Orthogonal Frequency Division Multiplexing Communication Research Articles

Performance Analysis of Selective Mapping in Underwater Acoustic Orthogonal Frequency Division Multiplexing Communication System

Under-Water Acoustic (UWA) communication networks are commonly formed by associating various independent UWA vehicles and transceivers connected to the bottom of the sea with battery-operated power modems. Orthogonal Frequency Division Multiplexing (OFDM) is one of the most vital innovations for UWA communications, having improved data rates and the ability to transform fading channels into flat fading. Moreover, OFDM is more robust on Inter-Symbol and Inter-Carrier Interferences (ISI and ICI respectively). However, OFDM technology suffers from a high Peak to Average Power Ratio (PAPR), resulting in nonlinear distortions and higher Bit Error Rates (BERs). Saving power of battery deployed modems is an important necessity for sustainable underwater communications. This paper studies PAPR in UWA OFDM communications, employing Selective Mapping (SLM) as a tool to mitigate PAPR. The proposed SLM with the oversampling factor method proves to be less complex and more efficient. Simulation results indicate that SLM is a promising PAPR reduction method for UWA OFDM communications reducing BER.

Index modulation for OFDM RadCom systems

This paper studies a novel Radar and wireless communication fusion system namely, RadCom, using orthogonal frequency division multiplexing index modulation waveforms. From the communication perspective, information is conveyed not only through modulated orthogonal frequency division multiplexing subcarriers but also by the active subcarrier indices that are dynamically updated at the transmitted symbol rates. When compared with conventional orthogonal frequency division multiplexing communication systems, only a subset of subcarriers is activated in the proposed orthogonal frequency division multiplexing index modulation system, which reduces signal peak-to-average ratios, leading to enhanced power amplifier efficiency. From the Radar sensing perspective, the Radar range-velocity profile obtained using compressed sensing approach is simulated and presented. It is shown that orthogonal frequency division multiplexing index modulation RadCom system can enhance the transmission data rates when low-order modulation schemes are adopted e.g. binary phase shift keying, and maintain similar Radar performance compared with the orthogonal frequency division multiplexing counterpart when the system is carefully designed.

An Adaptive Multi-Mode Underwater Acoustic Communication System Using OSDM and Direct Sequence Spread Spectrum Modulation

Dynamic underwater acoustic network is a research hotspot. Due to the complexity and time-varying of underwater acoustic channel, the performance of single-mode underwater acoustic communication system is often limited by the worst channel situation. Orthogonal signal division multiplexing (OSDM) is a new signal modulation technology, which can be regarded as a bridge between single carrier modulation and orthogonal frequency division multiplexing (OFDM) modulation. In order to improve the channel adaptive ability of underwater acoustic communication system and obtain a unified architecture of different carrier modulation, an adaptive multi-mode underwater acoustic communication system based on OSDM and direct sequence spread spectrum modulation (AMMUAC/OS) is proposed in this paper. Combined with spread spectrum modulation, the system can realize single carrier time domain spread spectrum (SC-TDSS), single carrier system with cyclic prefix (SC-CP), OSDM, multicarrier frequency domain spread spectrum (MC-FDSS), and OFDM communication system by adjusting the vector length M of OSDM. The addition of a variety of digital mapping methods enables the system to achieve multi-mode communication with a unified architecture. Based on the relationship between the OSDM vector length M and the underwater acoustic channel delay, a joint adaptive decision criterion based on the maximum channel delay and the channel signal-to-noise ratio (SNR) is proposed. Firstly, the OSDM vector length M is determined according to the maximum channel delay, and then the system communication mode is selected according to the channel SNR. Orthogonal matching pursuit (OMP) channel estimation algorithm is used to obtain channel information and minimum mean square error (MMSE) channel equalization algorithm is used for channel compensation. An adaptive multi-mode data frame structure is designed for the interaction between the sender and the receiver. The simulation results show that the proposed AMMUAC/OS can adapt to underwater acoustic channels with different channel delay and SNR.

Comparative Study of Artificial Neural Network Based Channel Equalization Methods for mmWave Communications

In this paper, we compare two artificial neural networks (ANNs) approaches designed to perform channel equalization for millimeter-wave (mmWave) signals operating in the 28 GHz frequency band. We used an in-house deterministic Three-Dimensional Ray-Launching (3D-RL) code to simulate the spatial structure of mmWave channels considering the material properties of the obstacles within the scenario at the frequency under analysis. We performed offline training of a multilayer perceptron (MLP) neural network with the simulated mmWave channels to equalize the received signal. We also performed online training of an extreme learning machine (ELM) neural network to directly get the equalized symbols at the receiver, given as input the received mmWave signal. The ANN solutions were tested in terms of the achievable spectral efficiency, bit error rate, and time to process. We compared the ANN techniques to the minimum mean square error and the zero-forcing equalizers, considering an orthogonal frequency-division multiplexing communication based on the 5G New Radio standard. We present numerical results on the performance of the proposed ANNs and show that the ELM strategy outperforms the MLP method, requiring significantly less processing time than the reviewed equalization methods.

Hardware-Efficient All-Digital Architectures for OFDM Backscatter Modulators

Orthogonal frequency-division multiplexing (OFDM) backscatter communication promises to enable ultralow-power wireless devices that are robust to time-varying multipath channels. Traditional OFDM transmitters require the use of power-hungry digital-to-analog converters (DACs) and vector modulators to realize the OFDM signal, adding to the complexity and power consumption of wireless sensor nodes. In this work, we compare three all-digital architectures for OFDM backscatter communication that use RF switches and discrete loads to implement digitally controlled single-sideband OFDM backscatter modulators. We present design analysis, including simulations and measurements, for a selected implementation using five subcarriers having binary phase shift keying (BPSK) modulation at a symbol rate of 250 kSymbols/s and a throughput of 1.25 Mbit/s with a modulator energy consumption of 160 pJ/bit. We also present a five-subcarrier over-the-air validation with 195-kb/s throughput. We demonstrate how the number of RF switch states and the choice of impedances impact the metrics of subcarrier interference ratio and sideband suppression ratio, and we explore how reduced-numeric-precision inverse fast Fourier transform (IFFT) structures impact the theoretical bit-error rate. The all-digital architecture and analysis presented in this article enable new avenues of low-cost, digital OFDM, and multiple-access backscatter communication systems for use in challenging multipath environments.

Impulse Noise Suppression based Iterative Hybrid Nonlinear in Power Line Communication

In power line orthogonal frequency division multiplexing (OFDM) communication systems, the presence of impulse noise can seriously affect system performance. One of the conventional solutions is to perform a mixed nonlinear processing on the time domain signal at the receiving end before performing subsequent OFDM demodulation. Therefore, this thesis studies an improved method based on iterative elimination of nonlinear distortion. Firstly, the received OFDM signal is subjected to hybrid nonlinear processing in the time domain. Secondly, the nonlinear distortion introduced by the time domain nonlinear processing is reconstructed by the symbols initially detected in the frequency domain, and the reconstruction accuracy is improved by iteration. Finally, the reconstructed nonlinear distortion is subtracted from the output of the hybrid nonlinear processing unit. The simulation results show that compared with the traditional hybrid nonlinear method, the performance of the proposed algorithm is much better than that of the traditional algorithm.

Joint low‐complexity equalisation and CFO compensation for DCT–OFDM communication systems based on SIC

In wireless radio communication systems, equalisation is one of the most important algorithms to improve the bit-error-rate performance. The linear zero-forcing equalisers suffer from noise enhancement. The linear minimum mean square error equalisers suffer from large complexity and require the estimation of the signal-to-noise ratio to work properly. In this study, the authors propose a joint low-complexity regularised zero-forcing equaliser with successive interference cancellation (JLRZF–SIC) based on discrete cosine transform (DCT) for orthogonal frequency division multiplexing (OFDM) systems. The proposed JLRZF–SIC scheme jointly performs the equalisation and carrier frequency offset (CFO) compensation with lower-complexity using the banded-matrix approximation concept. Simulation results show the important role of the proposed JLRZF–SIC scheme is to improve the system performance compared to the other equalisation schemes.

A Dual-Functional Massive MIMO OFDM Communication and Radar Transmitter Architecture

In this study, a dual-functional radar and communication (RadCom) system architecture is proposed for application at base-stations (BSs), or access points (APs), for simultaneously communicating with multiple user equipments (UEs) and sensing the environment. Specifically, massive multiple-input multiple-output (mMIMO) communication and orthogonal frequency-division multiplexing (OFDM)-based MIMO radar are considered with the objective to jointly utilize channel diversity and interference. The BS consists of a mMIMO antenna array, and radar transmit and receive antennas. Employing OFDM waveforms for the radar allows the BS to perform channel state information (CSI) estimation for the mMIMO and radar antennas simultaneously. The acquired CSI is then exploited to predict the radar signals received by the UEs. While the radar transmits an OFDM waveform for detecting possible targets in range, the communication system beamforms to the UEs by taking into account the predicted radar interference. To further enhance the capacity of the communication system, an optimum radar waveform is designed. Moreover, the network capacity is mathematically analyzed and verified by simulations. The results show that the proposed RadCom can achieve higher capacity than conventional mMIMO systems by utilizing the radar interference while simultaneously detecting targets.

Comparison of Correlation-Based OFDM Radar Receivers

Various correlation-based receivers have been proposed in passive bistatic and active monostatic radar exploiting orthogonal frequency-division multiplexing communications signals, but too little has been dedicated to establishing their relations and advantages over each other. Accordingly, this article provides an analytical comparison of the common filters in such waveform sharing scenarios, along with a performance analysis regarding three criteria: computational complexity, signal-to-interference-plus-noise-ratio, and resilience to ground clutter. The last two especially assess the possible detrimental effects of the random sidelobes (or pedestal) induced by the data symbols in the range-Doppler map. Although simulations show that none of the filters performs unanimously better, the ones employing circular correlations globally evidence attractive results.

Adaptive Transmission for Reconfigurable Intelligent Surface-Assisted OFDM Wireless Communications

Reconfigurable intelligent surfaces (RISs) have recently emerged as an innovative technology for improving the coverage, throughput, and energy/spectrum efficiency of future wireless communications. In this paper, we propose a new transmission protocol for wideband RIS-assisted single-input multiple-output (SIMO) orthogonal frequency division multiplexing (OFDM) communication systems, where each transmission frame is divided into multiple sub-frames to execute channel estimation simultaneously with passive beamforming. As the training symbols are discretely distributed over multiple sub-frames, the channel state information (CSI) associated with RIS cannot be estimated at once. As such, we propose a new channel estimation method to progressively estimate the associated CSI over consecutive sub-frames, based on which the passive beamforming at the RIS is fine-tuned to improve the achievable rate for data transmission. In particular, during the channel training, the RIS plays two roles of embedding training reflection states for progressive channel estimation and performing passive beamforming for data transmission on the data tones. Based on the estimated CSI in each sub-frame, we formulate an optimization problem to maximize the average achievable rate by designing the passive beamforming at the RIS, which needs to balance the received signal power over different sub-carriers and different receive antennas. As the formulated problem is non-convex and thus difficult to solve optimally, we propose two efficient algorithms to find high-quality solutions. Simulation results validate the effectiveness of the proposed channel estimation and beamforming optimization methods. It is shown that the proposed joint channel estimation and passive beamforming scheme is able to drastically improve the average achievable rate and reduce the delay for data transmission as compared to existing schemes.

Reconfigurable Intelligent Surface (RIS)-Enhanced Two-Way OFDM Communications

In this paper, we focus on the reconfigurable intelligent surface (RIS)-enhanced two-way device-to-device (D2D) multi-pair orthogonal-frequency-division-multiplexing (OFDM) communication systems. Specifically, we maximize the minimum bidirectional weighted sum-rate by jointly optimizing the sub-band allocation, the power allocation, and the discrete phase shift (PS) design of the reflecting elements at the RIS. To tackle the main difficulty of the non-convex PS design, we firstly formulate a semi-definite relaxation problem and further devise a low-complexity solution for the PS design by leveraging the projected sub-gradient method. We demonstrate the desirable performance gain for the proposed designs through numerical results, where the number of reflecting elements at the RIS and the deployment location of RIS are shown to play a key role in enhancing the performance gain.

Message authentication algorithm for OFDM communication systems

With the huge expansion in the telecommunications industry, the need for robust information security is becoming more critical than ever. Physical Layer Security has, recently, emerged as a promising candidate to secure emerging communication systems in a lightweight manner. This approach has proven to be superior to existing schemes, which has motivated many researchers to shift their work towards studying, understanding and exploiting the randomness of the physical layer. In this context, this paper presents a novel keyed-hash function for Orthogonal Frequency Division Multiplexing (OFDM) systems based on the physical characteristics and properties of wireless channels. More specifically, communicating entities sharing the same channel, extract common properties and mix them with a pre-shared secret key to produce a channel-based dynamic key. This key will later be used to generate the cipher primitives needed for performing source authentication and message integrity of the transmitted OFDM symbols. Since OFDM symbols, in the frequency-domain and time-domain, have different properties, the proposed keyed-hash function is modified and adapted to suit each case leading to two variants of the solution. To the best of our knowledge, this is the first work that proposes a keyed-hash function for complex OFDM symbols, based on physical layer conditions. Finally, different security and performance tests are conducted to prove the robustness and efficiency of this technique, in comparison to currently employed message authentication schemes.

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its good spatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabrication further leverage the use of TR wideband communication systems. The TR is generally carried out in the time domain and the focusing effect of TR comes from the use of a high transmit rate back-off factor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequency-division multiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TR precoding in combination with OFDM communication systems. Furthermore, most existing works on FD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. In this paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output (MISO) OFDM systems. In particular, we compare the communication performance of such system at the intended position and that at the unintended position. Based on the analysis, we demonstrate that increasing the BOF and/or the number of transmit antennas significantly improves the focusing effect at the intended position. In contrast, the unintended positions receive less useful power. We derive approximated mean-square-error (MSE) expressions of equalized received signals at both intended and unintended positions. The subsequent focusing gain is presented as a function of the BOF and the number of antennas, enabling us to gain insights in the contributions of each parameter to the system performance. Numerical simulations with multi-path Rayleigh fading channels are carried out to validate the MSE expressions.

Low-Complexity Doppler Compensation Algorithm for Underwater Acoustic OFDM Systems With Nonuniform Doppler Shifts

The Doppler effect severely degrades the orthogonality of orthogonal frequency division multiplexing (OFDM) systems, and is a severe problem in the underwater acoustic (UWA) communications, leading to nonuniform Doppler shifts for all subcarriers. In order to alleviate Doppler effect, resampling technique by oversized fast Fourier transform (FFT) can achieve fast and accurate resampling in frequency domain. Nevertheless, the accuracy of this conventional FFT based approach depends on the FFT size, which in turn increases receiver complexity. In order to overcome this problem, the Doppler compensation model in frequency domain is simplified by Taylor series expansion in this letter. Based on the simplified model, we propose the Doppler compensation algorithm by using FFT and interpolation to achieve higher compensation accuracy. Simulation results demonstrate that the proposed algorithm shows very similar bit error rate (BER) performance with that of Chirp Z-transform (CZT) based method. However, the complexity of our proposed algorithm is about half of CZT based method. Further, our proposed low-complexity Doppler compensation algorithm can also be applied to the sampling frequency offset (SFO) correction in terrestrial OFDM communication systems.

A hybrid OFDM–CDMA-based robust image watermarking technique

Digital watermarking is a process of embedding hidden information called watermark into different kinds of media objects. It uses basic modulation, multiplexing and transform techniques of communication for hiding information. Traditional techniques used are least significant bit (LSB) modification, discrete cosine transform (DCT), discrete wavelet transform (DWT), discrete Fourier transform (DFT), code division multiple access (CDMA) or a combination of these. Among these, CDMA is the most robust against different attacks except geometric attacks. This paper proposes a blind and highly robust watermarking technique by utilizing the basis of orthogonal frequency division multiplexing (OFDM) and CDMA communication system. In this scheme, the insertion process starts by taking DFT of host images, permuting the watermark bits in randomized manner and recording them in a seed as a key. Then PSK modulation follows inverse DFT (IDFT) that gives watermark information as OFDM symbols. These symbols are spread using spreading codes and then arithmetically added to the host image. Finally, scheme applies inverse DCT (IDCT) to get watermarked host images. The simulation results of the proposed scheme are compared with CDMA-based scheme in DCT domain. The results show that the robustness of the proposed scheme is higher than the existing scheme for non-geometric attacks.

Characterization of Orthogonal Frequency Division Multiplexing (OFDM) Communication Links

The demand for high data rate transmission is rapidly increasing in the area of wireless communications and Orthogonal Frequency Division Multiplexing (OFDM) is known to be a promising technique for high-rate transmission that can overcome the inter symbol interference (ISI) which results from the time dispersive nature of wireless channels. The idea is to utilize a number of carriers, spread regularly over a frequency band, in such a way so that the available bandwidth is utilized to maximal efficiency. This study presents the comparison of the performance of different modulation schemes: M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) over Additive White Gaussian Noise (AWGN), Rayleigh and Rician multipath fading channels in OFDM system. Bit error rate (BER) was used to evaluate the performance of the system; the number of fast Fourier transform (FFT) points used during the transmission was also examined. The results showed that for all the channels, lower order modulation schemes perform better than the higher order schemes. This comes at the detriment of the data rate, as lower order schemes have lower data rates compared with their higher order counterparts. In addition, it was observed that the system performed better in AWGN channel than Rayleigh and Rician fading channels for all the modulation schemes used. It was observed that the number of FFT points used during the transmission do not have much effects on the performance of the system.

An Efficient Implementation and Analysis of Tail-Biting Convolution Coding Algorithm for OFDM Based System in Terms of Speed, Memory and Peak-to-Average Power Ratio Using DSP

Fourth Generation (4G) mobile communication system uses Orthogonal Frequency Division Multiplexing (OFDM). With this technique, high data rate demands are achieved. Tail biting Convolution Coding (TBCC) avoids the data rate loss, hence it is widely used error control coding algorithm in OFDM and other various wireless communication technologies. But, OFDM has increased Peak to Average Power Ratio (PAPR). High PAPR signal consumes more power and makes system power inefficient. To gain supercomputing performance in terms of Speed, Memory and Power on mobile devices, an efficient implementation of algorithms on Digital Signal Processing (DSP) processor is an essential requirement. This requirement becomes more stringent for Fifth Generation (5G) mobile devices. For this, wide scope of knowledge as well as skills are required to understand the algorithm, DSP architecture, instruction set, optimization and performance measurement. In this article, we have implemented TBCC algorithm using Bit by Bit (BYB) and Look Up Table (LUT) approaches on Freescale StarCore SC140 based DSP platform and proposed an efficient algorithm implementation methodology by comparing the machine cycles and memory requirement. We have used coding rate (R) = 1/2, 1/3, 1/4 and constraint length (K) = 5 and K = 9 for implementation of TBCC. Using our proposed LUT approach, we have achieved average 45.82% Computational Complexity Reduction Ratio (CCRR) in machine cycles compared to BYB approach. Proposed LUT approach increases the TBCC execution speed. Our developed fixed point routines can be used for any K ≤ 9. TBCC is also analyzed for PAPR to get an overall profiling results and three dimensional optimization of TBCC algorithm in terms of Speed, Memory and Power.

Design and implementation of reconfigurable DCT based adaptive PST techniques in OFDM communication system using interleaver encoder

Objectives: To propose Reconfigurable Discrete Cosine Transform (RDCT) in Orthogonal Frequency Division Multiplexing (OFDM) high speed communication system to improve efficient spectrum utilization and mitigation of jitter,selective fading effects. The adaptive PTS method reduces Inter-Carrier-Interference (ICI), Inter-Symbol-Interference (ISI), PAPR and the computational intricacy of the communication system. To analyse and compare the performance of proposed Reconfigurable method with existing conventional OFDM communication system. Methods: This proposed technique employs interleaver encoder with both PTS and adaptive PTS with the Reconfigurable Discrete Cosine Transform (RDCT) in OFDM Communication system. Finding: A case study has been carried out and the proposed method is found to be better in CCDF performance, reliability, flexibility and accuracy. To reduce the autocorrelation between every modulated data bit, Forward error Correction in comparison with the conventional OFDM communication system. Novelty: The recent high-speed communication system most commonly using Orthogonal Frequency division multiplexing (OFDM) to enhance the quality of service(QoS) and Bandwidth on demand. Even though the OFDM scheme offers many advantages the signal of OFDM consists of a noise that causes variation in the amplitude resulting in high peak to average power ratio (PAPR) and since OFDM is a multi-carrier system, it is more susceptible to signal drifts. Various methods to reduce the PAPR in OFDM signal are already proposed, in this paper, a technique called interleaver encoder-based Partial Transmit sequence (IEPTS) is reviewed and the new proposed technique employs interleaving encoder utilizing in both PTS and A-PTS with reconfigurable discrete cosine transform (RDCT) in OFDM high-speed communication system. Keywords: Interleaver encoder; adaptive partial transmit sequence (A-PTS);orthogonal frequency-division multiplexing (OFDM); reconfigurable discrete cosine transform (RDCT); partial transmit sequence (PTS)

Doppler Compensation of Orthogonal Frequency Division Multiplexing for Ocean Intelligent Multimodal Information Technology

The era of intelligence has arrived. Intelligent multimodal information is essential for underwater engineering. Sound is an indispensable medium in underwater multi-modal information technology. Orthogonal frequency division multiplexing underwater acoustic communication technology can achieve high bit rate communication. However, the time-varying Doppler of the underwater acoustic channel will cause inter carrier interference in the OFDM system, destroy the orthogonality between subcarriers, and affect system performance. In this paper, we propose a method based on wavelet function to compensate for the Doppler distortions of the underwater acoustic channel for OFDM data. This method uses a set of orthogonal polynomials to perform channel estimation in both the time and frequency domains and uses the matched filtering for channel equalization. Compared with the traditional polynomials, the smoothly varying polynomials in Wavelet FFT greatly reduces the correlation of the processed data, enable more accurate channel estimation and equalization. Finally, this paper verifies the feasibility of the algorithm through the Songhua Lake underwater experiment.

Time-Varying Channel Equalization in Underwater Acoustic OFDM Communication System

In this paper, three time-varying channel equalization schemes are studied in the underwater acoustic (UWA) Orthogonal Frequency Division Multiplexing (OFDM) communication system. The equalization algorithms are the zero-forcing (ZF) equalization algorithm, and the minimum mean square error equalization (MMSE) algorithm and the serial interference cancellation (SIC) equalization algorithm. Among the schemes, there is a problem of needing a large amount of operation when obtaining the inversion of the channel matrix. Then, to reduce the computation complexity of channel matrix inversion, the band approximation of the channel matrix, the serial equalization and the LDLH decomposition are also studied. To evaluate the efficacy of the algorithms studied in this paper, numerical simulation and the field experiment are both conducted. The simulation results proof that each equalization algorithm can work appropriately under different time-varying conditions, and valid the reliability of each simplified algorithm under the same Doppler factor. The results of two sets of field experiment also prove that the simplified algorithm eliminates the influence of the residual narrow band Doppler to a certain extent, and a better effect is obtained while a channel estimation algorithm with higher accuracy is combined.