Wideband Orthogonal Frequency Division Multiplexing Research Articles

Channel Estimation With Reconfigurable Intelligent Surfaces—A General Framework

Optimally extracting the advantages available from reconfigurable intelligent surfaces (RISs) in wireless communications systems requires estimation of the channels to and from the RIS. The process of determining these channels is complicated when the RIS is composed of passive elements without any sensing or data processing capabilities, and thus, the channels must be estimated indirectly by a noncolocated device, typically a controlling base station (BS). In this article, we examine channel estimation for passive RIS-based systems from a fundamental viewpoint. We study various possible channel models and the identifiability of the models as a function of the available pilot data and behavior of the RIS during training. In particular, we will consider situations with and without line-of-sight propagation, single-antenna and multi-antenna configurations for the users and BS, correlated and sparse channel models, single-carrier and wideband orthogonal frequency-division multiplexing (OFDM) scenarios, availability of direct links between the users and BS, exploitation of prior information, as well as a number of other special cases. We further conduct simulations of representative algorithms and comparisons of their performance for various channel models using the relevant Cramér-Rao bounds.

An Efficient ICI Cancellation Scheme to Mitigate the Effect of ICI on OFDM Systems.

Inter-carrier interference (ICI) emerges in orthogonal frequency division multiplexing (OFDM) systems used for mobile communication as a consequence of the Doppler Effect's loss of orthogonality among subcarriers. Inter-Carrier Interference (ICI), which affects every subcarrier, drastically lowers performance. The performance of OFDM systems may be enhanced using a variety of ICI mitigation strategies. Comparable subcarrier frequency offsets are guaranteed by the premise that the OFDM transmission bandwidth is suitably modest in the majority of ICI mitigation strategies, on the other hand. The frequency offsets between each subcarrier might change, hence a wideband OFDM system in a situation with high mobility is investigated. Furthermore, the suggested ICI cancellation approach, Total ICI Cancellation, does not reduce bandwidth efficiency or transmission rate. As an example, the Total ICI Cancellation approach uses the ICI matrix's orthogonality to provide perfect ICI cancellation and a significant boost in BER at a linearly increasing cost. The suggested technique, which matches the BER performance of a wideband OFDM system without ICI, offers the best BER performance possible in the presence of frequency offset and time shifts in the channel, according to simulation findings in the AWGN and multipath fading channels.

Low-Cost Passive Beamforming for RIS-Aided Wideband OFDM Systems

Reconfigurable intelligent surface (RIS) has become a promising solution to improve the performance of future wireless systems with low energy consumptions. Concerning a RIS-aided orthogonal frequency division multiplexing (OFDM) system, we perform optimization with respect to the RIS passive beamforming such that the sum rate over all subcarriers is maximized. Different from an existing successive convex approximation (SCA) method, we propose a majorization-minimization (MM)-based iterative approach to optimize the RIS passive beamforming, where a closed-form solution is obtained in each iteration. Simulation results show that the proposed RIS passive beamforming achieves almost the same performance as the SCA-based method while with much lower computational complexity.

Rate maximizing OFDM parameter optimization on HF channels

In this work, we propose an algorithm which maximizes the bit rate of a given wideband OFDM (Orthogonal Frequency-Division Multiplexing) system operating in the HF (High Frequency) band. The proposed algorithm is built on the work given in the literature for estimating the electron densities in the ionosphere and estimating the propagation path of signals throughout the ionosphere. Algorithm uses the outputs of the ray tracing programs in order to calculate the channel parameters for using to optimize the parameters of the HF communication system. Proposed algorithm helps to determine the capacity of the subchannels, transmitter power, number of subchannels, carrier frequency, subchannel bandwidth and cyclic prefix for the OFDM in order to maximize the bit rate. Carrier frequencies produced by the proposed algorithm are compared with the frequencies suggested by the VOACAP (Voice of America Coverage Analysis Program) program.

A Spectral-Correlation-Based Blind Calibration Method for Time-Interleaved ADCs

With the quick expansion of signal bandwidth in communication systems, time-interleaved Analog-to-Digital Converters (TI-ADC) provide quick pathway to very high sampling speed. Nonetheless, mismatches among the sub-ADC channels are major performance limiting factors for TI-ADCs. With the scaling of CMOS processes, all-digital background calibration techniques with blind estimation have been actively pursued in recent years to compensate for the mismatches from TI-ADCs. In this paper, a new all-digital background calibration technique is introduced. The technique achieves blind estimation by exploiting the correlation difference between the signal component and the mismatch spurs. This method can process wideband signals and can be implemented with low calibration overhead. The technique is verified with extensive simulations over a 2GS/s 12bit 16-channel ADC. Simulation shows that, with ADC mismatch spurs over −40dBc, the new technique can reliably suppress the spurs to be lower than −80dBc within the duration of about $1.2\times 10^{6}$ samples with a typical wideband OFDM (orthogonal frequency-division multiplexing) signal.

Practical Modeling and Beamforming for Intelligent Reflecting Surface Aided Wideband Systems

Intelligent reflecting surface (IRS) has emerged as a revolutionizing solution to enhance wireless communications by intelligently changing the propagation environment. Prior studies on IRS are based on an ideal reflection model with a constant amplitude and a variable phase shift. However, it is difficult and unrealistic to implement an IRS satisfying such ideal reflection model in practical applications. In this letter, we aim to investigate the phase-amplitude-frequency relationship of the reflected signals and propose a practical model of reflection coefficient for an IRS-aided wideband system. Then, based on this practical model, joint transmit power allocation of each subcarrier and IRS beamforming optimization are investigated for an IRS-aided wideband orthogonal frequency-division multiplexing (OFDM) system. Simulation results illustrate the importance of the practical model on the IRS designs and validate the effectiveness of our proposed model.

Base Station Antenna Selection for Low-Resolution ADC Systems

For low-resolution analog-to-digital converter (ADC) systems, only high-complexity receive antenna selection has been developed and transmit antenna selection has been limited to a single antenna selection in prior work. In this paper, we propose low-complexity receive antenna selection algorithms and analyze transmit antenna selection by considering antenna selection at a base station with large antenna arrays and low-resolution ADCs. For downlink antenna selection, we show a selection criterion with zero-forcing precoding equivalent to a perfect quantization system; sum rate increases with number of selected antennas; derivation of the sum rate loss function from using a antenna subset; and sum rate loss reaches a maximum at a point of total transmit power and decreases beyond that point to converge to zero. For wideband orthogonal-frequency-division-multiplexing (OFDM) systems, our results hold when entire subcarriers share a common subset of antennas. For uplink antenna selection, we generalize a greedy antenna selection criterion; propose a quantization-aware fast antenna selection algorithm using the criterion; and derive a lower bound on sum rate achieved by the proposed algorithm. For wideband OFDM systems, we extend our algorithm and derive a lower bound on its sum rate. Simulation results validate theoretical analyses and show increases in sum rate over conventional algorithms.

Multi-Gigabit/s OFDM real-time based transceiver engine for emerging 5G MIMO systems

This paper presents a highly scalable multi-Gigabit/s Real-Time (RT) Wideband (WB) Orthogonal Frequency Division Multiplexing (OFDM) processing chain for 5G applications. It is aimed to significantly reduce the Hardware (HW) footprint of Multiple Input Multiple Output (MIMO) systems. In this context, implementation complexity results for MIMO configurations of 2 × 2, 4 × 4 and 8 × 8, enabled at sampling rates of 245.76, 122.88, 61.44 and 30.72 MHz, respectively, are compared with the ones from conventional MIMO architectures. For example, for a 8 × 8 MIMO–OFDM configuration implementation, savings of up to 87% are achieved when compared with conventional design methods, in terms of DSP48E1 slices. Moreover, in a Xilinx Virtex 7 XC7VX485T, it is shown that a second processing branch might be implemented, leading to a 5 Gbps (using 1024 Quadrature Amplitude Modulation (QAM)) Single-Input Single-Output (SISO) link or a 16 × 16 MIMO configuration. Finally, in order to validate the proposed architecture, the impact of its inclusion on a complete Field Programmable Gate Array (FPGA) 8 × 8 OFDM system, at LTE’s highest sampling rate of 30.72 MHz, is evaluated. Subsequently, it is demonstrated that this does not affect the performance of OFDM, even in the presence of Carrier Frequency Offset (CFO).

Parameter estimation based on factor graph in wide‐band OFDM systems

The channel estimation in a wide-band orthogonal frequency division multiplexing (OFDM) system is different from the narrow-band system, for the estimation parameters should include the Doppler scale factor, which is a particular parameter of the wide-band system. The underwater acoustic OFDM system is a typical wide-band system, and its channel has more than one scale factor usually. For this multi-scale case, the common estimation method is a factor-by-factor process with repeated calculation of the reproduced waveform. This article proposes a factor-graph-based estimation method, which fully uses the restriction relationship among the parameters, and estimates the parameters simultaneously. To reduce the computation load, the modified Bayesian propagation (BP) algorithm is proposed. Comparing to the common BP algorithm, the modified BP algorithm simplifies the calculation of the condition probability in the summation. The simulation results of the mean squared error show that the proposed method is an effective estimation method for the multi-scale wide-band system. The bit error rate performance of the proposed method is very close to the result of the known channel-state-information, and outperforms the fractional Fourier transform (FrFT) and the modified particle swarm optimisation (MPSO) methods for well channel condition.

Optical Comb Assisted OFDM RF Receiver

Orthogonal frequency-division multiplexing (OFDM) is a widely used signaling format in wireless communication and ultrawide band radio-frequency (RF) applications. An OFDM transceiver relies on a discrete Fourier transform (DFT) processor that matches both the resolution and the bandwidth of wireless link. A conventional wideband OFDM receiver necessarily relies on high-speed analog-to-digital converters (ADC) followed by a Fourier processing core. Both fundamental and practical ADC considerations that include bandwidth-resolution tradeoff impose strict limits on high precision digitization of the wideband OFDM signal. In addition to the quantization barrier, computational complexity of the high-speed DFT processor leads to high dissipation levels in conventional receivers. This report demonstrates an alternative OFDM receiver architecture that eliminates the need for high-speed ADC and digital DFT processor. The new design is based on hybrid, optoelectronic DFT processor, capable of generating DFT coefficients of wideband RF signal in real time. We demonstrate demodulation of a 4-quadratic amplitude modulation OFDM signal with 60 subcarriers positioned within 4–15.8 GHz band and separated by 200 MHz. The error vector magnitude is measured for all channels to characterize the performance of the comb-assisted OFDM RF receiver.

OFDM waveform design based on mutual information for cognitive radar applications

We propose a novel optimization method based on wideband orthogonal frequency division multiplexing (OFDM) signals to detect random extended targets with known covariance matrix in the presence of additive white Gaussian noise. Mutual information is used as our criterion for waveform design under transmitted power constraint. We utilize the advantage of OFDM signal to intelligently design the complex weights of the transmitted waveform. For making complete use of the transmission power, a novel iterative algorithm is introduced based on maximizing mutual information criterion between the target impulse response and the received echoes. We have derived the optimal Neyman–Pearson detector for the corresponding hypothesis testing problem and provided different numerical experiments to demonstrate the achieved performance improvement when the proposed method is applied.

High Range and Doppler Resolution by Application of Compressed Sensing Using Low Baseband Bandwidth OFDM Radar

Multicarrier waveforms such as orthogonal frequency-division multiplexing (OFDM) found their way into radar applications in the last few years. However, currently, typically only a fraction of the large baseband bandwidth required to obtain high resolution is available in practice due to hardware limitations. In this paper, we propose a frequency-agile sparse OFDM radar processing that allows the transmission of consecutive bandwidth-reduced OFDM pulses on different carriers, and thereby covering a much larger measurement bandwidth in a measurement frame. Through joint processing of multiple narrowband pulses and compressed sensing methods, high resolution and unambiguity in the joint range-velocity profile are obtained comparable to an equivalent wideband OFDM. It shows that a baseband bandwidth of 20% of the full channel bandwidth is sufficient to reliably obtain the same result as for an equivalent wideband OFDM signal. The proposed processing scheme is validated using simulations and radar measurements at 77 GHz.

Pilot Design for Sparse Channel Estimation in Orthogonal Frequency Division Multiplexing Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a well-known technique used in modern wide band wireless communication systems. Coherent OFDM systems achieve its advantages over a multipath fading channel, if channel impulse response is estimated precisely at the receiver. Pilot-aided channel estimation in wide band OFDM systems adopts the recently explored compressive sensing technique to decrease the transmission overhead of pilot subcarriers, since it exploits the inherent sparsity of the wireless fading channel. The accuracy of compressive sensing techniques in sparse channel estimation is based on the location of pilots among OFDM subcarriers. A sufficient condition for the optimal pilot selection from Sylow subgroups is derived. A Sylow subgroup does not exist for most practical OFDM systems. Therefore, a deterministic pilot search algorithm is described to select pilot locations based on minimizing coherence, along with minimum variance. Simulation results reveal the effectiveness of the proposed algorithm in terms of bit error rate, compared to the existing solutions.

A novel PAPR reduction scheme for VLC DCO-OFDM systems

In this study, we aim to reduce the peak-to-average power ratio (PAPR) for indoor visible light communication (VLC) orthogonal frequency division multiplexing (OFDM) systems and improve the system performance. By utilizing the spatial multiplexing (SMP) technique, we propose a subcarrier spatial grouping (SSG) scheme to address the high PAPR and LED restricted linear range problems. In this scheme, the high-PAPR wideband OFDM signals are partitioned into several low-PAPR narrowband independent signals that are simultaneously transmitted from multiple LED light sources. At the receiver, zero forcing (ZF) is used to reverse the impairments of the channel transform. The computer simulation results demonstrate that the proposed approach achieves lower PAPR and more robustness to LED nonlinearities leading to better bit error rate performance for the same spectral efficiency than that of the conventional DCO-OFDM scheme does in the whole signal-to-noise ratios range.

Reception and detection of a wide‐band OFDM signal in a Doppler spreading channel

This study focuses on the reception strategy and the detection method of a wideband orthogonal frequency-division multiplexing (OFDM) signal, which is passing through a Doppler channel. Most prior works regarded the Doppler effect as a reason of interchannel interference due to its destruction of the subcarrier orthogonality. Distinguishing from those works, the authors point out that the wideband OFDM signal is still an OFDM signal with subcarrier orthogonality after passing the channel with single Doppler scaling factor. Thus, the received OFDM signal passing the Doppler spreading channel is a linear superposition of a set of transmitted OFDM signals associated with different parameters. To make full use of the diversity gain offered by the superposition, the authors propose a strategy of diversity reception and a belief propagation detection method for the received wideband OFDM signal. The simulation results show that the proposed strategy improves the performance obviously with increase of the diversity order in a Doppler spreading channel. It offers different options for system demands. Furthermore, the proposed detection method can offer an acceptable performance even it combined with the conventional sampling process. It makes the conventional structure replacing the common resampling structure possible in a wideband OFDM system.

Improved Artificial Bee Colony Optimization Approach in UWBOFDM for Frequency Offset Estimation

Abstract Orthogonal frequency division multiplexing (OFDM) is considered a crucial and fundamental potential approach because of its significance and role with regards to broadband wireless communication systems. The swarm intelligence optimization approach has been deployed to facilitate frequency offset optimization. An improved artificial bee colony (IABC) based optimization approach is proposed in this work that will enable ultra wideband orthogonal frequency division multiplexing (UWBOFDM) system, which will then be deployed in the carrier frequency offset (CFO) joint estimation and also for sampling frequency offset (SFO). By proper selection of cyclic delay times, correlation existing between adjacent sub carriers, a joint estimation of both CFO and SFO has been derived by employing IABC. The performance of the proposed algorithm is compared with the existing estimators, and this comparison is carried out by MATLAB7.2 simulation, which clearly depicts that the performance of the proposed algorithm is superior in terms of estimation accuracy. The performances are measured in terms of the mean square error (MSE), signal-to-noise ratio (SNR) and normalized mean square error (NMSE). The proposed approach considered for transmitter (t = 4) and receiver (r = 4) with SNR is 30 dB, which achieves MSE of about 0.35 %. Similarly, when (t = 2) and (r = 2), the proposed IABC approach achieves MSE of about 0.4 %, whereas particle swam optimization (PSO) and maximum likelihood estimator (MLE) methods achieve 0.42 and 0.44 % MSE values, respectively. On the other hand, considering (t = 4) and (r = 4) with SNR of 8 dB, the proposed approach achieves NMSE of about 0.0008 %. However, considering (t = 2) and (r = 2), the NMSE values of IABC approach, PSO, and MLE are observed to be 0.002, 0.008, and 0.08 %, respectively, for Channel Model 4 (CM4).

Performance Analysis of Direct-Learning Digital Predistortion With Loop Delay Mismatch in Wideband Transmitters

Orthogonal frequency-division multiplexing (OFDM) modulation, which is commonly used in modern digital communication systems due to its flexibility and high spectral efficiency, is sensitive to transmitter nonlinearities introduced by the power amplifiers (PAs). As a key technology in wireless transmitting systems, digital predistortion (DPD) is widely applied to compensate for the transmitter nonlinearities and ensure high power-added efficiency in base stations and mobile devices for cellular systems. In a DPD architecture, the transmitter input and output signals are sampled, aligned, and processed to extract the DPD parameters that would be used to predistort the source signal before transmission to counteract the transmitter nonlinearities. However, the alignment accuracy in DPD is limited by nonideal electronic components and the associated circuitry, which introduces unknown loop delay mismatch and, thus, degrades the overall linearization performance. In this paper, we analyze the effect of a loop delay mismatch on PA model identification and, hence, on DPD linearization performance of the wideband OFDM signals. The expression and upper limit for the normalized mean square error (NMSE) are derived in terms of the loop delay mismatch, transmission signal bandwidth, and signal-to-noise ratio (SNR) of the feedback channel. The expression for the adjacent channel leakage ratio (ACLR) performance is also derived to predict the DPD linearization performance with the presence of the loop delay mismatch. The theoretical analysis reveals that the performance degradation increases with the bandwidth of the OFDM signal and the modulus of the loop delay mismatch. Experiments are performed on the Long-Term Evolution (LTE) Advanced signals with bandwidth ranging from 20 to 100 MHz to evaluate the degradation effect of the loop delay mismatch on the NMSE and ACLR performances.

Time-Domain LMMSE Channel Estimator Based on Sliding Window for OFDM Systems in High-Mobility Situations

Recently, wideband orthogonal frequency-division multiplexing (OFDM)-based systems such as Long-Term Evolution (LTE) have been proposed for future railway communication systems. In such high-speed situations, Doppler phenomena, related to the terminal's high speed, affects the use of OFDM systems by producing a loss of orthogonality between different carriers. Thus, the channel estimation and equalization become a challenging problem, particularly when the number of pilot carriers is limited. This paper investigates the use of time-domain channel estimation for OFDM systems in high-speed scenarios and for a sparse pilot distribution. A time-domain linear minimum mean square error (TD-LMMSE) filter is designed for channel estimation. The TD-LMMSE emulates the effect of a sliding window and takes into consideration the interference caused by the data field. The expression of the MSE of linear estimators is also developed. Furthermore, a simple technique for the estimation of the second-order channel statistics is proposed. Simulation results, showing a significant gain in terms of MSE and bit error rate, compared with other state-of-the-art algorithms, are reported over a wide signal-to-noise ratio (SNR) range.

Stepped OFDM radar technique to resolve range and doppler simultaneously

Recent developments involving the orthogonal frequency division multiplexing (OFDM) structure call for new potentials in radar. In this paper, a novel wideband OFDM based waveform is presented and the high range resolution (HRR) processing is derived.We show by means of comparisons with standard wideband waveforms the benefits of this concept. Radial velocities can be retrieved unambiguously and high velocities can be handled at no extra computational effort since the range migration constraint is related to the small pulse bandwidth as opposed to the large synthetic bandwidth. In our analysis, we inspect the impact of Doppler modulation on the processing and give recommendations for improvement in terms of the OFDM parameters. We address the problem caused by a correlation artefact and discuss the risk of intersymbol interference (ISI) typical with OFDM. Lastly, we show that our concept has inherent low probability of detection (LPD) characteristics.

Adaptive OFDM Radar Waveform Design for Improved Micro-Doppler Estimation

We analyze the performance of a wideband orthogonal frequency division multiplexing (OFDM) signal in estimating the micro-Doppler frequency of a rotating target having multiple scattering centers. The use of a frequency-diverse OFDM signal enables us to independently analyze the micro-Doppler characteristics with respect to a set of orthogonal subcarrier frequencies. We characterize the accuracy of micro-Doppler frequency estimation by computing the Cramer-Rao bound (CRB) on the angular-velocity estimate of the target. Additionally, to improve the accuracy of the estimation procedure, we formulate and solve an optimization problem by minimizing the CRB on the angular-velocity estimate with respect to the OFDM spectral coefficients. We present several numerical examples to demonstrate the CRB variations with respect to the signal-to-noise ratios, number of temporal samples, and number of OFDM subcarriers. The improvement in estimation accuracy due to the adaptive waveform design is also analyzed numerically. A grid-based maximum likelihood estimation technique is applied to evaluate the corresponding mean-squared error performance.