Figure 3 shows the classes and functions of the excitation/source instruments and receiving/analysis instruments commonly used in RF test. Among them, the vector signal generator (VSG) and signal analyzer provide the most comprehensive measurement and analysis of a communication system’s overall performance. The operating frequency range must cover from DC to approximately 110 GHz, while supporting 200 MHz to several GHz of vector signal bandwidth. Regarding frequency coverage, the Keysight E8267D PSG and the R&S FSW85 signal analyzer have reached 44 and 85 GHz, respectively. A breakthrough was announced at Mobile World Congress in February 2016: R&S exhibited the world’s first VSG (SMW200A) with a maximum frequency of 40 GHz and a modulation bandwidth up to 2 GHz. The maximum signal analysis bandwidth of two well-known signal analyzers products, i.e., FSW from R&S and UXA from Keysight, is 2 and 1 GHz, respectively. Further bandwidth expansion requires the assistance of other components. To generate/analyze signals with ultra-large (GHz) bandwidths, the main technical difficulties include RF channel equalization, high sampling rate ADCs, high speed digital signal processing and high data rate transmission.

For antenna array testing, the vector network analyzer (VNA) is a key instrument. Due to the lack of a single, 64-port VNA, three methods are usually adopted. The first is a step-by-step test using a single, multi-port VNA, which is relatively inexpensive but sacrifices test speed and ignores the coupling characteristics between antenna elements. The second involves cascading several multi-port VNAs, e.g., a 64-element antenna array is tested with eight, 8-port VNAs cascaded (see Figure 4). This approach can accurately test the actual S-parameters of each antenna element after calibration and greatly increases test speed. Still, there are some technical difficulties: the crosstalk between ports restricts dynamic range, and calibration time impacts test efficiency. The third scheme uses the conventional dual-port VNA with a switch matrix, which is a compromise between the prior two options. Cost is relatively low, but the speed is somewhat slow and the switch matrix introduces measurement errors. A few manufacturers are developing a single multi-port VNA to provide new solutions that address crosstalk between channels, fast calibration, cost and other aspects.

OTA test¹⁹ is another important aspect of 5G antenna array testing, for two reasons. First, directional indicators of the antenna array, e.g., effective isotropic radiated power (EIRP) and effective isotropic sensitivity (EIS), must be tested by OTA, which is consistent with 4G MIMO OTA test principles. Second, since 5G will use the microwave and mmWave bands, the antenna array and T/R elements will likely be integrated to reduce loss and improve matching. In this situation, most T/R component characteristics cannot be evaluated without wired conduction tests, and measures of performance such as RF circuit transmit power and sensitivity may interact with the characteristics of the antenna, making individual assessment difficult.

IC, Network and User Equipment Test

Although 5G-related technologies and standards are not yet clear, IC, UE and network equipment manufacturers, as well as operators, are in full swing conducting R&D of 5G prototypes to launch competitive solutions. Among the existing 5G prototype UEs, some support high speed transmission of several Gbps and some support as low as millisecond latency. The battery life of certain UEs (particularly IoT terminals) has been extended to nearly 10 years. Qualcomm, Spreadtrum, MediaTek and other IC manufacturers are developing 5G chips, and Qualcomm has announced prototypes. Because of the emerging 128-channel integrated network equipment, the corresponding test technology has been placed on the agenda. Three features, including UE diversity, scenario complexity and massive connections, challenge the testing of ICs, UEs and network equipment,²⁰ and available 4G/LTE test instruments can hardly fulfill these 5G tasks.