Future antenna designs are anticipated to be built on state-of-the-art technologies such massive multiple-input multiple-output (MIMO), which can greatly increase data rates and enhance the effectiveness of contemporary communications without requiring a large addition of power or spectrum. For next-generation wireless networks, multiple-antenna systems are essential for delivering high spectrum efficiency and enhanced quality of service. In light of this, this work offers important new information on how to build small, decoupled multiple antennas for effective MIMO communications. Utilizing the many types of diversity mechanisms to take advantage of the degrees of freedom between the multiple elements, massive MIMO antennas based on space–time block coding (STBC) have been proposed throughout this work. By utilizing their numerous resonant modes, the thesis explores the feasibility of combining multiple decoupled antennas into a small co-located multi-element antenna. In contrast to other cutting-edge methods, the suggested approach successfully lowers the symbol error rate (SER).