Software-defined PXI RF instrumentation
Richard McDonell, National Instruments
With each stage in their evolution, wireless communications systems aim to achieve higher data throughput than before. Historically, this was accomplished through wider channel bandwidths, spectrum utilization techniques such as orthogonal frequency-division multiplexing (OFDM), and more complex modulation types.
Today, one of the most recent innovations to increase wireless channel bandwidth is the development of multiple-input, multiple-output (MIMO) systems. This technology is implemented in a variety of wireless standards, including IEEE 802.11n, WiMAX, and Long-Term Evolution (LTE).
The premise of MIMO communication systems is that the data rate of a communication system using a finite spectrum bandwidth can be increased by using multiple “channels” within the same physical spectrum. To do this, a transmitter uses multiple transmit antennas, each transmitting a unique modulated signal.
The receiver also uses multiple antennas, and with a little signal processing, it can separate and decode each individual channel. This is called spatial multiplexing, and as one would expect, the maximum data rate of such a system scales with the number of channels. In today’s MIMO transceivers, common configurations range from 2x2 to 4x4, with the latter having four transmit and four receive antennas.
Accurate testing of MIMO transceivers requires advanced signal-processing algorithms to multiplex and demultiplex various spatial streams as well as tight synchronization between each channel of RF vector signal generators and analyzers. The challenge with MIMO system test is that separating each spatial stream is not trivial.
In the commercial world, MIMO transceivers can separate each spatial stream by applying a channel matrix to the received signal. The matrix is a set of phase and gain characteristics for each channel in the system. Hence, when testing MIMO devices, instrumentation must be able to separate each channel by applying a similar channel matrix.
The instrument synchronization requirements for MIMO test are some of the most difficult in the test industry. In a MIMO test system, each channel of a multichannel RF instrument must achieve true channel-to-channel phase coherency.
Achieving true phase coherency requires the synchronization of all synthesized local oscillators (LOs), analog-to-digital converter/digital-to-analog converter (ADC/DAC) sample clocks, and start triggers directly between each RF instrument. Fortunately, software-defined PXI instrumentation can easily address MIMO synchronization requirements with a modular architecture where all clock signals can be readily shared.
Using NI’s LabVIEW and PXI RF signal generators and analyzers, engineers can generate and analyze multichannel phase coherent RF signals (see the figure). Instruments such as the four-channel PXIe-5663E VSA and four-channel PXIe-5673 VSG achieve better than 0.1° channel-to-channel jitter. In addition, because both instruments share a common LO between each channel, all measurements are free of uncorrelated channel-to-channel phase noise.
Addressing multichannel RF test needs such as MIMO standards and beam-forming and direction-finding applications is a growing test challenge using existing instrumentation methods. Fortunately, NI LabVIEW and PXI-based software-defined instrumentation are suited for these applications due to their high data bandwidth, software-defined flexibility, and precise phase coherence of multiple RF signals.