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First, the deployment of the Long-Term Evolution (LTE) system will be concentrated on areas with high user traffic overlaying with the legacy second-generation (2G) or third-generation (3G) mobile system. Consequently, the limited LTE coverage will result in many inter-radio access technology (RAT) handovers from LTE to 3G systems and vice versa. Trouble-free operation of inter-RAT handovers requires the optimization of the handover parameters of each cell in both RATs. The current network planning and optimization methods provide a fixed network-wide setting for all the handover parameters of the cells. Cells that later show considerable mobility problems in operation mode are manually optimized with the aid of drive tests and expert knowledge. This manual optimization of the handover parameters requires permanent human intervention and increases the operational expenditure (OPEX) of the mobile operators. Moreover, the interoperability of several RATs increases further the parameter space of the handover parameters, which makes the manual optimization difficult and almost impracticable. To reduce OPEX and to achieve a better network performance, we propose in this paper a self-optimizing algorithm where each cell in a RAT updates its handover parameters in an autonomous and automated manner depending on its traffic and mobility conditions. The proposed algorithm uses a feedback controller to update the handover parameters as a means to providing a steady improvement in the network performance. In the context of control theory, the feedback controller consists of a proportional control block, which regulates the change in the magnitude of each handover parameter, and a gain scheduler, which modifies the parameters of the proportional control block depending on the mobility conditions in each cell. To benchmark the design of the proposed algorithm, we apply two general and nonself-optimization algorithms: Taguchi's method and simulated annealing to optimize the handover parameters. Simulation results show that the proposed self-optimizing algorithm reaches a stable optimized operation point with cell-specific handover parameter settings, which considerably reduce the number of mobility failure events in the network, compared with three fixed settings for the handover parameters. Moreover, it is presented that the proposed self-optimizing algorithm outperforms Taguchi's method and simulated annealing when applied to a mobility robustness optimization (MRO) problem. [ABSTRACT FROM PUBLISHER]

The combination of multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiple access, (OFDMA) is a promising solution to the flexible and spectrally efficient provision of data services in future wireless communication systems. However, adaptive resource allocation (HA) in frequency, time, and space in multiuser MIMO-OFDMA systems is very complex due to the inclusion of the space dimension and the large number of resources to be managed. Indeed, an optimal RA for maximizing the sum rate is usually too complex for practical application, and suboptimal strategies are required. In this paper, the performance, complexity, and fairness of suboptimal HA strategies aiming at the maximization of the sum rate are investigated. A model for suboptimal RA strategies is proposed, and two new HA strategies are introduced. The proposed strategies are compared in terms of the sum rate, complexity, and throughput fairness and are shown to present better performance-complexity and performance-fairness tradeoffs than some existing suboptimal strategies, as well as to achieve almost the same sum rate as that obtained through an exhaustive search. [ABSTRACT FROM AUTHOR]

In this article we review an important class of wireless cooperation protocols known as amplify- and-forward relaying. One or more low-complexity relay nodes assist the communication between sources and destinations without having to decode the signal. This makes AF relaying transparent to modulation and coding of the source/destination communication protocol. It is therefore a highly flexible technology that also qualifies for application in heterogeneous networks comprising many nodes of different complexity or even standards. Recently, two-way relaying was introduced, which is readily combined with AF relaying. It is a spectrally efficient protocol that allows for bidirectional communication between sources and destinations. In order to investigate the potential of wireless AF relaying, we introduce three distributed network scenarios that differ in the amount of cooperation between nodes. New challenges arise in those networks, and we discuss approaches to overcome them. For the most general case of a completely distributed system, we present coherent relaying solutions that offer a distributed spatial multiplexing gain even for single-antenna nodes. Based on real-world experiments, we validate the feasibility of all schemes in our laboratory. [ABSTRACT FROM AUTHOR]

Modeling and simulation is essential for performance evaluation of wireless systems and specific algorithms used, for instance, during resource allocation. For current and future wireless systems, several properties have to be considered such as data traffic, frequency selective fading channels, and radio resource management. Throughout this article, a new system-level simulator concept is presented for packet switched systems using OFDMA, which is called snapshot-based SLS and represents a compromise between state-of-the-art static and dynamic SLSs. The proposed methodology only considers short time intervals within the busy hour by their statistics so that RRM and data traffic can be considered, as in dynamic SLS, but with lower computational complexity. In this article a description is given of how the cellular setup and traffic generation are performed for the proposed snapshot concept. Furthermore, a new methodology is proposed for a quality measure of resource units that is applicable to future wireless systems using an interleaved subcarrier allocation. Exemplary simulation results show that the developed concept is able to evaluate the performance of OFDMA systems considering the impact of, for example, data traffic and resource allocation strategies. [ABSTRACT FROM AUTHOR]

In this paper, the throughput of an adaptive multiuser SISO-OFDMA/FDD system with channel quality information (CQI) signalled digitized over a feedback channel to the transmitter is investigated, where the instantaneous signal-to-noise ratio (SNR) of the different subcarriers is used as CQI to exploit multi-user diversity using adaptive subcarrier allocation. The CQI available at the transmitter is assumed to be imperfect due to estimation errors and quantization at the receiver side, time delay and feedback errors. In this paper, a closed form expression of the average throughput of an adaptive multi-user OFDMA system using imperfect CQI and uncoded M-QAM modulation is derived. Furthermore, a closed form expression of the average throughput of an OFDMA system exploiting frequency diversity, which does not require CQI at the transmitter, is presented. Both throughput performances are compared in order to identify the optimal transmission strategy depending on the grade of CQI imperfectness. [ABSTRACT FROM AUTHOR]