Mitigate Intersymbol Interference Research Articles

Steady-state performance of multimodulus blind equalizers

Multimodulus algorithms (MMA) based adaptive blind equalizers mitigate inter-symbol interference in a digital communication system by minimizing dispersion in the quadrature components of the equalized sequence in a decoupled manner, i.e., the in-phase and quadrature components of the equalized sequence are used to minimize dispersion in the respective components of the received signal. These unsupervised equalizers are mostly incorporated in bandwidth-efficient digital receivers (wired, wireless or optical) which rely on quadrature amplitude modulation based signaling. These equalizers are equipped with nonlinear error-functions in their update expressions which makes it a challenging task to evaluate analytically their steady-state performance. However, exploiting variance relation theorem, researchers have recently been able to report approximate expressions for steady-state excess mean square error (EMSE) of such equalizers for noiseless but interfering environment.In this work, in contrast to existing results, we present exact steady-state tracking analysis of two multimodulus equalizers in a non-stationary environment. Specifically, we evaluate expressions for steady-state EMSE of two equalizers, namely the MMA2-2 and the βMMA. The accuracy of the derived analytical results is validated using different set experiments and found in close agreement.

Space-time block code with equalization technology for underwater acoustic channels

In order to combat the effects of multipath interference and fading for underwater acoustic (UWA) channels, this paper investigates a scheme of the combination of space-time block code (STBC) and equalization technology. STBC is used in this scheme to reduce the effects of fading for the UWA channels, then equalization technology is used in this scheme to mitigate intersymbol interference. The performance of the scheme is analyzed with Alamouti space-time coding for the UWA channels. Our simulations indicate that the combination of STBC and equalization technology provides lower bit error rates.

Simulation study of molecular communication systems with an absorbing receiver: Modulation and ISI mitigation techniques

The research activities on Molecular Communication via Diffusion (MCvD) that can interconnect nanomachines, heavily depend on simulations to verify and evaluate the new communication paradigm. The existing simulation tools cannot be directly used for MCvD systems since the diffusion channel has different characteristics compared to classical communication channels. In an MCvD system, diffusion and demodulation processes have their own constraints and characteristics due to the impossibility of sending a negative number of molecules, the random movement of molecules, and a reception process of molecules that determines the signal. Therefore, a custom end-to-end MolecUlar CommunicatIoN (MUCIN) simulator for MCvD systems is presented. Compared to other simulators available in the literature, MUCIN simulator is an end-to-end simulator that considers first hitting process for the signal reception. It supports 1-D to 3-D environments, sending consecutive symbols, imperfect molecule reception, extendable modulation, and filtering modules. MUCIN simulator source code is available under BSD licensing for contributors from the nanonetworking community. Following the simulator analysis, a case study of inter-symbol-interference mitigation that utilizes the decision in one previous slot is introduced. The contribution of this paper is twofold; one is the modeling and the development of an end-to-end MCvD simulator, the other is the performance evaluation of the proposed inter-symbol interference filtering and demodulation techniques.

Optimal receiver design for diffusive molecular communication with flow and additive noise.

In this paper, we perform receiver design for a diffusive molecular communication environment. Our model includes flow in any direction, sources of information molecules in addition to the transmitter, and enzymes in the propagation environment to mitigate intersymbol interference. We characterize the mutual information between receiver observations to show how often independent observations can be made. We derive the maximum likelihood sequence detector to provide a lower bound on the bit error probability. We propose the family of weighted sum detectors for more practical implementation and derive their expected bit error probability. Under certain conditions, the performance of the optimal weighted sum detector is shown to be equivalent to a matched filter. Receiver simulation results show the tradeoff in detector complexity versus achievable bit error probability, and that a slow flow in any direction can improve the performance of a weighted sum detector.

A Study on Recent Advancements in MIMO-FBMC Systems

Multicarrier (MC) techniques when combined with Multiple Input Multiple Output (MIMO) technology increases the throughput and QOS of the system. OFDM with its unique capability of converting the frequency selective channel into a set of frequency flat channel have proved to be one of the best multicarrier technique that can be applied to MIMO channels. However, OFDM suffers from Bandwidth loss due to the addition of cyclic prefix. Filter bank multicarrier (FBMC) modulation system on the other hand, is highly bandwidth efficient but its application to MIMO channels is still a subject of study to various researchers for the past decade. So far, only FMT based FBMC system offers same flexibility as OFDM in adopting MIMO techniques. This paper presents the various advancements or methods proposed to enhance the flexibility of FBMC systems (FMT,CMT,OFDM-OQAM) while adopting MIMO techniques mainly highlighting the ISI and ICI mitigation methods through improved decoding techniques, equalization schemes and modified FBMC schemes.

Designing non‐contiguous orthogonal frequency division multiplexing transceiver based on wavelet transform and removable cyclic prefix for spectrum sharing in cognitive radio systems

Non-contiguous orthogonal frequency division multiplexing (NC-OFDM) technique has been proposed in cognitive radio literature in which the Fourier transform is used to provide orthogonality between subcarriers and a cyclic prefix (CP) with temporal length greater than channel delay spread is added to each NC-OFDM symbol to mitigate intersymbol interference. Using CP, however, degrades spectrum efficiency since it does not carry useful information. Besides, in traditional NC-OFDM-based cognitive radio systems, CP is an effective source of interference with adjacent primary users. To cope with these issues, a cognitive radio system based on wavelet transform and NC-OFDM is proposed. Wavelets have appropriate localisation, limited length and orthogonality both in time and frequency domains. Analysis of signal-to-noise ratio (SNR) show that the SNR gain increases when the number of nullified subcarriers assigned to the secondary users is increased. Also, simulation results of the proposed scheme indicate that the nullifying process improves the system performance in terms of bit error rate. To select the optimum wavelet filter order, the effect of filter order in orthogonal wavelet families is also investigated. Simulation results show that a wavelet transform with a lower filter order should be used.

Improving Receiver Performance of Diffusive Molecular Communication With Enzymes

This paper studies the mitigation of intersymbol interference in a diffusive molecular communication system using enzymes that freely diffuse in the propagation environment. The enzymes form reaction intermediates with information molecules and then degrade them so that they cannot interfere with future transmissions. A lower bound expression on the expected number of molecules measured at the receiver is derived. A simple binary receiver detection scheme is proposed where the number of observed molecules is sampled at the time when the maximum number of molecules is expected. Insight is also provided into the selection of an appropriate bit interval. The expected bit error probability is derived as a function of the current and all previously transmitted bits. Simulation results show the accuracy of the bit error probability expression and the improvement in communication performance by having active enzymes present.

Design and Measurement Techniques for an 80 Gb/s 1-Tap Decision Feedback Equalizer

A millimeter wave frequency mixed-signal design of a 1-tap half-rate look-ahead decision feedback equalizer for 80 Gb/s short-reach optical communication systems is presented. On-wafer tests are developed to determine the maximum operating bit rate of the equalizer. Results are also presented for intersymbol interference mitigation at 80 Gb/s for a 20 GHz bandwidth-limited channel. Further improvements on the architecture of the 80 Gb/s equalizer are discussed and used in the design and on-wafer measurement of a 110 Gb/s equalizer. The equalizers are designed in a 0.13 μm SiGe:C BiCMOS technology. The 80 and 110 Gb/s versions dissipate 4 and 5.75 W, respectively and occupy 2 and 2.56 mm2, respectively.

Frequency domain response estimation of underwater acoustic channel and noise estimation for single-carrier frequency domain equalization

Single-carrier frequency domain equalization (SC-FDE) has drawn increasing researches because of its lower computational complexity and better performance of mitigating intersymbol interference (ISI) as compared with the time domain equalization. The practical SC-FDE systems, however, require the knowledge of the channel frequency domain response and the noise power to calculate filter coefficients of the frequency domain equalizer. Improper channel and noise estimations have been shown to result in error floor at high signal-to-noise ratio (SNR), which may impede the utilization of SC-FDE in the high-rate underwater acoustic (UWA) communications. In this paper, we discuss the possible influence of inaccurate noise estimation on the error floor at high SNR, and propose a joint estimation scheme for the channel time domain and frequency domain responses and also the noise power based on Chu sequence. The channel energy concentration region is determined by the threshold and the sparse channel impulse response (CIR) of UWA channels, used in denoising process of the CIR and the noise power computation. The bite error ratio performance of the SC-FDE variation versus the threshold is simulated via the UWA models, and accurate noise power estimation is shown to be desirable to improve the SC-FDE performance. Numerical examples also indicate that our joint estimation scheme can eliminate the error floor and improve the performance of SC-FDE at high SNR.

Soft-Decision Feedback Turbo Equalization for LDPC-Coded MIMO Underwater Acoustic Communications

In this paper, a low-complexity turbo-detection scheme is proposed for single-carrier multiple-input-multiple-output (MIMO) underwater acoustic (UWA) communication systems that employ low-density parity-check (LDPC) channel coding. The proposed iterative soft-decision feedback equalization (SDFE) algorithm offers a novel approach to combat error propagation by utilizing the past soft decisions to mitigate intersymbol interference. In addition, we utilize an iterative data detection and channel estimation scheme to reduce the pilot overhead used in the data communication system. Performance of the proposed detection scheme is demonstrated through extensive communication data from two undersea experiments. The results show that the proposed SDFE algorithm provides robust detection for MIMO UWA communications with different modulations and different symbol rates, at different transmission ranges.

High-Order Intensity Modulations for OSTBC in Free-Space Optical MIMO Communications

Recently, Simon-Vilnrotter and Premaratne-Zheng modified the binary pulse-position modulation (PPM) to support free-space optical (FSO) multi-input multi-output (MIMO) systems with orthogonal space-time block coding (OSTBC) that requires the use of negative symbols. In this paper, we extend the investigation to optical OSTBC schemes with high-order intensity modulation. A necessary and sufficient condition for the high-order intensity modulation to maintain the orthogonality of the applied OSTBC is derived, and four Q-ary intensity modulation schemes satisfying this condition are proposed. In asynchronous FSO MIMO channels in which the signals from different transmitters arrive at the receiver with timing misalignment, the proposed modulation schemes can be used with shift-OSTBC to mitigate inter-symbol interference (ISI) and provide diversity gain.

Reducing Pattern Noise in Blind Energy Detection of Nonnegative PAM Symbols

In this paper, we consider blind initialization of decision-directed (BDD) parameter estimation for energy detection of nonnegative pulse-amplitude-modulated (PAM) symbols. Short pulses and guard intervals are used to mitigate intersymbol interference. In the previous work, no analytical model has been given to predict how the BDD approach converges, hindering the usage of the approach. We present analytical mean square error expressions to infer a sufficient observation interval that makes the effect of the pattern noise negligible in comparison with the noise component originated from the thermal noise of the receiver. Using a proper observation interval, the symbol error rate of the BDD method is demonstrated to be comparable with that of a corresponding data-aided estimator which, unlike the BDD approach, needs a periodical training signal.

Comparative study of classifiers to mitigate intersymbol interference in diffuse indoor optical wireless communication links

The maximum data rate that can be achieved in diffuse indoor optical wireless communication (OWC) is limited due to the effect of intersymbol interference (ISI). The adverse effect of ISI on the system performance can be minimised using a channel equaliser at the receiver. In this study, digital signal detection is formulated as a classification problem and hence a classifier is adopted at the receiver. The bit error performance of classifiers with non-linear decision boundary including a multilayer perceptron (MLP), a support vector machine (SVM), the radial basis function (RBF), and the Bayesian classifier is studied along with traditional equaliser and reported here. The MLP offers the best performance; however there is trade-off between the performance and complexity especially at highly diffuse channel.

Investigation of Phase Noise on the Performance of LMS-RLS Adaptive Equalizer

This paper investigates the effect of phase noise on equalization of communication channels using least mean square (LMS) and recursive least square (RLS) adaptive algorithms. The aim of the investigation is to mitigate inter-symbol interference (ISI) caused by the channel and to impose the bit error rate (BER) in the received signals. The equalizerusestwobasicadaptivealgorithms: LMS algorithmand RLS algorithm. Without LMS-RLS equalizer,theBER ismorethan when the system modelincludesLMS-RLS equalizer as indicated in table (1) and table (2). Equalizer algorithm is analyzed using MATLAB v.9 Communication Block Set.

Equalized near maximum likelihood detector

This paper presents new detector that is used to mitigate intersymbol interference introduced by bandlimited channels. This detector is named equalized near maximum likelihood detector which combines nonlinear equalizer and near maximum likelihood detector. Simulation results show that the performance of equalized near maximum likelihood detector is better than the performance of nonlinear equalizer but worse than near maximum likelihood detector.

Spatial Multipath Resolution with Space Time Block Codes

The use of space time block codes (STBCs) over frequency selective multipath multiple-input multiple-output (MIMO) channels is investigated in combination with spatial multipath resolution (SMR) [1], a novel spatial signal processing technique proposed by the authors for mitigating inter-symbol interference. Neither STBC nor SMR requires transmit channel state information. This fact, along with SMR not requiring any modification to the transmitter, makes the MIMO STBC-SMR hybrid effective. Numerical results are presented, illustrating how the scheme fares under the Alamouti STBC scheme, and highlighting the advantage of adaptive SMR - i.e., adapting the number of multipaths resolved based on the channel state. A practical multipath MIMO channel based on the IEEE 802.15.3c NLOS (CM4) model is used for simulation.

Fractional Fourier domain equalization for single carrier broadband wireless systems

In this paper, a novel single carrier equalization approach in the fractional Fourier domain (FRFD) is proposed. It can remove all the inter-symbol interference (ISI) and avoid the considerable noise enhancement of the frequency domain-zero forcing (FD-ZF) equalizer. As the fractional Fourier transform makes a chirp spread, the impulse response of the deep fading channel may be flattened in some orders of the FRFD while it would be greatly attenuated in the FD. By searching for an optimal order under certain criterion, we take advantages of the ZF algorithm to mitigate the effects of the ISI completely. This approach can overcome the contradiction between the ISI mitigation and the noise enhancement of the FD-ZF equalizer. Theoretical analysis and simulation results show that the proposed FRFD-ZF equalizer can achieve a significant performance and have the same computation cost O(N logN) as the conventional FD linear equalizer, especially in the frequency-selective deep-fading channels.

Experimental investigation on channel characteristics in tunnel environment for Time Reversal Ultra Wide Band techniques

The objective of this paper is to investigate the potential advantages of the Time Reversal (TR) technique applied to Impulse Radio Ultra Wide Band (UWB) signals for communications in tunnels. Indeed, in an environment with significant multipaths, it has already been outlined that this technique allows mitigating intersymbol interference and increases the peak power received at a target antenna. However, in a tunnel, as a result of the guiding effect of the structure, the spatial diversity degree decreases as the distance between the transmitter and receiver increases. An in‐depth analysis is therefore needed, and we first thus present the main characteristics of the UWB channel deduced from measurements made in a long straight arched tunnel and for a frequency band extending from 2.8 to 5 GHz. In the time domain, waveforms of the impulse radio signal are obtained through an inverse Fourier transform of the measured frequency response and examples are given for different distances varying from 50 m to 500 m. Delay spread and peak‐to‐peak gain are then studied, depending on the communication range. The case for multiple antenna transmission is also considered.

Digital Approaches to ISI-Mitigation in High-Resolution Oversampled Multi-Level D/A Converters

A new digital signal processing approach to shaping intersymbol interference (ISI) and static mismatch errors simultaneously in oversampled multi-level digital to analog converters (DAC) has recently been proposed. In this paper, a mathematical framework is established for analyzing ISI errors as well as comparing the ISI sensitivities of different mismatch shaping algorithms. The framework is used to analyze the fundamental problems of popularly used algorithms such as data-weighted-averaging (DWA) in the presence of nonlinear ISI: Large-signal even-order distortion and frequency modulated harmonics at low signal levels. The new ISI-shaping algorithm results in significant improvement over previous schemes including the modified Mismatch Shaper (MMS) which also addresses ISI error. The new ISI shaper, while increasing the digital complexity, practically eliminates the need for conventional ISI mitigation techniques such as time consuming, layout-critical, non-automated and process specific analog design methods. The advantages of ISI shaping is further verified on an experimental audio DAC with simple non-return-to-zero (NRZ) current steering segments implemented in a 45 nm CMOS process and running off a single-phase clock of only 3.072 MHz.

An efficient iterative frequency domain equalization for ATSC DTV receiver

In this paper, we present an efficient single carrier frequency domain equalization for Advanced Television Systems Committee (ATSC) digital television receiver. The proposed scheme employs iterative cyclic prefix reconstruction (CPR) combined with block-overlapping process to reduce inter-carrier interference (ICI) component caused by the absence of cyclic prefix in multipath fading channel. In addition, trellis decoder aided iterative decision feedback frequency domain equalizer is derived to further mitigate inter-symbol interference (ISI). Finally, low-complex iterative frequency domain channel estimation is proposed to predict and track the time-varying multipath fading channel coefficients. The computer simulation results reveal that the proposed scheme has a significant performance improvement in multipath fading channel with respect to the previous time domain equalization (TDE) and frequency domain equalization (FDE).