Investigation Of No Hit Zone FH Sequences,Quasi-Synchronous And Message-Driven FH Systems

Frequency hopping (FH) communication is one of the popular spread spectrum techniques. In conventional FH communication systems, the carrier frequencies of transmitter and receiver are hopped from one to another synchronously according to the FH sequences allocated. The Hamming correlation properties of the frequency hopping sequences have a great influence on the performance of the FH-CDMA communication systems. No hit zone (NHZ) FH sequences have ideal Hamming correlation properties within a correlation zone. If NHZ FH sequences are utilized in quasi-synchronous (QS) FH-CDMA systems, the multi-access interference (MAI) can be eliminated or reduced significantly, thus improving the system performance. In this thesis, some beneficial results are obtained by investigating NHZ FH sequence bounds, NHZ FH sequence design, and QS-FH-CDMA systems based on NHZ FH sequences. Different from conventional FH communication systems, message-driven frequency hopping (MDFH) communication systems use part of the message stream as the FH sequence to control the hopping of carrier frequencies in order to increase system spectral efficiency. The performance of an enhanced MDFH (E-MDFH) system is analyzed and its improvement is also researched in this thesis.Firstly, by analyzing the characteristics of NHZ sequences, a theoretical bound on NHZ FH sequence set is obtained, which gives the bounding inequality between the number of frequency slots, the number of FH sequences, and no hit zone size. Besides, by further considering the sequence length, another theoretical NHZ FH sequence bound is established. These two bounds are compared with an existing NHZ FH sequences bound, showing that the second bound proposed in this thesis is tighter than the others. Under the guidance of the theoretical bounds, three methods for designing NHZ FH sequences are presented, i.e. matrix transform method, mapping method and construction method of non-repeating NHZ FH sequences. The NHZ FH sequence sets obtained by these construction methods are optimal NHZ sequence sets which achieve the theoretical bound. The Hamming cross-correlation property of the general class of NHZ FH sequence set constructed is analyzed, showing that their Hamming cross-correlation functions possess the feature of mullion.Secondly, the idea of NHZ is extended to two-dimensional time delay and frequency shift no hit zone (TF-NHZ), and the definition of TF-NHZ is given. Based on the concept of TF-NHZ and the theoretical bound on1-dimensional NHZ FH sequences, an inequality is derived to reveal the bounding relationship of the number of frequency slots, the sequence length, the number of sequences and TF-NHZ size, resulting the theoretical bound on TF-NHZ FH sequences. Three construction methods of TF-NHZ FH sequences are presented, and two of them achieve the theoretical bound.Thirdly, a two-stage FFH/MFSK quasi-synchronous communication system using NHZ FH sequences is proposed. The users are divided into a number of groups here, and each group is assigned a unique NHZ FH sequence. The MAI between users of different groups can be eliminated even they are not perfectly synchronous, as long as their relative delays do not exceed the no hit zone. Therefore, residual MAI is only imposed on users belonging to the same user group. The multiuser detection technique of asynchronous system is utilized in the receiver to improve the detection performance further. The MAI analysis as well as the system model is provided. Simulation results show that the proposed NHZ-FFH/MFSK quasi-synchronous system exhibits better performance than a multistage fast frequency-hopping (MS-FFH) multiple-access system presented by Yang and Hanzo.Finally, an enhanced MDFH (E-MDFH) system is discussed. In particular, the BER performance of E-MDFH/MFSK system is investigated by theoretical analysis and simulation. An improved detection method is put forward to combat the drawback of carrier bits and ordinary bits detections in E-MDFH/MFSK system. It is shown by simulation that the proposed detection method can reduce the BER of information data under AWGN channel and Rayleigh fading channel. To improve the BER performance of E-MDFH system further, a modified transmitter and receiver model of E-MDFH/MFSK system is presented, and its BER performance is analyzed. Analysis and simulation results show that the BER performance of ordinary bits and carrier bits of the modified system are improved significantly compared with the existing E-MDFH system.