Near-Field Perturbation Effect on Constellation Error in Beam-Space MIMO

Beam-space MIMO has recently been proposed as a promising solution to enable transmitting multiple data streams using a single RF chain and a single pattern-reconfigurable antenna. Since in a beam-space MIMO system radiation pattern of the transmit antenna is exploited as extra dimension for encoding information, near-field interaction of the transmit antenna with its surrounding objects affects spatial multiplexing performance of the system. Through numerical simulations in the previous work, it has been concluded that under BPSK signaling beam-space MIMO is not more vulnerable to near-field coupling than its conventional counterpart. In this work, we extend the study to the case of higher-order modulation schemes, where the presence of external perturbation also affects the data constellation points transmitted by a beam-space MIMO antenna. To this aim, the error vector magnitude of the transmitted signal is evaluated when placing a QPSK beam-space MIMO antenna in close proximity to a hand model of the user. The obtained results emphasize the importance of reconsidering the decoding approach for beam-space MIMO systems in practical applications. Index Terms Beam-space MIMO, constellation distortion, error vector magnitude, near-field interaction, reconfigurable antenna, single-radio MIMO, user effect.