Multiple Orthogonal Frequency Division Research Articles

Robust Design for Intelligent Reflecting Surface-Assisted MIMO-OFDMA Terahertz IoT Networks

Terahertz (THz) communication has been regarded as one promising technology to enhance the transmission capacity of future Internet-of-Things (IoT) users due to its ultrawide bandwidth. Nonetheless, one major obstacle that prevents the actual deployment of THz lies in its inherent huge attenuation. Intelligent reflecting surface (IRS) and multiple-input-multiple-output (MIMO) represent two effective solutions for compensating the large path loss in THz systems. In this article, we consider an IRS-aided multiuser THz MIMO system with orthogonal frequency-division multiple (OFDM) access, where the sparse radio frequency chain antenna structure is adopted for reducing the power consumption. The objective is to maximize the weighted sum rate via jointly optimizing the hybrid analog/digital beamforming at the base station (BS) and reflection matrix at the IRS. Since the analog beamforming and reflection matrix need to cater all users and subcarriers, it is difficult to directly solve the formulated problem, and thus, an alternatively iterative optimization algorithm is proposed. Specifically, the analog beamforming is designed by solving a MIMO capacity maximization problem, while the digital beamforming and reflection matrix optimization are both tackled using semidefinite relaxation (SDR) technique. Considering that obtaining perfect channel state information (CSI) is a challenging task in IRS-based systems, we further explore the case with the imperfect CSI for the channels from the IRS to users. Under this setup, we propose a robust beamforming and reflection matrix design scheme for the originally formulated nonconvex optimization problem. Finally, simulation results are presented to demonstrate the effectiveness of the proposed algorithms.

Subspace-Based Estimation of Rapidly Varying Mobile Channels for OFDM Systems

It is well-known that time-varying channels can provide time diversity and improve error rate performance compared to time-invariant fading channels. However, exploiting time diversity requires very accurate channel estimates at the receiver. In order to reduce the number of unknown channel coefficients while estimating the time-varying channel, basis expansion models can be used along with long transmission frames that contain multiple orthogonal frequency division multiplexing (OFDM) symbols that experience the channel variation. The design of these OFDM frames need to judiciously incorporate training and data insertions in the transmitted signal while maintaining orthogonality. In this work, we propose an inter channel interference (ICI)-free training model depending on pilot symbols only and provide a corresponding time-varying channel estimation method. This scheme relies on an algorithm to determine the number of OFDM symbols per frame and the number of basis functions per path with minimal information about the Doppler bandwidth. As a performance benchmark, Bayesian Cramer Rao lower bound (CRLB) and the corresponding MSE bound are derived analytically for the proposed training model. Theoretical MSE expressions of the proposed estimation scheme are also derived as well as the MSE expressions in the presence of Doppler frequency mismatch. Simulations exhibit substantial MSE improvement and the corresponding Symbol Error Rate (SER) performances of the low complexity estimation scheme. They also corroborate that, unlike the common results in the literature, an OFDM system can perform better as the Doppler frequency increases with judicious design of training and channel estimation schemes.

Enhanced spectrum sensing using a combination of energy detector, matched filter and cyclic prefix

In this paper, we propose a sensing scheme based on energy detection, matched filter and cyclic prefix. Both Equal Gain Combining (EGC) and optimal combination of the aforementioned detectors are investigated in cooperative and non-cooperative spectrum sensing scenarios. In packet transmission systems such as OFDM (Orthogonal Frequency Division Multiple access) systems, the proposed scheme takes advantage of utilizing more samples than individual detectors, i.e., cyclic prefix, training or pilot samples, and data payload samples. The proposed combine-sensing scheme offers higher detection probability and lower false alarm probability, as compared with the performance of individual detectors over the same frame duration. Simulation results are congruent with the theoretical curves and confirm the validity of our derivations.

New Iterative Method for Estimation of Carrier Frequency in Multicarrier Systems

The key problem for OFDM (Orthogonal Frequency Division Multiple) systems is who to Estimate carrier frequency offset (CFO) with reduced complexity and acceptable performance. The CFO must be compensated before DFT (Discrete Fourier Transform) in order to restore data correctly and enhance the system performance. This paper, present a low complexity estimator of CFO with Semi-Blind (SB) criterion based on pilot tones and on virtual subcarriers, and with the aid of subspace based method. However, MUSIC and ESPRIT based semi-blind algorithms require a highly computational complexity. To overcome this drawback, we use Taylor’s series for the first order as developed in [1]. The present methods developed in this paper are very suitable for Multi- Carrier (MC) systems when the CFO are present. Simulation results demonstrated that the semi-blind (SB) approach outperforms the blind-based approach.

Outage Minimized Resource Allocation for Multiuser OFDM Systems With SWIPT

This paper considers a downlink multiuser orthogonal frequency division multiple (OFDM) system with simultaneous wireless information and power transfer (SWIPT), where the users decode information and harvest energy using the same received downlink signals. In particular, both power splitting (PS) and time switching (TS) SWIPT modes are considered for coordinating information transferring and energy harvesting. For multiuser information transmission, OFDM access (OFDMA) and time division multiple access (TDMA) are adopted for PS-SWIPT and TS-SWIPT, respectively. The outage performance of such a multiuser OFDM system with SWIPT is investigated. Particularly, we optimize the subcarrier allocation, power allocation and PS ratio for PS-SWIPT, and the power allocation and time allocation for TS-SWIPT, aiming at minimizing the fraction of outage over all users under the minimum harvest energy constraint along with the peak and total transmit power constraints. Suboptimal heuristic algorithms are obtained based on the Lagrange duality method. The outage performances of PS-SWIPT and TS-SWIPT are compared under various system parameter settings. We show that the outage performance of TS-SWIPT is better than that of the PS-SWIPT, except when the minimum required harvested energy is large, the total transmit power limit is small, or the number of users is small.

Enhanced Power Allocation for Sum Rate Maximization in OFDM-NOMA VLC Systems

This letter investigates the power allocation problem for the downlink of an orthogonal frequency division multiple (OFDM)-based non-orthogonal multiple access visible light communication (NOMA-VLC) system. Unlike the commonly used two-user OFDM-NOMA VLC model, in this letter, an arbitrary number of multiplexed users is considered when formulating the sum rate maximization problem. Moreover, both user-level and subcarrier-level power optimization are taken into account to improve the system performance. The corresponding theoretical analysis is presented, with the consideration of optimal decoding order and each user's required signal-to-interference-plus-noise ratio (SINR). Given a fixed power constraint on each OFDM subcarrier, we derive the optimal power ratio for an arbitrary number of multiplexed users in OFDM-NOMA VLC. Numerical results show that with a required SINR of 3 dB, the proposed enhanced power allocation algorithm has 39.59% and 40.11% sum-rate enhancement when compared to optimized fixed power allocation algorithm and gain ratio power allocation algorithm, respectively.

Tunable Multiwavelength Optical Comb Enabled WDM-OFDM-PON With Source-Free ONUs

In this paper, we propose and experimentally demonstrate a multiuser wavelength-division-multiplexing passive optical network (WDM-PON) system combining with orthogonal frequency division multiple (OFDM) technique. A tunable multiwavelength optical comb (MOC) is designed to supply flat optical lines for assisting the configuration of the proposed multiple source-free optical network units enhanced WDM-OFDM-PON system. Schematized by cascading a phase modulator and an intensity modulator with a recirculation loop, the MOC can output 29 ideal channel optical comb lines with the flatness of 0.85 dB. For the MOC enabled WDM-OFDM-PON system, source-free and interference-free multiuser upstream transmission over a single fiber can be efficiently supported, while downstream transmission channels can convey the baseband data stream and radio frequency OFDM signals simultaneously so that the proposed system can be connected to wire-line users and wireless users simultaneously.

Optimization of wireless information and power transfer in multiuser OFDM systems

Simultaneous wireless information and power transfer (SWIPT), which belongs to energy harvesting techniques, is an important research topic. In existing literature, two SWIPT schemes, namely the time switching (TS) scheme and the power splitting (PS) scheme are adopted. For multiuser orthogonal frequency division multiple (OFDM) systems, this paper proposes a new SWIPT scheme named as the subcarrier sharing (SS) scheme. Resource allocation algorithms for the SS scheme are then proposed for maximizing the sum rate under the minimum harvested energy constraint. We show that the SS scheme outperforms the existing TS and the PS schemes.

Performance Analysis of Diversity-Controlled Multi-User Superposition Transmission for 5G Wireless Networks.

In this paper, we propose a novel low-complexity multi-user superposition transmission (MUST) technique for 5G downlink networks, which allows multiple cell-edge users to be multiplexed with a single cell-center user. We call the proposed technique diversity-controlled MUST technique since the cell-center user enjoys the frequency diversity effect via signal repetition over multiple orthogonal frequency division multiplexing (OFDM) sub-carriers. We assume that a base station is equipped with a single antenna but users are equipped with multiple antennas. In addition, we assume that the quadrature phase shift keying (QPSK) modulation is used for users. We mathematically analyze the bit error rate (BER) of both cell-edge users and cell-center users, which is the first theoretical result in the literature to the best of our knowledge. The mathematical analysis is validated through extensive link-level simulations.

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.

Resource allocation for two-way amplify and forward OFDM relay networks

Physical-layer network coding (PNC) promises substantial theoretical gain to achieve the maximum system throughput in cooperative relay transmission. However, with the increasing global warming, how to reduce power consumption while satisfy system throughput requirement is becoming a vital issue. In this paper, we investigate energy-efficiency resource allocation (RA) based on PNC with amplify-and-forward (AF) protocol in orthogonal frequency division multiple (OFDM) bidirectional transmission. To minimize the overall transmit power consumption with required system throughput requirement, we consider joint subcarriers and power allocation and formulate the objective task into a constrained optimization problem where the best relay node is selected to minimize total transmit power. The closed form optimization power allocation solutions are acquired by analytical derivation. Based on derivation, we propose a novel optimal energy-efficient power allocation (OE-PA). Numerical results are given to evaluate the performance of the derived scheme as compared to other schemes and show that our scheme has significant improvement to energy saving.

Downlink Scheduling and Resource Allocation for 5G MIMO-Multicarrier: OFDM vs FBMC/OQAM

The definition of the next generation of wireless communications, so-called 5G networks, is currently underway. Among many technical decisions, one that is particularly fundamental is the choice of the physical layer modulation format and waveform, an issue for which several alternatives have been proposed. Two of the most promising candidates are: 1) orthogonal frequency division multiple (OFDM), a conservative proposal that builds upon the huge legacy of 4G networks and 2) filter bank multicarrier/offset quadrature amplitude modulation (FBMC/OQAM), a progressive approach that in frequency selective channels sacrifices subcarrier orthogonality in lieu of an increased spectral efficiency. The comparative merits of OFDM and FBMC/OQAM have been well investigated over the last few years but mostly, from a purely physical layer point of view and largely neglecting how the physical layer performance translates into user-relevant metrics at the upper-layers. This paper aims at presenting a comprehensive comparison of both modulation formats in terms of practical network indicators, such as goodput, delay, fairness, and service coverage, and under operational conditions that can be envisaged to be realistic in 5G deployments. To this end, a unifying cross-layer framework is proposed that encompasses the downlink scheduling and resource allocation procedures and that builds upon a model of the queueing process at the data-link control layer and a physical layer abstraction that can be chosen to model either OFDM or FBMC/OQAM. Extensive numerical results conclusively demonstrate that most of the a priori advantages of FBMC/OQAM over OFDM do indeed translate into improved network indicators, that is, the increase in spectral efficiency achieved by FBMC/OQAM makes up for the distortion caused by the loss of orthogonality.

Flexible Radio Access Beyond 5G: A Future Projection on Waveform, Numerology, and Frame Design Principles

To address the vast variety of user requirements, applications, and channel conditions, flexibility support is strongly highlighted for 5G radio access technologies (RATs). For this purpose, usage of multiple orthogonal frequency division multiplexing (OFDM) numerologies, i.e., different parameterization of OFDM-based subframes, within the same frame has been proposed in the third-generation partnership project discussions for 5G new radio. This concept will likely meet the current expectations in multiple service requirements to some extent. However, since the quantity of wireless devices, applications, and heterogeneity of user requirements will keep increasing toward the next decade, the sufficiency of the aforementioned flexibility consideration remains quite disputable for future services. Therefore, novel RATs facilitating much more flexibility are needed to address various technical challenges, e.g., power efficiency, massive connectivity, latency, spectral efficiency, robustness against channel dispersions, and so on. In this paper, we discuss the potential directions to achieve further flexibility in RATs beyond 5G, such as future releases of 5G and 6G. In this context, a framework for developing flexible waveform, numerology, and frame design strategies is proposed along with sample methods. We also discuss their potential role to handle various upper-level system issues, including the ones in orthogonal and nonorthogonal multiple accessing schemes and cellular networks. By doing so, we aim to contribute to the future vision of designing flexible RATs and to point out the possible research gaps in the related fields.

Fairness-Aware and Energy Efficiency Resource Allocation in Multiuser OFDM Relaying System

A fairness-aware resource allocation scheme in a cooperative orthogonal frequency division multiple (OFDM) network is proposed based on jointly optimizing the subcarrier pairing, power allocation, and channel-user assignment. Compared with traditional OFDM relaying networks, the source is permitted to retransfer the same data transmitted by it in the first time slot, further improving the system capacity performance. The problem which maximizes the energy efficiency (EE) of the system with total power constraint and minimal spectral efficiency constraint is formulated into a mixed-integer nonlinear programming (MINLP) problem which has an intractable complexity in general. The optimization model is simplified into a typical fractional programming problem which is testified to be quasiconcave. Thus we can adopt Dinkelbach method to deal with MINLP problem proposed to achieve the optimal solution. The simulation results show that the joint resource allocation method proposed can achieve an optimal EE performance under the minimum system service rate requirement with a good global convergence.

Joint energy‐efficient power allocation and subcarrier pairing in orthogonal frequency division multiple‐based multi‐relay networks

In this study, a resource allocation scheme for energy-efficient communication in a cooperative orthogonal frequency division multiple (OFDM) network based on subcarrier pairing is proposed. Different from traditional cooperative OFDM networks, in the concerned network, one subcarrier pair can be forwarded by multiple relays, and the source is allowed to retransmit the signals sent in the first phase. The problem aimed at maximising the system energy efficiency (EE) under total power constraint and minimum spectral efficiency constraint is formulated into a mixed-integer nonlinear programming (MINLP) problem, which in general has intractable complexity because of its combinatorial nature. The authors simplify the optimising model as a typical fractional programming problem to solve the complex the MINLP problem. The simplified fractional programming problem is proved to be quasi-concave, thus Dinkelbach's method consisting of outer iterations and inner iterations can be used to find the optimal solution to the proposed MINLP problem.

Joint Energy-Efficient Resource Allocation Scheme for AF-Based Multi-Relay OFDM Networks

In this paper, a joint resource allocation scheme for energy-efficient communication in cooperative orthogonal frequency division multiple (OFDM) networks based on subcarrier pairing (SP) is studied. The problem aimed at maximizing the system energy efficiency (EE) is formulated into a mixed-integer nonlinear programming (MINLP) problem. To solve the complex MINLP problem, we simplify the optimizing model as a typical fractional programming problem by defining the equivalent channel gain, thus Dinkelbach’s method consisting of outer iterations and inner iterations can be used to find the optimal solution to the MINLP problem proposed in polynomial time. Simulation results show that the proposed scheme can improve the system EE and ensure the quality of service (QoS) of users.

Optimized BS assignment and resource allocation in cooperative OFDM networks

Cooperative transmission (CT) and orthogonal frequency division multiple (OFDM) are promising technologies for extending coverage and increasing throughput in broadband wireless access (BWA) networks. Therefore, we propose a novel BWA network architecture, that can set up inter-cell collaboration using physical layer cooperative transmissions among distributed wired access networks with a powerful coordination capability at the central office. However, conventional base station (BS) assignment and resource allocation schemes cannot be used directly because a user can be serviced by more than one BS with cooperative transmission technology. This study proposes a novel framework of BS assignment and resource allocation in a cooperative OFDM network. We provide three approaches of resource allocation for minimizing bandwidth usage, minimizing transmission power consumption, and balancing resource costs respectively. An optimized resource allocation scheme can be implemented by flexibly choosing one of these approaches based on network load. The simulation results show the efficiency of the proposed mathematical formulations and linearization approach of our scheme. The performance benefit of CT technology on the bandwidth saving is demonstrated by comparing the new BS assignment and resource allocation scheme with conventional non-cooperative transmission.

A Novel Optical Network Units Source-Free Scheme Using Optical Frequency Comb Generator for OFDM-PON

The letter proposes a novel Optical Network Units (ONUs) source-free scheme for Orthogonal Frequency Division Multiple (OFDM)-based passive optical networks (PON), in which an optical frequency comb generator built by a conventional Mach-Zehnder modulator (MZM) is used to generate ultra-flat optical frequency comb spectra for the downstream and upstream transmission. The proposed architecture is presented and simulated to achieve simultaneously upstream transmission from multiple ONUs over the fiber. Simulation results show the receiver sensitivity reaches-18dBm for upstream transmission and-18.75dBm for downstream transmission.

A Joint Resource Allocation Scheme for Relay Enhanced Multi-cell Orthogonal Frequency Division Multiple Networks

This paper formulates resource allocation for decode-and-forward (DF) relay assisted multi-cell orthogonal frequency division multiple (OFDM) networks as an optimization problem taking into account of inter-cell interference and users fairness. To maximize the transmit rate of system we propose a joint interference coordination, subcarrier and power allocation algorithm. To reduce the complexity, this semi-distributed algorithm divides the primal optimization into three sub-optimization problems, which transforms the mixed binary nonlinear programming problem (BNLP) into standard convex optimization problems. The first layer optimization problem is used to get the optimal subcarrier distribution index. The second is to solve the problem that how to allocate power optimally in a certain subcarrier distribution order. Based on the concept of equivalent channel gain (ECG) we transform the max-min function into standard closed expression. Subsequently, with the aid of dual decomposition, water-filling theorem and iterative power allocation algorithm the optimal solution of the original problem can be got with acceptable complexity. The third sub-problem considers dynamic co-channel interference caused by adjacent cells and redistributes resources to achieve the goal of maximizing system throughput. Finally, simulation results are provided to corroborate the proposed algorithm.

Time-Frequency Spectrum Sharing Scheme for Next Generation OFDM Systems

Spectrum sharing and multiple-access techniques are challenging matters for future wireless communication systems. Orthogonal frequency division multiplexing (OFDM) has been considered as the most important physical layer candidate for the next generation wireless systems. Alternatively, multiband OFDM (MB-OFDM) approach has shown some novelty and efficiency for high-rate (HR) WPAN applications and so has been selected for future HR ultra wideband (UWB) systems. Based on an optimization problem formulation, we jointly consider resource allocation and scheduling to define a novel dynamic timefrequency spectrum sharing mechanism based on the MB-OFDM approach. Viewed as a cross-layer solution, the proposed MB-OFDM multiple-access (MB-OFDM-MA) scheme can achieve an efficient multiuser spectrum sharing under QoS constraints. While the optimization formulation results in a high-complexity time-frequency sharing solution, we propose a low-complexity suboptimal solution able to jointly provide fairness provision with the QoS support. Compared with traditional orthogonal frequency divisionmultiple access (OFDMA), the simple time-frequency MB-OFDM-MA approach ensures a high level of fairness among the users while satisfying high-priority users having strict QoS requirements.