Intersymbol Interference Research Articles (Page 1)

This paper presents the design, implementation, and performance evaluation of an Adaptive Joint SCAMP Filter and Relay Weight Optimization Scheme for a wireless Amplify-and-Forward (AF) cooperative relay network operating over frequency-selective fading channels. Conventional AF systems suffer from compounded noise and Inter-Symbol Interference (ISI) due to cascaded multi-tap channel effects. To address these limitations, this work employs a Joint Adaptive Filtering approach that simultaneously optimizes the source pre-coding filter and the relay amplification weight to minimize the end-to-end Mean Squared Error (MSE) and enhance the achievable data rate. The joint optimization problem is solved using the Projected Subgradient Method (PSGM), which provides robustness against non-linear constraints such as sparsity while maintaining low computational complexity. The algorithm is implemented and tested in a MATLAB simulation environment under a time-varying Auto-Regressive (AR(1)) fading model. Key performance metrics such as MSE convergence, filter characteristics, achievable rate, and robustness to parameter variations are analyzed. Simulation results demonstrate that the proposed adaptive joint scheme achieves 25–33% higher achievable rate than the conventional Fixed AF Relay and nearly double the throughput of a Direct Link transmission. The results validate that adaptive joint filtering provides superior spectral efficiency, improved ISI mitigation, and stable convergence, making it a practical and scalable solution for next-generation cooperative communication systems.

Generalized Frequency Division Multiplexing (GFDM) is a promising non-orthogonal waveform for 5G and beyond but suffers from high out-of-band emissions and inter-symbol interference. This work presents a novel signal processing framework integrating Squared Cosine Pulse Shape Filtering (SCos-PSF), an Extended Hadamard-based Wavelet Transform (Ex-HWT), and Quadrature Enhanced Spatial Modulation (QESM). The Ex-HWT enhances subcarrier separation and data recovery efficiency while reducing computational complexity relative to conventional wavelets. The proposed system, implemented in MATLAB, is evaluated using bit error rate (BER), peak-to-average power ratio (PAPR), and complementary cumulative distribution function (CCDF). Under Rayleigh fading conditions, it achieves a BER of 1.58 × 10−6 at 50 dB SNR, demonstrating robust, high-throughput, and low-latency performance. These results underscore the framework's suitability for real-time 5G and future 6G wireless communication systems.

This paper presents a comprehensive study on the modeling of power line communication (PLC) channels in a MIMO (Multiple Input Multiple Output) environment. PLC systems utilize existing electrical infrastructure to transmit data, but the complexity of these channels, characterized by multipath propagation and inter-symbol interference (ISI), poses significant challenges for designing robust and efficient systems. In this study, we rigorously compare two approaches to PLC channel modelling, which are as follows: the empirical approach and the deterministic approach. The empirical approach relies on the analysis of experimental data to derive statistical models of the channel, offering a realistic representation based on concrete observations. In contrast, the deterministic approach employs theoretical principles and electromagnetic equations to model the channel behavior, providing a detailed description of propagation phenomena. The obtained results demonstrate the significant impact of multipath propagation on the performance of PLC communication systems, highlighting the limitations of empirical models in various scenarios and the increased accuracy of deterministic models. This comparative study conducted in this work highlights the advantages and limitations of each approach and proposes solutions to optimize the performance of power line communication networks.

In this paper, a blind symbol synchronization algorithm is presented for orthogonal frequency-division multiplexing (OFDM) systems, and a timing function based on the redundancy of the cyclic prefix (CP) is introduced. The existing algorithms rely on the prior knowledge of the channel energy distribution i.e. channel power profile. In practical environment the channel power profile is unknown to the receiver and its statistics are expected to be highly changing. Nevertheless, the use of pilot symbols in channel profile estimation reduces efficiency as data subcarriers are used to carry pilots instead of payload. In this paper a timing function that accounts for early and late timing introduced errors together with channel estimation errors is introduced. The effects of symbol timing errors are quantified and an optimal OFDM symbol timing solution is derived using modified maximum likelihood (ML) method. Compared with existing schemes in the literature, the proposed approach does not rely on explicit detection of individual channel paths or the delay spread boundary and therefore greatly reduces timing complexity. The main contribution lies in modifying the ML metric to jointly account for intercarrier interference (ICI), inter symbol interference (ISI), and channel estimation error, leading to improved robustness in dispersive channels without requiring prior channel knowledge. Simulation results show that the proposed algorithm is robust and outperforms the existing CP-based algorithms, particularly in double dispersive channels, achieving up to 5 dB NMSE improvement, lower BER at low SNR, and a 33% reduction in computational complexity

In digital communication, data transmission is subject to distortion caused by physical characteristics such as channel multipath propagation and Doppler frequency shift, as well as noise interference. Among these, inter-symbol interference (ISI) induced by channel dispersion leads to energy overlap between adjacent symbols, severely limiting the reliability and rate of transmission. To enhance the reliability of digital communication and support high-speed transmission, it is essential to overcome the performance limitations of traditional equalization techniques in time-varying multipath channels and effectively suppress distortions such as intersymbol interference and Doppler frequency shift. This paper proposes a hybrid equalizer architecture combining a CNN-Transformer-GRU framework with a cyclic shift preprocessing technique. This model generates an 18-dimensional feature matrix by performing nine groups of cyclic shifts on the received signal, transforming the time-varying channel features into a spatial dimension representation. By combining CNN local feature extraction, Transformer global dependency modelling, and GRU dynamic temporal adjustment, efficient channel equalization is achieved. Experimental results carried out under the SUI-3 channel model indicate that, on the premise of the same bit error rate (BER), the proposed model achieves a signal-to-noise ratio (SNR) gain of approximately 1–8 dB and 1–6 dB in comparison with the traditional decision feedback equalizer (DFE) and the temporal model LSTM, respectively, and improves the performance by 1–3 dB compared with the CRNN-RES (Convolutional recurrent neural network with ResNet) model, while maintaining a robustness of 2–8 dB. The parallel computation of transformers improves training efficiency by 30%, while the GRU architecture reduces the number of parameters by 20%. This architecture provides a high-precision, low-complexity solution for high-speed dynamic channel equalization in 5 G/6G systems.

In this work, we consider a three-dimensional slow diffusive heterogeneous media-based mobile molecular communication (MC) system, with the communicating devices as point transmitters and passive spherical-shaped receiver nanomachines (NMs). For the considered slow diffusive MC system, we propose a time-varying stochastic diffusivity-based model for communicating devices and information-carrying molecules, and we characterize the mobile MC channel by the channel impulse response (CIR) and derive its mean. For the considered slow and stochastic diffusivity-based mobile MC system, we propose a novel silence-based multi-type hybrid transmission scheme, which combines communication through silence (CtS) with molecular shift keying (MoSK) and concentration shift keying (CSK) and we derive the closed-form expression for the average probability of error. For the slow diffusive environment, we compare the proposed transmission scheme with the position and concentration-based run-length aware, MoSK, and CSK transmission schemes. For the proposed silence-based multi-type hybrid and considered position and concentration-based run-length aware transmission schemes, we design their respective optimal threshold detectors. The proposed scheme outperforms and shows robust behavior in the presence of inter-symbol interference.

This work presents an 80 Gbps photonics-aided millimeter-wave (mm Wave) wireless communication system employing 16-Quadrature Amplitude Modulation (16-QAM) and a 1 × 2 single-input multiple-output (SIMO) architecture with maximum ratio combining (MRC) to achieve robust 87.5 GHz transmission over 4.6 km. By utilizing polarization-diverse optical heterodyne generation and spatial diversity reception, the system enhances spectral efficiency while addressing the low signal-to-noise ratio (SNR) and channel distortions inherent in long-haul links. A blind equalization scheme combining the constant modulus algorithm (CMA) and decision-directed least mean squares (DD-LMS) filtering enables rapid convergence and suppresses residual inter-symbol interference, effectively mitigating polarization drift and phase noise. The experimental results demonstrate an SNR gain of approximately 3 dB and a significant bit error rate (BER) reduction with MRC compared to single-antenna reception, along with improved SNR performance in multi-antenna configurations. The synergy of photonic mm Wave generation, adaptive spatial diversity, and pilot-free digital signal processing (DSP) establishes a robust framework for high-capacity wireless fronthaul, overcoming atmospheric attenuation and dynamic impairments. This approach highlights the viability of 16-QAM in next-generation ultra-high-speed networks (6G/7G), balancing high data rates with resilient performance under channel degradation.

Abstract With increasing demands for secure high-speed wireless communications, integration of quantum key distribution (QKD) into light fidelity (LiFi) systems is a promising method for boosting data confidentiality in optical wireless networks. The BB84 protocol and Mach–Zehnder modulator (MZM) are utilized with a pump-assisted continuous wave laser at 1,550 nm and polarization control components to encode quantum states onto photons. They are transmitted on free space optical (FSO) links with a length ranging from 2 km to 8 km and received using Avalanche photodiodes (APDs) and optical filters. Performance analysis conducted using bit error rate (BER) and Q-factor analysis. BER maintained at zero across all tested configuration and transmission distance up to 8 km indicating excellent signal integrity and security. A highest Q-factor achieved a maximum of 3,055.9, which ensures signal integrity and low interference noise. The system operates at a simulated data rate of 10 Gbps, suitable for high speed optical wireless communication. Consistently high eye height (∼172,210 units) and stable decision thresholds confirm minimal inter-symbol interference and accurate symbol recognition. And the Poincaré sphere showed even polarization stability for different optical power levels. This work presents a simulation-based implementation of a polarization encoded QKD system over a LiFi-enabled FSO communication channel, aiming to enhance secure optical wireless communication. Calculated based on quantum bit error rate (QBER) and included in the theoretical analysis to demonstrate practical secure throughput.

Faster-than-Nyquist (FTN) signaling is deemed a promising candidate for enabling high spectral efficiency in next-generation optical data center interconnects (DCIs). Trellis-based sequence detectors, such as maximum likelihood sequence estimation (MLSE) and maximum a posteriori (MAP) sequence estimation, operating with long memory lengths, are by far the most commonly adopted strategy for deciphering the original symbol from severe inter-symbol interference (ISI). Nevertheless, the inherent high complexity of this approach poses significant challenges to its practical implementation. In this paper, a partial-response decision feedback equalizer (PR-DFE) is proposed to realize ultra-low complexity and high-performance FTN signaling. The fundamental principle of the proposed scheme involves combining the transmitter-side precoding and receiver-side partial response equalization criteria, which significantly reduces the occurrence of the error propagation issue of the typical DFE in the FTN system. The effectiveness of the proposed scheme is experimentally verified in a 56Gbaud FTN 4-ary pulse amplitude modulation (PAM4) system with a compression factor of 0.8. According to the experimental results, PR-DFE achieves 2.4 dB and 1.5 dB receiver sensitivity improvement compared to the typical DFE and PR-FFE at a bit-error-rate (BER) of 4.5 × 10-3 (BER threshold of 6.7% overhead staircase hard-decision forward error correction code). Moreover, the implementation of the PR-DFE has successfully reduced the probability of two consecutive symbol errors from 4.83 × 10-3 to 0 compared to the typical DFE. According to experimental results, the proposed PR-DFE FTN signaling scheme is a promising candidate for ultra-low complexity and high spectral efficiency intra-datacenter interconnect (intra-DCI) applications.

Owing to digital signal processing, 50G passive optical networks (PON) in cost-sensitive scenarios can be implemented utilizing low-cost devices. However, the limited bandwidth of low-cost devices degrades the bit error ratio performance. Meanwhile, although 50G PON operates in the O band with a low chromatic dispersion (CD) coefficient, the cumulative CD cannot be ignored for a 50G on-off keying signal, especially in the 40km access scenario. This paper proposes a simplified Turbo equalizer based on maximum-log-maximum a posteriori (Max-Log-MAP) module for 50G PON to deal with the inter-symbol interference (ISI) caused by the low-cost devices and CD. The Max-Log-MAP module replaces the complicated logarithms and exponents with simple comparisons without degrading performance, greatly decreasing the complexity compared to the conventional Log-MAP module. The experimental results show that the receiver sensitivity improvement of approximately 0.1dB, 0.25dB, and 0.5dB can be obtained by the simplified Turbo equalizer for 0km, 20km, and 40km access scenarios, respectively. Therefore, the simplified Turbo equalizer shows great promise for the bandwidth-limited 50G PON.

Measurement While Drilling (MWD) technology plays a significant role in enhancing the geological steering and subsurface evaluation capabilities of extended-reach wells, challenging horizontal wells, and multilateral wells. With the increasing complexity of underground exploration, there is a heightened demand for the continuous wave mud pulse data transmission capacity. To address the inter-symbol interference(ISI) caused by the inherent inertia of the motor during high-speed data transmission using traditional modulation methods such as Frequency Shift Keying (FSK) and Phase Shift Keying (PSK), a novel approach has been proposed. This method employs Continuous Gradation Frequency Keying (CGFK) modulation combined with Convolution Neural Network (CNN) demodulation for continuous mud pulse data transmission. By controlling the waveform frequency to uniformly increase from zero to a predetermined value and then uniformly decrease back to zero within a symbol period, and utilizing the rate of frequency change as the feature for modulation and demodulation, this method effectively mitigates the issue of ISI caused by the motor’s inability to rapidly switch to the next speed due to its inertia during symbol transitions. Simulation tests indicate that, compared to the traditional Matched Filter method, Support Vector Machine (SVM), Long Short-Term Memory (LSTM) networks, and CNN exhibit superior performance in recognizing CGFK, with CNN demonstrating the best results. Physical tests show that CGFK, particularly when assisted by CNN demodulation, possesses the capability to avoid or reduce ISI caused by motor inertia, and achieves favorable information transmission rates and bit error rate (BER) compared to traditional FSK and PSK.

The plasma sheath surrounding high hypersonic vehicles traveling through the atmosphere can cause communication interruption, known as blackout. In this study, a multiphysics numerical solution model coupling the electromagnetic field and the plasma fluid field is proposed. The time-domain distortion of broadband digitally modulated signals in plasma sheaths is analyzed for the first time. The effect of plasma sheaths on communication systems is analyzed by studying the typical communication scheme for high hypersonic vehicles. Moreover, the modulation effect of a typical communication signal in plasma is studied. The results show that, although signals can reach the receiver, delay spread and frequency-selective fading occur, resulting in inter-symbol interference. Time-domain distortions such as sampling time offset and waveform envelope broadening are found to be important mechanisms leading to errors. It can cause synchronization problems and significantly increase the bit error rate of communication systems. Finally, a new design method for high hypersonic vehicle communication systems is proposed. Using the numerical solution model proposed earlier, frequency-domain compensation and time synchronization mechanisms are established to improve the receiver performance of the communication system.

ABSTRACTIn this work, we propose a novel Data Peninsular‐OTFS (DP‐OTFS) waveform technique to provide better resilience against intersymbol and intercarrier interferences in high‐mobility, doubly dispersive channels. We design the transmit symbol mapping in such a way that the circular convolution relationship is maintained between the transmit and receive grids in delay‐Doppler (DD) domain. This ensures the channel estimation and equalization to be performed directly in the DD domain with reduced complexity. To improve the channel estimation accuracy, we propose oversampling the receive grid in the DD domain. Further, by visualizing the transmit grid as a 2D image and the doubly dispersive channel as a point spread function (PSF), we propose a Wiener deconvolution‐based equalizer with reduced complexity. Simulation results are presented to validate the improvement in the bit error rate (BER) and normalized mean square error (NMSE) performances of the proposed DP‐OTFS system. We show that the proposed DP‐OTFS grid structure with oversampling and Wiener deconvolution equalizer proves to be efficient by decreasing the NMSE by about 10 to 100 times and improving the SNR by about 10 dB.

Secure transmission of Electrocardiogram (ECG) signals is critical in modern health monitoring systems, where protecting patient confidentiality and ensuring data integrity are essential. This study presents a hardware-efficient approach that integrates Lightweight Cryptography (LWC) with a Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) framework to protect ECG signal transmission. The proposed method leverages minimal logic resources and a simplified key schedule to achieve robust security with low computational overhead. To further improve reliability, turbo coding is applied to combat burst errors and Inter-Symbol Interference (ISI) inherent in wireless channels. Performance evaluation of the LWC-MIMO-OFDM system is carried out using ECG signals from the MIT Arrhythmia Database, with implementation and testing on a Spartan-6 FPGA platform. Results demonstrate the effectiveness of the proposed architecture in terms of area utilization, latency, power efficiency, and overall security performance, making it suitable for secure biomedical applications.

Visible Light Communications (VLC) is a promising communication technology designed to serve as a supplemental system to radio frequency communications for many demanding applications. VLC employs optical orthogonal frequency division multiplexing (O-OFDM) techniques to achieve higher data rates and resist intersymbol interference. Among these, asymmetrically clipped O-OFDM (ACO-OFDM) stands out as one of the most power-efficient and high-performing methods for VLC. However, like other O-OFDM techniques, ACO-OFDM encounters the challenge of a high peak-to-average power ratio (PAPR). VLC utilizes light-emitting diodes (LEDs) for data transmission at the transmitter and photodiodes (PDs) for data reception at the receiver. However, the high PAPR issue can drive LEDs to operate in their nonlinear region, reducing system performance and potentially causing LED damage or burnout. The paper aims to alleviate the high PAPR issue by proposing a hybrid combination of precoding technique and various nonlinear companding techniques. Compared to the standard ACO-OFDM, the proposed methodologies can reduce the PAPR issue by 5.5796 dB without a bit error rate (BER) degradation. The paper also aims to improve another aspect of the BER performance of ACO-OFDM. The paper proposes two noise cancellation receiver models to enhance the performance by 2.275 dB compared to standard ACO-OFDM to attain the same BER performance. The proposed PAPR reduction methodology with noise cancellation receiver model can achieve PAPR reduction by 6.0057 dB and enhanced performance by -0.309 dB to achieve the BER performance relative to standard ACO-OFDM. This paper systematically evaluates the proposed PAPR reduction approach by comparing it with established methods from the literature, ensuring a comprehensive assessment of its effectiveness and reliability.

To improve the reliability of information reception under the conditions of inter-symbol interference and inter-channel interference on the background of additive channel noise, it is proposed to use a correlator as an optimal receiver. The reference signal of the correlator is determined by minimizing the variance of the total output interference of the correlator at the reference moment of time, which is the sum of the square of the E-criterion and the variance of the output interference due to the action of additive channel noise. Calculations are given for the case of a reference signal, which is the product of the received channel symbol and a periodic signal. It is shown that there is a critical value of the signal-to-noise ratio, exceeding which allows to reduce the probability of error, compared to the classical method of selecting the reference signal. It is shown that the “price” for the simplicity of forming a periodic reference signal is an increase in the probability of error, compared to the case when the optimal reference signal is used.

To fully address the inter-symbol interference (ISI) for ultra-high bandwidth efficiency (BE) application, a hybrid integrated forward error correction (FEC) equalization (IFE) scheme is proposed and experimentally demonstrated in a coherent optical interconnection system. The estimation for the channel response affected with severe ISI is realized by an adaptive linear minimum mean square error (LMMSE) equalizer, assisted with soft-input soft-output (SISO) a posteriori information by low density parity check (LDPC) decoder, allowing simultaneously accurate ISI deduction and reliable FEC decoding. The proof-of-concept experiments are investigated in a 60 GBaud polarization division multiplexed 16 quadrature amplitude modulation system within a 5-GHz ultra-narrow bandwidth receiver. The results show that the proposed IFE scheme successfully obtains more than 3-dB optical signal-to-noise ratio (OSNR) gain compared with the serial, independent structure of the equalizer and the FEC decoder at the BER threshold of 1 × 10-2. The validity comparison of ISI reduction ability between traditional soft-decision maximum likelihood sequence estimation (MLSE) and the proposed IFE scheme has been experimentally investigated, with the proposed scheme demonstrating greater robustness.

To effectively mitigate the impairments of severe inter-symbol interference (ISI), a least-square-assisted Bahl, Cocke, Jelinek, and Raviv (LS-BCJR) detection algorithm is proposed and experimentally demonstrated in a bandwidth-limited intensity-modulation direct-detection (IM/DD) pulse amplitude modulation (PAM4) optical interconnection system. A training-based short-tap LS equalizer is utilized after the 1 + D post filter to dynamically reconstruct an effective channel response, thus assisting the BCJR estimator to optimize the transition metric calculation and obtain more accurate probabilistic information from the Markov for soft-symbol decision. The experimental results reveal the enhanced system performance with the 160 Gb/s PAM4 signals transmitted within a 19-GHz system bandwidth. The proposed LS-BCJR scheme improves the received optical power sensitivity by 1.2 and 0.9 dB under the validity comparison of the maximum likelihood sequence estimation (MLSE) and conventional BCJR schemes at the BER threshold of 2.4e-2. Furthermore, the LS-BCJR achieves the optimal receiver sensitivity improvement by 3 dB at the BER threshold of 3.8e-3, demonstrating a notable improvement over the conventional BCJR scheme.

In wireless communication, FSO promises to provide high-speed communication under extremely vulnerable atmospheric conditions. This study evaluates the effect of system design parameters for moderate rain conditions by simulating the proposed model using Optisystem 22.0. In this work, the divergence angle and the aperture diameter were varied, and their effects on the system were observed in terms of the signal quality, Bits in error, and the receiver's optical power. The findings show that increasing the divergence angle degrades system performance, whereas an increase in the receiver’s aperture diameter improves the signal quality at the receiver side. The observed eye diagram reveals that the received signal quality is improved as it has less Inter-Symbol Interference.

Inter-Symbol Interference Mitigation Using Adaptive Dataset Selection in the Parzen Window Method