Presence Of Intersymbol Interference Research Articles

Pulse Shaping for MC via Particle Size

In molecular communication (MC), combining different types of particles at the transmitter is a degree of freedom which can be utilized to improve performance. In this paper, we address the problem of pulse shaping to simplify time synchronization requirements by exploiting and combining the received signal characteristics of particles of different sizes. In particular, we optimize the mixture of particles of different sizes used for transmission in order to support a prescribed detection time period for on-off keying, guaranteeing on average 1) a sufficiently large received signal if a binary one is transmitted, and 2) a low enough received signal if a binary zero is transmitted even in the presence of inter-symbol interference. For illustration, we consider an optimization problem based on a free space diffusion channel model. It is shown that there is a tradeoff between the maximum feasible detection duration and the peak detection value for different particle sizes from the smallest particle size enabling the largest detection duration to the largest particle size minimizing the peak detection value at the expense of a limited detection duration.

Performance Analysis of Turbo Codes, LDPC Codes, and Polar Codes over an AWGN Channel in the Presence of Inter Symbol Interference.

This paper discusses the results of simulations relating to the performances of turbo codes, low density parity check (LDPC) codes, and polar codes over an additive white Gaussian noise (AWGN) channel in the presence of inter symbol interference, denoting the disturbances that altered the original signal. To eliminate the negative effects of inter symbol interference (ISI), an equalizer was used at the level of the receiver. Practically, two types of equalizers were used: zero forcing (ZF) and minimum mean square error (MMSE), considering the case of perfect channel estimation and the case of estimation using the least square algorithm. The performance measure used was the modification of the bit error rate compared to a given signal to noise ratio; in this sense, the MMSE equalizer offered a higher performance than the ZF equalizer. The aspect of channel equalization considered here is not novel, but there have been very few works that dealt with equalization in the context of the use of turbo codes, especially LDPC codes and polar codes for channel coding. In this respect, this research can be considered a contribution to the field of digital communications.

On the Design of Channel Coding Autoencoders With Arbitrary Rates for ISI Channels

This letter presents an autoencoder-based channel coding scheme in the presence of inter-symbol interference (ISI) and additive white Gaussian noise (AWGN), supporting arbitrary coding rates. Both the transmitter and receiver of the proposed autoencoder employ bi-directional gated recurrent unit (Bi-GRU) layers. Additional extra dense layers are applied at the end of the transmitter and at the beginning of the receiver, serving as learnable puncture and depuncture modules, respectively. Different code rates can be achieved by adjusting the output dimension of the extra dense layers. Experimental results demonstrate that the proposed autoencoder significantly outperforms conventional convolutional codes over ISI channels, for multiple code rates. The proposed autoencoder also outperforms LDPC codes in the low signal-to-noise ratio (SNR) regime. The neural codes still require improvement to be competitive in the high SNR regime.

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.

Time-Frequency Compressed FTN Signaling: A Solution to Spectrally Efficient Single-Carrier System

Faster-than-Nyquist signaling (FTNS) is capable of improving the spectral efficiency (SE) of communication systems. However, for conventional single-carrier FTNS (SC-FTNS) in which only symbol interval is reduced, the increase of SE is very limited due to the presence of inter-symbol interference (ISI) introduced by the FTNS. To deal with this problem, this paper proposes a new time-frequency compressed SC-FTNS (TFC-SC-FTNS) scheme that includes the conventional FTNS as a special case, to improve the SE via two dimensions simultaneously: time dimension by stacking symbols closer; frequency dimension by precoding to make the FTN signal spectrum more compact. Further, an optimization subject to a spectral mask constraint is performed on the precoder to suppress the ISI, according to a mean-square-error criterion, but the optimization problem is non-convex. A nontrivial contribution in the new scheme is that the non-convex problem is transformed into a convex one by a change of variable and an addition of admissibility constraint. Simulation results demonstrate that the proposed scheme significantly outperforms the conventional FTNS in terms of achievable SE or, equivalently, reception performance at a given SE. Further, with larger constellations applied, the gains of the TFC-FTNS increase.

Dynamic evolving neural fuzzy inference system equalization scheme in mode division multiplexer for optical fiber transmission

The performance of optical mode division multiplexer (MDM) is affected by inter-symbol interference (ISI), which arises from higher-order mode coupling and modal dispersion in multimode fiber (MMF). Existing equalization algorithms in MDM can mitigate linear channel impairments, but cannot tackle nonlinear channel impairments accurately. Therefore, mitigating the noise in the received signal of MDM in the presence of ISI to recover the transmitted signal is important issue. This paper aims at controlling the broadening of the signal from MDM and minimizing the undesirable noise among channels. A dynamic evolving neural fuzzy inference system (DENFIS) equalization scheme has been used to achieve this objective. Results illustrate that nonlinear DENFIS equalization scheme can improve the received distorted signal from an MDM with better accuracy than previous linear equalization schemes such as recursive‐least‐square (RLS) algorithm. Desirably, this effect allows faster data transmission rate in MDM. Additionally, the successful offline implementation of DENFIS equalization in MDM encourages future online implementation of DENFIS equalization in embedded optical systems.

On Flow-Induced Diffusive Mobile Molecular Communication: First Hitting Time and Performance Analysis

This paper investigates the problem of flow-induced diffusive molecular communication under various mobility conditions, such as: 1) both transmitter (TX) and receiver (RX) nanomachines are mobile; 2) TX is mobile and RX is fixed; and 3) TX is fixed and RX is mobile. In particular, a 1-D environment with a point TX and a fully absorbing point RX is considered. Closed-form expressions for the probability density function (PDF) of the first hitting time under the aforementioned mobile scenarios are derived, by characterizing the movement of the nanomachines and information molecules using Brownian motion with positive drift. The derived PDF expressions are validated through particle-based simulations. Based on these results, the performance of molecular communication with on-off keying (OOK) modulation in flow-induced diffusive channels is investigated. In particular, closed-form expressions for the probabilities of detection and false alarm with likelihood ratio test (LRT)-based decision rule, probability of error, and the achievable rate in the presence of inter-symbol interference, counting errors, and noise from the other sources are derived. Simulation results are presented to verify the theoretical results and to yield insights into the system performance for different mobility conditions.

Experimental study of performance enhanced IM/DD transmissions combining 4D Trellis coded modulation with precoding.

We propose to combine 4D trellis-coded modulation (TCM) with transmitter-side Tomlinson-Harashima precoding (THP) in IM/DD transmissions, and experimentally investigate the achieved performance improvement. Theoretically, THP can approximately produce an end-to-end additive white Gaussian noise (AWGN) channel even with severe bandwidth limitation, allowing TCM to maintain its coding gain in the presence of inter-symbol interference. In our experiments with off-the-shelf commercial components, which limit the 3 dB bandwidth of the system to be ~3.5 GHz, the combination of TCM and THP shows a better receiver sensitivity for various system bit rates from 56 Gbit/s to 112 Gbit/s, considering the KP4 threshold of BER = 2 × 10-4. In the 112 Gbit/s back-to-back (B2B) transmission, with the help of THP the receiver sensitivity is improved by 3.3 dB using 4D-PAM4 TCM at the KP4 FEC threshold compared with using conventional PAM4. In addition, combining TCM and THP also helps to lower the BER floor.

Novel Spectral Efficient Technique for MIMO-OFDM Channel Estimation with Reference to PAPR and BER Analysis

With emerge of increasing research in domain of future wireless communications, massive multiple input multiple output (MIMO) attracted most of researchers interests. Massive MIMO is nothing but high speed wireless communication standards. The performance of MIMO systems is based on techniques used for channel estimation. Efficient channel estimation leads to spectral efficient wireless communications. There are number of channel estimation techniques presented recently in literature with pros and cons. The recent method shows the spectral and bit error rate (BER) efficiency, however apart from this, there is need of improving the peak to average power ratio (PAPR). Recently we proposed, novel channel estimation method as the existing channel estimation techniques failed to effectively solve the inter symbol interference (ISI) problem. The presence of ISI in MIMO-OFDM may leads to worst performance. Our proposed blind channel estimation is combined with independent component analysis (ICA) hence this method is called as hybrid ICA (HICA) to minimize the ISI effect. The extensive simulation analysis of proposed HICA required to claiming the scalability as well as reliability. In this paper, proposed study on additional performance metrics such as PAPR and computational costs (energy) along with BER and spectral efficiency performances. The result claims that HICA is not improving the PAPR and energy performances significantly.

Mutual Information and Maximum Achievable Rate for Mobile Molecular Communication Systems.

Molecular communication (MC) enables conveying information at nano- to micro-scales via molecules or nanoscale particles. MC systems for fixed transmitter and receiver nanomachines have been extensively investigated. However, the scenarios for mobile MC are seldomly studied. In the paper, the mutual information and achievable rate in the presence of inter-symbol interference and noise for the mobile MC is investigated. The scenario that both the transmitter and the receiver are in independent random motions is considered. Due to the mobility, the channel impulse response varies and results in a varying received signal with varying signal-dependent noise. This further leads to the random variation of the probability density functions of received concentration for symbols which further generates varying error probability and therefore varying mutual information and maximum achievable rate. In this paper, the mutual information and maximum achievable rate for mobile MC scenario are derived and analyzed. The impacts of different parameters, such as the initial transmitter-receiver distance, the number of released molecules, symbol interval, the mobility of nanomachine, and priori probabilities of the transmitted symbol on the mutual information and maximum achievable rate in mobile MC scenario are studied by simulation investigations.

Abnormality Detection Inside Blood Vessels With Mobile Nanomachines

Motivated by the numerous healthcare applications of molecular communication within Internet of Bio-Nano Things (IoBNT) paradigm, this paper addresses the problem of abnormality detection in a blood vessel using multiple biological embedded computing devices called cooperative biological nanomachines (CNs), and a common receiver called the fusion center (FC). Due to blood flow inside a vessel, each CN and the FC are assumed to be mobile. In this paper, each CN performs abnormality detection with certain probabilities of detection and false alarm. The CNs report their local decisions to an FC over a diffusion-advection blood flow channel using different types of molecules in the presence of inter-symbol interference, multi-source interference, and counting errors. The FC employs the OR and AND logic-based fusion rules to make the final decision after decoding the local decisions using the sub-optimal detectors based on the approximation of the log-likelihood ratio. For the aforementioned system, probabilities of detection, and false alarm at the FC are derived. Finally, simulation results are presented to validate the derived analytical results, which provide important insights.

Concatenated Coding Schemes for High Areal Density Bit-Patterned Media Magnetic Recording

This paper proposes a new concatenated error correction coding scheme for 2-D magnetic recording (TDMR). The designed concatenated coding scheme consists of outer codes as binary polar codes and an inner code as non-binary low-density parity-check (LDPC) code. These error-correcting codes (ECCs) are considered to achieve large coding gains with small bandwidth expansion in the presence of intersymbol interference and inter-track interference. Polar codes have attracted much attention as one of the ECCs that provably achieve the optimal symmetric capacity of memoryless channels with an explicit construction and are decoded by the successive-cancellation decoding algorithm, whose computational complexity is comparatively low. Using systematic polar codes, we are able to design proper low-complexity ECCs with run-length-limited constraints for any TDMR schemes based on the selection criterion (which is called channel polarization ) for choosing parity and information symbols by the magnitude of the approximately calculated mutual information amount regarding each recorded symbol in a codeword. Concretely, the concatenated coding schemes are designed for four-track and three-track recording, and the block error rate (BER) performances of 2-D partial response equalized signal processing schemes with proposed concatenated ECCs for bit-patterned media recording are evaluated. As a result, it shows that the BER performances of the proposed schemes are superior to that of the conventional scheme using a 1-D high rate binary LDPC code.

Multi-Level Read/Write Signaling Using Pulse Width Modulation for High Density Perpendicular Magnetic Recording

In this letter, we propose a novel read/write (R/W) signal processing method by 3-ary amplitude shift keying for high density perpendicular magnetic recording. We also provide a 3B2T modulation to generate 3-ary signal, which will be converted to a pulse width modulation (PWM) signal, and then recorded on magnetic media. The PWM signal can be simply demodulated to derive the 3-ary signal through a low-pass filter at readback side. The demodulated signal in the presence of intersymbol interference is detected by the Viterbi algorithm as partial response signal. Nonbinary low-density parity-check codes over GF(8) are applied as error correcting codes in our evaluation. Our simulation result shows the proposed R/W method has +4% more user bit density gain than conventional binary recording.

Information theoretical performance analysis and optimisation of cooperative underwater acoustic communication systems

In this study, the authors investigate the information theoretical limits on the performance of single-carrier cooperative underwater acoustic communication systems in the presence of intersymbol interference. The authors assume decode-and-forward relaying and consider orthogonal half-duplex cooperation. Under the assumptions of sparse and frequency-selective Rician fading channel and non-white correlated Gaussian ambient noise, the authors derive expressions for bounds on the individual and achievable rates for both cases where channel state information is available at the transmitter or not. Using these expressions, the authors optimise input signalling and relay location to maximise average achievable rates.

Approximate Joint MAP Detection of Co-Channel Signals in Non-Gaussian Noise

Detection of co-channel signals is important as wireless communication systems become increasingly dense. A particularly challenging case is single antenna reception in both the presence of inter-symbol interference and co-channel interference. Optimal joint maximum a posteriori probability (MAP) detection of the co-channel signals is prohibitively complex. Therefore, in this paper, we propose a factor graph-based iterative receiver which approximates joint MAP detection. In the receiver, noise is modeled with a Gaussian mixture distribution since studies have shown that the noise affecting wireless communication systems is often impulsive. Furthermore, the parameters of the factor graph model (channel and noise parameters) are iteratively estimated within the receiver. The proposed receiver is shown to the outperform state-of-the-art receiver algorithms while having a lower complexity in both Gaussian and impulsive noise. We also show significant gains from iterative parameter estimation, especially in non-Gaussian noise.

Channel estimation and equalization based on origin analysis

The presence of inter-symbol interference (ISI) can significantly degrade the performance of digital communication systems. Generally, an ISI channel can be modeled as a time-varying FIR filter. The ISI effect can thus be removed by inversefiltering the distorted signal at the receiver. This paper, however, presents a different approach, which, other than recovering the source signal directly, attempts to adapt the decision rule in accordance with the channel statistics, so as to retrieve the loss of BER performance caused by ISI.

On the Use of Gaussian Approximation in Analyzing the Performance of Optical Receivers

The analytical calculation of the bit error rate (BER) of digital optical receivers that employ avalanche photodiodes (APDs) is challenging due to 1) the stochastic nature of the avalanche photodiode's impulse-response function and 2) the presence of intersymbol interference (ISI). At ultrafast transmission rates, ISI becomes a dominant component of the BER, and its effect on the BER should be carefully addressed. One solution to this problem, termed the bit-pattern-dependent (PD) approach, is to first calculate the conditional BER given a specific bit pattern and then average over all possible bit patterns. Alternatively, a simplifying method, termed the bit-pattern-independent (PI) approach, has been commonly used whereby the average bit stream is used to calculate the distribution of the receiver output, which, in turn, is used to calculate the BER. However, when ISI is dominant, the PI approximation is inaccurate. Here, the two approaches are analytically compared by analyzing their asymptotic behavior and their bounds. Conditions are found to determine when the PI method overestimates the BER. The BER found using the PD method exponentially decays with the received optical power, whereas for the PI approach, the BER converges to a constant, which is unrealistic. For an InP-based APD receiver with a 100-nm multiplication layer, the PI method is found to be inaccurate for transmission rates beyond 20 Gb/s.

Intersymbol Interference Reduction and Bit Error Rate Reduction in Wireless Channels Using Zero Forcing Equalizer

In telecommunication, Intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISI, and thereby deliver the digital data to its destination with the smallest error rate possible. In this paper, we design a zero forcing equalizer, which is a type of linear equalizer used to mitigate the effects of ISI (Intersymbol Interference). The zero forcing equalizer does this by finding the inverse of the impulse response of the channel. The ISI can be studied using eye diagram, which overlays many samples of a signal and can give a graphical representation of the signal characteristics. In this paper the results have been shown with proper eye diagrams. In this paper we also investigate the bit error rate, which is used to estimate the performance characteristics of the zero forcing equalizer.

Iterative Joint Estimation Procedure for Channel and Frequency Offset in Multi-Antenna OFDM Systems With an Insufficient Cyclic Prefix

This paper addresses a strategy to improve the joint channel and frequency offset (FO) estimation in multi-antenna systems, widely known as multiple-input–multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM), in the presence of intersymbol interference (ISI) and intercarrier interference (ICI) occasioned by an insufficient cyclic prefix (CP). The enhancement is attained by the use of an iterative joint estimation procedure (IJEP) that successively cancels the interferences located in the preamble of the OFDM frame, which is used for the joint estimation and initially contains the interferences due to a CP shorter than the channel length. The IJEP requires at certain steps a proper iterative interference cancellation algorithm, which makes use of an initial FO compensation and channel estimation obtained due to the use of a symmetric sequence in the preamble. After the iterative cancellation of interferences, the procedure performs an additional joint channel and FO estimation whose mean square error converges to the Cramér–Rao bound (CRB). Later on, this subsequent joint estimation permits the removal of the interferences in the data part of the frame, which are also due to an insufficient CP, in the same iterative fashion but saving iterations compared with the use of other estimation strategies. The appraisal of the procedure has been performed by assessing the convergence of the simulated estimators to the CRB as a function of the number of iterations. Additionally, simulations for the evaluation of the bit error rate (BER) have been carried out to probe how the utilization of the proposed IJEP clearly improves the performance of the system. It is concluded that, with a reduced number of iterations in the preamble, the IJEP converges to the theoretical bounds, thus reducing the disturbances caused by a hard wireless channel or a deliberately insufficient CP. Therefore, this eases the interference cancellation in the data part, leading to an improvement in the BER that approximates to the ideal case of a sufficient CP and, consequently, an improvement in the computational cost of the whole procedure that has been analyzed.

Mixed orthogonal frequency coded SAW RFID tags

Orthogonal frequency coded (OFC) SAW radio-frequency identification (RFID) tags are currently being explored as a multi-sensor platform because of their passive spread-spectrum operation, low loss, and resilience in harsh environments. Ongoing research continues to search for robust device embodiments that increase the number of identifiable codes, in the presence of intersymbol interference, while maintaining reasonable device lengths. This paper presents a technique that shortens the SAW response length while preserving code diversity and bandwidth by utilizing a multi-track SAW configuration. These new devices allow the time response of multiple OFC chips to overlap and yield a mixed-frequency chip having the sum of the chip bandwidths but shorter overall time response. The theoretical development is presented and examples are discussed for these new mixed orthogonal frequency coded (MOFC) SAW devices. Experimental results for MOFC sensors, fabricated on YZ-LiNbO3, with a 7% fractional bandwidth and five chip frequencies in three cells, provide a good contrast to similar OFC designs. Experimental results are presented for the simultaneous operation of eight wireless temperature sensors--four OFC and four MOFC--in a 915-MHz wireless correlator receiver system, highlighting the ability of these devices to operate in the same system.