RF Testing of MIMO Communication Systems

With each technological generation, wireless communication systems continue to achieve higher data throughput than before. Historically, this has been achieved through wider channel bandwidths, spectrum utilization techniques such as orthogonal frequency division multiplexing (OFDM), and more complex modulation types. One of the

more recent innovations to increase wireless channel bandwidth is the Multiple Input Multiple Output (MIMO) system. This technology is implemented in many wireless standards, including IEEE 802.11n, WiMAX, and Long Term Evolution (LTE). The

premise of a MIMO communication system is that the data rate of a communication system using limited spectrum bandwidth can be increased by using multiple "channels" within the same physical spectrum. To do this, the transmitter uses multiple transmit antennas, each transmitting a unique modulated signal. The

receiver also uses multiple antennas and can separate and decode the individual channels with only a small amount of signal processing, a technique known as spatial multiplexing. As one would expect, the maximum data rate of such a system is proportional to the number of channels. In current MIMO transceivers, typical configurations range from 2x2 to 4x4, the latter with 4 transmit antennas and 4 receive antennas.

Accurately testing MIMO transceivers requires advanced signal processing algorithms to multiplex and demultiplex the individual spatial data streams and achieve tight synchronization between the channels of the RF vector signal generator and analyzer. The challenge in testing MIMO systems is that separating each spatial data stream is complex.

Commercially, MIMO transceivers can achieve separation of each spatial data stream by applying a channel matrix to the receive signal. This matrix is ​​a set of phase and gain characteristics for each channel in the system, so when testing MIMO devices, the instrument must be able to separate each channel by applying a similar channel matrix. The

instrument synchronization requirements for MIMO testing are the most difficult to achieve in the test industry. In a MIMO test system, each channel of the multichannel RF instrument must achieve true channel-to-channel phase coherence.

To achieve true phase coherence, all synthesized local oscillators (LOs), analog-to-digital converters/digital-to-analog converters (ADC/DAC) sampling clocks, and start triggers must be directly synchronized between each RF instrument. Fortunately, software-defined PXI instruments can easily meet MIMO synchronization requirements. The instrument uses a modular architecture and all clock signals can be shared.

Using NI's LabVIEW and PXI RF signal generators and analyzers, engineers can generate and analyze multichannel phase-coherent RF signals (Figure). Instruments such as the four-channel PXIe-5663E VSA and four-channel PXIe-5673 VSG can achieve channel-to-channel jitter of less than 0.1°. In addition, because both instruments share a common local oscillator between each channel, all measurements are free of uncorrelated channel-to-channel phase noise.



Easily meet the challenges of MIMO RF testing

Meeting multichannel RF test requirements, such as MIMO standards and beamforming and direct discovery applications, is an increasing test challenge for existing instrument technology. Fortunately, NI's software-defined instruments based on LabVIEW and PXI are well suited for these applications because they provide high data bandwidth, software-defined flexibility, and precise phase coherence of multiple RF signals.