A.2 Multi-Probe Anechoic Chamber (MPAC)

38.1513GPPNRRelease 17TSUser Equipment (UE) Multiple Input Multiple Output (MIMO) Over-the-Air (OTA) performance requirements

A.2.1 System setup

MPAC test method is the reference methodology for FR NR MIMO OTA testing. By arranging an array of antennas around the Equipment Under Test (EUT), a spatial distribution of angles of arrival in MPAC system may be simulated to expose the EUT to a near field environment that appears to have originated from a complex multipath far field environment.

As illustrated schematically in Figure A.2.1-1, signals propagate from the base station/communication tester to the EUT through a simulated multipath environment known as a spatial channel model, where appropriate channel impairments such as Doppler and fading are applied to each path prior to injecting all of the directional signals into the chamber simultaneously through the antenna array. The resulting field distribution in the test zone is then integrated by the EUT antenna(s) and processed by the receiver(s) just as it would do so in any non-simulated multipath environment. MPAC system with 16 uniformly-spaced dual-polarized probes is permitted for NR FR1 MIMO OTA testing.

Figure A.2.1-1: MPAC system layout for NR FR1 MIMO OTA testing

A.2.2 Calibration procedure

The system needs to be calibrated by using a reference calibration antenna with known gain values in order to ensure that the downlink signal power is correct. In non-standalone (NSA) mode, the LTE link antenna provides a stable LTE signal without precise path loss or polarization control.

Unlike traditional TRP/TRS testing where the path loss corrections can all be applied as a post processing step to the measured data, the path loss for each probe in the MPAC system must be balanced at test time in order to generate the desired channel model environment within the test zone of the chamber. The imbalance of each path during testing would result in an alteration of the angular dependence of the channel model (i.e. varied characteristics of generated channel model) within the test zone of the chamber.

1. Place a vertical reference dipole in the centre of the test zone, connected to a VNA port, with the other VNA port connected to the input of the channel emulator unit.

2. Configure the channel emulator for bypass mode.

3. Measure the response of each path from each vertical polarization probe to the reference antenna in the centre of test zone.

4. Adjust the power on all vertical polarization branches of the channel emulator so that the powers received at the centre are equal.

5. Repeat the steps 1 to 4 with the magnetic loop or horizontally polarized reference dipole instead, and adjust the horizontal polarization branches of the channel emulator.

6. The worst-case path loss becomes the reference path loss of the entire system, this loss is used to compute the power in the centre of the test zone relative to the output power of the Base Station simulator. Besides, based on the reference path loss, the relative offset of each path loss shall be corrected.

Note: Calibration based on other antennas, e.g., horn antennas is not precluded.

A.2.3 Test procedure

Before throughput testing, the initial conditions shall be confirmed to reach the correct measurement state for each test case.

1. Ensure environmental requirements of Annex F are met.

2. Configure the test system according to Annex C, D and E for the applicable test case.

3. Verify the implementation of the channel model as specified in Annex C.3.

4. Position the UE in the chamber according to Annex A.3.

5. Power on the UE.

6. Set up the connection.

Note: For step 3, the verification of the channel model implementation is usually performed once for each channel model as part of the laboratory accreditation process, and will remain valid as long as the setup and instruments remain unchanged. Otherwise the channel model validation may need to be performed prior to starting each throughput test.

For throughput testing, the following steps shall be followed in order to evaluate NR MIMO OTA performance of the DUT:

1. Measure MIMO OTA throughput from one measurement point, the maximum downlink power P_RS-EPRE-MAX is defined in Clause 6.1.2. MIMO OTA throughput is the minimum downlink signal power resulting in a pre-defined throughput value, i.e., 70% and 90% of the maximum theoretical throughput. The downlink signal power step size shall be no more than 0.5 dB when RF power level is near the NR MIMO sensitivity level.

2. Rotate the UE around vertical axis of the test system by 30 degrees and repeat from step 1 until one complete rotation has been measured i.e. 12 different UE azimuth rotations.

3. Repeat the test from step 1 for each specified device orientation. A list of orientations is given in Annex A.3.

4. The postprocessing method to calculate the average MIMO Throughput is defined in Clause 6.

Note: For step 1 of throughput testing, the measurement is not needed to start from maximum downlink power each time. To save testing time, the starting downlink power can be set as a proper value (lower than maximum downlink power supported by test system) as long as all the throughput curve curves at 12 different UE azimuth rotations can reach at least 90% of the maximum theoretical throughput.

A.2.4 Minimum Range Length

The minimum range length of FR1 MPAC system is defined as the distance from the centre of the test zone to the aperture of the measurement probes/antennas, as illustrated in Figure A.2.4-1.

Figure A.2.4-1: Illustration of range length definition of FR1 MPAC

The minimum range length for NR FR1 MPAC OTA systems with 20cm test zone size is 1.2m. While for MPAC systems, the far-field requirements do not have to apply, it was shown that the spatial correlation can be impacted significantly for distances below 1.2m.

A.2.5 Preliminary MU budget of FR1 MPAC system

This clause defines the Preliminary measurement uncertainty (MU) budget for FR1 MPAC system, as shown in Table A.2.5-1.

Table A.2.5-1: Preliminary measurement uncertainty budget for FR1 MPAC system

UID	Description of uncertainty contribution	Example value (410MHz<f≤3GHz)	Example value (3GHz <f≤7.125GHz)	Distribution of the probability	Std Uncertainty (410MHz<f≤3GHz) [dB]	Std Uncertainty (3GHz <f≤7.125GHz) [dB]
Stage 2: DUT measurement
1	Mismatch for measurement process	0	0	U-Shaped	0	0
2	Measure distance uncertainty	0	0	Normal	0	0
3	Quality of quiet zone	0.6	0.6	Actual	0.6	0.6
4	Base Station simulator	1.5dB	2dB	Rectangular	0.87	1.15
5	Channel Emulator – absolute output power – output signal stability – output stability with temperature	1.5dB 0.5dB 0.4dB	1.5dB 0.5dB 0.4dB	Actual (normal- power; rect-stability)	0.84	0.84
6	Amplifier uncertainties	0.7dB	0.7dB	Rectangular	0.4	0.4
7	Random uncertainty	0.2dB	0.2dB	Normal	0.12	0.12
8	Throughput measurement: output level step resolution	0.25dB	0.25dB	Rectangular	0.14	0.14
9	DUT sensitivity drift	0.2	0.2	Rectangular	0.12	0.12
10	Signal flatness	0	0	Normal	0	0
Stage 1: Calibration measurement
11	Mismatch for calibration process – loopback cable path – system input path – reference antenna	0.2	0.2	U-Shaped	0.14	0.14
12	Reference antenna positioning misalignment	0	0	Normal	0	0
13	Quality of quiet zone	0.6	0.6	[Rectangular]	0.35	0.35
14	Total uncertainty of the Network Analyzer	0.5	0.5	Rectangular	0.29	0.29
15	Uncertainty of an absolute gain of the calibration antenna	1	1	Normal	0.5	0.5
16	Offset of the Phase Center of the Reference Antenna	0	0	Normal	0	0
Total Expanded Uncertainty, U, with 95% Confidence Interval					3.03	3.38

The detailed descriptions of each measurement uncertainty contributor are defined in Annex B.1.2 in [2].