Orthogonal Frequency Division Multiplexing Symbol Research Articles

Signal Detection Scheme Based on Adaptive Ensemble Deep Learning Model

Accurate signal detection is one of the most important requirements of wireless communication systems. The two most important processes of the signal detection are channel estimation and compensation. Since an orthogonal frequency-division multiplexing (OFDM) system among wireless communication systems uses orthogonal sub-carriers, the system has advantages of high bandwidth efficiency and simple channel compensation process compared with a single carrier system. However, due to the imperfect channel estimation and amplification of receiving noise by the channel compensation process, the reliability performance of the system is deteriorated according to the channel condition. This paper aims to introduce a new signal detection scheme based on deep learning. To address the challenges of signal detection, we propose the method which integrates the ensemble deep learning with the acquired received signals from multi-path channel according to the channel condition. The channel estimation corresponds to learning the deep neural network, and the channel compensation corresponds to assigning the received data to the learned network. Experiments on the OFDM symbol classification demonstrate that the proposed scheme has dramatically improved reliability performance compared with the conventional scheme.

CP-Less OFDM With Alignment Signals for Enhancing Spectral Efficiency, Reducing Latency, and Improving PHY Security of 5G Services

Although orthogonal frequency-division multiplexing (OFDM) is a widely accepted waveform in many standards and is expected to keep its dominance in future 5G systems with various types of parameterized waveforms, its performance in terms of spectral efficiency as well as transmission latency is usually degraded due to the excessive usage of cyclic prefix (CP). Particularly, in highly dispersive channels, CP rate might be very large in order to maintain the low-complex frequency-domain equalization. In this paper, we propose a novel method that can fit the low latency and high spectral efficiency requirements of future 5G wireless services by eliminating the need for inserting CP between successive OFDM symbols while keeping the whole detection process the same at the receiver side. In order to achieve that, we utilize specially designed alignment signals that can cancel the interference of one symbol on the other and add an additional signal component that makes the signal circularly convolved with the channel at the receiver side. Simulation results prove the superiority of the proposed scheme in terms of enhancing spectral and power efficiency, reducing latency, and improving physical-layer security against eavesdropping while using a low-complexity one-tap frequency-domain equalizer. These numerous, simultaneous, and desirable advantages have the potential to make the proposed technique a suitable fit for future 5G wireless services and applications including Internet of Things-based massive machine-type communication, ultra-reliable and low-latency communication, and enhanced mobile broadband.

Compressed Sensing Based Recursive Estimation of Doubly-Selective Channels for High-Mobility OFDM Systems

For an orthogonal frequency-division multiplexing (OFDM) system in high-mobility applications, channel suffers from both frequency-selective and time-selective fading introduced by Doppler shift. Large pilot overhead is needed to estimate the numerous parameters of doubly-selective channel, resulting in low-spectral efficiency. In this paper, considering the correlation of practical wireless channels in high-dimensional signal spaces, a recursive channel estimation scheme based on compressed sensing (CS) is proposed for high-mobility OFDM systems to reduce the pilot overhead. Specifically, by exploiting the sparsity of OFDM channel in basis expansion model (BEM), the sparse BEM coefficients is estimated instead of numerous channel taps. Then we theoretically verify that the BEM coefficients corresponding successive OFDM symbols also share a common support. To utilize this temporal correlation of BEM coefficients, a recursive channel estimation algorithm derived from the classical modified CS algorithm is proposed to improve the present estimation by prior channel information from previous estimation. Theoretical and simulation results demonstrate that the proposed recursive channel estimation scheme preforms better than conventional schemes in various scenarios, even with less pilot overhead.

Joint Time-Frequency Alignment for PAPR and OOBE Suppression of OFDM-Based Waveforms

Orthogonal frequency division multiplexing (OFDM) waveform has been adopted by many wireless standards due to its numerous advantages. However, OFDM symbols have two critical drawbacks: high out-of-band emission (OOBE) and high peak-to-average power ratio (PAPR). Cyclic prefix (CP) alignment is one of the promising methods that jointly suppresses the OOBE and PAPR of OFDM symbols without introducing extra receiver complexity. However, its suppression performance remains limited in practical scenarios as it only exploits the degrees of freedom (DoFs) provided by the CP part of the OFDM symbols. In this letter, we propose a novel approach which jointly exploits the time domain resources, i.e., CP, and the frequency domain resources that are not effectively used by the receiver, i.e., guard tones and the subcarriers faded by the multipath channel. Thus, the available DoFs are substantially increased and more efficiently utilized for alignment purpose. It is shown that the OOBE and PAPR suppression performance is significantly enhanced with the proposed method as compared with the original approach.

Improved Bootstrap Design for Frequency-Domain Signaling Transmission

In ATSC 3.0, a bootstrap is used to enable initial synchronization and carry transmission parameter signaling. The bootstrap consists of multiple orthogonal frequency division multiplexing symbols which enables variable signaling capacity. Signaling information is conveyed by applying cyclic shift in time-domain. However, the bootstrap suffers from performance loss in the channels of strong multipath and fast time-variation. In this paper, an improved bootstrap enabling separate design of synchronization part and signaling transmission part is proposed. Zero correlation zone sequence is employed in frequency-domain (FD) for signaling transmission by cyclically shifting the sequence in FD. At the receiver, a universal signaling decoding method regardless of the synchronization part design is presented. Besides, to eliminate the performance degradation introduced by non-linear distortion of channel transfer function, signaling validation and correction operations on the previous symbol are required. Numerical simulation indicates that the proposed design achieves better signaling transmission performance than current one in some corner cases.

A Novel Chaos-Based Encryption Approach for Future-Generation Passive Optical Networks Using SHA-2

A physical layer enhanced secure future generation passive optical network (PON) based on chaotic signal scrambling and the secure hash algorithm (SHA) is proposed and demonstrated. In this paper, the architecture of a wavelength-division multiplexed orthogonal frequency division multiplexing (OFDM) PON system based on a centralized light source using direct detection is analyzed. A logistic chaos map is employed here for the scrambling of OFDM symbols in the frequency and time domains. Before transmitting the signal over optical fiber, a message- digest of scrambled and descrambled OFDM signals is computed and compared at the optical line terminal and optical network unit, respectively, to verify the actual data recovered. For the downstream channel, a 10 Gbps 32 quadrature amplitude modulation OFDM signal is successfully transmitted over a distance of 50 km single-mode fiber within the limits of the peak-to-average-power value. The results show that the proposed scheme can protect the system from bad actors and eavesdroppers, while chaos encryption with SHA-2 provides a robust and promising secure strategy for future-generation PON systems.

Aligning Power in Multiple Domains for Pilot Decontamination in Massive MIMO

Pilot contamination in the uplink (UL) can severely hurt the channel acquisition performance at the base station (BS) in a massive multiple-input multiple-output (MIMO) system. To mitigate the interference, it is critical to explore all the allowed avenues to provide more orthogonal resources for the users to transmit non-interfering UL pilots. In this paper, with a realistic massive MIMO orthogonal frequency-division multiplexing (OFDM) system model, allowing the channels to be both frequency-selective and time-selective, we propose to mitigate the UL pilot contamination through aligning the channel power in multiple domains, i.e., the time, angular, and Doppler domains. First, we show how to align the power-delay profiles of different users so that the pilots sent within one common reference OFDM symbol are orthogonal. Second, we demonstrate our proposed time-varying pilots can asymptotically mitigate the pilot contamination by aligning the Doppler power spectra (DPSs) judiciously even when the UL channels vary with time. Furthermore, in the case of massive MIMO, as the angles of arrival of the UL paths are acquired by the BS, we show the pilot contamination can be reduced or eliminated by jointly aligning the DPSs and the angular power spectra. Extensive computer simulations convince us the proposed Power Aligning principle can serve as the baseline design philosophy for the UL pilots in massive MIMO.

A novel energy-efficient ICI cancellation technique for bandwidth improvements through cyclic prefix reuse in an OFDM system

Orthogonal frequency division multiplexing (OFDM), a very promising technique that is leading the evolution in wireless mobile communication to sideline the bandwidth scarcity issue in spectrum allocation, is severely affected by the undesirable effects of the frequency offset error, which generates inter carrier interference (ICI) due to the Doppler shift and local oscillator frequency synchronization errors. There are many ICI cancellation techniques available in the literature, such as self-cancellation (SC), maximum likelihood estimation (MLE), and windowing, but they present a tradeoff between bandwidth redundancy and system complexity. In this study, a new energy-efficient, bandwidth-effective technique is proposed to mitigate ICI through cyclic prefix (CP) reuse at the receiver end. Unlike SC and MLE where the whole OFDM symbol data is transmitted in duplicate to create redundancy at the transmitter end, the proposed technique uses the CP data (which is only 20% of the total symbol bandwidth) to estimate the channel, and it produces similar results with a huge bandwidth saving. The simulation results show that the proposed technique has a significant improvement in error performance, and a comparative analysis demonstrates the substantial improvement in energy efficiency with high bandwidth gain. Therefore, it outperforms the legacy ICI cancellation schemes under consideration.

Pilot Optimization for Estimation of High-Mobility OFDM Channels

Obtaining channel state information is very crucial for realizing high-performance high-rate wireless communications. For an orthogonal frequency-division multiplexing (OFDM) system operating in a high-mobility environment such as in high-speed trains, a sequence of pilot samples is inserted in each OFDM symbol to track the fast-varying channel responses. For such a high-mobility environment, the design of pilot sequence to minimize the mean squared error (MSE) of the channel estimate under a linear minimum mean squared error (LMMSE) estimator poses a difficult polynomial fractional optimization problem. In this paper, we develop a path-following optimization procedure, which improves the MSE in every iteration and quickly converges at least to its locally-optimal solution. Each iterative solution is given in a closed form with very low computational complexity. The developed path-following procedure can also be adapted to design pilot sequences for the least-square and maximum-likelihood estimators. Extensive simulation results demonstrate the effectiveness and superior performance of the proposed solutions and algorithms when compared to the state-of-the-art algorithms in the literature.

Analysis of the maximum likelihood channel estimator for OFDM systems in the presence of unknown interference

This paper is a theoretical analysis of the maximum likelihood (ML) channel estimator for orthogonal frequency-division multiplexing (OFDM) systems in the presence of unknown interference. The following theoretical results are presented. Firstly, the uniqueness of the ML solution for practical applications, i.e., when thermal noise is present, is analytically demonstrated when the number of transmitted OFDM symbols is strictly greater than one. The ML solution is then derived from the iterative conditional ML (CML) algorithm. Secondly, it is shown that the channel estimate can be described as an algebraic function whose inputs are the initial value and the means and variances of the received samples. Thirdly, it is theoretically demonstrated that the channel estimator is not biased. The second and the third results are obtained by employing oblique projection theory. Furthermore, these results are confirmed by numerical results.

A Majorization–Minimization Approach for Reducing Out-of-Band Radiations in OFDM Systems

This letter is concerned with the reduction of the out-of-band (OOB) components in spectrum of orthogonal frequency division multiplexing (OFDM) systems. We propose an efficient method to obtain a phase sequence that is required in a phase adjustment algorithm, an algorithm which reduces the OOB components by employing proper phase shifts in OFDM symbols. The proposed method is based on the majorization–minimization technique, and converges to stationary points of an NP-hard design problem (under some mild conditions). Simulation results illustrate that the proposed method outperforms its counterparts in view of the solution quality as well as the computational complexity.

Joint Blind Equalization and Decoding in OFDM Systems Using Particle Filtering

In this paper a sequential algorithm is proposed for joint blind channel equalization and decoding for orthogonal frequency-division multiplexing (OFDM) in frequency selective channels. This algorithm offers a recursive method to sequentially calculate the posterior probability for maximum a posteriori detection. Recursive calculations are done along the indexes in each OFDM symbol using a particle filter. By defining an appropriate importance function, and a proper prior probability distribution function for the channel tap coefficients (and marginalizing it), an efficient method is presented for joint equalization and channel decoding in OFDM based systems. Performance of the proposed detector is evaluated using computer simulations and its bit error rate is compared with the trained turbo equalizer and a conventional particle filter-based method. The results show that the proposed method outperforms the previously presented particle filter-based method without a need for training data.

Symbol error rate on fading self-interference channel in full-duplex

In-band full-duplex (FD) is being considered as a promising technology for the next generation wireless communication systems. In this paper, the performance of orthogonal frequency division multiplexing (OFDM) modulation system with different symbol duration and the code spreading system with different spreading sequence lengths under time-varying self-interference (SI) channel in FD mode is investigated respectively. Typically, the SI channel is estimated during the SI cancellation duration and used for SI suppression in the whole data transmission duration. First, the expressions of the residual SI power during the data transmission duration are derived under the classical, the uniform, and the two-way Doppler SI channels. Second, the signal-to-interference-and-noise-ratio after the SI mitigation is obtained. Third, the symbol error rates for the OFDM modulation and the code spreading systems are given. Simulation results show that OFDM symbol length should be selected longer when the symbol duration is significantly lower than the SI coherent time while the length of the coding spreading system should be chosen shorter.

Coded OFDM-IM With Transmit Diversity

In this paper, we propose a simple transmit diversity scheme for orthogonal frequency division multiplexing (OFDM) with index modulation (IM) in order to achieve a diversity gain for index detection, which can significantly improve the performance of OFDM-IM at the cost of the spectral efficiency. An optimal approach for active index detection is derived, which has a complexity, growing linearly with the size of OFDM symbol. A salient feature of the proposed transmit diversity scheme is that it can be easily employed in conjunction with a conventional coding scheme for data symbols. Consequently, we can implement coded OFDM-IM together with the proposed transmit diversity scheme in a straightforward manner, which can provide a good performance under a frequency-fading environment.

Minimizing the Effects of Inter-Carrier Interference Signal in OFDM System Using FT-MLE Based Algorithm

Orthogonal-frequency-division-multiplexing (OFDM) systems suffer from Inter-carrier interference (ICI) when the orthogonality between subcarriers is lost. The orthogonality between subcarriers is lost due to two factors. The first is the Doppler-frequency shift in the subcarriers due to the relative motion between the transmitter and receiver. The second is the miss-synchronizations between the local oscillators in the receivers and the received OFDM signal. This paper proposes two methods to reduce the variance of the ICI signal. The first method uses a Fourier-transform based maximum-likelihood estimator (FT-MLE) to estimate the Doppler-shift in the channel. The receiver estimates the frequencies of the subcarriers by estimating the parameters of pilot signal and exploiting the strong relation between the subcarriers in the OFDM signal. The second methods depends on decreasing the value of the normalized Doppler shift by increasing the OFDM symbol rate. No estimation for the Doppler shift is required in this method.

A Decision-Directed Channel Estimator for OFDM-Based Bursty Vehicular Communication

We consider bursty orthogonal frequency-division multiplexing (OFDM) signal transmission, where each signal burst (or packet) consists of a preamble followed by data symbols which may contain few pilots. Channel estimation in vehicular environments for such signals is a challenging issue, as evidenced by related studies for the IEEE 802.11p standard. Existing channel estimation methods often yield deficient performance or require relatively high latency to achieve better performance, unless additional pilots are introduced. We propose a technique that addresses this problem. The primary aspect of the design is to exploit the temporal correlation in channel responses over close-by OFDM symbols in a decision-oriented manner. A secondary aspect is to take advantage of any available pilots in the data symbols in a frequency-domain filtering built on the temporal processing results. Regarding the performance of the proposed method, we first analyze the proposed temporal processing technique under some simplistic channel conditions both for insight and for verification. Further simulation results based on the IEEE 802.11p specifications under a measurement-based fading multipath vehicle-to-vehicle (V2V) channel model show the superiority of the proposed method.

Time domain reshuffling for OFDM based indoor visible light communication systems.

For orthogonal frequency division multiplexing (OFDM) based indoor visible light communication (VLC) systems, partial non-ideal transmission conditions such as insufficient guard intervals and a dispersive channel can result in severe inter-symbol crosstalk (ISC). By deriving from the inverse Fourier transform, we present a novel time domain reshuffling (TDR) concept for both DC-biased optical (DCO-) and asymmetrically clipped optical (ACO-) OFDM VLC systems. By using only simple operations in the frequency domain, potential high peaks can be relocated within each OFDM symbol to alleviate ISC. To simplify the system, we also propose an effective unified design of the TDR schemes for both DCO- and ACO-OFDM. Based on Monte-Carlo simulations, we demonstrate the statistical distribution of the signal high peak values and the complementary cumulative distribution function of the peak-to-average power ratio under different cases for comparison. Simulation results indicate improved bit error rate (BER) performance by adopting TDR to counteract ISC deterioration. For example, for binary phase shift keying at a BER of 10-3, the signal to noise ratio gains are ~1.6 dB and ~6.6 dB for DCO- and ACO-OFDM, respectively, with ISC of 1/64. We also show a reliable transmission by adopting TDR for rectangle 8-quadrature amplitude modulation with ISC of < 1/64.

An Opportunistic Spectrum Access Scheme for Multicarrier Cognitive Sensor Networks

In this paper, we consider a broadband cognitive radio wireless sensor network operating in a crowded spectrum scenario, where a sensor transmitter (i.e., a sensor node that needs to transmit information to another sensor node or to a sink) opportunistically uses the channel of a primary system adopting an underlay paradigm. Both the primary and sensor systems employ an orthogonal frequency-division multiplexing (OFDM) modulation scheme. The proposed approach is reminiscent of additive superposition coding: specifically, the sensor transmitter behaves as an amplify-and-forward relay for the primary system, by sending a linearly weighted combination of the received primary signal and its own block of symbols. Such a superposition is performed by the sensor transmitter on the fly, in a single OFDM symbol of the primary system. Moreover, the proposed scheme does not require spectrum sensing at the sensor nodes and does not demand any cooperation between primary and sensor systems, thereby accounting for the inherent resource constraints of sensor nodes. With reference to both primary and sensor communications, we address the problem of power allocation by considering different amounts of channel state information and develop a worst case analysis of the achievable information rates. The developed algorithms and the derived formulas, which are corroborated by numerical Monte Carlo simulations, allow one to recognize and discuss interesting tradeoffs between the main parameters of the cognitive radio sensor transmission.

A Data Rate Maximization Algorithm for Multiuser OFDM Systems

In this paper, we present suboptimal integer bit allocation algorithm that maximizes overall data rate in multiuser orthogonal frequency division multiplexing (OFDM) systems implemented in wireless networks. Assuming knowledge of the instantaneous channel gains for all users, we propose a new multiuser OFDM loading algorithm, with constrains, that maximizes the total data rate in downlink transmission. This is done by solving the maximization problem in two steps. First step provides subcarrier assignment to users based on users' requests on quality of service (QoS) expressed in terms of a minimum signal-to-noise ratio (SNR) on each subcarrier. The second step provides bit and power allocation to subcarriers in order to maximize overall data rate. In order to reduce the computational complexity of the proposed problem wepropose a simple method, in the form of a theorem, which assigns subcarriers to users and distributes bits and power among them. We have tested the proposed algorithm in a multiuser environment for various subcarrier SNR values requested by users, and for various levels of interference in the OFDM system. The results show that our loading method obtains a better data rate in some situations than the algorithm described in [5], which states that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user. Results show that the total number of bits per OFDM symbol or data rate grows with the number of users in the system.

Modeling and Performance Analysis of Wireless Networks With Ambient Backscatter Devices

Ambient backscatter is an intriguing wireless communication paradigm that allows small devices to compute and communicate by using only the power they harvest from far-field radio-frequency (RF) signals in the air. Ambient backscattering devices reflect RF signals emitted by existing or legacy communications systems, such as digital TV broadcasting, cellular, or Wi-Fi ones, which are designed for transporting information and are not intended for RF energy transfer. This paper deals with mathematical modeling and performance analysis of wireless broadband networks operating over fading channels with ambient backscatter devices. After introducing a detailed signal model of the relevant communication links, we study the influence of physical parameters on the capacity of both legacy and backscatter channels, by considering different receiver architectures. We analytically show that, under reasonable operative conditions, a legacy system-employing an orthogonal frequency-division multiplexing (OFDM) modulation scheme-can turn the RF interference arising from the backscatter process into a form of multipath diversity that can be exploited to increase its performance. Moreover, our analysis proves that a backscatter system-transmitting one symbol per OFDM symbol of the legacy system-can achieve satisfactory data rates over relatively short distances, especially when the intended recipient of the backscatter signal is co-located with the legacy transmitter, i.e., they are on the same device.