OTA testing to gain importance with 5G
The next few years will see many more wireless users connect to wireless networks. Wireless users will expect a higher level of quality and accessibility from all their devices. Thus, carriers will need to provide better reliability in the network and in devices. The result: Testing will, therefore, need to evolve to more closely emulate actual usage conditions. Over-the-Air (OTA) testing will become essential for engineers to evaluate and certify the reliability and performance characteristics of wireless devices, both for mobile and fixed location.
Testing the components that will support 5G will be vastly different than for 4G/LTE. Connecting mobile devices to test equipment through cables is convenient and cost-effective, but it can't mimic the actual condition these devices encounter. OTA testing lets engineers see what truly happens as the radio waves propagate over the air from the user equipment to the base station and back.
Two main drivers will necessitate OTA testing. The level of integration of the device under test (DUT) will increase significantly, particularly as devices add antenna arrays. Greater integration will make connecting DUTs to test equipment by cables physically impossible. Second, at millimeter wave frequencies, signal absorption rates are much higher, requiring the need for beam focusing or forming to boost the gain. Test setups are needed for beam characterization and for checking beam acquisition and beam tracking performance. OTA testing will become essential.
The current state of OTA testing
OTA testing of wireless devices is required by numerous regulatory agencies, standards organizations, industrial bodies, and carriers. To attain global access and interoperability of mobile systems, certification tests have been developed so that manufacturers around the world provide the same level of quality in all new mobile devices.
Cellular Telephone Industries Association (CTIA) has set standards for OTA testing of 3G and 4G/LTE devices, and has certification labs all around the world. Minimum performance requirements for OTA behavior have been defined in terms of transmitter power levels and receiver sensitivity levels. In the US, wireless carriers have also established industry performance requirements that a device must meet before it will run on a network.
OTA testing is typically used during the R&D phase for all equipment that radiates electromagnetic waves. In current mobile phones, for example, testing is designed to ensure that the signal is homogeneous, in that the same signal is transmitted or received from all directions (Figure 1). It is important that the antenna radiates in all directions, so that the mobile device user does not need to face in a particular direction to get a good signal, nor should a call be dropped as the user passes by a tall building.
Figure 1. Today's mobile phone are designed amd tested for a uniform field.
OTA testing at cm and mm wave frequencies
For the wireless industry to attain space for these additional users and the wider bandwidth and higher data rates they require, mobile operators need access to higher frequencies. In going to frequencies of 30, 40, 50, 60, and even 90 GHz, the devices enter the cmWave and mmWave ranges. As the wavelength becomes shorter, the transmission distance for a given power level also shrinks. 5G technology must compensate for losses such as free space path loss, atmospheric absorption, scattering due to rain and gases, and line-of-sight issues. The new devices for these applications will become so highly integrated that using cable connections for testing will be very difficult, if not physically impossible, making OTA testing critically important for 5G.
Based on the above losses, the signal absorption becomes much higher at higher frequencies. To achieve the necessary communication distance, providers either need to increase the transmitter power or focus the radiated energy from the mobile device into a sharp, narrow beam (Figure 2).
Figure 2. Mobile devices will need to focus their transmitter beams to maximize transmit power at mmWave frequencies.
Creating narrow beams will require new antenna structures and arrays to ensure proper that beams focusing. There will be a spatial or directional component to the focused beams, which can ensure the beam is pointing in the right direction and switch the beam if there is a blocked communication channel. This beam forming technique will extend the multiple antenna concept known as multiple input multiple output (MIMO) by sending data to different user devices simultaneously. Doing so exploits their uncorrelated locations. Beam forming will also reduce energy consumption; it will target individual user equipment and specifically leave out others with their assigned signal.
Figure 3. Beamforming can direct power where it's needed while minimizing interference to other devices.
Using connectors for testing won't be practical because of their high costs, high losses, and the degree of coupling. Also, in the case of massive MIMO systems, the radio transceivers are integrated directly with the antennas (Figure 4), resulting in a loss of RF test ports, meaning that the DUT radio and antenna performance can only be measured over-the-air.
Figure 4. A 5G capable device could consist of an array of polarized antennas, making wired test ports impractical or impossible.
OTA testing will be a prerequisite for the new designs and their certification. For 5G test systems, the basic components are expected to remain essentially the same, but they must be adapted for the higher frequencies, which will mean smaller antennas.