Accurate 5G Millimeter-Wave Signal Analysis with Advanced Strategies

Millimeter-wave (mmWave) technology is an essential element that sets 5G apart from 4G Long Term Evolution (LTE). mmWave frequencies provide the ultra-wide bandwidths 5G networks need to deliver 20 times faster connection speed and 10 times the throughput capacity than 4G LTE. Today, dozens of 5G devices support mmWave technology on the market. 5G service providers around the world are accelerating their pace to deploy mmWave sites to expand coverage.

mmWave technology brings many challenges across the device and network workflow from design and manufacturing to deployment. At mmWave frequencies, path loss makes radio-frequency (RF) power limited and costly, and wide bandwidths introduce more noise and frequency responses.

Path loss represents the power loss of an electromagnetic wave propagating through space. Path loss increases as the frequency increases. Significant power loss results in a decreased signal-to-noise ratio (SNR). A low SNR makes signal analysis measurements such as error vector magnitude (EVM), adjacent channel power (ACP), and spurious emissions, challenging.

Frequency response is the measure of the output spectrum of a system from a stimulus. Frequency responses can occur at different frequencies and introduce amplitude and phase errors degrading modulation quality. Many components in a signal analyzer and external testing fixture can generate frequency responses and they increase with signals with wide bandwidths at high frequencies.

Path loss and frequency responses make achieving accurate and repeatable testing results more difficult at mmWave frequencies. Measurement complexity and uncertainty are major challenges for chipset and device makers, network equipment manufacturers, and operators, but you can use various strategies to overcome these challenges.

Strategy #1: Reduce Signal Path Loss by Selecting the Right Signal Path

Advanced signal analyzers such as Keysight’s N9042B UXA X-Series and PXA provide the flexibility to select the most suitable signal path for different testing scenarios. The standard path is ideal for low-level signals with a bandwidth under 45 MHz. The microwave preselector bypass path allows wideband analysis and a flat spectrum response. Bypassing the preselector also improves amplitude accuracy by eliminating the amplitude drift and passband ripple of the preselector. The low-noise path enables high-power-level testing. This path reduces path loss and eliminates frequency responses. With a Keysight UXA or PXA signal analyzer, you can also combine the low-noise path with the microwave preselector bypass to achieve low path loss, high signal fidelity, and high measurement sensitivity. It has up to 10 dB less loss than the default path.

Strategy #2: Improve Signal Condition with Input Mixer Optimization

Figure 1. Vector Signal Analyzer (VSA block) diagram

Optimizing the input mixer level and adjusting intermediate frequency (IF) gain for the digitizer can improve the signal condition significantly. You can optimize the input mixer level in two ways. The first method consists of attenuating the power level at the first mixer to ensure the high-power input signal does not distort the signal analyzer. This method is suitable for low-input-level test scenarios.

Alternatively, you can add an external low-noise amplifier (LNA) at the front end as in Figure 1. This method is better suited for high-input-level test scenarios, a more robust solution for testing 5G mmWave devices. The Keysight UXA X-Series signal analyzer features an internal LNA in the signal path and an internal preamplifier, providing the benefits of the LNA without the additional hardware.

Strategy #3: Calibrate at the Reference Plane

The location of the reference plane plays an important role in the test results. As shown in Figure 2, calibrating the amplitude and phase of the test receiver path is necessary to offset the impact from frequency responses.

With Keysight’s N9042B UXA X-Series signal analyzer, you can manually configure complex corrections, including amplitude and phase corrections. The Keysight U9361 RCal receiver calibrator functions as a receiver system calibrator that can inject tones at the desired calibration plane, which becomes the new reference plane. The calibrator generates continuous wave tones of known amplitude and phase. The signal analyzer measures each tone’s amplitude and phase at the output of the test network and compares them to the known amplitude and phase. The signal analyzer then corrects differences, stores the corrections, and applies them as needed.

Figure 2. Test network elements for channel correction

Improving mmWave Measurement Accuracy

Despite the great benefits and unlimited potential of mmWave technology, there are still many challenges in mmWave workflow. Signal analysis accuracy becomes more crucial. Selecting the appropriate signal path can reduce path loss, adjusting the input power level can improve the signal condition, and moving the reference plane can eliminate frequency responses. These new strategies can significantly improve mmWave measurement accuracy for 5G device testing?.

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