The Research Of Multiple Access Schemes In Ultra Wideband Communication Systems

Ultra-Wideband technology makes use of ultra-short duration (<1ns) pulses, which yield ultra-wide bandwidth (up to several GHz) to transmit signals. So UWB is particularly promising for short-range wireless communication, and it's the hotspot of the present research. The multiple access (MA) of UWB is the key technology. Unlike conventional wireless communications systems, UWB systems of multiple access has the characteristic such as small coverage of networks, small amount of users, simultaneous operating piconets (SOP) and so on.The thesis introduces the basic principles of UWB systems. Then we expatiate multiple access schemes for UWB such as time-hopping with pulse position modulation (TH-PPM), direct-sequence spread-spectrum (DSSS), pseudo-chaotic time hopping (PCTH) and chaotic sequence spread- spectrum (CSSS). We compare TH-PPM and DS-UWB, and give a standard for judge which one has a better performance, which depends on the waveform of the pulse transmitted. Then the multiple access performances of pseudo-chaotic time hopping and conventional time hopping (TH-PPM), spread spectrum with chaotic sequence and that with pseudo-random sequence are compared, in terms of the complexity of receiver, theoretical analysis of bit error rate and simulation results It is shown that, there is a tradeoff between the performance of bit error rate (BER) and complexity of receiver. As the system BER is concerned, pseudo-random sequence spread spectrum is better than that of chaotic sequence, and conventional time hopping is better than pseudo-chaotic time hopping. As the complexity of receiver is concerned, chaotic sequence spread spectrum and conventional time hopping are superior.Furthermore, we study the performance of PCTH systems. It is found that the chaotic mapping has effect on the bit rate, while the modulation of multiple access affects the symbol error rate (SER). N-way Bernoulli shift and N-way tent mapping can improve the bit rate, for example, the data rate of double-tent mapping is increased as twice as that of the Bernoulli mapping and single-tent mapping. Within the same chaotic mapping, modulation of BPSK can get lower SER than that of TH for multiple access. So we improve the PCTH for the double-tent mapping in using BPSK modulation, and this manipulation provides a better SER performance. It is proved that the improved system gets as twice bit rate as the traditional system as well as a better SER performance by analysis in theory and simulation results.Finally, we discuss the system of simultaneous operating piconets(SOP), taking anexample of four piconets that multiple access across piconet is CDMA while multiple access within piconet is time-hopping. The model of the system is given, and the performance is analyzed. Then, it is showed that orthogonal code for CDMA can efficiently reduce the interference across piconet. The cyclic codes based on pseudo random sequence have well correlation performance, as well as high efficiency of the codes. And simulation result supports that.