Ensuring information security in the current era is becoming increasingly important and attracting significant attention from researchers. MIMO and STBC will remain among the key technologies in next-generation networks such as B5G and 6G. In this paper, a new method is proposed that combines MIMO and STBC encoding scheme to enhance Physical Layer Security (PLS) efficiency for next-generation networks. In the proposed method, the conventional STBC encoding patterns will be modified into various alternative patterns while maintaining orthogonality and preserving the benefits. At each transmission time, one of the STBC patterns is chosen according to the RSSI value between the transmitter and the legitimate receiver. The RSSI value changes randomly and is only known by both transmitter and legitimate receiver. This value is not known by the illegitimate receiver or eavesdroppers. Therefore, the RSSI value can be used as a secret key at the transmitter and legitimate receiver. The selection of the STBC encoding pattern is designed to ensure randomness at the transmitter after each signal transmission cycle. This makes it difficult for illegitimate receivers to determine the transmitted code pattern, thereby enhancing the security of the system. As a result, the analysis and simulation results clearly show that the BER between the source node and the destination node is still maintained, while the BER at the eavesdropper increases significantly. This leads to a substantial improvement in security for networks.