OFDM Signal Research Articles

Performance Enhancement of Probabilistically Shaped OFDM Enabled by Precoding Technique in an IM-DD System

Probabilistically shaped orthogonal frequency-division multiplexing (PS-OFDM) signal is proposed and experimentally demonstrated in a low-cost intensity-modulation and direct-detection (IM-DD) system for optical access networks. The concatenation of constant composition distribution matcher (CCDM) and low-density parity-check (LDPC) code sequentially implements probabilistic amplitude shaping (PAS) and channel coding. Thanks to the precoding scheme, all data subcarriers can be treated equally by one optimized probabilistic distribution. Experimental results demonstrate that constant amplitude zero autocorrelation sequence (CAZAC) precoding can offer optimal performance compared to orthogonal circulant transform (OCT) precoding and discrete Fourier transform spread (DFT-spread) in terms of increasing generalized mutual information (GMI) and reducing PAPR. Moreover, probabilistically-shaped 64-QAM OFDM signal can provide shaping gains of 1.04/0.94/0.64 dB at 4.0/3.6/3.0 bits/QAM symbol compared to traditional 64-QAM OFDM signal. Meanwhile, compared to traditional 16-QAM OFDM, it can offer shaping gains of 1.64/0.64 dB at 3.6/3.0 bits/QAM symbol. In addition, net data rate ranging from 32.66 to 43.55-Gb/s over 20-km single mode fiber (SMF) can be achieved by only adjusting the probabilistic distribution.

Efficiency Study of Kravchenko Weight Windows at Receiving Signal Structures Based on OFDM Signals in the Presence of Spectrum-Concentrated Interference

In this paper, we consider signal constructions based on OFDM signals and error-correcting codes that are resistant to the effects of spectrum-concentrated interference. An algorithm for receiving these signal constructions using weight windows and a criterion for optimal weight windows are given. A number of Kravchenko weight windows based on atomic functions are shown to belong to practically optimal windows. The probabilistic characteristics for the reception of the considered signal structures formed using the convolutional code with a code rate of 1/2 are specified.

A Highly Linear Wideband Polar Class-E CMOS Digital Doherty Power Amplifier

This article presents the first application of a digital-intensive intrinsically linear digitally controlled class-E technique in a Doherty configuration. By careful nonlinear segmentation and multiphase RF-clocking along with overdrive-voltage control and automatic duty-cycle correction, it is shown that even the nonlinearities related to Doherty operation can be fully handled by the underlying design such that digital predistorion (DPD) can be, in principle, omitted. The nonlinearity behavior of the whole digital Doherty power amplifier (PA) is analyzed, and closed-form equations are given to predict the AM-AM and AM-phase modulation (PM) curves. In addition, time/phase mismatch between the peak and main branches and the AM and PM signals is accurately compensated. In order to achieve the maximum intrinsic linearity, two separate chips with the same architecture, but different design parameters, are fabricated as the main and peak amplifiers in 40-nm bulk CMOS. To achieve a large RF bandwidth and high passive combiner efficiency, a differential low-loss, wideband Marchand balun-based Doherty power combiner, implemented using reentrant coupled lines with independent second-harmonic control is proposed, and together with the matching network is fabricated on a two-layer PCB. The measured peak/6-dB power backoff P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</sub> , drain efficiency/power-added efficiency at 2.4 GHz are 17.5 dBm/12.2 dBm, 57%/52% and 36%/25% with VDD main/peak = 0.6 V/0.7 V. Measured results without using DPD show -41-dBc adjacent channel power ratio (ACPR) and -36-dB error vector magnitude (EVM) for a 16-MHz OFDM signal at 2.5 GHz. By using DPD, the measured ACPR and EVM of a 16-MHz /32-MHz OFDM signals are -52 dBc/-48 dBc and -50 dB/-48 dB, respectively.

Ultra-Wideband Dual-Mode Doherty Power Amplifier Using Reciprocal Gate Bias for 5G Applications

A novel architecture to extend the bandwidth of the Doherty power amplifier (DPA) is presented in this article. It is illustrated that two DPA modes at different frequency bands can be realized by simply swapping the gate biases of the transistors without changing the matching circuits, and hence, ultrawide bandwidth can be achieved by using a single load modulation network in DPA. A dual-mode DPA with 2.8–4.1-GHz bandwidth for Mode I and 2.2–2.7-GHz/4.2–4.8-GHz bandwidth for Mode II using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated DPA attains a measured 7.5–11.7-dB gain and 39.2–41-dBm saturated power. 35.0%–49.7% drain efficiency is obtained at 6-dB output power back-off for the designed dual-mode bands. When driven by a ten-carrier 200-MHz OFDM signal with 7.7-dB peak-to-average power ratio, the proposed DPA achieves adjacent channel leakage ratio of better than −50 dBc after digital predistortion at 2.5 GHz/3.5 GHz/4.5 GHz with an average efficiency of 46.0%/35.7%/33.0%. This simple configuration provides a promising solution for 5G, where multiple frequency bands in sub-6 GHz will be deployed.

CP‐based OFDM radar–communications signal using interval linear phase compression modulation

In this study, a signal design algorithm for radar–communications system is investigated. An arbitrary pulse length orthogonal frequency-division multiplexing with sufficient cyclic prefix (CP-based OFDM) signal is designed as the radar–communications signal, the communication symbols are embedded by proposing an interval linear phase compression (ILPC) modulation. Firstly, the authors construct the model of CP-based OFDM signal, and analyse the impact of the communication symbols embedding. Then, to decrease the impact caused by the communication symbols embedding, the ILPC modulation is proposed. To guarantee the performances of the CP-based OFDM radar–communications signal, a time/frequency domain iterative filtering and clipping method is used. This algorithm can be used to design a CP-based OFDM signal that has a balanced performance in communications and radar. Finally, numerical results are provided to verify the validity of the proposed algorithm.

PAPR and security in OFDM-PON via optimum block dividing with dynamic key and 2D-LASM.

An approach for peak-to-average-power ratio (PAPR) reduction and security improvement in an orthogonal frequency division multiplexing passive optical network (OFDM-PON) is proposed by using optimum block dividing with 2-dimensional logistic adjusted sine map (2D-LASM) and dynamic key assignment technique. One frame of OFDM signal can be regarded as a symbol matrix. All the divisors of length and width of matrix can be calculated out, and each divisor of length and width can be arbitrarily combined to divide the matrix into blocks. A 4D hyperchaotic system is applied to generate a cipher book for 2D-LASM. And we assign different dynamic key groups from the cipher book for 2D-LASM to encrypt different block dividing situations. Different encrypted divisor blocks can obtain different values of PAPR. The optimum dividing situation is obtained by calculating out the minimum value of PAPR (VPAPR). The values of optimum encryption signal (VPAPR-op), the original signal (VPAPR-or) and the optimization ratio (η) are gradually equal to 146, 269 and 1.82, respectively, with the number of quadrature amplitude modulation (QAM) symbols in each subcarrier increasing. Simulating 1000 sets of 120×200 QAM symbols, the distribution of η approximately meets the Rayleigh distribution and its central distribution is around 1.7. The processing time decreases with the number of QAM symbols increasing in each block. The performance of PAPR reduction is more than 3 dB between the secure optimum signal and the original signal. In addition, the hyper-threading technique with two algorithms is applied to improve the performance of the encryption method. They promote the processing time by 38.6% and 50%, respectively. Finally, a 22.06 Gb/s optimum encryption OFDM signal transmits through a back-to-back (BTB) system and a 25-km standard single mode fiber (SSMF). These experimental results verify that the proposed approach is a promising candidate for solving both of PAPR reduction and security improvement in access network systems.

Hardware co-simulation of 1024-point FFT and it's Implementation, in Simulink, Xilinx Vivado IDE on Zynq-7000 FPGA

In this paper a 1024-point FFT Algorithm is implemented on Zynq-7000 FPGA device. The design implementation uses Hardware co-simulation in Simulink and Xilinx Vivado environments with Zynq-7000 FPGA target evaluation board using JTAG setup. The power parameter for the configured FFT IP core for 1024 point and the signal source DDS block are estimated. The DDS with both sine and cosine signal outputs enabled, consume a power of 0.277 W, whereas, the 1024 point FFT core consume a power of 0.044 W. Further when 2 DDSs were instanced to generate orthogonal sine and cosine sources for OFDM signals of same frequency 1MHz each, a total of 0.277W power is consumed. When a single DDS core is configured for both sine or cosine signal only configuration by instancing a 1024-point FFT core the total power consumed is 0.268W and 0.267W, respectively, a 1mW higher to cosine case. Further, when 1024 point FFT core power alone is calculated it is found to be 0.044W (or 44mW). When a single DDS is instanced for OFDM signal generation by opting both the sine cosine signals, it consumed a total power of 0.233 W saving a power of 0.044W or 44mW by sine or cosine data re-use from the LUT ROM of DDS. Thus saving a power of 44mW by using data re-use through LUT’s of DDS. This is a significant power saving. In this, hardware co-simulation process, Xilinx system generator tool (Sysgen) is used. This implementation is coded using Verilog HDL, verified on Xilinx Vivado platform on the Zynq-7000 FPGA device. Note that Zynq-7000 is supporting hardware co-simulation, hence the 1024-point FFT has been implemented on this device. The simulation results are captured on Xilinx signal viewer for a proper conclusion.

Hardware co-simulation of 1024-point FFT and it's Implementation, in Simulink, Xilinx Vivado IDE on Zynq-7000 FPGA

In this paper a 1024-point FFT Algorithm is implemented on Zynq-7000 FPGA device. The design implementation uses Hardware co-simulation in Simulink and Xilinx Vivado environments with Zynq-7000 FPGA target evaluation board using JTAG setup. The power parameter for the configured FFT IP core for 1024 point and the signal source DDS block are estimated. The DDS with both sine and cosine signal outputs enabled, consume a power of 0.277 W, whereas, the 1024 point FFT core consume a power of 0.044 W. Further when 2 DDSs were instanced to generate orthogonal sine and cosine sources for OFDM signals of same frequency 1MHz each, a total of 0.277W power is consumed. When a single DDS core is configured for both sine or cosine signal only configuration by instancing a 1024-point FFT core the total power consumed is 0.268W and 0.267W, respectively, a 1mW higher to cosine case. Further, when 1024 point FFT core power alone is calculated it is found to be 0.044W (or 44mW). When a single DDS is instanced for OFDM signal generation by opting both the sine cosine signals, it consumed a total power of 0.233 W saving a power of 0.044W or 44mW by sine or cosine data re-use from the LUT ROM of DDS. Thus saving a power of 44mW by using data re-use through LUT’s of DDS. This is a significant power saving. In this, hardware co-simulation process, Xilinx system generator tool (Sysgen) is used. This implementation is coded using Verilog HDL, verified on Xilinx Vivado platform on the Zynq-7000 FPGA device. Note that Zynq-7000 is supporting hardware co-simulation, hence the 1024-point FFT has been implemented on this device. The simulation results are captured on Xilinx signal viewer for a proper conclusion.

Multilayered optical OFDM for high spectral efficiency in visible light communication system

This paper presents two new schemes for improving the spectral efficiency of an optical orthogonal frequency-division multiplexing-based visible light communication (OFDM-VLC) system. The proposed schemes employed the generalized spatial modulation (GenSM) along with multi-input multi-output (MIMO) techniques as well as modified the coded modulation optical OFDM (CMO-OFDM) approach to attain more bandwidth utilization and higher spectral efficiency. The proposed multilayered/enhanced CMO-OFDM (LeCMO-OFDM) systems ensure the real-valued optical OFDM signals without using Hermitian symmetry and utilize the redundant codes existed in CMO-OFDM for further improvements in system’s spectral efficiency while ensuring acceptable bit error rate (BER) performance. LeCMO-OFDM scheme transmits multilayer optical signals after converting them to unipolar form either by using DC bias (DC-LeCMO-OFDM) or by separating them through employing the GenSM with no DC bias (NDC-LeCMO-OFDM). We present analytical and simulation results to evaluate the effectiveness of the proposed systems and demonstrate the additional capacity gain in terms of spectral efficiency. In addition, the impact of the spacing distance between the light emitting diodes has been addressed. Detailed performance comparison in terms of spectral efficiency and BER between the proposed techniques and other recent works is provided for validation purpose. Finally, results show that our schemes can achieve better spectral and power efficiencies performance with acceptable BER performance when compared with the related methods.

A Novel Signal Scrambling Technique for PAPR Reduction in OFDM Systems

OFDM forms the basis of the upcoming next generation technologies so as to achieve higher data rates within a given bandwidth effectively. One of the major issues associated with OFDM is Peak to Average Power Ratio (PAPR) which needs to be minimized to get an efficient performance. The random variation in the signal amplitude of the OFDM signal leads to additional interference in the system and hence affecting the performance of HPA in non-linear region. In this paper, we propose a technique for the reduction of PAPR in OFDM systems with some increased complexity which works for any modulation type and any number of subcarriers. The simulation results show performance improvement with respect to the existing signal scrambling techniques

Impact of Rayleigh-Distributed PAPR on the Performance of a Pre-Clipped DCO-OFDM System

Abstract The paper investigates the impact of Rayleigh-distributed statistical behavior of peak-to-average power ratio (PAPR) associated with a pre-clipped signal on the performance metrics of a direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system. The analytical model for the system takes into consideration a pre-clipped and dc-shifted baseband OFDM signal, driving an optical source over its linear operating range. The model employs a bias-scaling factor, which is heuristically varied over the entire range (0 to 1) to examine improvement in overall power efficiency. Further, it utilizes the cumulative distribution function (CDF) of the pre-clipped signal to get a weighted estimate of the available signal power within the clipped PAPR. The model also takes into consideration the clipping noise effects due to limited linearity of the optical source during electrical-to-optical conversion of baseband OFDM signal. Using this model, the paper aims to arrive at a realistic estimate of the system behavior in terms of bit error rate, electrical power-efficiency and spectral efficiency. Using theoretical simulation results, for a given set of operating parameters viz., signal power, PAPR, bias-scaling factor, modulation order and sub-carrier count, the paper examines the trade-offs involved in optimizing the performance metrics over appropriate dynamic range of the DCO-OFDM transmitter.

Signal Constructions Based on OFDM Signals Resistant to the Influence of Spectrum-Concentrated Interference

A description is given of a class of signal structures that are resistant to the effects of narrowband interference based on OFDM signals and error-correcting coding. Algorithms for receiving these signal structures are given using the weighted processing of input implementations in the presence of channel interference under consideration and the criterion of weight windows is given. It is shown that the Kaiser and Dolph–Chebyshev weight windows belong to the class of practically optimal windows. The error-performance characteristics for a number of considered signal constructions formed using a convolutional code with a code rate of 1/2 are given.

Prospects of using the UFMC technology In 5g / Imt-2020 networks

&lt;p&gt;&lt;span&gt;The article considers the technology of frequency multiplexing with universal filtering UFMC, planned to be introduced in the fifth generation of mobile communication networks, which allows maximizing the rate of decay of the side lobes of the multifrequency signal spectrum that cause out-of-band emissions. As a method of investigation, a computational experiment was chosen. The parameters of the OFDM and UFMC signals were compared to determine the gain of the UFMC technology in the occupied bandwidth of the signal spectrum, as well as the number of arithmetic operations, required to generate a data symbol compared to the OFDM technology, on the basis of which, conclusions were made about the practical application of UFMC technology in networks mobile communication of the fifth generation. The conducted analysis can help to select the optimal number of sub-channels in groups in order to minimize the amount of computations during the UFMC symbol generation process.&lt;/span&gt;&lt;/p&gt;

Methods of Frequency Synchronization of OFDM Signals in an Underwater Acoustic Channel

Application of signals with orthogonal frequency division multiplexing in underwater communication systems allows efficient use of the information transfer channel bandwidth and thereby increase the carrying capacity of the system. Among the main distinguishing features of the underwater channel there are the relatively low speed of sound propagation in water, multiple reflections from the water surface and the bottom of the reservoir and the Doppler effect, which leads to compression / stretching of the signal in time. The model of the underwater acoustic channel was developed on the assumption that the signal at the receiver input is a superposition of the signals which are copies of the transmitted signal, but passed through different paths from the transmitter. Each signal has its own amplitude, time delay and degree of compression / stretching in time. For correct demodulation of the orthogonal frequency division of the channel-signal, the receiver must first perform time and frequency synchronization. Time synchronization is performed to determine the beginning of the packet and the symbols’ boundaries, and frequency synchronization is necessary for matching the receiver and transmitter sampling frequency to eliminate interchannel interference.For frequency synchronization in a hydroacoustic channel of orthogonal frequency division type, either the preambles invariant to Doppler effect or pilot components of the channel of the orthogonal frequency division type are used. The method based on the synchronization preamble and on a bank of matched filters uses a non-invariant to the Doppler effect preamble at the beginning of the packet. Each filter is matched with a preamble having compression / stretching in time. The autocorrelation method assumes that two identical symbols are included in the transmitted data block for signals with orthogonal frequency division multiplexing, which are used to estimate the scale of signal stretching / compression. The conclusions on the advantages of using orthogonal frequency division multiplexing in an underwater acoustic channel are given.

A 28-GHz CMOS Linear Power Amplifier With Low Output Phase Variation Over Dual Power Modes

This letter presents a 28-GHz CMOS linear power amplifier (PA) for 5G beamforming front-end that has dual power modes. The proposed PA consists of parallelly connected cascode cells, which are used to linearize the PA in the high-power mode (HPM). Power modes are implemented by splitting the cascode cells again. To have a low-power mode (LPM), half of the cells are turned off while halving the supply voltage simultaneously. This concurrent change makes it possible to keep the phase of the PA unchanged over two power modes while the dc power consumption is reduced by about half in the LPM. In addition, the good linearity is maintained in the LPM. The PA has gains of 18/14 dB, saturated output powers of 18.5/12.2 dBm, P1dBs of 16.5/10.2 dBm, and peak PAEs of 27.3%/17.8% with a 28-GHz continuous wave (CW) signal. The maximum linear output powers that satisfy EVM $\times100$ -MHz 64-QAM OFDM signal.

Convolutional Neural Networks for Pilot-Induced Cyclostationarity Based OFDM Signals Spectrum Sensing in Full-Duplex Cognitive Radio

Convolutional Neural Networks for Pilot-Induced Cyclostationarity Based OFDM Signals Spectrum Sensing in Full-Duplex Cognitive Radio

Multi-Target Position and Velocity Estimation Using OFDM Communication Signals

In this paper, we consider a passive radar system that estimates the positions and velocities of multiple moving targets by using OFDM signals transmitted by a totally un-coordinated and un-synchronizated illuminator and multiple receivers. It is assumed that data demodulation is performed separately based on the direct-path signal, and the error-prone estimated data symbols are made available to the passive radar receivers, which estimate the positions and velocities of the targets in two stages. First, we formulate a problem of joint estimation of the delay-Doppler of reflectors and the demodulation errors, by exploiting two types of sparsities of the system, namely, the numbers of reflectors (i.e., targets and clutters) and demodulation errors are both small. This problem is non-convex and a conjugate gradient descent method is proposed to solve it. Then in the second stage we determine the positions and velocities of targets based on the estimated delay-Doppler in the first stage. For the second stage, two methods are proposed: the first is based on numerically solving a set of nonlinear equations, while the second is based on the neural network, which is more efficient. The performance of the proposed algorithms is evaluated through extensive simulations.

A Wideband 28-GHz Transmit–Receive Front-End for 5G Handset Phased Arrays in 40-nm CMOS

This paper reports a transmit-receive (Tx-Rx) front-end (FE) in bulk CMOS targeting fifth-generation (5G) mobile user equipment (UE)-phased arrays. Block-level specifications are first reviewed based on link budget and UE array beam steering considerations, and compared against measured standalone power/low-noise amplifier and phase shifter test circuit performances. Then, the codesign of the Tx-Rx switch and antenna impedance matching is investigated for the important case of wideband dual-resonance networks, leading to a proposed topology that is experimentally verified using silicon passive test structures. A fully integrated FE prototype is fabricated in 1.1-V 1P6M 40-nm bulk CMOS and characterized in detail across digital gain and phase settings. Measurements show Tx-mode P1 dB/PAE1 dB ≥ 14.6 dBm/21.9% across 27-30 GHz, and peak Pout/PAE = 6.5dBm/8.8% at 27 GHz for an 8 x 100-MHz carrier-aggregated 64-QAM OFDM signal. In Rx-mode, a noise figure of 5.5-6 dB and an input-referred third-order intercept point of -8.5 to -6 dBm are achieved across 27-33 GHz. Normalized to area/power consumption, the achieved performance is the state of the art relative to the most recent CMOS/SiGe FEs.

A new method for CE-OFDM phase demodulation

CE-OFDM technology is used to modulate the phase of OFDM signal, and it ensures the range of signal envelope to be constant and overcomes the disadvantage of high OFDM signal peak-to-average ratio. This essay puts forward a kind of phase demodulation mode based on Costas loop for CE-OFDM system, and makes a comparison with the original demodulation modes by MATLAB simulation.

Sparse Reconstruction Off-grid OFDM Time Delay Estimation Algorithm Based on Bayesian Automatic Relevance Determination

Aiming at the problem of single measurement vector (SMV) OFDM signal delay estimation in complex environment, a sparse reconstruction off-grid delay estimation algorithm based on Bayesian automatic correlation determination (OGBARD) is proposed. The algorithm adopts Bayesian framework, from the perspective of further mining useful information, introduces asymmetric autocorrelation to determine a priori parameters and off-grid parameters, and integrates them into the parameter estimation process, which improves the delay estimation accuracy under SMV and low signal-to-noise ratio (SNR) condition. Firstly, based on the channel frequency response (CFR) estimation value of the OFDM signal physical layer protocol data unit, the algorithm constructs a sparse representation model with off-grid parameters, and then introduces the probability hypothesis of noise vector, off-grid parameters and sparse coefficient vector in the model. Finally, based on Bayesian inference, the expectation maximization (EM) algorithm is used to estimate the hyperparameters. Simulation experiments show that the proposed algorithm has better estimation performance than the comparison algorithm and is closer to the Cramer–Rao Bound (CRB).