Importance of OTA Measurements in Ensuring 5G Networks Meet KPIs

November 18, 2019

The advent of 5G is finally here. In countries all over the world, spectrum allocations have been made, with 5G services generally sitting in and around 3.5 GHz or in the 28 GHz and 39 GHz millimeter-wave (mmWave) bands. There are significant architectural difference between 4G and 5G, which require network operators and device makers to make considerable investments of time, money, and manpower to enable 5G. For example, while 4G networks use omni-directional antennas to transmit ubiquitous signals, 5G networks utilize active antenna systems (AAS) with beamforming and massive MIMO to help focus strong signals toward each device in the coverage area.

Figure 1: Conducting OTA measurements will help ensure 5G networks meet KPIs

To make transmissions, 5G networks will be more densely packed with the new antenna systems that leverage beamforming technology. With this technology, power is focused into a narrow, directional beam rather than transmitting it in all directions, like with LTE. The beam is aimed and energy transmitted directly toward the mobile user or other device connected to the base station. Because these beams are transmitted with concentrated power, there are inherent radiated power characteristic that need to be monitored and their potential effects minimized to avoid interference issues. With this higher level of complexity, there needs to be a new generation of test solutions that conduct analysis differently and perform over-the-air (OTA) measurements.

5G OTA Testing

Because of its new and unique architecture, deploying and verifying 5G networks must be carried out differently than any of its predecessors. Traditional 4G/LTE test methods typically involve transmitting the expected power level while a network technician conducts measurements using equipment physically connected to the base station via a coaxial cable to an RF test connector. This type of hard connection allows calibrated measurements for channel power and modulation quality, as well as decoding some basic cell site information to be conducted. The design of an LTE macro site is usually comprised of a radio base station in an equipment room at ground level, with long RF cables feeding the antenna atop a tower. These cables are generally 10 meters or more in length. The losses and reflections in this feed to the antenna has been a common source of performance issues that require testing.

For 5G, this approach will not work, as the base station has changed in the traditional sense. There is no remote radio head (RRH) with a single test port monitoring the RF output to a common antenna. Now there is an antenna array with typically 64 elements – and there is no single test port that can monitor all of those. Testing needs to be performed on a fully formed beam OTA. For a 3.5 GHz radio, this would be at least 10 meters away. If this AAS is mounted to a rooftop or somewhere difficult to gain access to, then testing must be done at an even greater distance. OTA testing allows technicians to position themselves in the far field distance from a base station, wirelessly connecting to conduct new tests.

OTA Performance: Validation and Test

There are several critical tests to ensure 5G networks are transmitting properly, including:

Equivalent Isotropic Radiated Power (EIRP) – EIRP indicates the amount of power that would be required to be fed into an omni-directional antenna to give the same signal strength as that measured at a distance and direction from the antenna. It shows the effectiveness of beamforming technology. Essentially, knowing the EIRP measurement helps network operators validate that their transmitters are performing as promised.

Interference – The wider-band signals associated with 5G means signals are more prone to overlap, making interference an even greater concern than in 4G (as impossible as that sounds considering the worry associated with interference in existing LTE networks). Field technicians need to verify signal boundaries and determine how much a signal may be bleeding into neighboring bands.

Harmonic and Spurious – Beamformed signals can generate side lobes that can be radiated in any direction from a base station during transmission. OTA test solutions can effectively measure these signals to ensure that when high-powered signals are transmitted other signal attributes being generated are below a strength level threshold where they would cause interference. Figure 2 is a display of such a measurement taken with the Field Master Pro™ MS2090A.

Figure 2: Power measurement taken with the Anritsu MS2090A.

Other Considerations

OTA 5G measurements are typically made on the primary synchronization signals in the 5G sync signal block (SSB). The beams in the SSB are continuously transmitted and can be used to perform basic RF measurements in the field. Measurements on data channels will always be more challenging, as beams are only formed after a UE has established a data channel with the base station, and the beams are directed to the UE. So, test solutions must have signaling or UE capability to create and align with these beams to conduct necessary measurements.

Using OTA tests for all these needs will help network operators and their partners prove that new 5G networks meet key performance indicators (KPIs). That means greater consumer experience and the ability to effectively roll out 5G while also opening the door to new applications and user experiences. To learn more, download this Proving 5G: How OTA Testing Helps Enable a New Mobile Era application note.

 

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