Research On Suppression Of Near-Far Effect For Co-Band Positioning System

The Co-Band positioning system based on the communication network can realize high-precision positioning of indoor and outdoor environments.Its positioning signal and communication signal are carried at the same frequency.The positioning signal is embedded in the background noise of the communication signal with extremely low power.Using this mode,the positioning signal neither occupies a new frequency band nor interferes with the communication signal.The Co-Band positioning signal adopts the CDMA modulation method.Due to the bound theory,different pseudo-random codes in the code group face the problem that they are not completely orthogonal to each other.When the terminal receives the positioning signals from the distant and nearby base stations at the same time,the strong signal will cause multiple access interference to the demodulation of the weak signal.These problems have become bottlenecks that prevent terminals from using weak signals to perform positioning calculations.In recent years,researchers have tried to solve the near-far effect problem from the system level and signal processing level.On the one hand,they seek to change the positioning signal system,making that different pseudo-random codes in the code group tend to be orthogonal.On the other hand,they try to reduce the influence of strong signals on the demodulation of weak signals in signal processing algorithms.The breakthroughs in these two aspects have improved the ability of Co-Band positioning terminals to use weak signals to perform positioning calculations in interference environments.This paper will conduct research on multiple access interference and pseudo-random code orthogonality in Co-Band positioning system.The main research contents and innovations are as follows:1.Aiming at the problem of multiple access interference caused by difference in the power of the signals received by the terminal from the distant and nearby base stations,this paper proposes a near-far effect suppression technology based on the Intermediate Frequency Interference Cancellation(IFIC)algorithm.Reconstruct the strong signal and then cancel it in the intermediate frequency signal.The cancellation result enters a new channel to capture and track the weak signal.This process will no longer be affected by the strong signal.The simulation results show that the IFIC algorithm has better noise performance and lower carrier loop phase error than the serial interference cancellation method and the subspace projection method,and effectively reduces the multiple access interference in the signal processing process.2.Aiming at the problem of incomplete orthogonality between different pseudo-random codes in the Co-Band positioning system,this paper proposes a local pseudo-random code generation based on transform coefficient(PGTC)model.By weighting the transform coefficient to a standard pseudo random code,the receiver replicates the new constructed pseudo random code locally.The new code tends to be orthogonal to the pseudo random code of the strong signal in the received signal,thereby suppressing the cross-correlation interference.Simulation results show that this model has lower correlation function sidelobe value and lower code phase measurement error than serial interference cancellation method and subspace projection method.It effectively solves the problem of incomplete orthogonality between different pseudo-random codes in the Co-Band positioning system.3.This paper completes the design and implementation of the software and hardware of the Co-Band positioning signal generating unit based on the FPGA+AD9361 chips.A Co-Band positioning test platform is built to verify the IFIC algorithm and PGTC model proposed in this paper.The test results show that compared with the serial interference cancellation method and the subspace projection method,the IFIC algorithm proposed in this paper has a smaller noise in the IQ demodulation result of the weak signal tracking loop,and its carrier frequency,Doppler frequency results are more accurate.It illustrates that the strong signal component in the weak signal solution channel is effectively suppressed.Compared with the serial interference elimination method and the subspace projection method,the PGTC model proposed in this paper achieves stable tracking of weak signals in the tracking loop with the least time.By using PGTC model,the tracking loop can quickly and accurately lock the weak signal autocorrelation peak,and the phase detection error of the code ring is small.It illustrates that PGTC model effectively solves the problem of incomplete orthogonality of the pseudo random codes.The test results verifies the performance of the joint scheme of the IFIC algorithm and the PGTC model.Compared with the single scheme,the joint scheme further improves the near-far effect suppression effect.