Using Earth Station Time Synchronization System to Achieve Large-Capacity Communication under the Constraint of High-Precision Ranging

The CDMA earth station time synchronization networks using GEO satellite transponder are based on Two-Way Satellite Time and Frequency Transfer. The spectrum of the ranging and communication signals is overlap, and each signal power is equal. As the effect of multiple access interference, the equivalent carrier to noise ratio of the earth station received signal is not linear growth with the increase of single signal transmission power, but exist a maximum. The maximum value of equivalent carrier to noise ratio of the received signal decreases when the number of earth station increases, which will reduce the ranging accuracy and increase the bit error rate, so the capacity of system communication reduced. To solve this problem, we propose a new method in the condition of fixed transponder bandwidth. First, it uses the signal processing method of parallel interference cancellation in the receiver to suppress the multiple access interference. Second, in the aspect of the system design, under the premise of ensuring high-ranging accuracy, appropriately reducing the spreading code rate, and partially separating the spectrum of the ranging signals and communication signals. After separating the spectrum, the user number of each spectrum is half of the total user numbers. It reduces the multiple access interference of the received signal in each spectrum. It obtained the optimal spread code rate by analyzing in this paper. It only need eliminate the multiple access interference within a single spectrum after separate the spectrum. As the absence of eliminating the multiple access interference between two spectrums, comparing with directly using parallel interference cancellation, the received equivalent carrier to noise ratio decreased to less than 1dB after interference cancellation, while the computational complexity reduced to one-third of the original.