Estimation Of Timing And Frequency Offset For NC-OFDM

With the rapid development of wireless communication, spectrum resources in every country have been almost allocated completely, but they are not utilized efficiently due to the fixed spectrum assignment policy. Therefore, the dynamic spectrum access (DSA) technology in Cognitive Radios has been attracting more and more attention, and the concept of Non-contiguous OFDM (NC-OFDM) is proposed. The NC-OFDM not only achieves high data rates via collective usage of a large number of non-contiguous subcarriers in wireless fading channel, but also has enough flexibility in frequency combination, cutting and assignment. So it has been a promising candidate of the transmission scheme for flexible spectrum pooling system.NC-OFDM system breaks through the limitation that the traditional OFDM system must transmit signal in a contiguous frequency-band, but the consequent problem is that the synchronization technique becomes more complicate. In traditional OFDM systems, Most of the synchronization methods are based on the assumption that a user utilizes a contiguous bandwith to transmit data, so how to synchronize efficiently will be an important technical challenge in NC-OFDM system.Based on the existing researches, this thesis discussed the characteristics and some technique problems of NC-OFDM. Especially, the synchronization in non-contiguous frequency bands and the interference removal of the primary user are studied in deail from theoretical analyses and simulations. The main work is as follows:1. Study on the frequency synchronization in NC-OFDM1) In view of no-contiguous frequency bands assignment, an effective blind CFO estimator has been proposed, and a simple closed-form solution is derived from the 4th order characteristic of OFDM signal. the proposed method is suited for random non-contiguous frequency bands due to the flexibility of utilization of the datas on each subcarrier.2) In view of interference from the primary users, the approach of pre-filtering for interference removal is investigated, and the estimation performance is analyzed in kinds of scenarios. The study results indicate that the pre-filtering would suppress the interference efficiently. Moreover, the estimation performace of the CFO after pre-filtering is close to the one in non-interference from the primary users.2. Study on the timing synchronization in NC-OFDMThe timing synchronization in the NC-OFDM is studied, and a new timing acquisition method is proposed. The simulation results indicate that, the proposed method is more robust than the traditional one in the NC-OFDM scenarios.3. Study on the joint timing and frequency synchronization in NC-OFDM:The method for joint estimation of timing and CFO is investigated in the NC-OFDM scenarios. On account of the high computational complexity of the 2-D exhaustive search, an alternative 1-D iterative search scheme is presented. The simulation results verify that, after few data update and iteration, the 1-D iterative estimation achives almost identical performance to that of the more expensive 2-D exhaustive serach.