Signal Analysis in 5G NR Base Station Transmitters: Part 4

In the first, second, and third parts of the 5G signal analysis (spectrum analysis) conformance testing blog series, we discussed base station transmitter characteristics and required tests to comply with 3rd generation partnership project (3GPP) standards. In the last part of this blog series, we discuss the 5G NR conformance test challenges and the latest industry signal analyzer (spectrum analyzer) solutions to overcome these testing challenges.

5G NR Release 16 Conformance Test Challenges

According to conformance test requirements, all frequency range 2 (FR2) device and base station tests, and some FR1 base station tests, are radiated. This process results in over-the-air (OTA) testing, which introduces additional test challenges. Whether you are assessing transmitters, troubleshooting receivers, or analyzing OTA signals, flexibility in the hardware and software of your spectrum analyzer (signal analyzer) is very important to enable you to find the right solution for your needs. Let’s have a look at the main testing challenges:

Excessive path loss is one of the main challenges in higher frequencies such as millimeter wave (mmWave). The components designed for mmWave devices are compact and highly integrated, with no place to probe. Unlike traditional cabled tests, this results in the need for radiated or OTA tests, which introduce many new challenges, including path loss. Greater path loss and higher measurement uncertainties make it difficult to achieve measurement accuracy. A test solution for mmWave designs must have an adequate signal-to-noise ratio (SNR) to accurately detect and demodulate 5G signals. The excessive path loss at mmWave frequencies between instruments and devices under test (DUT) results in a lower SNR for signal analysis, making transmitter measurements, such as EVM, adjacent channel power (ACP), and spurious emissions, challenging. Minimizing any possible path loss is critical for mmWave testing.

Another challenge in mmWave testing is the wideband noise, which increases the test complexity and measurement uncertainties. Even though mmWave can access higher frequencies, higher data rates, and wider bandwidths, it introduces more noise. The reason is that the transmit signal must compete with the channel’s noise floor to increase the receiver sensitivity. As bandwidth increases, a signal can travel over a channel faster so that the receiver can receive it. However, introducing more noise to the spectrum analyzer (signal analyzer) reduces its SNR, making mmWave measurements more complex.

The frequency response is another major mmWave testing challenge. The goal of a test system is to characterize a DUT. A system must isolate the DUT’s measured results from the effects of all other segments. The frequency response of a test system comes from components like mixers, filters, and amplifiers between a spectrum analyzer (signal analyzer) and a DUT. These responses have different frequencies and include amplitude and phase errors.

Signal Analysis Solutions

All wireless standards specify transmitter measurements at the maximum output power. You can optimize the input level by using an external low noise amplifier (LNA) at the mixer input. The Keysight N9042B UXA X-Series signal analyzer provides a built-in LNA and preamplifier for various test scenarios. Figure 13 shows how the two-stage gain balances noise and distortion for optimizing the best low-input-level measurement performance. Also, Figure 1 shows a 5G demodulation analysis example, in which turning on the LNA has improved the EVM from 5.75% to 1.99%.\

*Figure 1. A comparison between the EVM measurements when LNA is OFF and ON *

Keysight’s V3050A external frequency extender with an integrated preselector and RF switch has a seamless operation interface that integrates with the N9042B UXA X-Series signal analyzer. This solution enables a sweeping power spectrum from 2 Hz to 110 GHz without managing band breaks and images. The U9361 RCal receiver calibrator, when used with the X-Series signal analyzers, can help move the reference plane to the DUT for accurate and efficient calibration. This way, you can calibrate the linear impairments in your test receiver system caused by fixtures, cables, and adapters in a compact and powerful device. Therefore, you can establish a calibration plane where the test receiver system physically connects to the DUT output.

The Keysight N9032B PXA X-Series signal analyzer delivers the widest analysis bandwidth in a 4U form factor with superior performance to satisfy your wide bandwidth requirements at lower frequencies. The N9032B signal analyzer offers up to 2 GHz of bandwidth (four times more than other units) for all 8.4, 13.6, and 26.5 GHz frequencies. With the same LNA and preamp as the UXA X-Series, this analyzer offers the best swept displayed average noise level (DANL) and the best EVM residuals and sensitivity. Figure 15 is an example of a 3 to 7 dB improvement in the DANL because of the advanced front-end design in N9032B PXA X-Series signal analyzer. By pairing industry leading measurement software such as Keysight’s PathWave X-Series measurement applications, Keysight PathWave vector signal analysis software (89600 VSA), plus the RCal receiver calibrator, and the Keysight VXG signal generator you can get a complete solution to help you stay ahead of the ever-changing regulations.

Figure 2. A comparison between the N9032B DANL signal analyzer with the Keysight N9030B and N9040B signal analyzers