Discrete Multitone Signalling for Wireline Communication

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 resourceor 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 fsrms of jitter, 1.26 mVrms 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 × 10^-4 over a physical channel with severe notches that is very challenging for 4-PAM.

Interpolated decision feedback equalization for long intersymbol interference channels

In orthogonal frequency division multiplexing (OFDM), Fourier Transform (FT) is used as a modulation technique. To avoid inter-symbol interference (ISI) in OFDM by a time dispersive channel, a cyclic prefix (CP) is used which also reduces its spectral efficiency. wavelet packet transform (WPT) as a modulation scheme has been proposed for wireless communication. In this paper, WPT based OFDM is presented and proposed using minimum mean square error (MMSE) equalizer. Equalization is a major factor in wavelets. MMSE is proposed for wavelet based OFDM (WOFDM) based on WPT and compared with zero forcing equalizer (ZFE). The performance is measured for arbitrary magnitude finite impulse response (FIR) channels in the presence of additive white Gaussian noise (AWGN) and impulse noise. Simulation results show a clear improvement in bit error rate (BER) for MMSE over ZFE equalization technique in the presence of noise.

As four‐level pulse‐amplitude‐modulation (PAM‐4) signal becomes widely adopted for high‐speed wireline communication, robust equalization is crucial due to reduced eye‐opening and increased sensitivity to channel loss, compared to PAM‐2. Utilizing analog‐to‐digital converters and digital signal processing with a feed‐forward equalizer (FFE) and a decision‐feedback equalizer (DFE), conventional equalizer adaptation may result in suboptimal bit‐error‐rate (BER) performance. This letter introduces an improved training algorithm to obtain the optimal equalizer configuration (i.e., main tap position in FFE and the DFE tap coefficient). The experimental results demonstrate superior BER performance of the proposed algorithm compared to the conventional approaches.

Rapid internet traffic growth has fueled the demand for bandwidth in metro networks and data centers and pushed the serial link data rate into 25Gb/s territory, populated by such electrical interface as OIF CEI-25G, CEI-28G [1], IEEE 802.3bj 100G-KR4. To cope with severe channel impairments at 25Gb/s with up to 30dB loss at Nyquist, a feed-forward equalizer (FFE)/decision feedback equalizer (DFE) based transceiver without power-hungry analog-to-digital converter (ADC) provides robust performance. This work presents a low-power and area-efficient transceiver that employs a 14-tap adaptive DFE at the receiver (RX) and a 5-tap FFE at the transmitter (TX) for multi-standard applications up to 28Gb/s in 28nm CMOS.

The paper derives a minimum bit error rate (BER) solution for the decision feedback equaliser (DFE) that employs a linear combination of the channel observations and the past decisions. We show that by using a geometric translation the DFE is reduced to a simpler linear equaliser. A BER expression for the linear equaliser is obtained under the assumption of linearly separable decision regions, and a method is proposed to optimally set the linear-combiner coefficients of the DFE. This minimum BER solution is superior to the usual minimum mean square error (MSE) solution.

In a companion paper, we have studied the zero-forcing (ZF) transceiver with decision feedback equalizer (DFE) over slowly time-varying narrowband multiinput multioutput (MIMO) channels. The space-time generalized triangular decomposition (ST-GTD) was used for the design of ZF-DFE transceivers. The space-time geometric mean decomposition (ST-GMD) ZF transceiver minimizes both the arithmetic mean square error (MSE) at the feedback detector and the average uncoded bit error rate (BER) in moderate high signal-to-noise ratio (SNR). This paper addresses the design problem of DFE transceiver without zero-forcing constraint. In the first part, a channel independent temporal precoder is superimposed on the conventional block-wise GMD-based minimum mean square error (MMSE) DFE transceiver to take advantage of the temporal diversity. In the second part, ST-GTD is applied for the design of MMSE DFE transceivers. With accurate channel prediction and space-time powerloading, the proposed ST-GMD MMSE transceiver minimizes the arithmetic MSE at the feedack detector, and maximizes Gaussian mutual information. For practical applications, the ST-GTD MMSE transceiver which does not require channel prediction but shares the same asymptotic BER performance with the ST-GMD MMSE system is also developed. In the convex region, our analysis shows that the proposed MMSE transceivers has better BER performance than the conventional GMD-based MMSE transceiver; the average BERs of the proposed systems are nonincreasing functions of the ST-block size. The superior performance of ST-GMD MMSE transceiver over the ST-GMD ZF transceiver is also verified analytically.

Skin effect and dielectric loss in super-long cable will cause nonlinear attenuation at different signal frequency, and in addition, coupling noise and thermal noise also cause signal distortion at the receiver end. These factors seriously affect the signal transmission speed in the super-long cable. Especially, in the field of exploration of shale gas and bed methane, the transmission cable is also used to transport high-precision synchronization pulse signal, and the synchronization pulse must reach the microsecond accuracy, which is used for data phase calibration. A synchronization signal is a high frequency signal, which suffers more severe attenuation and noise interference. At the receiving end, the sync pulse signal will be drowned in the noise environment, and so it is difficult to restore the original signal.#br#Although fiber can achieve a high transfer rate, but the fiber cable cannot transmit power energy; in addition, the tensile strength and heat resistance of the fiber are much worse than copper cable, these weaknesses limit its application in such industry. Therefore, an effective balancing algorithm is necessary to overcome the propagation effects and interference in a super-long copper cable. However, conventional equalization techniques have well-balanced effect for the short-range communications, but for the long-distance communication, they often have poorly balanced results. In order to solve the above problem and improve the long cable signal transmission speed, this paper presents a new balanced portfolio structure; the new structure uses feed-forward equalizer (FFE) as the pre-stage, and decision-feedback equalizer (DFE) as the post stage to form a new structure. The combination structures can effectively utilize the flexibility of FFE and overcome the problem of error diffusion in DFE. By mathematical modeling and simulation, this paper gives the best combination factors. Furthermore, based on the improved structure, a new convergence algorithm is proposed, which uses the arc tangent function combined with three error converge factors to form a converging function, and it has the advantages of fast convergence and steady-state error. Simulation results show that the FFE-DFE combination equalizer has low computational complexity, fast convergence, and strong channel tracking capability; in addition, it can speed up the data processing speed, and better respond to the real variation of the channel. Simulation results show also that the performance is improved by 50% by eliminating inter-symbol interference and noise.#br#The real circuit board based on the new algorithm have been tested in the East China Petroleum Bureau, the test results show that the algorithm can rectify 160 dB signal distortion, and the transmission speed can reach 5 Mbps in 6 dB signal to noise ratio.

It is shown that depending on the channel and the operating signal to noise ratio, the onset of error propagation in a decision feedback equaliser (DFE) is a detectable event. With the exploitation of this event using error correcting schemes the run length distribution of the feedback errors can be altered so as to improve the average bit error rate (BER). A novel fractionally spaced DFE using feedback error detection is presented which offers BER improvement over the conventional fractionally spaced DFE with only a small increase in complexity. A number of error correcting schemes are explored, the simplest of which has the lowest BER.

Addresses problems related to the conceptual design of a fully digital synchronous DFE (decision feedback equalizer) and investigates the effects of the necessary quantization of the input signals and coefficients on DFE performance in static and dynamic operation. The analysis is performed taking into account the following topics: the influence of the dynamic range and quantization levels of the input signal and coefficients: the tracking and acquisition algorithm capabilities; comparison of DFE structures; and the extension of robust algorithms to DFEs. An experimental setup including a digital baseband equalizer is described. Results are reported for two kinds of DFE structures and a 16-AQM (quadrature amplitude modulation) modulation format. Specific problems related to the operation of DFEs in digital radio modems are discussed. It is established that the DFE structure is able to track continuous minimum-phase to nonminimum-phase transitions and vice versa without getting out of lock. This is an important result since the DFE outperforms current linear equalizers. >

In recent years, there has been an increasing interest in the use of the discrete Hartley transform (DHT) for orthogonal frequency-division multiplexing (OFDM). Previous studies have not reported an improvement in bit error rate (BER) when using a DHT-based OFDM without substantial complexity increase due to the fact that the DHT cannot diagonalize a circulant channel matrix but rather produces a unique structured matrix that has only a main diagonal and anti-diagonal. In this paper, the coupling between symmetric carriers is exploited by introducing a frequency-domain subcarrier diversity receiver for DHT-based OFDM systems. The proposed simple receiver structure takes advantage of the coupling to enhance the BER performance of the DHT-OFDM system by increasing the diversity gain between subcarriers. A detailed statistical and performance analysis of the system is presented, in which closed-form expressions of the average BER were derived. The proposed system performance was compared to the conventional discrete Fourier transform OFDM (DFT-OFDM), the generalized DHT-OFDM, and the DHT-precoded OFDM systems in terms of average BER and peak-to-average power ratio (PAPR). The proposed DHT-OFDM system outperforms the generalized DHT-OFDM system by 12 dB and outperforms the conventional DFT-OFDM and the DHT-precoded OFDM systems by 17 dB at an average BER of 10−5. In terms of PAPR, the proposed DHT-OFDM system suffers from an approximately 5.5, 2, and 1 dB increase in PAPR when compared to the DHT-precoded OFDM, the conventional DFT-OFDM, and the generalized DHT-OFDM systems, respectively.

The increasing demand for higher network data rates by new businesses and entertainment has never been fulfilled. Mixed-signal PAM - 4 transceivers prevail over their ADC - DSP counterparts in energy efficiency and chip area, but they have difficulties operating over high - loss links. Typically, a continuous-time linear equalizer (CTLE) and a multi-tap decision-feedback equalizer (DFE) are implemented in a mixed-signal receiver (RX). However, when the data rate reaches 112Gb/s, the implementation of the DFE suffers from stringent feedback timing. Direct DFE works only at 100Gb/s in an optical receiver [1], leaving no room for feedforward error correction (FEC). A speculative 1 - tap DFE is implemented in [2], but it requires an 8-tap feedforward equalizer (FFE) at the transmitter (TX) to generate a 1+0.5D response; this may be impractical without knowing the characteristics of the entire channel. Another drawback of a speculative DFE is the large 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> -tap latency, which brings about challenges in realizing two or more taps. In addition, the DFE does not compensate for pre-cursor inter-symbol interference (ISI), which becomes significant for channels with higher loss. Without a DFE, the CTLE only covers a small loss of up to 10dB [3,4]. This paper presents a 112Gb/s mixed-signal transceiver using an RX analog FFE with adaptive pre- and post-cursor ISI equalization in 28nm CMOS, compensating for 20.8dB loss at a power efficiency of 2.29pJ/b.

Recently, researches on Orthogonal Frequency Division Multiplexing (OFDM) based Free Space Optics (FSO) communication system gain interest since it combines the advantages of both techniques by mitigating the multipath propagation effects to improve the transmission data rate. The performance of such systems depends on base band modulation used, number of subcarriers, turbulence modeling, nature of laser beam and so on. In this paper, a performance comparison is carried out considering three different levels (weak, medium and strong) of atmospheric turbulence with log-normal and Gamma- Gamma scintillation model for plane, spherical and Gaussian beams. The Gamma-Gamma distribution analysis shows that for all kinds of laser beams, Bit Error Rate (BER) is high for weak turbulence which decreases further for medium and strong. The only difference is that the BER variation for Gamma-Gamma distribution with Gaussian beam is more prominent than for plane beam. The decrease in BER is clearly observed at higher SNR value. This kind of improvement in BER is observed with log-normal PDF at low SNR with weak turbulence. Thus we can say that weak atmospheric turbulence can be modeled by log-normal PDF and medium to strong condition can be modeled by Gamma-Gamma PDF.

The author proposed soft decision feedback equalisation for orthogonal frequency division multiplexing (OFDM) transmission over doubly selective channels which is used for turbo equalisation. The doubly selective channel is approximated using the basis expansion model (BEM). The decision feedback equaliser (DFE) is obtained and updated for every sub-carrier which results in a per-tone DFE. On the basis of the Gaussian approximation, expressions for the a priori, the a posteriori and the extrinsic information have been derived for quadrature phase shift keying (QPSK) modulation. For turbo equalisation, where iterative joint channel equalisation and channel decoding is executed, in the first iteration, estimates are based on the feed-forward filter which are fed to the channel decoder. These estimates are not reliable and may incur error propagation and poor turbo equalisation performance. For more reliable estimates in the first iteration, the DFE is initialised as a linear minimum mean square error (LMMSE) equaliser. The per-tone LMMSE soft-output equaliser is also derived, and expressions for the a priori, a posteriori and extrinsic information are obtained viewing the LMMSE equaliser as a special case of the DFE. The extrinsic information in both cases was derived based on the Gaussian approximation. Numerical results for bit-error rate performance assuming the BEM channel model and Jakes' channel model are shown to confirm our findings.

Optical communication networks (OCNs) provide promising and cost-effective support for the ultra-fast broadband solutions, thus enabling them to address the ever growing demands of telecommunication industry such as high capacity and end users’ data rate. OCNs are used in both wired and wireless access networks as they offer many advantages over conventional copper wire transmission such as low power consumption, low cost, ultra-high bandwidth, and high transmission rates. Channel effects caused by light propagation through the fiber limits the performance, hence the data rate of the overall transmission. To achieve the maximum performance gain in terms of transmission rate through the OCN, an optical downlink system is investigated in this paper using feed forward equalizer (FFE) along with decision feedback equalizer (DFE). The simulation results show that the proposed technique plays a key role in dispersion mitigation in multi-channel optical transmission to uphold multi-Gb/s transmission. Moreover, bit error rate (BER) and quality factor (Q-factor) below 10 − 5 and above 5, respectively, are achieved with electrical domain equalizers for the OCN in the presence of multiple distortion effects showing the effectiveness of the proposed adaptive equalization techniques.