Study On Time Hopping Multi-access Technology Of Ultra Wide Band

With the development of wireless communication technology, various kinds of wireless communication systems come forth one after another, which causes the available frequency resource becoming more and more rare.Howerver, people frequently increase their demands on wireless communication system, and they hope that the system can provide higher data transfer rate with low cost and low power. Ultra Wide Band (UWB) technology attracts people's attention at this background and becomes the focus of research and development in short range wireless communication field.UWB is regarded as one of key technologies of the next generation wireless communication. According to FCC's definition, the concept of UWB is not limited to pulse transmittion. UWB can be extended to continious transmittion technology as long as the signal bandwith is larger than 500MHz. At present, there are three kinds of fulfill schemes of UWB communication system, that is carrier-free pulse scheme, singal carrier DS-CDMA and MB-OFDM. Study on multi-access technology in this paper is based on carrier-free pulse scheme.In wireless communication system, multi-access makes it possible to accommodate multiple users on the same channel. Multiple access performance has close relationship with user number and transfer rate. It is not only a major standard to value the performance of the system, but also a key factor to influence its application and generalization of the technology. The purpose of the study of ultra wideband time hopping multiple access technology is to increase the performance of the system via improving multiple access technology.UWB pulse communication is different from continuous carrier modulation technology.The system uses very narrow pulse of time domain to carry information.Time hopping multi-access technology is firstly applied to UWB system.It makes use of the specialty of low duty cycle of UWB signal and divides the pulse repetition period into N h separate time slots Tc . Controlled by a set ofunique time hopping sequences, each transmitter selects one slot from N h time slots as the sending position of pulse. Receiver uses the same time hopping sequence as the aimed transmitter to demodulate the information.Pulse sequences from different users will not collide with each other so long as the applied time hopping sequences have good orthogonal property.Thus the MUI is eliminated and multi-user access is realized. We can learn from the information above that UWB time hopping multi-access technology is realized through time hopping sequences.So the research work on multi-access technology in this paper is carried out mainly revolving around time hopping sequences.Though UWB is studied much earlier in foreign, the reseach work on time hopping multi-access technology is a few. Comparatively speaking, there are lots of literatures on time hopping multi-access technology in domestic. At present, the concept of time hopping sequence design with zero correlation zone (ZCZ) or low correlation zone (LCZ) for quasi-synchronous system is much popular. The achievements on time hopping sequence research work are obvious, yet there are deficiencies too. The general problem in time hopping code design is that people usually care about only the number and the cross-correlation property of time hopping sequence and overlook the period of the sequence. Time hopping sequence acts the double role of multi-access and smoothing the power spectral density of the signal in UWB time hopping communication system. Increasing the period of time hopping sequence can effectively eliminate the discrete components in power spectrum of UWB signal, and which is very helpful for improving the performance of the whole system. Time hopping codes play an important role in time hopping communication system. So these sequences need to have following properties: large number of sequences to accommodate more users; good auto-correlation property to eliminate ISI and improve the synchronization of the system; good cross-correlation to minimize MUI; long period to smooth the power spectral density of the signal. According to these basic points, new UWB time hopping multiple access schemes are proposed depending on the relative delayτbetween different users in a multiuser system, threepossible scenarios: synchronous transmission, quasi-synchronous transmission and asynchronous transmission are considered. The achievements on time hopping sequence research are analyzed and summarized in this paper. Furthermore improved scheme of UWB time hopping multi-access and new method of time hopping sequence design are proposed.For synchronous transmission scheme, the way of increasing system user number through the method of improving time hopping sequences is limited. The more effective method to increase system user number is to use CDMA technology which is used in ceil telecommunications system. The proposed method in this paper is to take THMA plus CDMA mode. For Walsh sequences with period length of N, the proposed method can increase the system user number and gain to N times comparing with the mere time hopping system.For quasi-synchronous transmission scheme, the main consideration of time hopping codes design method is to maintain a zero correlation zone or low correlation zone. In this paper, the two stage time hopping codes with zero correlation zone generating method was improved. The improved sequence has the same family size and zero correlation zone as the codes generated by the original method. The proposed method prolongs the period of the sequences effectively which means a lot to smooth the power spectral density of the signal.For asynchronous transmission scheme, the proposed TH codes generating method in this paper is based on optimal pair theory of m-sequence. The TH codes generated by the proposed method have the same period as m sequences and similar correlation performance to linear congruence codes. The overall performance of the improved sequences is obviously better than linear congruence codes. Of course, the proposed method has flaws too, the major shortcoming is that the method failed to limit the family size and correlation value of the generated sequences in the form of math expression.Experiment is an effective way of testifying results in theory and a way of promoting principle research. A lot of simulation works are carried out on Matlab/Simulink platform in this paper. Based on achieved theory, functionmodules of UWB time hopping communication system are written and Simulink simulation models of the system are constructed. Besides a lot of simulation programs used for data processing are compiled.Research achievements in this paper can divided into two parts. On one side, the multiple access method of UWB time hopping system is improved in theory. On the other side, the UWB time hopping communication system simulation models are constructed and simulation programs used for data processing are compiled in the process of experiments.