Study Of Narrowband Interference Suppression In MB OFDM System

The multi-band orthogonal frequency-division multiplexing ultra-wideband (MB-OFDM UWB) system combining OFDM with frequency hopping is a promising candidate for the future short-range mobile communication systems. The narrow-band interference suppressing and anti-multipath fading techniques in the MB-OFDM systems are studied in this thesis. The main content of the thesis are as following:Based on the traditional interference suppressing OFDM (IS-OFDM) UWB system, a new UWB system is proposed by using the carrier grounping way for combating the narrow-band interferences, which exist in the spectrum range of a UWB signal. In the proposed IS-OFDM UWB system, the coded bit streams are spread over the sub-carriers through making full use of the frequency diversity across all the sub-carriers. This increases diversity gain and provides the robustness to narrowband interference for MB-OFDM UWB system. And, the value of grouping number may be optimized in terms of the channel environment for enhancing the performance of the system more effectively. Furthermore, the binary signature sequences with minimum total squared correlation (TSC) are used to shape the spectrum of the transmitted signal. Both theoretic analysis and numerical results obtained from simulations show that this method could further promote the robustness of the system to narrow-band interference.Convolutional coding and bit interleaving are employed to exploit the diversity in the MB-OFDM UWB system, but they are not suitable for the system with a high data rate. The diversity gain of the system with high rate, could be improved by utilizing spreading transform. However, the multipath component in a channel would not be used efficiently with a common orthogonal spreading matrix. Therefore, a new MB-OFDM UWB system based on a higher order rotation spreading matrix is proposed to suppress the bad effects of narrowband interference on the MB-OFDM UWB system. The advantage of this new spreading matrix over more traditional spreading matrix is its flexibility in determining different structures of matrixes with the simple angle rotation. These matrixes can advance the correlation between the transmitted symbols and improve the utilization rate of the multipath component in UWB channel. And, the diversity of the higher order rotation spreading matrix with different angles are analyzed by the method of error weight distribution. Both theoretic analysis and simulation results indicate that through utilizing the diversity of partial sub-carriers in a sub-band of the MB-OFDM UWB system, the higher order rotation spreading matrixes with different angles (exceptionπ/2) can be used to enhance the robustness of the system to narrow-band interference effectively.When the high rate MB-OFDM UWB signal is transmitted through a channel, their sub-carriers will experience frequency selective fading and the adjacent subcarriers are very much correlated. Hence, correlation would result in a small frequency diversity for spreading. For the problem, a combining scheme based on block spreading and chip interleaving is investigated. In the scheme, the spreading datas are reordered to achieve frequency diversity and randomization effect in frequency domain and moreover, the delay latency becomes less.