A cost-effective method for MIMO antenna system to evaluate human exposure

This paper proposes a cost-effective method for evaluating the electric fields generated from antennas in a two-branch Multiple Input Multiple Output (MIMO) system at a long-term evolution (LTE) base station using the large-delay Cyclic Delay Diversity (CDD) or Tx Diversity transmission mode for assessing the human exposure to radio frequency (RF) fields in a test environment such as an anechoic chamber. In the proposed method, the electric fields from two branches in the MIMO system are measured separately instead of considering the simultaneous transmissions of the two branches in an actual MIMO system. The uncorrelated sum method given in IEC/TR 62630 is applied as the method for combining the measured electric fields from the two branches, because the signals from the MIMO branches using the transmission mode have no correlation. In addition, a continuous wave (CW) is applied as a test signal instead of the modulated signal based on the LTE specifications in 3GPP Release 8. Two types of antenna systems are discussed: horizontally spaced and dual polarized. The results show that the combined-measured electric field distribution is less than 0.9-dB higher on average compared to the measured electric field distribution of a MIMO system replicating an actual LTE base station. Therefore, the cost of the transmission system for the proposed method is much lower than replicating that for an actual LTE base station. This is because the proposed method needs only one generator regardless of the number of branches in the MIMO system, no function relating the generators as a MIMO system, and no modulated signal. Furthermore, the evaluated electric field strength is conservative but not too large compared to that for an actual LTE base station.