| With the rapid development of aerospace industry in our country,various types of space exploration satellites have been more and more widely used in weather forecasting,disaster prevention,environmental protection,ocean observation,deep space exploration,and satellite communication.Vast space exploration data need to be transmitted to the ground in real time.Therefore,today's society puts forward higher requirements for the transmission rate of the satellite-to-earth communication,especially for low-earth orbit(LEO)StE optical communication with low orbit height,short transmission delay.It is expected that the downstream transmission rate of LEO StG will reach 40Gbps or even 100Gbps,LEO satellite direct to ground station optical communication without relay is an effective way to realize downstream large capacity information transmission.However,in the LEO satellite direct to ground station optical communication link without relay,the received optical signal is very weak due to the power loss caused by space loss,atmospheric turbulence,transceiver loss and so on.Thus,these reasons make enough power budget become the key factor to achieve reliable LEO satellite direct to ground station optical communication without relay(abbreviated as Low-earth-orbit Satellite-to-Ground(LEO StG)optical Communication).Although the current Gbps level StG optical communication based on intensity-modulation direct detection(IM/DD)has many advantages,such as wide available spectrum,no frequency application,and strong anti-interference,due to the low receiver sensitivity determined by direct detection method,the power budget is difficult to meet the requirements of StG optical communication systems when the information rate is as high as 40Gbps or even 100Gbps.Fortunately,the receiver sensitivity of the coherent optical communication systems is usually more than 20dB higher than that of IM/DD systems by mixing the received optical signal and local oscillator.Coherent optical communication has become an effective technical to realize 100Gbps StG optical communication.Therefore,it is necessary to utilize the advantages of coherent communication technology to develop 100Gbps StG coherent optical communication systems and design the digital signal processing(DSP)algorithms.At the same time,despite the advantage of high receiver sensitivity brought by coherent detection,the power loss of space channel without relay is serious,coherent detection still cannot meet the aggressive power budget requirements of 100Gbps LEO StG optical communication.Thus,one of the key problems in LEO StG optical communication is how to effectively improve the receiver sensitivity and suppress the atmospheric turbulence effect.Focusing on the development requirements and technical problems of LEO StG optical communication,this thesis has conducted in-depth research on the key technical problems of high-speed StG coherent optical communication systems.The main research work and innovations are as follows.1.Considering that the IM/DD is difficult to meet the power budget demands of 100Gbps LEO StG communication and the atmospheric turbulence seriously deteriorates the system performance,the constellation shaping and diversity reception based polarization multiplexing 16-ary quadrature amplitude modulation coherent optical orthogonal frequency-division multiplexing(PM-16QAM-CO-OFDM)system and DSP compensation algorithms based on training sequences are designed for downstream 112Gbps and upstream 28Gbps LEO StG optical communications.Based on the phase screen simulation of atmospheric turbulence,the downstream 112/upstream 28Gbps PM-MQAM-CO-OFDM LEO StG simulation platform is built,and the performance of proposed system scheme and DSP algorithms are simulated and analyzed.The simulation results show that the designed 112/28Gbps PM-16QAM-CO-OFDM communication system can meet the power budget requirements of LEO StG optical communication under the conditions of downstream "constellation shaping combined with 4 diversity reception,launch power 1W/30dBm,receiving aperture of 31.8cm and divergence angle of 30urad";upstream "constellation shaping,launch power 3 W/34.8dBm,receiving aperture of 20cm and divergence angle of 15urad",and there is still a power margin of downstream 3.2dB/upstream 2.0dB under strong turbulence condition(the atmospheric refractive index structure constant Cn2 is 6.5×10-14m-2/3).2.In response to the urgent needs to improve the receiver sensitivity,especially the sensitivity to phase noise in CO-OFDM LEO StG optical communication systems.In this thesis,we propose a phase uniformly distributed circular MQAM combined with probabilistic shaping(PS)scheme.In such scheme,the constellation points of each ring are uniformly distributed to maximize the phase space of adjacent constellation points,and the radius ratio of the circular 16/32QAM is optimized to maximize the minimum Euclidean distance(MED)under the limitation of the average transmit power.Besides,to further increase the MED under the constraint of average transmit power,the probability shaping is performed on such circular 16/32QAM constellation.The proposed circular QAM hardly increases the implementation complexity of CO-OFDM system,while the complexity of DSP algorithm increased by PS is less than 5%.The simulation results show that at the same information entropy,the linewidth times symbol duration product ?v·Ts tolerance is increased by 12.5%and 50%at the cost of 1dB power penalty for constellation shaping based 14GBaud PM-16/32QAM-CO-OFDM downstream StG optical communication systems,and the receiver sensitivity of the shaped 16/32QAM is improved by about 1.2dB and 2.2dB.In addition,the off-line experimental platform is built by using the space light modulator to simulate atmospheric turbulence,the experimental results are consistent with the simulation results,which verifies the effectiveness of the proposed scheme.3.In the high-speed downstream LEO StG optical communication diversity reception systems,due to the independent atmospheric channels,the phase between different branches is not synchronized,and thus,there is inter branch interference caused by relative phase offset(RPO),which decreases the diversity gain.In this thesis,the influence of the number of diversity channels,atmospheric turbulence intensity and receiving aperture on the diversity gain is theoretical analyzed,and a shared local oscillator(LO)single input multiple output(SIMO)diversity reception PM-CO-OFDM system based on group timing synchronization(GTS)and diversity branch phase correction(DBPC)in StG optical communications is proposed.In GTS,the relative delay caused by timing synchronization error is eliminated by finding the peak value of the sum of autocorrelation function that from all branches;taking the characteristic of that the phase noise introduced by frequency offset and linewidth between different branches is the same for shared LO diversity reception systems,in DBPC,the received branch signal with largest signal-to-noise ratio is used as the reference signal to calibrate the phase difference between the other branch signals and the reference signal.The 112Gbps PM-16QAM-CO-OFDM downstream StG optical communication simulation results show that the proposed GTS and DBPC mechanisms can effectively eliminate the RPO between different branches.The simulation results also show that the diversity gain under different conditions is as follows:at the forward error correction threshold of 3.8×10-3,the receiver sensitivity is improved by 2dB,10.3dB and 8.1dB under weak,moderate and strong turbulence condition(the atmospheric refractive structure constant Cn2 is 6×10-16m-2/3,5×10-15m-2/3 and 6.5×10-14m-2/3 respectively)when the receiving aperture is 31.8cm and 4 diversity reception is employed. |
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