Multiuser OFDM Research Articles

On Relay Selection and Subcarrier Assignment for Multiuser Cooperative OFDMA Networks With QoS Guarantees

This paper investigates quality of service (QoS)-aware relay selection and subcarrier assignment in multiuser OFDMA relay networks. In contrast to some existing works, which solve the sum-rate maximization problem directly, we first simplify it by exploiting the structure of the optimal solution and then consider the simplified problem from the perspectives of both primal and dual optimality. To characterize the optimal sum-rate performance, we solve the simplified problem optimally by the branch-and-cut method. In addition to the sum-rate maximization problem, we demonstrate that its two variants, namely, the worst-user rate maximization problem and the mixed rate maximization problem, can be also similarly simplified and then solved optimally by the branch-and-cut method. The optimal method serving as the benchmark is particularly suitable for problems of moderate scale. Furthermore, we study the simplified sum-rate maximization problem comprehensively from a dual perspective and propose three types of subgradient methods for the dual problem. The dual subgradient methods obtaining near-optimal solutions can be implemented in a distributed manner and thus are quite suitable for high-dimensional problems. To provide more in-depth analysis, we further consider the scenario in which subcarrier pairing is incorporated and suggest the natural generalization from the one-source-node case to the multiple-source-node case. Finally, two groups of simulations are conducted to validate the optimality of the branch-and-cut method and the effectiveness of the dual subgradient methods.

Multiuser Resource Allocation for OFDM Based on AFSA with User Bandwidth Limitation

According to the issue that user bandwidth limitations is always ignored within the resources allocation algorithm of OFDM downlink, a multiuser OFDM adaptive resource allocation algorithm based on AFSA under the condition of limited users bandwidth is proposed in this paper. Under the condition of user bandwidth limitations, the maximum gain of channels is obtained and the bandwidth of each user is optimized by seeking optimal behaviors of AFSA, iteration and convergence. As a result, the resources for OFDM downlink link can be allocated more efficiently. The results of simulation experiments show that the proposed resource allocation strategy based on swarm intelligence, compared with similar algorithms, can carry out the global optimal convergence. Simultaneously, better system performance can be achieved in the given three different cases.

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.

Joint Transmitter and Receiver Energy Minimization in Multiuser OFDM Systems

In this paper, we formulate and solve a weighted-sum transmitter and receiver energy minimization (WSTREMin) problem in the downlink of an orthogonal frequency division multiplexing (OFDM) based multiuser wireless system. The proposed approach offers the flexibility of assigning different levels of importance to base station (BS) and mobile terminal (MT) power consumption, with the BS being connected to the grid and the MT relying on batteries. To obtain insights into the problem, we first consider two extreme cases separately, i.e., weighted-sum receiver-side energy minimization (WSREMin) for MTs and transmitter-side energy minimization (TEMin) for the BS. It is shown that Dynamic TDMA (D-TDMA), where MTs are scheduled for single-user OFDM transmissions over orthogonal time slots, is the optimal transmission strategy for WSREMin at MTs, while OFDMA is optimal for TEMin at the BS. As a hybrid of the two extreme cases, we further propose a new multiple access scheme, i.e., Time-Slotted OFDMA (TS-OFDMA) scheme, in which MTs are grouped into orthogonal time slots with OFDMA applied to users assigned within the same slot. TS-OFDMA can be shown to include both D-TDMA and OFDMA as special cases. Numerical results confirm that the proposed schemes enable a flexible range of energy consumption tradeoffs between the BS and MTs.

Adaptive Resource Allocation Algorithms for Multi-user MIMO-OFDM Systems

Radio resource allocation to the system users is the key challenge issue in multi-user orthogonal frequency division multiplexing (MU-OFDM) systems. In this paper, an efficient and low-complexity proportional rate-adaptive radio resource (sub-carrier and power) allocation algorithm for MU-OFDM is proposed to maximize the sum-rate capacity of the system and achieve acceptable users' rates fairness. Three-dimensional (spatial, frequency, and multi-user) greedy power allocation (GPA) algorithms for the MU multi-input multi-output OFDM (MU MIMO-OFDM) systems are proposed. The proposed algorithms start with spatial sub-carrier allocation followed by an optimal two-dimensional spatial-frequency GPA (SFGPA) step, which exploits both the spatial and frequency diversities. Therefore, spatial, frequency, and multi-user diversities are exploited by the proposed three-dimensional GPA algorithms. Several experiments are carried out to test the performance of the proposed three-dimensional GPA algorithms in terms of sum-rate capacity. The optimal solution is achieved by the three-dimensional dynamic sub-carrier-SFGPA with average gain (DS-SFGPA-AG) algorithm, which achieves the best sum-rate capacity performance compared with the other power allocation algorithms. The performance of the DS-SFGPA-AG algorithm is also analyzed and compared for the four OFDM-based systems; single-user (SU) OFDM, SU MIMO-OFDM, MU OFDM, and MU MIMO-OFDM. The MU MIMO-OFDM system with adaptive sub-carrier and power allocation outperforms the other systems.

Full duplex random access for multi-user OFDMA communication systems

Random medium access control (MAC) plays a critical role in OFDMA wireless networks to support services with bursty traffic. However, the existing OFDMA random access protocols suffer from low efficiency due to the constraints in half-duplex communications. In this paper, the capability of full-duplex communications is leveraged to propose novel mechanisms such as full duplex carrier sensing, collision detection, and collision jamming. With these mechanisms, a full-duplex carrier sense multiple access with collision detection (FD-CSMA/CD) protocol is developed for OFDMA wireless networks. It is based on a hierarchical design: (1) with full-duplex communications, operation of subchannels is decoupled, and a CSMA/CD-like protocol is implemented in each subchannel as a random access scheme; (2) on top of random access per-subchannel, a simple but effective subchannel selection scheme is executed locally on each client to harvest multi-user diversity. Both theoretical analysis and simulations are carried out to validate the effectiveness and efficiency of our protocol.

Multiuser Successive Maximum Ratio Transmission (MS-MRT) for Video Quality Maximization in Unicast and Broadcast MIMO OFDMA-Based 4G Wireless Networks

In this paper, we propose a novel low-complexity beamforming algorithm for multiuser successive maximum ratio transmission (MS-MRT) toward space-division multiplexing to achieve video quality maximization in the downlink of a multiuser (MU) multiple-input-multiple-output (MIMO) orthogonal frequency-division multiple-access (OFDMA)-based fourth-generation (4G) wireless network. We compare the performance of this algorithm in terms of system throughput and video quality with the two popular precoding techniques, namely, block diagonalization (BD) and successive optimization (SO). We also compare its performance with the generalized versions of these two algorithms termed coordinated transmit-receive processing (CTR)-BD and CTR-SO algorithms, respectively. Further, we propose an extension of the MS-MRT scheme to broadcast scenarios, termed broadcast successive maximum ratio transmission (BS-MRT), which computes the optimal broadcast beamforming vector to maximize the video quality at each of the broadcast group members while maintaining orthogonality to the previously scheduled user groups. We also demonstrate that the proposed MS-MRT and BS-MRT schemes can be naturally adapted to the context of various MU OFDMA scheduling algorithms such as proportional fairness (PF) and round robin (RR), to name a few, to maximize video quality through optimal scheduling in the presence of a large number of users. We discuss implementations of the proposed MS-MRT scheme in association with these scheduling algorithms for MU unicast and broadcast video transmission scenarios. The simulation results presented in this work rely on the rate models derived using the Joint Scalable Video Model (JSVM) software developed by the Joint Video Team (JVT) and are, thus, readily applicable in practice. The presented results clearly demonstrate the ability of the proposed algorithm to maximize the video quality in comparison with the other competing MU MIMO precoding techniques. Further, employing the proportional fair scheduling algorithm in conjunction with the MS-MRT results in an overall enhancement in the received video quality compared with the RR and max-rate algorithms.

Resource Allocation in OFDMA Networks With Imperfect Channel State Information

This letter investigates the effect of imperfect channel state information (CSI) on the performance of radio resource management for a downlink multi-user OFDMA system. It is shown that the imperfect CSI introduces a signal-and-power dependent noise term, which makes the symbol error rate (SER) constellation-point dependent, and yields an error floor on the SER. In addition, a two-step multi-user channel, power and constellation allocation algorithm is proposed to maximize the total network throughput.

An Improved Frequency Offset Cancellation for Uplink Multi-User OFDMA System Based on TF-RLS Algorithm

Time and frequency recursive least squares algorithm (TF-RLS) is proposed to cancel the interference due to the frequency offset (FO) in uplink Orthogonal Frequency Division Multiple Access (OFDMA) system. TF-RLS algorithm is composed of two stages, which is TD-RLS scheme and FD-RLS scheme. In the first stage, TD-RLS scheme is selected to pre-cancel the frequency offset in the time domain, and then the interference induced by residual frequency offset has been eliminated by the FD-RLS scheme in the frequency domain. The result of bit error rate (BER) shows that its performance is robust for cancellation as comparison criteria, even though the frequency offset is 0.45. The 16QAM constellation figures are also simulated to observe the improvements from the proposed suppression schemes.

An LP based Fair Subcarrier Allocation Algorithm for Cooperative OFDMA Systems with Grouped Users

Cooperative communication and orthogonal frequency division multiplexing (OFDM) technology are both promising candidates for next generation wireless communication systems. In this paper a Linear Programming (LP) based subcarrier allocation algorithm for cooperative multiuser OFDM system with grouped user is proposed. The proposed algorithm maximizes the data rate of all users over downlink under given constraints over non-sharing of carriers and fairness of data rate among all users. In this proposed algorithm the non sharing constraint of subcarriers have been relaxed initially into sharing constraint so that the non–convex optimization problem becomes a convex optimization problem, and thus, can solved efficiently using standard convex optimization tools. After maximizing total capacity of all active users in the system, the subcarrier allocation constraint is, then converted back to non-sharing constraint. The performance of proposed algorithm is studied and simulation results show that higher total capacity can be achieved using the proposed algorithm.

An Efficient Packet Scheduling and Resource Allocation Algorithm for Practical Multi-Layer Systems

In this paper, we research the wireless resource allocation problem with packet scheduling in both MAC layer and physical layer in multi-user orthogonal frequency division multiplexing (OFDM) systems. Under the practical environment where mobile users receiving signal with different SNR levels in a cell or around a Wi-Fi hot spot, our designed algorithm can execute relatively fair packet scheduling from MAC layer consideration and make efficient use of wireless resources at physical layer. simulation results show that our proposed scheme satisfy most users in various SNR and have better average performance in packet drop rate, packet delay and total throughput.

Fairness-Based Resource Allocation with Minimum Rate Guarantee in a Multiuser OFDMA System

We propose a resource allocation scheme based on Proportional Fairness (PF) for minimum bit rate guarantee in a multiuser, frequency-selective frame-based OFDMA system. Instead of considering subcarrier-and-bit allocation, we consider a more realistic scheduling which is performed at the beginning of each frame, while allowing time-slots multiplexing among active users. Our scheme aims to guarantee different minimum bit rates to different users, while throughput fairness among users is maintained. We formulate the optimization problem based on a multichannel proportional fairness objective function. Minimum bit rate guarantee is taken as the constraint. The optimization problem is solved by using the Lagrange multiplier method. Then, we use our fast cross-layer approach integrated to our optimization solution to schedule packets from finitely backlogged queues. Simulations with user mobility are conducted. The results show that the proposed scheme can serve users with satisfied minimum bit rate requirement. Furthermore, the simulation results also confirm throughput fairness among users.

Wireless Information and Power Transfer in Multiuser OFDM Systems

In this paper, we study the optimal design for simultaneous wireless information and power transfer (SWIPT) in downlink multiuser orthogonal frequency division multiplexing (OFDM) systems, where the users harvest energy and decode information using the same signals received from a fixed access point (AP). For information transmission, we consider two types of multiple access schemes, namely, time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA). At the receiver side, due to the practical limitation that circuits for harvesting energy from radio signals are not yet able to decode the carried information directly, each user applies either time switching (TS) or power splitting (PS) to coordinate the energy harvesting (EH) and information decoding (ID) processes. For the TDMA-based information transmission, we employ TS at the receivers; for the OFDMA-based information transmission, we employ PS at the receivers. Under the above two scenarios, we address the problem of maximizing the weighted sum-rate over all users by varying the time/frequency power allocation and either TS or PS ratio, subject to a minimum harvested energy constraint on each user as well as a peak and/or total transmission power constraint. For the TS scheme, by an appropriate variable transformation the problem is reformulated as a convex problem, for which the optimal power allocation and TS ratio are obtained by the Lagrange duality method. For the PS scheme, we propose an iterative algorithm to optimize the power allocation, subcarrier (SC) allocation and the PS ratio for each user. The performances of the two schemes are compared numerically as well as analytically for the special case of single-user setup. It is revealed that the peak power constraint imposed on each OFDM SC as well as the number of users in the system play key roles in the rate-energy performance comparison by the two proposed schemes.

Energy-Efficient Resource Allocation in OFDM Systems With Distributed Antennas

In this paper, we develop an energy-efficient resource-allocation scheme with proportional fairness for downlink multiuser orthogonal frequency-division multiplexing (OFDM) systems with distributed antennas. Our aim is to maximize energy efficiency (EE) under the constraints of the overall transmit power of each remote access unit (RAU), proportional fairness data rates, and bit error rates (BERs). Because of the nonconvex nature of the optimization problem, obtaining the optimal solution is extremely computationally complex. Therefore, we develop a low-complexity suboptimal algorithm, which separates subcarrier allocation and power allocation. For the low-complexity algorithm, we first allocate subcarriers by assuming equal power distribution. Then, by exploiting the properties of fractional programming, we transform the nonconvex optimization problem in fractional form into an equivalent optimization problem in subtractive form, which includes a tractable solution. Next, an optimal energy-efficient power-allocation algorithm is developed to maximize EE while maintaining proportional fairness. Through computer simulation, we demonstrate the effectiveness of the proposed low-complexity algorithm and illustrate the fundamental tradeoff between energy- and spectral-efficient transmission designs.

Outage Probability Analysis for Multi-User Single-Relay OFDMA DF Networks in Frequency Selective Rayleigh Fading Channels

In this letter, we study a multi-user single-relay decode-and-forward orthogonal frequency division multiple access system, where the channel model is Rayleigh frequency selective fading. We provide analytical approximations for the global outage probability in the sense that the outage event occurs when at least one link goes into outage. The main contribution of this letter is the consideration of the statistical correlation between the link capacities which is created by the relay-destination channel that is common to all the links. The analysis is carried out by fitting the multi-variate normal distribution to the joint probability distribution function of the link capacities.

On the Complexity of Joint Subcarrier and Power Allocation for Multi-User OFDMA Systems

Consider a multi-user Orthogonal Frequency Division Multiple Access (OFDMA) system where multiple users share multiple discrete subcarriers, but at most one user is allowed to transmit power on each subcarrier. To adapt fast traffic and channel fluctuations and improve the spectrum efficiency, the system should have the ability to dynamically allocate subcarriers and power resources to users. Assuming perfect channel knowledge, two formulations for the joint subcarrier and power allocation problem are considered in this paper: the first is to minimize the total transmission power subject to quality of service constraints and the OFDMA constraint, and the second is to maximize some system utility function (including the sum-rate utility, the proportional fairness utility, the harmonic mean utility, and the min-rate utility) subject to the total transmission power constraint per user and the OFDMA constraint. In spite of the existence of various heuristics approaches, little is known about the computational complexity status of the above problem. This paper aims to fill this theoretical gap, i.e., characterizing the complexity of the joint subcarrier and power allocation problem for the multi-user OFDMA system. It is shown in this paper that both formulations of the joint subcarrier and power allocation problem are strongly NP-hard. The proof is based on a polynomial time transformation from the so-called 3-dimensional matching problem. Several subclasses of the problem which can be solved to global optimality or $\epsilon$-global optimality in polynomial time are also identified. These complexity results suggest that there are not polynomial time algorithms which are able to solve the general joint subcarrier and power allocation problem to global optimality (unless P$=$NP), and determining an approximately optimal subcarrier and power allocation strategy is more realistic in practice.

An Adaptive Allocation Scheme in Multiuser OFDM Systems with Time-Varying Channels

Previously, a scheme in [1] is proposed for the subcarrier, bit, and power allocation problem to minimize the total transmit power for multiuser orthogonal frequency division multiplexing systems in downlink transmission. However, it is a batch mode which may not be so efficient in terms of computational complexity for slowly time-varying channel environments. The solution of the current frame can be obtained with slight modification from that of the previous frame in an adaptive fashion. By utilizing this property, we propose a scheme to obtain the solution in the adaptive fashion, which offers comparable performance with a reduced complexity compared to the previously proposed method and other existing suboptimal methods. Based on a derived expression composed of the channel gains, the numbers of assigned subcarriers, and the data rates along with the new proposed processing procedures, the proposed adaptive scheme is able to track the channel variation for the solution adjustment in a faster speed compared to the original batch mode method. Simulation results reveal that the proposed adaptive scheme has the competitive performance compared with those of the optimal and the existing schemes while the computational complexity and the number of iterations are both reduced.

Resource Block Assignment to Reduce Peak to Average Power Ratio in Multi-User OFDM

SUMMARY This paper proposes the assignment of resource blocks (RBs) to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) in a multi-user OFDM system. This system ranks the users according to the channel state information (CSI) for RB assignment. In our proposed technique, an RB is assigned to either the first- or second-ranked mobile station (MS) to minimize the PAPR of the OFDM signal. While this process reduces the PAPR, the throughput is also reduced because of the user diversity gain loss. A PAPR-throughput tradeoff is then established. Theoretical analyses and computer simulations confirm that when the number of MSs becomes large, the PAPR-throughput tradeoff is eased because of the minimal effect of the diversity gain loss. Therefore, significant PAPR reduction is achieved with only a slight degradation in the throughput.

Distributed spectrum sharing scheme for multiuser orthogonal frequency-division multiplexing cognitive radio networks

This paper presents a novel spectrum sharing design aiming at optimising the performance of a Multiuser Orthogonal Frequency-Division Multiplexing (MU-OFDM) Cognitive Radio Network (CRN) that consists of multiple secondary Transmitter-Receiver (Tx-Rx) pairs. For most MU-OFDM systems, the Exclusive Subchannel Assignment (ESA) is an efficient resource allocation method. Nonetheless, it is inappropriate for the network considered in this paper, because subchannels sharing among secondary Tx-Rx pairs can further improve the system performance. We investigate the Weighted Sum Rate (WSR) maximization problem under the Shared Subchannel Assignment (SSA), where each subchannel is shared by multiple secondary Tx-Rx pairs. With Lagrangian duality technique, we decompose the original resource allocation problem into several sub-problems on each subchannel and propose a duality-based subchannel sharing approach. For practical realisation in the cognitive systems without central control entity, a distributed duality-based WSR maximization scheme is presented. Simulation results manifest that the proposed scheme achieves significantly better performance than ESA duality scheme.

Efficient and Fair Power and Subchannel Allocation in Multiuser OFDM Networks

We present an efficient and fair scheme for allocating resources in the downlink of multiuser OFDMA cellular networks based on Nash Bargaining Solution (NBS). Although NBS provides a fair and optimum approach in order to maximize the total rate of the network, there is no simple solution for the case of K users. The proposed algorithm satisfies the minimum rate requirement of each user first and allocates the excess subchannels by searching over the K × N subchannels to noise ratio matrix based on the bargaining process happening between the base station and the users. Hence, the complexity reduces from order O(K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sup> ) to O(N × K).