In the case of grant free, due to the lack of synchronous time signaling, the problem of asynchronous reception in Sparse Code Multiple Access（SCMA）system cannot be ignored. The existing Belief Propagation Message Passing Algorithm (BP-MPA) can detect asynchronous SCMA, but the complexity is high. Aiming at the problem of high complexity of detection algorithm in uplink asynchronous SCMA system, this paper proposes an Improved Belief Propagation Message Passing Algorithm (IBP-MPA) by selecting some highly reliable user nodes not to participate in the iteration of message passing algorithm according to the judgment of each resource on the probability of user sending codeword. The simulation results show that under the same conditions, IBP-MPA can greatly reduce the complexity of BP-MPA while maintaining the detection performance.

Polar codes is the only coding scheme in the world that has been proved that the Shannon threshold is reachable. Because of the high time delay caused by the serial characteristic of Successive Cancellation (SC) decoding, the Belief Propagation (BP) decoding has received more attention because of its higher throughput. However, because its decoding performance is not as good as SC decoding, Bit-Flip (BF) decoding is used in BP. Aiming at the inaccurate construction of flip set in current bit flipping decoding, this paper proposes a new bit error rate BP BF (BPF) decoding algorithm, which is used to construct flip set. We also propose rough flip set and fine flip set Concept of flip set. The rough flip set is constructed by the variance of the characteristics of data stability. The range of the flip set is reduced by the difference in bit error rate, that is, the fine flip set. The research results show that the algorithm proposed in this paper is superior to the current best decoding algorithms such as BP, Enhanced BPF (EBPF) and Log Likelihood Ratio BPF (BPF-LLR). At the same time, the number of iterations of the algorithm is significantly reduced compared with other bit-flipping decoding algorithms.

For millimeter-wave large-scale Multiple-Input Multiple-Output (MIMO) systems with hybrid precoding, due to the hardware defects in the system, random phase shifts will be introduced in the received pilots, which will make the performance of traditional channel estimation algorithms become very poor. In some extreme conditions, even the channel information cannot be estimated. In order to solve this problem, this paper uses pilots from different radio frequency chains to have the same phase offset in the same time period, and pilots in different time periods have different phase offset characteristics to estimate the channel information with an Iterative Reweight (IR) based Partially Coherent-Compressive Phase Retrieval (PC-CPR) (IR-PC-CPR) algorithm. The algorithm first performs phase offset compensation on the received coherent pilots, and then uses an iterative reweight (IR) algorithm to estimate channel parameters to achieve high-precision channel estimation. The simulation results show that the algorithm proposed in this paper has better performance than the PC-CPR algorithm.

In view of the complexity of theoretical calculation and long calculation time of traditional evaluation method of optical fiber link transmission performance in coherent optical communication system, this paper uses machine learning technology to build correlation model and accurately evaluate the transmission performance of coherent optical transmission system with different modulation format and input optical power parameters. Through the training model, the results show that the machine learning model can replace the complex theoretical calculation, and can more accurately predict the predicted value, The overall average error between the theoretical value and the predicted Q value is 0.27 dB. The overall average error between the theoretical value and the predicted Optical Signal to Noise Ratio (OSNR) value is 0.33 dB. The calculation time of machine learning model is about 4.4 s, while the theoretical calculation time increases with the increase of the number of interference channels.

Future information networks will place higher requirements on the accuracy and reliability of time synchronization. In order to study the potential security risks of the time synchronization protocol, an experimental simulation analysis of the packet attack and delay attack of the Precise Time Protocol (PTP) is carried out. In order to solve the problem that the traditional encryption method cannot resist the delay attack of the time synchronization protocol, a quantum-enhanced secure synchronization protocol is proposed, which combines PTP with quantum key distribution technology to satisfy future network synchronization reliability requirements. On the basis of defending against message attacks by encryption, it implements detection and early warning of delay-type attacks and triggers a protection mechanism. The quantum-enhanced secure synchronization protocol can actively prevent security attacks against time synchronization protocols, and realizes secure transmission of time synchronization information.

In order to solve the problems of three-dimensional active optical network, such as long path configuration time, high insertion loss of optical router and high network hardware overhead, this paper designs adaptive routing algorithm and non-blocking, low-loss optical router. The adaptive routing algorithm uses the XYZ determines the routing algorithm to make the first judgment of the port. Then it make a second judgment according to the blocking condition, and finally choose the non-blocking and short distance port as the node output port. Optical routers with 6 ports in the layer and 3 ports in the layer are designed to replace optical routers with 7 ports for three-dimensional communication. The experimental results show that when the adaptive routing algorithm is used for routing calculation, the delay of path establishment is small and the blocking is low. The number of waveguides and micro-ring resonators required by optical routers is small, and the insertion loss is small. The average insertion loss is only 0.88 dB and the optical network hardware has low overhead.

Aiming at the problem of long-distance laser receiving and alignment, a laser bridge system using large area avalanche photo diode is proposed in this paper, which can be used as the extension of Wide Area Network （WAN）and the connection between Local Area Networks（LAN）. The software of the laser bridge system needs a computer as a server for information processing. The hardware connection is realized through the switch and electro-optic conversion, and based on two-way laser lenses placed in two buildings for transmitting and receiving signal. The distance between the two buildings is about 1 km, and the geometric loss of the laser is about 26 dB/km. By adjusting the relative position of the two lasers until the optical power received is less than -30 dBm, the threshold optical power required is realized, which can perform two-way video communication successfully.

With the construction of State Grid informatization, cyberspace security incidents occurs frequently. Therefore, it is important to develop secure and reliable quantum communication technologies for the power grid. In this paper, from the perspective of the multi-terminal quantum key application, a handheld meter reading solution based on quantum private communication technology is proposed to effectively achieve the technological upgrading. First, from the technical theory and application fields, the current situation of quantum private communication technology and its feasibility of combining with classical facilities are introduced and analyzed. Then, considering the quantum key management system as the core, we design and implement quantum key secure storage and business security process application model. Finally, based on the overall environment of quantum key distribution, the architecture design of multi-terminal quantum key application verification is implemented to verify the quantum key business application process.

The fiber Bragg grating sensor measures the value according to the wavelength variation. Therefore, it is of great significance to improve the wavelength demodulation accuracy of the fiber grating. Most of the traditional demodulation methods have a single structure, which results in poor measurement accuracy for overlapped spectrum and distortion spectrum, and limits the improvement of demodulation accuracy. The high-precision wavelength demodulation method mainly solves the problems of poor system temperature stability and low peak-finding accuracy. This paper introduces the application of fiber grating wavelength demodulation technology in communication, and reviews the high-precision fiber grating wavelength demodulation methods in recent years, including hardware methods of reference fiber gratings, Michelson interferometers and gas cells, and software methods based on neural networks and particle swarms. The principle of each demodulation method and its advantages and disadvantages in the field of fiber grating sensing are described, and the problems and development trends of fiber grating demodulation methods are analyzed.

Due to the small mode field of silicon photonic chips, the large coupling loss is a road-blocker for their applications. In this work, the coupling efficiency between a silicon photonic modulator chip and a single-mode optical fiber is investigated. The mode field matching theory is analyzed at first. Then the mode field test of silicon chip and single-mode optical fiber for coupling is carried out. Next, a low-cost coupling method to improve coupling efficiency is proposed. The experimental results show that the coupling efficiency of the single-mode optical fiber and silicon photonic chip reaches 70%. The optical module is made by using this scheme, and the 4 Pulse Amplitude Modulation (PAM4) eye diagram is tested. The experimental results show that better eye diagram can be obtained.

In order to obtain a high-quality ultrashort pulse light source, we combine the self-similar pulse with the grating pair, and analyze the evolution and compression process of the self-similar pulse in detail through a numerical simulation method. We first launch the Full Width at Half Maximum (FWHM) of 1 ps, energy of 45 pJ Gaussian pulses into a dispersion decreasing fiber to generate the chirp self-similar pulse with strong linear characteristic. Then we use the grating self-similar pulse chirp compensation compressor to obtained an ultrashort pulse with FWHM of 80 fs and peak power of 507.6 W. The compression quality parameter Q is 0.96, and the compression factor is 12.5. On this basis, we analyze the influence of the third-order dispersion in the declining fiber on the compression quality of the grating, and find that as the third-order dispersion increases, the compressed pulse quality will be significantly reduced. The results show that when the third-order dispersion effect in the system is small, high-power ultrashort pulses can be obtained by using self-similar pulses and grating pairs. This result is helpful for the acquisition of ultrashort pulses.

In inter-satellite laser communication, four-quadrant detector is often used in precise tracking unit for spot location. The positioning error directly affects the angular measurement accuracy of fine tracking unit. In order to achieve the tracking precision of micro-radian magnitude, this paper studies various noise factors that affect the positioning accuracy of the detector, simplifying the analysis according to the practice of inter-satellite laser communication, and putting forward the specific calculation method of positioning error. Finally, we evaluate the influence degree of each factor through a specific example. The results show that the signal-to-noise ratio of the received signal increases with the increasing of emitting power in a certain range with reduced positioning error. However, positioning error does not decrease linearly with the increase of transmitting power. When the transmitting power increases to a certain extent, the reduction of tracking error is limited. At this time, it will be very expensive to improve the tracking precision simply by increasing the transmitting power.

Some nodes and other network nodes are within the communication range of each other in the normal data transmission process, which may lead to interference in the data transmission. In order to solve the problem of data interference in the Media Access Control (MAC) protocol of multi- Personal Area Network （PAN） Terahertz Wireless PAN （WPAN） in the normal channel time allocation period, a low interference multi-PAN MAC protocol for Terahertz WPAN is proposed. By classifying normal nodes, the concept of edge node type is proposed to find out the normal nodes that may cause interference. Adaptive normal node type reporting mechanism is applied, which may inform the Piconet Coordinator （PNC）of the node type of the normal node. A new mechanism for normal node slot allocation is also used, which enable the PNC to allocation slot resources according to the normal type. Therefore, the interference of conventional nodes in the process of data transmission is reduced. The data transmission success rate and network throughput are improved effectively, and the average delay is reduced.

Antennas play a key role in communication systems, but single-band antennas no longer meet today's needs. Therefore, the study of multi-band antennas is necessary. In this paper, a multi-band microstrip antenna based on coplanar waveguide feeding is proposed. The microstrip antenna consists of an annular slit etched on the conductive surface of the substrate as the initial structure. Then, electric dipole, metamaterial Complementary Split Ring Resonator (CSRR) with Inter-Digital Capacitance Loaded Loop Resonator (IDCLLR), and a U-shaped slot are loaded at different positions of the substrate to achieve multi-frequency resonance of microstrip antenna. The results show that the antenna is capable of operating simultaneously in the Wireless Local Area Network (WLAN) band (2.42~2.51 GHz), World Interoperability for Microwave Access (WIMAX) band (3.63~3.74, 5.45~5.66 GHz), the uplink frequency for satellite communication services (3.94~4.08 GHz), International Mobile Telecom System(IMT-2020 (5G)) band ( 4.84~4.98 GHz), satellite communication service downlink frequency (5.88~6.24 GHz) and X-band (7.19~7.85 GHz) resonance in seven frequency bands. The return loss of the antenna is tested by vector network analyzer, and the test results are in good agreement with the simulation results, which has good engineering significance.