摘要
跳频是一种扩频通信技术(FHSS),频点的随机跳变使得整个系统工作在较宽频带内；跳频也可以作为一种码分多址接入方式(FH-CDMA或FHMA),多个用户按照预先分配的跳频序列共享这个频带。跳频系统具有抵抗干扰、抵抗多径衰落和多址接入能力强等优点。基于这些优点,跳频系统可以方便地与现有通信系统相结合,实现高保密性、较好通信性能的混合通信系统。设计适合未来高速无线通信需求的跳频多址接入系统是本文研究重点之一。多用户干扰(MAI)是影响码分多址系统性能的主要因素,如何消除多用户干扰并提高跳频系统误码率性能是本文研究另一重点。最后,本文将传统的跳频技术应用于一类新型的能源网络——智能电网,设计出适合智能电网通信需求的跳频通信网络结构和跳频序列。
在高速无线通信系统中,以OFDM为代表的多载波技术能有效地抵抗信道衰落和符号间干扰。基于OFDM和FH两种通信技术的优势,本文提出一种新型的跳频多址接入方式——OFDM-FHMA通信系统。为了降低频带占用宽度并提高误码率性能,该混合通信系统采用相位调制(PSK)和相干解调方式。另外,为进一步消除多径衰落对系统的影响,在OFDM-FHMA系统中设计了分集发送方式和合并接收机。对OFDM-FHMA的研究主要分成三个部分：第一部分,研究随机跳频方式下同步接入OFDM-FHMA系统在AWGN信道和平坦Rayleigh慢衰落信道时的理论误码率性能；建立误码率与跳频频点个数、用户数、分集级数和信道等参数之间的数值关系。这部分研究对OFDM-FHMA系统的设计具有普遍的指导意义。第二部分,由于在跳频频点的随机跳动,很难保证系统接收端完全实现相位相干解调,不可避免地会产生相位误差。因此,进而研究相位误差对部分相干OFDM-FHMA系统性能的影响,并提出采用最大比值合并接收(MRC)方法减少相位误差对系统性能影响。最后,研究准同步接入方式的OFDM-FHMA通信系统误码率性能。为了降低多用户干扰,提出将一类新型的无碰撞区(NHZ)跳频序列应用于该系统。通过仿真分析给出三种典型跳频序列——完全随机跳频方式、NHZ序列和RS码,在不同接入时延下的误码率性能。通过这三个部分的研究,比较全面地解决了基于相位调制OFDM-FHMA系统的一系列问题。
目前,大多数对跳频系统性能的理论研究均采用完全随机跳频方式。然而,采用随机跳频方式的FHMA系统的多址干扰(MAI)完全受到跳频频点集合大小限制,只有增加跳频频点个数才能从根本上提高系统性能。在无线通信系统中无限地增加频点个数是不现实的,因此采用具有较好汉明相关特性的伪随机跳频序列是提高系统性能最有效的方法之一。本文在准同步接入的频率调制跳频多址接入系统(FSK/FHMA)中采用无碰撞区跳频序列(NHZ)。着重分析了特定NHZ跳频序列在无碰撞区内和略超过无碰撞区时多用户干扰模型,并给出MFSK/FHMA-NHZ系统判决变量表达式。采用特征函数方法推导了基于NHZ跳频序列的准同步接入BFSK/FHMA系统误码率性能。本文结论充分体现了相对接入时延、跳频序列汉明互相关特性和系统误码率之间的数值关系。
智能电网是下一代电力传输网络的发展方向。它通过融合当今先进的传感器技术、控制理论和无线通信技术,以提高现有电网中电能使用率,减小断电率并使其具有自动检测等功能。智能电网中通信网络的安全性和多级QoS传输保障是两大主要问题。基于跳频技术安全性高等优势,本文设计出基于跳频技术的智能电网无线通信物理层结构,并从通信需求、数据传输方式、序列构造、系统性能分析等方面系统地阐述了该新型通信网络结构。首先,根据智能电网中有用信息的产生特点,提出了一种传输效率高、频带占用少的智能电网数据发送模型。通过对电能消耗的实地测量,该数据发送模型可以近似建模为Poisson过程。另外,为实现智能电网中多级QoS要求,本文提出一种具有多级汉明互相关值的新型跳频序列,并基于NHZ序列的构造思路给出一类具有两级汉明互相关值的跳频序列构造算法。基于该新型跳频序列和数据模型,推导了采用BFSK调制时智能电网跳频通信网络的误码率性能。为了简化计算复杂度,利用平均碰撞概率近似伪随机跳频序列的实际碰撞概率。通过数值计算与仿真分析可以看出,该新型跳频系统可以为用户提供多级的误码率性能,即实现了智能电网通信系统多级QoS传输要求；同时也证实了本文提出的跳频技术通信网络与智能电网能够平滑融合。
Frequency-hopping (FH) is one kind of spreading spectrum (SS) communication technologies, where the frequency is hopped randomly from one slot to another within a wide range of spectrum. FH is also considered as a code devision multiple access (FH-CDMA, or FHMA) technology, in which the entire spectrum is seprated and assigned to the different users by using their presigned hopping codes. Due to the inherent advantages of FH technology, such as anti-fading, anti-jamming, multiple-access capacity and so forth, it is desired to combining the FH technology with the exisiting communication system to form novel hybrid communication systems with high security and good performance. One of main insterets in our paper is to design a novel hybrid FH system which could meet the demands in future communication systems. It it well known that multiple-access interference (MAI) is the predominant factor reducing the CDMA system performance. Thus, another interest of our paper is to analysis the performance improvement of FHMA systems with a general orthogonal hoppin sequence. Finally, based on security and QoS requirements, the FH technology is firstly proposed for the wireless transmission scheme in a promising power grid——smart grid. The infrastuacture of the proposed network and the specific FH sequence code, which are suitable to the smart grid, are designed in our paper.
In high-speed wireless communication systems, the orthogonal frequency devision multiplex (OFDM) is considered as one of most promisisng technologies due to its advantages of anti-fading and compressing inter-symbol interference. Based on the advantages of FH and OFDM, a novel FHMA system named OFDM-FHMA system is proposed in our paper. The phase shift keying (PSK) modulation and coherent demodulation scheme are usually adopted in this system for the small spectrum width and the low error probability.
The study of OFDM-FHMA system in this paper includes the following three sub-parts. Firstly, the theoretical performance of synchronous OFDM-FHMA system with maximum ratio combining (MRC) diversity technique is analysed over AWGN and slow Rayleigh channels. The theoretical results show the relationship between the error rate and other parameters, such as the number of users, the order of diversity, the number of frequency slots and so forth. Our results provide the performance criteria for designing the OFDM-FHMA system. Secondly, due to the presence of AWGN in carrier recovery loop (e.g., phase-locked loop) and the random hopping of the frequency, the resulting steady-state phase error is appreared definitely. The effect of phase error on the OFDM-FHMA system is analysed. MRC diversity technique is proposed in this system to compress the effect of phase error. Finally, the theoretical performance of OFDM-FHMA system with quasi-synchronization multiple-access scheme and random hopping pattern is studied. In order to reduce the MAI effectively, a novel general orthogonal hopping sequence, e.g. no-hit zone (NHZ) code, is proposed into the system. The error probability of the system with the following three classic hopping sequences, i.e., random pattern, NHZ code and Reed-Solomen code, are presented respectively for various multiple-access delays by using the simulation method. Through above three parts of research, the related issues of OFDM-FHMA system have been addressed throughoutly.
Most of researches on the theoretical performance of FHMA systems employ the completely random hopping pattern; however, the performances (or MAI) of such systems are uniquely restricted by the size of the frequency slots, which imply that more slots, better performance. It should be noted that the frequency slots could not be incresead without any limitation in wireless communications. Thus, to employing the hopping sequence with the idea Hamming properties in the FHMA system is one of most effective methods to reduce the MAI. In our paper, we propose the NHZ code for the MFSK/FHMA system. Based on the specific NHZ properties, the MAI of MFSK/FHMA-NHZ systems are derived detailedly when the delay is restricted within or slight-outside the no-hit zone. Then the decision variable of the non-coherent MFSK/FHMA-NHZ system is presented, which is utilized to estimate the error probability through semi-analytic Mente Carlo simulations. Subsequently, the theoretical bit error rate (BER) of quasi-synchronous BFSK/FHMA-NHZ system is derived by characteristic function method. The BER expression is the function of cross-correlation value of NHZ code, the relative delay and other system parameters. For comparison, the performance of MFSK/FHMA system with Markov hopping pattern is analyzed as well.
Smart grid has been considered as a promising technology in the next generation electric power system, which can smoothly integrate advanced sensing technologies, automated control methods and modern communication technologies into the power grid to address the issues introduced in the exiting power grid. Security and quality-of-service (QoS) are two critical issues in smart grid communication networks. Considering the security in FH technology, we design a novel physical-layer infrasctructure of smart grid communication, which is named the FH-based smart grid communication system. Based on the characteritics of useful data in smart grid, the specific transmission scheme, which occupies small portion spectrum, is proposed in our paper. By using the real measurement, the statistic characteristics of the data collected from power grid is modeled as a general Poisson process. In addtion, to realize the multi-level QoS requirement, we propose a new hopping sequence set, which has multi-level of cross-correlation properties. A design algorithm of sequence set with two-level cross-correlation properties is presented. Finally, by using the proposed sequence set and data model, the theoretical BER of the FH-based smart grid communication system with BFSK modulation over slow Rayleigh fading channel is derived. For reducing the complexity of computation, we use the note of the average hit rate to approximate the real hit rate of the specific sequences. In a word, some critical issues in FH-based smart grid communication network, such as, communication requirements, transmission scheme, sequence design, system performance and so forth are throughoutly analyzed. Moreover, the real simulation and numerical results verify that the FH technology could smoothly integrate with smart grid.
引文
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