Electronics & Scientific Guide, Measuring & Testing

What is Over The Air (OTA) testing

Terminal devices and base stations for the fifth mobile generation are much more integrated than LTE models. There are also considerably more device variants. In their development, OTA tests will play a crucial role.

Over-the-air (OTA) tests can be used to assess and certify the reliability and performance of wireless devices such as cell phones, tablets, and base stations. It is essential to perform such OTA tests in an environment that is as close as possible to the actual conditions of use. The testing of components for the future 5G environment will be significantly different from those for 4G / LTE.

A simple and inexpensive process is to connect the mobile devices to the test equipment via cables. However, the tester does not simulate the actual behavior of the devices under real operating conditions. With increasing degree of integration of the tools, this method is less and less. Network operators, who need to use higher frequencies to obtain the necessary bandwidth for 5G, are facing significant challenges. To test mobile devices under realistic conditions, the tests must be wireless over the air interface. The developers can thus check the actual propagation of the radio waves through the air from one terminal to the base station and vice versa from the base station to the terminal.

Two reasons for an OTA test at 5G

For the development of 5G OTA tests are necessary for two main reasons: Firstly, the degree of integration of the test items increases considerably, so that it is technically no longer possible to connect the test items to the test equipment via cables. That’s why there is no alternative to OTA testing.

Second, the signal absorption rates at millimeter-wave frequencies are much higher. To achieve a sufficient signal level here, a focusing and shaping of the transmit beam are required (beam focusing / -forming) (Figure 1). Accordingly, test structures for the beam characterization and the examination of the beam detection and tracking are necessary. Only OTA test systems offer this functionality.

The current state of the OTA test technology

Numerous regulatory agencies, standards organizations, industry bodies and network operators impose testing of mobile devices over the air interface. To ensure global accessibility and interoperability of mobile systems, certification tests have been developed to help manufacturers worldwide maintain the same quality standards across all new mobile devices. The Cellular Telephone Industries Association (CTIA) has set standards for OTA testing of 3G and 4G LTE devices and had certification labs around the world. Minimum OTA performance requirements have been defined regarding the transmit radiated power and receiver sensitivity so that all calls are received under predefined conditions.

As a rule, all devices that emit electromagnetic waves are tested via the air interface during the R & D phase. With current mobile phones, for example, the tests should ensure that the signal is homogeneous, i.,e. That the same signal is transmitted in all directions and conversely received from all directions (Fig. 2). It is important that the antenna transmits in all directions so that the user does not have to turn in a particular direction to get a good signal. Besides, the radio link must not break off when the user passes a tall building. In particular, OTA test equipment designed for R & D is useful for identifying potential problems at an early stage of product development.

The effects of 5G on OTA testing

To accommodate the extra users and the required higher bandwidths and higher data rates, mobile operators need to resort to higher frequency centimeter and millimeter wave bands. They are at 30, 40, 50, 60 or even 90 GHz. Just as the wavelength is inversely proportional to the frequency, so does the transmit range for a given power level with frequency. New developments are intended to reduce the losses caused by free space damping, air absorption, scattering by rain and gases and shading. This significantly increases the degree of integration of 5G components. In such devices, tests on cable connections are extremely difficult or even physically impossible to perform. Therefore, OTA tests will play a crucial role for 5G.

The signal absorption associated with the loss sources described above increases significantly at higher frequencies. However, to ensure the necessary communication range, manufacturers must either increase transmitter power or focus the radiated energy from the mobile device to a robust and narrow beam. For this new antenna structures and antenna arrays are needed, which ensure that the poles are focused correctly. The focused beams require a spatial or directional component to ensure that the beam is pointing in the right direction and that the system can switch the beam when a communication channel is blocked. The beamforming method extends the multi-antenna concept known as MIMO (Multiple Input Multiple Output) because a base station can now simultaneously send data to several, independently positioned terminals. Beamforming reduces power consumption because the main lobe directly addresses a terminal with the signal assigned to it, while other terminals are explicitly excluded (Fig. 3). These are thus not disturbed by the sign.

The use of connectors for testing will not be possible due to the high costs, the high losses and the degree of coupling (Figure 4). Also, so-called massive MIMO systems with 64 to 128 polarized antennas, the transceivers are integrated into the antennas, which leads to a loss of RF test ports (Figure 5). Here, the radio and antenna performance of the DUT can be measured via the air interface alone. Therefore, OTA tests are of fundamental importance to 5G. They are the prerequisite for the development of new designs and their certification. The test systems for 5G are expected to have substantially the same basic components as before but adjusted for the higher frequencies.

The system components for an OTA test

The key components of an OTA test system are a test chamber with positioning equipment, signal generation and analysis instruments, and measuring antennas. Also, there is a control and reporting software for automating the measurements (Fig. 6). The communication between the device under test and the measuring antenna must be set up so that the device under test sends and receives the signals correctly. Today, OTA tests are carried out in an ideal, i.e., a shielded and encapsulated environment within a reflection- and echo-free absorber chamber. This chamber is lined with foam pyramids for absorbing reflected signals. Their size depends on the object and the frequency ranges to be tested. The tests take into account the radiation characteristics of the devices,

Examples of typical OTA measurements

Although different usage scenarios such as the use of a mobile device indoors or outdoors, in urban or rural surroundings, in open spaces or forests, stationary or in motion and the vicinity of other mobile devices are relevant in practice, they can not be quantified for testing purposes , Therefore, such real-life conditions are simulated using a variety of well-defined test scenarios, which are currently being developed further for 5G applications. For certification testing, test chambers of the specified specification are used to provide accurate, repeatable and reproducible measurements.

Test engineers use OTA tests to test performance factors such as signal path, antenna gain and radiation characteristics, as well as the radiated power and sensitivity of the device under test to internal components and other devices. There are also tests for reliability and safety aspects. In May 2015, CTIA’s Test Plan for Wireless Device Over-the-Air Performance defined the test, configuration, and measurement methods for OTA measurements. These include only the measure of performance and performance.

Key OTA tests currently include Total Radiated Power (TRP), Total Isotropic Sensitivity (TIS), Total Radiated Sensitivity (TIA), Total Radiated Sensitivity (TIS). TRS, according to the 3GPP specification), Equivalent Isotropically Radiated Power (EIRP), and Radiated Receiver Sensitivity on Intermediate Channels (RSIC) of a device. The TRP is an indicator of transmitter performance, while TIS and TRS relate to the receiver. Further measurements are made to determine the radiation pattern and efficiency of the antenna. Coexistence measurements are used to evaluate the sensitivity loss in parallel operation of multiple mobile technologies. When OTA tests are performed during the development of a device, developers can solve the identified issues early and optimize device performance.

What should be considered when measuring 5G-OTA

The construction of an OTA test system and the performance of the measurements involve various challenges. Some of this concerns the antenna system. Thus, in the context of the 5G introduction, a suitable measurement setup and the correct positioning of the 3D antennas for testing the mobile beams must be determined to take into account interference signals and scattering. This requires a new dimension of measurement – space or power versus radiation direction. In particular, the devices must also take into account the blocking effect of the human body on the radiation pattern. For this purpose, phantoms are used in the OTA tests (Fig. 7).

OTA tests to determine the three-dimensional antenna pattern can be performed either in the near field or in the far field. Measurements in the near field have the advantage that smaller absorber chambers can be used. However, they require a measuring setup with which both the phase and the amplitude can be measured with high spatial accuracy, and make post-processing, the so-called near-field, far-field transformation, necessary (Figure 8).

Another challenge is that each transceiver in the active antenna system must be characterized via an OTA interface. Measurements must be made for both the transmitter and the receiver. For personal verification, each transceiver must be powered up and a group of transceivers for a common assessment.

Dynamic beam measuring systems necessary

A third challenge concerns beamforming, which plays an essential role in 5G. Due to the high radio field attenuation and limited range of a millimeter-wave radio system, it is crucial that the devices generate the transmit beam precisely, able to capture and track it quickly. Static beam pattern characterization is sufficient for existing cellular technologies, but future millimeter wave systems require dynamic beam measurement systems to characterize beam tracking and beam steering algorithms accurately.

Some particular challenges related to devise testing for RF compliance. Today, well-characterized cable-based test port connections are used for this purpose, allowing repeatable measurements. Since 5G devices have no RF test ports with connectors, the appropriate test setup and calibration for an OTA environment must be defined.

Tests in the production

A similar problem arises during production. Radiation measurements are mandatory for all sparking devices. With extremely high production speeds, OTA test systems need to be flexible and adapt quickly to the testing needs of future and unpredictable devices without sacrificing quality or thoroughness of test procedures. The production tests include functional tests of the completely assembled unit as well as the calibration of the antenna system. This ensures that the mismatch between the RF signal paths remains below a certain threshold.

The selection of an OTA test system

When choosing an OTA performance test system, flexibility and scalability are the most important criteria for making it suitable for a variety of frequency ranges, device sizes, and usage patterns. An OTA test system can be a collection of individual components that the user himself assembles and integrates. But more useful is a turnkey system. Given the complexity of the methods and the necessary integration of many different components, the acquisition of a turnkey system is the fastest way to success. First, the components are coordinated. Second, the OTA test system vendor can assist in selecting the features that are needed, suggesting additional features for specific customer needs, and managing all phases from installation to use.

With 5G, new procedures will emerge for OTA testing. The equipment must be adaptable to future mobile device designs that are far from over. To keep an eye on the latest trends and overarching market requirements, the provider of the OTA test system should be in close contact with the device and component manufacturers and should be involved in the relevant standardization committees and workshops.

Rohde & Schwarz offers this for OTA tests

Rohde & Schwarz’s Performance Test System for OTA R & S TS8991 is a one-stop, one-stop wireless test system that meets industry and regulatory certification requirements. It includes an absorber chamber, positioning equipment, testing equipment and automated measurement software. It is compliant with the CTIA, CTIA & Wi-Fi Alliance, and 3GPP-compliant test plans. The test system is available in different sizes. Also, the modular design allows customized configurations. The methods can be tailored to individual customer requirements regarding size, functionality, frequency range and possible applications. First versions for 5G test applications are already installed.

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