Orthogonal Frequency Division Multiplexing Block Research Articles

Deep Learning-Based Joint Pilot Design and Channel Estimation for OFDM Systems

We propose a non-uniform joint pilot design and channel estimation (JPDCE) scheme in closed-loop orthogonal frequency-division multiplexing (OFDM) systems. Specifically, the encoder of the proposed JPDCE scheme is composed of two sub-networks which are used for pilot location assignment and pilot power allocation, respectively. To ensure accurate learning of the pilots, the OFDM block consisting of intelligently customized layers is designed to constrain the output of the encoder. Furthermore, the decoder is used to learn the channel state information (CSI) based on the output of OFDM layers by minimizing the mean square error (MSE) of the channel estimation. The numerical results indicate that the JPDCE scheme considerably outperforms the traditional methods as well as two state-of-the-art deep learning (DL)-based ones, demonstrating its excellent ability to learn the statistical characteristics of the wireless channel. In addition, we demonstrate that the JPDCE scheme shows excellent robustness against various channel distortion and interference: when (i) there are limited pilots, (ii) the cyclic prefix (CP) is removed, and/or (iii) clipping noise is introduced.

Performance Analysis of SIMO–OFDM Systems With Pilot-Aided Channel Estimation

Pilot symbols are commonly modulated on uniformly spaced subcarriers to aid channel estimation, thereby facilitating data demodulation in practical orthogonal frequency-division multiplexing (OFDM) systems operating over quasi-static multipath channels, where the channel frequency response (CFR) remains static within one block. Aided by uniformly spaced pilot symbols, the direct least-square CFR estimation can enable reliable symbol-by-symbol data demodulation within the same OFDM block, based on the principle of generalized likelihood ratio test (GLRT). Such GLRT-based OFDM can be easily incorporated with single-input multiple-output (SIMO) antenna technique to enhance data demodulation over quasi-static channels exhibiting random multipaths. In this article, an upper bound to the average bit error probability (BEP) is analytically derived for this GLRT-based SIMO–OFDM system over quasi-static random multipath channels. The analysis is generally applicable to arbitrary two-dimensional component modulations when the quasi-static SIMO channel consists of independent but not necessarily identically distributed random channel paths with Rician or Rayleigh distributed path response magnitudes. The BEP upper bound facilitates the error performance characterization of the GLRT-based SIMO–OFDM system with respect to data-to-noise power ratio, data-to-pilot power ratio, antenna diversity, pilot density, and pilot power allocation.

BER Reduction and Capacity Enhancement with Novel Guard Sequence Selection for Multi-Carrier Communication

Orthogonal frequency division multiplexing (OFDM) is an efficient multicarrier scheme that uses different types of guard intervals such as cyclic prefix (CP) and known symbol padding (KSP) (zero padding (ZP), unique word (UW), etc.) in block formation. Among these guard intervals, CP varies for each block, while other guard intervals remain fixed from block to block. These guard intervals efficiently perform channel estimation, synchronization and remove inter-block interference (IBI); nevertheless, none of the existing schemes develop any relationship between the guard interval (sequence) and the data symbols on different subcarriers of the OFDM block. We present a new idea of selecting the guard interval based on the data symbols of a subset of subcarriers in the block and exploit the high auto-correlation of the selected guard sequence to improve the bit error rate (BER) performance of the system. The results based on a fair comparison show that our enhanced orthogonal frequency division multiplexing (eOFDM) scheme inherits significant improvements in BER and the capacity of a multicarrier system as compared to the existing techniques.

Reduced-Rank Filtering-Based Semiblind MIMO-OFDM Sparse Channel Estimation

This article derives and analyzes novel, semiblind data/channel estimation algorithms for block-based multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) transmission in sparse channels. A novel reduced equalizer (with few active taps) is introduced and the semiblind sparse channel/data estimation problem is solved via reduced-rank filtering methods. A novel arrangement of the “full” equalizer matrix allows data/channel estimation, with fewer received OFDM blocks (in channels with short coherence time). The “reduced” equalizer then further reduces the number of OFDM blocks for estimation. The novel, reduced-rank filters are analyzed and shown to be equivalent to some nonblind Bayesian and “natural gradient” methods, widely used for sparse estimation. Thus, the novel method can be considered a blind Bayesian method, which is not available in the literature, and whose perturbation analysis is performed. Simulation results illustrate that the novel semiblind estimators perform much better than existing blind/training-based sparse methods (even including the popular compressed sensing and Bayesian methods), when few training subcarriers are available (which may occur in futuristic, pilot-starved massive MIMO-OFDM systems), at much reduced complexity.

Adaptive Channel Prediction, Beamforming and Scheduling Design for 5G V2I Network: Analytical and Machine Learning Approaches

The fifth-generation wireless network (5G) is expected to support vehicle to infrastructure (V2I) communication which requires an accurate understanding of its highly mobile and dynamic propagation environment for efficient physical layer design. Towards this end, this paper proposes a joint design of adaptive channel prediction, beamforming and scheduling for 5G V2I communications. The channel prediction algorithm is designed without using pilot signals and assuming the channel impulse response (CIR) model. In this regard, we first propose an adaptive recursive least squares (RLS) CIR prediction approach that uses few of the past estimated CIRs to predict one or more future orthogonal frequency division multiplexing (OFDM) block CIR coefficients. Then, we jointly design beamforming and vehicle equipment (VE) scheduling for each sub-carrier to maximize the uplink channel average sum spectrum efficiency (SE) by employing the predicted CIR. The beamforming problem is formulated as a Rayleigh quotient optimization where its global optimal solution can be obtained. Moreover, the VE scheduling design is formulated as an integer programming problem solved using a greedy search. To enhance the prediction performance, we propose a model-free Q-learning technique to learn a strategy for selecting the best CIR predictor at the current OFDM block. In addition, as the CIR coefficients might contain undesired taps with zero (negligible) average powers, we further examine a dominant CIR tap index identification problem (formulated as convex) from the past observed signals. We carry out extensive simulations to validate analytical expressions and examine the effects of different parameters including the VE speed on the achievable sum SE. Numerical simulations corroborate the superiority of the proposed channel prediction and scheduling algorithms over the existing ones and the relevance of identifying dominant CIR taps.

Security and Energy Harvesting for MIMO-OFDM Networks

We consider a multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) network in which a source node, Alice, communicates with an energy-harvesting destination node, Bob, in the presence of a passive eavesdropper. To secure the wireless transmission, Alice generates a hybrid artificial noise (AN) in both frequency and time domains. Moreover, in order to collect more energy, Bob splits the received signal power of the cyclic prefix of each OFDM block. We then propose two non-convex optimization problems to balance both the need for security and the need for harvesting energy at Bob. While one considers maximizing the secrecy rate, the other approach aims at maximizing the harvested energy. Path-following algorithms of low computational complexity are developed and evaluated. Our numerical results show the gain of our proposed scheme and the effectiveness of our proposed algorithms.

Full-Duplex Relay-Assisted Orthogonal Frequency-Division Multiplexing With Index Modulation

In this paper, we propose a full-duplex relay-assisted orthogonal frequency-division multiplexing (OFDM) with index modulation system, in which a complete transmission from source to destination is forwarded by a full-duplex decode-and-forward relay. By introducing full-duplex relaying, we are able to achieve a higher end-to-end capacity, as long as the power of the residual self-interference can be mitigated to an appropriate level. To investigate the proposed system, we assume that the maximum-likelihood detection is adopted at both relay and destination for decoding the received OFDM block. Then, we derive or approximate the average outage probability, block error rate, and end-to-end capacity in a closed form. All analyses are verified by numerical results generated by Monte Carlo simulations and comparisons between half-duplex and full-duplex relaying schemes are also provided to show the performance superiority of the proposed system.

Successive Multipath Interference Cancellation for CP-Free OFDM Systems

Traditional orthogonal frequency-division multiplexing (OFDM) systems mitigate multipath interferences with the help of a cyclic prefix (CP) appended between two adjacent OFDM blocks. The length of the CP must be made longer than channel delay spread to avoid intersymbol interference (ISI). The use of CPs degrades spectrum efficiency significantly, which is a deal price paid in all existing OFDM systems. Motivated to deal with this issue, this paper proposes a CP-free OFDM scheme with successive multipath interference cancellation (SMIC), which does not require CPs and removes ISI before fast Fourier transform (FFT) at a receiver using stored feedback equalization (SFE). The SFE operation leaves a CP gap between adjacent OFDM blocks, and the CP is regenerated with estimated signals. In this way, we convert linearly shifted OFDM blocks induced by multipath propagation back to cyclically shifted OFDM blocks for successful FFT operation. SMIC-OFDM achieves a much higher spectrum efficiency than traditional OFDM owing to the save of CPs in transmission signals. The performance of SMIC-OFDM in terms of the bit error rate, capacity, and computational complexity is compared to that of traditional OFDM to verify the effectiveness of the proposed SMIC-OFDM scheme.

BER Reduction Using Precoded OFDM in Power-line Channel

Power-line Communications (PLC) effectively uses the pervasive power grid as a high data rate communication medium. Despite its advantages, the presence of severe multipath fading and susceptibility to impulsive noise make reliable communication in a PLC system challenging. In PLC systems, orthogonal frequency-division multiplexing (OFDM) is an attractive choice mainly due to its resilience to multipath fading. However, OFDM signals suffers from large peak-to-average power ratio (PAPR) thus resulting in non-linear signal distortions at the transmitter. Therefore, for an OFDM-based PLC system, the combined effect of large PAPR and the presence of impulsive noise results in high bit error rate (BER). This paper proposes the use of precoding at the transmitter to reduce PAPR, and blanking non-linearity at the receiver to mitigate impulsive noise. Precoding is a data-independent and robust PAPR reduction scheme that essentially distributes the power of the individual OFDM symbols over the entire OFDM block. Blanking is an effective impulsive noise mitigation technique that results in better BER performance at the receiver. Simulation results show that, with the combination of precoding and blanking, significant gains in BER performance for an OFDM based PLC system can be obtained with minimal increase of transceiver complexity.

Joint CFO and I/Q Imbalance Estimation for OFDM Systems Exploiting Constant Modulus Subcarriers

In this paper, we propose blind methods to jointly estimate carrier frequency offset and in-phase/quadrature-phase imbalance for orthogonal frequency-division multiplexing (OFDM) systems exploiting constant modulus subcarriers. The proposed algorithms support fully loaded transmissions, which enhance the spectral efficiency. Other advantages of the proposed algorithms include: 1) they work well with only one received OFDM block and thus belong to the category of one-shot estimation methods; 2) they are computationally efficient; and 3) they outperform the existing methods. Simulation results and Cramer–Rao bound are provided to demonstrate the superiority of our methods compared to the existing methods.

A PTS Technique With Non-Disjoint Sub-Block Partitions in M-QAM OFDM Systems

This paper presents a modified partial transmit sequence (PTS) algorithm for peak-to-average power ratio (PAPR) reduction of M-QAM orthogonal frequency division multiplexing (OFDM) signals by partitioning an OFDM block into non-disjoint OFDM sub-blocks. Since an M-QAM constellation can be written as sum of several QPSK constellations, we can then apply different disjoint partitions on the QPSK OFDM blocks, which result in an equivalent non-disjoint partition on the M-QAM OFDM block. Compared to a disjoint sub-block partition in conventional PTS, simulation results show that the modified PTS with a non-disjoint partition achieves better PAPR reduction in adjacent, interleaved, and random partitioning schemes.

Adaptive Relay Selection and Power Allocation for OFDM Cooperative Underwater Acoustic Systems

The increasing use of relays in underwater acoustic (UWA) communications is a driving force to explore efficient techniques that can significantly improve the system performance. To fully exploit the benefits of cooperative systems, efficient management of resources including relays and power is required. In this paper, both optimal relay selection and power loading issues are investigated for an orthogonal frequency division multiplexing (OFDM) cooperative transmission over UWA channel. In our analysis, we consider amplify-and-forward (AF) relaying with perfect channel state information (CSI) at the source, relay, and destination nodes. Moreover, we assume sparse and frequency-selective Rician fading in the presence of colored Gaussian ambient noise. Unlike previous works on this topic, in our study, the power of noise is not identical for all subcarriers at both the relay and destination nodes. We solve two optimization problems that rely on the minimization of the bit error rate (BER) and maximization of the system capacity. In each problem, both optimal relay selection and power loading issues are addressed in two dependent phases. In the first phase, an unconstrained optimization problem is solved to determine the optimal relay out of multiple relays vertically located at different depths of water. We adopt all-subcarrier (AS) basis approach in our OFDM-based transmission model in which a single relay is engaged to transmit the entire OFDM block to the destination. In the second phase, after selecting the optimal relay, another optimization problem is solved to obtain the optimal power allocation. This is jointly done at both the source and relay nodes under total power constraint and fixed subcarrier rate. Extensive simulations are conducted to evaluate the performance of proposed algorithms under different scenarios.

Sparse Bayesian Learning for Channel Estimation in Time-Varying Underwater Acoustic OFDM Communication

In this paper, we study the sparse Bayesian learning (SBL) framework for channel estimation in underwater acoustic (UWA) orthogonal frequency-division multiplexing (OFDM) communication systems, which provides a desirable property of preventing structural error with fewer convergence errors for sparse signal reconstruction compared with the compress sensing (CS)-based methods. First, we design a SBL-based channel estimator for block-by-block processing using the channel sparse structure independently in each block. Then, we propose a joint channel model after Doppler compensation for multi-block joint processing, where the delays of the channels for several consecutive blocks are similar and the path gains exhibit temporal correlation, and we denote a temporal correlation coefficient for path gains to evaluate the strength of the correlation. Furthermore, we propose the temporal multiple SBL (TMSBL)-based channel estimator to jointly estimate the channels by taking advantage of the channel coherence between consecutive OFDM blocks. Results of numerical simulation and sea trial demonstrate the effectiveness of the SBL and TMSBL channel estimator algorithms in time-varying UWA channel, which achieve better channel estimation performance and lower bit error rate compared with the existing CS-based methods, such as orthogonal matching pursuit (OMP) and simultaneous OMP, especially the TMSBL estimator achieves the best performance in strong temporal correlated channels and maintains robustness in weak temporal correlated channels.

Symbol Cyclic Shift Equalization PAM-OFDM—A Low Complexity CP-Free OFDM Scheme

In traditional orthogonal frequency division multiplexing (OFDM) systems, a cyclic prefix (CP) must be used in every block for intersymbol interference (ISI) suppression in time-dispersive channels at a price of severely impaired spectrum efficiency. To deal with this issue, the symbol cyclic shift equalization (SCSE) algorithm was proposed to implement a CP-free OFDM system. In this paper, we aim to further reduce the computational complexity of the SCSE algorithm. In particular, a pulse amplitude modulation (PAM) is used at a transmitter to construct conjugate symmetric OFDM blocks, while only partial samples in an OFDM block are involved in the SCSE algorithm at the receiver side, leading to a significantly reduced implementation complexity if compared to an SCSE quadratic-amplitude modulation(QAM)/phase-shift keying (PSK)-OFDM system, where all samples in an OFDM block must be involved in the detection process. The performance of the proposed SCSE PAM-OFDM scheme is analyzed and compared to traditional OFDM systems. The results obtained from this paper verify the effectiveness of the proposed scheme.

Cyclic-feature based Doppler scale estimation for orthogonal frequency-division multiplexing (OFDM) signals over doubly selective underwater acoustic channels

Underwater acoustic (UWA) communications enable underwater wireless networks to be applied in various applications, such as oceanographic research, pollution early-warning, disaster prevention, and military systems. However, a major challenge in UWA communications is to combat the Doppler distortion caused by doubly selective (time and frequency selective) channels. Most Doppler scale estimators rely on training data or specially designed packet structures. These methods have fundamental limitations in transmission rate and spectral efficiency. Different from these methods, this paper presents a Doppler scale estimation approach exploiting the redundant information contained within the cyclic prefix (CP) or cyclic suffix (CS) of orthogonal frequency-division multiplexing (OFDM) signals. We analyze the cyclic features of OFDM signals over doubly selective underwater channels in order to demonstrate the relationship between the cyclic features and the Doppler scale. Based on the theoretical analyses, we find that the Doppler scale can be estimated from the extremums of the cyclic autocorrelation function (CAF) of the received signal. Simulations validate our theoretical analyzes and the performance of the proposed Doppler scale estimator. Apart from the high estimation performance, we also highlight the utility of our method when only few OFDM blocks are available.

An Artificial-Noise-Aided Hybrid TS/PS Scheme for OFDM-Based SWIPT Systems

We propose a new artificial-noise aided hybrid time-switching/power-splitting scheme for orthogonal frequency-division multiplexing (OFDM) systems to securely transmit data and transfer energy to a legitimate receiving node. In our proposed scheme, the cyclic prefix has two more benefits in addition to the cancellation of the inter-symbol interference between the OFDM blocks. First, it enables the legitimate transmitter to send artificial-noise (AN) vectors in a way such that the interference can be canceled at the legitimate receiver prior to information decoding. Second, its power is used to energize the legitimate receiver. We optimize the cyclic prefix length, the time-switching and power-splitting parameters, and the power allocation ratio between the data and AN signals at the legitimate transmitter to maximize the average secrecy rate subject to a constraint on the average energy transfer rate at the legitimate receiver. Numerical results demonstrate that our proposed scheme can achieve up to 23% average secrecy rate gain relative to a pure power-splitting scheme.

Superposition Coding for Downlink Underwater Acoustic OFDM

Superposition coding (SC) is a nonorthogonal scheme for downlink communications, in which all users are allowed to use the full degrees of freedom of the channel and successive interference cancellation (SIC) is adopted for user decoding. Combining SC with orthogonal frequency-division multiplexing (OFDM) modulation, the optimal resource allocation depends on the perfect channel state information (CSI) of all users at the transmitter, which is hard to obtain for underwater acoustic (UWA) channels. In this paper, we propose a practical OFDM-modulated SC scheme for downlink UWA communications, where the transmitter splits the power between two users based on statistical CSI. The expressions to characterize the boundary of the ergodic rate region achievable by the proposed scheme over long codewords are presented first, followed by the analysis of outage probability when coding is applied within one OFDM block. Then we examine the performance of SC in an OFDM-modulated system with practical coding and modulation pairs. Simulation results show that the OFDM-modulated SC scheme outperforms the orthogonal frequency-division multiple access (OFDMA) in performance of both block error rate (BLER) and spectral efficiencies under different data rate pairs. Recorded data from both medium-range and short-range sea tests verify that channel statistics are stable over a long period of time and can be used to assist resource allocation for the proposed scheme. Compared with OFDMA, considerable increase of spectral efficiencies has been found for SC based on experimental data when both users with disparate channels have nonzero data rates.

Analytic Comparison for Channel Response Estimation Based on Time- and Frequency-Domain Pilot Signals

For the orthogonal frequency-division multiplexing (OFDM) system, one can estimate the channel responses (CRs) based on frequency-domain (FD) pilot symbols. One can also estimate the time-domain (TD) channel impulse response (CIR) by inserting a unit pulse ahead of each OFDM block. However, there is no analytic comparison between the CR estimation based on FD pilot symbols and TD unit pulses. In this paper, we first consider the 1-D blocks of CRs and give the analytic comparison of mean-squares error (MSE) for the CR estimation based on TD and FD pilot signals, respectively. For the linear minimum MSE (LMMSE) algorithm, which attains the minimum MSE for the CR estimation based on FD pilots, we derive an MSE lower bound among all possible distributions of pilot symbols. This MSE lower bound coincides with the MSE attained by the optimal CR estimation based on TD unit pulses, under the condition that both FD and TD pilot signals have the same power. Our analysis also indicates that when the number of FD pilots is smaller than the rank of correlation matrix of CRs, it results in an error floor. We further extend our analysis to 2-D blocks of CRs and show that the MSE formulas for CR estimation based on FD and TD pilot signals share the same forms and that they have close numerical values.

A Delay-Tolerant Asynchronous Two-Way-Relay System over Doubly-Selective Fading Channels

We consider design of asynchronous orthogonal frequency division multiplexing (OFDM) based diamond two-way-relay (DTWR) systems in a time-varying frequency-selective (doubly-selective) fading channel. In a DTWR system, two users exchange their messages with the help of two relays. Most of the existing works on asynchronous DTWR systems assume only small relative propagation delays between the received signals at each node that do not exceed the length of the cyclic-prefix (CP). However, in certain practical communication systems, significant differences in delays may take place, and hence existing solutions requiring excessively long CPs may be highly inefficient. In this paper, we propose a delay-independent CP insertion mechanism in which the CP length depends only on the number of subcarriers and the maximum delay spread of the corresponding channels. We also propose a symbol detection algorithm that is able to tolerate very long relative delays, that even exceed the length of the OFDM block itself, without a large increase in complexity. The proposed system is shown to significantly outperform other alternatives in the literature through a number of specific examples.

Blind Identification of SM and Alamouti STBC-OFDM Signals

This paper proposes an efficient identification algorithm for spatial multiplexing (SM) and Alamouti (AL) coded orthogonal frequency division multiplexing (OFDM) signals. The cross-correlation between the received signals from different antennas is exploited to provide a discriminating feature to identify SM-OFDM and AL-OFDM signals. The proposed algorithm requires neither estimation of the channel coefficients and noise power, nor the modulation of the transmitted signal. Moreover, it does not need space-time block code (STBC) or OFDM block synchronization. The effectiveness of the proposed algorithm is demonstrated through extensive simulation experiments in the presence of diverse transmission impairments, such as time and frequency offsets, Doppler frequency, and spatially correlated fading.