The fifth generation (5G) of mobile communications gives birth to the evolution of the base stations. They progressively include arrays of antennas in the sub-6 GHz frequency band worldwide. Moreover, 5G takes advantage of the beamforming capabilities to gain power efficiency and higher throughputs. Within this framework, achieving high decoupling performance between any pairs of antennas and compactness at the same time become a major challenge. The present paper describes an advanced solution based on the interposition of a thin and low-profile absorbing wall operating in the 3.4-3.6 GHz frequency band. The absorption is provided by a frequency selective surface whose elementary pattern size is first optimized. Then, the compact absorbing wall that associates back-to-back pieces of absorber is interposed between elements of an antenna array. Moreover, the fabrication of both antenna array and decoupling device involve lightweight and low-cost composite technology. The results show that such a compact decoupling device achieves excellent decoupling capabilities while keeping up the antenna array performance.