The Pilot Optimization Design In NC-OFDM Systems

Noncontinuous orthogonal frequency division multiplexing (NC-OFDM) modulation technique is one of main data transmission mode for IEEE802.22, and its available subcarriers have the characteristics of non-continuous distribution compared with traditional OFDM systems, which brings new technical challenges. Especially, the pilot design is one of the key issues that has to be solved in order to improve the performance of NC-OFDM systems in cognitive radio context. Because of the noncontinuous and dynamic characteristics of available spectrums, it is necessary to modify, improve or/and innovate the existing research of OFDM systems. In traditional OFDM systems, equi-powered and equi-spaced pilots are optimal, however, for satisfying primary users(PUs)’interference constrains, the optimal pilots of NC-OFDM systems will be non-uniform with available subcarriers’ limited power distribution and non-continuous spectrums. The proposed pilot optimization design rules under the above two conditions can reduce the computation complexity of pilot designs, adapt to the requirements of dynamic pilot pattern, and maximize the BER performance.The pilot optimization design of the NC-OFDM systems based on the convex optimization theory has been analyzed and summarized in this dissertation. The main contents include the following three aspects:1. The pilot optimization design of a NC-OFDM system based on convex mapping has been studied.Because of the discontinuity of the optimization variable set, the optimal pilot design is a non-convex optimization problem, which is very difficult to obtain the optimal solution by the numerical method. It is necessary to seek the convex mapping methods to this nonconvex problem, and then obtain the feasible solution by convex optimization tools.Firstly, the pilot design of multi-objective optimization problem is found, the non-convex problem is solved by convex mapping in single PU scenarios amd more PU scenarios. Then we can obtain the optimal pilot pattern based convex optimization. The suboptimal pilot design in different Scenarios are also presented for reducing the computation complexity.2. A optimization design of the pilot positions under a power constraint of available subcarriers has been studied.In short-distance communication scenes beween different SUs, generally the lower power amplifier can meet the requiements of channel transmission, which presents linear characteristic and doesn’t bring nonlinear distortion for NC-OFDM signals and out-of-band(OOB) interference. Assuming ideal HPA characteristics, the pilot optimization design only need to consider the total power constraint of available subcarriers and the sub-channel power constraints from PU interference constraints。Under a total power constraint and sub-channel power constraints, the pilot optimization design is studied. Firstly, the mutual interference between SU and PU transmissions is analyzed and the PU interference temperature model is found. Mapping the PU interference temperature constraints into the sub-channels’ transmission power constraints, the optimal subcarrier power allocation is obtained by the Lagrange method. Then, we reconstruct the pilot optimization design under the above available subcarriers’power allocation, and obtain the suboptimal soluton using a upper bound of the channel estion MSE instead of MSE. Theoretical analysis has shown that the suboptimal pilot positions is a function of the available subcarriers’powers and the distance between different pilots.3. The optimization design of pilot positions under a HPA nonlinear interference constraint has been studied based on the analysis of nonlinear HPA.For the poor channel conditions or long-distance communication between SUs, the high power amplifier(HPA) is neccssary to obtain higher signal power for meeting the requirements of NC-OFDM systems, however, large dynamic range of NC-OFDM signals after HPA will lead to serious in-band distortions and OOB spectrum leakages, the latter will increase the interference temperature in PU band. In this scene, it is necessary to consider the nonlineality of HPA for pilot optimization design.Firstly, a mathematical model for measuring the OOB radiation in NC-OFDM systems is presented, and the interference power in adjacent PUs’ bands are computed in this dissertation. Based on this mathematical model, the HPA nonlinear interference constraint among HPA nonlinear parameter, the number of guard subcarriers, PUs’ occupancy rate and PU interference temperature limits is obtained by solving the nonlinear programming (NLP) problem. Then, the closed expression of pilot-assisted channel estimation MSE consideringr the HPA nonlinearity is deduced, and the pilot optimaization design problem under a HPA nonlinear interference constraint is described and solved by the numerical method..According to the exploration and study of these key technologies, we have shown some pilot optimization designs which can reduce the computational complexity and improve the channel estimation performance of a NC-OFDM system receiver, thereby, optimizing the BER performance and providing a theoretical and technical basis for a widely application.