Research On Suppressing Interference In Spread Spectrum Communication Systems Based On Subspace Analysis

Spread spectrum communication systems have many advantages, including low probability of intercept transmission, multiple accessing, multipath signal suppression. It has widely used in personal communication network, wireless mobile communication and military tactical communication. For the restriction of spread spectrum band width, spread spectrum system can't suppress the interference only depending on spread spectrum gain especially in the presence of strong interference. As is well known, the inherent processing gain of spread spectrum systems can provide the system with interference rejection capability. But if the level of interference is too high, the interference immunity can be improved significantly by using signal processing techniques that complement the spread spectrum modulation.First, the thesis introduces basic theory of direct sequence spread spectrum, frequency hopping spread spectrum and direct sequence/ frequency hopping communication system, analyses their principle of interference suppression and the models of narrowband interference, and simulates and analyses the interference suppression performance of them at last.Then, a low complexity method of interference suppression is proposed, based on singular value decomposition (SVD) technique and iteration. The analysis of correlation matrix of the received signal proves that the energy of DSSS signal is uniformly distributed in the whole signal space, while the energy of narrowband interference (NBI) is only in the low-dimensional subspace. NBI suppression can be achieved by estimating the interference subspace and projecting the received signal to the corresponding orthogonal complementary subspace. In this paper, we propose a low-complexity interference suppression algorithm. The algorithm first computes the dimensions of the interference subspace and estimates this subspace iteratively. At last, we project the received signal to it's orthogonal complementary subspace to suppress the interference. Simulation results show that this algorithm has low computational complexity and can reach the idea performance gradually by iterating.Applying above algorithm to DS/FH slow frequency hopping system, the simulation results show that it can also suppress the narrowband interference. Frequency hopping communication has good ability of suppressing the narrowband interference, and this ability is enhancing as the jumping speed becomes higher. The above iterative SVD technique can not apply in the DS/FH fast frequency hopping system (per bit distributes two hops at least). Algorithm here utilizes SVD and diversity combining technique in DS/FH fast frequency hopping system, using the singular value decomposition technique and the diversity feature of fast frequency hopping system to suppress narrowband interference of hopping signals. After the detection, it will combine the hopping signals that are not jammed by interference to despread. The simulation results show that this algorithm can suppress the narrowband interference effectively.