The Research On TH Sequences For Ultra Wideband Communication System

The current meaning of UWB is also called Impulse radio, which emerged in sixties last century, but it is limited to military, radar location and telemeters ect. In Feb.14 2002, this wireless technology is firstly permitted to be used in civil application by FCC, consequently it was concerned by the whole world.It is known to all that the traditional communication technologies carry the signals from baseband to the carrier, while the UWB technology in this dissertation is a carrier-free technology. The UWB system transmit a train of baseband pulses named Gaussian monocycle pulses or simply impulses with very short duration in the order of sub-nano seconds which make its bandwith reach GHz. Also because of the short duration of the pulse, the signals have strive capacities to distinguish multipath channels, so in the wireless indoor environments, it makes it possible that multipath is separated and processed form the multipath channels, enhance the system performance greatly. These pulses appear in time randomly which is implemented by the time hopping sequences. The design and realization of the time hopping sequences are very important in UWB communication system. Because the TH sequences are the sources of the Multiple-Access in the UWB system, the number of the TH sequences decides the number of the users that the UWB system can contain. Under multiple-user conditions, the receiver tells the different transmitted signals by the TH sequences, so it is required that the TH sequences have good correlation properties. The better correlation properties are, that is the number of the collision among different users is less, the better the performance of system is.The good TH sequences must have the properties like white noise, which can smooth the PSD of the signals, enhance the anti-detection and intercept and capture capacities, and also it constitute a reliable source for synchronization and channel estimation. So the TH sequences technology is a critical technology in UWB system. The performance of the TH sequences will effect on the system performance.So this dissertation mainly researches the generation of the TH sequences and their capabilities in the UWB system. This dissertation has accomplished these works as follows: Firstly, very large numbers of literatures are consulted and make the foundational principle of the UWB system clear, and found the foundation for the TH sequences design and research. Compare with other wireless systems, UWB system has so many advantages such as good ulterior properties, protecting system form detection, high process gain and large capacitity.In this dissertation we follow the research technique of the FH sequences to research the generation and performance of the TH sequences. As spread spectrum sequences, the selection of TH sequences must comply with the following rules: the signals with TH modulation should have good correlation properties, the TH sequences should have long period and enough numbers to implementation the Multiple-Access, should be easy to generate, duplicate and control. On the basis of the FH sequences, according to the a frame a pulse structure, this dissertation presents three TH sequences which based on Gold sequences, Permutation Sequences and Quadratic Congruential Sequences.The correlation properties of the TH sequences are very important to the whole system performance. In the dissertation the correlation properties are analyzed and the correlation function of the TH sequences is given, and the upper-bound of correlation value is derived. Through the analysis of presented TH sequences, we can see the upper limits of both the maximum self correlation value and maximum cross correlation value of TH sequences based on Permutation Sequences are 4, while those of TH sequences based on Quadratic Congruential Sequences are 2 and 4. So the latter has a better performance. Better correlation properties bring out lower bit error rate. This dissertation addresses an optimum algorithm which can decrease the maximum value of the correlation function to a half.The TH sequences have two actions in the UWB system, generally speaking, one of action is to smooth the spectrum, and another is to allow for Multiple-Access. We have known that the lower the power is and more smooth the PSD is, the signals have better shelter properties. This paper takes the 2nd derivative Gaussian pulse as the example, analyzes the PSD of a single pulse and derives the PSD of pulses with TH sequences modulation or not. Though the simulation, the performance of three sequences that smooth the spectrum is achieved and we point out that TH sequences based on Permutation Sequences has the best performance in these three TH sequences. And for the signals with TH modulation, an effective method is put forward to enhance the performance of anti-detection is to increase the period of the TH sequences.How to enhance the capacity of anti-interfere is a very important issue, especially to military communication systems. Follow the methods of the traditional spread spectrum technique, this dissertation analyzes the impact of the TH sequences and emphasizes on the single frequency sinusoidal interference and barrage jamming interference. As for single frequency sinusoidal interference, the period of TH sequences affect the anti-interference capacity directly. While as for the barrage jamming interference, the BER performance is independent of TH sequences. Simultaneity, by increasing the pulse numbers of every data bit, the capacity of anti-interference can also be enhanced. And above all, we can draw the conclusion that the TH sequences based on Permutation Sequences are the best in three TH sequences presented in this dissertation.At the end of the dissertation, a simulation link model is built in Simulink. In this link, it is mainly composed of data transmitter block, TH modulation block, UWB signal detection block, acquisition block, template signal generator block and demodulator block. By the simulation results, the TH sequences are verified in the link, and it make the link run in the course of naturenormally, which indicates that the presented TH sequences are suitable for the UWB communication system, Simultaneity, the result shows the proposed model can make UWB communication feasible in Simulink.