Achieving Wireless Cable Testing for MIMO Devices With a Novel Condition Number Reduction Method

Wireless cable is a promising solution for testing highly integrated multiple-input multiple-output (MIMO) devices in an over-the-air (OTA) manner. It has attracted significant attention from academia, industry, and standardization bodies. By measuring and calibrating the transfer matrix between the device under test (DUT) antenna ports and probe antennas, wireless cables, i.e., OTA direct links, can be established between the probe antennas and the corresponding DUT antenna ports, thereby enabling conductive cabled testing functionality. However, achieving high-quality wireless cables requires a transfer matrix with a small condition number. Otherwise, implementation errors in the measurement setup can be significantly amplified, degrading the quality (isolation level) of the realized wireless cables. For this issue, this paper proposes a novel condition number reduction method for wireless cable connections. By employing the precoding matrix, a new composite coupling matrix with a small condition number can be generated and used for constructing high-quality wireless cables. Different from existing methods that require random mechanical movement of DUT or brute-force probe selection, the proposed method can be explicitly performed based on the obtained ill-conditioned transfer matrix, saving measurement time. The proposed method is validated through Monte Carlo simulations and experimental measurements. Excellent isolation levels of above 22 dB are achieved in two different measurement scenarios, demonstrating the effectiveness and robustness of this solution.