OFDM Block Research Articles

Index Modulation OFDM Systems with Direct Pattern of Subcarrier Activation

OFDM-IM is a spatial modulation technique applied to OFDM's block subcarriers, resulting in a modified version of OFDM. Compared to traditional OFDM, OFDM-IM boasts several benefits, such as enhanced bit-error rate (BER) performance and power efficiency. When working with OFDM-IM, the specification of subcarrier-activation-patterns (SAPs) is done by utilizing either look-up tables (LUTs) or a combinatorial approach to conform with data. However, the data-to-SAP mapping methods of both suffer from increased complexity as the number of subcarriers expands. The paper proposes a direct data-to-SAP mapping that eliminates the need for any type of look-up tables. Through the presented analysis and computer simulation results, it is demonstrated that the OFDM-IM system utilizing the proposed mapping scheme outperforms equivalent traditional OFDM-IM systems in both complexity and bit error rate (BER) performance.

Optimization of system’s parameters for wavelength conversion of E-band signals

Current and future wireless communication systems are designed to achieve the user’s demands such as high data rate and high speed with low latency and simultaneously to save bandwidth and spectrum. In 5G and 6G networks, a high speed of transmitting and switching is required for internet of things (IoT) applications with higher capacity. To achieve these requirements a semiconductor optical amplifier (SOA) is considered as a wavelength converter to transmit a signal with an orthogonal frequency division multiplexing with subcarrier power modulation (OFDM-SPM). It exploits the subcarrier’s power in conventional OFDM block in order to send additional bits beside the normally transmitted bits. In this paper, we optimized the SOA’s parameters to have efficient wavelength conversion process. These parameters are included the injection current (IC) of SOA, power of pump and probe signals. A 7 Gbps OFDM-SPM signal with a millimeter waves (MMW) carrier of 80 GHz is considered for signal switching. The simulation results investigated and analyzed the performance of the designed system in terms of error vector magnitude (EVM), bit error rate (BER) and optical signal-to-noise ratio (OSNR). The optimum value of IC is 0.6 A while probe power is 9.45 and 8.9 dBm for pump power. The simulation is executed by virtual photonic integrated (VPI) software.

Nonlinear suppression for polarization division multiplexed optical OFDM/OQAM systems

Optical orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) increases the system spectral efficiency compared with the conventional optical OFDM by removing the cyclic prefix (CP) inserted between consecutive optical OFDM blocks. For optical OFDM/OQAM, the orthogonal condition is relaxed from the complex domain to the real field. Thus, nonlinear suppression for this kind of system is much more complicated than for optical OFDM. In this paper, we proposed a dual Volterra expansion based nonlinear equalization (DVNE) method for the suppressing of nonlinear interference for polarization division multiplexed (PDM) optical OFDM/OQAM system. In this work, nonlinear interference concentration property of the filter banks is systemically discussed for PDM OFDM/OQAM system. Then, pilot-aided DVNE nonlinear equalization method is proposed to improve system robustness against nonlinear interference by using specially designed pilot sequences. As shown in numerous Montel Carlo simulation results, system robustness against nonlinear interference have been improved significantly for both 59.97 Gb/s 4-QAM CO-OFDM/OQAM and 59.97 Gb/s 16-QAM CO-OFDM/OQAM system respectively thanks to the proposed method.

MIMO-OFDM LTE system based on a parallel IFFT/FFT on NoC-based FPGA

The growing demand for wireless devices capable of performing complex communication processes has imposed an urgent need for high-speed communication systems and advanced network processors. This paper proposes a hardware workflow developed for the Long-Term Evolution (LTE) communication system. It studies the multiple-input, multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) LTE system. Specifically, this work focuses on the implementation of the OFDM block that dominates the execution time in high-speed communication systems. To achieve this goal, we have proposed an NoC-based low-latency OFDM LTE multicore system that leverages Inverse Fast Fourier Transform (IFFT) parallel computation on a variable number of processing cores. The proposed multicore system is implemented on an FPGA platform using the ProNoC tool, an automated rapid prototyping platform. Our obtained results show that LTE OFDM execution time is drastically reduced by increasing the number of processing cores. Nevertheless, the NoC’s parameters, such as routing algorithm and topology, have a negligible influence on the overall execution time. The implementation results show up to 24% and 76% execution time reduction for a system having 2 and 16 processing cores compared to conventional LTE OFDM implemented in a single-core, respectively. We have found that a 4×4 Mesh NoC with XY deterministic routing connected to 16 processing tiles computing IFFT task is the most efficient configuration for computing LTE OFDM. This configuration is 4.12 times faster than a conventional system running on a single-core processor.

Signal Space Diversity for Improving the Reliability Performance of OFDM with Subcarrier Power Modulation

As the demand for higher data rates continues to increase exponentially, there is a shift towards exploring contemporary techniques that are likely to offer higher spectral efficiency. A new modulation technique termed as orthogonal frequency division multiplexing with sub‐carrier power modulation (OFDM‐SPM) has recently been introduced. It is an original technique that is still largely under-explored and aims at transmitting more bits per sub‐carrier by manipulating the power of the sub‐carriers in an OFDM block, in addition to those bits that are usually transmitted by conventional modulation schemes such as M-PSK. During the investigation and theoretical testing stages, it was found that the additional data stream conveyed by subcarriers' power has higher bit error rate (BER) performance compared to the data stream conveyed by conventional modulation schemes. To overcome this shortfall, signal space diversity (SSD) along with coordinate interleaving is proposed in this work to be integrated with OFDM‐SPM to help improve the overall BER performance. By doing so, it is shown that a BER performance of 10-3 can be obtained at SNR of 15 dB, which means achieving an improvement of more than 5 dB compared to the case of using OFDM‐SPM without SSD. Furthermore, the performance results of OFDM‐SPM‐SSD is compared with those of OFDM‐SPM with maximal ratio combining (MRC) and the obtained results show that OFDM‐SPM‐SSD offers superior performance. Additionally, a study of the effect of the constellation rotation on OFDM‐SPM is provided and an in‐depth analysis is also carried out for different power polices of sub‐carrier power modulation with regards to two main performance metrics; namely BER and the throughput. Simulation results show that SSD provides a considerable improvement in the BER over both plain OFDM‐SPM and OFDM‐SPM‐MRC. The analysis also reveal that when considering the different power policies of SPM, the BER results are optimum in the case of using power‐reassignment policy.

Continuous phase Flip-OFDM in optical wireless communications

Intensity Modulated (IM) optical communication over an LED channel requires the use of a non-negative signal that can also cope with the low-pass nature of LEDs. For this purpose, dedicated schemes such as Flip-OFDM and Assymetrically Clipped Optical (ACO)-OFDM have been proposed. We derive a common mathematical description on which both schemes rely. Exploiting this insight, we propose Continuous Phase Flip-OFDM (CP-Flip-OFDM) as an enhancement to Flip-OFDM. It ensures phase continuity at the transition between the two successive copies of the OFDM blocks, thereby it obviates the Cyclic Midfix between the first OFDM block and its flipped copy. Simultaneously, we derive a less compute-intensive way to generate and detect ACO-OFDM. Instead of creating an Hermitian-symmetry at the transmit Inverse FFT input, which is common in optical IM OFDM, we use zero padding. After the transmit IFFT, a phase ramp-up, i.e., a multiplication with a complex-valued exponential, is applied before truncating to a real and non-negative signal.

Combined phase offset channel estimation method for optical OFDM/OQAM

On the basis of coherent optical orthogonal frequency-division-multiplexing (CO-OFDM), CO-OFDM/ offset quadrature amplitude modulation (CO-OFDM/OQAM) improves the spectrum efficiency by utilizing specially designed filter banks to eliminate the cyclic prefix (CP) inserted between consecutive OFDM blocks and decreases the demanding of guard band between two channels, being a promising candidate for the next generation optical transmission link. Due to the relaxation of the orthogonal condition from the complex field to the real field, chromatic Dispersion (CD) and polarization mode dispersion (PMD) bring serious intrinsic imaginary interference (IMI) to an optical OFDM/OQAM system. Channel estimation schemes based on the interference approximation method (IAM) have been proved to be effective in suppressing the IMI with low complexity. For the IAMs, increasing the power of the pseudo pilot could decrease the effects of amplified spontaneous emission (ASE) noise and other frequency domain residual error on the channel estimation accuracy. In most of the previous reported IAM works, pilot blocks were designed according to the structure of the center pilot block loaded, while the two side pilots nulled for simplicity. However, the power of the pseudo pilot under this method did not reach the maximum, resulting in non-optimal channel estimation accuracy. In this paper, channel estimation method based on the combined phase offset (CMPO) is proposed and systematically discussed. For the proposed method, three consecutive full loaded blocks are treated as the pilot blocks and the optimal pilot structure have been obtained by utilizing the maximum likelihood algorithm. As validated by numerous Montel Carlo simulations results, system robustness against IMI, phase noised induced interference, and nonlinear effect have been improved significantly, thanks to CMPO.

P.648 Pre-weaning maternal rat antibiotic exposure leads to long-term neurobehavioral and neurochemical outcome in rat offspring

On the basis of coherent optical orthogonal frequency-division-multiplexing (CO-OFDM), CO-OFDM/ offset quadrature amplitude modulation (CO-OFDM/OQAM) improves the spectrum efficiency by utilizing specially designed filter banks to eliminate the cyclic prefix (CP) inserted between consecutive OFDM blocks and decreases the demanding of guard band between two channels, being a promising candidate for the next generation optical transmission link. Due to the relaxation of the orthogonal condition from the complex field to the real field, chromatic Dispersion (CD) and polarization mode dispersion (PMD) bring serious intrinsic imaginary interference (IMI) to an optical OFDM/OQAM system. Channel estimation schemes based on the interference approximation method (IAM) have been proved to be effective in suppressing the IMI with low complexity. For the IAMs, increasing the power of the pseudo pilot could decrease the effects of amplified spontaneous emission (ASE) noise and other frequency domain residual error on the channel estimation accuracy. In most of the previous reported IAM works, pilot blocks were designed according to the structure of the center pilot block loaded, while the two side pilots nulled for simplicity. However, the power of the pseudo pilot under this method did not reach the maximum, resulting in non-optimal channel estimation accuracy. In this paper, channel estimation method based on the combined phase offset (CMPO) is proposed and systematically discussed. For the proposed method, three consecutive full loaded blocks are treated as the pilot blocks and the optimal pilot structure have been obtained by utilizing the maximum likelihood algorithm. As validated by numerous Montel Carlo simulations results, system robustness against IMI, phase noised induced interference, and nonlinear effect have been improved significantly, thanks to CMPO.

Complementary Coded CDMA Systems With CP-Free OFDM

In a frequency division complementary coded CDMA (FD-CC-CDMA) system, multiple tuners have to be used for multi-carrier modulation, which has a very high complexity and power consumption. In this paper, we propose a CP-free OFDM scheme to implement multi-carrier modulation in an FD-CC-CDMA system, which is named as CP-free OFDM-CC-CDMA system. The transmitter in the CP-free OFDM-CC-CDMA system offers a unique flexibility to adjust the amount of data processed at the same time to ensure that the OFDM block size exceeds the delay spread of a multipath channel, which attributes to effective mitigation of multipath induced inter-symbol interference (ISI). At the receiver of a CP-free OFDM-CC-CDMA system, minimum mean square error (MMSE) equalization and MMSE combining are used to mitigate multi-user interference (MUI) and inter-carrier interference (ICI). The obtained results show that a CP-free OFDM-CC-CDMA system offers a much higher transmission efficiency than FD-CC-CDMA and OFDM-DS-CDMA systems. In addition, the proposed CP-free OFDM-CC-CDMA system outperforms OFDM-DS-CDMA system in terms of bit error rate due to its inherent powerful diversity gain.

Channel prediction based temporal multiple sparse bayesian learning for channel estimation in fast time-varying underwater acoustic OFDM communications

In recent years, the multi-blocks joint channel estimation methods for underwater acoustic (UWA) OFDM systems have been proposed to improve the performance by using time correlation across consecutive OFDM blocks. However, the assumption of common sparse support (multi-path delays are invariant across consecutive blocks) in these methods is difficult to be met in fast time-varying channels. Therefore, we propose a channel prediction based joint channel estimation method for UWA fast time-varying channels, where multi-path delays and gains change significantly across consecutive OFDM blocks. Firstly we define a channel offset parameters model by the clustering property of UWA channels, and adopt Orthogonal Matching Pursuit (OMP) algorithm to estimate the channel offset parameters. Then we reconstruct a virtual current received signal based on the prediction method. Finally we combine the virtual received signal and the actual one into a joint estimation model, and utilize temporal multiple sparse Bayesian learning (TMSBL) method to jointly estimate the channel. Results of simulations and sea trial demonstrate the effectiveness of the proposed method in fast time-varying UWA channels, which achieves better performance than both the TMSBL method which is based on the joint processing of multi-blocks, and the OMP method which is based on block-by-block processing.

Blind channel identification for cyclic-prefixed MIMO-OFDM systems with virtual carriers

This paper applies the repetition index scheme (RIS) to the channel identification of cyclic prefixed (CP) multiple-input Multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with virtual carriers (VCs) in the environment of the number of receive antennas being no less than that of transmit antennas. The VCs will cause a rank deficiency problem in computing the subspace information. With the subcarrier mapping matrix, the received signal is simplified to remove the rank deficiency. We use the RIS scheme to generate many times of equivalent symbols so the channel identification can converge with few received OFDM blocks. The RIS scheme will convert the white noise into non-white noise. With the Cholesky factorization, a noise whitening technique is developed to turn the non-white noise back to white noise. We further analyze the necessary conditions of identifiability of channel estimation. Simulations are performed to show the superiority of the proposed method.

An Emergence Alert Broadcast Based on Cluster Diversity for Autonomous Vehicles in Indoor Environments

This paper addresses an emergency alert broadcast for autonomous vehicles in indoor environments. For a rapid alert broadcast, a novel cluster diversity is proposed. Using the cluster diversity, the alert broadcast can significantly reduce the number of rebroadcasts. This leads to the rapid delivery of emergency alerts. The cluster diversity consists of clustering and diversity combining. For the clustering, the cluster diversity technique utilizes the location information of autonomous vehicles. Using the location information, a cluster head is selected in each cluster. The cluster head rebroadcasts the emergency alert to the autonomous vehicles. Therefore, the clustering can lessen the number of rebroadcasts. In order to further reduce the number of rebroadcasts, a diversity combining technique is proposed in this paper. The diversity approach is based on the clustering. For the diversity, the emergency alert packet is repeated in the transmitted OFDM block. For each cluster, the emergency alert packet can orthogonally be allocated to the corresponding frequency sub-band in the OFDM block. In the received OFDM block, the autonomous vehicle utilizes the emergency alert packet with the maximum power in order to achieve a diversity. The experimental results exhibit that the proposed cluster diversity can considerably reduce the number of rebroadcasts in indoor environments. The results also show that the presented approach substantially outperforms the conventional technique in terms of the number of outage vehicles in the cases of rebroadcasts under indoor environments.

Signal-Code Constructions Based on Index Modulation Aided OFDM and Space-Time Block Coding for Frequency-Selective and Time Varying Wireless Communication Channels

New spectrally and energy-efficient signal-code construction (SCC) based on Index Modulation aided OFDM and Space-Time Block Coding, and also its noise-immune version in phase instability conditions with Inter-Channel Interference compensation operator, built into Alamouti’s coder structure, and increasing signals frequency diversity operator (Walsh-Hadamard) are proposed. In a complicated interference environment, first of all, under the influence of electronic suppression means, such methods increase simultaneously the data rate and reliability of digital information transmission. The SCCs enhance the noise immunity of the new generation wireless systems, as well as modern wireless communication networks of special purpose, which require high-quality implementation of bulky multimedia applications with high energy efficiency, in conditions of limited energy resources with spectral efficiency up to 4 bits/s/Hz. Using two transmit and no more than a pair of receive antennas guarantees the minimum weight-dimensional characteristics of mobile stations while providing the necessary level of antenna space decorrelation. The highest informational efficiency of such methods is achieved by applying reference look-up tables for short subblocks and small signal constellations, which allows the practical implementation of an optimal Maximum Likelihood Detector based on modern digital signal processors.

Modelling the OFDM-based PHY Layer in SoC for Visible Light Communication

In this paper, we report a System-on-Chip (SoC) architecture for OFDM-based Visible Light Communication (VLC). The OFDM block was implemented as VLC PHY layer. The OFDM block comprises of transmitter and receiver. In transmitter block, there are Reed-Solomon encoder, modulator, IFFT, and preamble generator. While in receiver block, there are Reed-Solomon decoder, demodulator, FFT, and synchronizer. In SoC, these blocks are designed as IP cores. The industry standard AXI4-Stream protocol was used for data exchange between IP cores. The OFDM model in SoC was verified by comparing with a MATLAB simulation.

Analysis of the Time-Frequency Localization Property of the Filter Banks for Optical OFDM/OQAM Systems

Orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) relaxes the orthogonal condition from the complex domain to the real field. Therefore, the cyclic prefix (CP) inserted between consecutive optical OFDM blocks, for combating the inter-symbol interference (ISI) induced by chromatic dispersion (CD) and polarization-mode-dispersion (PMD), could be removed for optical OFDM/OQAM to promote the system spectral efficiency. The ISI and the inter-carrier-interference (ICI) could be combated by using the filter banks with good time-frequency localization (TFL) property. Thus, the accuracy of channel estimation and system transmission performance largely depend on the TFL property of the filter banks for optical OFDM/OQAM. Recently, numerical experimental demonstrations and theoretical discussions of optical OFDM/OQAM have been reported. However, to our best knowledge, discussions on the TFL property of the filter banks have not been studied for optical OFDM/OQAM to date. In this paper, we systematically analyze the TFL property of the filter banks for optical OFDM/OQAM for the first time. We evaluate the TFL property of the filter banks with energy concentration, orthogonal basis, and isotropic property. Three typical filters as the Gaussian filter, the extended Gaussian function (EGF) filter and the isotropic orthogonal transfer algorithm (IOTA) filter have been employed for comparison. According to our theoretical analysis, with the use of the IOTA filter, optical impairments and channel estimation complexities could be decreased evidently thanks to the promising isotropic property. As shown in the numerous Montel Carlo simulations of the coherent optical OFDM/OQAM, system robustness against IMI, phase noise induced interference (PNII) and nonlinear effect could be improved evidently with the use of the IOTA filter.

Fractional Wavelet Transform based PAPR Reduction Schemes in Multicarrier Modulation System

The future mobile communication system stimulates researcher to improve the spectral efficiency with reduced data loss by carrying out several works. High peak to average power ratio (PAPR) of the transmitted signal is the major drawback in the multicarrier transmission like orthogonal frequency division multiplexing. This study presents a novel way to improve the performance of the multicarrier transmission system with the reduction of PAPR. It is achieved by reconstructing the Fourier-based OFDM block with the help of fractional wavelet transform. This transform provides the accurate joint information of time and fractional frequency contents of the signal which highlights the proposed system. In this system, the wavelet transform is utilized to remove the usage of cyclic prefix and to improve the bandwidth efficiency. In addition, the analysis of PAPR in the FrWT-based OFDM is carried out with the help of selective mapping (SLM) and partial transmit sequences (PTSs) techniques. The simulated results are compared with the existing PAPR analysis of Fourier and wavelet-based OFDM systems. The proposed system shows reduced PAPR of 6.6 dB with SLM scheme and 4.8 dB with PTS scheme to achieve complementary cumulative distribution function of 10−3. The overall performance of the FrWT-OFDM transceiver is analyzed by means of bit error rate (BER) plots and it also shows BER reduction of 0.0028 with SLM scheme and 0.0012 with PTS scheme at high SNR of 16 dB.

Distributed Processing for Multi-Relay Assisted OFDM With Index Modulation

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth-generation (5G) wireless communications and has thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual subcarriers. In this regard, we propose a distributed processing scheme for multi-relay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDM-IM systems. Specifically, a single selected relay only needs to decode partial information carried on certain active subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on active subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.

Priori-Information Hold Subspace Pursuit: A Compressive Sensing-Based Channel Estimation for Layer Modulated TDS-OFDM

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has been widely recognized as a fundamental OFDM block transmission scheme because of its advantages in spectral efficiency. However, it suffers from severe performance loss under strong frequency selective channels and thus has difficulty supporting high-order modulations such as 256QAM. In this paper, we proposed a compressive sensing (CS)-based channel estimation (CE) algorithm for layer modulated TDS-OFDM (LM-TDS-OFDM). In the proposed CE algorithm, the low-order modulated symbols will be recovered and then sent to a feedback loop for precise CE. The priori-information hold subspace pursuit algorithm is investigated to achieve accurate estimation of channel. Analyses and simulations show that the proposed algorithm can successfully obtain high-accuracy CE and significantly reduce the computational complexity. Based on the proposed CE scheme, LM-TDS-OFDM can well support 256QAM transmission under severe channel conditions.

Low Complexity Pilot Allocation Scheme for a Large OFDM Block with Null Subcarriers

A low complexity pilot allocation scheme is proposed for a large OFDM block with null subcarriers. The proposed scheme allocates pilots to the edge part of the active subcarrier region according to the 2nd order polynomial and to the middle part of the active subcarrier region according to the 1st order polynomial or the comb‐type pilot pattern. To find a parameter of the 2nd order polynomial, the proposed scheme applies exhaustive search of a single parameter by using an integer unit resolution. It is shown by simulation that the proposed scheme is a close‐to‐optimal pilot allocation scheme yielding better symbol error rate (SER) performance than the traditional 3rd and 5th order polynomial‐based schemes although the proposed scheme has lower computation and implementation complexity.

Multi-segmental OFDM signals equalization with piecewise linear channel model over rapidly time-varying channels

For rapidly time-varying channels, the performance of (orthogonal frequency division multiplexing) OFDM systems with the conventional one tap equalizer will be significantly degraded. Because the orthogonality between subcarriers is destroyed, the conventional way to combat the inter-carrier interference (ICI) is employing the banded minimum mean square error (MMSE) equalizer, which can save computational efforts introduced by a large number of subcarriers. However, the width of the banded channel matrix is mainly determined by the normalized Doppler frequency in the sense that with the high Doppler frequency the complexity of equalization for one OFDM block will significantly increase with the band width D. In order to reduce the equalization complexity, the authors proposed multi-segmental OFDM signal equalization method with piecewise linear model (PLM) to approximate the time variations and mitigate the corresponding ICI. Its complexity is significantly reduced with the small segments. Furthermore, an alternative MMSE method with the iterative rank-1 matrix updates is proposed to further reduce the complexity. We also derive the theoretical pre-equalized and equalized signal to interference ratio (SIR) for different normalized Doppler frequencies and segment numbers, which implies that the larger segment number can achieve the better performance. Simulation results demonstrate that the proposed method outperforms the conventional banded MMSE equalizer and the partial fast Fourier transform (FFT) method in terms of bit error rate (BER) with almost the same complexity.