Research On Sidelobe Suppression For NC-OFDM Systems

In recent years, with the growing of high-speed data communication services, the existing wireless spectrum resource was very scarce. However, it was found by the authoritative organizations observing on the current available spectrum that, the spectrum efficiency of the authorized users (primary users) is very low, the allocated spectrum is often vacant for a long time, or only parts of the allocated spectrum resource are used frequently. Therefore, the temporary shortage of spectrum resources is partially due to the current spectrum policy. It's necessary to introduce new wireless transmission mechanism and communication technologies to improve the spectrum utilization efficiency. The "Cognitive Radio" based on dynamic spectrum access (DSA) is one of these technologies. It improves spectrum efficiency through the unauthorized users"borrowing"the primary users'vacant spectrum temporarily.Orthogonal frequency division multiplexing (OFDM) technology is recognized as one of the most competitive candidate technologies for cognitive radio because of its fast, effective against multipath fading and other special characteristics. Non–contiguous OFDM (NC-OFDM) system, inheriting the advantages of OFDM system, controlling the in-band subcarriers on and off according to the selective information provided by spectrum sensor, is very flexible and can be well applied in cognitive radio environments. Unfortunately,both NC-OFDM and OFDM systems are based on fast Fourier transform (FFT) signal processing technology, there is a large transmitted signal sidelobe leakage which will interfere adjacent systems.In this paper, the existing research is reviewed, and the technical features and new challenges in the NC-OFDM systems are also described. Specially, the Sidelobe Suppression (SS) of OFDM and NC-OFDM system is fully explored. Some practical ideas and methods are given as following by theoretical analysis and simulation experiments:1. A sidelobe suppression method in OFDM system based on SER constraintThe adaptive power allocation method combined with subcarrier weighting (SW) sidelobe suppression method is employed, and a multi-objective optimization model is proposed. Then the problem is simplified into a single objective nonlinear optimization problem by turning the optimal goal into constraint, which the optimal solution of can be obtained by nonlinear optimization algorithm. Besides, according to the power of sidelobe mainly affected by the both sides subcarriers, we propose a reduced complexity sub-optimal implementation method.2. The joint sidelobe suppression between frequency blocks in NC-OFDM system In the NC-OFDM system, the joint sidelobe suppression method and independent sidelobe suppression method are proposed respectively. Besides, the power distribution, sidelobe suppression and compute complexity of proposed algorithms are compared through theoretical analysis and simulation results. particularly in NC-OFDM system, the frequency covering is very wide, and different frequency block's modulation pattern and noise power may be varied. So the influence of different modulation pattern and noise power to power allocation and sidelobe suppression of the proposed method is analyzed, when the sidelobe is jointly suppressed between different frequency blocks, and the simulation results are given to prove.