MIMO

Testing MIMO

Multiple input/multiple output (MIMO) is a technology that can be used to increase effective wireless range, data throughput, or both. It is used in WiFi (802.11n), WiMAX (802.16e-2005) and LTE.

In simple terms, the same data can be sent using multiple antennas, and by using dynamic phase and gain adjustments, an optimal signal can be “steered” to the receiving device. This is called “beamforming” or “precoding,” and requires that the system first sample the channel to assess its current state.

An alternative approach for range expansion, called “diversity coding,” is to send the same signal on each antenna, using space-time coding, and use diversity reception by multiple antennas and receivers by the receiving device.

The data throughput increase, called “spatial multiplexing,” involves splitting a high-data-rate signal into different lower-rate streams, and sending each stream over a different transmitter and antenna, simultaneously. Upon arrival at the multiple antennas and receivers of the receiving device, the composite signals with each’s unique spatial signature can be separated using DSP and recombined into a reproduction of the original high-rate signal.

In the development phase of a MIMO-enabled device, the receivers must be exercised using precisely structured, multiple, transmit (TX) signals. Similarly, the transmitters signals must be captured and precisely analyzed. In a 4x4 MIMO device (one having four transmitters and antennas, and four receivers and antennas), true, multi-stream development testing requires having four signal sources (e.g. VSGs) and four analyzers (e.g. VSAs) plus the ability to precisely synchronize them.

LitePoint’s IQnxnplus provides complete and comprehensive development test capabilities for WiFi and WiMAX MIMO (802.11n and 802.16e). The basic VSA/VSG units are IQmax-750s which can be used to test both WiFi and WiMAX MIMO devices. For development of WiFi MIMO only LitePoint offers its IQnxn.

For manufacturing testing of MIMO devices, where design is considered stable, a WiFi test system (e.g IQflex or WiMAX test system (e.g IQmax) can be used in conjunction with a multiport adapter (e.g. LitePoint MPTA) to provide switched VSG signals to the device’s receivers and capture switched device TX signals for analysis by the VSA.

For more information about MIMO read “The Challenges of Testing MIMO” [LINK] in RF Design.