Communications
H. Alizadeh Ghazijahani; M. Atashbar
Abstract
Background and Objectives: The combination of multiple-input-multiple-output (MIMO) with a Visible light communication (VLC) system leads to a higher speed of data transmission named the MIMO-VLC system. In multi-user (MU) MIMO-VLC, an LED array transmits signals to users. These signals are categorized ...
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Background and Objectives: The combination of multiple-input-multiple-output (MIMO) with a Visible light communication (VLC) system leads to a higher speed of data transmission named the MIMO-VLC system. In multi-user (MU) MIMO-VLC, an LED array transmits signals to users. These signals are categorized as signals of private information for each user and signals of public information for all users. Methods: In this research, we design an omnidirectional precoding to transmit the signals of public information in the MU-MIMO-VLC network. We aim to maximize the achievable rate which leads to maximizing the received mean power at the possible location of the users. Besides maximizing the achievable rate, we consider equal mean transmission power constraints in all LEDs to achieve higher power efficiency of the power amplifiers used in the LED array. Based on this, we formulate an optimization problem in which the constraint is in the form of a manifold, and utilize a gradient method projected on the manifold to solve the problem. Results: Simulation results indicate that the proposed omnidirectional precoding can achieve superior received mean power besides more than 10x bit error rate reduction compared to the classical form without precoding utilization. Conclusion: In this research, we proposed an omnidirectional precoding for transmitting the public signals in the MU-MIMO-VLC system. The proposed optimization problem maximizes the received mean power constrained with equal transmission mean power of LEDs in the array.
Communications
B. Norsabbaghi; G. Baghersalimi; A. Pouralizadeh; O. Mohammadian
Abstract
Background and Objectives: High peak-to-average power ratio (PAPR) in Orthogonal Frequency Division Multiplexing (OFDM)-based Underwater Optical Wireless Communication (UOWC) systems is one of the most important reasons for out-of-band power and in-band distortion leading to the declination of system ...
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Background and Objectives: High peak-to-average power ratio (PAPR) in Orthogonal Frequency Division Multiplexing (OFDM)-based Underwater Optical Wireless Communication (UOWC) systems is one of the most important reasons for out-of-band power and in-band distortion leading to the declination of system performance. Therefore, different approaches have been suggested and implemented for decreasing high PAPR of OFDM signals in UOWC systems that is the main aim of this paper.Methods: In this research, the performance of an OFDM-based UOWC system is investigated by employing Repetitive Clipping and Filtering (RCF) technique in clear open ocean water. The Monte Carlo Modeling of Light (MCML) approach with the Henyey Greenstein (HG) model of the scattering phase function is used to simulate the UOWC channel. Results: First, the CCDF performance of the suggested system with RCF method for different CR values is investigated. Also, the proposed system performance is examined in terms of bit error rate (BER) and error vector magnitude (EVM) at two different depths for link lengths of 1m and 5 m.Conclusion: The results showed that the system performance is limited by increasing the link length, the number of subcarriers, and depth. Also, it is shown that the RCF method significantly leads to reduction of the PAPR in the DCO-OFDM UWOC system and enhance BER performance up to 10 dB.
Communications
R. Karimzadeh Baee; A. Rahati Belabad; H. Moazzen; A. Ahmadi
Abstract
Background and Objectives: In order to compensate the nonlinearity, several linearization approaches such as feedback, feed-forward, and predistortion have been proposed. Among these linearization methods, the analog predistortion (APD) method is very suitable for the power amplifier and TWTA linearization, ...
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Background and Objectives: In order to compensate the nonlinearity, several linearization approaches such as feedback, feed-forward, and predistortion have been proposed. Among these linearization methods, the analog predistortion (APD) method is very suitable for the power amplifier and TWTA linearization, because it has a good compromise between complexity, linearity improvement and bandwidth. On the other hand, analog predistortion circuit can be added to existing amplifiers as a separate circuit. Many of the reported linearizer circuits are capable of linearizing power amplifiers with high gain and phase variations. However, these circuits are complex and bulky due to the use of distortion producer, phase shifter, adjustable attenuator, combiner/divider, and control unit. The objective of this paper is increasing the dynamic range of the predistorter by a simple proposed circuit. Hence, linearization capability of the linearizer is boosted.Methods: A new analog predistortion linearizer based on a coupler and a Schottky diode is presented. The proposed structure satisfies linearization requirements of the solid-state power amplifiers (SSPAs) and traveling wave tube amplifiers (TWTAs). Using a parallel combination of two Schottky diodes along with transmission line in port 2 and 3 of the coupler, the dynamic range of reflection coefficient of each port is increased compared to using a single diode in each branch. By increasing the range of reflection signal, the linearization capability has been increased to linearize power amplifiers and TWTAs. The AM/AM and AM/PM of the linearizer is controlled by changing the bias voltage of power supply.Results: The simulation results show that about 30° phase shift and 5 dB gain expansion can be compensated for power amplifiers and TWTAs. Because of utilizing two diodes, the linearization capability of the new linearizer is augmented compared to the conventional linearizer.Conclusion: A novel analog predistortion circuit based on the hybrid coupler and Schottky diodes is presented. The suggested topology increases the dynamic range of reflected power from each port. Using the proposed idea, AM/AM and AM/PM characteristics are expanded such that this characteristic can linearize the power amplifier with high linearity.
Communications
M. Sheikh-Hosseini
Abstract
Background and Objectives: Design of low-complexity receiver for space-time block coded (STBC) transmission over multiple-input multiple-output (MIMO) multiple-access channels has been subject of interest over the years. In this regard, zero-forcing receiver, as a low complexity receiver needing as many ...
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Background and Objectives: Design of low-complexity receiver for space-time block coded (STBC) transmission over multiple-input multiple-output (MIMO) multiple-access channels has been subject of interest over the years. In this regard, zero-forcing receiver, as a low complexity receiver needing as many receive antennas as the numbers of users, has received increasing attention.Methods: This paper investigates multiuser detection for STBC transmission over a flat-fading MIMO multiple-access channel consisting of co-channel users each with antennas and a zero-forcing coherent receiver equipping with receiving antennas. For the cases in which , it was previously claimed that it is impossible to extend this receiver to general scenarios of orthogonal STBC transmission with and .Results: We provide a theorem allowing this extension to any scenarios satisfying the theorem condition. Describing in more details, we first prove that zero-forcing receiver of antennas can successfully extend to any STBC transmission over MIMO multiple-access systems which provides an Alamouti-like structure for the inner product of equivalent channels between different receive antennas and users. Then, in order to gain more insight, the theorem role on extending zero-forcing receiver for transmission of orthogonal STBC over MIMO multiple-access systems with and , and also to other STBC schemes like generalized coordinate interleaved orthogonal design and Quasi-orthogonal STBC is investigated in more details. Finally, the average symbol error rate of considered scenarios are numerically evaluated and compered for different STBC schemes with various numbers of and .Conclusion: Generally speaking, it is concluded that extension of zero-forcing receiver to any scenarios of OSTBC transmissions over MIMO multiple-access channels exactly depends on satisfaction of the provided theorem and this receiver can be successfully employed in all scenarios providing an Alamouti-like structure for the inner product of equivalent channels between different receiving antennas and users.
Communications
N. Danesh; M. Sheikhan; B. Mahboobi
Abstract
Background and Objectives: To achieve significant throughput, interference alignment (IA) is an encouraging technique for wireless interference networks. In this study, we design an aligned beamformer based on the interference leakage minimization (ILM) method to reduce the interference power for a ...
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Background and Objectives: To achieve significant throughput, interference alignment (IA) is an encouraging technique for wireless interference networks. In this study, we design an aligned beamformer based on the interference leakage minimization (ILM) method to reduce the interference power for a multiple-input multiple-output interference channel (MIMO-IC).Methods: To deal with the non-convexity of ILM problem, we used a non-convex programming method (i.e., difference of convex [DC]). In this way, the interference leakage function is reformulated to a DC function including difference of two convex terms. Then, an additive function is defined that includes the DC objective function and a penalty function.Results: We propose a novel DC-based IA algorithm that uses solutions of an upper bound of the additive function in each iteration; as the initial state for the next iteration. Through an iterative manner and for the large values of the penalty factor, the solutions of upper bound function converge to the solutions of the original DC objective function (i.e., interference leakage function).Conclusion: In contrast to the frequent IA methods, the proposed DC-based IA algorithm updates transmit- and receive-beamformers in each iteration jointly (not alternately). Simulation results indicate that the proposed method outperforms some competitive IA algorithms by providing more throughputs and less interference leakage.
Communications
F. Yousefi Moghadam; B. Afzali; F. Nadi; R. Zallbeygi
Abstract
Background and Objectives: In this paper, an ultra-wide stopband microstrip low pass filter (LPF) with sharp roll-off and compact size is designed and fabricated. Also, proposed filter is designed, simulated, optimized, fabricated and measured and we can see a good agreement between the simulations and ...
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Background and Objectives: In this paper, an ultra-wide stopband microstrip low pass filter (LPF) with sharp roll-off and compact size is designed and fabricated. Also, proposed filter is designed, simulated, optimized, fabricated and measured and we can see a good agreement between the simulations and measurement results. Methods: T-shaped and stepped impedance have been used, because these structures have a very small dimensions. This method provides a suitable cut off frequency. Results: The cut off frequency at -3dB is 3.1GHz. In the proposed structure, the insertion loss is lower than 0.12dB and the return loss is greater than 15dB. In order to achieve a -20dB attenuation level in the stopband, a modified L-shaped structure is utilized. This filter has an ultra-wide stopband that is expanded from 3.37GHz to 37.5GHz. Also, the suppression level is greater than -20dB. The fabricated LPF has a size about 16.6 × 13.5 mm2 which is equal to 0.28ƛg × 0.22ƛg, where ƛg is the guided wavelength at 3.1GHz, which is very small. Conclusion: A microstrip LPF with 3.1GHz cut-off frequency consisting of T-shaped and stepped impedance resonators has been proposed, simulated and fabricated. The transition band (-3 to -40dB) is 0.38GHz, which is a very sharp response. The stopband of the proposed filter is from 3.37 to 37.5 GHz which is equal to 12.1fc. There was a significant agreement between the simulation and the measured results. With all these features, these filters are widely used in wireless communication systems.======================================================================================================Copyrights©2018 The author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, as long as the original authors and source are cited. No permission is required from the authors or the publishers.======================================================================================================
Communications
M. Safkhani
Abstract
Background and Objectives: To overcome the security flaw of RAPP authentication protocol, Zhuang et al. proposed a novel ultralightweight RFID mutual authentication protocol, called R2AP. In this paper, we first propose a new desynchronization attack against this protocol. Methods: To extract the secret ...
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Background and Objectives: To overcome the security flaw of RAPP authentication protocol, Zhuang et al. proposed a novel ultralightweight RFID mutual authentication protocol, called R2AP. In this paper, we first propose a new desynchronization attack against this protocol. Methods: To extract the secret ID, linear cryptanalysis is used, which is a tool mostly for attack block ciphers. Results: Our proposed desynchronization attack succeeds with the probability almost 1 and requires an adversary to initiate 1829 sessions of the protocol with the tag. On the other hand, the protocol updates the tag and the reader secretes to provide the tag holder privacy. However, it is shown that a passive adversary who eavesdrops only two sessions of the protocol can trace the tag with the probability of 0.921. In addition, passive attack for which the adversary can extract the secret ID of the tag is presented assuming that the adversary eavesdropped 128 sessions of the protocol, its success probability would be 0.387. Conclusion: It was shown that R2AP suffers from desynchronization, traceability, and disclosure attacks, where the two later attacks work in passive adversary model.======================================================================================================Copyrights©2018 The author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, as long as the original authors and source are cited. No permission is required from the authors or the publishers.======================================================================================================
