阅读说明：本技术 一种基于ppm调制的差分空间调制系统的构建方法 (Construction method of differential spatial modulation system based on PPM modulation ) 是由 王惠琴 毛一聪 曹明华 马玉昆 豆红霞 吕佳芸 侯文斌 杨丽荣 于 2020-04-14 设计创作，主要内容包括：一种基于PPM调制的差分空间调制系统的构建方法,主要包括差分空间调制的比特映射和差分传输过程,接收端信号的检测与解映射、基于PPM的差分光空间调制系统误码率上界的计算。其中,比特映射是将输入的比特信息分为两部分,一部分选择空间域激光器的索引顺序,另一部分映射为激活激光器上的PPM星座映射点。将上述映射所产生的空时映射矩阵块经过差分过程后再进行信息传输。接收端采用最大似然序列检测算法完成信号的检测,并经反映射即可恢复出原始比特。在此基础上,推导了基于PPM调制的差分空间调制的误码率上界。该差分空间调制系统的构建可以有效避免复杂的信道估计,同时可提高系统的传输速率、改善系统的误码性能。(A construction method of a differential space modulation system based on PPM modulation mainly comprises the bit mapping and differential transmission processes of the differential space modulation, the detection and demapping of a signal at a receiving end, and the calculation of the upper bound of the bit error rate of the differential light space modulation system based on PPM. The bit mapping is to divide the input bit information into two parts, wherein one part selects the index sequence of the space domain laser, and the other part is mapped to the PPM constellation mapping point on the active laser. And carrying out information transmission after carrying out a difference process on the space-time mapping matrix block generated by the mapping. The receiving end adopts the maximum likelihood sequence detection algorithm to complete the detection of the signal, and the original bit can be recovered through reflection. On the basis, the error rate upper bound of differential space modulation based on PPM modulation is deduced. The construction of the differential spatial modulation system can effectively avoid complex channel estimation, and simultaneously can improve the transmission rate of the system and improve the error code performance of the system.)

1. A construction method of a differential space modulation system based on PPM modulation is characterized in that input bit information is simultaneously mapped into PPM constellation modulation symbols and orthogonal space-time coding matrixes meeting a differential process, each active laser carries different PPM constellation mapping points, and generated coding matrix blocks are sent out after the differential process and an optical antenna; the receiving end detects and de-maps the signal by using a maximum likelihood detection algorithm, and can recover original bit information; and calculating the upper bound of the theoretical bit error rate of the system under the turbulent flow channel.

2. The PPM-based differential spatial modulation method according to claim 1, characterized by the specific steps of:

the method comprises the following steps: at the transmitting end, the input random bit stream is divided into two parts, one part selects the index sequence of the space domain laser, and the other part is mapped to n through PPM modulation mode_tOn a respective active laser; generating a transmission matrix block S and a space-time mapping matrix block X which meet the requirement of differential spatial modulation through mapping;

step two: carrying out differential operation on the space-time mapping matrix block X and the transmission matrix block S to generate a new transmission matrix, namely S_t＝S_t-1X_t(ii) a That is, the transmission matrix S at time (t-1)_t-1And time t space-time mapping matrix block X_tMultiplication as the actual transmission matrix block S in the channel at the current time t_t；

Step three: the modulation signal transmitted by the log-normal turbulence channel is detected at a receiving end by utilizing a maximum likelihood sequence detection criterion, and the original bit information can be recovered after reflection; the adopted criteria are:

step four: calculating the theoretical bit error rate of the DOSM system by a method of a united boundary technology and classification discussion; part of the results generated by classification is subjected to approximate calculation through an approximate probability density function, and the other part of the results is subjected to approximate calculation by adopting a Gaussian kernel density estimation algorithm; obtaining an average bit error rate expression of each type of errors, namely:

wherein, ω is_nAnd a_nRespectively representing the gaussian-hermitian expansion weight and the abscissa factor, gamma representing the number of terms used in the expansion,andrespectively, is a Gaussian random variable U₁、U₂And U₃α represents the number of variables in the gaussian kernel density estimate,μ_KDEand b represents the mean of the gaussian distribution and the gaussian kernel density estimation window, respectively, where b is 5000,combining the above formula and calculating the false Hamming distance of each type to obtain 2 × n under the turbulent flow channel_rThe ABER upper bound of DOSM-2PPM of (A) is expressed as:

Technical Field

The invention relates to a differential space modulation method based on PPM modulation, which combines the differential space modulation technology with the PPM modulation technology, does not need complex channel estimation at the receiving and transmitting ends, further improves the error code performance of a differential space modulation system, and belongs to the technical field of wireless optical communication.

Background

With the increasing demand of terminal traffic and the number of terminals, the radio access network technology is facing a serious challenge. As a new access network technology, wireless optical communication has attracted attention of researchers due to its advantages of high transmission rate, high security, unlimited frequency band, flexibility and easy architecture. However, the optical carrier is susceptible to severe link fading caused by scattering and flicker during transmission, which further degrades the error performance of the system. In order to overcome the influence of atmospheric turbulence, the optical MIMO technology is developed. However, when all the lasers are activated at the same time, problems such as inter-channel mutual interference (ICI) and inter-antenna synchronization (IAS) are caused, thereby limiting the advancement of the optical MIMO technology. Optical Spatial Modulation (OSM), as a new MIMO technique, effectively overcomes the problems of ICI and IAS, since only one laser is activated at any time. In addition, the OSM system also carries information through a space domain and a symbol domain, and the transmission rate of the system is effectively improved. In other words, a portion of the input bits are used to select the index order of the lasers, while another portion is mapped onto the selected lasers by digital modulation. Compared with the traditional optical MIMO system, the OSM additionally transmits the null bit information in the form of the index of the laser, thereby effectively improving the transmission rate of the system.

Currently, all OSM systems are based on one assumption: accurate Channel State Information (CSI) can be obtained at both the transmit and receive ends of the system. This assumption holds when the channel changes slower than the transmission rate. In a scenario of high-speed movement, high-speed variation of the channel results in an unstable channel state, such as underwater optical communication and atmospheric laser communication. In a real communication system, channel estimation is very difficult due to the time-varying and complexity of the channel. In particular, the complexity of channel estimation also increases as the number of lasers increases. Even if accurate CSI can be obtained, the original bits may not be fully recoverable.

Differential Spatial Modulation (DSM) is effective in avoiding CSI, but conventional DSM systems are not directly applicable to the field of WOC, so the first introduction of DSM into WOC by scholars has proposed DOSM-PAM and DOSSK systems. These methods, while effective in solving the complex channel estimation, limit the transmission rate of the system by transmitting the same information on all active lasers in a transmission matrix block. In addition, as the PAM modulation order increases, the error rate of the system is further degraded. Therefore, we need to explore new improved schemes for DOSM.

Currently, intensity modulation/direct detection techniques are commonly employed in WOC systems. Common intensity modulation/direct detection methods are OOK, PPM, etc. Among them, PPM is a popular modulation technique due to high energy efficiency and inherent orthogonality. In the invention, a new DOSM method is constructed by combining PPM and space-time coding blocks. In the method, the spatial domain and the symbol domain are simultaneously subjected to differential processing, and different PPM constellation points are transmitted on all the active lasers, so that the transmission rate of the system is maximized.

Disclosure of Invention

The invention aims to provide a construction method of a differential space modulation system based on PPM modulation.

The invention relates to a construction method of a differential space modulation system based on PPM modulation, wherein input bit information is simultaneously mapped into PPM constellation modulation symbols and orthogonal space-time coding matrixes meeting the difference process, each activated laser carries different PPM constellation mapping points, and generated coding matrix blocks are transmitted out after the difference process and an optical antenna. The receiving end detects the signal by utilizing a maximum likelihood detection algorithm, and original bit information can be restored through demapping; calculating the upper bound of the theoretical bit error rate of a differential space modulation system based on PPM modulation under a turbulent flow channel; the method of the invention not only avoids the complex channel estimation at the receiving and transmitting ends, but also improves the performance of the differential system; the method comprises the following specific steps:

the method comprises the following steps: at the transmitting end, the input random bit stream is divided into two parts, one part selects the index sequence of the space domain laser, and the other part is mapped to n through PPM modulation mode_tOn the corresponding activated laser. Generating a transmission matrix block S and a space-time mapping matrix block X which meet the requirement of differential spatial modulation through mapping;

step two: carrying out differential operation on the generated space-time mapping matrix block X and the transmission matrix block S to generate a new transmission matrix, namely S_t＝S_t-1X_t；

That is, the transmission matrix S at time (t-1)_t-1And time t space-time mapping matrix block X_tMultiplication as the actual transmission matrix block S in the channel at the current time t_t；

Step three: detecting the modulated signal passing through the channel at a receiving end by using a maximum likelihood sequence detection criterion, and then recovering original bit information through reflection; the adopted criteria are:

step four: calculating the theoretical bit error rate of the DOSM system by a method of a united boundary technology and classification discussion; part of the results generated by classification is subjected to approximate calculation through an approximate probability density function, and the other part of the results is subjected to approximate calculation by adopting a Gaussian kernel density estimation algorithm; obtaining an average bit error rate expression of each type of errors, namely:

combining the above formula and calculating the false Hamming distance of each type to obtain 2 × n under the weak turbulence channel_rThe ABER upper bound of DOSM-2PPM of (A) is expressed as:

the invention has the advantages that: a difference technology is introduced, a space-time coding matrix with orthogonal property is designed, and a PPM modulation-based differential spatial modulation system is constructed by combining the characteristics of PPM modulation. The method greatly improves the error code performance and the transmission rate of the system, and effectively avoids channel estimation at the same time. The method provides an effective measure for optical communication of high-speed scenes or time-varying channels, and has a certain reference value.

Drawings

Fig. 1 is a differential spatial modulation system model, fig. 2 is a flow diagram of the method of the present invention, fig. 3 is a comparison of theoretical results and simulation results of error rate of a PPM-based differential spatial modulation system, fig. 4 is a comparison of error performance of DOSM schemes with different laser numbers, and fig. 5 is an error performance of the PPM-based differential spatial modulation system and other schemes at the same transmission rate.

Detailed Description

The invention provides a construction method of a differential space modulation system based on PPM modulation. The introduction of the differential technology effectively avoids complex channel estimation. Meanwhile, the adoption of the space-time coding block and PPM modulation greatly improves the transmission rate and the error rate performance of the differential system. The present invention will be described in detail below with reference to the accompanying drawings.

The invention is achieved by the following techniquesThe technical measures are adopted to achieve the following steps: for one has n_tA laser, n_rThe differential space modulation system of each photoelectric detector is characterized in that a system model is shown in figure 1, a sending end carries out DOSM modulation on an input binary bit stream and then maps the binary bit stream into a modulation signal of a space-time coding matrix block with orthogonality, wherein different PPM constellation mappings are adopted on an activated laser in the matrix block, the matrix block is converted into a transmission signal in an atmospheric channel through a differential process, and a receiving end carries out demapping after maximum likelihood sequence detection (M L SD) to restore original bit information.