A key feature of upcoming 4G wireless communication systems is multiple-input-multiple-output (MIMO) technology. To make the best use of MIMO, the antenna correlation between adjacent antennas should be low (< 0.5). In this context, we propose a correlation reduction technique suitable for closely spaced antennas (distance, d < λ/40). This technique reduces mutual coupling between antennas and concurrently uncorrelates antennas' radiation characteristics by inducing the negative group delay at the target frequency. The validity of the technique is demonstrated with a USB dongle MIMO antenna designed for LTE 700 MHz band. Measurement results show that the antenna correlation is reduced more than 40% using the proposed technique.
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