In the reverse link of OFDMA, different users (subcarriers) may have different frequency offsets, which causes severe performance degradation. It is also difficult to estimate/compensate these frequency offsets in the reverse link. Even if the frequency offsets are correctly estimated, the compensation of one subcarrier (or user) may worsen the inter-carrier interference from the frequency offsets of other subcarriers (users). One of the key ideas in this letter is to merge frequency offset compensation into the FFT matrix. The new inverse transform matrix is used in lieu of regular FFT. A single MMSE-based transform is used to perform both inverse Fourier transform and frequency offset compensation. Compared to the conventional methods such as circular convolution, this approach has lower computational complexity with comparable performance.

A simple DFT-based noise variance estimator for orthogonal frequency division multiplexing access (OFDMA) systems is proposed. The conventional DFT-based estimator differentiates the channel impulse response and noise in the time domain. However, for partial frequency response, its time domain signal will leak to all taps due to the windowing effect. The noise and channel leakage power become mixed. In order to accurately derive the noise power, we propose a novel symmetric extension method to reduce the channel leakage power. This method is based on the improved signal continuity at the boundaries introduced by symmetric extension. Numerical results show that the normalized mean square error (NMSE) of our proposed method is significantly lower than that of the conventional DFT method.

We formulate the resource allocation problem to maximize the sum rate, subject to constraints on total power, bit error rate, and rate proportionality among the users for proportional resource allocation in orthogonal frequency division multiple access (OFDMA) systems. For this we propose a novel scheduling scheme based on weighted normalized user's channel state information (CSI). We then adopt the linear power distribution method and waterfilling technique to allocate power among the users and across the subcarriers respectively.

We propose a control-channel-hopping scheme to mitigate scrambling attacks in orthogonal frequency division multiple access (OFDMA) systems. A scrambling attack can be realized by jamming specific frames after monitoring the control channel or by jamming the control channel itself. This letter details a situation in which the control channel is scattered among OFDMA subcarriers. The scattered control channel has a two-dimensional hopping sequence with a mixed order. Simulation results show that our scheme can prevent a jammer from monitoring the control channel and from attacking the channel itself.

In this letter, we propose a novel resource allocation scheme to enhance downlink system performance for orthogonal frequency division multiple access (OFDMA) and time division duplex (TDD) based femtocell networks. In the proposed scheme, the macro base station (mBS) and femto base stations (fBSs) service macro user equipments (mUEs) and femto user equipments (fUEs) in inner and outer zones in different periods to reduce interference substantially. Simulations show the proposed scheme outperforms femtocell networks with fractional frequency reuse (FFR) systems in terms of the system capacity and outage probability for mUEs and fUEs.

In this letter, a novel resource allocation method is proposed for Discrete Fourier Transform Spread Orthogonal Frequency Division Multiple Access (DFT-s-OFDMA) systems in Long Term Evolution (LTE). The proposed method is developed based on a minimal metric loss criterion and performs better than the commonly used Recursive Maximum Expansion (RME) method.

This letter studies cellular controlled short-range communication in OFDMA networks. The network needs to decide when to allow direct communication between a closely located device-to-device (D2D) pair instead of conveying data from one device to the other via the base station and when not to, in addition to subchannel and power allocation. Our goal is to maximize the total network throughput while guaranteeing the rate requirements of all users. For that purpose, we formulate an optimization problem subject to subchannel and power constraints. A scheme which combines a joint mode selection and subchannel allocation algorithm based on equal power allocation with a power reallocation scheme is proposed. Simulation results show that our proposed scheme can improve the network throughput and outage probability compared with other schemes.

In orthogonal frequency division multiple access (OFD-MA) uplink, the distortions introduced by both multiple carrier frequency offsets (CFOs) and in-phase and quadrature-phase (IQ) imbalances will severely degrade the system performance. With both CFOs and IQ imbalances, signal detection at the receiver becomes hard, if not impossible. In this letter, a linear receiver is proposed to cope with the distortions at a slight drop in system transmission rate. The analysis and simulations demonstrate the effectiveness of the proposed approach.

Orthogonal frequency division multiple access (OFDMA) is adopted as a multiuser access scheme in recent cellular systems such as long term evolution (LTE) and WiMAX. In those systems, the performance improvement on cell-edge users is crucial to provide high-speed services. We propose a new resource allocation scheme based on multiple input multiple output – orthogonal frequency division multiple access – code division multiplexing (MIMO-OFDMA-CDM) to achieve performance improvements in terms of cell-edge user throughput, bit error rate, and fairness among users. The proposed scheme adopts code division multiplexing for MIMO-OFDMA and a modified proportional fairness algorithm for CDM, which enables the fairness among users and a higher throughput. The performance improvements are clarified by theoretical analysis and simulations.

Secrecy on the physical layer is receiving increased research interest due to its theoretical and practical importance. In this letter, a subcarrier allocation scheme is proposed for physical-layer security in cooperative orthogonal frequency division multiple access (OFDMA) networks that use the Amplify-and-Forward (AF) strategy. We consider the subcarrier pairing and assignment to maximize overall system rates subject to a secrecy level requirement. Monte Carlo simulations are carried out to validate our analysis.

In multi-cell OFDMA-based networks, co-channel interference (CCI) is inevitable when the frequency reuse scheme is used. The CCI affects the performance of users, especially that of cell edge users. Several frequency reuse schemes and subcarrier allocation algorithms have been proposed to solve the CCI problem. Nevertheless, it is difficult to improve both the cell capacity and the performance of cell edge users since they have a trade-off. In this paper, we propose a new balanced frequency reuse (BFR) as a new frequency partitioning scheme that gives more power to the users in the outer region and allocates more subcarriers to the users in the inner region. In addition, we propose ordering and directional subcarrier allocation (ODSA) for our frequency partitioning proposal to mitigate the CCI effectively when cells have heterogeneous traffic loads. The performance of the proposed BFR with the ODSA algorithm is investigated via analyses and simulations. Performance evaluation shows that the proposed BFR with the ODSA algorithm can increase both the spectral efficiency and the performance of cell edge users if the transmission power is appropriately handled.

Recently, Li and Xia proposed a physical-layer security design to guarantee a low probability of interception (LPI) for asynchronous cooperative systems without relying on upper-layer data encryption. The proposed scheme utilizes diagonal unitary codes to perform different encoding in the frequency domain over subcarriers within each OFDM block to randomize the transmitted signals. To build on their idea, in this letter, a subcarrier-reference (SR) transmission scheme is proposed with deliberate signal randomization to achieve LPI in multiuser MISO-OFDMA systems. For each user, one of the allocated subcarriers is chosen by the transmitter to send reference signals, and others are chosen to send the user's information symbols. By some deliberate signal randomization, the eavesdropper cannot detect the transmitted symbols, while the authorized users can operate the system successfully without knowledge of the channels by subcarrier-reference demodulation. Extensive simulations are conducted to demonstrate the scheme's effectiveness.

In orthogonal frequency division multiple access (OFD-MA) systems, soft frequency reuse (SFR) and distributed antenna system (DAS) are two effective techniques to avoid excessive inter-cell interference (ICI). To gain the advantages of both, in this letter, we build a new cell architecture by jointly taking DAS and SFR into consideration to achieve the goal of high and well-balanced capacity. Furthermore, to rectify the shortfall in the literature, the capacity and outage probability in the multi-cell environment are derived by taking the complete channel effects into account, including the path loss, shadowing and Rayleigh fading. Simulations verify the superior performance and exactness of the analytical results.

In this article, we study different methods to enhance the capacity of Voice-over-IP (VoIP) in Evolved UTRAN (E-UTRAN) downlink. According to previous E-UTRAN system level simulation results, VoIP capacity with dynamic scheduling and semi-persistent scheduling is limited by available Physical Downlink Control CHannel (PDCCH) resources and Physical Downlink Shared CHannel (PDSCH) resources respectively. The different behavior of these two packet scheduling schemes and specific bottleneck on capacity lead us to formulate two distinctly different performance enhancement strategies, which are the main focus of this paper. For dynamic scheduling, we propose a bi-directional power control algorithm to rationalize PDCCH resource allocation, thus greatly improving VoIP capacity. For semi-persistent scheduling, we introduce Adaptive Transmission Bandwidth (ATB) to rationalize PDSCH resource allocation and further apply dynamic packet bundling in highly-loaded network, thus improving VoIP capacity in a considerable manner. The proposed enhancement methods are validated through large-scale system level simulations and the obtained system simulation results further confirm effectiveness of these enhancement methods.

This paper proposes a new resource allocation algorithm for uplink OFDMA-based cooperative relay networks, assuming multiple user nodes, multiple relay nodes and a single destination. The aim is to maximise the total sum of the users' data rates, while guaranteeing fairness among them with different QoS requirements. Assuming perfect channel state information (CSI) at the resource allocation controller, the optimisation problem is formulated such that each user is assigned a weight factor based on its QoS requirements. The ones with higher weights are given higher priorities to select their resources (relay stations and subcarriers) first. Once the required QoS is achieved for all users, the weight factor for all users is reduced to a small uniform value. The remaining resources are then allocated to the users with higher instantaneous rates in order of magnitude. The results show that the proposed algorithm outperforms the greedy and static algorithms in terms of outage probability and fairness, and at the same time outperforms Jeong's algorithm by 58% in terms of total sum rate, with an average 74% reduction in system complexity.

Most existing works on resource allocation in cooperative OFDMA systems have focused on homogeneous users with same service and demand. In this paper, we focus on resource allocation in a service differentiated cooperative OFDMA system where each user has a different QoS requirement. We investigate joint power allocation, relay selection and subcarrier assignment to maximize overall system rates with consideration of QoS guarantees and service support. By introducing QoS price, this combinatorial problem with exponential complexity is converted into a convex one, and a two-level dual-primal decomposition based QoS-aware resource allocation (QARA) algorithm is proposed to tackle the problem. Simulation results reveal that our proposed algorithm significantly outperforms previous works in terms of both services support and QoS satisfaction.

Efficient resource allocation for delay-sensitive traffic, such as telephony and video streaming, in Orthogonal Frequency Division Multiple Access (OFDMA) networks is needed to increase system performance. In our system, users try to achieve a low queuing delay and buffer space usage by competing for transmission over the subchannels. We formulate this problem as a bargaining game and use the Nash Bargaining Solution (NBS) to realize a fair and efficient subchannel allocation for the users. Simulation results show performance improvements, with regard to packet dropping and delay distribution, over other algorithms.

A novel adaptive cyclic prefix (CP) transmission scheme is proposed for the uplink of orthogonal frequency division multiple access (OFDMA) systems to reduce the power consumption of mobile stations (MSs). In the proposed scheme, an MS adaptively changes its CP length in each frame, while the guard interval is maintained at a fixed duration to avoid frame synchronization problem and the interference problem with frames of other users. Using the proposed scheme, MSs can save power by not transmitting signal during the time difference between the guard interval and the duration of the adaptive CP. We numerically analyze the performance of the proposed scheme in terms of achievable capacity, the amount of power saving, and the feedback overhead of CP values. The result shows that the proposed scheme can reduce MS power consumption by about 20% with a small amount of additional feedback overhead.

This paper proposes an iterative peak power reduction method with adaptive intermediary over-sampling which uses the necessary minimum bandwidth according to iteration number for wireless OFDMA systems. The required bandwidth to each iteration number is evaluated by computer simulation, and over-sampling numbers in iterative processing are controlled by using the simulation results. The results show that the required bandwidth is 1.6, 2.0, and 2.7 times of the used signal bandwidth at the iteration number of 1, 2, and 3, respectively. The proposed adaptive over-sampling method can reduce its multiplication number by 13%.

A differential cross-correlation cell ID identification algorithm is proposed for IEEE 802.16e OFDMA cellular system. The cell ID represents the number of the preamble selected by the base station in downlink mode. First, we construct the downlink (DL) preamble structure and signal model with carrier frequency offset (CFO) and channel effects. Next, in order to achieve the initial synchronization, a differential receiver with cross correlation for all preamble patterns is proposed to search for cell ID. Simulation results confirm that the proposed structure is suitable for ITU fading channels and outperforms the conventional cell search system.