Orthogonal Frequency Division Multiple Access Systems Research Articles

Spectrum Refarming: A New Paradigm of Spectrum Sharing for Cellular Networks

Spectrum refarming (SR) refers to a radio resource management technique which supports different generations of cellular networks to operate in the same radio spectrum. In this paper, an underlay SR model is proposed for an Orthogonal Frequency Division Multiple Access (OFDMA) system to share the spectrum of a Code Division Multiple Access (CDMA) system through intelligently exploiting the interference margin provided by the CDMA system when operating with a low system load. The asymptotic signal-to-interference-plus-noise ratio (SINR) of the CDMA system is used to quantify the interference margin, which interestingly does not depend on the instantaneous information (spreading codes and channel state information) of the CDMA system, thanks to its internal power control. By using the transmit power constraints together with the derived interference margin, the uplink resource allocation problem for OFDMA system is formulated and solved through dual decomposition method. The proposed SR system only requires high level system parameters from the CDMA system, hence, the upgrading of legacy CDMA system is not needed. Simulation results have verified our theoretical analysis, and validated the effectiveness of the proposed resource allocation algorithm and its capability to protect the legacy CDMA users.

Low Complexity CFO Compensation in Uplink OFDMA Systems With Receiver Windowing

Orthogonal frequency division multiple access (OFDMA) systems in the uplink suffer from multiple access interference (MAI) due to their high sensitivity to frequency misalignments between different users. In this paper, we propose the application of time domain receiver windowing methods to confine the leakage caused by multiple carrier frequency offsets (CFOs) to a few neighboring subcarriers with almost no additional computational burden. The CFO effects can be translated into a linear system of equations with a coefficient matrix that is called interference matrix. As a result of receiver windowing, we can approximate the interference matrix with a quasi-banded one by neglecting its small elements outside a certain bandwidth. This allows us to propose a class of low complexity CFO compensation techniques. These techniques are applicable to the generalized subcarrier allocation scheme (G-CAS). The complexity reduction in the proposed solutions is substantial when compared to the existing ones in the literature. It is worth mentioning that this substantial complexity reduction is achieved in expense of some bandwidth efficiency loss. Hence, there exists a tradeoff between bandwidth efficiency and complexity. Solutions based on both zero forcing (ZF) and minimum mean squared error (MMSE) criteria are proposed and compared. Simulation results demonstrating the effectiveness of the proposed algorithms in approaching the optimal performance are also presented.

Subcarrier and Power Allocation Algorithm for Spectral Efficiency Maximization in Superposition Coding OFDMA Systems

This paper addresses the optimization problem on subcarrier and power allocation of orthogonal frequency division multiple access (OFDMA) system under spectral efficiency (SE) metric when deploying superposition coding (SC) transmission strategy. An algorithm with polynomial time complexity, of the order of (UN log 2(N)) has been proposed for sub-optimal SE maximization. Results indicate that the system SE increases with the use of SC technique. Besides, the throughput gain with SC adoption increases when the number of users (U) approaches the number of subcarriers (N) available in the system.

Adaptive Resource Allocation in OFDMA System: A Review

The need for high capacity downlink in wireless mobile network in future is very much essential and efforts are being made to identify appropriate techniques to attain high capacity downlink. Latest approaches indicated that the orthogonal frequency division multiple access (OFDMA) technique is emerging as a potential technique. While the other techniques result inter-carrier interference, OFDMA technique over comes this problem due to orthogonal location of sub-carriers. This gives advantage by obviating the need for inter-carrier guard band. The issues of efficient utilization of bandwidth, minimizing power consumption and fair distribution of resources among users are central to determine the appropriate technique to be used for resource allocation. This paper discusses various techniques suggested by the researchers for resource allocation and concludes that adaptive algorithm is more efficient compared to static TDMA/FDMA system.

Resource Allocation for Joint Transmitter and Receiver Energy Efficiency Maximization in Downlink OFDMA Systems

This paper investigates the joint transmitter and receiver optimization for the energy efficiency (EE) in orthogonal frequency-division multiple-access (OFDMA) systems. We first establish a holistic power dissipation model for OFDMA systems, including the transmission power, signal processing power, and circuit power from both the transmitter and receiver sides, while existing works only consider the one side power consumption and also fail to capture the impact of subcarriers and users on the system EE. The EE maximization problem is formulated as a combinatorial fractional problem that is NP-hard. To make it tractable, we transform the problem of fractional form into a subtractive-form one by using the Dinkelbach transformation and then propose a joint optimization method, which leads to the asymptotically optimal solution. To reduce the computational complexity, we decompose the joint optimization into two consecutive steps, where the key idea lies in exploring the inherent fractional structure of the introduced individual EE and the system EE. In addition, we provide a sufficient condition under which our proposed two-step method is optimal. Numerical results demonstrate the effectiveness of proposed methods, and the effect of imperfect channel state information is also characterized.

Advances in Cooperative Single-Carrier FDMA Communications: Beyond LTE-Advanced

In this paper, we focus our attention on the cooperative uplink transmissions of systems beyond the LTE-Advanced initiative. We commence a unified treatment of the principle of single-carrier frequency-division multiple-access (FDMA) and the similarities and dissimilarities, advantages, and weakness of the localized FDMA, the interleaved FDMA, and the orthogonal FDMA systems are compared. Furthermore, the philosophy of both user cooperation and cooperative single-carrier FDMA is reviewed. They are investigated in the context of diverse topologies, transmission modes, resource allocation, and signal processing techniques applied at the relays. Benefits of relaying in LTE-Advanced are also reviewed. Our discussions demonstrate that these advanced techniques optimally exploit the resources in the context of cooperative single-carrier FDMA system, which is a promising enabler for various uplink transmission scenarios.

Sparse channel estimation and pilot optimization for underwater acoustic orthogonal frequency division multiple access uplink communications

Considering that the conventional channel interpolation method with sparse and irregular spaced pilots will lead to an error floor in underwater acoustic (UWA) orthogonal frequency division multiple access (OFDMA) uplink communications, a method for sparse channel estimation and pilot optimization is proposed in this paper. A compressed sensing (CS) algorithm is utilized for sparse channel impulse response estimation, which performs well in sparse and irregular spaced pilots and significantly decreases the channel estimation error. Besides, the pilots’ pattern and power joint optimization algorithm based on the random search technique is proposed for the minimum mutual coherence criterion in CS theory, which further improves the performance of CS estimation algorithm. During each iteration step, we randomly pick a pilots’ pattern from the subcarrier index set and a pilots’ power subset from the available power set. Then we perform this step iteratively within a certain searching time. Finally, the local optimal solution of the objective function for minimizing mutual coherence is considered as the feasible pilots’ pattern and power. Simulation results show that the convergence performance of the pilots’ pattern and power joint optimization algorithm is much better than that of the pilots’ pattern optimization algorithm. Furthermore, the channel estimation error of the proposed method is much lower than that of conventional least-squares channel estimator based on linear interpolation, CS channel estimator without pilot optimization, and CS channel estimator merely with pilots’ pattern optimization in channels of different multipath delay spreads. Finally, performance of the proposed method is demonstrated in the UWA uplink OFDMA systems with interleaved and generalized carrier assignment schemes respectively in the two-user case in a pool experiment. Experimental results show that the proposed method decreases dramatically the bit error rate in both carrier assignment schemes, and simultaneous reception for two users is achieved when signal noise ratio is larger than 10 dB.

Resource Reservation to Support Service Continuity in OFDMA Systems

When the load in a multi-cell orthogonal frequency division multiple access (OFDMA) system is allowed to excessively increase in face of frequent handover, the cell area becomes smaller than the designed size, and thus continuity of quality of service (QoS) for handover requests cannot be guaranteed. To efficiently support the mobility of a mobile terminal (MT), we should adaptively cope with the resource demand of handover calls. This paper proposes a twofold resource-reservation scheme for OFDMA systems to guarantee continuity of QoS for various mobile multimedia services during MT handover from lightly to heavily loaded cells. Our twofold scheme attempts to guarantee service continuity for handover and to maximize resource allocation efficiency. We performed a simulation to evaluate our scheme in terms of outage probability, handover failure rate, total throughput, and blocking rate.

Inphase and Quadrature Utilization for Pairing Diversity and Interference Exploitation in Uplink OFDMA

This paper proposes a new pairing cooperative technique to achieve spatial diversity and constructive interference exploitation in uplink orthogonal frequency-division multiple access (OFDMA) systems. The underlying principle is to pair users such that their symbols are combined into a single constellation, according to a predefined rule, so that the same symbol is transmitted by each pair of users so their signals can be coherently summed at the receiver, hence maximizing the received signal power. To achieve such constructive combination of both signal copies, single-dimensional OFDMA is utilized in the inphase and quadrature dimensions. It will be shown that such transmission method not only maximizes the received power but also greatly reduces sensitivity to timing misalignment between the cooperating users. Both time synchronous and asynchronous transmission scenarios will be considered. In the asynchronous case, interference reduction will be applied at the base station to reinstate the pairs' orthogonality and hence minimize co-channel interference. The performance of this technique is evaluated through simulations and mathematical analysis. It will be shown that the results from both models closely match and that the proposed technique provides significant bit-error-rate reductions, particularly at low signal-to-noise ratio, and throughput improvements relative to existing cooperative transmission techniques.

Complexity Analysis of Joint Subcarrier and Power Allocation for the Cellular Downlink OFDMA System

Consider the cellular downlink Orthogonal Frequency Division Multiple Access (OFDMA) system where a single transmitter transmits signals to multiple receivers on multiple discrete subcarriers. To adapt fast channel fluctuations, the transmitter should be able to dynamically allocate subcarrier and power resources. Assuming perfect channel knowledge, we formulate the joint subcarrier and power allocation problem as two optimization problems: the first is the one of minimizing the total transmission power subject to quality of service constraints, and the second is the one of maximizing a system utility function subject to power budget constraints. In this letter, we show that both the aforementioned formulations of the joint subcarrier and power allocation problem are generally NP-hard. We also identify several subclasses of the problem which are polynomial time solvable.

New Energy Efficiency Metric With Imperfect Channel Considerations for OFDMA Systems

Considering recent important efforts on green radio technologies, this letter deliberates on imperfect channel realizations to propose a new effective-energy-efficiency metric for downlink transmissions in multi-user multi-carrier orthogonal frequency-division multiple access (OFDMA) systems. To comply with the traditional energy efficiency formula under the assumption of perfect channel knowledge available, this new metric is designed on the fractional relation between the effective throughput and the total power consumed at the transmitter. Mathematical proofs are presented to confirm such compliance in terms of the impact of the channel coefficients on the energy efficiency performance. In order to prove the property, this letter also presents profound insight into the monotonicity of the non-centralized chi-squared distribution.

Composite differential evolution aided channel allocation in OFDMA systems with proportional rate constraints

Orthogonal frequency division multiple access (OFDMA) is a promising technique, which can provide high downlink capacity for the future wireless systems. The total capacity of OFDMA can be maximized by adaptively assigning subchannels to the user with the best gain for that subchannel, with power subsequently distributed by water-filling. In this paper, we propose the use of composite differential evolution (CoDE) algorithm to allocate the subchannels. The CoDE algorithm is population-based where a set of potential solutions evolves to approach a near-optimal solution for the problem under study. CoDE uses three trial vector generation strategies and three control parameter settings. It randomly combines them to generate trial vectors. In CoDE, three trial vectors are generated for each target vector unlike other differential evolution (DE) techniques where only a single trial vector is generated. Then the best one enters the next generation if it is better than its target vector. It is shown that the proposed method obtains higher sum capacities as compared to that obtained by previous works, with comparable computational complexity.

Noisy chaotic neural network for resource allocation in high-speed train OFDMA system

High-speed train communication system is a typical high-mobility wireless communication network. Resource allocation problem has a great impact on the system performance. However, conventional resource allocation approaches in cellular network cannot be directly applied to this kind of special communication environment. A multi-domain resource allocation strategy was proposed in the orthogonal frequency-division multiple access (OFDMA) of high-speed. By analyzing the effect of Doppler shift, sub-channels, antennas, time slots and power were jointly considered to maximize the energy efficiency under the constraint of total transmission power. For the purpose of reducing the computational complexity, noisy chaotic neural network algorithm was used to solve the above optimization problem. Simulation results showed that the proposed resource allocation method had a better performance than the traditional strategy.

Imperfect CSI-Based Joint Resource Allocation in Multirelay OFDMA Networks

Radio resource allocation is very important in relay orthogonal frequency-division multiple-access (OFDMA) networks. Since perfect channel state information (CSI) is hardly obtained, the robust design that takes into account the CSI imperfection plays a crucial role in the development of efficient resource-allocation schemes. As to the chunk-based resource-allocation scheme, this paper focuses on the imperfect CSI-based resource-allocation problem to jointly optimize the chunk assignment, transmission link selection (direct or relay link), and power allocation in multirelay OFDMA systems. The optimization formulation minimizes the total transmit power with transmission requirement constraints and is considered a mixed-integer optimization problem, and a dual-maximization-based joint resource-allocation algorithm is proposed. The proposed dual-maximization-based algorithm is shown to be optimal. We also develop a low-complexity but near-optimal resource-allocation algorithm. The numerical results validate the performance gains offered by the proposed joint resource-allocation algorithms. The results also show that the impact of imperfect CSI is nonnegligible, which highlights the importance of robust designs in practical wireless networks.

Proportional fair resource allocation based on hybrid ant colony optimization for slow adaptive OFDMA system

Proportional fair resource allocation plays a critical role to enhance the performance of slow adaptive orthogonal frequency division multiple access (OFDMA) system. In slow adaptive OFDMA system, the subcarrier allocation is updated at the beginning of every time window and the channel gain of users at the time of the subcarrier allocation could not be known precisely. This leads to the challenge for designing the resource allocation algorithm for slow adaptive OFDMA system. In this work, we formulate a proportional fair resource allocation problem based on chance-constrained programming for slow adaptive OFDMA system which maximizes the average sum-rate in an adaptive time window and guarantees the Jain fair index (JFI) requirement with the target probability. In order to solve the chance-constrained resource allocation problem, we propose hybrid ant colony optimization (HACO) which combines ant colony optimization (ACO) and support vector machine (SVM). Simulation results demonstrate that HACO not only yields higher average sum-rate, but also guarantees the chance-constrained condition very well compared with other algorithm.

Sensitivity Analysis of OFDMA and SC-FDMA Uplink Systems to Carrier Frequency Offset

Orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA) are two technologies for the uplink transmission of present and next generation of broadband wireless systems. This paper studies and compares OFDMA and SC-FDMA in terms of sensitivity to carrier frequency offset (CFO) in the uplink. In order to calculate signal-to-interference ratio (SIR), we use a simple superposition principle approach where the contributions of different users are studied separately. We derive closed-form mathematical expressions for the desired signal, interference terms, and consequently SIR for both OFDMA and SC-FDMA systems in the uplink. It is pointed out that there is a strong relationship between sensitivity analysis to CFO and subcarrier allocation schemes. Also, we prove that the derived expressions for both systems are reduced to very simple forms in interleaved subcarrier allocation. Finally, the theoretical analysis are verified using Monte Carlo simulations in block, interleaved, and block-interleaved subcarrier allocations, and the two systems are compared upon these set of results.

Novel Cross-Layer QoE-Aware Radio Resource Allocation Algorithms in Multiuser OFDMA Systems

The assurance of quality of experience (QoE) and provisioning of high throughput of the system represent the main goals of future wireless and mobile networks. This paper presents novel and practical cross-layer QoE-aware radio resource allocation (RRA) algorithms for the downlink of a heterogeneous orthogonal frequency division multiple access (OFDMA) system. The objective of the proposed algorithms is to assure the appropriate level of QoE for each user of the system by incorporating application-layer parameters and subjective human perception of quality into the RRA process. We propose two user-oriented joint subcarrier and power allocation algorithms with low complexity for real-time and interactive services. The first algorithm dynamically allocates resources by assuring the same level of QoE to all users of the system, whereas the second algorithm introduces the efficient trade-off between the user's QoE and the spectral efficiency of the system. By considering application-layer parameters and user's perception of quality, high users' QoE and explicit control of data rates can be achieved. The numerical results show that the proposed algorithms achieve significant increase in the level of QoE compared to previous works, a fair distribution of capacity among users and near to optimal solution of QoE for the OFDMA system.

CAPACITY IMPROVEMENT OF MIMO-OFDMA SYSTEM USING ADAPTIVE RESOURCE ALLOCATION ALGORITHM

The user demands for wireless communication are increasing but the existing networks are not able to fulfill the high data rate of future communication services. More bandwidth can support high data rate wireless access. Due to spectral limitations, it is very expensive to increase. Therefore, the available bandwidth is needed to use efficiently. Orthogonal Frequency Division Multiplexing (OFDM) has been explored as a promising air interface technology to utilize the bandwidth effectively. OFDM can be exploited as a multiple access technique, called Orthogonal Frequency Division Multiple Access (OFDMA) that can support both frequency and multiuser diversity. In order to serve the user population, one of the most sophisticated techniques, resource allocation technique, can be integrated into the OFDMA system. Multiple Input Multiple Output (MIMO) technology can increase the spectral efficiency because multiple data streams can be simultaneously transmitted over the channel. The combination of MIMO technology and OFDMA system can achieve both higher spectral efficiency and better Quality of Service (QoS) such as capacity, fairness, and Bit Error Rate (BER). In this paper, a MIMO-OFDMA system is developed for downlink transmission within one single cell. Two optimization methods such as subcarrier allocation and power allocation are used for resource allocation. To decompose the MIMO channel into parallel single channels, Eigen Value Decomposition (EVD) is used. Subcarrier selection is based on the priority of the user by using the dominant eigen channels. Water-filling algorithm is adapted for power distribution among users. The aim is to maximize the system capacity subject to constraints on total power and proportional fairness. The performance of the system is evaluated by comparing the static allocation method (equal subcarrier allocation with equal power) and two adaptive allocation methods (adaptive subcarrier allocation with equal power and adaptive subcarrier allocation with adaptive power).

Feedback Optimization Schemes for Downlink MISO-OFDMA Systems With Heterogeneous Users

This paper proposes feedback optimization schemes for multiple-input single-output orthogonal frequency-division multiple access (MISO-OFDMA) systems with heterogeneous users. The proposed scheme adjusts the unit and resolution of the feedback to efficiently exploit the long-term channel statistics, including the path loss, delay spread, and mobility. To develop an analytical optimization framework, we first derive the sum-rate of MISO-OFDMA systems when the channel-state information (CSI) at the transmitter is imperfect because of the quantization error, channel variation, and feedback delay. Based on the derived results, we present effective solutions to the sum-rate maximization problem for distributed and centralized scenarios. When the channel statistics of the users are not available at the base station, each user can improve the sum-rate of the system by minimizing the distortion on CSI feedback under a fixed per-user feedback rate. With the channel statistics of the users, the base station can further control the user set and the feedback rate in addition to the feedback granularity. We evaluate the performance of both distributed and centralized schemes over practical wireless environments, where each user has different long-term channel statistics. Simulation results show that both methods significantly improve the performance of MISO-OFDMA systems.

Spectral efficiency improvement in OFDMA systems with modified adaptive subcarrier spacing based resource allocation algorithm

Future broadband wireless communication standards are expected to offer high-quality services under all mobility conditions. Achieving high-quality services in a wireless fading environment is a challenging task. To combat the effect of frequency selective fading, recent wireless standards prefer multicarrier modulation schemes such as orthogonal frequency division multiplexing (OFDM) instead of single carrier modulation. The high mobility of the users leads to a larger amount of inter-carrier interference (ICI) in orthogonal frequency division multiple access (OFDMA) systems and thus the spectral efficiency reduces. Achieving higher spectral efficiency by combating the effect of ICI with reasonable complexity is the essence of this paper. Considering carrier frequency offset (CFO) pre-correction schemes along with adaptive subcarrier spacing (ASS)–adaptive bit loading (ABL) algorithms, we present the modified adaptive subcarrier spacing (MASS) resource allocation algorithm to cancel ICI and to increase s...