filter by: Publication Year
(Descending) Articles
Ghahramani Almanghadim H. ,
Karimi, B. ,
Valizadeh S. ,
Ghaedi, K. ,
Forouzan, A.R. ,
Akbari s., S. ,
Sheikhbahaei, M. ,
Maimani h.r., Non-coding RNA Research (24680540) pp. 70-90
Recently, the various regulative functions of long non-coding RNAs (LncRNAs) have been well determined. Recently, the vital role of LncRNAs as gene regulators has been identified in the immune system, especially in the inflammatory response. All cells of the immune system are governed by a complex and ever-changing gene expression program that is regulated through both transcriptional and post-transcriptional processes. LncRNAs regulate gene expression within the cell nucleus by influencing transcription or through post-transcriptional processes that affect the splicing, stability, or translation of messenger RNAs (mRNAs). Recent studies in immunology have revealed substantial alterations in the expression of lncRNAs during the activation of the innate immune system as well as the development, differentiation, and activation of T cells. These lncRNAs regulate key aspects of immune function, including the manufacturing of inflammatory molecules, cellular distinction, and cell movement. They do this by modulating protein-protein interactions or through base pairing with RNA and DNA. Here we review the current understanding of the mechanism of action of lncRNAs as novel immune-related regulators and their impact on physiological and pathological processes related to the immune system, including autoimmune diseases. We also highlight the emerging pattern of gene expression control in important research areas at the intersection between immunology and lncRNA biology. © 2024 The Authors
Janatyan, N. ,
Dezhtaherian, M. ,
Esteki M. ,
Forouzan, A.R. ,
Janbaz, S. ,
Oboudi, M.R. International Journal of Quality and Reliability Management (0265671X) pp. 401-408
Purpose: Companies look for their customers’ requirements (CRs) due to the importance of providing customer satisfaction in creating a competitive advantage. The purpose of this study is to introduce and categorize methods of prioritizing CRs based on company-oriented to customer-oriented approaches spectrum. Design/methodology/approach: Three methods, i.e. traditional FMEA, the Madzik and Kormanec method and the FAHP-FTOPSIS method, have been used to prioritize CRs in an auto-parts company. In this study, the most important CRs were identified and prioritized based on the opinions of senior managers to increase customer satisfaction in an auto-parts company. So, the 10 CRs were prioritized via customer-oriented and company-oriented approaches by considering the opinions of the managers of the company. Requirement curves were depicted with the help of average start and endpoint for each CR. Moreover, Risk Priority Number (RPN) and values of the effort and effect indexes were calculated using customer orientation and FAHP-FTOPSIS methods. Findings: The results of this research showed that CR1 (High quality of the part) is in first place of priority in traditional FMEA, CR3 (High quality of lining of the part) is in the first priority in the FAHP-FTOPSIS method and CR10 (After-sales service and warranty) is the most important in the Madzik and Kormanec method. The difference in results is because of the difference in their approach to data collection from customers or managers of the company. Originality/value: The originality of this study is to consider the methods of prioritizing the CRs of products due to the spectrum of company-oriented to customer-oriented approaches. © 2025, Emerald Publishing Limited.
IET Communications (17518628) 19(1)
In this paper, we investigate optimal receiver filter design with respect to shot noise in both non-coherent and coherent fibre optic communication systems. We derive analytical expressions for the moment-generating function of filter output and the signal-to-shot noise power ratio (SSNR) for intensity modulation direct detection (IM/DD) and phase diversity homodyne (PDH) optical fibre communication systems, with arbitrary waveform and receiver filter considerations in the presence of shot noise. Closed-form formulas are provided for the receiver filter structures that maximise the SSNR. The performance of SSNR is evaluated using return-to-zero (RZ) coding with rectangular and Gaussian pulse shapes across various receiver filter structures, including the ideal integrator, the RC model, and the matched filter. Our results indicate that the integrator, or its variations, is either optimal or near-optimal for diverse IM/DD systems. However, matched filtering to the received pulse power becomes superior when dark current or d.c. bias power is prominent. For coherent PDH systems, matched filtering to the complex received pulse amplitude approaches near-optimal performance provided the local oscillator (LO) power significantly exceeds the received signal power. When the LO power is much greater than the signal power, such systems can achieve double the SSNR of IM/DD systems, equivalent to twice the number of converted photoelectrons per symbol, thus nearing the quantum-limit performance considering the photodetector quantum efficiency. © 2025 The Author(s). IET Communications published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
The low cost, energy efficiency, and resilience of constant-envelope modulations to power amplifier nonlinearity make them appealing for multi-antenna multiuser systems in 5G and beyond. This paper proposes a linear zero-forcing (ZF) and a non-linear ZF Tomlinson-Harashima precoder (THP) based precoding scheme for downlink multiuser MIMO systems with arbitrary-sized PSK modulation. The proposed scheme achieves perfect persymbol multi-user interference (MUI) cancellation, enabling high throughput with minimal delay, cost, and complexity. The precoder structure is relatively easy to implement, involving straightforward matrix calculations. Analytical and simulation results demonstrate that the MUI is perfectly cancelled, and the bit-error rate decreases unboundedly with increasing SNR. © 2024 IEEE.
In the context of medical body sensor networks that rely on visible light communication (VLC), adaptive modulation plays a crucial role. Despite VLC's advantages, challenges arise due to fluctuating signal strength caused by patient movement. To address this, we propose an adaptive modulation system that adjusts based on link conditions, specifically the signal-to-noise ratio (SNR). Our approach involves an uplink channel for feedback, allowing the receiver to select the appropriate modulation scheme based on measured SNR after noise mitigation. The analysis focuses on various medical situations and investigates machine learning algorithms. The study compares adaptive modulation based on supervised learning with that based on reinforcement learning. By implementing a bi-directional system with real-time modulation tracking, we demonstrate the effectiveness of adaptive VLC in handling environmental changes (interference and noise). Notably, the use of the Q-learning algorithm enables real-time adaptation without prior knowledge of the environment. Our simulation results show that photodetectors placed on the shoulder and wrist benefit significantly from this approach, experiencing improved performance. © 2024 IEEE.
Iranian Journal Of Electrical And Electronic Engineering (17352827) 20(4)
Visible Light Communication, a key optical wireless technology, offers reliable, high-bandwidth, and secure communication, making it a promising solution for a variety of applications. Despite its many advantages, optical wireless communication faces challenges in medical environments due to fluctuating signal strength caused by patient movement. Smart transmitter structures can improve system performance by adjusting system parameters to the fluctuating channel conditions. The purpose of this research is to examine how adaptive modulation performs in a medical body sensor network system that uses visible light communication. The analysis focuses on various medical situations and investigates machine learning algorithms. The study compares adaptive modulation based on supervised learning with that based on reinforcement learning. The findings indicate that both approaches greatly improve spectral efficiency, emphasizing the significance of implementing link adaptation in visible light communication-based medical body sensor networks. The use of the Q-learning algorithm in adaptive modulation enables real-time training and enables the system to adjust to the changing environment without any prior knowledge about the environment. A remarkable improvement is observed for photodetectors on the shoulder and wrist since they experience more DC gain. © 2024, Iran University of Science and Technology. All rights reserved.
Analytical expressions for the signal to shot noise power ratio (SSNR) of phase diversity homodyne (PDH) optical fiber communication systems with arbitrary waveform and receiver filter are derived. The receiver filter structures maximizing the SSNR is calculated in closed form formulas. The SSNR performance of the systems is evaluated for rectangular and Gaussian pulse shapes and receiver filter structures such as matched filtering, the ideal integrator, and the photodetector RC model. As a side outcome, we show that the SSNR is upper-bounded by twice the converted electrons of the local oscillator. Numerical results show that the proposed optimal filter outperforms the ideal integrator and the integrator plus RC receiver by a few dBs. However, matched filtering to the complex received pulse amplitude is near optimal provided that the local oscillator power is much larger than the received power. © 2024 IEEE.
Probabilistic constellation shaping (PCS) has attracted a lot of attention for increasing the spectral efficiency (SE) in coherent fiber optic communication. Unfortunately, increasing the transmit power could increase non-linear self- and cross-phase modulations due to the Kerr effect in fiber optic. Owing to smaller peak-to-average power ratio (PAPR), higher angular distance between constellation points, and lower amplitude levels, amplitude and phase shift keying (APSK) can be more robust to phase noise than quadrature amplitude modulation (QAM). A new PCS scheme is proposed for the standard APSK 16 and 64 DVB-S2/S2X modulation formats. Simulation results in additive white gaussian noise (AWGN) channels show a close bit error rate performance for the proposed system compared to probabilistically-shaped QAM. © 2021 IEEE.
IEEE Transactions on Communications (00906778) 69(5)pp. 2851-2862
Bit rate fairness is an important concern in current G.fast and the forthcoming fifth generation DSL systems where the frequency is extended to hundreds of MHz and crosstalk couplings reach unprecedented levels. In this paper, we study the price of fairness (PoF) in dynamic spectrum management (DSM) enabled DSL systems. We propose optimal low complexity PoF-constrained max-min fairness (MMF), weighted max-min fairness (WMMF), proportional fairness (PF), and (p,α) -PF algorithms. The proposed algorithms inherently provide weight factors which can be used to reduce the computational complexity of user encoding or decoding order optimizing algorithms used in nonlinear vectoring. Our simulation results show that although PoF grows exponentially with the minimum bit rate, fairness can be improved considerably with a relatively small price in DSM level 2 spectrum balancing and particularly in DSM level 3 using minimum mean square error (MMSE) generalized decision feedback equalizer (GDFE). It is also seen that the proposed optimal PoF-constrained MMF algorithm can reach the solutions of PF and (p,α) -PF measures for some PoF. That is, the PoF-constrained MMF or WMMF algorithms can be used instead of the non-linear PF and (p,α) -PF measures, which often result in computationally intensive solutions. © 1972-2012 IEEE.
IEEE Communications Letters (10897798) 24(3)pp. 672-675
Pilot contamination (PC) is a practical drawback of massive multiple-input multiple-output (mMIMO) systems. It prevents the exploitation of the inherent spectral-efficiency (SE) of mMIMO systems, especially in dense networks. In this letter, we consider multi-antenna users in a multi-cell network and propose a novel approach based on interference alignment (IA) to remove PC. We show that the extra degrees of freedom (DoFs), provided by multi-antennas at the users' side, can be used efficiently when employed for pilot decontamination, especially in heavy-loaded networks. Thus, while the intra-cell interference is removed due to the properties of mMIMO the out-of-cell interference is eliminated by the proposed IA-based pilot decontamination. The proposed algorithm uses linear zero-forcing (ZF) and needs no continuous cooperation between the base stations (BSs). Moreover, the users need no channel state information (CSI). Our simulations show that the proposed method outperforms the conventional PC precoding algorithms significantly, particularly in heavy-loaded networks. © 1997-2012 IEEE.
AEU - International Journal of Electronics and Communications (16180399) 110
Interference alignment (IA) is an effective scheme for counteracting multi-user interference in wireless networks. Unfortunately, IA is sensitive to channel-state-information (CSI) imperfections. Achieving perfect CSI knowledge at a central node in large scale antenna wireless networks implies a huge feedback which is prohibitive. In this paper, we assume a distributed multicellular scenario, where there is no central node that knows global CSI and optimizing IA's precoder and combiner matrices is done by exchanging the local channel information between users and base stations (BSs) in several iterations. By using low-rank matrix approximation theory, we propose an efficient method to iteratively optimize precoder and combiner matrices for distributed IA. In each iteration, only a part of the CSI is fed back to BSs. More precisely, based on the latest available CSI and certain performance criteria, a few columns of the effective channel are sent back to the transmitters to approximate the interference covariance matrix which is then used to update the precoder matrices. We also propose a new method for quantizing the channel information matrix non-uniformly, which improves upon the conventional channel feedback quantization techniques. We evaluate the proposed methods by simulating a cellular network with various number of BS antennas and different feedback channel capacities. Simulation results show that our methods outperform both the conventional and improved channel feedback quantization algorithms significantly. © 2019 Elsevier GmbH
IET Communications (17518628) 13(5)pp. 610-619
Dirty paper coding (DPC) is a non-linear transmit scheme in presence of known interference, which is capacity achieving. Unfortunately, the implementation complexity of DPC is usually prohibitive. A sub-optimal yet practical method to implement DPC in inter-symbol interference channels is Tomlinson-Harashima precoding (THP). However, THP is not applicable to the constant-envelope phase shift keying (PSK) modulation. In this study, a modified Tomlinson-Harashima precoder is proposed to implement DPC for PSK modulation. The technique involves design of decision region at the receiver side and an algorithm to select the phase of transmitted symbol optimally at the transmitter side accordingly. Various patterns, namely, striped, checked, hexagonal, and radial are proposed and examined. Simulation results show that by using the proposed equaliser, the error floor is eliminated and significantly smaller bit-error rate is achieved. Numerical and simulation results show that the striped pattern outperforms other decision region designs. Moreover, its implementation and computational complexities is almost the same as the Tomlinson-Harashima precoder. © The Institution of Engineering and Technology 2018.
A real-Time algorithm for spectrum balancing (SB) of digital subscriber lines (DSL) is proposed. A real-Time SB algorithm is an algorithm for which the PSD and power regulatory constraints are always met after each iteration. The algorithm is based on fixed-point iterations and has a low computational complexity. Our simulations show that the proposed technique converges much faster than a recently proposed real-Time SB algorithm. Moreover, the achievable bit rates by the algorithm are near-optimal. Particularly the algorithm outperforms the iterative SB (ISB) algorithm in large scenarios. © 2018 IEEE.
Transactions on Emerging Telecommunications Technologies (21613915) 28(6)
In this paper, we consider the weighted sum rate (WSR) maximization problem in a partially coordinated multiple-input multiple-output multicell broadcast channel with constraints on base stations' transmit powers where the communication is based on dirty paper coding. To be able to obtain a computationally efficient solution for this non-convex optimization problem, we solve it by a fast algorithm for WSR maximization under per-user power constraints. The main idea is to define virtual per-user power constraints that add up to the per-base station power budgets in each cell and optimize the per-user power constraints to maximize the WSR. We propose two computationally efficient power allocation algorithms to find the per-user power constraints, namely, the waterfilling-based power update and the gradient descent–based power update algorithms. In the latter one, we find a closed-form expression for the derivative of the maximum of WSR wrt the per-user power constraints resulting in a considerable reduction in the complexity. The resulting algorithms are computationally efficient, and our simulation results show that they achieve significantly higher bit rates than the current algorithms at the same signal-to-noise power ratio. Copyright © 2016 John Wiley & Sons, Ltd.
IEEE Communications Letters (10897798) 20(10)pp. 2015-2018
A cyclostationary feature-based wideband spectrum sensing is proposed in which cyclic frequency offset is considered. The received signal passes through a rough and flexible filter which its effective band tuned to a specific part of the received signal spectrum. It belongs to a target signal, which possibly exists in the received signal. Some cyclic frequencies of the target are employed to derive the normalized least mean square algorithm that estimates the filter output from its frequency shifted samples. Based on the weight estimate's norm, the algorithm is tuned to find the accurate cyclic frequencies. If it succeeded, it means that the target signal is present. On the other hand, a complete search without major weights estimate means that the target signal is absent. In addition, the overall system, named cyclic frequency lock loop can be reformed as a demodulator for sinusoidal frequency modulation signals. The method is very easy to implement and has less computational complexity comparing with its other spectrum sensing counterparts. Simulation results validate the efficiency of the proposed method. © 2016 IEEE.
Characterizing the satellite beam footprint boundary can be used to measure the overlap of the beam coverage area of a satellite with that of other communication systems. A crucial application of this is in emerging cognitive satellite communication systems. In this paper, an analytical model based on ray tracing is proposed for specifying the boundary and the size of the satellite beam footprint and the distance of the footprint center from the beam center on Earth's surface. These characteristics are primarily a function of four parameters, namely, the coordinates of satellite, the beam cross-sectional shape, the coordinates of beam center, and the beamwidth. These parameters are reduced to only two parameters of Earth central angle between the nadir and the beam center and the beamwidth. Numerical results show that by increasing any of the two parameters the asymmetry of the footprint, its size, and the distance between the footprint center and the beam center increase. © 2016 IEEE.
Transactions on Emerging Telecommunications Technologies (21613915) 27(10)pp. 1380-1395
In this paper, we propose an efficient spectrum leasing scheme for cognitive radio networks in which the leasing process is carried out by employing two generally non-identical secondary users. The first secondary user is selected for cooperating with the primary network, and the second one is offered the released spectrum to perform the secondary transmission. The two mentioned secondary users are selected independently. Both the decode-and-forward and the amplify-and-forward relaying protocols are considered in this paper. Moreover, a protocol selective relaying scheme named decode- or amplify-and-forward (DAF), which smartly switches between the two protocols, is applied. We show that the disjoint secondary user selection and the protocol selective relaying result in an enhanced outage performance for both the primary and secondary networks as well as an increased achievable transmission rate for the secondary network. Numerical and simulation results are presented to evaluate the performance improvement of the proposed scheme. Using the simulation results, we also investigate the effect of disjoint secondary user selection on fairness considerations of the secondary network. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
IET Communications (17518628) 10(13)pp. 1655-1664
The authors study the power spectrum density (PSD) estimation of wideband wide-sense stationary (WSS) signals with sub-Nyquist sampling rate. Owing to the large bandwidth, Nyquist rate sampling of such signals needs very high rate analogue-to-digital converters. It is important to note that PSD estimation does not necessarily require reconstruction of the original signal. Indeed, the power spectrum can be directly obtained from sub-Nyquist samples. In this study, a new method for reconstructing the power spectrum in the frequency domain for WSS signals is presented. The main idea is to divide the whole spectrum into N equal-length segments and calculate the average PSD in each segment using a frequency-domain representation of sub-Nyquist samples. In addition, the capability of the proposed method, as a detector of spectrum holes, is studied using receiver operating characteristic (ROC) curves. Then, the analysis of false alarm probability is provided. Simulation results for the additive white Gaussian noise channel and the slowly fading frequency selective channel show that the proposed method considerably outperforms available techniques. © The Institution of Engineering and Technology 2016.
One of the main impairments for advanced digital subscriber line (DSL) systems is crosstalk. Crosstalk can be effectively cancelled using vectoring. However, some challenges such as implementation and computational complexities associated with full users' coordination when the number of DSL lines is large or when they are not co-located at any end make full vectoring impractical in many scenarios. In this paper, we consider interference alignment (IA) as an alternative technique when full vectoring is not practical. We apply this technique to both non-coordinated and partially coordinated very fast digital subscriber line 2 (VDSL2) systems. The computational complexity of IA can be prohibitive, however, we reduce it by applying IA to subsets of tones resulting in a computational complexity much smaller than that of vectoring. We also use iterative IA algorithms for performance improvement. Simulation results show that IA increases the achievable rates of VDSL2 loops considerably without a need for signal coordination among users. Moreover, when IA and partial vectoring are applied together, the users can achieve bit rates moderately close to that of a crosstalk-free network. © 2016 IEEE.
Electronics Letters (1350911X) 52(8)pp. 617-619
A well-known property of the Z transform is the differentiation in z-domain property, which states that if X(z) Z{x[n]} is the Z transform of a sequence x[n] then the Z transform of the sequence nx[n] is Z{nx[n]}=-z(dX (z)/dz). It is generally believed that the regions of convergence (ROC) for the two Z transforms are the same. It is shown that this is not true in the general case where X(z) is not rational and an example, in which the ROC is different for X(z) and Z{nx[n]}, is given. © The Institution of Engineering and Technology 2016.
IET Communications (17518628) 9(12)pp. 1442-1449
In this study, the authors examine resource allocation in an orthogonal frequency-division multiple-access-based cognitive radio (CR) network which dynamically senses primary users (PUs) spectrum and opportunistically uses available channels. The aim is resource allocation such that the CR network throughput is maximised under the PUs maximum interference constraint and cognitive users (CUs) transmission power budget. This problem is formulated as a mixedinteger non-linear programming problem which is NP-hard in general and infeasible to solve in real-time. To reduce the computational complexity, the authors decouple the problem into two separate steps. After initial power allocation, in the first step, an adaptive algorithm is employed to assign subcarriers to the CUs toward throughput maximisation by using these initial powers. In the second step, power is allocated optimally to the assigned subcarriers. Simulation results show that the proposed method nearly achieves the optimal solution in a small number of iterations meaning significant reduction in the computational complexity. © The Institution of Engineering and Technology 2015.
In this paper, we propose a new approach for signal level estimation in binary phase shift keying (BPSK) modulation based on the empirical characteristic function (ECF), when the probability density function (PDF) of the noise is unknown. Then, we compare our proposed method with two other estimators that are suggested for systems with known noises. Numerical results show that, in the presence of Laplace noise, the ECF estimator has a better performance in low signal-to-noise ratios (SNR) in comparison with previously proposed methods. Moreover, the proposed method does not require the knowledge of noise PDF and works without any training sequence. © 2015 IEEE.
Spectrum Sensing (SS) is the first step to establish a cognitive radio network. Current non-cooperative spectrum sensing methods exhibit a poor performance in certain applications. Therefore, cooperation can be pursued to improve the sensing performance. In addition, some applications need overall space-frequency perspective of spectrum in which spectrum mapping can be applied instead of point-by-point spectrum sensing. Generally, the spectrum mapping algorithms lead to computationally extensive optimization problems. Reducing the computational costs of algorithms, would extend the application domain of these methods. In this paper we propose a solution to attain space-frequency spectrum map of cognitive radio networks with a low computational complexity using the past behavior of space and frequency variations in time. The proposed algorithm offers a solution to reduce complexity and estimation error of spectrum sensing by forming a sensing time queue. © 2014 IEEE.
IEEE Transactions on Communications (00906778) 62(1)pp. 302-315
There are two types of crosstalk in digital subscriber line (DSL) systems, namely near-end crosstalk (NEXT) and far-end crosstalk (FEXT). NEXT is usually much stronger than FEXT. Therefore, high-speed DSL systems transmit in preplanned disjoint downstream (DS) and upstream (US) frequency bands to avoid NEXT. Although easy for implementation, such a fixed bandplan can lead to inefficient bandwidth usage, depending on the DS and US bit rate requirements and the loop topology, particularly in so-called vectored DSL systems, which include signal coordination for FEXT cancellation. In dynamic bandplanning (DBP), each frequency band is allocated to either DS, US, or to both directions depending on the aforementioned parameters. In this paper, we consider optimal DBP for vectored DSL with linear as well as with nonlinear transmitter/receiver structures. We propose an optimal DBP algorithm for systems with disjoint DS and US bands. For systems with overlapping bands, the problem of finding the optimal transmitter/receiver filters is nonconvex and we propose two iterative algorithms based on recent optimum spectrum balancing schemes from the literature. We also study the effect of echo and US NEXT cancellation on the system performance. Finally, simulation results are provided to demonstrate that our algorithms can indeed significantly increase the achievable bit rates. © 1972-2012 IEEE.
Previous works in spectrum leasing for centralized cognitive radio networks (CRNs) are based on selecting a secondary user as a relay to cooperatively transmit the primary data and then leasing the released spectrum due to the cooperation to the selected user. In this paper, we propose a new spectrum leasing scheme to improve the throughput and thus the spectrum efficiency of the secondary system. The proposed approach selects two secondary users for cooperation and secondary transmission independently, which are the best users towards the primary and secondary systems, respectively. We show that the outage performance of the secondary system is enhanced as a result of the independent secondary user selection. Analytical and simulation results are presented to verify the efficiency of our approach and performance improvement in the secondary system. © 2014 IEEE.
IEEE Signal Processing Letters (10709908) 20(11)pp. 1006-1009
In this letter, we derive formulas for optimal discrete-time pulse shaping for communications in multiband channels. Furthermore, we propose a technique for extending a short signal segment for the purpose of power spectral estimation using the available information about the signal stopbands. The later technique can be also used for minimizing the energy leakage into stopbands of a signal, e.g., as in windowing for orthogonal frequency division modulation (OFDM). Simulation results show that when the pulse duration is long enough, the stopband energy can be practically reduced to zero using the proposed technique. The proposed power spectral estimation technique can improve the resolution and reduce the energy leakage from signal passbands into adjacent stopbands. Moreover, it can reduce the out-of-band power leakage significantly when used for windowing in OFDM. © 1994-2012 IEEE.
IEEE Transactions on Communications (00906778) 60(10)pp. 3111-3122
In this paper, we investigate joint level 2 and 3 dynamic spectrum management (joint DSM 2/3) for downstream (DS) DSL. We consider a DS scenario in which users are divided into a few separate groups, where vector encoding based signal coordination can be applied in each group and spectrum coordination is possible for all users. This can be seen as a mixed interference/broadcast channel (IF/BC) scenario. In order to obtain the optimal transmitter structure, we develop a generalized duality between the vector broadcast and multiple-access channel (MAC) for scenarios in which partial signal coordination is available among users. This theory together with optimal spectrum balancing (OSB) is exploited to calculate the jointly optimal filters and transmit powers for non-linear vector dirty paper coding structures (in the form of Tomlinson-Harashima precoder (THP)) in the groups. The proposed scheme is compared to several other joint DSM 2/3 algorithms for DS DSL. Simulation results show that the proposed scheme (referred to as the IF/BC-OSB algorithm) achieves considerably higher bit rates than the other schemes. IF/BC-OSB encompasses the earlier developed BC-OSB algorithm as a special case. A simplified version of IF/BC-OSB avoiding exhaustive search with near-optimal performance is also proposed. © 2012 IEEE.
Proceedings - IEEE Global Communications Conference, GLOBECOM (25766813) pp. 3086-3091
Different precoder and equalizer structures have been proposed in literature for vectored digital subscriber lines (DSLs). In this paper, we assess the relative merit of advanced complex precoders and equalizers in practical DSL scenarios. Using extensive computer simulations, the achievable bit rates are investigated in the presence of alien crosstalkers, assuming a beta distribution model for crosstalk couplings. Vectoring in conventional as well as phantom mode (PM) VDSL2 transmission is considered. Vectoring techniques at the access-node (AN) are studied such as the zero-forcing equalizer (ZFE), ZF generalized decision feedback equalizer (ZF-GDFE), linear minimum mean squared error (MMSE), and MMSE-GDFE for upstream (US) scenarios and zero-forcing precoder (ZFP) for downstream (DS) scenarios. Vectoring is also considered at the customer premises (CP) in PM transmission. The results show that when there are no alien crosstalkers, the low-complexity ZFE allows reaching crosstalk-free rates in US direction. However using an MMSE-GDFE and if the noise spatial covariance information is assumed static, considerably higher bit rates can be achieved for long loops in the presence of alien crosstalkers. This rate increase comes at the expense of increased delay. In DS direction, the ZFP can achieve crosstalk-free rates in the absence of alien crosstalkers. The results also show that the use of vectoring techniques at the CP in PM transmission does not increase the achievable bit rates in US or DS direction when vectoring is already employed at the AN. © 2012 IEEE.
2013 National Conference on Communications, NCC 2013 (15503607) pp. 2559-2564
In this paper, we consider optimal dynamic spectrum management (DSM) for a downstream (DS) DSL scenario in which users are divided into a few separate groups, where vector encoding based signal coordination can be applied in each group and spectrum coordination is possible for all users. This can be seen as a mixed interference/broadcast channel (IF/BC) scenario. We investigate several candidates for vector encoding the signals inside the groups, including the linear zero-forcing (ZF) pre-compensator, ZF Tomlinson-Harashima pre-coder (THP), optimal linear pre-compensator (OLP), and THP with optimal transmit filters. The calculation of the optimal transmit filters (for both the linear pre-compensator and THP) is a non-convex problem. To resolve this problem, we develop a generalized duality between the IF/BC and IF/multiple-access channel (MAC). In order to achieve the highest data rates, optimal spectrum balancing (OSB) is applied to all users on top of the vector encoding inside the groups. Simulation results show that the grouped THP with optimal transmit filters achieves considerably higher bit rates than the other schemes. The resulting algorithm is referred to as the IF/BC-OSB algorithm, and encompasses the OSB (i.e., IF-OSB) and the earlier developed BC-OSB algorithm as special cases. © 2012 IEEE.
IEEE Transactions on Communications (00906778) 59(10)pp. 2851-2861
Dynamic spectrum management (DSM) refers to a wide range of techniques for counteracting crosstalk in digital subscriber line (DSL) networks. DSM is categorized into three levels based on the degree of coordination among users. In this article, we investigate optimal joint level 2 and 3 DSM for upstream DSL. We will discuss the difficulties of finding the universally optimal solution and we propose an optimal algorithm, referred to as IF/MAC-OSB, under some practical and implementation assumptions for this problem. Using computer simulations, we show that IF/MAC-OSB is capable of increasing the user bit rates considerably compared to several other DSM techniques. The proposed algorithm involves using the minimum mean squared error (MMSE)-generalized decision feedback equalizer (GDFE) together with Lagrange dual optimization. We address several aspects of the problem including the optimal decoding order in the GDFE receiver, GDFE error propagation, and the computational complexity of the algorithm. We also study effects of channel model randomness and upstream power back-off utilization on the performance of the algorithm. © 2011 IEEE.
Conference Record - International Conference on Communications (05361486)
Lagrange dual optimization technique (LDO) is a powerful tool for solving constrained optimization problems in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing systems (OFDM) and is generally considered to be optimal in the literature. LDO relaxes a constrained problem into an unconstrained dual problem using Lagrange multipliers. To solve the dual problem, the optimal value of the Lagrange multipliers should be found. The Lagrange multipliers are usually determined in an iterative process and reducing the number of iterations is of crucial importance to obtain systems with manageable computational complexity. In this paper, we show that for the LDO to be optimal in optimal spectrum balancing of DSL, the joint rate and power region (JRPR) should be strictly convex. Moreover, we propose a new LDO based algorithm with two basic advantages. Firstly, the computational complexity of the algorithm is logarithmic in the desired precision. Secondly, the algorithm can be used to find the optimal solution even for the cases with non-strictly convex JRPR. Our results can potentially be generalized to a wider range of optimization problems in the context of MIMO-OFDM and other complex separable systems. © 2011 IEEE.
IEEE Transactions on Communications (00906778) 59(9)pp. 2562-2573
Orthogonal spreading codes play an essential role in code-division multiple-access (CDMA) systems by eliminating multiple-access interference (MAI). In this paper, a novel orthogonal spreading code has been proposed for spectrally-encoded (SE) CDMA, a.k.a., spread-time (ST) CDMA with arbitrary pulse shape. It has been shown that it is possible to retain the orthogonality of the code in the presence of tail truncation by time windowing and in a general multipath fading channel in which users experience different frequency selectivity just by modifying the user codewords. Simulation results show that the proposed codes can achieve single user performance when the code length is twice the number of users. © 2006 IEEE.
The minimum mean squared error (MMSE)- generalized decision feedback equalizer (GDFE) provides a high multi-user crosstalk cancellation performance in upstream digital subscriber lines (DSL) scenarios. The weighted sum rate maximization approach can be used to find the points located on the boundary of the corresponding rate region (RR). The performance of the MMSE-GDFE depends on the decoding order of the users. Unfortunately, unlike in ideal multiple access channels (MAC), the optimal decoding order is not determined by the order of weight factors in practical DSL MACs. To reach the boundary of the RR, a tonal exhaustive search (TES) with computational complexity O(N2N!) is required for each tone, where N is the number of users. In this paper, we investigate the main deviations of the DSL receiver from the ideal MAC receiver and model their effects as a set of update factors on the weight factors. The order of the updated weight factors is then used to determine the decoding order of the users in an iterative fashion. The computational complexity of our updated weight factor ordering (UWFO) algorithm is O(N4) which is significantly smaller than that of TES. Moreover, our simulation results show that the algorithm achieves bit rates very close to the boundary of the RR leading to approximately the same performance as TES. UWFO can be applied to wireless MACs and zero-forcing GDFEs as well. © 2010 IEEE.
IEEE Transactions on Signal Processing (19410476) 57(7)pp. 2558-2568
Recently, the problem of optimal spectral balancing (OSB) for digital subscriber lines (DSL) with constrained transmit power has been solved using Lagrange's dual optimization technique and a weighted sum rate maximization (WSRM) approach. In many cases, the total power constraint is not binding. Although, this means a huge computational complexity reduction, the algorithm fails to reach certain points on the rate region (RR). In this paper, an in-depth analytical view of the WSRM approach is provided, and it is shown that when the RR is not strictly convex, the WSRM approach fails to reach certain points on the RR. Moreover, using N-dimensional geometry, a novel iterative facet dividing algorithm (IFDA) capable of reaching any point on the RR is proposed. Analytical and simulation results show that our technique is much more reliable and considerably faster than current algorithms. Moreover, it can be used for a wide range of problems which use WSRM approach, including OSB in the general case. © 2009 IEEE.
Eurasip Journal on Advances in Signal Processing (16870433) 2007
Line selection (LS), tone selection (TS), and joint tone-line selection (JTLS) partial crosstalk cancellers have been proposed to reduce the online computational complexity of far-end crosstalk (FEXT) cancellers in digital subscriber lines (DSL). However, when the crosstalk profile changes rapidly over time, there is an additional requirement that the partial crosstalk cancellers, particularly the LS and JTLS schemes, should also provide a low preprocessing complexity. This is in contrast to the case for perfect crosstalk cancellers. In this paper, we propose two novel channel matrix inversion methods, the approximate inverse (AI) and reduced inverse (RI) schemes, which reduce the recurrent complexity of the LS and JTLS schemes. Moreover, we propose two new classes of JTLS algorithms, the subsort and Lagrange JTLS algorithms, with significantly lower computational complexity than the recently proposed optimal greedy JTLS scheme. The computational complexity analysis of our algorithms shows that they provide much lower recurrent complexities than the greedy JTLS algorithm, allowing them to work efficiently in very fast time-varying crosstalk environments. Moreover, the analytical and simulation results demonstrate that our techniques are close to the optimal solution from the crosstalk cancellation point of view. The results also reveal that partial crosstalk cancellation is more beneficial in upstream DSL, particularly for short loops.
Spectrally encoded (SE) CDMA systems, a. k. a. spread time (SL) CDMA systems, have several advantages over their frequency-time dual, direct-sequence CDMA, such as the abilities to shape the transmitted spectrum and to avoid narrow-band interference. However, no orthogonal spreading code has been proposed to efficiently avoid multiple-access interference (MAI). In this paper, we propose a novel orthogonal spreading code for SE-CDMA. We study undesirable channel conditions under which the orthogonality property of the code could be lost. We propose a chip normalizing scheme (CNS) to restore the orthogonality property of the code in these cases. Our simulation results show that the encoded system can achieve the performance of interference-free channels. Furthermore, using our CNS the users can achieve a higher bit rate at the same bit error rate as the unnormalized case. © 2007 IEEE.
Conference Record - International Conference on Communications (05361486) 7pp. 3258-3263
Line selection (LS), tone selection (TS), and joint tone-line selection (JTLS) partial crosstalk cancellers have been proposed to reduce the online computational complexity of far-end crosstalk (FEXT) cancellers in DMT DSL. However, in contrast to perfect crosstalk cancellers, partial crosstalk cancellers should also provide a low pre-processing complexity when crosstalk profile changes rapidly in time. In this paper, we propose two novel fast classes of JTLS algorithms, namely, a family of sub-sort JTLS algorithms and the Lagrange JTLS. The computational complexity of our algorithms is much lower than that of the optimal greedy algorithm. Simulation results show that the performances of the proposed sub-sort algorithms are very close to the optimal greedy algorithm. More importantly, our Lagrange JTLS algorithm is optimal for the DSL channels we consider. © 2006 IEEE.
IEEE Transactions on Communications (00906778) 53(5)pp. 863-875
A class of generalized optical orthogonal codes (OOCs), namely, frame time-hopping (FTH) patterns with an extremely large cardinality, are studied for implementing multirate and multiservice (MR/MS) optical CDMA (OCDMA) networks. Conventional MR/MS methods, namely variable spreading rate and parallel mapping, are considered. Using FTH patterns, the problem of low OOC code cardinality in conventional MR/MS schemes is removed. Moreover, several new multicode methods, using subcode concatenation scheme, are proposed for MR/MS OCDMA. The proposed multicode schemes present flexibility for supporting MR/MS applications such as lower implementation complexity and cost, less need for service synchronization, and finally lower link power budget. Multiple-access performances of the systems are evaluated using saddle-point approximation methods considering photodetector shot-noise, dark current, and circuit thermal noise. The results show that the conventional parallel mapping outperforms the other schemes in high received powers, and the proposed multicode method, using Walsh subcode along with difference modulation, presents the best performance in low received powers for the cases considered. © 2005 IEEE.
Loop loss and crosstalk are the main impairments in DSL. Recently, novel crosstalk rejection and reduction techniques have been proposed to achieve the maximum capacity of the DSL channel. The performance of these techniques depends strongly on the degree of coordination among the users. In this paper, we consider maximizing the throughput of DSL when the users are coordinated at different levels. Moreover, we propose a novel centralized power allocation algorithm by generalizing a recently proposed iterative spectrum balancing (ISB) algorithm. Our generalized ISB (GISB) algorithm provides a superior performance to the ISB with polynomial complexity in the number of users. This complexity is much lower than that of the optimal spectral balancing (OSB) algorithm. Moreover, it can be selected adaptively based on the available processing power. © 2005 IEEE.
IEEE Microwave and Wireless Components Letters (15311309) 13(8)pp. 308-310
The interference issues related to ultrawideband (UWB) radio pose tight restrictions on the maximum data rate of UWB radio telecommunication systems. A possible solution is to reduce the required signal to interference ratio (SIR) that gives satisfactory performance to the UWB system. In this letter, we propose coded M-ary UWB radio communication systems. Two classes of convolutional codes, namely, low-rate superorthogonal codes and high-rate punctured codes are considered for this purpose. Simulation results on the bit error rate of the proposed system indicates that the system is capable to work in lower SIR's and therefore supports higher data transmission rates in a real interference environment compared to the previously proposed UWB communication systems.
IEEE International Symposium on Information Theory - Proceedings (21578096) pp. 326-326
Two new receiver structures for fiber-optic code-division multiple-access (FO-CDMA), namely chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard-limiter are proposed. Performance of these new receivers for a binary pulse position modulation frame time-hopping FO-CDMA network is compared to performance of some basic receiver structures such as correlation receiver and chip-level detector. The results predict that chip-level detector with optimum comparator threshold is superior to chip-level detector for received low signal powers. Furthermore, the error floors of correlation receiver with an electrical hard-limiter are significantly lower than those of all other considered receiver structures in a wide range of the received power.
Iterative median filtering for restoration of images corrupted by impulsive noise is considered. A modified version of median filtering that can be also applied iteratively is also proposed. The methods are compared with an iterative method suggested by Marvasti for images with high pixel loss. The results show that applying the median filter iteratively results in well sighted resulting images. Moreover, despite the implementation simplicity, the proposed modified median filtering scheme provides a considerably higher convergence speed which intends a lower numerical complexity. © 2003 IEEE.
IEEE Transactions on Communications (00906778) 50(12)pp. 1971-1983
In this paper, a new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-access (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC (λ = 1). To obtain the performance of such systems, we use a recently introduced communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. It is discussed that systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the performance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC (λ = 1), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard limiter, are also proposed. Performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, optimum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures.
IEEE Transactions on Wireless Communications (15361276) 1(4)pp. 671-681
In [Scholtz (1993)], an ultra-wide bandwidth time-hopping spread-spectrum code division multiple-access system employing a binary PPM signaling has been introduced, and its performance was obtained based on a Gaussian distribution assumption for the multiple-access interference. In this paper, we begin first by proposing to use a practical low-rate error correcting code in the system without any further required bandwidth expansion. We then present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates for the uncoded scheme. Furthermore, it indicates that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate, the coding scheme increases the number of users by a factor which is logarithmic in the number of pulses used in tune-hopping spread-spectrum systems. © 2002 IEEE.
Conference Record - International Conference on Communications (05361486) 10pp. 3017-3021
In [1], an ultra-wide bandwidth time-hopping spread-spectrum code division multiple access system employing a binary PPM signaling has been introduced, and its performance was obtained based on a Gaussian distribution assumption for the multiple access interference. A coded scheme of this system has been proposed in [2], which shows much better performance in comparison to the uncoded scheme under the above Gaussian assumption. In this paper, we present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates.
In this paper, we propose to use a practical low rate error-correcting code in a time-hopping spread spectrum multiple access system employing a binary PPM signaling. To this end, superorthogonal codes, which are near optimal and have relatively low implementation complexity, are considered. The performance analysis shows that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate the proposed scheme increases the number of users by a factor which is logarithmic in the number of pulses used in time hopping spread spectrum system.
