OTA Testing Basics | R&S 5G OTA Testing Solutions

This app note covers OTA Testing basics. It mentions 5G OTA testing requirement as per 5G NR standard and describes Rohde and Schwarz solutions for OTA testing.

Introduction:
5G NR (New Radio) system is latest in the 3GPP series of wireless cellular technologies designed to deliver very high data rates both in the uplink and downlink directions. To address this requirement, 5G base station (gNB) and mobile phones use massive MIMO antenna arrays. This helps in achieving high capacity and high energy efficiency.

5G base staion uses separate RF front end for each of the antennas used in the array. RF front end consists of RF transceiver, RF power amplifier, up converter, down converter etc. 5G NR supports various use cases viz. eMBB, URLLC and mMTC. The measurements are categorized into conducted and radiated. Conducted measurements use cable or wires where as radiated measurements are performed over the air using EM (Electromagnetic) wave transmission and reception.

What is OTA (Over The Air) ?

Radiated tests are performed Over the Air without any cables/wires. It is categorized into two major tests viz. near field and far field based on distance between measurement system and DUT (i.e. UE or gNB). The region upto distance of 2*D²/λ is known as near field region where as distance above 2*D²/λ is known as far field region. Here D is antenna diameter and λ is wavelength. In near field, power decay follows 1/R⁴ equation where as in far field power is proportional to 1/R² from original towards far end from antenna. Hence in near field, power intensity falls off very rapidly as we measure away from the source.

Image Courtesy: Rohde & Schwarz

The figure-1 depicts 5G OTA testing setup from Rohde & Schwarz. The setup uses R&S spectrum analyzer.

5G OTA testing

The 5G NR RAN working group is working on radio performance and protocol testing under series of standards TS 38.101 to 38.307. This group is responsible to finalize minimum requirements for radio and protocol conformance testing during transmission and reception. The standard defines EMC requirment, radio link test requirement, cell selection/re-selection, performance tests and so on.

OTA testing should cover both the frequency bands (FR1 and FR2) supported by 5G NR devices. FR1 ranges from 450 MHz to 6 GHz and FR2 ranges from 24.25 to 52.6 GHz. FR1 and FR2 uses bandwidth of 100 MHz and 400 MHz respectively.

Following are the OTA test methods finalized for 5G NR UE devices.
• Direct Far Field (DFF)
• Indirect Far Field (IFF)
• Near Field to Far Field Tranformation (NFTF)

Image Courtesy: Rohde & Schwarz

The figure-2 depicts PWC200 (Plane Wave converter) from Rohde & Schwarz. It consists of bidirectional phased array with 156 vivaldi antennas. It is housed in compact test chamber to perform realtime transmitter and receiver measurements. To feed the signal to the array or to test signal reception part, a combiner is used. The power combiner merges all signal paths to a single port where test instrument is connected.

R&D measurements include gain , radiated power (ERP or EIRP or TRP), receiver sensitivity, transceiver characterization, beam steering & beam tracking etc. Production tests mainly include functional tests, calibration etc. Following table mentions transmitter and receiver tests as part of transceiver characterization.

Device and Test type	Description of test cases
UE Radiated Transmitter Tests	• Transmitter power • Output power dynamics • Transmit signal quality • Output RF spectrum emissions • Spurious emissions • EVM
Base Station Radiated Transmitter Tests	• Radiated transmit power • Base station output power • Output power dynamics • Transmitted signal quality (EVM) • Occupied bandwidth • Adjacent channel leakage ratio(ACLR) • Operating band unwanted emissions • Transmitter spurious emissions • Transmitter intermodulation
UE Radiated Receiver Tests	• Diversity characteristics • Reference sensitivity power level • Maximum input level • Adjacent channel selectivity • Blocking Characteristics
Base Station Radiated Receiver Tests	• Sensitivity • Dynamic range • In-band selectivity and blocking • Out-of-band blocking • In-channel selectivity

R&S solutions for 5G OTA Testing

Following table mentions solutions from R&S for 5G OTA testing.

R&S Solution	Description
CTIA OTA (TS8991/WPTC)	• Frequency: 0.4 to 18 GHz • Minimum Size: 250 x 250 x 220 cm • Fields: Near & Far • Signals: Modulated/CW • Parameters: EIRP, EiS, Gain, EVM etc.
OTA R&D (WPTC Spiral Scanner)	• Frequency: 0.4 to 40 GHz • Minimum Size: 250 x 250 x 220 cm • Fields: Near & Far • Signals: Modulated /CW • Parameters: EIRP, EiS, Gain, EVM etc.
OTA R&D and Production (ATS1000)	• Frequency: 0.4 to 90 GHz • Minimum Size: 85 x 100 x 180 cm • Fields: Near & Far • Signals: Modulated /CW • Parameters: EIRP, EiS, Gain, EVM etc.
OTA R&D (DST 200)	• Frequency: 0.4 to 40 GHz • Minimum Size: 77 x 76 x 70 cm • Fields: Far Field (UEs) • Signals: Modulated / CW • Parameters: EIRP, EiS, Gain, EVM etc.
OTA Production (NRPM OTA Power Sensors)	• Frequency: 28 to 75 GHz • Minimum Size: 45 x 40 x 48 • Fields: Near & Far • Signals: Modulated / CW • Parameters: EIRP at single points
PWC200 (Plane Wave converter)	• Frequency: 2.3 to 6 GHz • Bandwidth: 200 MHz • Dimensions: 70.86 in x 70.86 in • Quiet zone size: 100 m • EVM, EiS, EIRP, Gain, Antenna calibration etc.

References

➤TS 38.101 - User Equipment Radio Transmission and Reception
➤TS 38.104 - BS Radio Transmission and Reception
➤TS 38.141 - BS Conformance testing

Also refer 5G test cases and 5G test equipments from Keysight technologies >>.

5G NR Numerology | 5G NR Terminology

• 5G NR PHY timing unit • 5G NR SLIV • 5G NR Mini Slot • 5G NR Bandwidth Part • 5G NR CORESET • 5G NR DCI Formats • 5G NR UCI • 5G NR slot formats • 5G NR RRC IEs • 5G NR RSRP RSRQ SINR

5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences

• 5G NR SSB, SS, PBCH • 5G NR PRACH • 5G NR PDCCH • 5G NR PUCCH • 5G NR Reference Signals • 5G NR m-Sequence • 5G NR Gold Sequence • 5G NR Zadoff Chu Sequence

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