Dispersive Channels Research Articles

Coherent Multiple-Antenna Block-Fading Channels at Finite Blocklength

In this paper, we consider a channel model that is often used to describe mobile wireless scenarios: multiple-antenna additive white Gaussian noise channels subject to random (fading) gains with full channel state information at the receiver. The dynamics of the fading process are approximated by a piecewise-constant process (frequency non-selective isotropic block fading). This paper addresses the finite blocklength fundamental limits of this channel model. Specifically, we give a formula for the channel dispersion—a quantity governing the delay required to achieve capacity. The multiplicative nature of the fading disturbance leads to a number of interesting technical difficulties that required us to enhance traditional methods for finding the channel dispersion. Alas, one difficulty remains: the converse (impossibility) part of our result holds under an extra constraint on the growth of the peak-power with blocklength. Our results demonstrate, for example, that while the capacities of $n_{t}\times n_{r}$ and $n_{r} \times n_{t}$ antenna configurations coincide (under fixed received power), the coding delay can be sensitive to this switch. For example, at the received SNR of 20 dB, the $16\times 100$ system achieves capacity with codes of length (delay) which is only 60% of the length required for the $100\times 16$ system. Another interesting implication is that for the MISO channel, the dispersion-optimal coding schemes require employing orthogonal designs such as Alamouti’s scheme—a surprising observation considering the fact that Alamouti’s scheme was designed for reducing demodulation errors, not improving coding rate. Finding these dispersion-optimal coding schemes naturally gives a criteria for producing orthogonal design-like inputs in dimensions where orthogonal designs do not exist.

Practical Pulse-Shaping Waveforms for Reduced-Cyclic-Prefix OTFS

In this paper, we model $M\times N$ orthogonal time frequency space modulation (OTFS) over a $P$ -path doubly dispersive channel with delays less than $\tau _{\max }$ and Doppler shifts in the range $(\nu _{\min },\nu _{\max })$ . We first derive in a simple matrix form the input–output relation in the delay-Doppler domain for practical (e.g., rectangular) pulse-shaping waveforms, next generalize it to arbitrary waveforms. This relation extends the original OTFS input–output approach, which assumes ideal pulse-shaping waveforms that are bi-orthogonal in both time and frequency. We show that the OTFS input–output relation has a simple sparse structure that enables one to use low-complexity detection algorithms. Different from previous work, only a single cyclic prefix is added at the end of the OTFS frame, significantly reducing the overhead, without incurring any penalty from the loss of bi-orthogonality of the pulse-shaping waveforms. Finally, we compare the OTFS performance with different pulse-shaping waveforms, and show that the reduction of out-of-band power may introduce nonuniform channel gains for the transmitted symbols, thus impairing the overall error performance.

Spectrally Augmented Hartley Transform Precoded Asymmetrically Clipped Optical OFDM for VLC

In this letter, layered discrete Hartley transform (DHT)-spread asymmetrically clipped optical-orthogonal frequency division multiplexing (LDHTS-ACO-OFDM) is presented; it uses DHT for multiplexing/demultiplexing and pulse-amplitude modulation (PAM) alphabets. LDHTS-ACO-OFDM yields several concrete enhancements over classical layered asymmetrically clipped O-OFDM (LACO-OFDM), such as low peak-to-average power ratio (PAPR), lower computational complexity, improved bit error rate (BER), and lower optical power penalty in a dispersive channel. Besides, LDHTS-ACO-OFDM augments the spectral efficiency relative to DHTS-ACO-OFDM with each addition of layer and ousts the inefficient bandwidth utilization limitation of DHTS-ACO-OFDM.

Mapping of discharge channels in void creating effective partial discharge area

This paper describes a novel approach to understanding partial discharge (PD) dynamics and mapping of the discharge channels in a void. During long term PD exposure, discharges develop inside channels within a void, leading to a variation in the discharge area on the void wall as well as a clustering effect. In this paper, the development of a method for identifying localized changes in the dielectric structure, through the detection of dispersed channels with reduced resistivity which occur due to multichannel discharges, is described. Discharge spots in selected ranges of surface resistivity, occurring due to the energetic impact of PD, are mapped. This mapping shows both the distribution of local centers on the surface of the specimen, characterized by a reduced value of surface resistivity. Furthermore, during aging tests this surface resistivity exhibited a progressive reduction by four orders of magnitude, justifying the occurrence of an effective discharge area in the void source. This additionally impacts PD pattern modulation and can be a possible initiation of the formation of cross channels in the bulk isolation which, in the long term, can lead to complete discharge.

Performance Evaluation of SFBC MIMO–OFDM Using FrFT Under TWDP Fading Channel

Multiple-input–multiple-output orthogonal frequency division multiplexing (MIMO–OFDM) is a promising 4G technology to increase data rate and capacity of a system. Further, the space–frequency-block-coding (SFBC) technique in MIMO–OFDM system has increased reliability of system in high mobility channels. Fourier transform in conventional OFDM system can obtain either in time or frequency domain, hence it can be acceptable to reduce inter-carrier-interference while it fails for doubly dispersive channels where both the domain are changing simultaneously. Another mathematical tool named as fractional Fourier transform (FrFT) cancels the effect of rapidly varying time–frequency-distortions by considering new rotating axis \(\left( {u,v} \right)\) at an angle \(\alpha\) from conventional time–frequency plane. In this paper, the performance of SFBC encoded MIMO OFDM system using FrFT in place of FFT has been analyzed under two-wave-diffused-power (TWDP) frequency selective fading channel. In this paper, the novel closed form expression for average bit-error-rate (BER) for coded and un-coded FrFT based MIMO–OFDM system under TWDP has also been presented in this paper. The results carried out using simulation shows that the proposed model achieves BER of 10−2 at 5 dB SNR whereas conventional achieves the same at 14 dB hence, concluded that the novel system performance improved by nearly 9 dB.

Design and Simulation of 1.28 Tbps Dense Wavelength Division Multiplex System Suitable for Long Haul Backbone

Abstract Wavelength division multiplex (WDM) system with on/off keying (OOK) modulation and direct detection (DD) is generally simple to implement, less expensive and energy efficient. The determination of the possible design capacity limit, in terms of the bit rate – distance product in WDM – OOK –DD systems is therefore crucial, considering transmitter/receiver simplicity, as well as energy and cost efficiency. A 32-channel WDM system is designed and simulated over 1000 km fiber length using Optsim commercial simulation software. The standard channel spacing of 0.4 nm was used in the C–band range from 1.5436 to 1.556 nm. Each channel used the simple non return to zero – on/off keying (NRZ – OOK) modulation format to modulate a continuous wave (CW) laser source at 40 Gbps using an external modulator, while the receiver uses a DD scheme. It is proposed that the design will be suitable for long haul mobile backbone in a national network, since up to 1.28 Tbps data rates can be transmitted over 1000 km. A bit rate length product of 1.28 Pbps.km was obtained as the optimum capacity limit in 32 channel dispersion managed WDM – OOK – DD system.

MmWave and VLC-Based Indoor Channel Models in 5G Wireless Networks

MmWave and VLC are two promising technologies for future 5G indoor communications to enable gigabit-per-second data transmission and to meet the ever increasing capacity requirements. Only with a solid understanding of the channels on mmWave and VLC can researchers and industry practitioners further explore the appropriate architectures and efficient communication schemes. In this article we focus on two recent indoor channels in future 5G networks based on mmWave and VLC. The article first discusses their operating frequency, room dimension, and transceiver configuration, which are among the key factors deciding the channel characteristics and impacting the communication performance. The detailed analysis on path loss and time dispersion for both mmWave and VLC channels are then elaborated. Finally, comparisons between mmWave and VLC channels and discussions on VLC and mmWave applications are presented. The study in this article shows that VLC and mmWave channels share some common characteristics while differing in others, which makes them appealing in different application scenarios.

Dispersion in two-dimensional periodic channels with discontinuous profiles.

The effective diffusivity of Brownian tracer particles confined in periodic micro-channels is smaller than the microscopic diffusivity due to entropic trapping. Here, we study diffusion in two-dimensional periodic channels whose cross section presents singular points, such as abrupt changes of radius or the presence of thin walls, with openings, delimiting periodic compartments composing the channel. Dispersion in such systems is analyzed using the Fick-Jacobs (FJ) approximation. This approximation assumes a much faster equilibration in the lateral than in the axial direction, along which the dispersion is measured. If the characteristic width a of the channel is much smaller than the period L of the channel, i.e., ε = a/L is small, this assumption is clearly valid for Brownian particles. For discontinuous channels, the FJ approximation is only valid at the lowest order in ε and provides a rough, though on occasions rather accurate, estimate of the effective diffusivity. Here we provide formulas for the effective diffusivity in discontinuous channels that are asymptotically exact at the next-to-leading order in ε. Each discontinuity leads to a reduction of the effective diffusivity. We show that our theory is consistent with the picture of effective trapping rates associated with each discontinuity, for which our theory provides explicit and asymptotically exact formulas. Our analytical predictions are confirmed by numerical analysis. Our results provide a precise quantification of the kinetic entropic barriers associated with profile singularities.

Decomposition and analysis of signals sparse in the dual polynomial Fourier transform

The acoustic waves transmitted through a dispersive environments can be quite complex for decomposition and localization. A signal which is transmitted through a dispersive channel is usually non-stationary. Even if a simple signal is transmitted, it can change its characteristics (phase and frequency) during the transmission through an underwater acoustic dispersive communication channel. Commonly, several components with different paths are received. In this paper, we present a method for decomposition of multicomponent acoustic signals using the dual polynomial Fourier transform and time-frequency methods. In real-world signals, some disturbances are introduced during the transmission. Common form of disturbances are the sinusoidal signals, making some of the frequency domain signal samples unreliable. Since the signal components can be considered as sparse in the dual polynomial Fourier transform domain, these samples can be omitted and reconstructed using the compressive sensing methods. The acoustic signal decomposition and its reconstruction from a reduced set of frequency domain samples is demonstrated on examples.

Efficient T-OOFDM System to Mitigate the Dispersion of Long-Haul Optical Fiber Channel at WANs

Due to its attractive features, the utilization of fiber optics as a transmission medium with various applications is increased rapidly. In despite, when signals are transmitted with high data rates through ultra-long haul distances of single-mode fiber (SMF), which is usually used at wide area networks (WANs), the nonlinear dispersion of signals is raised. This phenomenon leads digital pulses to interfere with the adjacent pulses. In this paper, an optical orthogonal frequency division multiplexing based T-transform (T-OOFDM) system is proposed to mitigate the effect of fiber dispersion significantly and reduce the peak-to-average power ratio (PAPR) of the transmitted signal when compared with conventional optical OFDM (OOFDM) system. Simulations results confirmed by the analytical analysis demonstrated that the detrimental effects arising from fiber channel dispersion on the subcarrier orthogonality of the transmitted signals can be efficiently minimized by using T-OOFDM system. Moreover, the peak of the transmitted signal will be considerably reduced whilst preserving the average power of signals.

Beta–Beta Bounds: Finite-Blocklength Analog of the Golden Formula

It is well known that the mutual information between two random variables can be expressed as the difference of two relative entropies that depend on an auxiliary distribution, a relation sometimes referred to as the golden formula. This paper is concerned with a finite-blocklength extension of this relation. This extension consists of two elements: 1) a finite-blocklength channel-coding converse bound by Polyanskiy and Verdu, which involves the ratio of two Neyman–Pearson $\beta $ functions (beta–beta converse bound); and 2) a novel beta–beta channel-coding achievability bound, expressed again as the ratio of two Neyman–Pearson $\beta $ functions. To demonstrate the usefulness of this finite-blocklength extension of the golden formula, the beta–beta achievability and converse bounds are used to obtain a finite-blocklength extension of Verdu’s wideband-slope approximation. The proof parallels the derivation of the latter, with the beta–beta bounds used in place of the golden formula. The beta–beta (achievability) bound is also shown to be useful in cases where the capacity-achieving output distribution is not a product distribution due to, e.g., a cost constraint or structural constraints on the codebook, such as orthogonality or constant composition. As an example, the bound is used to characterize the channel dispersion of the additive exponential-noise channel and to obtain a finite-blocklength achievability bound (the tightest to date) for multiple-input multiple-output Rayleigh-fading channels with perfect channel state information at the receiver.

The influence of envelope features on interunit dispersion around a naturally ventilated multi-story building.

This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.

Damped Zero-Pseudorandom Noise OFDM Systems

This paper proposed a new OFDM scheme called damped zero-pseudorandom noise orthogonal frequency division multiplexing (DZPN-OFDM) scheme. In the proposed scheme, ZPN-OFDM non-zero part damped to reduce the guard interval energy as well as the mutual interference power in-between the data and training blocks, and conservative the pseudo-noise conventional properties required for channel estimation or synchronization. The motivation of this paper is the OFDM long guard interval working in wide dispersion channels, where significant energy waste if conventional ZPN-OFDM is used as well as the BER performance degradation. Also, to solve the ZPN-OFDM spectrum efficiency loss problem, the proposed scheme doesn’t duplicate the guard interval. Both detailed performance analysis and simulation results show that the proposed DZPN-OFDM scheme can, indeed, offer significant bit error rate, spectrum efficiency as well as energy efficiency improvement.

Hydrodynamic dispersion in thin channels with micro-structured porous walls

Flow and transport within porous- and microtextured-walled channels is relevant to a number of natural and industrial processes. Designing and optimizing the topology of the pores and/or microstructure to achieve target performance at the system scale (or macroscale) is still an open question. In this work, we study whether hydrodynamic dispersion in microfluidic channels with walls structured by obstacles can be modeled by dispersion in channels with porous walls described as continuous porous media of zero or finite permeability. We perform single phase microfluidic non-reactive flow experiments in channels embedded in micropatterns with different topologies. Specifically, we focus on transverse riblets and arrays of pillars as examples of impermeable and permeable obstructions, respectively. We compare the experimental results with three models: 3D pore-scale simulations which resolve the micropattern geometry explicitly and two upscaled models which treat the micropattern as a continuum of zero or finite permeability. This study demonstrates that polydimethylsiloxane micromodels with appropriately patterned surfaces can be successfully employed to validate various continuum-scale modeling approximations in different physical regimes, identified by the order of magnitude of the Péclet number and the obstruction permeability.

Performance Investigation of Different Modulation Schemes in RoF Systems under the Influence of Self Phase Modulation

Abstract The Radio over Fiber (RoF) system has emerged out a promising solution to the last mile connectivity along with higher data rates making it the preference for future communication networks. But fiber nonlinearities induce crosstalk and degrade the performance of the system. This paper presents a simulative investigation & comparison of Mach Zehender Modulator (MZM) and Optical Phase Modulator (OPM) including the effects of Self Phase Modulation. A single channel RoF system has been designed using MZM and OPM respectively. The model is analyzed for different ranges of dispersion and input channel power. It is observed that as the dispersion modulus increases the SPM induced pulse broadening also increases. The pulse broadening also increases with increase in the channel input power. Further it is revealed that MZM offers 0.3 dB more channel output power compared to OPM when dispersion is increased from 2 to 10 ps/nm/km. With the increase in channel input power from 10 to 17.5 dBm, OPM suppress SPM effects better than MZM and provides 2 dB more channel output power.

Fiber Bragg Grating and Channel Spacing Effect in WDM Radio over Fiber System Using DPSK Modulation Format

Fiber Bragg Grating (FBG) is distributed reflector written in short length of optical fiber to reflect specific wavelengths of light and transmit the other components of spectra. By using FBG method in dispersion compensation can boost significantly the system performance. High capacity transmission and low effective cost of Radio over Fiber technique (Rof) are resulted from the combination of the optical communication capacity and the mobility of wireless networks. In this research article, an investigation of radio over fiber transmission for 50 km-long optical fiber operated at 10 Gb/s data rate and modulated under efficient DPSK modulation technique. The effect of Fiber Bragg Grating as chromatic dispersion compensation and channel spacing in WDM on optical communication system is analyzed and evaluated. The system performance is evaluated in terms of eye diagram. The outcomes of our simulation shows that Four unequal spacing channels radio over fiber system with ideal dispersion FBG has the best performance in terms of Q-factor and BER.

Performance evaluation of hybrid optical amplifiers in WDM system

There is a huge demand of bandwidth and gain in today’s high tech era. To fulfil this demand proper amplification is required. Hybrid Optical Amplifier made a remarkable change in present optical networks in order to amplify the signals for better performance of a system. In this paper, we have proposed the different configurations of hybrid optical amplifiers for better performance of Wavelength Division Multiplexed (WDM) systems. This paper also provides a crucial insight into the performance of different configurations of hybrid optical amplifiers (such as EDFA–Raman, EDFA–EDFA and SOA–EDFA) at 10 Gbps in WDM systems with 0.4 and 0.8 nm channel spacing. Work capacities of the systems were compared on the basis of various parameters viz., fiber length, dispersion, bit error rate, quality factor, eye diagram and output power. Attempts were taken to design 8 × 10 Gbps Dense Wavelength Division Multiplexed system with reduced channel spacing and 16 × 10 Gbps WDM system with 100 GHz channel spacing. It was assessed that for WDM system with 16 inputs and channel spacing of 0.8 nm, Raman–EDFA configuration gives better results as compared to EDFA–EDFA and SOA–EDFA configurations. Further, it was observed that SOA–EDFA configuration with reduced channel spacing (0.4 nm) provided maximum output power and better eye opening with lowest BER at a distance of 100 km. Moreover, it was seen that with increase in channel dispersion from 2 to 8 ps/nm/km, the quality of the signal started decreasing.

Improvement of bit error rate performance of MC-CDMA system by using adaptive digital modulation schemes

In Wireless Communication, the transmission channel suffers due to multipath propagation and channel dispersion due to Inter Symbol Interference (ISI), thereby degrades the system performance greatly. Future communication requires high data rate and effective utilization of bandwidth. Hence, there is a need to improve the Bit Error Rate (BER). The proposed work is Adaptive Modulation (AM), combined with Multi Carrier Code Division Multiple Access (MC-CDMA) based transmission system. It is a promising way to increase the data rate and it uses bandwidth in a efficient manner. The Adaptive Modulation based MC-CDMA systems applied to a wide-area environment, can achieve high data rate up to 100Mbps. In this paper M-ary PSK, M-ary QAM and M-ary CPM modulation techniques are used. The Adap-tive modulation based MC-CDMA systems perform well for around 10 number of users. As a whole, the adaptive MC-CDMA system is found to give the optimum BER performance of given digital modulation schemes for fourth generation environment.

Coded color shift keying with frequency domain equalization for low complexity energy efficient indoor visible light communications

The adaptive tri-chromatic color shift keying (CSK) modulation standardized in IEEE 802.15.7 and the advanced quad-chromatic CSK are unable to utilize their entire spectral efficiency range over dispersive indoor VLC channels and incur large power penalties without the use of channel equalization and forward error correction. This degrades the energy efficiency and limits the throughput capability of CSK schemes to approximately half of the data-rates specified in the IEEE standard for multimedia services in wireless personal area networks. To comply with the desire for low latency and minimal implementation complexity in the CSK standardization framework, we consider an industry standard binary convolutional code and frequency domain equalization (FDE) which provide one-shot data packet processing. Our results show that when operating over hybrid indoor visible light communication (VLC) links, the FDE based rate-adaptive coded modulation CSK scheme achieves a significant 11 dB SNR gain over an uncoded, unequalized system.

Novel Adaptive Filter Design for Filter Bank Multicarrier System Under Doubly Dispersive Channel Conditions

ABSTRACTFilter bank multicarrier (FBMC) has emerged as the favourite multicarrier communication technique since it meets the demands of the next-generation standards, especially for communication under doubly dispersive channels. In order to combat the undesirable channel effects like inter symbol interference and inter carrier interference, the prototype filters used in FBMC systems must satisfy orthogonality conditions and be well localized simultaneously in both time and frequency domains. Even though conventional doubly localized filters are optimum in the absence of channel, they fail to satisfy orthogonality and localization criteria under doubly dispersive channels. In the present study, an adaptive filter design method is proposed in which the prototype filter is designed as the weighted sum of a time-limited Nyquist filter, a band-limited Nyquist filter, and doubly localized Gaussian filter based on the analysis of discrete ambiguity function (DAF). The filter is designed to vary its characteristics according to the maximum time and frequency domain dispersions of the channel, so that signal-to-interference ratio is maximized. In order to ensure distortion-less transmission, the perfect reconstruction criterion for distortion-less transmission is mapped to specific conditions for DAF according to two-dimensional channel scattering function. The superiority in performance of the proposed adaptive filter is analysed in terms of signal-to-interference ratio and bit error rate, and is compared with the conventional filters for various channel scattering conditions.