OFDM Signal Research Articles

Імпульси Найквіста в системі з OFDM

Orthogonal frequency division multiplexing or OFDM technology allows converting a single high-speed data stream into parallel low-speed streams, where each stream is transmitted in a channel with its own subcarrier frequency. Unlike FDM, where there is a protective band gap between channels, channels in an OFDM system overlap, what requires orthogonal properties between base pulses of transmitter and receiver to avoid intersymbol (ISI) and inter-channel (ICI) interference. In traditional OFDM, a rectangular pulse is used as envelope of base pulses of the transmitter and receiver. Choice of the rectangular envelope is due to simplicity of its shape and additionally allows implementing the process of receiving OFDM signal using fast Fourier transform technology (FFT). Disadvantage in this case is the increased level of out-of-band emission in frequency domain, as well as increased sensitivity of system to the Doppler effect. Main feature of spectrum of radio pulse with rectangular envelope is the maximum spectral density at the carrier frequency and many zeros located at equal frequency intervals, what means that spectrum is selective. The paper shows that this feature of spectrum for OFDM is not accidental. In turn, Nyquist pulses are a type of pulses that have one maximum and many zeros located at equal time intervals. That is, the spectrum of radio pulse with rectangular envelope can be considered as a Nyquist pulse if time and frequency are replaced. The paper considers general criteria for shape of continuous base pulses of the transmitter and receiver. Requirements are set for shape of receiver base pulses if transmitter base pulses have selective spectrum. Usage features of fast Fourier transform technology for transmitter base pulses with selective spectrum have been investigated.

Secure OFDM‐Based NOMA for Machine‐to‐Machine Communication

Machine‐to‐machine communication (M2M) has obtained increasing interest in recent years. However, its enhancement and broadcasting characteristics produced a new security challenge. We have suggested a novel dynamic Quadrature Amplitude Modulation (QAM) scheme for a totally elastic and dynamic mapping of user data by using chaos. This paper analyses physical layer security methods in Orthogonal Frequency Division Multiplexing‐based Nonorthogonal Multiple Access (OFDM‐NOMA) and introduces a secure data transmission mechanism created by dynamic QAM. The security robustness given by the suggested encryption scheme is assessed, where an overall keyspace of ~10163 is achieved, which is sufficient to provide security against exhaustive attacks. The result of the scheme is verified through MATLAB simulation, where the bit error rate performance of our proposed scheme is compared with an unencrypted OFDM signal, and the performance of our proposed scheme is analyzed for an illegal user. The suggested dynamic mapping fulfills the fundamental obligations of cryptography for data security. Moreover, it enhances the level of security in OFDM‐NOMA.

Multipath number estimation based on QR decomposition

Accurate multipath number estimation is the prerequisite for MUSIC algorithm to achieve excellent superresolution performance. To solve the problem that the existing multipath number estimation algorithms based on eigenvalue distribution usually require a large amount of computation, this paper proposed a fast multipath number estimation algorithm based on QR decomposition under the OFDM signal model. This algorithm only needs few times QR iteration decomposition to obtain the pseudo-eigenvalues of the matrix, and uses them to estimate the multipath number through two-step comparison. Due to the avoidance of accurate eigenvalues solution, compared with the traditional algorithms based on eigenvalue distribution, proposed algorithm’s computation is smaller. Simulation results showed that the proposed algorithm has better detection performance in low SNR and few snapshots than existing algorithms.

Robust Digital Signal Recovery for LEO Satellite Communications Subject to High SNR Variation and Transmitter Memory Effects

This paper proposes a robust digital signal recovery (DSR) technique to tackle the high signal-to-noise ratio (SNR) variation and transmitter memory effects for broadband power efficient down-link in next-generation low Earth orbit (LEO) satellite constellations. The robustness against low SNR is achieved by concurrently integrating magnitude normalization and noise feature filtering using a filtering block built with one batch normalization (BN) layer and two bidirectional long short-term memory (BiLSTM) layers. Moreover, unlike existing deep neural network-based DSR techniques (DNN-DSR), which failed to effectively take into account the memory effects of radio-frequency power amplifiers (RF-PAs) in the model design, the proposed BiLSTM-DSR technique can extracts the sequential characteristics of the adjacent in-phase (I) and quadrature (Q) samples, and hence can obtain superior memory effects compensation compared with the DNN-DSR technique. Experimental validation results of the proposed BiLSTM-DSR with a 100 MHz bandwidth OFDM signal demonstrate an excellent performance of 11.83 dB and 9.4% improvement for adjacent channel power ratio (ACPR) and error vector magnitude (EVM), respectively. BiLSTM-DSR also outperforms the existing DNN-DSR technique in terms of the ACPR and EVM by 2.4 dB and 0.9%, which provides a promising solution for developing deep learning-assisted receivers for high-throughput LEO satellite networks.

Maximum Likelihood Localization Method With MIMO-OFDM Transmission

In this research, we propose to estimate the location of mobile users by using the maximum likelihood (ML) method with statistical properties of the transmission signal angle of departure (AOD) and received signal strength (RSS) from access points (APs) to user equipment (UE). Location estimation (LE) is performed at each UE by using a signal from a multiple-input, multiple-output (MIMO) antenna system at the AP, which transmits specially designed, MIMO-orthogonal frequency division multiplexing (MIMO-OFDM), beamforming signals. The ML localization method is derived from statistical models of AOD and RSS of the OFDM signal. We also derive the theoretical root mean square error (RMSE) given the statistical models. Based on the results, the ML with the AOD and RSS methods has a lower RMSE than the other methods and can achieve close to the theoretical RMSE. The RMSE can also be significantly reduced by using a higher number of APs along with proper AP placement. In addition, the LE performance increases as the number of antennas and the number of subcarriers increases but with diminishing effectiveness. The developed RMSE calculation tool in this paper can be an important instrument to investigate and plan the deployment of APs for localization and can be further extended into larger-scale studies.

An mm-Wave Multi-Mode Asymmetric Power Amplifier With Back-off Efficiency Enhancement

This paper presents a multi-mode asymmetric power amplifier (MMA-PA) with back-off efficiency enhanced. The back-off efficiency is improved without a lossy impedance inverter or signal separator which is used in conventional Doherty or outphasing amplifiers. The design considerations are provided in detail. In addition, the proposed MMA-PA operates in three different modes according to the input power level. The PA is fabricated in a 65-nm bulk CMOS technology. In the high-power (HP) mode, the MMA-PA exhibits a small-signal gain of 18.4 dB at 30 GHz and a 3-dB bandwidth of 4.5 GHz from 27.7 to 32.2 GHz. The output saturation power ( $\text{P}_{\text {sat}}$ ) and 1-dB compression power are 16.7 and 15.1 dBm, respectively. While the peak power-added efficiency (PAE) is 24.7 %, the 6-dB back-off PAE keeps a high value of 14.1 %. In the medium-power (MP) mode, $\text{P}_{\text {sat}}$ and peak PAE are 12.3 dBm and 21 %, respectively with a similar gain of 18.9 dB. In the low-power (LP) mode, the gain, $\text{P}_{\text {sat}}$ and peak PAE are 11.5 dB, 7.5 dBm and 16.5 %, respectively. With a 100-MHz 64-QAM OFDM signal following the 3GPP 5G NR standard, the average output power in the HP mode is 6 dBm with EVM of −26 dB and ACLR < −34 dBc. In the MP and LP modes, the average output power are 3.3 and 1.1 dBm, respectively with EVM of −26.6 and −26.1 dB.

OFDM 신호에 대한 스펙트럼 센싱 기술

OFDM 신호에 대한 스펙트럼 센싱 기술

2차 통계량과 RNN을 이용한 OFDM 신호 검출

2차 통계량과 RNN을 이용한 OFDM 신호 검출

Nonlinear Distortion Mitigation in Multi-IF over Fiber Transmission Using Modulation-Based Adaptive Power Allocation

We propose a modulation-based adaptive power allocation (MBAPA) technique for nonlinear distortion mitigation in intermediate frequency over fiber (IFoF) systems. The technique allocates the spectral power of each IF band according to the required signal-to-noise power ratio (SNR) of the modulation format. To demonstrate the performance of the technique, transmission experiments were performed in 10 km and 20 km with 24-IF bands using OFDM signals. The feasibility of the proposed MBAPA technique was experimentally verified by reducing inter-modulation distortion (IMD) power and enhancing channel linearity.

Design of a Ka-Band Cascode Power Amplifier Linearized With Cold-FET Interstage Matching Network

A Ka-band CMOS cascode power amplifier (PA) linearized with a cold-FET-based interstage matching network is presented, which is designed in a 65-nm CMOS process. Since it is difficult to make a cascode PA matched to the optimum output and input impedances at high frequencies, a matching network has to be introduced at the node between the common-source (CS) and common-gate (CG) stages. The cold-FET-based matching network improves the linearity and the input and output impedance matchings, which is analyzed and optimized with its simple model. It makes the PA have gain expansion and phase lag with the power, which allows the PA to have less amplitude-to-amplitude (AM-AM) and amplitude-to-phase (AM-PM) distortions. In addition, it improves the return losses of the PA by making the impedances for power matching and conjugate matching located closely. The implemented PA achieves the peak power-added efficiency (PAE) of 38.2% and the saturated output power (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) of 17.1 dBm at 31 GHz while occupying the chip area of 0.16 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . It is also shown that the OP1 dB is improved by 1.7 dB, and the AM-PM distortion is reduced to only 1.1° due to the linearization technique. It is tested with 64-quadrature amplitude modulation (QAM) signals, which has a 400-MHz channel bandwidth (BW) and a 9.7-dB peak-to-average power ratio (PAPR). It achieves average output powers of 9.6/7.7 dBm with error vector magnitudes (EVMs) of -25/-30 dB for 64-QAM OFDM signals, efficiencies of 17.7%/12%, and the adjacent channel leakage ratio (ACLR) of -28.2/-32.8 dBc, respectively.

A Quadrature Class-G Complex-Domain Doherty Digital Power Amplifier

This article presents an efficient quadrature digital power amplifier (DPA) based on a complex-domain Doherty (CDD) architecture. The proposed CDD architecture allows a PA to achieve high efficiency through Doherty operation in a complex domain using two independent vectors with different amplitudes and phases. It demonstrates high efficiency when two vectors have in-phase components in the complex domain by introducing an additional efficiency peak. The proposed DPA based on a switch-capacitor PA (SCPA) employs the CDD and dual-supply Class-G techniques to enhance system efficiency (SE) and adds three additional efficiency peaks down to 12-dB power back-off (PBO). An additional efficiency peak at 6-dB PBO and two additional efficiency peaks at 2.5- and 12-dB PBOs are associated with CDD and Class-G, respectively. The prototype, fabricated in a 65-nm CMOS process, achieves 27.8-dBm peak output power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</sub> ) with a peak SE of 32.1%. It exhibits an error vector magnitude (EVM) of -42.0 dB (-43.3 dB) at a P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</sub> of 14.7 dBm (15.4 dBm) for an 802.11ax 40-MHz (20-MHz) 1024-QAM OFDM signal with 13.1-dB (12.4-dB) peak-to-average power ratio (PAPR) at 2.2 GHz. The average SE measured with a 20-MHz single-carrier 1024-QAM signal with 6.8-dB PAPR at a P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</sub> of 21 dBm is 18.4%.

Full duplex dispersion compensating system based on chromatic dispersion in analog RoF links

Abstract Radio over Fiber technology is a promising solution for next-generation wireless communication networks. In radio over fiber systems, signal distortions caused by modulator nonlinearity is a major concern. In this paper, a full duplex dispersion compensating system using chromatic dispersion is proposed. A system using 16 QAM OFDM signal transmission is presented. Distortion compensation is observed when a required amount of dispersion is added. Bit Error Rate (BER) and constellation diagrams for different amount of dispersion are analyzed and its comparison is presented. Since the framework utilizes just a single persistent wave laser for upstream and downstream transmission the base station is additionally disentangled and it diminishes the general framework cost. Signal distortions are reduced when the dispersion is adjusted to 640 ps/nm for uplink transmission and 760 ps/nm for downlink transmission.

Chaotic constant composition distribution matching for physical layer security in a PS-OFDM-PON

This paper proposes a probabilistic shaping orthogonal frequency division multiplexing passive optical network (PS-OFDM-PON) based on chaotic constant composition distribution matching (CCDM). With the implementation of a four-dimensional hyperchaotic Lv system, probabilistic shaping and chaotic encryption are realized with low complexity on the process of signal modulation, so as to enhance the system performance in the presence of bit error rate (BER) and security. An 8.9 Gb/s encrypted PS-16 quadrature amplitude modulation (QAM)-OFDM signal transmission over a 25 km standard single mode fiber (SSMF) is experimentally demonstrated. And experimental results indicate that compared with conventional uniform 16QAM-OFDM, the encrypted PS-16QAM-OFDM can obtain a 1.2 dB gain in receiver sensitivity at a BER of 10-3 under the same bit rate. Moreover, the key space of the proposed scheme is 1.98 × 1073, which is a large enough number to effectively guard against any malicious attacks from illegal optical network units (ONUs). The combined superiority of BER and security performance enables a promising prospect for the proposed PS chaotic encryption scheme in a future low-cost optical access network.

Full-Duplexing With SDR Devices: Algorithms, FPGA Implementation, and Real-Time Results

In this paper, we present a novel nonlinear digital self-interference canceller algorithm, its implementation details on a software-defined radio (SDR) platform, and performance results of real-time full-duplex experiments on both device and link level. The canceller algorithm is based on an augmented Hammerstein model, with a nonlinear part modeling the transmitter non-idealities followed by a linear filter to model the self-interference (SI) channel. The nonlinear part includes a spline-based model for the nonlinear power amplifier, a polynomial model for baseband nonlinearities, as well as models for I/Q mismatch and LO leakage. The canceller is implemented on an FPGA as a part of an OFDM transceiver testbed for real-time measurements. Extensive real-time measurements show excellent performance: (1) the digital canceller, together with an RF isolator, can suppress the SI to within 1-2 dB's of the receiver noise floor, with total SI suppression of up to 103 dB; (2) digital cancellation of up to 46 dB is evidenced, which is among the highest real-time cancellations in literature; (3) system-level measurements with OFDM signals demonstrate the benefit of utilizing the proposed canceller in a two-way communication scenario, showing up to 90 % increase in sum-rate compared to half-duplex communication.

Noise-Immune Communication with Orthogonal Frequency Division Multiplexing Using Kravchenko Weight Function Processing

Signal structures based on OFDM signals and error-correcting codes resistant to the influence of spectrum-concentrated noise are described. An algorithm for receiving such signal structures with the use of weight functions is presented. It is shown by theoretic analysis and simulations that the class of Kravchenko weight functions based on atomic functions represents nearly optimal windows in terms of the minimum error reception probability.

Method of advanced channel estimation in OFDM systems by regression technique

The article considers the task of increasing the efficiency of estimating the parameters of multipath radio communication channels used in the design of mobile remote access systems with orthogonal frequency division with OFDM multiplexing. In the conditions of an ever-growing number of subscribers, an increase in transmission speed and noise immunity of mobile radio communication systems, the task of increasing the efficiency of channel estimation in multipath signal propagation remains an urgent one. It is also essential that the evaluation function was performed with minimal complexity and with high accuracy of signal recovery. Within the framework of the work, the analysis of existing algorithms, models, and methods for estimating the communication channel parameters of OFDM systems is carried out, the main differences and disadvantages are noted. A technique has been developed for estimating the complex envelope of an OFDM signal in a multipath communication channel based on regression analysis. A mathematical model for estimating a discrete multipath channel by a regression method using pilot symbols is presented. The type of the approximating function is selected, taking into account the requirements for the complexity of implementation and performance indicators. We present an algorithm for estimating the complex envelope of an OFDM signal in a multipath communication channel by the regression method.

A novel ant colony optimization algorithm for PAPR reduction of OFDM signals

SummaryThe high peak‐to‐average power ratio (PAPR) is the main challenge of orthogonal frequency division multiplexing (OFDM) systems. Partial transmit sequence (PTS) is a useful approach to diminish the PAPR. Although the PTS approach significantly decreases the PAPR, it requires to explore all possible sequences of phase weighting factors. Hence, the computational cost exponentially increases with the number of divided subblocks. This paper proposes a novel PTS technique based on ant colony optimization (ACO) to diminish the high PAPR and computational cost of OFDM systems. By the new representation of phase factors as a graph, the improved ACO algorithm is combined with the PTS method to explore the optimal compound of the phase rotation factors. Simulation results represent that the proposed ACO‐based PTS approach significantly reduces the PAPR and improves the computational cost at the same time. A comparative analysis of the other meta‐heuristics shows that the ACO‐PTS approach outperforms the genetic algorithm, particle swarm optimization, and gray wolf optimization in terms of reducing PAPR.

PAPR reduction of MIMO-OFDM system with reduced computational complexity SLM scheme

Multiple input and multiple output orthogonal frequency division multiplexing (MIMO-OFDM) is a prominent technology for fourth generation mobile radio communication. OFDM is a special case of multi-carrier-modulation (MCM) technique that divides the high data rate input bit streams into large number of lower rate sub streams and each sub stream is transmitted over orthogonal subcarriers. The main advantage of MIMO-OFDM system is the efficient utilization of band width, multipath delay spread tolerance and mitigation of inter symbol interference (ISI) in frequency selective fading channels. However, the main drawback of MIMO-OFDM system is high peak to average power ratio (PAPR) for large number of sub carriers which causes serious degradation in performance. One of the well-known peak to average power ratio (PAPR) reduction schemes for MIMO-OFDM systems is selected mapping (SLM) scheme. In this work computational complexity of MIMO-OFDM system is reduced by adding mapping signal sequences to the OFDM signal sequences. It also increases the PAPR reduction performance of the MIMO-OFDM system.

Color-Shift-Keying Embedded Direct-Current Optical-Orthogonal-Frequency-Division-Multiplexing (CSK-DCO-OFDM) for Visible Light Communications (VLC)

We propose and experimentally demonstrate a color-shift-keying embedded direct-current-optical-orthogonal-frequency-division-multiplexing (CSK-DCO-OFDM) scheme for the VLC systems. The proposed scheme can provide not only non-flickering aggregated optical power to reduce human health concerns, but also offers additional modulation dimension or flexibility to the CSK by including high data rate OFDM signal. The data rate is increased from 10 Mbit/s in the original CSK signal to 70.43 Mbit/s in the CSK-DCO-OFDM signal, and both the CSK and OFDM signals satisfy the forward-error-correction (FEC) threshold.

Broadband RF-Input Continuous-Mode Load-Modulated Balanced Power Amplifier With Input Phase Adjustment

This article presents the theory and design methodology of broadband RF-input continuous-mode load-modulated balanced power amplifier (CM-LMBA) by introducing the CM output-matching networks in the LMBA architecture. It is illustrated that the CM impedance condition can be achieved by properly adjusting the phase difference between the different PA branches in the proposed CM-LMBA during the entire load modulation process. An RF-input CM-LMBA with 1.45-2.45-GHz bandwidth using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated CM-LMBA attains a measured 11.2-13.4-dB gain and around 40-W saturated power. Power-added efficiency (PAE) of 46.4%-56.5% and 43.2%-50.3% is achieved at 6- and 8-dB output power back-offs throughout the designed band. When driven by a 100-MHz OFDM signal with an 8-dB peak-to-average power ratio (PAPR), the proposed CM-LMBA achieves better than -46-dBc adjacent channel leakage ratio (ACLR) and higher than 45% average PAE after digital predistortion at 1.8 and 2.1 GHz.