In this paper, we use the Xilinx Zedboard 7000 software-defined radio (SDR) plat-form with the AD9361 RF module to build up a full-duplex (FD) decode-and-forward relay system prototype based on the long-term evolution (LTE) downlink standard. Two directional antennas are used at the relay node for passive self-interference (SI) suppression in the radio-frequency (RF) analog part. In addition, two adaptive SI cancellation (SIC) algorithms, namely stochastic gradient descent (SGD) and recursive least-square (RLS) methods, respectively, are investigated in the baseband with a power minimization principle. Frame synchronization is also designed for identifying the SI and desired signals and facilitates the execution of the digital SIC. Both computer simulations and real experiments are conducted to demonstrate the interplay between the antenna separation of directional antennas and the digital SIC as well as their impact on the total SI suppression performance. The experiment results show that the proposed scheme can achieve an SI suppression gain up to 55 dB in total.