Significant Intersymbol Interference Research Articles

Decision Feedback Equalization-Based Low-Complexity Interference Cancellation and Signal Detection Technique Based for Non-Orthogonal Signaling

FTN signalling is an effective communication method that achieves a high spectral efficiency. However, employing a symbol rate faster than the Nyquist rate disrupts the orthogonality between symbols, leading to unavoidable inter-symbol interference (ISI). To mitigate the effects of ISI, interference cancellation and signal detection processes are essential for FTN receivers. Conventional ISI reduction techniques often utilize trellis-based algorithms. However, as the number of states increases due to additional interference symbols, the complexity of these algorithms grows exponentially. To address this challenge, this paper explores a matrix computation-based interference cancellation technique tailored for FTN communication systems, aiming to significantly reduce the complexity of the ISI mitigation process. To execute ISI cancellation and signal detection more precisely, the proposed technique includes iterative interference cancellation and a signal detection process. When six interference symbols are considered, the complexity of the proposed technique is reduced by 97% compared with that of the conventional Viterbi algorithm. Furthermore, in the case of τ = 0.85, the performance of the proposed technique is about 1 dB better than that of the Viterbi algorithm at BER = 10−4.

Higher Order Derivative-Based Receiver Preprocessing for Molecular Communications

While molecular communication via diffusion experiences significant inter-symbol interference (ISI), recent work suggests that ISI can be mitigated via time differentiation pre-processing which achieves pulse narrowing. Herein, the approach is generalized to higher order differentiation. The fundamental trade-off between ISI mitigation and noise amplification is characterized, showing the existence of an optimal derivative order that minimizes the bit error rate (BER). Theoretical analyses of the BER and a signal-to-interference-plus-noise ratio are provided, the derivative order optimization problem is posed and solved for threshold-based detectors. For more complex detectors which exploit a window memory, it is shown that derivative pre-processing can strongly reduce the size of the needed window. Extensive numerical results confirm the accuracy of theoretical derivations, the gains in performance via derivative pre-processing over other methods and the impact of the optimal derivative order. Derivative pre-processing offers a low complexity/high-performance method for reducing ISI at the expense of increased transmission power to reduce noise amplification.

Recurrent Neural Network Equalization for Wireline Communication Systems

Satisfying the demand for ever increasing data rates in wireline communication systems is challenging due to high frequency-dependent channel insertion loss causing significant inter-symbol interference (ISI). This motivates the use of ADC-based receiver architectures that allow for digital equalizer implementations. However, the large tap count feed-forward equalizers (FFEs) required at high data rates causes noise enhancement that can degrade signal-to-noise ratio (SNR) and high complexity is required to implement multi-tap decision feedback equalizers (DFEs). Moreover, analog front-end (AFE) nonlinearity can limit the performance of these traditional FFE-DFE architectures. This brief investigates an alternative approach with recurrent neural network (RNN) equalization based on long short-term memory (LSTM) and gate recurrent unit (GRU) cells. 64 Gb/s PAM4 modeling results show that the RNN topologies offer significant BER improvement over a wide SNR range for a channel with 30 dB loss at 16 GHz and enable operation over a channel with a large notch at 10 GHz that the traditional FFE-DFE topology cannot support. Synthesis results utilizing a 22 nm FinFET process yield only a 34% and 29% increase in receiver power consumption for the LSTM and GRU implementations, respectively, to enable this improved performance.

Delay-Doppler domain decision feedback turbo equalization for OTFS modulation

Emerging evidence indicates that orthogonal time–frequency space (OTFS) modulation is a potential candidate modulation scheme for high mobility wireless communications. However, OTFS may experience significant inter-symbol interference (ISI) and inter-Doppler interference (IDI) in the receiver. In this paper, we propose a soft decision feedback turbo equalization for OTFS transmission over delay-Doppler channels to jointly combat both interferences. A novel block decision feedback equalization (BDFE) algorithm is constructed using the band feature of the channels in the delay-Doppler domain. The feedforward and feedback filters are designed by the delay-time channels coefficients. According to the designed filter, an equivalent system model is employed to allow turbo equalization. The posterior probability is established using the soft prior information and feedback filter, and then fed back to the channel decoder as external information. Both theoretical analysis and simulation results demonstrate the effectiveness of the proposed scheme in improving the bit error rate (BER) performance and combat various interference. Numerical simulations are finally provided to justify the validity of the proposed scheme in improving the bit error rate (BER) performance and combating various interference.

Accurate Characterization for Continuous-Time Linear Equalization in CMOS Optical Receivers

Recently published CMOS optical receivers consist of a limited-bandwidth first-stage transimpedance amplifier (TIA) followed by an equalizer. Limiting the TIA’s bandwidth improves the gain and reduces the noise but introduces a significant inter-symbol interference (ISI) that is dealt with by the subsequent equalizer. Continuous-time linear equalizer (CTLE) is a commonly used equalizer in both electrical and optical links. However, recent research reported different findings about CTLE-based optical receivers. Some research papers concluded that CTLEs boost high-frequency noise compared to a full-bandwidth design. Other publications reported that high-frequency noise remains unaffected while white noise is significantly reduced. This work aims to solve this discrepancy by presenting an accurate analysis for CTLE-based optical receivers considering noise, gain, and jitter. We show that the noise performance depends on the pole/zero locations of the limited-bandwidth (LBW)-TIA and the follow-on equalizer. A properly designed CTLE-based receiver achieves a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5 \times $ </tex-math></inline-formula> higher gain and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.74\times $ </tex-math></inline-formula> better noise than the full-bandwidth design. The CTLE is also compared to the well-known decision feedback equalizer (DFE). The noise performance of the CTLE-based receiver lies between that of finite and infinite impulse response DFE-based receivers but achieves better gain than both architectures.

Capacity of underwater optical wireless communication systems over salinity-induced oceanic turbulence channels with ISI.

Point-to-point underwater optical wireless communication (UOWC) links are mainly impaired by scattering due to impurities and turbidity in the open water, resulting in a significant inter-symbol interference (ISI) that limits seriously both channel capacity and the maximum practical information rate. This paper conducts, for the first time, the channel capacity analysis of UOWC systems in the presence of ISI and salinity-induced oceanic turbulence when the undersea optical channel is accurately modeled by linear discrete-time filtering of the input symbols. In this way, novel upper and lower bounds on channel capacity and mutual information are developed for non-uniform on-off keying (OOK) modulation when different constraints are imposed on the channel input. The results show that the capacity-achieving distribution, which is computed through numerical optimization, is discrete and depends on the optical signal-to-noise-ratio (SNR). Moreover, a non-uniform input distribution significantly improves the channel capacity of such systems affected by ISI and oceanic turbulence, especially at low optical SNR. Monte Carlo techniques are employed to test the developed bounds for different undersea optical channels with one, two and three casual ISI coefficients.

A Concentration-Time Hybrid Modulation Scheme for Molecular Communications

Significant inter-symbol interference (ISI) challenges the achievement of reliable, high data-rate molecular communication via diffusion. In this paper, a hybrid modulation based on pulse position and concentration is proposed to mitigate ISI. By exploiting the time dimension, molecular concentration and position modulation (MCPM) increases the achievable data rate over conventional concentration and position-based modulations. In addition, unlike multi-molecule schemes, this hybrid scheme employs a single-molecule type and so simplifies transceiver implementations. In the paper, the optimal sequence detector of the proposed modulation is provided as well as a reduced complexity detector (two-stage, position-concentration detector, TPCD). A tractable cost function based on the TPCD detector is proposed and employed to optimize the design of the hybrid modulation scheme. In addition, the approximate probability of error for the MCPM-TPCD system is derived and is shown to be tight with respect to simulated performance. Numerically, MCPM shows improved performance over standard concentration and pulse position-based schemes in the low transmission power and high bit-rate regime. Furthermore, MCPM offers increased robustness against synchronization errors.

Экспериментальное исследование самоорганизующегося акустического канала связи

This paper presents the results of an experimental study of the adaptive communication system with phase separation. Adaptation is carried out by methods of machine learning and consists in transmitters phases regulation. An acoustic waveguide with a tunable metasurface is used as a propagation medium. In the case of multi-beam propagation and the presence of significant inter-symbol interference the transmission rate of 1 kbit/s was achieved. The adaptation time of the investigated system to sudden changes in the channel parameters does not exceed 10 cycles of training, even in case of significant fading.

Low-Complexity Symbol Detection and Interference Cancellation for OTFS System

Orthogonal time frequency space (OTFS) is a two-dimensional modulation scheme realized in the delay-Doppler domain, which targets the robust wireless transmissions in high-mobility environments. In such scenarios, OTFS signal suffers from multipath channel with continuous Doppler spread, which results in significant inter-symbol interference and inter-Doppler interference (IDI). In this article, we analyze the interference generation mechanism, and compare statistical distributions of the IDI in two typical cases, i.e., limited-Doppler-shift channel and continuous-Doppler-spread channel (CoDSC). Focusing on the OTFS signal transmission over the CoDSC, our study firstly indicates that the widespread IDI incurs a computational burden for the element-wise detector like the message passing in the state-of-the-art works. Addressing this challenge, we propose a block-wise OTFS receiver by exploiting the structure and characteristics of the OTFS transmission matrix. In the receiver, we deliberately design an iteration strategy among the least squares minimum residual based channel equalizer, reliability-based symbol detector and interference eliminator, which can realize fast convergence by leveraging the sparsity of channel matrix. The simulations demonstrate that, in the CoDSC, the proposed scheme achieves much less detection error, and meanwhile reduces the computational complexity by an order of magnitude, compared with the state-of-the-art OTFS receivers.

Optimal Photon Counting Receiver for Sub-Dead-Time Signal Transmission

In many practical scenarios, single-photon avalanche diodes (SPADs) are good solutions to improve the performance of optical communication systems due to their high sensitivity to photon arrival. SPAD receivers can be implemented in large arrays to achieve higher data rates and additional protection against background light; however, they suffer from a significant intersymbol interference (ISI) if the SPAD dead time is comparable or larger than the symbol duration, i.e., sub-dead-time signal transmission. This work proposes a novel detection scheme designed for high-speed SPAD-based systems to effectively mitigate the degradation induced by ISI. Different from traditional receivers, in the proposed scheme, the information extracted from both the counts and arrival times of photons are utilised for the optimal symbol detection in the presence of non-linear and random ISI effect due to dead time. Our extensive numerical and experimental results demonstrate the superiority of the proposed photon time information based detection (PTID) scheme in terms of both BER performance and background light tolerance of the communication link. In addition, a linear approximation of the SPAD-based channel is investigated, which illustrates that the traditional equalization methods are effective under some specific circumstances.

Noise Analysis and Design Considerations for Equalizer-Based Optical Receivers

Optical receiver front ends that are intentionally designed to have a bandwidth low enough that significant inter-symbol interference (ISI) is introduced are becoming commonplace. Although the resultant ISI must be removed using an equalizer, the lower bandwidth allows for higher gain in the front-end’s first stage, lower input-referred noise, and fewer gain stages. With fewer main-amplifier stages, power dissipation is reduced. The noise analysis of these front ends presents several challenges. This paper derives integrated input-referred noise for inverter-based shunt-feedback transimpedance amplifiers from first principles and highlights the importance of correctly estimating the gain and noise bandwidth of the receiver. The notion of the effective gain of the receiver is introduced, which is lower than the midband gain typically used in noise calculations. The analysis of the inverter-based TIA is used to discuss the important design tradeoffs depending on the type of equalizer used. Integrated input-referred noise is derived and compared for front ends using decision-feedback equalizers (DFEs), continuous-time linear equalizers, and feed-forward equalizers. The simulation results show that a DFE-based receiver achieves the lowest input-referred noise.

Enhancing Signal Detection in Frequency Selective Channels by Exploiting Time Diversity in Inter-symbol Interference Signal

Inter-symbol interference (ISI) causes many errors in modulated symbols, when they are transmitted through band-limited channels. According to the multipath delay profile of the channel, the modulated symbol may disperse over more than one symbol period casing interferences among the received symbols. In this paper, the ISI signal is not considered as a problem, but it is exploited as a source of time diversity for the modulated symbols. The ISI signal is considered as if the modulated symbols are transmitted more than one time through different symbols periods. The number of periods over which the modulated symbol extends, represents the time diversity gain of the proposed system. In the proposed receiver, the interfering symbols are considered as symbols from different users. A multiuser detector is used to demodulate these interfering symbols. Moreover, a signal combiner is used to collect the detected symbol from the output of the multiuser detector through the dispersion period of the channel. The output of the combiner represents the decision variable to the baseband detector. The diversity gain in the decision variable in the proposed receiver is proportional to the dispersion period of the channel.

ISI Free Channel Equalization for Future 5G Mobile Terminal Using Bio-inspired Neural Networks

This article presents the concept of future 5th generation (5G) wireless communications as an existing beyond 4G systems like long term evolution (LTE) which means the indicate of 5G scenarios can be introduced in near future. Therefore, this paper deals of a future 5G mobile terminal for applications to uplink transmission in a multiuser LTE scheme. Unfortunately, LTE-uplink inherently generates significant inter-symbol interference especially high bandwidth scenarios. The result is a rise to mutual interference among active users with an increased error rate. This incidence eventually causes non-orthogonal user spreading codes. Moreover, this drawback is known as the multiple access interference episodes which demonstrate high computational complexity and enhances symbol error rate at the receiving end and degrades the communication quality. Most of the related work has been claimed iterative linear minimum mean square error (LMMSE) detection requires a matrix inversion role which has a high computational complexity and contains a combinatorial optimization problem. Consequently, the LMMSE does not meet the requirement to implement real-time detection with low complexity and thus limiting its application. Therefore, we propose an acceptable bio-inspired neural network (NN) in the case of single and multilayer NNs with supervised learning particularly Levenberg---Marquardt backpropagation learning algorithm to improve the convergence speed. Simulation results performed with highest approaches highlights a better act for the proposed system.

A wide band QAM receiver based on DFE to reject ISI in wireless time varying fading channel

Quadrature Amplitude Modulation (QAM) system with high data rate suffers from Inter-Symbol-Interference (ISI) in frequency-selective wireless channel. To reduce this ISI signal, Decision Feedback Equalizer (DFE) is used. When the channel impulse response is time varying, the channel's coefficients need to be adaptively estimated. Adaptive implementation of the forward filter on the conventional DFE can track the time variation of the channel coefficients, but its tracking capability depends on the covariance matrix of the receiver vector and the speed of channel variation. In this paper, wide band QAM receiver is proposed with new arrangement of the DFE filters. Data diversity at the receiver is achieved by using N antennas at its input. The forward filter of the DFE will be used as a Maximum-Ratio-Combiner (MRC) as well as a channel compensator. By first removing the ISI signal from the received vector, the tracking performance of the forward filter will be enhanced. The new performance of the proposed receiver will be compared with the conventional DFE receiver. The tracking performance of the forward filter using LMS and RLS algorithms in the new arrangement is compared with that of the conventional DFE receiver.

Bit-Loaded PAPR Reduction for High-Data-Rate Through-Metal Control Network Applications

Data transmission through metallic structures is commonly required in industrial control applications. In a number of these applications, mechanically penetrating the structure to pass cables and establish a wired communication link is either impossible or undesirable. Examples of such structures include metal bulkheads, pressure vessels, or pipelines. Ultrasonic signaling has been proposed as a solution for through-metal data transfer without penetrating the structure. The reverberant nature of the through-metal channel, however, can lead to significant intersymbol interference, limiting the data rate achievable by conventional single-carrier communication techniques. In this paper, we describe a through-metal communication technique that exploits the slow-varying nature of the ultrasonic channel to implement an orthogonal-frequency-division-multiplexing-based rate-adaptive peak-to-average power ratio (PAPR) reduction algorithm. Measurements of the proposed adaptive algorithm have demonstrated transmitted throughput rates of up to 14 Mbps while reducing PAPR by up to 3 dB and maintaining a bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">- 5</sup> at average transmit powers of roughly 6 dBm. This enhancement provides the required throughput and error rate to support high-rate network applications in otherwise data-limited environments.

Intersymbol interference cancellation in CDMA 1xEVDO network

SUMMARYThis paper presents the BER performance of the physical layer of the code division multiple access 1x Evolution Data Only (EVDO) standard for different data transmission formats, and shows the improvement on the BER performance of the EVDO system over intersymbol interference channel by adapting the low complexity turbo equalization scheme at the receiver. The radio frequency, channel condition, and mobile user data speeds were extracted from a mobile user with a drive experiment on a live wireless EVDO network. Then, the real data were utilized to simulate the actual mobile user's data speeds in our simulations to show the overall BER performance of the EVDO system application. The simulation results are shown over Gaussian and Proakis A channels and the results indicate that significant intersymbol interference cancellation is obtained with an adapted code division multiple access system and the advantages of turbo equalization on the EVDO system are discussed in this research. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Canceling the ISI Due to Finite S/H Bandwidth in a Circular Buffer Forward Equalizer

In an analog discrete-time circular buffer forward equalizer (FE), finite bandwidth in the sample-and-hold (S/H) circuit can introduce significant intersymbol interference (ISI) that degrades the FE performance. This brief investigates the effect of the finite S/H bandwidth and presents two methods to equalize the ISI introduced by the finite S/H bandwidth. The first method uses a decision-feedback equalizer (DFE) that has nonuniform delays between its taps. The second method uses a DFE that has uniform delays between its taps, allows more time in the first DFE feedback loop, and requires fewer DFE taps at the expense of additional FE taps. Simulation results show that when the proposed methods equalize a 100-m CAT5 cable and a backplane channel, the signal-to-noise ratio improvement is at least 1 and 0.8 dB, respectively, for bit error rates <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> .

Performance of a novel LED lamp arrangement to reduce SNR fluctuation for multi-user visible light communication systems

This paper investigates the performance of our recently proposed LED lamp arrangement to reduce the SNR fluctuation from different locations in the room for multi-user visible light communications. The LED lamp arrangement consists of 4 LED lamps positioned in the corners and 12 LED lamps spread evenly on a circle. Our studies show that the SNR fluctuation under such a LED lamp arrangement is reduced from 14.5 dB to 0.9 dB, which guarantees that users can obtain almost identical communication quality, regardless of their locations. After time domain zero-forcing (ZF) equalization, the BER performances and channel capacities of 100-Mbit/s and 200-Mbit/s bipolar on-off-keying (OOK) signal with most significant inter-symbol interference (ISI) are very close to that of the channel without any ISI caused by this LED lamp arrangement.

Adaptive Channel Estimation for Multiuser Underwater Communication Systems

Problem statement: Multiple Access Interference (MAI) signals and poor estimation of the unknown channel parameters in the presence of limited training sequences are two of the major problems that degrade the systems’ performance. Two synchronous multiuser receivers with Rake reception of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems, in conjunction with channel estimation, are considered for communication over different short range shallow water acoustic channels. Approach: The proposed hard/soft chip channel estimation and carrier phase tracking are jointly optimized based on the Mean Square Error (MSE) criterion and adapted iteratively by the reconstructed MAI signal. This is generated from exchanged soft information in terms of Log-Likelihood Ratio (LLR) estimates from the single-users’ channel decoders. The channel parameters and error estimation are used to enable the chip cancellation process to retrieve an accurate measurement of the detrimental effects of Intersymbol Interference (ISI) and MAI. Results: The performance of the proposed receiver structures with small processing gain are investigated and compare with 2 and 4 synchronous users using memoryless Quadrature Phase-Shift Keying (QPSK) at an effective rate of 439.5 bps per user. Conclusion: The results demonstrate that the performance is limited by MAI and ISI signals and the IDMA performance outperforms long code CDMA and short code CDMA.

Multi-Branch Fractional Multi-Bit Differential Detection of Continuous Phase Modulation with Decision Feedback

Differential detection of continuous phase modulation suffers from significant intersymbol interference. To reduce bit error rate, multi-branch fractional multi-bit differential detection (MFMDD) with decision feed-back is proposed. By introducing decision feedback in multi-bit differential detected signals, severe inter-symbol interference can be removed. Simulation results show that the proposed structure can greatly im-proves the performance compared with MFMDD without decision feedback, and the performance of 9 FMDD is very near to the performance of the coherent detection.