【作者】 褚振勇；

【导师】 易克初；

【作者基本信息】 西安电子科技大学 ， 信号与信息处理， 2005， 博士

【摘要】 所谓卫星重叠通信(简称SOC)是这样一种通信方式,它是隐蔽地与其他卫星通信系统共享透明转发器提供的信道,其信号重叠在那些系统的信号之上,但比它们弱得多而不影响那些系统的正常运行。因此,它可用作为一种应急通信手段,在未经允许的情况下借用可视空域内任一通信或广播电视系统的卫星链路进行通信。SOC系统是一种隐蔽通信系统,因而可以有效地避免敌方对卫星链路的摧毁、有意干扰和窃听。由于SOC的信号极其微弱,信干比很低,所以必须采用一些特殊技术,其中M元扩频技术就是一种有用的技术。本文重点研究M元扩频技术中的一些关键问题,论文的主要贡献如下：采用时频分析方法推导了理想信道条件下单码片成形直扩信号及其解扩信号的短时频谱特性。针对M元扩频码的性能要求,提出了一种通过计算机搜索来获得M元扩频码的方法。同时还提出了一种利用小波包函数构造扩频码的方法,它使用一组正交的小波包基函数作为扩频码的基,用以构造正交的扩频码。详细分析了频偏对零中频非相干解扩解调器误码性能的影响,并提出了一种改进型非相干解扩解调器。仿真结果表明：改进型非相干解扩解调器比常规的解扩解调器能够容忍更大的频偏。此外,还依据向量范数等价性原理,提出了一种基于1范数计算、能有效简化计算的非相干解扩解调算法。传统的M元扩频方式存在选码难度大、接收机复杂度高的问题,这就限制了M元扩频技术的广泛应用。这对该问题,提出了一种基于循环扩频码的M元扩频系统。在该系统中,只需给每个用户分配一条原型扩频码,各个用户的原型扩频码是相互正交或准正交的。每个用户将原型扩频码进行循环移位和极性翻转,构造出M条扩频码,分别对应于K比特数据信息,从而实现M元扩频。采用这种方式所构造的扩频码被称为正交循环码(OCCs)或双正交循环码(BOCCs)。接收机利用一个基于FFT的变换域匹配滤波器就可实现对M元扩频信号的解扩解调。提出了一种适用于时分双工(TDD)上行链路的基于预均衡和正交循环码的M元MC-CDMA系统。移动用户发端使用正交循环码进行M元扩频可降低选码难度。上行链路的M元MC-CDMA信号在传输前经过了预均衡处理,这就使各用户信号在到达基站接收机后的能够保持良好的正交性。基站接收机将接收信号分别与各用户的本地码相乘,然后利用OFDM解调器同时实现M元解扩和多载波解调。与传统的M元MC-CDMA系统或MC-CDMA系统相比较,新系统在误码性能、解扩处理运算量,频带利用率,选码难度和抗多径衰落等方面都具有较强的优势。提出了一种基于单片SAWTDL器件实现的M元扩频接收机方案,它使用正交循环码进行M元扩频,接收端只使用一个SAWTDL器件即可完成M元解扩,避免了器件一致性问题的困扰。同时还提出了一种采用修正的正交循环码的、低复杂度的次最优M元扩频接收机方案,它使用修正的正交循环码进行M元扩频,降低了组合相关干扰,同时使用单片低位SAWTDL器件实现对长扩频码的M元解扩,解决了SAWTDL器件位长受限的问题,而且具有较强的抗频偏能力。更多还原

【Abstract】 So-called satellite overlap communication (abbreviated as SOC) is such a communication mode that covertly shares a channel provided by a transparent transponder with other communication systems, whose communication signal is overlapped on the signals of the others but much weaker than them so that it does not affect their normal running. Therefore, SOC system can be employed as an emergency communication instrument and runs by using, without the owner’s permission, the links of any communication or broadcast satellite as long as they are located in the space area where they can be used by the user. The SOC system is a covert communication system so that it can easily keep away from destroy, jamming or intercept. Since its signal is extreme weak, it is necessary to use some special techniques, where M-ary spread spectrum technique is a very useful candidate. This dissertation researches on several key issues about M-ary spread spectrum system. The author’s main contributions are as follows.The short time spectra of single-chip waveform coded DSSS (Direct Sequence Spread Spectrum) signals and their despreaded signals are analyzed with time-frequency analysis method. A computer searching method of designing M-ary spreading codes is proposed. A wavelet packet construction method of spreading codes is also proposed, where a group of wavelet packets is employed as the radices of the spreading codes to construct a set of orthogonal codes.After thoroughly analyzing the effect of frequency-offset on zero-IF noncoherent despread/demodulator (abbreviated as NDD), a modified NDD is proposed and analyzed. Through comparing the BER performance of the proposed NDD with the conventional NDD, the result suggests that the conventional zero-IF NDD is very sensitive to frequency-offset while the proposed NDD is able to tolerate greater frequency-offset. Meanwhile, based on the equivalent property theory of vector norm, a simplified algorithm for calculating the energy of NDD signals is proposed with 1-norm method.An M-ary spread spectrum system is based on many orthogonal spreading codes, so that its despreading/demodulation appears to be quite complex computationally, which limits its wider applications. A novel M-ary spread spectrum system based on cyclic spreading codes is presented, where only one prototype spreading code is assigned to each user and the codes for different users are orthogonal or quasi-orthogonal with each other. The M spreading codes for each user to represent K bits data are generated by circularly shifting the assigned code and/or reversing its polarity. The spreading codes generated as above are called as Orthogonal Cyclic Codes (OCCs) or BiOrthogonal Cyclic Codes (BOCCs). At the receiver of the system, a transform domain matched-filter implemented by means of Fast Fourier Transform (FFT) is employed to despread and demodulate the received signals.A novel M-ary MC-CDMA system applied to the uplink of TDD (Time Division Duplication) mode is proposed, which is based on OCCs and pre-equalization. At the transmitter, every mobile user employs the OCCs as M-ary spreading codes, which can reduce the spreading codes selection difficulty of M-ary MC-CDMA system. In order to keep good orthogonal property between every user’s signals, a pre-equalizer is employed to process the M-ary MC-CDMA signal before transmitted. The received signal is multiplied by the local sequence, and then despreaded & demodulated by base-station OFDM demodulator at the same time. The proposed scheme has better performances than traditional M-ary MC-CDMA system and MC-CDMA system on BER performance, bandwidth efficiency, convenience in the spreading codes selection and anti-fading potential.A scheme of M-ary spread spectrum receiver based on OCCs and single chip SAWTDL device is proposed, which can bypass the difficulty in high consistency requirement for multiple SAWTDL devices. At the same time, a quasi-optimal M-ary spread spectrum receiver scheme is also proposed, which can despread long M-ary spreading codes with one chip short-delay-time SAWTDL device. Meanwhile, the influence of combination-correlation interfere can be reduced by employing the presented MOCCs for an M-ary spread spectrum system. The analysis of the system performance in the case of carrier frequency with offset shows that quasi-optimal M-ary spread spectrum receiver has much stronger ability against frequency offset than the optimal receiver.更多还原