Multichannel RF test
16 June 2010
In this five-part series, National Instruments draws upon its knowledge and experience of a wide range of industries to identify key technologies impacting test and measurement in 2010. This second instalment focuses on testing next-generation wireless devices.
With each incremental evolution of wireless communications systems, RF engineers strive to attain higher data throughput than before. Although each new generation of wireless system delivers benefits to consumers, it creates challenges for today’s test engineers.
Until recently, increased data rates have been achieved through widening channel bandwidths, advanced spectral utilisation techniques and complex modulation types. This added complexity results in increased development costs and longer test times, forcing test engineers to consider alternative approaches.
A recent innovation set to simplify increased wireless throughput is the development of smart antenna technology, such as multiple-input, multiple-output (MIMO) systems. As a means of increasing channel capacity, MIMO technology is already being implemented in next-generation smartphones and several emerging communication standards including IEEE 802.11n, Mobile WiMAX Wave 2 and 3GPP Long Term Evolution (LTE). MIMO allows RF engineers to improve communication performance, whilst conserving predefined bandwidth and transmission power. This is achieved by utilising multiple antennas at both the transmitter and receiver, affording the parallel transmission of numerous “channels” within the same physical spectrum.
The multi-channel RF generation and acquisition equipment required to test MIMO systems is fundamentally difficult to configure and presents two significant technical hurdles for test engineers to overcome. Namely, the scalability of the multichannel RF test systems and the phase cohesion of all data channels.
A 2x2 MIMO system uses two transmitters and two receivers to facilitate the increase in data throughput. As one would expect, the maximum data rate of such a system scales with the number of channels, so the 2x2 or 4x4 systems emerging in current MIMO systems will undoubtedly become the 8x8 or 16x16 MIMO systems of tomorrow. Consequently, the scalability of next-generation RF testers is paramount if engineers want to minimise the cost and complexity of upgrading their test systems to meet future requirements.
The synchronisation requirements for MIMO test are some of the most stringent in the test industry, as a “MIMO-ready” test instrument must ensure true channel-to-channel phase cohesion. The traditional approach of sharing reference clocks and occasional start triggers is sufficient to ensure the simultaneous acquisition of signals, but it does not guarantee true phase coherency.
On the generation side, the baseband sample clocks, start triggers and LO (Local Oscillators) must be shared between multiple RF vector signal generators. On the acquisition side, analysers must share each and every LO used as part of the downconversion processes. To add further complexity, path lengths must be carefully considered when sharing the high-frequency clock sources required for RF test. Even the smallest mismatch in cable length will introduce an interchannel phase skew.
This requirement for interchannel synchronisation was noted by Professor Robert W. Heath, Jr., from the Department of Electrical and Computer Engineering at the University of Texas: “Prototyping and testing MIMO wireless communication systems will require a low-cost and adaptable multichannel RF architecture that supports comparable bandwidths and a variety of modulation formats, as well as permitting multichannel synchronisation.”
The traditional three-stage superheterodyne architecture of benchtop RF vector signal analysers does not lend itself to this level of synchronisation. For this reason, National Instruments have been expanding their range of high-performance, software-defined instrumentation with a significant investment in RF and wireless test. In particular, PXI-based instrumentation can easily address MIMO synchronisation requirements with a modular architecture that allows all clock signals to be readily shared amongst all transmitters and receivers.
NI multichannel RF Vector Signal Generators and Analysers provide a solution to engineers seeking a flexible, cost-effective alternative to the high cost and high complexity of traditional MIMO instrument configurations. The graphical development environment, National Instruments LabVIEW, can be used to implement advanced signal processing to multiplex and demultiplex the spatial streams. Combining these bespoke algorithms with the PXI modular RF hardware allows test developers to create a ruggedised, integrated system with a small physical footprint. Although the software-defined instrumentation offered by NI is inherently customisable, the systems do include LabVIEW example code, which provides a ready-to-run, out-of-the-box experience for multichannel RF signal generation and acquisition.
Steve Seiden, VP of Cal-Bay, a global test system integrator who has worked with companies such as IBM and National Semiconductors, recently commented: "The RF instrumentation from National Instruments has been an excellent, cost-effective solution for our four-channel record-and-playback system. Using LabVIEW example code and PXI modular instruments from NI, we configured each channel to be completely phase-coherent. Our system, based on the NI PXIe-5663 6.6GHz four-channel RF VSA, implemented a common local oscillator between modules to achieve better than 0.1° channel-to-channel jitter at a 1GHz carrier frequency."
Ultimately, multichannel test architectures will reduce aggregate test times, increase test throughput and improve instrument usage. Software-defined PXI offers all the flexibility required to test multiple frequencies with the same hardware. In addition, it forms the basis of a highly scalable phase-coherent RF test system, accommodating up to 4x4 MIMO configurations within a single PXI chassis and up to 16x16 when daisy-chaining multiple PXI chassis.
The NI solution saves engineers time and money because it is preconfigured to meet advanced MIMO prototyping and test requirements. It can help reduce instrument footprint and power consumption, and eliminates the time-intensive troubleshooting involved with configuring proprietary MIMO development and test systems.
For more information regarding MIMO technology and the National Instruments MIMO test configurations, please take a look at: www.ni.com/automatedtest/mimo.
Richard Roberts is Technical Marketing Engineer, National Instruments UK & Ireland
Contact Details and Archive...
Related Articles...
Most Viewed Articles...