Study Of Shortwave Differential Frequency Hopping Multicarrier Communication Systems

The broadband Frequency Hopping(FH) technology with high-speed data rate is a trend for the shortwave communication technology today, including Differential Frequency Hopping (DFH) technology and the multicarrier shortwave FH technology. The DFH technology is an advanced FH technology, it does not use pseudo-random code to control the generation of hopping-frequency, but its hopping-frequencies is determined by the G function and message to be transmitted. Therefore, there is certain correlation between the adjacent hopping-frequency, the system is capable of error-correction. Now, the Differential Frequency Hopping technology could work with up to5000hops/s in HP communications, it is especially suited to the military communications. In the case, the National Natural Science Fund Project "Study of software-based high-speed anti-jamming shortwave radio frequency-hopping communication system "(No.60372056), and Two main ways of decoding with symbol-by-symbol decoding and sequential decoding are studied for the DFH. Meanwhile, the National Natural Science Fund Project "Study of software-based radio shortwave frequency hopping communication system with OFDM technology "(Grant No:60672043),"Study of Anti-jamming technology for DFH&multicarrier communications with Coherent receiver"(Grant No:61072041) are funded to carry out the short-wave multi-carrier frequency-hopping technology, especially orthogonal frequency division multiplexing (OFDM) technology with advantages of high frequency-utilization, high-speed communications, and resistance inter-symbol interference(ISI), and it is the key technologies for HF communications with higher data rate. With the wider band for the shortwave OFDM system, it will be susceptible to interference from traditional shortwave radio, and a variety of human jamming for military communications; For these problems, it is necessary to study the transceiver method for the shortwave FH/OFDM system and its ability to resist all kinds of human jamming.First, An overview of the HF channel characteristics and shortwave communication technology development is introduced in this dissertation, especially shortwave differential frequency-hopping system components and its key technologies, and the construction of several G functions and error-jump correction principle are introduced too, includes differential detection method on FH signal. According to the characteristics of the DFH, the DFH transfer process is to be as a Markov process, and a set of frequencies constitute the Markov chain, Transition probability between two arbitrary states is only determined by the message source and irrelevant to the current time. For the discrete memoryless sources, the DFH process coulde be seen as a homogeneous Markov chain. Three types of G function structure are introduced in this dissertation, such as G constructed by simple linear function, by convolution coding technology and by Turbo encoding technique, and According to the characteristics of differential frequency hopping signal, and the effection of HF channel multipath delay and Doppler frequency shift, some algorithm s on DFH signal detection are researched such as fast fourier transform(FFT), short-time Fourier transform(STFT), multiple signal classification(MUSIC) and Estimating signal parameters viarotational invariance techniques (ESPRIT), and Further to search better detection methods for shortwave DFH system frequency,All these are based for demodulation method on the DFH system.Secondly, considering the DFH system without coding, methods on the demodulation and decoding for DFH system are researched, such as the symbol-by-symbol decoding and sequential decoding. As a basis, Iterative decoding and demodulation method on DFH is proposed throught STFT. only adjacent transfer relationship between the two hopping frequency is considered by symbol-by-symbol decoding, but the sequence decoding could search for the best possible frequency shift path by all of the hops in accordance with the received signals in the G function trellis, so as sequential decoding, it is more full use of the correlation between the transmission frequency, and therefore its performance is superior compared to symbol-by-symbol decoding. Iterative algorithm can greatly improve system’s performance, the symbol-by-symbol MAP decoding algorithm is introduced to detect DFH signals with the G function by Tubro coding, the STFT algorithm and MAP decoding algorithm for frequency detection are combined to detect the differential signal frequency hopping sequence. And the detection performance is studied under the HF channel conditions, respectively multi-path interference, group delay, Doppler effect, it is better performance by G function with Turbo codes to convolutional code.Meanwhile, in order to further improve the data rate of HF communication, the OFDM technology is applied to the shortwave hopping-frequency communication systems, and demodulation and decoding method for shortwave FH/OFDM system is researched, such as demodulation algorithm based on iterative demapping and decoding, as well as subcarrier fast-hopping system based on the bit interleaving demodulation&decoding algorithm. After subchannel estimation and interpolation methods are analyzed and simulated by the comb pilot, channel estimation DFT algorithm and interpolation method are proposed to suitable for HF channel transmission. For OFDM PAPR problem, a nonlinear curve expansion PAPR reduction method is proposed in this paper. When differential detection (or non-coherent detection) is adopted, then the channel estimation will not be required, only two adjacent signals by comparing the phase and amplitude can accurately recover the baseband signal, thereby greatly reducing the cost of their implementation of the system and complexity, therefore, Differential modulation for the transmitter in frequency-domain normally to OFDM, Non-coherent sequence detection(NSD) with iterative decoding and iterative demodulation, not only could be an effective method to improve the system Bit Error Rate(Ber) performance, and without the accurate channel estimation. In extreme cases, especially partial-band jamming or multi-tone jamming, bit-interleaved coded modulation (BICM) and pre-encoded sub-carrier fast-hopping technology can significantly improve the bit error rate performance for FH/OFDM system under the conditions of fading channel.Finally, anti-jamming performance is analyzed for the shortwave FH/OFDM system under the Gaussian channel and fading channel in this paper, further, the effects on FH/OFDM system is analyzed to the partial-band jamming and multi-tone jamming, and how to improve the system BER performance is analyzed by the channel estimation, jamming status information too. A constant envelope spectrum FH/OFDM system is introduced with anti-jamming ability to partial-band jamming and multi-tone jamming.