## 1 Introduction  
The increasing lack of spectrum resources has become a bottleneck, restricting the development of the fifth generation (5G) of mobile communication technology [1; 2; 3]. Therefore, alleviating the shortage of wireless spectrum resources and improving the utilization of spectrum resources has become one of the important problems of current wireless communication research. Co-time Co-frequency Full Duplex (CCFD) transmission technology, one of the key 5G technologies, can alleviate this lack of spectrum resources to a certain extent [4]. Transceivers in CCFD systems can occupy the same frequency band to transmit and receive data from the uplink and downlink at the same time. Compared to traditional half-duplex systems, such as a frequency division duplex (FDD) system and a frequency division duplex (TDD) system, the spectrum efficiency of the CCFD systems can be double. This increase has a significant advantage on throughput by reducing congestion [5; 6]. However, the data from the uplink and downlink are transmitted while in the same frequency band, and the near-end receiver of the full duplex system will be subjected to high-power interference from the local transmitter. We called this interference self-interference (SI) [7]. Suffering from the limited quantization dynamic range of analog-to-digital converters (ADCs), the interference to signal ratio (ISR) of the ADC input cannot be too large or it will cover the desired signal with additional quantization noise [8]. Therefore, in order to ensure that the ADC is not blocked, the receivers of the CCFD system need to suppress SI signals in the radio frequency (RF) domain before quantization of the ADC [9; 10]. Thus, the purpose of this paper is to suppress strong SI signals in the RF domain of the CCFD system. At present, most of the studies in the field of RF SI cancellation in the CCFD system make use of direct radio frequency coupling cancellation (DRFCC) structures [11]. The basic idea of SI using DRFCC is to directly couple a part signal of the RF channel to the transmitter as a reference signal. By adjusting the phase, amplitude, and delay of the reference signal, the SI signal can be reconstructed, so that both the linear and non-linear SI can be effectively suppressed [12; 13; 14]. Compared with indirect radio frequency coupling cancellation (IRFCC), which needs an additional separate transmitting channel to reconstruct the interference signal, the DRFCC does not require extra runtime for the separate channel, thereby reducing the quantity of memory data that have to be transmitted at run-time, so that the runtime overhead of DRFCC can reduce greatly. The Least Mean Square (LMS) algorithm is an optimization extension of the Wiener filtering theory using the fast descent method [15]. It has the advantages of a simple principle, low computational complexity, and easy implementation [16; 17]. However, there is an obvious disadvantage of the LMS algorithm: the convergence speed and steady-state error (SSE) are mutually constrained [18]. Reference [19] proposed a Variable Step Size LMS (VSSLMS) algorithm to overcome the drawbacks of the LMS algorithm. VSSLMS uses a prediction error signal to control the step size factor. However, the practicability of the algorithm is poor, and in the process of convergence, a steady-state misalignment (SSM) easily occurs, which does not guarantee precision. Reference [20] improved the VSSLMS algorithm based on the sigmoid function (SVSLMS). The algorithm has the characteristics of a fast convergence speed and small SSE, but the algorithm has high computational complexity and a large computational load, and the step size of the error will be mutated when the value approaches near zero, which is bad for the stability of the algorithm.