Intersymbol Interference Equalization Research Articles

Channel modeling and achievable data rate for 1-D molecular communication in bounded environments

In this paper, we propose the analytical propagation model for diffusive molecular communication in an one-dimensional bounded environment, and determine the available data rate for a given limit of interference. Molecular communication is a promising solution for applications when the typical electromagnetic communication is not available. However, due to the randomness of the molecule movement, it is challenging to analyze the channel in molecular communication. Taking into account the impact of an absorbing boundary to the one-dimensional vessel-like channel, we model the time-varying distribution of the releasing molecules. Accordingly, we derive the hitting probabilities for an information molecule to separately reach the receiver or the boundary, and present the impulse response of the channel at a given time t. Moreover, under the same environment, we consider the time duration between transmissions of a binary modulation system for a given level of inter-symbol interference. Our analysis shows that, for a certain degree of interference, the available data rate of the system can be expressed as 2Dαth2S2, where S is the distance between the receiver and the boundary, and D is the diffusion coefficient of the channel, and αth2 is the corresponding index to the interference degree. The authenticity of our analysis are verified via simulations.

RNN-Based Demodulation Framework for Spectrum Narrowing in Ultra-Dense WDM Networks

To cope with the continuous traffic growth, spectral efficiency needs to be enhanced. Ultra-dense WDM systems, where narrower guard-bands are inserted between wavelength-division-multiplexed (WDM) signals than those used in conventional dense WDM (DWDM) systems, are expected to offer higher spectral efficiency. However, such systems are hindered by the spectrum narrowing induced by wavelength selective switches (WSSs) at nodes. As a result, intersymbol interference (ISI) can be excessive. Linear digital filters widely used in typical digital coherent receivers can equalize the ISI. However, ISI equalization yields noise enhancement due to the interaction between ISI and amplifier noise. Sequence estimation can alleviate the impact of this interaction; however, to introduce sequence estimation into real systems, an ISI-imposing filter is needed because the adaptive filter used for polarization recovery automatically equalizes the ISI. Although the optimum ISI-imposing filter can be created with the proper spectrum model, its characteristics change frequently in real optical path networks. Therefore, we need to develop a demodulation framework that does not necessitate any spectrum models. In this letter, we propose a novel demodulation framework that uses a recurrent neural network (RNN) to simultaneously realize the ISI-imposing filter and sequence estimator. Extensive computer simulations show that our proposal increases the transmission distance in ultra-dense WDM networks.

The Residual ISI for Which the Convolutional Noise Probability Density Function Associated with the Blind Adaptive Deconvolution Problem Turns Approximately Gaussian.

In a blind adaptive deconvolution problem, the convolutional noise observed at the output of the deconvolution process, in addition to the required source signal, is—according to the literature—assumed to be a Gaussian process when the deconvolution process (the blind adaptive equalizer) is deep in its convergence state. Namely, when the convolutional noise sequence or, equivalently, the residual inter-symbol interference (ISI) is considered small. Up to now, no closed-form approximated expression is given for the residual ISI, where the Gaussian model can be used to describe the convolutional noise probability density function (pdf). In this paper, we use the Maximum Entropy density technique, Lagrange’s Integral method, and quasi-moment truncation technique to obtain an approximated closed-form equation for the residual ISI where the Gaussian model can be used to approximately describe the convolutional noise pdf. We will show, based on this approximated closed-form equation for the residual ISI, that the Gaussian model can be used to approximately describe the convolutional noise pdf just before the equalizer has converged, even at a residual ISI level where the “eye diagram” is still very closed, namely, where the residual ISI can not be considered as small.

A 4.5 Gb/s/pin transceiver with hybrid inter‐symbol interference and far‐end crosstalk equalization for next‐generation high‐bandwidth memory interface

A 4.5 Gb/s/pin transceiver capable of eliminating the inter-symbol interference (ISI) and far-end crosstalk (FEXT) in a hybrid scheme with low power and small area for next-generation high-bandwidth memory (HBM) interfaces is presented. Built around the combination of two ISI and FEXT equalization topologies, the transmitter (TX) energy efficiently reduces data-dependent jitter (DDJ) and crosstalk-induced jitter (CIJ) by using the compensation signal generated from edge detectors (ED) to ensure the sampling margin. The prototype transceiver, implemented using a 28-nm complementary metal-oxide semiconductor (CMOS) process, operates over a 3-mm mimicked silicon interposer channel with 21.2-dB loss. It achieves a data rate per density of 9 Gb/s/μm at a bit error rate (BER) < 10–12 with 0.23 unit interval (UI) eye width for pseudorandom binary sequence (PRBS)15 data while consuming only 1.46 pJ/bit.

On the Effect of Compactness of Pulse Shaping Function on Inter-Symbol Interference

In this letter we theoretically and numerically investigate the effectiveness of a compact pulse in terms of producing less inter-symbol interference (ISI) and thus less symbol error rate (SER). We present theoretical equations to express the signal to interference plus noise ratio (SINR) for different levels of ISI as well as theoretical SER in terms of quantified compactness of the pulse. In a single compact form equation, we expressed the relationship that connects all the aforementioned terms with each other and with the spectral efficiency. A well-studied &#x2018;better-than-raised cosine&#x2019; (BTRC) pulse shape in quadrature amplitude modulation (QAM) is considered and the performance are compared with standard pulses such as Gaussian and raised cosine pulse shapes. Performance comparison results are presented showing that the BTRC pulse outperforms the conventional pulses in both SNIR and SER metrics.

BER Performance of UWOC With APD Receiver in Wide Range Oceanic Turbulence

In this paper, the bit error rate (BER) performance of underwater wireless optical communication (UWOC) systems with on-off keying (OOK) modulation and an avalanche photodiode (APD) detector are analyzed in a wide range of oceanic turbulence environment, such as lognormal, Gamma, K, Weibull, and Exponentiated Weibull distributions. To achieving this goal, a new mathematical system model is established, which has not only considered the aggregated degrading phenomena, namely the absorption, scattering, and turbulence, but also been suitable for describing UWOC with APD receiver. Then the closed-form expressions of the BER performances of UWOC with APD for different turbulence distributions are derived in detail by subdividing the working situation into high and lower inter-symbol interference (ISI) scenarios. Theoretical analysis and computer simulations show that no matter what the ISI levels are, the derived closed-form expressions for system BER under various turbulences can approximate the accurate integral ones well. This is obviously helpful to the design and performance evaluation of UWOC systems.

Molecular-Type Permutation Shift Keying in Molecular MIMO Communications for IoBNT

Molecular communication (MC) is a bio-inspired communication paradigm, which lays the foundation for the Internet of Bio-NanoThings (IoBNT) in the medical field. As a high energy-efficient information transfer method, MC via diffusion (MCvD) is envisioned as a promising candidate for IoBNT but suffers from low date rates due to the long tail of the channel impulse response (CIR). To this end, the multiple-input–multiple-output (MIMO) technique has been introduced to MCvD. However, the intersymbol interference (ISI) and interlink interference (ILI) deteriorate the bit error rate (BER) performance of MIMO MCvD systems. In this article, molecular type permutation shift keying in the space domain (MTPSK-SD) and time-interleaved MTPSK-SD are proposed for MIMO MCvD systems to improve the BER performance by reducing ILI. The principle of MTPSK-SD is further generalized to the spatiotemporal domain, yielding three spatiotemporal modulation schemes, which can provide desirable BER performance without requiring any CIR information in the communication scenarios affected by different levels of ISI and ILI. Two low-complexity detectors are proposed to obtain different tradeoffs between anti-ILI and anti-ISI performance. Furthermore, a complementary coding scheme, which can effectively reduce the ILI under the considered symmetrical system topology, is designed and applied to all the proposed modulation schemes. Additionally, the BER upper bound is analyzed. Numerical simulations on BER corroborate the analysis and show that the proposed schemes are promising multimolecule modulation alternatives, which outperform the existing MIMO MCvD modulation schemes.

A Novel Technique for Achieving the Approximated ISI at the Receiver for a 16QAM Signal Sent via a FIR Channel Based Only on the Received Information and Statistical Techniques.

A single-input-multiple-output (SIMO) channel is obtained from the use of an array of antennas in the receiver where the same information is transmitted through different sub-channels, and all received sequences are distinctly distorted versions of the same message. The inter-symbol-interference (ISI) level from each sub-channel is presently unknown to the receiver. Thus, even when one or more sub-channels cause heavy ISI, all the information from all the sub-channels was still considered in the receiver. Obviously, if we know the approximated ISI of each sub-channel, we will use in the receiver only those sub-channels with the lowest ISI level to get improved system performance. In this paper, we present a systematic way for obtaining the approximated ISI from each sub-channel modelled as a finite-impulse-response (FIR) channel with real-valued coefficients for a 16QAM (16 quadrature amplitude modulation) source signal transmission. The approximated ISI is based on the maximum entropy density approximation technique, on the Edgeworth expansion up to order six, on the Laplace integral method and on the generalized Gaussian distribution (GGD). Although the approximated ISI was derived for the noiseless case, it was successfully tested for signal to noise ratio (SNR) down to 20 dB.

Effects of Framing Errors on the Performance of Molecular Communications With Memory

In conventional digital communication systems, synchronous transmission is achieved by embedding the clocking information into the data signal. However, the implementation of this technique in molecular communication systems, which rely on the diffusion of molecules as information carriers, becomes very complex due to the randomness of the diffusion process. Hence, in this paper we consider the molecular communication between two nanoscale devices with no exchange of any clock signal. To initiate the communication, the transmitter sends a special molecular symbol called beacon in order to trigger the detection process in the receiver. Therefore, this beacon symbol is equivalent to the start bit used for framing in asynchronous serial communication systems. We assume that both transmitter and receiver clocks are perfect, but not synchronized. Accordingly, the analysis focuses on the effects of framing errors on the performance of the molecular channel, measured via the symbol error probability. These errors are inherent to the random nature of the beacon arrival instant, which tends to degrade the alignment between the transmitter and receiver frames. We also assume a molecular channel with any level of inter-symbol interference and the use of different types of molecules to encode information symbols. We validate the derived SEP expression by means of extensive simulation experiments, and finally we develop a design scheme for the beacon symbol that satisfactorily mitigates the effects of framing errors.

Performance of modified and low complexity pulse shaping filters for IEEE 802.11 OFDM transmission

ABSTRACTThe most commonly used multicarrier modulation method in 802.11 wireless communication systems is Orthogonal Frequency Division Multiplexing (OFDM). However, OFDM is easily affected by Inter Symbol Interference (ISI). In this paper, different pulse shaping filters are employed in the OFDM system and three new filters are presented to further reduce ISI effects. The first one is a Modified Parametric Exponential Pulse (MPEXP) filter which employs a different transfer function that is implemented at the transmitter side. The second one is a Better Than Modified Flipped Exponential Pulse (BTMFEXP) filter which is derived by modifying the transfer function of Modified Flipped Exponential Pulse (MFEXP). The third one is a hybrid of BTMFEXP and MPEXP (HBTMFPEXP). Lastly, low complexity forms of the MFEXP and BTMFEXP, derived from Taylor Series, are implemented. The OFDM system was tested with the existing and proposed pulses over AWGN and Fading channels. The proposed filters show better Bit Error Rate (BER) performance. Under a high ISI level, BTMFEXP and HBTMFPEXP showed a gain of 0.23 and 0.57 dB respectively over MFEXP with AWGN channel. With a fading channel at high ISI level, the proposed BTMFEXP and HBTMFPEXP demonstrated a gain of 2.33 and 5.33 dB respectively over MFEXP.

The criterion for the existence of the weight of the orthogonality of equidistant elementary signals

To reduce the level of intersymbol interference and interchannel interference, it is proposed to form the transmitted signals using the basis functions obtained by shifting the impulse responses of linear systems by multiple time intervals. An algorithm for calculating the weight of the orthogonality of basis functions is proposed. A criterion for the existence of the indicated weight of orthogonality is formulated.

Effect of Degradation in Molecular Communication: Impairment or Enhancement?

In the nanonetworking literature, many solutions have been suggested to enable the nanomachine-to-nanomachine communication. Among these solutions, this paper focuses on what constitutes the basis for molecular communication paradigms—molecular communication via diffusion. In this paper, the channel for a spherical absorbing receiver under messenger molecule degradation is analytically modeled and the formulations are shown to be in agreement with the simulation results of the same topology. Next, the paper identifies how signal characteristics such as pulse peak time and pulse amplitude are affected by degradation. Indeed, it is analytically shown how signal shaping is achieved through degradation. This paper also compares communication under messenger molecule degradation with the case of no-degradation and electromagnetic communication in terms of channel characteristics. Lastly, the paper evaluates the communication performance of the scenarios having various degradation rates. Here, the system performance is assessed according to the traditional network metrics such as the level of intersymbol interference, detection performance, bit error rate, and symbol rate. The results indicate that introducing degradation significantly improves the system performance when the rate of degradation is appropriately selected. The analysis is done by taking different detection thresholds, symbol durations, and communication distances into account.

Analytical characterisation and optimum performance of DC‐biased optical orthogonal frequency division multiplexing signals

Orthogonal frequency division multiplexing (OFDM) schemes are being considered for both optical fibre and wireless optical communications mainly because their ability to cope with the high inter-symbol interference levels associated to dispersive channels. DC biased optical OFDM (DCO-OFDM) is a promising technique that meet the particularities of intensity modulated OFDM schemes. To reduce the required DC component OFDM signals need to be submitted to an asymmetric clipping. However, the resulting non-linear distortion effects can lead to significant performance degradation and/or undesirable out-of-band radiation. Therefore it is important to evaluate the impact of an asymmetric clipping on DC-biased optical OFDM signals. In this work, the authors take advantage of the Gaussian-like nature of OFDM signals and they study analytically the impact of an asymmetric clipping operation in the performance of DCO-OFDM schemes. The authors present accurate theoretical expressions for the power spectral density and the non-linear distortion at the subcarrier level, which can be used for obtaining the bit error rate of conventional receivers. The authors also study the optimum performance of these non-linear OFDM schemes and the authors show that for an optimum maximum likelihood detection the non-linear distortion instead of leading to performance degradation can actually improve it, even outperforming conventional, linear OFDM schemes.

Feed-forward carrier phase recovery for offset-QAM Nyquist WDM transmission.

Due to the half symbol delay between in-phase and quadrature components for offset quadrature amplitude modulation (OQAM) signal, phase noise cannot only lead to constellation rotation but also introduce additional crosstalk. Therefore, OQAM signal has very poor tolerance to the laser linewidth. Here, we carry out a semi-analytical investigation of phase noise induced crosstalk during OQAM Nyquist WDM transmission, and find that the carrier phase recovery (CPR) has to be implemented prior to the inter-symbol-interference (ISI) equalization. Then, after a function separation of polarization de-multiplexing and ISI equalization, we propose a new DSP flow with a linewidth-tolerant blind feed-forward CPR scheme for OQAM signal. Its effectiveness is verified under the scenario of 5-channel 28-Gbaud polarization multiplexing (PM) OQAM Nyquist WDM systems. A tolerance of linewidth and symbol duration products of 6.5×10(-4) and 1.1×10(-4) is secured for 4-OQAM and 16-OQAM, respectively, given 1-dB required-OSNR penalty at BER = 10(-3).

BER-Optimal Analog-to-Digital Converters for Communication Links

In this paper, we propose low precision BER-optimal analog-to-digital converters (ADC) where quantization levels and thresholds are set nonuniformly to minimize the bit-error rate (BER ) in a high data rate communication link. This is in contrast to how ADCs are used today, in which they act as transparent waveform preservers. Simulations for various communication channels show that for 2-PAM modulation, the BER-optimal ADC achieves shaping gains that range from 2.5 dB for channels with low intersymbol interference (ISI) to more than 30 dB for channels with high levels of ISI. Moreover, a 3-bit BER -optimal ADC achieves the same or even lower BER than a 4-bit uniform ADC. With a DFE and a high-ISI channel, a 3-bit BER -optimal ADC achieves lower BER than a 5-bit uniform ADC.

Electronic Impairment Mitigation in Optically Multiplexed Multicarrier Systems

In order to improve the performance of optically multiplexed multicarrier systems with channel spacing equal to the symbol rate per carrier, we propose and systematically investigate an electronic signal processing technique to achieve near-interchannel crosstalk free and intersymbol-interference (ISI) free operation. We theoretically show that achieving perfect orthogonality between channels in these systems, together with ISI free operation as needed in generic communication systems, requires the shaping of the spectral profiles of not only the demultiplexing filter, but also the signal of each channel before demultiplexing. We develop a novel semianalytical method to quantitatively analyze the levels of residual crosstalk and ISI arising from nonideal system response in these systems. We show that by prefiltering the signal to ensure that the system impulse response before channel demultiplexing approaches the targeted condition, the residual crosstalk due to imperfect orthogonality can be significantly mitigated and the necessity for carrier phase control in single-quadrature format-based system can be relaxed. Further combining prefiltering and receiver-side postfiltering to adaptively trim the demultiplexing filter enhances the performance. The use of the combined digital signal processing (DSP) in coherent-detection quadrature phase-shifted keying (QPSK)-based optically multiplexed multicarrier system shows that this method outperforms conventional QPSK-based multicarrier system without DSP or with only receiver-side DSP, especially when the responses of the transmitter and the demultiplexing filter are not precisely designed and the sampling rate of the analogue-to-digital converter is not sufficiently high. In addition, the inclusion of ISI free operation, with this aspect similar to the reshaping method in conventional wavelength-division-multiplexing systems, allows the relaxation of the modulation bandwidth and chromatic dispersion compensation.

Minimum BER Block-Based Precoder Design for Zero-Forcing Equalization: An Oblique Projection Framework

This work devises a minimum bit error rate (BER) block-based precoder used in block transmission systems with the proposed cascaded zero-forcing (ZF) equalizer. The study framework is developed as follows. For a block-based precoder, a received signal model is formulated for the two redundancy schemes, viz., trailing-zeros (TZ) and cyclic-prefix (CP). By exploiting the property of oblique projection, a cascaded equalizer for block transmission systems is proposed and implemented with a scheme, in which the inter-block interference (IBI) is completely eliminated by the oblique projection and followed by a matrix degree-of-freedom for inter-symbol interference (ISI) equalization. With the available channel state information at the transmitter side, the matrix for ISI equalization of the cascaded equalizer is utilized to design an optimum block-based precoder, such that the BER is minimized, subject to the ISI-free and the transmission power constraints. Accordingly, the cascaded equalizer with the ISI-free constraint yields a cascaded ZF equalizer. Theoretical derivations and simulation results confirm that the proposed framework not only retains identical BER performance to previous works for cases with sufficient redundancy, but also allows their results to be extended to the cases of insufficient redundancy.

Modelling and Comparative Performance Analysis of a Time-Reversed UWB System

The effects of multipath propagation lead to a significant decrease in system performance in most of the proposed ultra-wideband communication systems. A time-reversed system utilises the multipath channel impulse response to decrease receiver complexity, through a prefiltering at the transmitter. This paper discusses the modelling and comparative performance of a UWB system utilising time-reversed communications. System equations are presented, together with a semianalytical formulation on the level of intersymbol interference and multiuser interference. The standardised IEEE 802.15.3a channel model is applied, and the estimated error performance is compared through simulation with the performance of both time-hopped time-reversed and RAKE-based UWB systems.

Joint Interference Cancellation and Channel Shortening in Multiuser-MIMO Systems

In this paper, a two-stage receiver structure for interference cancellation in multiuser spatially multiplexed multiple-antenna systems is presented. A space-time equalizer is used in the first stage for joint coantenna/cochannel-interference suppression and shortening of the effective channel for each transmit stream of the desired user. The channel shortening, combined with independent detection, helps reduce the complexity of the second stage Viterbi equalizer, which is used for separate intersymbol-interference equalization for each of the streams. Three objective functions are proposed for determining the coefficients of the space-time equalizer using a direct training data-based approach, which does not require estimation of the interferer's channel. Simulation results show good symbol error performance as compared to existing algorithms with asynchronous multiple-input multiple-output interferers

Turbo Transceivers for MIMO Wireless Communications and Their Performance Verification via Multi-Dimensional Channel Sounding

The discovery of the Turbo codes has driven research on the creation of new signal detection concepts that are, in general, referred to as the Turbo approach. Recently, this approach has made a drastic change in creating signal detection techniques and algorithms such as equalization of inter-symbol interference (ISI) experienced by broadband single carrier signaling over mobile radio channels. A goal of this paper is to provide readers with broad views and knowledge of the Turbo concept-based Multiple-Input Multiple-Output (MIMO) signal transmission techniques. How the techniques have been developed in various applications and how they perform in real-field environments are introduced.