Algorithm Study On Narrowband Interference Suppression In OFDM Systems

Because of the highly efficient utilization of frequency band and the ability of mitigating the multipath effect, Orthogonal Frequency Division Multiplexing (OFDM) has invoked a popular research interest and been adopted as the physical layer transmission technology in more and more wireless communication systems. However, some OFDM systems operate in unlicensed bands, for instance 802.11a WLAN system on the 5GHz UNII band. Since unlicensed bands are opened to all legal communication systems, OFDM signals have to share the same band with others and inevitably interfere each other. Due to the effect of "spectral leakage" in discrete Fourier transform (DFT), which is used in OFDM receivers, narrowband interference will be spread in the OFDM spectrum, thus, strongly impacts adjacent subchannels. We can disable the severely interfered subchannels to improve the system's BER (bit error rate) performance while suffering the decrease of throughput. The goal of our research is to suppress the narrowband interference (NBI) while maintaining the system throughput at the same time.In this dissertation, we summarize NBI suppression techniques and divide them into two categories: receiver window techniques and NBI cancellation techniques. We make contributions in both areas:First, we take the symmetry of the long preamble in IEEE 802.11a physical layer frame into account and propose a Hanning window based channel estimation. We also investigate the selection of window function for data reception based on the measurements of indoor wireless environment. The joint window-based receiver scheme satisfies the 802.11a standard and doesn't require any modifications at the transmitter. Theoretical analysis and simulations demonstrate the effectiveness of this technique in improving the system performance. For example, when the signal interference ratio (SIR) is 5dB, the average BER of OFDM signal can be decreased from 10-3 to 10-4. Second, we assume that the NBI problems are caused by narrow band digital communication systems, and propose a novel NBI cancellation method. A practical OFDM system is designed for the new algorithm. We discuss the estimations of NBI central frequency and its bandwidth, and employ zoom FFT technique to reduce the calculation complexity. Simulation results show that this method can achieve about 6dB signal to interference plus noise ratio (SINR) gain when the interference bandwidth is as wide as two OFDM subcarriers and the system SIR is as strong as 0dB. As the bandwidth of NBI decreases, more SINR gain will be obtained.