Frequency Division Multiple Access Downlink Research Articles

Two-phase downlink subcarrier allocation for multicell OFDMA systems

To increase spectrum efficiency, the radio resource management with frequency reuse factor = 1 is usually adopted in multi-cell orthogonal frequency division multiple access systems. Such a reuse-one strategy results in severe co-channel interference, especially penalizing mobile users near the cell border. This paper presents an effective subcarrier-assignment algorithm to mitigate the co-channel interference for multi-cell orthogonal frequency division multiple access downlink systems. The proposed algorithm consists of an initial assignment phase and a compensation phase. In the first phase, each subcarrier is assigned to mobile users with higher channel gains and lower mutual co-channel interferences. The second phase further compensates mobile users who do not get adequate resources to meet the requirement of quality of service in the first phase. System simulation results reveal that the proposed algorithm can significantly improve the number of QoS-satisfied users compared with previous used scheme.

Energy- and Spectral-Efficiency Trade-Off in OFDMA Downlink Channels

In this paper, a trade-off metric of unified energy efficiency (EE) and spectral efficiency (SE) is investigated in orthogonal frequency division multiple access downlink networks. The weighted product of the EE and SE of users is adopted to balance EE and SE. A maximization problem of the minimum weighted utility is formulated, and the original problem is divided into two sub-problems; one is a subcarrier allocation (SA) problem, and the other is power allocation (PA). For the SA problem, a sub-optimal algorithm employing a search method is utilized. To solve the PA problem, quasi-concavity of the utility function with respect to total transmit power in the PA sub-problem is shown. Based on the quasi-concavity, an iterative search algorithm of the optimal PA is developed. Through simulations, the efficiency of the algorithm is examined.

On the Sum Rate of Fair Resource Allocation With Selective Feedback

Opportunistic scheduling exploits the multiuser diversity to improve the performance of wireless communications. The scheduling gain is enabled by the channel feedback sent from the receiver to the transmitter. In this paper, we consider the so-called opportunistic cumulative distribution function (CDF) scheduling with threshold-based selective feedback in the orthogonal frequency division multiple access downlink system. Among opportunistic scheduling techniques, the CDF scheduling is known to provide multiuser diversity gain while maintaining fair radio resource sharing among users. We first derive the exact and asymptotic average sum rates assuming antenna selection at transmitters. The expressions are valid for arbitrary number of inter-cell interference and number of transmit antennas. Moreover, a closed-form approximation for the user rate distribution is calculated using the exact moments of the user sum rate. The approximation is shown to provide an accurate estimate for the rate distribution, and the results can be utilized in rate adaptation at the transmitter.

Robust carrier frequency offset and channel estimation for orthogonal frequency division multiple access downlink systems in the presence of severe adjacent‐cell interference

This study investigates joint estimation carrier frequency offset (CFO) and channel impulse response (CIR) in the problematic orthogonal frequency division multiple access (OFDMA) downlink scenario at the cell boundary, where a maximum supposition of three adjacent base stations preambles can be received. A novel scheme is proposed for estimating the CFOs and CIRs of different cells. First, the highly efficient joint maximum-likelihood (JML) algorithm is used to enable robust CFO estimation by exploiting the idempotent property of the projection matrix. Next, in order to estimate the precise CIR, the composite CFO-based and circularly shifted preamble signatures are proposed by applying the constrained minimum variance (CMV) algorithm to suppress adjacent-cell interference. Finally, the CFO and CIR estimations are enhanced by an iterative cancellation scheme. To the best of the authors research, there is no publication study proposed joint JML-CFO and CMV-CIR estimators for OFDMA downlink systems with adjacent-cell interference. Simulation results show that the proposed algorithms provide better performance than the conventional estimators and approach the theoretical Cramer–Rao lower bound at the cell boundary over frequency-selective fading channels.

Dynamic Orthogonal Frequency Division Multiple Access resource management for downlink interference avoidance in two‐tier networks

SUMMARYIn two‐tier networks, which consist of macrocells and femtocells, femtocells can offload the traffic from macrocells thereby improving indoor signal coverage. However, the dynamic deployment feature of femtocells may result in signal interference due to limited frequency spectrum. The tradeoff between broad signal coverage and increased signal interference deserves further exploration for practical network operation. In this paper, dynamic frequency resource management is proposed to avoid both co‐tier and cross‐tier Orthogonal Frequency Division Multiple Access downlink interference and increase frequency channel utilization under co‐channel deployment. A graph‐based non‐conflict group discovery algorithm is proposed to discover the disjoint interference‐free groups among femtocells in order to avoid the co‐tier interference. A macrocell uses the femtocell gateway for frequency resource allocation among femtocells to avoid cross‐tier interference. We formulate the optimized frequency resource assignment as a fractional knapsack problem and solve the problem by using a greedy method. The simulation results show that the average data transfer rate can be increased from 21% to 60%, whereas idle rate and blocking rate are decreased in the range of and , respectively, as compared with conventional graph coloring and graph‐based dynamic frequency reuse schemes. Copyright © 2013 John Wiley & Sons, Ltd.

Performance analysis of dynamic channel allocation based on the greedy approach for orthogonal frequency-division multiple access downlink systems

This paper presents a performance analysis of dynamic channel allocation (DCA) based on the greedy approach (GA) for orthogonal frequency-division multiple access downlink systems over Rayleigh fading channels. The GA-based DCA achieves its performance improvement using multiuser diversity. We analyze the statistics of the number of allocable users that represents the multiuser diversity order at each allocation process. The derived statistics are then used to analyze the performance of GA-based DCA. The analysis results show that the number of subcarriers allocated to each user must be equal to achieve the maximum system performance based on outage probability and data throughput. Copyright © 2011 John Wiley & Sons, Ltd. (The greedy approach-based dynamic channel allocation achieves its performance improvement using multiuser diversity. We analyze the statistics of the number of allocable users that represents the multiuser diversity order at each allocation process. The derived statistics are then used to analyze the performance of the greedy approach-based dynamic channel allocation.)

An Adaptive Hot-Spot Operating Scheme for OFDMA Downlink Systems in Vertically Overlaid Cellular Architecture

In vertically overlaid cellular systems, a temporary traffic concentration can occur in a hot-spot area, and this adversely affects overall system capacity. In this paper, we develop an adaptive hot-spot operating scheme (AHOS) to mitigate the negative effects from the nonuniform distribution of user location and the variation in the mixture of QoS requirements in orthogonal frequency division multiple access downlink systems. Here, the base station in a macrocell can control the operation of picocells within the cell, and turns them on or off according to the system overload estimation function. In order to determine whether the set of picocells is turned on or off, we define an AHOS gain index that describes the number of subcarriers saved to the macrocell by turning a specific picocell on. For initiating the picocell OFF procedure, we utilize the changes in traffic concentration and co-channel interference to the neighboring cells. According to computer simulation, the AHOS has been proved to have maximize system throughput while maintaining a very low QoS outage probability under various system scenarios in both a single-cell and multi-cell environments.

Effect of traffic channel configuration on the orthogonal frequency division multiple access downlink performance

This paper studies traffic channel design tradeoffs for downlink orthogonal frequency division multiple access (OFDMA) systems. For fixed and portable users where the propagation channels are quasistatic, we show that the aggregated rate can be maximized by exploiting through intelligent channel allocation. In this case, the traffic channel should be configured so that its corresponding achievable data rate has the highest variance among users. The capacity upper and lower bounds are derived to quantify the impact of different traffic channel configurations. For mobile services where multiuser diversity cannot be efficiently exploited due to the signaling overhead, we show that the system outage capacity is maximized when the traffic channels are configured with maximum frequency diversity. Using the results presented, designers of OFDMA system can determine the optimum traffic channel configuration based on the types of services supported by the network.