containment at the transmitter and spectral selectivity at the receiver is proposed. These systems have the attractive feature that they provide spectral shaping at both ends of the transceiver without requiring the cyclic prefix to be longer than the order of the channel impulse response. The windows are designed in a channel independent manner and are constrained to produce subchannel outputs that are free from (intrasubchannel) intersymbol interference (ISI). Furthermore, the design allows the intersubchannel interference (ICI) to be controlled in such a way that it can be mitigated using a relatively simple minimum mean-square error (MMSE) successive interference cancellation scheme. Under a realistic model for a digital subscriber line (DSL) environment, the achievable bit rate of the proposed system is significantly higher than that of the conventional DMT system and some established windowed DMT systems with receiver-only windowing. This performance gain is a result of the capability of the proposed system to combat the near-end crosstalk (NEXT) at the transmitter and receiver and to mitigate the narrowband noise at the receiver, without the requirement of excess cyclic prefix.

Impulse Noise Mitigation Based on Computational Intelligence for Improved Bit Rate in PLC-DMT

This paper introduces a modified version of the discrete multi-tone transceiver (DMT) to increase the data rate in broadband power-line communications (PLC). Basically, a computational-intelligence technique trained by a second-order optimization method is applied to mitigate the high-power impulse noise, while the DMT copes with the severe intersymbol interference observed in power-line channels. The masking of the impulse noise enhances the signal-to-noise ratio in each DMT subchannel. As a result, a high number of bits can be allotted to each subchannel for a given error probability. The simulation results reveal that the proposed PLC-DMT solution outperforms, in terms of the data rate, traditional PLC-DMT over different environments, especially in the presence of additive high-power impulse noise.

Zero-Forcing Frequency-Domain Equalization for Generalized DMT Transceivers with Insufficient Guard Interval

We propose a zero-forcing frequency domain block equalizer for discrete multitone (DMT) systems with a guard interval of insufficient length. In addition to the insufficient guard interval in the time domain, the equalizer takes advantage of frequency domain redundancy in the form of subcarriers that do not transmit any data. After deriving sufficient conditions for zero-forcing equalization, that is, complete removal of intersymbol and intercarrier interference, we calculate the noise enhancement of the equalizer by evaluating the signal-to-noise ratio (SNR) for each subcarrier. The SNRs are used by an adaptive loading algorithm. It decides how many bits are assigned to each subcarrier in order to achieve a maximum data rate at a fixed error probability. We show that redundancy in the time domain can be traded off for redundancy in the frequency domain resulting in a transceiver with a lower system latency time. The derived equalizer matrix is sparse, thus resulting in a low computational complexity.

Minimum Mean-Squared Error Impulse Response Shortening for Discrete Multitone Transceivers

We reformulate the minimum mean squared error (MMSE) solution for time domain equalization (TEQ) in multitone transceivers. We show that the maximum shortening signal-to-noise ratio (MSSNR) and minimum intersymbol interference (min-ISI) TEQs are particular cases of our new formulation. With good channel estimation, the MMSE-TEQ achieves near-optimal rate performance. Our algorithm is less sensitive to channel estimation errors than other methods.

A frequency-domain training approach for equalization and noise suppression in discrete multitone systems

In discrete multitone (DMT) transceivers, a time-domain equalizer (TEQ) is used to shorten the effective channel impulse response such that a shorter cyclic prefix can be used. In this paper, we pose the TEQ design problem completely in the frequency domain by defining a frequency-domain weighted least-squares cost function. The definition of the frequency-domain TEQ design criterion, we show, allows for the introduction of a weighting function to control the spectral shape of the TEQ, and facilitates important extensions particularly useful for the DMT system. The TEQ can be used to suppress the noise and interference in the DMT system, a feature especially useful in the frequency division multiplexing-based DMT system. Also, in the echo-cancellation-based DMT system, the TEQ can be used to jointly shorten the echo response thus reducing the complexity of the echo canceler. We present corresponding algorithms for these two objectives and simulation results, which show that the weighted frequency-domain least-squares algorithm and the two extensions we derive can achieve the shortening (channel or channel and echo) as well as noise suppression objectives effectively.

A discrete multitone transceiver at the heart of the PHY layer of an in-home power line communication local area network

A networked home is a micro-version of the Internet, connecting home appliances, PCs, and other smart devices not only to one another but also to a globally connected world through the Internet. For consumers to embrace home networking, a solution that utilizes the existing infrastructure within the home, such as electrical wiring, is significant. However, the use of power lines as a home networking platform presents quite a troublesome and noisy environment that fluctuates with varying load impedance and time, and is also plagued by impulse noise. The latest developments in VLSI and DSP technologies have enabled power line communication (PLC) networks to compensate for the impairments of the environment. This article discusses the use of discrete multitone technology at the PHY layer of an in-home PLC network, particularly with reference to bit-loading techniques.

An enhanced joint equalization algorithm for full-duplex DMT transceivers

An objective function incorporating the factors that affect the system throughput is proposed, and an enhanced joint time-domain equalization algorithm is developed for full-duplex discrete multitone (DMT) transceivers. This algorithm is able to shorten the channel and echo impulse responses using a single filter while keeping the system throughput high.

Smith form of FIR pseudocirculants

The pseudocirculant matrices have been found to be a very useful tool in the analysis and design of communication systems, e.g., precoding systems and discrete multitone transceivers. In these systems, a scalar channel P(z) is recast into a pseudocirculant channel matrix. Many important channel properties have been derived from the Smith form and the decomposition of pseudocirculants. In this letter, we will show that the Smith form of a pseudocirculant matrix can be given in terms of the zeros of the underlying finite-impulse response (FIR) channel P(z). Once the zeros of P(z) are known, the Smith form of the corresponding pseudocirculant matrix can be obtained in closed form.

Joint impulse response shortening for DMT transceivers

A new algorithm is proposed for joint impulse response shortening in full-duplex discrete multitone transceivers. In comparison with the previous approach, the proposed algorithm takes into account both the presence of noise and the flexibility of having different signal/noise power distributions.

Design methods for time-domain equalizers in DMT transceivers

Time-domain equalizers (TEQ) are used in the discrete multitone (DMT) transceivers in order to reduce the duration of the overall response of the transmission system, so that a shorter-length cyclic prefix could be used. The optimum TEQ is the one that results in maximum bit allocation to each block of the DMT. However, the optimum design of TEQs turns out to be a very difficult task. We give the general guidelines that one should follow in the design of TEQ to achieve a good performance. Based on the suggested guidelines, we first propose an eigenapproach design method which results in TEQs with comparable performance to those of a previously reported method, but at a much lower computational cost. Further study of the proposed guidelines reveals that the choice of target-impulse response in the design of TEQ only weakly depends on the channel response. Noting this, we propose a second design method that is even simpler than our first method, but still results in comparable designs to those of our first method and also those obtained from the much more complex methods of the present literature.

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.

Optimal impulse response shortening for discrete multitone transceivers

The authors have derived a new algorithm for the optimal shortening of a channel impulse response in discrete multitone (DMT) transceivers. This algorithm uses eigenvalues and eigenvectors to generate the coefficients of the shortening impulse response filter (SIRF). In comparison with the previous approach, this new algorithm can calculate the optimal settings of an SIRF with arbitrary length.

Mismatched finite-complexity MMSE decision feedback equalizers

Closed-form expressions that quantify the degradation in the decision-point signal-to-noise ratio of the finite-length minimum mean-square-error decision feedback equalizer (MMSE-DFE) due to channel and noise mismatch conditions are derived. The analysis is further extended to other receiver structures, namely, the MMSE-DFE with an adaptive feedforward filter, the MMSE linear equalizer, and the discrete multitone transceiver, all under a finite-complexity constraint. The limiting case of infinite-length filters is also analyzed. In addition, we present computer simulation results that compare the performance of the various receiver structures under study, assuming a particular channel and noise mismatch model for the high-bit-rate digital subscriber loop environment. Finally, several methods that can be used to mitigate the effects of mismatch are outlined.

Wavelet and subband transforms: fundamentals and communication applications

Subband and wavelet transforms have been a subject of great interest, especially in the fields of signal processing and applied mathematics. This article presents a tutorial on this subject, emphasizing the fundamentals and the reason for its success, importance, and potential. Subband and wavelet transform applications in communications are also reviewed. These application areas include interference excision, spread spectrum codes based on subband transform bases, CDMA, and discrete multitone transceivers.