The role of PXI in 5G new radio RF tests
01 May 2019
5G promises faster, more reliable communications. To enable mobile broadband communications, 5G uses existing & new technologies to achieve extreme data throughputs. With limited 4G LTE spectrum, 5G new radio (NR) introduces mmWave operating bands, wider channel bandwidths & complex multi-antenna configurations that all bring new demands for test.
This article was originally featured in EPDT's 2019 PXI for T&M supplement, included in the May 2019 issue of EPDT magazine [read the digital issue version of this article]. Sign up to receive your own copy of EPDT each month.
Eric Hsu, Product Marketing Manager at T&M experts, Keysight Technologies explains how industry standard PXI instrumentation can help design & test engineers address these challenges.
New mmWave spectrum frequency bands
To accommodate new spectrum requirements, the 5G NR standard specifies two frequency ranges (FR): sub-6GHz (FR1: 410 MHz to 7.125 GHz) and millimetre-wave (mmWave) bands (FR2: 24.25 to 52.6 GHz). This means 5G devices and test instruments need to support one or both frequency ranges. The use of mmWave frequencies poses challenges related to path loss and signal propagation. At mmWave frequencies, the path loss between instruments and devices-under-test (DUTs) is higher than at sub-6 GHz. In addition, over-the-air (OTA) test methods for performance metrics at sub-6 GHz and mmWave frequencies make it more difficult to achieve accurate and repeatable results.
Performance margin vs. test cost
To reduce measurement uncertainties at high frequencies in every dimension, high-performance signal generators and signal analysers are essential, to ensure the errors do not originate from the test instruments themselves. Integrating the design verification test (DVT) systems for 5G mmWave frequency bands requires high-performance test capabilities, such as high output power, error vector magnitude (EVM) performance, adjacent channel power (ACP), dynamic range and complicated system integration. The test system integration and test costs create higher barriers, from R&D to volume production.
An in-band test solution will cover specific frequency bands for specific test applications or standards. A 6 GHz vector signal analyser (VSA) and a vector signal generator (VSG) cover all current FR1 operating band test cases. The VSA and VSG can also operate as an intermediate frequency (IF) signal analyser and signal generator, when coupled with an external mmWave transceiver for FR2 in-band tests, as shown in Figure 1. The unique PXI platform offers the integration of multiple instruments into one chassis for R&D DVT system and volume production. Move the head as close as possible to the antenna to shorten the mmWave signal routing, to reduce insertion loss between the test system and DUT. This in-band test solution offers the precision required for in-band design validation in all new 5G NR bands, at a lower cost, using a high-performance microwave VSA and VSG.
Measurement acceleration for wider channel bandwidth
5G NR maximum channel bandwidth is 400 MHz for FR2, and the maximum(contiguous) aggregated channel bandwidth is up to 1.2 GHz. In addition, to characterise the non-linear performance of power amplifiers (PAs) using advanced measurements, such as digital pre-distortion (DPD) techniques for RF PAs, you need signal generation and analysis instruments with up to 1.2 GHz bandwidth – 3x the channel bandwidth to evaluate DPD. It takes time to transfer the measured data to a computer for further signal processing and stream the processed data back to the generator for such wide bandwidth signals.
The PXIe new PCIe Gen3 platform delivers high data bandwidth (8 GB/s slot-to-slot) to enable measurement acceleration in a high-performance PXI FPGA processing module and stream measured/processed data between modules. The custom FPGA may be accessed to implement your unique computing algorithms and test applications, such as DPD and envelope tracking (ET) signal processes. FPGA hardware processing and data transferring via the PXIe backplane significantly speeds up DPD/ET test applications. Applications can experience as much as a 20x improvement in measurement speed for the fastest DPD/ET measurements.
Complex multi-antenna test configurations
5G technologies adopt multi-antenna techniques, such as multiple-input, multiple-output (MIMO) and beamforming to improve spectral efficiency and radio coverage. The initial 5G NR standard specifies 8x8 MIMO, which increases the complexity of the multi-channel RF test system setup. To accurately assess the multi-channel signal performance, the timing and phase between test channels must be precisely aligned.
Baseband timing synchronisation
When the number of synchronised channels increases, cabling between instruments becomes much more complicated and setting up the test takes more time. PXI instruments can share clocks and route trigger signals through a backplane bus. This makes implementing synchronisation and more repeatable trigger events easier, because of the fixed test environment and minimal cabling.
Multi-antenna RF systems employ a common local oscillator (LO) that distributes to multiple channels. A similar structure is needed to simulate or analyse multi-antenna RF systems. However, a commercial signal generator or signal analyser has its own independent synthesizer, which makes it difficult to achieve tight phase coherence. By using a shared, common LO between multiple instruments, it is possible to calibrate static time and phase skew, which come from cable lengths and connectors. New RF synthesizers use a direct digital synthesis (DDS) method that benefit from low power consumption, a small package, fast frequency sweep, low phase noise and adjustable phase. PXI instrumentation for 5G phase coherence tests provide these advantages. Figure 2 shows a dual-channel phase-coherent 44 GHz vector signal generator with 2 GHz modulation bandwidth in a single PXI chassis.
True multiport test architecture
As test margins tighten for multiport RF components, such as RF front-end modules (FEMs) and transceivers, a true multiport test architecture is needed to provide exceptional performance, regardless of the number of ports utilised. A true multiport test architecture maintains higher performance than a switch-based solution, and provides faster measurement speed due to simultaneous sweeps. A PXI module platform, will enable the ability to cascade multiple PXI vector network analyser modules to get up to 50 ports in a single chassis for developing a multiport or multi-site measurement solution, as shown in Figure 3.
5G NR technology shifts and test requirements introduce demanding new test challenges. As a result, mmWave frequency bands, wider channel bandwidths and multi-element antenna arrays create greater complexity and the need for higher test equipment performance. To integrate the design verification test systems for 5G, multiple frequency bands, wide channel bandwidths, and multi-channel synchronisation and phase coherence must be supported. PXI modular instrumentation provides flexibility and scalability by integrating systems to enable thorough testing of complex 5G NR devices.
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