Interference Mitigation And Performance Enhancement For Cellular Systems

Orthogonal frequency division multiple access (OFDMA) becomes a key technology in the future cellular mobile communication systems. For the sub-carriers in the intra-cell are orthogonal with each other, the intra-cell interference can be avoided efficiently. However, the inter-cell interference problems may become serious since many co-frequency sub-carriers are reused among different cells. Under this background, how to mitigate inter-cell interference and improve the performance for cellular users become more urgent. In this dissertation, the critical technologies about cellular interference mitigation and performance enhancement are researched. The work can be divided into two parts: inter-cell interference mitigation strategies and cellular performance enhancement strategies.Especially, the inter-cell interference mitigation strategies include three schemes, which respectively are interference coordination, interference prediction and interference cancellation.1. Frequency coordination and power coordination play important roles in the inter-cell interference coordination. For frequency coordination, the current research about frequency reuse schemes are analyzed, and its theoretical basis are explained from the point of the graph theory, the algebraic analysis theory and the extension theory. Moreover, a novel frequency reuse scheme is introduced in this dissertation, called as soft fractional frequency reuse. Also we derive its frequency reuse factor. By this scheme, simulation results show the throughputs in cell-edge are efficiently improved compared with soft frequency reuse scheme. For power coordination, the current research about power control are analyzed. On this basis, both the inter-cell power control method and the exponential iterative kernel equation are proposed. Compared with the usual arithmetic iterative equation, the convergence speed is faster for the exponential iterative kernel equation. Simulation results show such inter-cell power control algorithm improves the users' SINR, improve the throughputs for cell-center users and reduce the blocking rate for cell-edge users, which shows better performance than DPC and DB algorithm.2. The inter-cell interference prediction is proposed in this dissertation, which is an active interference mitigation method. Moreover, we give its theoretical basis, which is the optimal estimation theory. It includes two parts:time series and the optimal filter estimation. Besides, the reliability is also analyzed by means of prediction accuracy, which is based on the relationship of the coherent time and the time delay. In addition, we analyze change trend for the actual measured radio signals, with AR model, MA model and ARIMA model. The analytical results show time series model can efficiently predict the radio signals change.3. For inter-cell interference cancellation, two major technologies are described in this dissertation, which respectively are space interference suppression and interference reconstruction/subtraction. Based on the independent component analysis (ICA) technology in blind source separation, a semi-blind interference cancellation algorithm is proposed, written as Max-SINR ICA, which aims to improve the output SNR and optimize the innitial iterative separation matrix. Simulation results show that the iterative convergence speed for Max-SINR ICA algorithm is faster than the traditional Fast-ICA algorithm. Besides, we consider the input SINR and the output SNR in two scenarios, respectively fix the processing frame and fix the thermal noise. By the Max-SINR ICA algorithm, the inter-cell interference can be efficiently cancelled in a semi-blind state, especially with lower input SINR, higher input SNR and longer processing frame.On the other hand, the cellular performance enhancement strategies also include three parts, which respectively are multi-cell resource allocation, multi-cell selection optimization and coordinated multi-point transmission.1. For many existing multi-cell resource allocation algorithms, the resource allocation usually depends on current channel state information (CSI). However, as the CSI varying, such resource allocation method may hardly reach to the optimal object at the next time slot. According to this problem, a novel resource allocation based on prediction information is proposed to eliminate such time lag. Take Kalman filter as example, it allocates the sub-carriers based on the optimal estimated CSI in the next timeslot by filter prediction. Specially, we detect the gain to interference ratio (GIR), and predict the GIR in the next timeslot by Kalman filter, then allocate the sub-carriers according to the predicted GIR. Compared with time series and Wiener filter, Kalman filter reduces the complexity.2. For multi-cell selection optimization strategy, this dissertation analyzes the theoretical basis of multi-cell selection, then introduces the AHP and GRA principle, and applys such priciples into multi-cell selection. Considering the criteria for Fast Cell Selection (FCS) usually depend on single factor, which is lack of flexibility and fairness. In order to improve FCS performance, a novel algorithm is proposed. Based on extension theory, a flexible mapping table is constructed for Quality of Experience (QoE) evaluation parameters, and some actual performance parameters are mapped into corresponding calibration interval. Then the mapping values are calculated. On this basis, a judgment matrix is constructed by means of Fuzzy Analytic Hierarchy Process (FAHP) analysis, and its consistency is tested. Finally, the total utility function values of each cell are calculated by the weight vectors. According to such values, the optimal FCS scheme can be found. Simulation analysis shows that the weight factors in each cell have a directly effect on utility function. Compared with the existing algorithms, the proposed algorithm makes the judgment of performance parameters to be comprehensive with the proper increase of computing complexity, which reduces the blocking rate and raises the throughputs. 3. For coordinated multi-point transmission technology, this dissertation analyzes four senarios:intra-cell single user, intra-cell multi-users, inter-cell single user and inter-cell multi-users. Moreover, we propose a slide coordinated cell cluster model. Considering the existing frequency reuse schemes aims to interference avoidance while hardly supports inter-cell coordination, we also give a novel frequency reuse scheme supporting coordination. Simulation results show that this scheme enables to improve the throughputs in cell-edge. On the other hand, in order to solve power coordination problem in coordinated scenario, a new power allocation algorithm is presented for coordinated cell, which is based on game theory. Specially, we establish the power allocation game model, prove the existence and uniqueness of Nash equilibrium, and gives a non-cooperative game power allocation algorithm. Simulation results show that the proposed algorithm improves the throughputs in cell-center and cell-edge, and reduces the blocking rate for cell-edge users.