Signal Design, Multiaccess Interference And Synchronization Analysis For Quasi-Synchronous Frequency Hopping Systems

In frequency hopping code-division multiple-access (FH-CDMA) communication systems, the Hamming autocorrelation and cross-correlation properties of the frequency hopping sequences employed play an important role on the level of the self-interference (SI), multiple access interference (MAI), therefore directly influence the performance and capacity of the FH-CDMA systems. The idea of no hit zone (NHZ) for frequency hopping sequence gives a new choice to enhance the multiple access performance. In this thesis, some beneficial results are obtained by investigating on the techniques of quasi-synchronization and synchronization for frequency hopping system based on the concept of NHZ frequency hopping sequences.Firstly, by employing the periodic zero correlation zone(ZCZ) binary spread spectrum sequences obtained, this thesis generates a new class of frequency hopping sequence sets which not only have no hit zone Hamming correlation performance but also approach or achieve the theoretic bound. There are eight subsets that can be mapped to NHZ frequency hopping sequences and these mapping shows up the relationshp between NHZ sequences and ZCZ sequences. The proposed four methods where methods III and IV with complementary sequences as starter are optimal NHZ sequences which achieve the theoretical bound under some conditions.Secondly, based on the four hit states Markov model which takes into account the marginal effect of the multiple access interferences (MAI) in frequency-hopping communication systems, combining the Hamming cross-correlation performance of NHZ frequency hopping sequences and the multiple access characteristic of quasi-synchronous (QS) FH-CDMA system, the MAI performance analyses model for NHZ QS-FH-CDMA system is given. The thesis analysis and compares the multiple access interference performances for NHZ QS-FH-CDMA system, synchronious FH-CDMA system and asynchronious FH-CDMA system. The results shown that the MAI performance of NHZ quasi-synchronous system used NHZ sequences outperforms the asynchronous system employing random hopping frequency sequences.Thirdly, the conditional deviation of multiple access interferences of K(K>1) users which have different time delays in the asynchronious FH system and the probability distribution function(PDF) of signal noise plus interference ratio(SINR) are analyzed, then the exact closed form expressions of error probability of asynchronious FH system are obtained, which is the function of frequency full hit probability, frequency partial hit probability, time delay of interference signal and the number of user. Based on the Hamming cross-correlation performance of NHZ frequency hopping sequences, the frequency full hit probability and frequency partial hit probability of NHZ-QS-FH-CDMA system and the conditional deviation of multiple access interference for NHZ-QS-FH-CDMA system are analyzed, and then the exact closed form expressions of error probability of NHZ-QS-FH-CDMA system are also obtained. Besides the above variables, the expression involves also the length of sequences and the size of no hit zone. Compared with Gaussian approximate method, the obtained closed form expression is much tight.Then, the effect of multipath fading on orthogonality of signal tones in FH/MFSK and in SFH/MFSK with non-coherent demodulation and time and frequency synchronization error are analyzed. By using the gate function to represent the valid length of signal in time synchronization error and multipath time delay, a general expression of orthogonality loss factor Djnl which is appropriate in various conditions is derived. The results shown that the Djn is determined not only by the multipath delays, signal strength fading and synchronization time error, but also by the phase variations caused by multipath time delays. Further, the scenario of FH/MFSK is the special case of SFH/MFSK system. Numerical results demonstrate that the effects vary with different phase variations. This result will further affect BER performance of FH/MFSK systems. Thus it is no longer appropriate to consider that the MFSK non-coherent demodulation is independent of the phase variations in the case of multipath fading for frequency hopping system.Finally, the effect of phase variations caused by multipath time delays on the estimation performance of time and frequency synchronization errors for FH/MFSK system with non-coherent demodulation in full-band and partial-band noise jamming are analyzed. The expressions for mean and deviation of time and frequency synchronization errer spectral estimation are derived under various conditions. Numerical results demonstrate that the phase variations caused by multipath time delays may influence accuracy of the mean of time synchronization error spectral estimation.