Tech Briefs Magazine - December 2022 - 15

methods and requirements for NR base
stations in the technical specifications
TS 38.141-1 (conducted testing) and
TS 38.141-2 (radiated testing). Chapter
six of the specifications lays out the
requirements for transmission tests in
base station transmitters.
For 3GPP standards, EVM measurement
is one of the primary metrics to
assess the quality of the transmitted
signal from the base station. The error
vector is the vector difference between
the in-phase and quadrature (IQ) reference
signal and the measured signal
at a given time. EVM measurement is
sensitive to any signal impairment affecting
the magnitude and phase trajectory
of a signal for any digital modulation
format. Therefore, EVM is an
ideal tool for diagnosing problems in
the baseband, the intermediate frequency
(IF), or the RF sections of a
communication system.
Many factors influence the quality
of a signal, including baseband signal
processing, modulation, filtering, and
up-conversion. Orthogonal properties
in orthogonal frequency division multiplexing
(OFDM) systems prevent interference
between overlapping carriers.
However, impairments such as IQ impairments,
phase noise, linear compression
(AM to AM), and nonlinear compression
(AM to PM), as well as frequency errors
can distort the modulated signal.
To avoid the impact of signal degradation
on design performance, device
designers need to overcome the physical
challenges present in wide bandwidth
and high-frequency signals. Measurement
and characterization of signal
quality require test solutions with better
performance than the device under test
(DUT) without introducing new signal
impairments. In Figure 1, see examples
of IQ modulation impairments that
cause constellation symbols to deviate
from their defined locations. These deviations
provide information about the
nature of the impairments based on the
type and intensity of deviations.
EVM measurements during conformance
testing verify modulation quality
is within the limits specified by the minimum
requirements. Antenna connectors
and transceiver array bound (TAB)
connectors must meet the minimum reTech
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quirements when testing one at a time
or in parallel. It is necessary to repeat
tests for all antenna and TAB connectors.
For example, EVM for NR carriers
of different modulation schemes
on a physical downlink shared channel
(PDSCH) must demonstrate less than
the limits that appear in Table 1. It is
necessary to perform EVM measurements
for all bandwidths on all allocated
resource blocks and downlink slots within
10 ms measurement periods.
Challenges for Testing
5G Private Network Base
Station Transmitters
Confronting Path Loss
It is possible to perform OTA tests for
either the near-field or far-field. While
near-field measurements may be appropriate
for some applications, 5G cellular
communication
requires
far-field
assumptions.
The far-field distance and
path loss associated with radiated waves
increase with frequency due to the nature
of the waves. Figure 2 shows how the
characteristics of these signals change as
they propagate from antenna arrays and
develop further in the far-field region.
The greater the path loss and the larger
the distance between the DUT and
the probe antenna, the more challenging
the test. Due to the excess path loss,
getting accurate OTA measurements is
challenging using higher frequency devices.
Because of the excessive path loss
between instruments and DUT, signal
analysis
measurement becomes even
more challenging due to the low signalto-noise
ratio (SNR). Low SNR degrades
base station transmitter measurements
such as EVM and misrepresents the performance
of the DUTs. Therefore, 5G
private networks require engineers to rethink
their design and testing processes.
Measurement Uncertainties
The wideband noise that contributes
to measurement uncertainties and
complexity represents another testing
challenge for 5G private network base
station transmitters at higher frequencies
and wider bandwidths. With higher
frequencies and wider bandwidths, it is
possible to achieve faster data rates, lower
latency, higher data throughput, bet15
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Tech Briefs Magazine - December 2022

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