Understanding fundamental limits of the antiinterference performance of direct sequence spread spectrum (DSSS) systems is essential for efficient state-of-the-art antiinterference implementation. Existing research contributions on anti-interference implementation mainly aims at blocking interference and DSSS with long period sequences (LPS-DSSS). However, broadband interference, due to high energy efficiency, is a more effective interference mode to DSSS systems. Conventional bit error rate (BER) models bring errors during the despreading process in DSSS with short period sequences (SPS-DSSS). Therefore, in this paper, we focus on the BER performance of SPS-DSSS systems. More particularly, we propose the maximum matching decision criteria (MMDC) based statistical BER model that is capable of accurately evaluating the BER performance for both LPS and SPS systems. Firstly, explicit mathematical model of the despreading of MSK signals in DSSS systems is established; Then, mathematical models of despreading under typical interference modes, i.e., multi-tone and pseudo-random noise (PN) code MSK interference, are derived. Finally, statistical BER model is proposed using the MMDC scheme to evaluate the BER performance. Extensive simulation and experimental results reveal that: 1) the proposed statistic BER model outperforms conventional BER model for SPS-DSSS systems, and is consistence with that for LPS-DSSS; 2) for MSK DSSS systems, PN code MSK interference outperforms multi-tone interference in the BER performance; 3) practical tests demonstrate that the same PN code length interference is capable of obtaining higher BER performance than different PN code lengths interference, due to the strong correlation between the interference signal and the desired signal in DSSS systems.

The interference cancellation ratio (ICR) is a key performance indicator of digital-to-analog hybrid radio frequency (RF) interference cancellation systems. Aiming at the low convergence speed of a digital-to-analog hybrid RF interference cancellation system based on a multi-tap structure (MDARFICS), a novel, fast, and robust variable-step-size least-mean-square (LMS) algorithm based on an improved hyperbolic tangent function (IHVSS-LMS) is proposed. The IHVSS-LMS algorithm adopts an improved hyperbolic tangent function and uses adjustable parameters and the iteration number to jointly adjust the step size, which improves the convergence speed and reduces the computational complexity. Moreover, by using the prior information of the input signal, the non-linear relationship between the step size and the input signal power is established, which enhances the robustness and the ability to suppress interference with mutable power. The IHVSS-LMS algorithm is applied to the MDARFICS. Through theoretical derivation, the convergence speed and the steady-state expressions of the interference cancellation ratio of the MDARFICS are obtained. The simulation results show that under the conditions of high and low signal-to-noise ratio (SNR), the robustness, convergence speed, and steady-state error performance of the IHVSS-LMS algorithm are better than the existing variable-step-size algorithm. The experimental results show that using the IHVSS-LMS algorithm, the MDARFICS can not only effectively accelerate the convergence speed by at least three times but can also improve the ICR by more than 3 dB.

Adaptive interference cancellation is an effective means to fulfill the anti-interference requirements of communication systems. The radio frequency (RF) front-end gain plays a crucial role since it determines the communication sensitivity and anti-interference performance, especially in the presence of both high-power interference signals and weak communication signals. In this paper, the influences of the RF front-end gain on communication sensitivity as well as interference cancellation ratio (ICR) are thoroughly analyzed. Thereinafter, we derive the mathematical model of the RF front-end gain constrains taking communication sensitivity, ICR requirement and analog-to-digital converter quantization accuracy into consideration. Based on the upper and bottom boundaries gain analysis, we propose the gain design criterion for adaptive interference cancellation in communication systems. Extensive simulations demonstrate that: 1) the ICR performance improves with the increase of RF front-end gain when lower than the gain obtained from upper boundary analysis, and is saturated afterward; 2) the gain obtained from bottom boundary analysis can ensure the communication sensitivity. Experimental results verify that the gain design criterion ensures the communication sensitivity and satisfies the requirements of anti-interference performance under high-power interference signals.