Introduction
To meet the increased demand on high data rate applications, new wireless technologies such as LTE, LTE-Advanced, and Mobile WiMAX (Worldwide Interoperability for Microwave Access) require the use of multiple antennas in mobile terminals. Multiple-Input and Multiple- Output (MIMO) technology in a wide sense covers any multi-antenna technology, such as Spatial Multiplexing (SM), Beam Forming (BF), and spatial diversity. MIMO offers signifi cant increases in data throughput, quality of service (QoS) and cell coverage without additional bandwidth or transmit power. Communication performance is improved by exploiting the characteristics of the propagation channel in which the device is operating. As a result, a MIMO device will adapt to the RF (Radio Frequency) environment. Hence, there is a need of an advanced testing methodology which is capable of creating such propagation conditions in a repeatable fashion.
In MIMO systems, spatial correlation, which is a function of both antenna and propagation characteristics, plays a key role. Indeed, the level of correlation cannot be determined based on propagation characteristics without knowing the characteristics of the antenna. Similarly, the level of correlation cannot be determined based on the antenna characteristics without knowing the propagation characteristics. Therefore, it is necessary to include both antenna and propagation characteristics at the same time when testing multi-antenna terminals.
In order to understand the end-to-end reception performance of a MIMO device, Over-the-Air (OTA) testing is needed. Due to the complexity of multiple antenna setups, a fl exible and fast accurate testing solution becomes a major asset in the antenna design cycle and fi nal product verifi cation (and consequently, Time To Market). To date three fundamentally different approaches are being studied by the wireless industry through 3GPP (3rd Generation Partnership Project), COST2100 (European Cooperation in Science and Technology), and CTIA (International Association for the Wireless Telecommunication Industry).