5G Testing: 3 Considerations for MIMO Test Setups

2020-03-27  |  8 min read 

The 5G New Radio (NR) standard offers the potential for increased data rates by using higher-order modulation, wider modulation bandwidths at higher millimeter-wave (mmWave) frequencies, and multiple-input/multiple-output (MIMO) technology.

While MIMO is complex for 4G and wireless local area network (WLAN), it can be significantly more complex for 5G NR in the frequency range 2 (FR2) band because of wide modulation bandwidths at mmWave frequencies. As you migrate from single-antenna single-input/single-output (SISO) implementations to two-channel MIMO, a multitude of design and testing challenges emerge that impact peak data rates and make it difficult to troubleshoot and debug hardware performance issues.

Ensuring optimal performance in a multichannel MIMO implementation requires comprehensive multichannel MIMO measurements, such as error vector magnitude (EVM), a key metric for transmitter performance. Radio frequency (RF) and baseband impairments such as timing errors, local oscillator (LO) phase noise, power amplifier gain/phase distortion, and intermediate frequency (IF)/RF filter group delay can degrade transmitter EVM.

In addition, propagation loss at mmWave frequencies necessitates the use of phased-array technology and beam steering to achieve enough signal-to-noise ratio (SNR) and link quality for 5G NR and other emerging applications such as 802.11ay. This adds a layer of complexity in validating mmWave MIMO system performance under real-world scenarios.

Your testbed should take into consideration the following 3 factors to address these challenges:

  • Extreme bandwidths
  • Phase noise
  • Speed

1. Extreme bandwidths

Emerging high-band mmWave applications demand extreme bandwidths. 802.11ay, for example, specifies a two-channel bonded configuration for 2 * 2.16 GHz, or 4.32 GHz of channel bandwidth. This standard also specifies other optional channel bonding configurations for 6.48 GHz and 8.64 GHz of channel bandwidth, as well as some channel aggregation configurations. In addition, standards bodies are considering an optional MIMO feature for 802.11ay.

Your MIMO measurement testbed needs to be flexible and scalable enough to address a multitude of frequency bands, extreme frequency bandwidths, and multiple channels to address demanding emerging mmWave test challenges. Keysight’s R&D mmWave testbed uses a new ultra-performance, real-time, 110 GHz oscilloscope (UXR) to directly digitize and analyze wide-bandwidth, high-frequency mmWave signals.

5G NR MIMO test setup

Figure 1. 5G NR 28 GHz MIMO test setup

2. Phase Noise

Low phase noise is essential for making accurate mmWave measurements. MIMO measurements require a low-phase-noise clock distributed to multiple channels without degradation. The length (10 mS) and wide bandwidth (800 MHz) of 5G NR mmWave signals demand a measurement with both excellent close-in phase noise and high-offset phase noise. Broadband noise increases with bandwidth by 10 * log (BW increase). Phase noise increases with center frequency (CF) by 20 * log (CF increase). Moving a 5G carrier from 3 to 39 GHz will therefore increase phase noise by more than 22 dB, if there is no improvement in the frequency reference.

Consider the instruments in your MIMO test setup carefully to address this challenge. Keysight’s test setup uses the new UXR oscillocope, which has the low noise, wide bandwidth, multiple channels, and flat frequency response to measure 5G NR MIMO mmWave signals. It also features a clean 8 GHz clock (–130 dBc/Hz @ 100 kHz offset) and distributes this to all four channels. The clock is then multiplied up to 128 GHz for the samplers to collect data at 256 GSa/s. This fixed multiplication chain allows the use of proprietary Keysight amplifiers and filters to nearly eliminate phase noise beyond the 20 * log (CF increase).

Fixed multiplication maintains tight coherency between channels. The jitter added by making multiple channel measurements over single channel measurements is specified as < 10 fs rms. A 39 GHz carrier run into two channels of a UXR, measured with 1 GHz instantaneous BW, will show < 1/2 deg rms of jitter between them.

This performance makes the UXR a good choice for mmWave EVM measurements and enables excellent phase noise measurements. Multiple channels and cross-correlation remove the noise contribution of the channels, generating very low-noise, very wide (many gigahertz) offset phase noise measurements.

UXR phase measurements

Figure 3. UXR phase noise measurements

3. Speed

High signal fidelity and a low noise floor measurement require a high sample rate for signal acquisition. The slow processing speed of high sample rates makes an oscilloscope with standard hardware inadequate for 5G NR EVM measurements. Oversampling results in a more accurate and lower EVM result but increases the EVM demodulation processing time.

Once again, consider the instruments in your MIMO test setup carefully. Keysight’s UXR oscilloscope hardware supports a technique called digital down conversion (DDC) that enables the high-frequency content of an RF signal to be appropriately down-sampled before storing it in the oscilloscope memory.

The oscilloscope hardware first applies a bandpass filter to a user-specified frequency span that prevents out-of-band noise from aliasing into the passband region during the down-sampling process. Half-band decimators perform frequency shifting and decimation of the data.

Smaller frequency spans enable more decimation to be applied, resulting in lower output sample rates and less data being stored to memory than would be required when capturing the full sample rate data record. This smaller data size also requires less data transfer and post-processing time.

The UXR oscilloscope hardware supports a series of discrete frequency spans, ranging from 40 MHz to 2.16 GHz. The VSA software can calculate measurements such as EVM several orders of magnitude faster than it could on a full sample rate data record.

DDC feature on UXR oscilloscope

Figure 3. DDC feature on the UXR oscilloscope captures and decimates signals into memory for VSA processing.

Learn more about making quick and accurate phase-coherent MIMO measurements by downloading Keysight’s application note MIMO and Wideband Millimeter-Wave Testing with the UXR Application Note available on the 5G Network Equipment Manufacturers webpage.