Yung-Fang Chen

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 allocation for multiple input multiple output-orthogonal frequency division multiplexing -based space division multiple access systems

The problem of the system throughput maximisation and the fairness issue among users is considered for the downlink multiuser multiple input multiple output-orthogonal frequency division multiplexing system with MMSE precoding. Since finding the optimal solution by using the exhaustive search has very high computational burden, the authors propose a low-complexity user selection strategy plus the optimal power allocation in this study. With resorting to the mathematical simplification process, the proposed user selection scheme is designed to compare the related value of SINR. The power allocation is performed after the user selection. However, the power allocation is not in interference-free environment but in the presence of crosstalk, which is a nonlinear non-convex optimisation problem. In this study, the authors use an optimisation technique to obtain the optimal solution of the constrained non-convex nonlinear problem by replacing the objective function with a difference of two convex functions. A bisection-based strategy to solve this problem with interference-awareness to find local optimal solutions is presented in this study. Simulation results show that the performance of the proposed algorithm is close to that of the optimal solution. The trade-off between the system throughput and the fairness among users is also considered.

Improved Subcarrier and Power Allocation Schemes for Wireless Multicast in OFDM Systems

A capacity maximization problem subject to a total power constraint in OFDM-based wireless multicast systems is investigated in this paper. A subcarrier and power allocation method is proposed based on a property observed from the simulation in the multicast system. We design a criterion to efficiently assign subcarriers to multicast groups. Simulation results reveal that the proposed method outperforms the existing suboptimal methods. With the help of the improved mechanism and the designed criterion, the performance of the proposed method is almost the same as that of the optimal solution.

A multiuser subcarrier and power allocation scheme in localized SC-FDMA systems

This paper presents a novel allocation scheme to solve a subcarrier and power allocation problem for multiuser SC-FDMA uplink systems with a localized subcarrier mapping mode. In the localized mode, subcarriers assigned to a user are required to be consecutive. The limitation makes the problem considered in this paper different from those of conventional multiuser OFDM systems. The proposed scheme contains the chunk and the iterative subcarrier assignments. The chunk assignment utilizes an interior term to allocate a set of subcarriers to a user. The iterative assignment re-assigns the adjacent subcarriers between users to other users to improve the achievable rates. Furthermore, a constant power allocation approach is employed in conjunction with the proposed scheme. Simulation results show that the proposed scheme outperforms the existing schemes.

Improved Schemes for Subcarrier, Bit, and Power Allocation in Multiuser OFDM Systems

In this paper, the subcarrier, bit, and power allocation problem for minimizing total transmit power in multiuser OFDM systems is considered and a near-optimal allocation scheme is developed. The heuristic algorithms based on initial subcarrier assignment and iterative improvement two steps have been proposed by Wong et al. and Chen et al. for real-time services [4- 5]. However, an appropriate method for the initial assignment should be selected or designed to offer even better performance. In this paper, one subcarrier allocation algorithm proposed by Zhang [6] is adopted as an initial subcarrier assignment method for the allocation problem. Its advantage is that there is no need to pre-determine the number of the assigned subcarriers for each user, and it outperforms the constructive initial assignment method proposed by Wong et al.. We adopt the subcarrier allocation algorithm proposed by Zhang along with iterative improvement schemes to obtain better performance. Detailed discussions of the subcarrier allocation algorithm proposed by Zhang are presented in this paper. As indicated in the simulation results, it reveals that the better initial subcarrier assignment with the iterative improvement schemes achieves better performance. We utilize the linear integer programming-based branch-and-bound algorithm to provide the optimum solution as the performance bound. The simulation results show that the performance is improved with the new proposed heuristic polynomial time algorithm and is close to that of the optimum solution.

Adaptive schemes and analysis for blind beamforming with insufficient cyclic prefix in single carrierfrequency division multiple access systems

When the duration of the cyclic prefix (CP) is shorter than that of the channel impulse response in single carrier-frequency division multiple access systems, inter-symbol interference and inter-carrier interference will degrade the system performance. Previously, one solution of this problem while considering the effect of carrier frequency offsets (CFOs) and the co-channel interference is a blind received beamforming scheme based on eigenanalysis in a batch mode. As the capability in suppressing the multipath signal with the delay larger than the CP length has not previously been analysed theoretically for the scheme, the theoretical analysis regarding the capability in suppressing the long-delayed multipath signal is provided in this study. The analysis provided in this study is also utilised to design an adaptive processing scheme. The adaptive algorithm is developed to find the beamforming weight vector updated on a per symbol basis without using reference signals. The proposed adaptive algorithm reduces the computational complexity of and shows competitive performance under the insufficient CP, the CFOs, the co-channel interference and the time-varying scenarios. The simulation results reveal that the proposed adaptive algorithm provides better performance than the previously proposed algorithm.

Genetic Algorithm-Based Power Allocation for Multiuser MIMO-OFDM Femtocell Networks with ZF Beamforming

In this paper, a power allocation algorithm is proposed for multiuser MIMO-OFDM femtocell networks on downlink, where an objective considers the maximization of the SINR of a macro user that is near the femto base station. Since the macro and the femto base stations share the same spectrum, there is a co-channel interference problem. We propose a power allocation method based on genetic algorithms to enhance the SINR of the macro user. In addition, we also utilize the zero-forcing beamforming design to alleviate the co-channel interference. The simulation results show that the proposed genetic algorithm-based power allocation method with zero-forcing beamforming reveals the best performance, and outperforms the method with equal power allocation.

Effects of adaptive prediction algorithms on adaptive QAM in flat Rayleigh fading channels

The long-range prediction (LRP) algorithm was investigated by Duel-Hallen et al., (2000) such that we can predict future channel state information for more than ten milliseconds ahead. However, its adaptive implementations to avoid the matrix inversion have not been investigated in detail. This paper presents the performance comparison among the various adaptive prediction algorithms for the LRP purpose. The adaptive QAM transmission schemes with utilizing the algorithms are thus proposed for the use in the FDD-based adaptive transmission under the scenario of flat Rayleigh fading channels. The scheme is also built with the pilot symbol assist modulation and the speed zone detection techniques. The adaptive modulation schemes by utilizing the speed zone detection lead in performance improvement in the selection of the optimal prediction order under the different mobile speeds. The efficacies of the schemes are presented through the simulation study.

Modified decision feedback method for mobile OFDM channel estimation

In some frame-based OFDM applications such as WLAN and WMAN, the channel estimation is accomplished through the aid of the preamble. By means of the preamble, the estimated channel is only well applied under the assumption of a static channel status within the frame. If we consider a time-varying channel status in those applications, the channel tracking has to be performed without the aid of pilots. A past idea for channel tracking was to use a decision-feedback loop with a smoothing method to reduce the drawback of decision-error propagation. In our work, we design the error detectors for detecting error propagation and propose some compensation methods for reducing the influence of error propagation. In simulation results, the performance of the proposed decision-error detectors and compensation methods is compared. We show that the smaller bit error rate can be achieved for the proposed time domain decision feedback algorithm and thus, bandwidth efficiency can remain without the pilots used for channel tracking.

Recursive Parameter Estimation for Regression Channel Model in Pilot-Aided OFDM Systems

OFDM model-based channel estimation techniques conventionally apply the least squares method to estimate parameters in a regression model through uniformly distributed pilots in a local region. However, the model estimation must use as many pilots as possible to reduce the effect of noises with the penalty of increasing the storage size for received OFDM symbols. We observed that the model parameters between neighboring local regions are correlated. Hence, some recursive methods are proposed to adaptively estimate model parameters such that the required number of pilots can be reduced, and thus, the required storage size is reduced for interpolating the symbols used in the regression model. Theoretical analysis and simulations show that a better performance is obtained as well by using the proposed parameter estimation methods.