Adaptive Allocation Algorithms And Their Computational Complexity In OFDM System

Adaptive technique is an effective scheme to combat the fading channel in Orthogonal Frequency Division Multiplexing (OFDM) systems. The basic premise of adaptive techniques is a real-time balancing of the link budget in flat fading through adaptive variation of several parameters. Thus, without wasting power or sacrificing BER, the system can obtained a higher average link spectral efficiency (bps/Hz). A great amount of literature has been presented to demonstrate the results of various adaptive schemes, and most of them have focused on the improvement of performance theoretically, while ignoring the computational complexity and feasibility of the adaptive algorithms. Regarding them as the important parameters, this thesis has analyzed the computational complexity and feasibility of some typical adaptive allocation algorithms, and provided some adaptive algorithms with lower computational complexity and better performance. The results of this work can be referenced for employing adaptive allocation algorithms in practical systems.In this thesis, we first choose some typical adaptive allocation algorithms of single-user or multiple-user system, and analyze their computational complexity. These chosen algorithms must be either the optimal algorithms, which have theoretical significance, or the algorithms with lower computational complexity, which can be easily realized. The simulation results of the computational complexity of the chosen algorithms have been presented, and the time they cost in current DSP are calculated. This work can be used as a reference for employing adaptive allocation algorithms in OFDM systems.In order to reduce the computational complexity further, we provide a new adaptive allocation algorithm with blocked subcarriers for single-user systems, which is based on the water-filling theorem. The theoretical derivation and simulation analyses for its performance have been given. The subcarriers have been divided into blocks according to the correlation between the adjacent subcarriers and those subcarriers in a block use the same modulation mode. Thus, the amount of computation is decreased premising an acceptable tradeoff between the computational complexity and the cost of capacity.Then, we employ the subcarrier-blocked scheme into multi-user OFDM systems and analyze its performance. The subcarriers have been divided into blocks according to the correlation between the adjacent subcarriers and allocated to different user in block. The optimization target is to minimize the total transmit power while guaranteeing the quality of service for each user. If the total transmit power is fixed, then the optimization target is to maximize the number of user the system can support. The simulation results show that this algorithm can reduce the amount of computation greatly; meanwhile, the overhead to transfer the bit allocation vector is less.In this paper, equal-power-allocating algorithm has also been studied taking advantage of multiuser diversity. Given a maximal number of bit that a subcarrier can carry, we derivate a formula to demonstrate the probability of finding at least one suitable subchannel, in which the maximal number of bit can be transferred, among the same subchannels of multiple users. We also provide two kinds of scheme to realize the equal-power-allocating algorithm, optimizing the number of users and the users' data rate respectively. At last, some simulation results are presented.In the last chapter, the effects of imperfect channel information on the performance of the adaptive allocation have been discussed, and some combating measures have been summarized.