【作者】 陈韵；

【导师】 蔡平；

【作者基本信息】 哈尔滨工程大学 ， 通信与信息系统， 2012， 博士

【摘要】 水声信道因其特殊时空频变特性，严重的信道衰落效应和复杂的水下环境噪声而成为最为复杂的无线信道。因此，追求一种可靠有效高速的水下信息传播的调制解调方法成为水声通信的主要内容。分数阶Fourier变换是近年来兴起的一种针对chirp类信号的处理算法，可以理解成为基于chirp基分解或者是时频面的旋转过程，在合适的分数阶域能够实现chirp类信号的能量聚集出现峰值的现象。因而将分数阶Fourier变换技术引入到水声通信中，发展基于分数阶Fourier变换的水声通信方案能够在应对复杂水下噪声环境和抑制水声信道衰落效应方面具有积极的意义。传统的正交多载波(OFDM)技术具有通信速率和频带利用率高的特点，且借助于FFT技术可以实现快速的调制解调，简化系统复杂度，成为未来第四代无线通信技术的基础。但是OFDM技术引入到水声领域之后，水声信道较窄的可利用频带范围和梳状的幅频响应特性不仅使得OFDM的优点没有得到充分的发挥，还受到频率选择性衰落的影响而降低性能。针对这些问题，本文提出基于分数阶Fourier变换的正交多载波(FRFT-OFDM)水声通信方案，采用正交的LFM信号作为子载波信号，利用分数阶Fourier变换作为信息的调制解调方法。宽带的LFM子载波相比于基于正弦信号子载波，不易于被信道所形成的深陷频率零点所衰落，因而可以可靠的传输信息。且LFM信号具有较大的多普勒容限，相比OFDM技术，FRFT-OFDM具有一定的抗多普勒效应的能力。分数阶Fourier变换的chirp基分解特性说明，一个LFM信号在其对应的阶次的分数阶域上会出现能量聚焦而形成冲激函数，在除此之外的分数阶域均不会产生这种效果。外界干扰只有在具有与LFM信号近似的时频分布特性时才会产生干扰作用，且噪声可以认为均匀散布在整个时频面中，任何阶次的分数阶域均不会形成干扰，因此本文提出基于分数阶Fourier变换的脉冲位置调制(FRFT-PPM)技术，将要传输的信息调制到分数阶域的峰值位置当中，具有较强的抗噪声和抗干扰性能。在此基础上，根据正负斜率的LFM信号只会聚焦于相对应的正负阶次分数阶域而不会产生相互之间的干扰，提出正负斜率LFM信号作为载波的分数阶域脉冲位置调制技术，在不额外占用系统带宽的前提下倍增FRFT-PPM系统通信速率，在保证了系统稳健性的前提下提高了系统有效性能。水声信道严重的多途扩展效应是通信系统产生码间干扰(ISI)的主要原因，因此采用接收均衡技术能够有效的抑制多途效应，减少误码的产生。多途效应对信号实际上是增加了时间延迟，根据分数阶Fourier变换的时移特性，当信号在时间域上发生移动时，信号变换的象函数在分数阶域上也会发生移动。为了减少干扰，本文提出了在系统中添加时域保护间隔和分数阶域保护间隔，以减少这种码间干扰的影响。然而这种方法在一定程度上降低了系统的效率，且水声信道多途扩展十分严重，完全依靠保护间隔抑制ISI将使系统性能变得不可接受，因此本文提出采用Rake接收机和时间反转镜技术来抑制水声信道的多途影响。Rake接收机是一种特殊的路径分集技术，而分数阶Fourier变换的时间移动特性完全能够在分数阶域分离出信号的独立衰落路径，因此将Rake接收机应用于基于分数阶Fourier变换的水声通信系统具有理论基础，并且本文讨论了各种合并方式的性能。相对于Rake接收机，时间反转镜技术无需知道信道的确切信息，通过探测信号估计出大致的时反信道和接收信号进行卷积，达到去除多途信道影响，突出主要路径的能量，提高接收信号的信干噪比，从而实现减小误码的产生。通信平台的相对运动产生了多普勒效应，使得接收信号在时间尺度上产生压扩效应，在频域上出现频率弥散。根据分数阶Fourier变换的尺度变换特性，信号在时间尺度的变换造成其在分数阶域的尺度的变换，根据这个性质可以得知，多普勒效应并不改变LFM载波在分数阶域正交特性，只是产生统一的分数阶域偏移和尺度变换，因此本文提出分数阶域的多普勒效应补偿方法，通过测定多普勒因子，从而在分数阶域实现多普勒效应的补偿。通过大量的仿真实验和湖试试验验证基于分数阶Fourier变换的水声通信方案的有效性与可行性，具有较为广阔的应用前景。更多还原

【Abstract】 The underwater acoustic channel has spatial-temporal-frequency dependent characteristic,severe channel fading effect and complex ambient noise, which makes it become the mostcomplex wireless channel. Therefore, the pursuit of a reliable and effective high speedunderwater acoustic modulation and demodulation method is becoming the main content ofunderwater acoustic communication. The fractional Fourier transform (FRFT) is a recentdevelopment algorithm for chirp signal, which can be interpreted as chirp basisdecomposition or time-frequency plane rotation and chirp signal energy accumulation peakformation in the appropriate fractional domain. Hence, there is positive significance inintroduction of FRFT to underwater acoustic communication and developing a novelcommunication method based on FRFT to deal with complex underwater ambient noise andsuppression fading channel effects.The traditional orthogonal frequency division multiplexing (OFDM) technology has highspeed communication rate and bandwidth efficiency, by means of FFT technique OFDM caneasily achieve fast modulation and becomes foundation of the4G wireless communicationtechnology. However, when OFDM was introduced to underwater acoustic field, the narrowbandwidth, comb frequency-amplitude response and frequency selective fading of underwaterchannel makes its performance degradation. For these problems, this paper presented a novelorthogonal multi-carrier modulation based on FRFT, which used orthogonal LFM signals assub-carriers and used FRFT as modulation and demodulation method. Comparing tosinusoidal sub-carrier, the LFM sub-carrier can not be faded by null steering of underwaterchannel, thus this method can reliably transmit information. And comparing to OFDM system,the FRFT-OFDM system has a better anti-Doppler effect because of the large Dopplertolerance of LFM signal.The chirp decomposition characteristic of FRFT shows that the LFM signal’s energy willfocus and form an impulse function in corresponding fractional domain but will not in theother order fractional domain. Interference signal only has the time-frequency distributionapproximately like LFM signal will produce interfering effect and the ambient noise can beseem as evenly spread in the whole time-frequency plane, any order FRFT of ambient noisewill not produce interference. Therefore, in this paper, the pulse position modulation based onFRFT (FRFT-PPM) was presented, and the transmitted information was modulated in thepeak position in fractional domain, which can have a strong ability of anti-noise andanti-interference. On this basis, according to the positive and negative slope LFM signalenergy focused on corresponding order of fractional domain without mutual interference, this paper presented the positive and negative slope LFM signal FRFT-PPM communicationsystem, doubled FRFT-PPM communication rate without doubled the system bandwidth,which enhanced the robustness and performance of the system.Severe multi-path spread of underwater acoustic channel causes inter-symbolinterference (ISI), thus the equalizer at receiving end can effectively suppress multi-patheffect and reduce error. Actually the multi-path effect is to increase the signal time delay, andaccording to time delay shift of FRFT, the shift of signal in time domain causes transformedsignal shift in fractional domain. In order to reduce interference, this paper presented time andfractional domain guard interval (GI) to reduce ISI. However, this method reduced systemefficiency, and because of the underwater channel severe multi-path spread, completelyreducing ISI with GI will be unacceptable. In this paper, the Rake receiver and the timereverse mirror (TRM) technique was presented to suppression the impact of multi-pathchannel. Rake receiver is a special path diversity technique, and time delay shift of FRFT cancompletely isolate independent fading channel path in fractional domain, thus the Rakereceiver used in FRFT communication system has theoretical basis, and the combiningmethods were discussed in this paper. Comparing to Rake receiver, the TRM dose not needthe exact channel information and suppresses the channel effect, enhances main path energyand signal to interference and noise ratio (SINR) and reduces error by the probe signal’sapproximate reverse channel impulse function convolution with receiving signal.Relative motion of communication platform generates Doppler effect, which makesreceiving signal compression and expansion in time domain, and frequency dispersion infrequency domain. According to scale transformation of FRFT, the change of signal time scalemakes its FRFT fractional domain scale change. So the Doppler effect just causes unifiedoffset and scale of fractional domain change rather than change the LFM signal orthogonalproperty, in this paper, the novel fractional domain Doppler effect compensation method waspresented to suppress Doppler effect by estimating Doppler factor and change the scale andresample in fractional domain.By a large number of simulation and lake experiment to verify the effectiveness andfeasibility of the underwater acoustic communication scheme based on the fractional Fouriertransform, which has broad application prospects.更多还原