The effective use of multiple antenna systems in compact terminal devices is plagued by the problems of mutual coupling and spatial correlation. Consequently, increasing the number of antennas per unit volume results in rapidly diminishing returns after an upper limit is reached. We review recent advances on this topic of closely coupled antennas, including several existing techniques that may be used to push the boundary further, such that closely spaced antennas can still give good

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... ive good performance. A new approach of decoupling closely spaced antennas is also introduced.