A conventional method to model the radiation pattern of the entire array is to multiply the pattern of one element by an array factor. The array factor is the radiation pattern that would result if the antenna elements were isotropic. Thus, Equation (22) can be expressed as:
Equation (23) is referred to as a pattern multiplication, where Eelement is the amplitude of the element’s field and AF is the array factor given by:
The array factor does not depend on the type of elements used in the array but rather depends on their spacing and the phase difference between the radiating elements. Figure 9 shows the simulated radiation patterns of 7-element and 13-element antenna arrays. Note that increasing the number of elements from 7 to 13 produces a narrower antenna beam, which helps in reducing interference with other co-located radio systems.
Figure 9 Simulated array factor for 7-element (a) and 13-element (b) linear arrays.
OTA CONFORMANCE TESTS OF 5G BASE STATIONS
In OTA testing of 5G mmWave base stations transceivers (i.e., operating in FR2 band),5,11,12 the test specifications are defined at the radiated interface boundary (RIB). Figure 10 shows the RIB for OTA testing of base stations Type 1-O and Type 2-O.2
Figure 10 RIB for base station Type 1-O OTA testing.
OTA conformance testing of 5G mmWave base stations is mandatory. The base station’s transmitter and receiver OTA tests are presented and explained in the following sub-sections.
OTA Conformance Tests of 5G NR Transmitters
Table I lists the mandatory OTA conformance tests for 5G mmWave base station transmitters.
TABLE I - TEST REQUIREMENTS FOR 5G MMWAVE BASE STATION TRANSMITTERS
In Table I, the OTA base station output power is the total radiated power (TRP) defined at the RIB during the transmitter ON period. It must be met for any beamforming setting for which the beam peak direction is within the OTA peak direction set.
The OTA total power dynamic range is the difference between the maximum and the minimum transmit power of an orthogonal frequency division multiplexing (OFDM) symbol for a specified reference condition. This requirement applies at each RIB supporting transmission in the operating band.
The OTA transmitter transient period is the time during which the transmitter changes from the OFF period to the ON period or vice versa. This requirement applies at each RIB supporting transmission in the operating band.
The OTA frequency error is the measure of the difference between the actual base station transmit frequency and the assigned frequency. It is defined as a directional requirement at the RIB and is met within the OTA coverage range.
The Modulation quality is defined as the difference between the measured carrier signal and a reference signal. It is expressed as EVM and is defined as a directional requirement at the RIB to be met within the OTA coverage range.
The OTA Time Alignment Error (TAE) is the largest timing difference between any two different NR signals. It is defined as a directional requirement at the RIB and is met within the OTA coverage range.
The OTA occupied bandwidth is the width of a frequency band such that, below the lower and above the upper-frequency limits, the mean powers emitted are each equal to 0.5 percent of the total mean transmitted power. This requirement applies during the transmitter ON period for a single transmitted carrier. It is defined as a directional requirement and is met at the RIB.
The OTA adjacent channel leakage power ratio (ACLR) is the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency. The measured power is TRP. This requirement is defined at the RIB during the transmitter ON period.
OTA operating band unwanted emissions (OUBE) define all unwanted emissions in the downlink operating band plus the frequency ranges 10 MHz above and 10 MHz below the band.
OTA transmitter spurious emission limits apply from 30 MHz to the second harmonic of the upper-frequency edge of the downlink operating band, excluding the frequency range from ΔfOBUE below the lowest frequency of the downlink operating band, up to ΔfOBUE above the highest frequency of the downlink operating band (e.g., for base station type 2-O, the ΔfOBUE is 1500 MHz).
OTA transmitter intermodulation (IM) is a measure of the capability of the transmitter to prevent the generation of signals in its non-linear elements caused by the presence of the wanted signal and an interfering signal reaching the transmitter via the radio distribution network and antenna array from a co-located base station. The test applies at each RIB supporting transmission in the operating band.
OTA Conformance Tests of 5G NR Receivers
In OTA testing of 5G base station receivers, receiver performance parameters are evaluated by measuring the throughput; the throughput should be ≥ 95 percent of the maximum possible. Table II lists required receiver OTA tests for 5G mmWave base stations.
TABLE II - OTA TEST REQUIREMENTS FOR 5G MMWAVE BASE STATION RECEIVER
The reference sensitivity power level (PREFSENS) is the equivalent isotropic sensitivity (EIS) level at the center of the QZ in the receiver beam peak direction. The throughput should meet or exceed the requirements for the specified reference measurement channel. The reference receive sensitivity is defined assuming a 0 dBi reference antenna located at the center of the QZ. It is a directional requirement that is given by:
and
Where:
BW = channel bandwidth in Hz
NF = receiver’s noise figure in dB
The receiver becomes more sensitive as its antenna gain increases. Thus, the receiver’s highest sensitivity is in its antenna beam’s peak direction. Therefore, receiver performance should be tested in the beam peak direction (θmax, φmax).
The receiver’s OTA dynamic range is a measure of its capability to receive a wanted signal in the presence of an interfering signal inside the received channel bandwidth. The requirement applies at the RIB when the angle of arrival (AoA) of the received incident wave and the AoA of the interfering signal are from the same direction. The wanted and interfering signals apply to all supported polarizations, under the assumption of polarization match.
OTA adjacent channel selectivity (ACS) is a measure of the receiver’s ability to receive an OTA-wanted signal, at its assigned channel frequency, in the presence of an OTA adjacent channel signal. The requirement applies at the RIB when the AoAs of the incident wave of a received signal and the interfering signal are from the same direction. The wanted and interfering signals apply to all supported polarizations, under the assumption of polarization match. The throughput must be ≥ 95 percent of the maximum throughput of the reference measurement channel.
OTA in-band blocking is a measure of the receiver’s ability to receive an OTA-wanted signal in its assigned channel in the presence of an unwanted OTA interferer, which is an NR signal with one resource block (RB). The requirement applies at the RIB when the AoA of the incident wave of a received signal and the interfering signal are the same. The wanted and interfering signals apply to all supported polarizations, under the assumption of polarization match. The throughput is ≥ 95 percent of the maximum throughput of the reference measurement channel with OTA-wanted and OTA-interfering signals.
OTA out-of-band blocking is a measure of the receiver’s ability to receive a wanted signal at the RIB in its assigned channel in the presence of an unwanted interferer. The requirement is at the RIB when the AoA of the incident wave of the received signal and the interfering signal are the same. The wanted signal applies to all supported polarizations, under the assumption of polarization match. The interferer is polarization-matched in-band and the polarization is maintained for out-of-band frequencies. The throughput is ≥ 95 percent of the maximum throughput of the reference measurement channel with OTA-wanted and OTA-interfering signals.
OTA receiver spurious emissions are the power of the emissions radiated from the antenna array from the receiver. It is characterized by measuring the TRP, and it is defined at the RIB.
OTA receiver intermodulation is the third and higher-order mixing of the two interfering RF signals that can produce an interfering signal in the band of the desired channel. Intermodulation response rejection is a measure of the capability of the receiver to receive a wanted signal on its assigned channel frequency in the presence of two interfering signals that have a specific frequency relationship to the wanted signal. It is defined as a directional requirement at the RIB.
OTA in-channel selectivity (ICS) is a measure of the receiver’s ability to receive a wanted signal at its assigned RB in the presence of an interfering signal received with a larger power spectral density. In this condition, a throughput requirement must be met for a specified reference measurement channel. The interfering signal is time-aligned with the wanted signal.
CONCLUSION
The basic concept of OTA testing for characterizing 5G mmWave base station transceivers is discussed. DFF and IFF CATR test methods are explained as well as antenna basics essential for performing OTA testing
References
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