Narrowband Interference Suppression In Direct Sequence Ultra-Wideband Systems

The ultra-wideband (UWB) technology has been studied extensively in recent years, which offers many advantages such as simple transceiver architecture, low power consumption and high data transmission rate. UWB has been considered as an important candidate technology at the physical layer of the wireless personal area networks (WPAN). The UWB systems occupy so wide bandwidth that they must co-exist with other legacy narrowband systems. In order to minimize the impact of UWB signals on pre-existing narrowband systems, the transmission power of the UWB devices is strictly limited. However, these narrowband systems may cause severe interference to the UWB systems. Therefore, narrowband interference (NBI) suppression is crucial to UWB receiver design.The NBI cancellation methods include NBI avoidance techniques and NBI suppression techniques. The underlying idea of NBI avoidance techniques is to design the special UWB pulse waveform which is orthogonal to the NBI in frequency domain, thus, it is easy for the receiver to separate the desired signals from the interference. For the NBI suppression techniques, signal processing methods are used to extract the desired component from the received signal. NBI suppression in direct-sequence (DS) UWB is studied in the thesis.Firstly, NBI suppression in additive white Gaussian noise (AWGN) is investigated. The conclusion that DS spread spectrum (DS-SS) serves no helps against AWGN is proven strictly in terms of bit error rate (BER) and signal-to-noise ratio (SNR). In many cases, the power of the NBI is a few tens of dBs higher than that of the UWB signals, so NBI suppression in UWB is a more challenging problem. After the code-aided technique is extended to DS-UWB, a new type of spreading sequence is proposed here which significantly improves the NBI suppression capability of the code-aided technique. It is shown that the new spreading code is more efficient with more powerful NBI and larger processing gain. It also can be shown that using the new spreading code will decrease the transmission power and the chip rate greatly. A spreading code selection scheme is introduced which can ensure the code-aided technique achieves suboptimal performance, while the spreading code has good autocorrelation.Secondly, NBI suppression in frequency-selective channel is examined. A RAKE receiver based on code-aided technique is introduced. It is shown that the performance of the RAKE receivers with maximal ratio combining (MRC) reception degrades greatly when the residual interference is strong and the noise terms in different fingers are highly correlated. To solve this problem, a minimum mean-square error combining (MMSEC) technique is proposed. The results show that MMSEC outperforms MRC and selective diversity (SD) significantly when the NBI is strong. The performance of the RAKE receivers using spreading code selection scheme is also studied in the thesis.Finally, the performance of the linear predictor, interpolator and code-aided technique against autoregressive (AR) and passband interferers is evaluated. Especially, the performance between linear interpolator and the traditional code-aided technique against strong NBI is compared. The computational complexities of adaptive versions of these two techniques are analyzed. A conclusion is drawn that the interpolator is more suitable for rejection of strong NBI in UWB. The impact of poles and orders of AR interferers, the number of filter taps, the central frequency of passband interferers and the number of interferers on the performance against NBI is also considered in this work, and some important results are found. The frequency responses of linear predictor and interpolator are also investigated.